Mechanical Engineering Interview Questions Answers

Mechanical Engineering Interview Questions Answers

 

Mechanical Engineering Interview Questions Answers Part 1

1. Where Multi-stage pump used?
Pressure washing of Aircraft, Trains, Boats and Road vehicles as well as Spray washing of industrial parts and Electronic components.

2. What is the function of Scoop in BFP (Boiler Feed Water Pump) in Thermal Power Station?
The Function of Scoop tube is regulating the varying amount of oil level in the coupling during operation of infinite variable speed.

3. In the Thermal Power Plant why Deaerator (D/A) placed on height?
To build a Very high pressure and the temperature for a boiler feed water pump and it discharge high pressure water to the boiler. And to provide the required Net Positive Suction Head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feed water during rapid changes in BFP.

4. How to determine the capacity of Refrigeration system? How we use Condenser coils, Compressor, Capillarity?
To determine the capacity of Refrigeration system, test of C.O.P is used as well as the Condenser coils, Compressor, Capillarity, based on the properties.

5. In orifice why the Pressure and Temperature decreases?
Orifice is a small hole like a nozzle. When a high pressure fluid passes through the orifice, Pressure gets reduced suddenly and the velocity of the fluid gets increased. Also the heat transfer rate increases. We know that Heat transfer rate is directly proportional to the difference in temperature, Area and the Heat transfer coefficient. Heat transfer Coefficient remains constant for a fluid at a particular temperature.

 Q= KA (T1-T2)

If the heat transfer rate increases, it seems the difference in temperature gets increased. There is no way in reduction of inlet temperature of the orifice. As a result, the outlet temperature of the orifice gets reduced. Hence the pressure and temperature gets reduced when it passes through orifice.

6. What is the difference between Bolt and Screw?
The main difference is based on the load acting on it, and the size. For smaller loads, screws are enough but in case of greater-loads, bolts are to be used. In bolt we give centrifugal force or tangential force and screw we give axial force for driving.

7. How to calculate force required to bend a metal sheet into pipe?
It depends upon the thickness of sheet and required diameter to be formed from the sheet.

8. A bearing is designated as 6205 , what is it’s bore diameter?
Generally for bearing bore diameter is 5 times the last number in its designation i.e. for designation 6205, Bore diameter should be 25.

9. where half nut is use?
half nut is used in lathe machine. In half nut mechanism is engaged during the thread cutting operation.and it connect carriage and lead screw.it have 2 portion one half is fixed and one movable.

10. why rails are in the form of I-beam?
It’s flexural rigidity is high and it’s cross section consume less metal.

11. Difference between Performance and Efficiency?
• The accomplishment of a given task measured against preset known standards of accuracy, completeness, cost, and speed is called as Performance.
• Efficiency is defined as the input given and the work obtained from that input like money, time, labour etc. It’s the main factor of productivity.

12. Difference between Anti friction Bearing and Journal Bearing
Generally, journal bearings have higher friction force, consume higher energy and release more heat, but they have larger contact surface, so normally used in low speed, high load applications. In anti friction bearings friction is less. One object just rolls over each other.

13. What is a Cotter joint?
A cotter joint is used to connect rigidly two co-axial rods or bars which are subjected to axial tensile or compressive forces. Here shaft is locked in place by a smaller pin that passes through the side of the lug and partly or completely through the shaft itself. This locking pin is named as cotter.

14. What is difference between Hardness and Toughness?
• Toughness is the ability of a material to absorb energy.
• Hardness is the ability of a material to withstand wear.

15. What is Auto Dosing?

Auto dosing is an automated system of feeding the equipment with liquid products. It is the ideal way to ensure the correct calibrated dose at the right time every time in auto.

16. Difference between Codes, Standards and Specifications?

  • Code is procedure of acceptance and rejection criteria.
  • Standard is accepted values and compare other with it.
  • Specification is describing properties of any type of materials.

17. What are differences between Welding & Brazing?
In Welding concentrated heat (high temperature) is applied at the joint of metal and fuse together.
In Brazing involves significantly lower temperatures and does not entail the melting of base metals. Instead, a filler metal is melted and forced to flow into the joint through capillary action.

Mechanical Engineering Interview Questions Answers Part 2
What is OEE?

OEE means Overall Equipment Effectiveness. This terminology widely used in Total productive maintenance, which is used to calculate the effectiveness of machines in manufacturing. Basically it captures the losses of machines in production and try to improve defects on machines. Higher the OEE, more capable is the machine.

Why In-volute Curve is used in Gear?

Involute curve is the path traced by a point on a line as the line rolls without slipping on the circumference of a circle. Involute curve has a contact angle between two gears when the tangents of two gears pass through the contact point without friction.

What is Bearing stress?

The stress which acts on the contact surface area between two members is known as Bearing stress. An example for this is the stress between nut and the washer.

Which is hard material Cast Iron or Mild Steel?

Cast iron.  Due to the excess carbon content than mild steel it is harder. The more carbon content, the more hardness will be. But it reduces the weldability due to this hardness. It is brittle too.

What is the difference between a Shaper machine and a Planner machine?

In Shaper machine tool is having reciprocating motion and work piece is clamped on table which is stationary. It is mostly suitable for light duty operation. In Shaping large cutting force is transferred to tool.

In Planer machine tool is having stationary and work piece is clamped on table which is reciprocating motion. It is mostly suitable for Heavy duty operation. In planner large cutting force is transferred to table.

How will you calculate the tonnage of Mechanical Press?

F = S x L x T divided by 1000
F = Force in kilo Newton
S = shear stress of material in MPa
L = the total length of peripheries being cut/sheared in mm

T = thickness of material in mm

How many governors are needed for safe turbine operation? Why?

Two independent governors are needed for safe turbine operation:

  • One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
  • The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.

How will you detect that misalignment is the probable cause of excessive vibration?

  • Coupling to the driven machine is to be disconnected.
  • The turbine is to be run alone.
  • If the turbine runs smoothly, either misalignment, worn coupling or the driven equipment is the cause of the trouble.

In which part of the steam turbine does corrosion fatigue occur?

In the wet stages of the LP cylinder.

In which part of the steam turbine does stress corrosion cracking (SCC) occur?

In the wet stages of the low-pressure turbine.

In which zone of steam turbines has temperature-creep rupture been observed?

Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is, it has been found to occur in the control stages of the high-pressure and intermediate-pressure turbines where steam temperature sometimes exceed 540°C. In the reheat stage, it has been observed that creep has caused complete lifting of the blade shroud bands.

What are the types of thrust bearings?

  • Babbitt-faced collar bearings
  • Tilting pivotal pads
  • Tapered land bearings
  • Rolling-contact (roller or ball) bearings

What are the types of turbine seals?

  • Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
  • Labyrinths mated with shaft serrations or shaft seal strips.
  • Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
  • Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.

What are the basic causes of the problem of rotor failure?

  • Normal wear.
  • Fatigue failure due to high stress.
  • Design deficiency.
  • Aggressive operating environment

What are the differences between impulse and reaction turbines?

The impulse turbine is characterized by the fact that it requires nozzles and that the pressure drop of steam takes place in the nozzles.

The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row of blades mounted on a drum. The drum blades are separated by rows of fixed blades mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of correcting the direction of steam onto the moving blades.

In the case of reaction turbines, the pressure drop of steam takes place over the blades. This pressure drop produces a reaction and hence cause the motion of the rotor.

Mechanical Engineering Interview Questions Answers Part 3

Why Entropy decreases with increase in temperature?
ds=dQ/T Entropy is inversely proportional to the temperature so, as temperature increases, entropy decreases.

Why different types of sound are produced in different bikes, though they run on SI Engines?
Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsibility for sound variations.

Explain Bicycle Rear Wheel Sprocket working?
Rear wheel sprocket works under the principle of ratchet and pawl.

Definition of Octane Number and Cetane Number?

Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

Which Mechanism is used in Automobile gearing System?
Differential mechanism

When Crude Oil is Heated, Which Hydro Carbon comes first?
Natural gas (Gasoline)… at 20 Celsius

How to calculate Bearing number ti Diameter of the inner and outer?

Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according to type of load we have to choose the type of bearing and that will give prior number of the bearing.

The Fatigue life of a part can be improved by?
Improving the surface finish by Polishing & providing residual stress by Shot peening.

What happens if gasoline is used in a Diesel Engine, diesel Engine will work?

No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

Poisson Ratio is Higher in, Rubber/Steel/Wood?

When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson’s ratio is a measure of the Poisson effect.
For rubber = 0.5
For steel = 0.288
For wood < 0.2
Thus Poisson’s ratio is higher in RUBBER.

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Mechanical Engineering Interview Questions Answers Part 4

Why the Centrifugal Pump is called High Discharge pump?

Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formula:

Centrifugal force F= (M*V^2)/R.
Where,
M-Mass
V-Velocity
R-Radius

How Cavitation can be eliminated by Pump?

  • Cavitation means bubbles are forming in the liquid.
  • To avoid Cavitation, we have to increase the Pump size to One or Two Inch
  • To increase the pressure of the Suction Head, or
  • Decrease the Pump Speed.

Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?

The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.

Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?

The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.
One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

Mechanical Engineering Interview Questions Answers Part 5

What is Difference between stamina and strength?

Strength is capability over a short length of time and Stamina is the ability to keep going continuously.

What is Hydrostatic System?

Hydrostatics is the study of fluid bodies that are

  • At rest
  • Moving sufficiently slowly so there is no relative motion between adjacent parts of the body

For hydrostatic situations

  • There are no shear stresses
  • There are only pressure forces that act perpendicular to any surface.

It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies energy to motor and motor gives return energy to pump supply.

How is the excess discharge pressure prevented?

Discharge pressure prevented by a pressurized spike cushion. Here the system employs a pressurized cushion of air and a two o-ring piston, which permanently separates this air cushion from the water system. When the valve closes and the water flow is suddenly stopped, the pressure spike pushes the piston up the arrester chamber against the pressurized cushion of air. The air cushion in the arrester reacts instantly, absorbing the pressure spike that causes water hammer.

What is the position of Piston Ring?
In 180 degree angle the Top ring, Second ring and Oil ring are fixed. Position the ring approximately 1 inch gap below the neck.

What is meant by One Ton Air-Conditioner?
1 ton refrigeration means 210 kJ/min extraction of heat from the system.

State 1st Law of Thermodynamics?
Heat and mechanical work are mutually convertible. Energy can neither be created nor destroyed but it can be transferred from one form to another form.

If you heat a steel pipe with the hole at center, does heat affects the hole diameter?
It gets bigger.

Mechanical Engineering Interview Questions Answers Part 6

What is the difference between Technology and Engineering?

Engineering is application of science. Technology shows various methods of Engineering. A bridge can be made by using beams to bear the load,by an arc or by hanging in a cable; all shows different technology but comes under civil engineering and science applied is laws of force/load distribution.

How to Measure Temperature in Wet Bulb Thermometer?

Wet bulb temperature is measured by covering the thermometer bulb with a wick and wetting it with water. It corresponds to the dew point temperature and relative humidity.

What are the Advantages and Disadvantages of using LPG in Car?

Advantages

  • Complete combustion
  • Fuel saving
  • Homogeneous combustion

Disadvantages

  • As complete combustion is occurring ,more heat liberated,not advised for long journey, engine will be over heated
  • Installation is difficult
  • Reduce engine life efficiency

What is the difference between Rated Speed and Economic Speed?

The rated speed tells us about the maximum speed which can be achieved by a vehicle or some other machine but the economical speed means the speed limit at which the machine works efficiently with least consumption of fuel. e.g. in normal bikes (not racing),the max.speed limit shown on speedometer is upto 120 kmph but companies always advice their customers to drive such bikes at around 60 kmph to have maximum mileage.

What is Powder Technology?

Powder technology is one of the ways of making bearing material. In this method metals like bronze, Al, Fe are mixed and compressed to make an alloy.

State all the laws of Thermodynamics?

There are three laws of the thermodynamics.

First Law: Energy can be neither created nor destroyed. It can only change forms. In any process in an isolated system, the total energy remains the same.

Second Law: When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at some time allowed to interact, breaking the isolation that separates the two systems, and they exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of the final exchanging systems. In the process of reaching a new thermodynamic equilibrium, entropy has increased, or at least has not decreased.

Third Law: As temperature approaches absolute zero, the entropy of a system approaches a minimum.

State the difference between Unilateral and Bilateral Tolerance?

A unilateral tolerance is tolerance in which variation is permitted only in one direction from the specified direction.e.g. 1800 +0.000/-0.060

Bilateral tolerance is tolerance in which variation is permitted in both direction from the specified direction.e.g. 1800 +0.060/-0.060

What is the abbreviation of welding rod 7018 means?

7018 =

70=tensile strength 70000psi
1= welding position
8=current flux

Which has more Efficiency Diesel Engine or Petrol Engine?

Diesel engine has the better efficiency out of two.

Mechanical Engineering Interview Questions Answers Part 7
What is important to remember about radial bearings?

A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder. These bearings must be accurately aligned to maintain the close clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.

 

What is meant by critical speed?

It is the speed at which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the vibration is caused by critical speed and not by some other trouble).

How is oil pressure maintained when starting or stopping a medium-sized turbine?

An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.

Besides lubrication, what are two functions of lubricating oil in some turbines?

In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

Do you stop cooling-water flow through a steam condenser as soon as the turbine is stopped?

You should keep the cooling water circulating for about 15 minutes or more so that the condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.

How can the deposits be removed?

  • Water soluble deposits may be washed off with condensate or wet steam.
  • Water insoluble deposits are removed mechanically after dismantling the turbine.
  • Experience shows that water soluble deposits are embedded in layers of water-insoluble deposits. And when the washing process is carried out, water soluble parts of the deposit dissolve away leaving a loose, friable skeleton of water-insoluble deposits which then break loose and wash away.

How can the fatigue damage on high-pressure blades be corrected?

Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc admission. This can be corrected by switching over to full arc admission technique.

Mechanical Engineering Interview Questions Answers Part 8

What are the different types of fits? Explain?

On the basis of standards fits can mainly be categorized into three groups:

Clearance Fit: These types of fits are characterized by the occurrence of a clearance between the two mating parts. The difference between the minimum size of the hole and the maximum size of the shaft is called the minimum clearance, the difference between the maximum size of the hole and the minimum size of the shaft is known as maximum clearance.

Interference Fit: In these types of fits the size of the mating parts are predefined so that interference between them always occurs. The tolerance zone of the hole is completely below the tolerance zone of the shaft.

Transition Fit: As the name suggests this type of fit has its mating parts sized limited to allow either clearance or interference. The tolerance zone of the hole and the shaft overlaps in case of such fits.

For a shaft designated as 40 H8/f7, calculate the tolerances.

Given: Shaft designation = 40 H8/f7

The shaft designation 40 H8/f 7 means that the basic size is 40 mm and the tolerance grade for
the hole is 8 ( i. e. I T 8) and for the shaft is 7 ( i. e. I T 7).

Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean diameter,
D = Square root of (30 x 50) = 38.73 mm
We know that standard tolerance unit,
i = 0.45 x Cube root of (D) + 0.001 D
i = 0.45 × 3.38 + 0.03873 = 1.559 73 or 1.56 microns
i = 1.56 × 0.001 = 0.001 56 mm …(1 micron = 0.001 mm)

The standard tolerance for the hole of grade 8 (IT8)
= 25 i = 25 × 0.001 56 = 0.039 mm
The standard tolerance for the shaft of grade 7 (IT7)
= 16 i = 16 × 0.001 56 = 0.025 mm

What are the factors that can affect the Factor of safety selection?

The factor of safety is used in designing a machine component. Prior to selecting the correct factor of safety certain points must be taken into consideration such as:

> The properties of the material used for the machine and the changes in its intrinsic properties over the time period of service.
> The accuracy and authenticity of test results to the actual machine parts.
> The applied load reliability.
> The limit of stresses (localized).
> The loss of property and life in case of failures.
> The limit of initial stresses at the time period of manufacture.
> The extent to which the assumptions can be simplified.

The factor of safety also depends on numerous other considerations such as the material, the method of manufacturing , the various types of stress, the part shapes etc.

What is heat treatment and why is it done?

Heat treatment can be defined as a combination of processes or operations in which the heating and cooling of a metal or alloy is done in order to obtain desirable characteristics without changing the compositions. Some of the motives or purpose of heat treatment are as follows:

In order to improve the hardness of metals.
> For the softening of the metal.
> In order to improve the machinability of the metal.
> To change the grain size.
> To provide better resistance to heat, corrosion, wear etc.

Heat treatment is generally performed in the following ways:

Normalizing
> Annealing
> Spheroidising
> Hardening
> Tempering
> Surface or case hardening

What are the rules that must be kept in mind while designing castings?

Some of the points that must be kept in mind during the process of cast designing are as follows:

To avoid the concentration of stresses sharp corners and frequent use of fillets should be avoided.

> Section thicknesses should be uniform as much as possible. For variations it must be done gradually.
> Abrupt changes in the thickness should be avoided at all costs.
> Simplicity is the key, the casting should be designed as simple as possible.
> It is difficult to create true large spaces and henceforth large flat surfaces must be avoided.
> Webs and ribs used for stiffening in castings should as minimal as possible.
> Curved shapes can be used in order to improve the stress handling of the cast.

What are the points that should be kept in mind during forging design?

Some of the points that should be followed while forging design are:
> A radial flow of grains or fibers must be achieved in the forged components.
> The forged items such as drop and press forgings should have a parting line that should divide the forging into two equal halves.
> The ribs in a forging should not be high or thin.
> In order to avoid increased die wear the pockets and recesses in forgings should be minimum.
> In forgings the parting line of it should lie as far as possible in a single plane.
> For ease of forging and easy removal of forgings the surfaces of the metal should contain sufficient drafts.

Describe briefly the different cold drawing processes.

Some of the important cold drawing processes are as follows:
> Bar and Rod Drawing: In the case of bar drawing the hot drawn bars are at first pickled, washed and coated to prevent oxidation. Once this is done a draw bench is used for the process of cold drawing. In order to make an end possible to enter a drawing die the diameter of the rod is reduced by the swaging operation. This end is fastened by chains to the draw bench and the end is gripped by the jaws of the carriage. In this method a high surface finish and accuracy dimensionally is obtained. The products of this process can be used directly without any further machining.

> Wire Drawing: Similar to the above process the bars are first pickled, washed and coated to prevent any oxidation. After this the rods are passed through several dies of decreasing diameter to provide a desired reduction in the size ( diameter ). The dies used for the reduction process is generally made up of carbide materials.

> Tube Drawing: This type of drawing is very similar to the bar drawing process and in majority of cases it is accomplished by the use of a draw bench.

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What are the different theories of failure under static load, explain briefly?

The main theories of failure of a member subjected to bi-axial stress are as follows:
> Maximum principal stress theory ( Rankine’s theory): This theory states that failure occurs at a point in member where the maximum principal or normal stress in a bi-axial system reaches the maximum strength in a simple tension test.
> Maximum shear stress theory ( Guest’s or Tresca’s theory): This theory states that failure occurs when the biaxial stress reaches a value equal to the shear stress at yield point in a simple tension test.
> Maximum principal strain theory ( Saint Venant theory): This theory states that failure occurs when bi-axial stress reaches the limiting value of strain.
> Maximum strain energy theory ( Haigh’s theory): This theory states that failure occurs when strain energy per unit volume of the stress system reaches the limiting strain energy point.
> Maximum distortion energy theory ( Hencky and Von Mises theory): This theory suggests that yielding of a ductile material begins when the second deviatoric stress invariant reaches a critical value. It is part of plasticity theory that applies best to ductile materials, such as some metals. Prior to yield, material response can be assumed to be of a nonlinear elastic, viscoelastic or linear elastic behavior.

Mechanical Engineering Interview Questions Answers Part 9

What are the assumptions made in simple theory of bending?

The assumptions made in the theory of simple bending are:
> The material of the beam is homogeneous this implies that it is uniform in density, strength and have isotropic properties meaning possessing same elastic property in all directions.
> Even after bending the cross section of the beam remains constant.
> During the initial stages the beam is straight and unstressed.
> All the stresses in the beam are within the elastic limit of its material.
> The layers of the beam are free to contract and expand longitudinally and laterally
> On any cross section the perpendicular resultant force of the beam is zero.
> Compared to the cross-sectional dimension of the beam the radius of curvature is very large.

Why is stress considered important in a shaft?

The following types of stresses are prevalent in shafts:
> At the outermost surface of the shaft the max shear stress occurs on the cross-section of the shaft.
> At the surface of the shaft on the longitudinal planes through the axis of the shaft the maximum longitudinal shear stress occurs.
> At 45 degrees to the maximum shearing stress planes at the surface of the shafts the major principal stress occurs. It equals the max shear stress on the cross section of the shaft.
> For certain materials where the tensile and compressive strengths are lower in measure as compared to the shear strength, then the shaft designing should be carried out for the lowest strengths.
> All these stresses are of significance as they play a role in governing the failure of the shaft. All theses stresses get generated simultaneously and hence should be considered for designing purposes.

What do you understand by the Hooke`s Coupling what are its purposes?

The Hooke`s coupling is used to connect two shafts whose axes intersect at a small angle. The two shafts are inclined at an angle and is constant. During motion it varies as the movement is transferred from one shaft to another. One of the major areas of application of this coupling is in gear boxes where the coupling is used to drive the rear wheels of trucks and other vehicles. In such usage scenarios two couplings are used each at the two ends of the coupling shaft. they are also used to transfer power for multiple drilling machines. The Hooke`s coupling is also known as the Universal coupling. The torque transmitted by the shafts is given by :

T= (pie/16) * t * d^3

Where T = torque, t = shear stress for the shaft material and d the diameter of the shaft.

What kind of materials should be used for shafts manufacturing?

Some of the qualities that should be present in materials for shafts are as follows:
> The material should have a high index of strength.
> Also it should have a high level of machinability.
> The material should possess a low notch sensitivity factor.
> The material must also have wear resistant properties.
> Good heat treatment properties should also be present

The common material used to creates shafts of high strengths an alloy of steel like nickel is used. The shafts are manufactured by hot rolling processes and then the shaft is finished using drawing or grinding processes.

Why should a chain drive be used over a belt or rope driven drive? State pro`s and con`s?

The advantages of using a chain drives are:
> In a chain drive no slip occurrence takes place.
> The chains take less space as compared to rope or belts as they are made of metal and offer much strength.
> The chain drives can be used at both short and long ranges and they offer a high level of transmission efficiency.
> Chain drives can transmit more load and power as compared to belts.
> A very high speed ratio can be maintained in one step of chain drives.

Some of the cons of using a chain drive are:
> The cost of producing chain drives is higher as compared to that of belts.
> The chain drives must be serviced and maintained at regular intervals and henceforth their cost of ownership is high comparatively.

What are the different types of springs and explain them briefly?

Springs can be broadly classified into the following types:
> Helical Springs: These springs as their name suggests are in coil form and are in the shape of helix. The primary purpose of such springs are to handle compressive and tensile loads. They can be further classified into two types: compression helical spring and tension helical spring each having their own unique areas of application.

> Conical and volute springs: Both these spring types have specialized areas of usage where springs with adaptable rate according to the load is required. In case of conical springs they are wound so as to have a uniform pitch while on the other hand volute springs are wound in a slight manner of a parabloid.

> Torsion Springs: The characteristics of such springs is that they tend to wind up by the load. They can be either helical or spiral in shape. These types of springs are used in circuit breaker mechanisms.

> Leaf springs: These types of springs are comprised of metal plates of different lengths held together with the help of bolts and clamps. Commonly seen being used as suspensions for vehicles.

> Disc Springs: As the name suggests such types of springs are comprised of conical discs held together by a bolt or tube.

> Special Purpose Springs: These springs are all together made of different materials such as air and water.

During the design of a friction clutch what are the considerations that should be made?

In order to design a friction clutch the following points must be kept in mind:
> The material for the contact surfaces must be carefully selected.
> For high speed devices to minimize the inertia load of the clutch, low weight moving parts must be selected.
> The contact of the friction surfaces must be maintained at all the times without the application of any external forces.
> Provisions for the facilitation of repairs must be there.
> In order to increase safety the projecting parts of a clutch must be covered.
> A provision to take up the wearing of the contact surfaces must be present.
> Heat dissipaters to take away the heat from the point of contacting surfaces must be there.

What are the different types of brakes and explain them briefly?

Brakes can be classified on the basis of their medium used to brake, they are as follows:
> Hydraulic Brakes: These brakes as their name suggest use a fluid medium to push or repel the brake pads for braking.
> Electric Brakes: These brakes use electrical energy to deplete or create a braking force.
Both the above types of breaks are used primarily for applications where a large amount of energy is to be transformed.
> Mechanical Brakes: They can be further classified on the basis of the direction of their acting force: Radial Brakes: As their names suggests the force that acts on the brakes is of radial direction. They can further be classified into internal and external blades. Axial Brakes: In these types of brakes the braking force is acting in an axial direction as compared to radial brakes.

On what basis can sliding contact bearings be classified? Explain?

Sliding contact bearings can be classified on the basis of the thickness of the lubricating agent layer between the bearing and the journal. They can be classified as follows:
> Thick film bearings: These type of bearings have their working surface separated by a layer of the lubricant. They are also known as hydrodynamic lubricated bearings.
> Thin film bearings: In this type of bearings the surfaces are partially in direct contact with each other even after the presence of a lubricant. The other name for such type of bearings is boundary lubricated bearings.
> Zero Film Bearings: These type of bearings as their name suggests have no lubricant present between the contact layers.
> Externally or hydrostatically pressurized lubricated bearings: These bearings are able to without any relative motion support steady loads.

What are the basis on which the best material for Sliding Contact Bearings manufacturing?

Some of the important properties to lookout for in the material for sliding contact bearings are as follows:
> Compressive Strength: In order to prevent the permanent deformation and intrusion of the bearing the material selected should be possess a high compressive strength to bear the max bearing pressure.
> Fatigue Strength: the material selected for the bearing should be able to withstand loads without any surface fatigue cracks getting created. This is only possible if the material has a high level of fatigue strength.
> Comfortability: The material should be able to adjust or accommodate bearing inaccuracies and deflections without much wear and heating.
> Embeddability: The material should allow the embedding of small particles without effecting the material of the journal.
> Bondability: The bearings may be created by bringing together ( bonding ) multiple layers of the material. Due to the above reason the bondability of the material should be sufficiently high.
> Thermal conductivity and corrosion resistance: Thermal conductivity is an essential property for bearing materials as it can help in quickly dissipating the generated heat. Also the material should have a level of corrosion resistance against the lubricant.

Briefly explain the advantages of Cycloidal and Involute gears?

The advantages of the Cycloidal gears are as follows:
> Having a wider flank as compared to Involute gears they are considered to have more strength and hence can withstand further load and stress.
> The contact in case of cycloidal gears is between the concave surface and the convex flank. This results in less wear and tear.
> No interference occurs in these types of gears.

The advantages of Involute gears are as follows:
> The primary advantage of involute gears is that it allows the changing of the centre distance of a pair without changing the velocity ratio.
> The pressure angle remains constant from start to end teeth, this results in less wear and smooth running of the gears.
> The involute gears are easier to manufacture as they can be generated in a single curve ( the face and flank ).

How can the reaction of support of a frame be evaluated?

Generally roller or hinged support are used to support the frames. The conditions of equilibrium are used to determine the reaction support of a frame. The condition of equilibrium takes place when the sum of the horizontal and vertical forces sum equal to zero. The system must form a state of equilibrium even after considering the external loads and the reactions at the supports. For equilibrium to be prevalent in the system the following conditions are required to be in occurrence:
> Summation of V = 0. This implies that the summation of all the forces in the vertical direction results to zero.
> Summation of H = 0 . This implies that the total of all the forces acting in horizontal direction is also zero.
> Summation of M = 0. The sum of all the moment of forces around a point must be zero.

Explain in an orderly manner how the force in the member of a truss be detected using the method of joint.

The steps required to calculate the force are as follows:
> The reaction at the support has to be first calculated.
> Once the reaction is calculated the direction of force of the member is made to make it tensile. On getting the result to be negative the direction assumed is wrong and this implies the force being compressive in nature.
> A joint needs to be selected whose 2 members are not known. The lami`s theorem is used on the joint on which less than three forces are acting.
> After the above process is complete the free body diagrams of the joint needs to be made. Since the system is in equilibrium the condition of Summation of V and H must result in zero.
> After the above step the resolution of forces method needs to be used on the joint on which more than 4 forces are acting.

In order to derive the torsional formulas what are the assumptions taken?

The torsion equation is derived on the basis of following assumptions:
> The shaft material is uniform, throughout the shaft.
> Even after loading the shaft circular remains circular.
> After the application of torques the plain section of a shaft remains plain.
> Any twist that occurs in the shaft remains uniform and constant.
> After the application of torque the distance between any two cross-sectional references remains constant.
> The elastic limit value of a shaft is never exceeded even after the shear stress induced because of torque application.

What are Bevel Gears and what are its types?

Bevel gears are the type of gears in which the two shafts happen to intersect. The gear faces which are tooth bearing are conical in shape. They are generally mounted on shafts which are 90 degrees apart but they can be made to work at other angles as well. The bevel gears are classified into the following types on the basis of pitch surfaces and shaft angles:
> Mitre Gears: These types of gears are similar to each other ie. they have the same pitch angles and contain the same number of teeth. The shaft axes intersect at 90 degrees angle.
> Angular bevel gears: When two bevel gears connect at any angle apart from 90 degrees.
> Crown bevel gears: When the two shaft axes intersect at an angle greater than 90 and one of the bevel gears have a pitch angle of 90 degrees they are known as crown bevel gears.
> Internal bevel gears: In these type of gears the teeth on the gears is cut on the inside area of the pitch cone.

What are the different values that need to be determined in order to design a cylinder for an ICE?

The following values are needed to be determined:
> Thickness of the cylinder wall: The cylinder walls in an engine is made witness to gas pressure and the side thrust of a piston. This results in two types of stresses: longitudinal and circumferential stress. Both the types of stresses are perpendicular to each other and hence it is aimed to reduce the resulting stress as much as possible.
> Length and bore of the cylinder: The length of the cylinder and the length of the stroke is calculated on the basis of the formula: length of cylinder L = 1.15 times the length of the stroke (l). L = 1.15(l)
> Cylinder flange and studs: The cylinders are always cast integral as a part of the upper crankcase or in some cases attached to it by means of nuts and bolts. The flange is integral to a cylinder and henceforth its thickness should be greater than that of the cylinder wall. The thickness of flange should generally be between 1.2t-1.4t where t is the cylinder thickness.
The stud diameter is calculated by equating gas load ( due to max pressure ) to the grand total of all the resisting forces of the studs.

What are considerations taken into account while creating a piston head?

The piston head is designed on the basis of the following considerations:
> The crown should have enough strength to absorb the explosion pressure inside the engine cylinder.
> The head must always dissipate the heat of the explosion as quickly as possible to the engine walls. The thickness of the head is calculated on the basis of another formula which takes into consideration the heat flowing through the head, the conductivity factor of the material. The temperature at the center and edges of the head.
> The thickness of the piston head is calculated on the basis of the Grashoff`s formula which takes into consideration the maximum gas pressure of an explosion , the permissible bending and the outside diameter of the piston.

Mechanical Engineering Interview Questions Answers Part 10

What is the function of a thrust bearing?

Thrust bearings keep the rotor in its correct axial position.

What are some conditions that may prevent a turbine from developing full power?

  • The machine is overloaded.
  • The initial steam pressure and temperature are not up to design conditions.
  • The exhaust pressure is too high.
  • The governor is set too low.
  • The steam strainer is clogged.
  • Turbine nozzles are clogged with deposits.
  • Internal wear on nozzles and blades.

What is a stage in a steam turbine?

In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a “stage.” A single Curtis stage may consist of two or more rows of moving blades.

What is a diaphragm?

Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragm is fitted into the top of the casing, the other half into the bottom.

What are topping and superposed turbines?

Topping and superposed turbines arc high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.

What is a combination thrust and radial bearing?

This unit has the ends of the Babbitt bearing extended radically over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.

What is the differences between gas turbine and a steam turbine?

Gas turbine works on Bryton cycle where as steam turbine works Rankine cycle. Construction, operation of a gas turbine are entirely different to steam turbine. Gas turbine has a compressor to compress the combustion air, a combustion chamber to burn the fuel and a turbine section to extract the work for burning fuel. Steam turbine is just has a turbine section to extract the work from steam.

What is operating pressure?

The amount of pressure nearest the point of performing work at the output end of a pneumatic system. The system operating pressure is used to specify the capability of valves and actuators.

What are the safety valves? How many on each boiler?

A valve opening automatically to relieve excessive pressure, especially in a boiler.

There are normally two to six safety valves provided in the drum depending upon the capacity.

The super heater outlet will have one to three safety valves on either side of the boiler. There will be an electromatic relief valve on the super heater pipe in addition. This valve will be set at lower pressure than the lowest set safety valve on the super heater.

The super heater outlet will have one to three safety valves on either side of the boiler. There will be an electromatic relief valve on the super heater pipe in addition. This valve will be set at lower pressure than the lowest set safety valve on the super heater.

The reheater pipes both at the inlet and outlet side will also have safety valves which can range from two to eight both in the inlet and outlet of the reheater put together.

What is a sentinel valve?

Sentinel valves are simply small relief valves installed in some systems to warn of impending over pressurization. Sentinel valves do not relieve the pressure of the system. If the situation causing the sentinel valve to lift is not corrected, a relief valve (if installed) will lift to protect the system or component. If a relief valve is not installed, action must be taken quickly to secure the piece of equipment or system to reduce the pressure.

What is the function of Hydrogen seals work on a generator?

Provide a seal between the generator housings and rotor shaft to maintain the pressurized hydrogen gas inside the generator. Also, provides a trap-vent system to prevent the release of hydrogen into the turbine generator lube oil system and building atmosphere.

How is the excess discharge pressure prevented?

Pressure relief valves on the discharge side of each seal oil pump relieves back to seal oil system.

Which two seal oil pumps are driven from the same motor?

Main seal oil pump and re-circular seal oil pump; both are driven by Main seal oil pump motor.

When does Emergency Seal Oil Pump automatically start?

When its pressure switch senses Main Seal Oil Pump discharge pressure reduced to 78 PSI.

What is the consequence of not maintaining hydrogen (or air) pressure in generator casing at a value above atmospheric pressure when seal oil system is in service?

Failure to do so will cause excessive seal oil to be drawn into the generator.

Mechanical Engineering Interview Questions Answers Part 11
How can problems of “excessive vibration or noise” due to piping strain be avoided on steam turbines?
  • The inlet as well as exhaust steam lines should be firmly supported to avoid strains from being imposed on the turbine.
  • Adequate allowance should be made for expansion of steam pipes due to heat.

How the misalignment of Flanges be rectified?

The bolts holding the flanges together are to be tightened. The coupling is to be checked for squareness between the bore and the face. At the same time axial clearance is to be checked.

How the problem of excessive speed variation due to throttle assembly friction be overcome?

The throttle should be dismantled. Moving parts should be checked for free and smooth movement. Using very fine-grained emery paper, the throttle valve seats and valve steam should be polished.

How the problems of vibration and fatigue arise in steam turbine blades?

  • These arise due to flow irregularities introduced because of manufacturing defects, e.g. lack of control over tolerances.
  • System operating parameter, e.g. low flow may excite various modes of vibration in the blades.

How does solid-particle erosion occur?

Solid-particle erosion, i.e. SPE occurs in the high-pressure blades. And it takes place when hard particles of iron exfoliated by steam from superheater tubes, reheater tubes, steam headers and steam leads strike on the surface of turbine blades.

How does the internal efficiency monitoring lead to the detection of turbine deposits?

  • Process heat drop.
  • Adiabatic heat drop.
  • The process heat drop and adiabatic heat drop are obtained from a Mollier-Chart for the corresponding values of steam parameters – pressure and temperature – at initial and final conditions.

 

 

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Real Time Face Detection and Tracking Robot using Arduino

Ever wanted to build a Face Tracking Robotic Arm or Robot by simply using Arduino and not any other programming like OpenCV, visual basics C# etc? Then read along, in this project we are going to implement face detection by blending in the power of Arduino and Android. In this project, the mobile camera will move along with your face with the help of servos. The advantage of using the Android Mobile Phone here is that you do not need to invest on a camera module and the whole image detection work can be done in the phone itself, you do not need your Arduino connected to your computer for this to work. Here we have used Bluetooth Module with Arduino to communicate with Mobile wirelessly.

The Android application used in this project was created using Processing Android, you can either directly install the application by downloading the APK file (read further for link) or put on your programming cap and make your own more appealing Android Application using the Processing Code given further in the Tutorial. Learn more about Processing by checking our previous Processing Projects.

By the end of this tutorial you will have a Mini Tilt and Span Robotic Arm that could track your faceand move along with it. You can use this (with further advancement) to record your vlog videos or even take a selfie with the rear camera of your mobile phone since it positions your face exactly at the centre of your mobile screen. So!! Sounds interesting? Check the Demo Video at the end this tutorial to see it working. Let’s see how we can build one…

I have tried my best to make this project to work as simple as possible, anyone with minimum knowledge on hardware or coding can use this guidelines to make this project work in no time. However once you make it I suggest you to get behind the codes so that you can really know what makes this thing work and how.

 

Real Time Face Detection Tracking Robot using Arduino

Real Time Face Detection and Tracking Project using Arduino

Materials Required:
  1. Arduino Nano
  2. Servo motor SG90 – 2Nos
  3. Android Phone with decent camera
  4. HC-05/HC-06 Bluetooth Module
  5. Computer for programming
  6. 3D printer (optional)
  7. 9V Battery
3D Printing the Required Parts (Optional):

In order to pan and tilt our mobile phone we need some mechanical structures like a mobile holder and a few servo brackets. You can use a cardboard to make one, since I have a 3D printer I decided to 3D print these parts.

3D printing is an amazing tool that can contribute a lot when building prototype projects or experimenting with new mechanical designs. If you have not yet discovered the benefits of a 3D printer or how it works you can read The beginners Guide to 3D printing.

If you own or have access to a 3D printer then you can use the STL files which can be downloaded from here to directly print and assemble them. However few parts like the mobile phone holder might need some modifications based on the dimensions of your phone. I have designed it for my MOTO G mobile phone. I have used a very basic printer of mine to print all the parts. The printer is FABX v1 from 3ding which comes at an affordable price with a print volume of 10 cubic cm. The cheap price comes with a trade off with low print resolution and no SD card or print resuming function. I am using software called Cura to print the STL files. The settings that I used to print the materials are given below you can use the same or change them based on your printer.

 

READ COMPLETE BLOG BY CLICKING LINK BELOW

https://circuitdigest.com/microcontroller-projects/arduino-face-tracking-robot

Properties of Discretisation Schemes in CFD

In theory numerical results may be obtained that are indistinguishable from the ‘exact’ solution of the transport equation when the number of computational cells is infinitely large, irrespective of the differencing method used. However, in practical calculations we can only use a finite – sometimes quite small – number of cells, and our numerical results will only be physically realistic when the discretisation scheme has certain fundamental properties.

  • Finite size of control volume introduce numerical issues.
  • In order to analyze numerical errors, the numerical discretization scheme are tested for the following three properties.

• Conservativeness
• Boundedness
• Transportiveness

Conservativeness

Integration of the convection–diffusion equation over a finite number of control volumes yields a set of discretised conservation equations involving fluxes of the transported property φ through control volume faces. To ensure conservation of φfor the whole solution domain the flux of φ leaving a control volume across a certain face must be equal to the flux of φentering the adjacent control volume through the same face. To achieve this the flux through a common face must be represented in a consistent manner – by one and the same expression – in adjacent control volumes.

  • To ensure conservation of φ for the whole solution domain, the flux of φ leaving a control volume across a certain face must be equal to the flux of φ entering the adjacent control volume through the same face.
  • For example, consider the one-dimensional steady state diffusion problem without source terms shown below:

 

Properties of discretisation schemes in CFD

The fluxes across the domain boundaries are denoted by qA and qB. Let us consider four control volumes and apply central differencing to calculate the diffusive flux across the cell faces. The expression for the flux leaving the element around node 2 across its west face is Γw22 −φ1)/δx and the flux entering across its east face is     Γe23−φ2)/δx. An overall flux balance may be obtained by summing the net flux through each control volume, taking into account the boundary fluxes for the control volumes around nodes 1 and 4:

Properties of discretisation schemes in CFD

 

Since Γe1w2, Γe2w3 and Γe3w4 the fluxes across control volume faces are expressed in a consistent manner and cancel out in pairs when summed over the entire domain. Only the two boundary fluxes qA and qB remain in the overall balance, so above equation expresses overall conservation of property φ. Flux consistency ensures conservation of φ over the entire domain for the central difference formulation of the diffusion flux.
Inconsistent flux interpolation formulae give rise to unsuitable schemes that do not satisfy overall conservation. For example, let us consider the situation where a quadratic interpolation formula, based on values at 1, 2 and
3, is used for control volume 2, and a quadratic profile, based on values at points 2, 3 and 4, is used for control volume 3.

  • Flux consistency ensures conservation of φ over the entire domain for the central difference formulation of the diffusion flux.
  • Inconsistent flux interpolation schemes are pron to numerical errors such as quadratic interpolation formula.

As shown in Figure below, the resulting quadratic profiles can be quite different.

Properties of discretisation schemes in CFD

 

Consequently, the flux values calculated at the east face of control volume 2 and the west face of control volume 3 may be unequal if the gradients of the two curves are different at the cell face. If this is the case the two fluxes do
not cancel out when summed and overall conservation is not satisfied. The example should not suggest to the reader that quadratic interpolation is entirely bad. Further on we will meet a quadratic discretisation practice, the so-called QUICK scheme, that is consistent.

paidverts22

 Boundedness

The discretised equations at each nodal point represent a set of algebraic equations that needs to be solved. Normally iterative numerical techniques are used to solve large equation sets. These methods start the solution
process from a guessed distribution of the variable φ and perform successive updates until a converged solution is obtained. Scarborough (1958) has shown that a sufficient condition for a convergent iterative method can be expressed in terms of the values of the coefficients of the discretised equations:

  • For iterative solvers, the matrix must be diagonally dominant.

Properties of discretisation schemes in CFD

  • All coefficients of the discretized equations should have the same sign (usually all positive).

Here a′is the net coefficient of the central node P(i.e. aP−SP), and the summation in the numerator is taken over all the neighbouring nodes (nb). If the differencing scheme produces coefficients that satisfy the above criterion
the resulting matrix of coefficients is diagonally dominant. To achieve diagonal dominance we need large values of net coefficient (aP−SP) so the linearisation practice of source terms should ensure that SP is always negative. If this is the case −SP is always positive and adds to aP.
Diagonal dominance is a desirable feature for satisfying the ‘boundedness’ criterion. This states that in the absence of sources the internal nodal values of property φ should be bounded by its boundary values. Hence
in a steady state conduction problem without sources and with boundary temperatures of 500°C and 200°C, all interior values of T should be less than 500°C and greater than 200°C. Another essential requirement for boundedness is that all coefficients of the discretised equations should have the same sign(usually all positive). Physically this implies that an increase in the variable φ at one node should result in an increase in φ at neighbouring nodes. If the discretisation scheme does not satisfy the boundedness requirements it is possible that the solution does not converge at all, or, if it does, that it contains ‘wiggles’.

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 Transportiveness

The transportiveness property of a fluid flow (Roache, 1976) can be illustrated by considering the effect at a point P due to two constant sources of φ at nearby points W and E on either side as shown in Figure below. We define the non-dimensional cell Peclet number as a measure of the relative strengths of convection and diffusion:

  • Relative strength of convection and diffusion is measured by Peclet number.

Properties of discretisation schemes in CFD

The lines in Figure below indicate the general shape of contours of constant φ (say φ=1) due to both sources for different values of Pe. The value of φ at any point can be thought of as the sum of contributions due to the two
sources.

  • Consider the effect at a point P due to two constant sources of φ at nearby points W and E on either side

 

Properties of discretisation schemes in CFD

Let us consider two extreme cases to identify the extent of the influence at node P due to the sources at Wand E:
• no convection and pure diffusion (Pe→0)
• no diffusion and pure convection (Pe→∞)
In the case of pure diffusion the fluid is stagnant (Pe→0) and the contours of constant φ will be concentric circles centred around W and E since the diffusion process tends to spread φ equally in all directions. Figure (a) shows that both φ=1 contours pass through P, indicating that conditions at this point are influenced by both sources at W and E. As Pe increases the contours change shape from circular to elliptical and are shifted in the direction of the flow as shown in Figure (b). Influencing becomes increasingly biased towards the upstream direction at large values of Pe, so, in the present case where the flow is in the positive x-direction, conditions at P will be mainly influenced by the upstream source at W. In the case of pure convection (Pe→∞) the elliptical contours are completely stretched out in the flow direction. All of property φ emanating from the sources at W and E is immediately transported downstream. Thus, conditions at P are now unaffected by the downstream source at E and completely dictated by the upstream source at W. Since there is no diffusion φis equal to φW. If the flow is in the negative x-direction we would find that φP is equal to φE. It is very important that the relationship between the directionality of influencing and the flow direction and magnitude of the Peclet number, known as the transportiveness, is borne out in the discretisation scheme.

References

  1. An Introduction to Computational Fluid Dynamics, THE FINITE VOLUME METHOD, Second Edition
    H K Versteeg and W Malalasekera.
  2. Computational Fluid Dynamics-I, Lecture-9 by Dr. Tariq Talha, College of EME, NUST, Pakistan.

Overview of Welding Technology

In this article, we consider the processes that are used to join two or more parts into an assembled entity. The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts–a joint that cannot easily be separated. The term assembly usually refers to mechanical methods of fastening parts together. Some of these methods allow for easy disassembly, while others do not.

Welding is a materials joining process in which two or more parts are coalesced at their contacting surface by a suitable application of heat and/or pressure. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of a heat and pressure; and still others by pressure alone, with no external heat applied. In some welding processes a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called weldment. Welding is most commonly associated with metal parts, but the process is also used for join plastics. Our discussion of welding will focus on metals.

Welding is relatively new process. Its commercial and technological importance derives from the following:

  • Welding provides a permanent joint. The welded parts become single entity.
  • The welding joint can be stronger than the parent materials if a filler material is used that has strength properties superior to those of the parents, and if proper welding techniques are used.
  • Welding is usually the most economical way to join components in terms of material usage and fabrication costs. Alternative mechanical methods of assembly require more complex shape alterations (e.g., drilling of holes) and addition of fasteners (e.g., rivets or bolts). The resulting mechanical assembly is usually heavier than a corresponding weldment.
  • Welding is not restricted to the factory environment. It can accomplished “in the field”.

Although welding has advantages indicated above, it also has certain limitations and drawbacks (or potential drawbacks):

  • Most welding operations are performed manually and are expansive in terms of labor cost. Many welding operations are considered “skilled trades,” and the labor to perform these operation maybe scarce.
  • Most welding processes are inherently dangerous because they involve the usage of high energy.
  • Since a welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly. If the product must occasionally be disassembled (e.g., for maintenance or repair), then welding should not be used as assembly method.
  • The welded joint can suffer from certain quality defects that are difficult to detect. The defects can reduce the strength of joint.
Overview Of Welding Technology

Welding involves localized coalescence or joining together of two metallic parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that are to be joined.

Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar materials.

Types of welding processes

Some 50 different types of welding operations have been cataloged by the American Welding Society. They use various types or combinations of energy to provide the required power.  We can divide the welding processes into two major groups: (1) fusion welding and (2) solid-state welding.

Fusion welding

Fusion welding processes use heat to melt the base materials. In many fusion welding operations, a filler material is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. A fusion welding operation in which no filler material is added is referred to as an autogenous weld. The fusion category includes the most widely used welding processes, which can be organized into the following general groups (initials in parentheses are designations of the American Welding Society):

  • Arc welding (AW). Arc welding refers to a group of welding processes in which heating of the metal is accomplished by an electric arc, as shown in fig. some arc welding operations also apply pressure during process and most utilize a filler metal.

Welding Technology

 

  • Resistance welding (RW). Resistance welding achieves coalescence using heat from electrical resistance to a flow of a current passing between the faying surfaces of two parts held together under pressure.
  • Oxyfuel gas welding (OFW). These joining processes use an oxyfuel gas such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal, if one is used.
  • Other fusion welding processes. Other welding processes that produce fusion of the metal joined include electron beam welding and laser beam welding.

Certain arc and oxyfuel processes are also used for cutting metals.

Solid-state welding

Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure.

If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilized. Some representative welding processes in this group include:

 

Welding Technology

 

Diffusion welding (DFW). Two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state fusion.

Friction welding (FRW). Coalescence is achieved by the heat of friction between two surfaces.

Ultrasonic welding (USW). Moderate pressure is applied between the two parts and an oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surfaces films and achieve atomic bonding of the surfaces.

 

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

There are a number of issues that must be resolved before the proper HVAC system can be designed, whether it is intended for the isolation rooms, surgical suite, the patient rooms, or the administration offices. Initially, the proper ambient design conditions must be selected. Too often, only the peak cooling design conditions are considered for sizing the capacity requirements of the system. These ambient conditions are listed in the ASHRAE Handbook – Fundamentals as the dry-bulb temperatures with mean coincident wet-bulb temperatures, representing conditions on hot, mostly sunny days. These conditions are used in sizing cooling equipment such as chillers or package equipment for cooling control. In some climates, this might be satisfactory; however, in geographic areas known for higher humidity levels, considering only this cooling condition might not be sufficient. Extreme dew-point temperature conditions may occur on days with moderate dry-bulb temperatures, resulting in high relative humidity’s and peak absolute moisture loads from the weather. These values from tables found in the Fundamentals Handbook are useful for humidity control applications, such as desiccant cooling and dehumidification, cooling-based dehumidification, and fresh air ventilation systems. These values can also be used as a checkpoint when analyzing the behavior of cooling systems at part load conditions, particularly when such systems are used for humidity control as a byproduct of temperature control.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS
Type of HVAC System – Isolation Rooms and Critical Examination Rooms

For the critical areas such as isolation rooms, intensive care units and operating rooms, critical diagnostic and examination rooms, consider only the centralized HVAC system encompassing “all air systems”. In all air systems, the outdoor air enters the system via a low – efficiency or “roughing” filters, which removes the large particulate matter. It is mixed with the return air and is made to pass the fine filters, which removes small size particles and many microorganisms. The air is than conditioned and delivered to each zone of the building. After the conditioned air is distributed to the designated space, it is withdrawn through a return duct system and delivered back to the HVAC unit. A portion of this “return air” is exhausted to the outside while the remainder is mixed with outdoor air and filtered for dilution and removal of contaminants. In some critical areas the air again filtered through HEPA filters located downstream the cooling/heating coil or at the terminal end of the duct. All air systems can be classified as single-zone, multi-zone, dual-duct and reheat systems. Single-zone systems: Single-zone systems serve just one zone having unique requirement of temperature, humidity and pressure. This is the simplest of all air systems. For this type of system to work properly, the load must be uniform all through the space, or else there may be a large temperature variation.[AdtoAppearHere]

Multi-zone systems: Multi-zone systems are used to serve a small number of zones with just one central air handling unit. The air handling unit for multi-zone systems is made up of heating and cooling coils in parallel to get a hot deck and a cold deck. For the lowest energy use, hot and cold deck temperatures are, as a rule, automatically changed to meet the maximum zone heating (hot deck) and cooling (cold deck) needs. Zone thermostats control mixing dampers to give each zone the right supply temperature.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

Dual-duct systems: Dual-duct systems are much like multi-zone systems, but instead of mixing the hot and cold air at the air handling unit, the hot and cold air are both brought by ducts to each zone where they are then mixed to meet the needs of the zone. It is common for dual-duct systems to use high-pressure air distribution systems with the pressure reduced in the mixing box at each zone.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

 

Reheat systems: Reheat systems supply cool air from a central air handler as required to meet the maximum cooling load in each zone. Each zone has a heater in its duct that reheats the supply air as needed to maintain space temperatures. Reheat systems are quite energy-inefficient and have been prohibited by various codes. Energy may though be saved through the recovery of the refrigeration system’s rejected heat and the use of this heat to reheat the air.

reheat

Caution

Air from infectious patient rooms is normally NOT recirculated and is exhausted directly to the outside via a HEPA filter. Use of terminal heating and cooling units such as fan coil units is NOT acceptable in isolation rooms, surgical suites and other critical areas where maintaining the room pressure relationships is important.

Type of HVAC System – Normal Patient Care Rooms, Administrative and Noncritical Areas

For the patient bedrooms and other non-critical areas, any one of the following HVAC systems can be used.

  1. All air systems as discussed above.
  2. Terminal heating and cooling units, such as fan coil units or radiant ceiling panels.
  3. Radiant heating and cooling system

The amount of outdoor air and how it is supplied to the occupied spaces would depend upon the type of HVAC system used. When the fan coil units or radiant ceiling panels are used, a central ventilation unit supplies conditioned air to the spaces. With this arrangement, the source of outdoor air being external to the principle cooling and heating equipment, it is possible to ensure the predetermined amount of outdoor air distribution to all the spaces.

CHILLERS

The chiller is the heart of an air conditioning plant. In a typical water-cooled chiller plant, it accounts for as much as 60% of the total HVAC power requirement. It is even higher (at 80%) in an air-cooled chiller plant. Chillers are specified by their design capacity in tons (1 ton = 12,000 Btu/hr) and their design efficiency in kW/ton.

Today chillers are available to operate at as low as 0.470 kW per ton. Given that annual energy costs for a chiller may amount to as much as one-third of their purchase price, even a modest improvement in efficiency can yield substantial energy savings and attractive paybacks. ASHRAE Standard 90.1 establishes minimum energy efficiency levels.

Four types of electrical chillers dominate the market:

  1. Reciprocating compressors

Reciprocating compressors are driven by a motor and use pistons, cylinders and valves to compress the refrigerant. These compressors are available in hermetic, semi-hermetic or externally driven versions.

  • In a hermetic unit, the motor and compressor are enclosed in a common housing, which is sealed. Because the components are not accessible for repair, the entire compressor unit must be replaced if it fails.
  • In the semi-hermetic unit the motor is also part of the unit, however it is not sealed so it is serviceable.
  • In a direct drive unit the motor and compressor are separated by a flexible coupling. These types of units utilize older technology and are not commonly used today.

rec-chiller

  1. Scroll compressors

Scroll compressors perform at higher efficiency levels than reciprocating compressors. The compressors operate without cylinders, pistons or valves so it offers:

  • Low maintenance and high reliability
  • Low noise and vibration levels
  • Low space requirements
  • Relatively low weight

Inside the scroll compressor, two spiral-shaped members fit together forming crescent shaped gas pockets. One member remains stationary while the other orbits relative to first. This movement draws gas into the outer pocket and seals off an open passage. As the spiral movement continues, gas is forced toward the center of the scroll design, creating a nearly continuous compression cycle.

scrollcomp-300x294 ScrollCompressor8

  1. Screw compressors

A screw compressor’s moving parts include a main and secondary rotor. It also has significant benefits:

  • Dramatic reduction of compressor parts
  • Low maintenance and high reliability
  • Low noise and vibration levels
  • Low space requirements
  • Relatively low weight

The screw compressor’s suction, compression and discharge all occur in one direction. Suction gas is pressed into one grooved rotor by the second similar rotor. The screw-like rotor motion continues toward the end of the compressor’s working space. In this way, refrigerant volume steadily reduces or compresses until it reaches the stationary end of the compressor. These chillers are common in high capacity ranges up to 1000 tons and are available in both air-cooled and water cooled options. [AdtoAppearHere]

  1. Centrifugal compressors

Centrifugal compressors are used in chillers with typical capacities of 150 to 2,000 tons. Centrifugal chillers are the most efficient of the large-capacity chillers but are ONLY used in water cooled configurations. The most effective chiller is primarily a function of chiller size and in general the following guidelines apply:

<=100 tons

1st Choice – Reciprocating

2nd Choice – Scroll

3rd Choice – Screw

100 -300 tons 

1st Choice – Screw

2nd Choice – Scroll

3rd Choice – Centrifugal

300 tons   

1st Choice – Centrifugal

2nd Choice – Screw

Chillers operate more efficiently when they are loaded close to their full rating than when they are only lightly loaded. It is imperative to determine which portion of the total load required 24 hours operations.

Air Handling Equipment Sizing Criteria

Air must be delivered at design volume to maintain pressure balances. The air handling equipment must be sized in accordance with the following guidelines:

  1. Load Calculations: Heat gain calculations must be done in accordance with the procedure outlined in the latest ASHRAE Handbook of Fundamentals. The calculations performed either manually or with a computer program.
  2. The calculated supply air shall be the sum of all individual peak room air quantities without any diversity.
  3. Safety Margin: A safety factor of 5 percent shall be applied to the calculated room air quantity to allow for any future increase in the room internal load.
  4. The adjusted supply air shall be, thus, 5 percent in excess of the calculated supply air.
  5. Air leakage: The air leakage through the supply air distribution ductwork shall be computed on the basis of the method described in the SMACNA Air Duct Leakage Test Manual. The maximum leakage amount shall not exceed 4 percent of the adjusted supply air.
  6. Supply Air Fan Capacity: The capacity of the supply air fan shall be calculated per the following example:
  7. Calculated Supply Air Volume = 20,000 CFM
  8. Safety Margin = 5 percent of item (a) = 1,000 CFM
  9. Adjusted Supply Air Volume = 21,000 CFM
  10. Duct Air Leakage = 4 percent of item (a) = 840 CFM
  11. Supply Air Fan Capacity = 21,840 CFM
  12. Equipment Selection: selection of the supply air fan, cooling coil, preheat coil, energy recovery coil (if any), filters, louvers, dampers, etc., shall be based on the supply fan capacity, 21,840 CFM calculated in the example above. A psychrometric chart shall be prepared for each air-handling unit. Make sure heat gains due to the fan motor and duct friction losses are taken into account for sizing cooling coils.
  13. Air Distribution:
  • The main supply air ductwork shall be sized to deliver the supply air fan capacity, 21,840 CFM as calculated in the example above.
  • The individual room air distribution system including supply, return, exhaust air ductwork, air terminal units, reheat coils and air outlets/inlets shall be sized and selected on the basis of the adjusted supply air volume, 21,000 CFM.
  • The fan and motor selection shall be based on the supply air fan capacity and static pressure adjusted, as necessary, for the altitude, temperature, fan inlet and discharge conditions, and the AMCA 201 System Effect Factors. The fan selection shall be made within a stable range of operation at an optimum static efficiency. The fan motor W (BHP), required at the operating point on the fan curves, shall be increased by 10 percent for drive losses and field conditions to determine the fan motor horsepower. The fan motor shall be selected within the rated nameplate capacity and without relying upon NEMA Standard Service Factor.
  1. Motor Voltages: Motor Voltages shall conform to NEMA/ANSI standard as follows:

pICTUER

pICTRUE2

 

Air Handling Units Specifications

The air handling equipment requires special attention to disinfection, and cleanliness; clusters of infections due to Aspergillus spp., Pseudomonas aeruginosa, Staphylococcus aureus, and Acinetobacter spp. have been linked to poorly maintained and/or malfunctioning air conditioning systems.

The failure or malfunction of any component ofthe HVAC system may subject patients and staff to discomfort and exposure to airborne contaminants. AIA guidelines prohibit United States hospitals and surgical centers from shutting down their HVAC systems for purposes other than required maintenance, filter changes, and construction. Often the routine maintenance and troubleshooting functions need to be addressed without necessarily disabling the units.

The following key elements need to be addressed when procuring these units.

  1. Specify the cabinet construction with stainless steel or galvanized steel sheets polyester-coated both from the inside and outside. Ensure cabinet framework is constructed from aluminum profiles for increased rigidity.
  2. Specify a layer of non-flammable mineral wool between the inside and outside sheets for the cabinet casing.
  3. Specify oblique floors for the air handling unit, tubs for the cooling units and drip channels made of stainless steel construction. Specify vacuum seal P-trap on the drain pan.
  4. Specify all edges and offsets to be filled with fungicidal silicon certified for hygienic applications in health care facilities which precludes formation of the microbe expansion centers.
  5. Specify provision for pressure gauges on the filter section casing of AHU along with audible alarm. This is to confirm that NO air stream will elude filtration, if openings are present because of filter damage or poor fit.
  6. Specify access and inspection openings with the lighting elements installed in covers of the sections for humidification, filtration, heat exchangers and fans.
  7. Specify modular construction with all the subunits to be assembled in a manner enabling their washing from all sides. All subunits and materials shall be resistant to commonly used disinfecting agents.
  8. Specify a drum fan with an inspection flap and an outflow pipe which enables the drum cleaning OR a centrifugal and axial-flow fan with an open rotor.
  9. Specify driving motor manufactured in the IP class, enabling washing and disinfection.
  10. Specify multistage filtration with minimum of MERV 14 final filtration installed in plastic frames and mounted in frameworks made of resistant materials. The filters shall be provided with differential pressure gauge and pollution level indicators.
  11. Specify UV bactericidal lamp ensuring disinfection of the recirculated air.
  12. Specify cable glands providing connection of motors and the lighting system, ensuring the appropriate tightness and cleanliness class.

Exhaust Fans

Exhaust fan must be selected to produce the rate of airflow required by the exhaust system. The flow must be developed against the total system resistance, including pressure losses through the air distribution network including air cleaning devices. A fan of proper size and operating speed should than be selected from the ratings published by the fan manufacturer.

HVAC515_whlbldgvent7_Rudd_1-10-13

The exhaust fan should be located downstream of the air cleaning filter and as close to the discharge point as possible. The preferred location for an exhaust fan is outdoors, normally on the roof. A straight duct section of at least 6 equivalent duct diameters and 3 equivalent duct diameters should be used when connecting to the fan inlet and outlet respectively before any bend or fittings. When this is impractical due to space constraints, corrective devices such as turning vanes or flow dividers should be used, or the associated loss must be accounted for. Fan selection should consider long term contaminant effects on the fan and the fan wheel. Where severe conditions of abrasion or corrosion are present, special lining or metals could be used in fan construction. Safe means should be provided to allow the wheel of an exhaust fan to be examined without removing the connecting ducts.

A flexible sleeve or band should be incorporated onto the fan inlet and outlet ducts to minimize vibration of the ductwork.

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Air Distribution Ductwork

Recommended Elements:

  1. In an effort to save installation dollars, the return duct is often deleted from the plans and the interstitial space between the suspended ceiling and the roof assembly, or the floor assembly above, is used as a return plenum. Open return air path directly over the false ceiling is NOT recommended for isolation rooms or elsewhere in health care facilities.
  2. Any air leakage through duct joints will disrupt the pressure balance raising possibility of infectious material entraining into the air supply. The supply and return air ducts should be properly sealed and insulated during construction. On the return side of the equipment, leaky ducts will draw in far more moisture than the cooling coils were designed to remove. The result is a higher than designed and desired humidity level in the space.
  3. Supply and exhaust systems should be designed as failsafe (for example, using duplex fans) to prevent contamination of any area within the facility in the event of fan failure.
  4. The ductwork of a negative pressure isolation room must not communicate with the ductwork of the rest of the hospital. Ductwork should be designed to reduce the possibility of cross contamination in the event of fan failure. This can be accomplished by ducting each negative pressure isolation room separately from the air-handling unit.
  5. The exhaust fan should be located at a point in the duct system that will ensure that the entire duct is under negative pressure within the building.
  6. Position the exhaust discharge duct to prevent the contamination of intake air. In acute cases, the discharge plume may need to be modeled to prevent entrainment.
  7. Round duct should be used for the construction of the exhaust system. Rectangular ducts, if used, should be as square as possible.
  8. All branches should enter the main duct at gradual expansions at an angle not exceeding 45 and preferably 30 or less. Connections should be to the top or side of the main and directly opposite each other. Elbows and bends should be at a minimum of 2 gauges heavier than straight length ducts of equal diameter and have a centerline radius of at least 2 and preferably 2.5 times the duct diameter. The smaller branches should enter the main near the high suction end, closer to the fan inlet.
  9. Exhaust stacks should be vertical and terminated at a point where height or air velocity would preclude re-entry of the contaminated air into the work environment.
  10. Duct velocities should be sufficient to prevent the settling of dry aerosols. The recommended minimum duct velocity for most areas of the healthcare facility is 2500 fpm.
  11. Ductwork should be located so that it is readily accessible for inspection, cleaning and repairs; Keep provisions for routine test ports for appropriate airflow and pressure balance.
  12. Labeling the ductwork helps prevent unnecessary exposure to maintenance personnel who may unknowingly cut into the ductwork for the purpose of testing airflow or repairing equipment. Using a HEPA filter at the point of exhaust in the room allows you to use non-sealed ductwork (after the HEPA), which may be on a shared exhaust run. The ductwork located after the HEPA filter does not need to be labeled as potentially contaminated.

Insulation

The dew-point temperature of the air surrounding the cooler ducts and pipes could easily be higher than the surface temperature of the ducts and pipes. Condensation will occur when this happens. If the ducts and piping happen to be in the ceiling space, the condensate can drip onto a surface that is loaded with mold food (ceiling tiles, dry wall boards, insulation, plywood, etc.) and all of the necessary elements are there for mold growth.

AC-Duct-Edited

Care must be taken to ensure that the supply air ducts, the chilled water lines (supply and return) and the refrigerant lines are well insulated with non-flammable mineral wool.

Noise Criteria

  1. The noise level should be restricted to 35 NC level for all patient rooms, operating rooms (major or minor), diagnostic rooms, audio suites, examination rooms, conference rooms, large offices, lobbies and waiting areas.
  2. The noise level should be restricted to 40 NC level for all small private offices, nursing stations, auditoriums, treatment areas, corridors, pharmacy and general work rooms.
  3. The noise level should be restricted to 45 NC level for all laboratories, Dining, Food Service/Serving, Therapeutic Pools.
  4. The noise level should be restricted to 50 NC level for all gymnasiums, recreation rooms, laundries and HVAC plant rooms.

Duct Sizing Criteria

Duct systems should be designed in accordance with the general rules outlined in the latest ASHRAE Guide and Data Books, SMACNA Manuals and Design Guide Section of the Associated Air Balance Council Manual.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

  1. Supply duct system, with total external static pressure 2 inches – w.g and larger, shall be designed for a maximum duct velocity of 2500 fpm for duct mains and a maximum static pressure of 0.25 inch-w.g. per 100 ft duct length. Static pressure loss and regain shall be considered in calculating the duct sizes. Size supply branch ducts for a maximum duct velocity of 1500 fpm.
  2. All other duct systems such as return and exhaust, including branch ducts, shall be designed for a maximum velocity of 1500 fpm for the duct mains and a maximum static pressure of 0.10 inch- w.g. per 100 ft duct length, with the minimum duct area of 48 sq in ( or 8 in x 6 in) size.
  3. Indicate Duct Static Pressure Construction Classification according to SMACNA (1/2″, 1″, 2″, 3″ and 4″) on drawings.

Pipe Sizing Criteria

All piping required for HVAC systems shall be sized based on the following criteria:

Water losses, pressure loss, etc., for sizing piping shall be based on “Cameron Hydraulic Data”: With C = 100 for open (cooling tower) systems and C = 150 for closed systems.

For closed systems, the maximum friction loss shall be 4 ft of water per 100 ft of pipe with maximum velocity of 14 fps for systems in occupied areas, and up to 8 fps for mains and large branches. For open systems, the maximum friction loss shall be 4 ft of water per 100 ft of pipe and a maximum velocity of 8 to 10 fps. The minimum pipe size shall be 3/4-inch.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

HVAC EQUIPMENT LOCATION AND INSTALLATION

Equipment shall be located to be accessible for installation, operation and repair. Mechanical spaces shall be of suitable size to permit inspection and access for maintenance, and to provide space for future equipment when required. The effect that equipment noise or vibration might have on areas adjacent to, above, and below equipment shall be considered. Design shall comply with specified room sound ratings.

Location of equipment remote from sound sensitive areas should be emphasized. Make provisions for all necessary stairs, cat walks, platforms, steps over roof mounted piping and ducts, etc., that will be required for access, operation and maintenance. Access to roofs by portable ladder is not acceptable.

Air Handling Equipment

Air handling units and similar equipment shall be housed in a mechanical equipment room or in a mechanical penthouse enclosure. Penthouse type of fully weatherized roof top units constructed in standard sections of modules would be acceptable in lieu of the mechanical equipment rooms or mechanical penthouses. These units shall provide excess sections for walk through servicing, maintenance, and shall ensure that the piping connections and electrical conduits are fully enclosed within the units.

 HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

 

Cooling Towers

Select and locate cooling towers to avoid problems with aesthetics, noise, vibrations, air recirculation or drift. Include a noise analysis of the proposed cooling tower relative to adjacent occupancies and consider alternative cooling tower selections, if necessary, to meet noise level of 60 dB(A) at 15 m (50 feet) which may be lowered for critical locations. Consider provisions for security and maintenance lights and receptacles. Provide a permanent service platform and ladders for access to cooling tower basin access doors. Water treatment of cooling tower water is very important because the cooling tower operation is associated with Legionella disease and lower respiratory tract infections. Effective methods for disinfecting the hospital water supply include chlorination, thermal eradication, UV light, ozone treatment and metal (copper –silver) ionization system.

HVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERSHVAC SYSTEM & EQUIPMENT DESIGN IN HEALTH CARE CENTERS

Air Intakes and Outlets

  1. Ensure that air intakes and exhaust outlets are located properly in construction of new facilities and renovation of existing facilities.
  2. Locate exhaust outlets >25 ft from air-intake systems.
  3. Locate outdoor air intakes >6 ft above ground or >3 ft above roof level… (The air intake shall be located as high as practical or not less than stated).
  4. Locate exhaust outlets from contaminated areas above roof level to minimize recirculation of exhausted air.
  5. Operating Room system air intakes shall be at least 30 ft above the ground.
  6. Laboratory and Research exhaust shall be terminated at the highest point of the building (NFPA 99, 5-3.3.4).
  7. Outside air intake shall not be near hot exhaust discharging horizontally or deflected down, nor be near plumbing vents, animal room exhausts, generator exhausts, loading docks, automobile entrances, driveways, passenger drop-offs, cooling towers, incinerator and boiler stacks.
  8. Louvers shall be designed for a maximum velocity of 750 fpm through the free area of 35 percent. Drainable louvers may be designed for a maximum velocity of 1000 fpm and 45 percent free area.
  9. Ensure that the intakes are kept free from bird droppings, especially those from pigeons.

 

References:
  1. UCSF Medical Center Design Guidelines HVAC, June 2015.
  2. HVAC Design for Healthcare Facilities, Course No: M06-011, Credit: 6 PDH by A. Bhatia, Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980.

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