The usual need to look up values from various tables and charts makes the conventional hand calculation
quite laborious, time consuming and prone to errors and inaccuracies because of the tendency to simplify
truncate or interpolate tabulated values. Listed below are the suggested applications and methodology of
two computer programs Elite CHVAC and TRACE 700 that may be used in the cooling and heating load
CHVAC is a commercial Heating, Ventilation and Air Conditioning software platform developed by Elite
Software. This computer program calculates the maximum heating and cooling loads in commercial and
Listed below are capabilities of CHVAC:
- Calculates peak heating and cooling loads
- Calculates both heating & cooling airflow CFM requirements
- Calculates run out and main trunk duct sizes
- Automates compliance with ASHRAE Standard 62
- Provides overall building envelope report
- Spreadsheet file compatibility
- Performs complete psychrometric analysis
- Prints bar graphs and exploded pie charts
- Exterior shading handles overhangs, fins, & glass tilt
- Uses exact ASHRAE CLTD procedures
- Built-in design weather data for hundreds of cities
- Analyzes up to 12 months per calculation
- Calculates 24 hours per design day
- Allows unlimited number of zones per project
- Zones may be grouped under 100 air handlers
- Zones may be optionally grouped under VAV boxes
- Allows 12 walls, 12 windows, and 5 roofs per zone
- Allows simultaneous infiltration and ventilation
- Allows different summer and winter air rates
- Allows varying indoor conditions within a project
- Allows 6 master roof types,8 master wall types, 8 master partition types, and 20 master glass types
- Allows glass to be tilted from 0 to 180 degrees
- Allows for roof and wall color effects
- Provision for both VAV and constant volume systems
- Proper handling of return air plenum loads
- Accounts for people diversity in total building load
- Computes supply fan horsepower and heat gains
- Accounts for both draw-thru and blow-thru fans
- Calculates reheat requirements if necessary
- Computes supply and return duct gains and losses
- Allows direct specification of supply CFM quantities
- Allows specification of minimum supply air quantities
- Allows heating only, cooling only, or both
- Excess supply air can be handled as reheat, reserve capacity, or by adjusting the leaving coil conditions
- Leaving coil conditions can be specified with a desired dry bulb temperature or a relative humidity
- Calculates chilled and hot water coil flow rates
- Allows for pretreated outside air
- Allows heating and cooling safety factors
- Lighting & equipment watts along with no. of people can be entered directly or on a per square foot basis
CHVAC performs calculations using the CLTD/CLF procedures described in the ASHRAE Handbook of Fundamentals. The programs use exact CLTD and MSHGF table values where possible, otherwise direct
calculations are made. This calculation technique allows the programs to calculate for any building
orientation and still produce output results that can be easily verified by hand.
CHVAC is a true Windows program that uses all the latest data entry techniques such as toolbars, hyper linked help, and form tabs. All input data is checked at the time of entry so that no improper data can be entered. Five types of data are requested: general project data, outdoor design data, building material data, air handler data, and specific zone data. The general project data includes the project and client name, designer, building opening and closing hours, internal operating load schedules, and any desired safety factors. The outdoor design data includes the summer and winter outdoor design conditions (automatically looked up for you if a city reference is given) and the desired ventilation and infiltration rates. The building material data includes the definition of master building material types for roofs, walls, partitions, glass sections, and exterior shading. A user defined material library is available for saving the data on common material types. The air handler data includes the fan and terminal type, the desired heating and cooling supply air temperatures and data for duct heat gains and losses. The zone data includes the zone name, floor length and width, number of people, equipment watts, lighting watts, external shading data, and specific roof, wall, partition, floor, and glass data.
The CHVAC program provides eleven types of reports,which can be selectively previewed onscreen or printed as desired. CHVAC supports all printers that work with Windows and numerous full color reports are available.The reports are: General Project Data, Air Handler Input Data, Zone Input Data, Detailed Project Zone Loads, Air System Zone Summary, Total Building, Air System, and Zone Load Profiles, Air System Total Load Summary, Air System Psychrometric Analysis, Overall Building Envelope Report, Pie Charts, Bar Graphs, and the Total Building Load Summary. Air system summary data can be exported to your favorite spreadsheet.
TRACE ® 700
The TRACE Load ® 700 program is a commercial Heating, Ventilation and Air Conditioning software platform developed by Trane’s CDS Group.
The Load phase of the program computes the peak sensible and latent zone loads, as well as the block sensible and latent loads for the building. In addition, the hourly sensible and latent loads, including weather-dependent loads, are calculated for each zone, based on the weather library. The building heating/cooling load calculations, used in the load phase of the program for annual energy consumption analysis, are of sufficient detail to permit the evaluation of the effect of building data such as orientation, size, shape and mass, heat transfer characteristics of air and moisture, as well as hourly climatic data. The Design phase of the TRACE program calculates the design supply air temperatures, heating and cooling capacities, and supply air quantities given the peak load files generated by the Load phase. For applications where the building design parameters are known, you can override the calculation of these values using optional entries to the System phase. This gives you the ability to simulate existing buildings
with installed equipment that may not be sized according to the loads calculated in the program’s Load Phase.
Beyond this, the calculations used to simulate the operation of the building and its service systems through a full-year operating period, are of sufficient detail to permit the evaluation of the effect of system design, climatic factors, operational characteristics and mechanical equipment operating characteristics on annual energy usage. Manufacturers’ data is used in the program for the simulation of all systems and equipment. The calculation procedures used in TRACE are based upon 8,760 hours of operation of the building and its service system. These procedures use techniques recommended in the appropriate ASHRAE publications or produce results that are consistent with such recommended techniques. The following are the program features:
Project Navigator View
- Organizes entries by task to lead you through the modeling process
- Displays the status of each modeling step
- Accommodates up to 4 alternatives per project
Project Tree View
- Organizes all rooms, systems, and plants in a hierarchical list
- Displays all information about a system, zone, or room on 1 screen
- Supports cut, copy, and paste to save entry time
- Displays cooling set point for every room in the project on 1 screen
- Makes it easy to check and edit your work
Task-oriented display guides you through the modeling process as follows:
Select weather information
- Provides both design and typical weather data by location
- Choose from 400 climate locations
- Import standard weather files for a full-year (8760) analysis
- Describe the construction, airflows, thermostat settings, heat sources, and schedules by room
Create airside systems
- Choose from more than 30 methods of air distribution
- Add energy recovery, economizers, and dedicated ventilation/makeup air
Assign rooms to systems
- Create thermal zones and assign them to systems
- Determine airflows, coil loads, and fan sizes for each airside system
If the only requirement is to calculate the cooling and/or heating loads and the project does not require energy analysis and economic evaluation, it is recommended that the Program Load ® 700 be utilized instead of Full TRACE 700.The only difference is that Trace Load ® 700 users only have access to the Load Design section (from Project Information to Assign Rooms to Systems). Full TRACE 700 users will have full access to the Load, Energy and Economic sections.
The advantage of using only Trace Load ® 700 is that all the added features and capabilities (applicable to load design) in full TRACE 700 program are also available to the Trace Load ® 700 users. Also, same file extensions and libraries will enable users of both programs to transfer archived files back and forth without any additional steps needed.
The fundamentals of Sound and Vibrations are part of the broader field of mechanics, with strong connections to classical mechanics, solid mechanics and fluid dynamics.
Dynamics is the branch of physics concerned with the motion of bodies under the action of forces.
Vibrations or oscillations can be regarded as a subset of dynamics in which a system subjected to restoring forces swings back and forth about an equilibrium position, where a system is defined as an assemblage of parts acting together as a whole. The restoring forces are due to elasticity, or due to gravity.
The subject of Sound and Vibrations encompasses the generation of sound and vibrations, the distribution and damping of vibrations, how sound propagates in a free field, and how it interacts with a closed space, as well as its effect on man and measurement equipment. Technical applications span an even wider field, from applied mathematics and mechanics, to electrical instrumentation and analog and digital signal processing theory, to machinery and building design. Most human activities involve vibration in one form or other. For example, we hear because our eardrums vibrate and see because light waves undergo vibration. Breathing is associated with the vibration of lungs and walking involves (periodic) oscillatory motion of legs and hands. Human speak due to the oscillatory motion of larynges (tongue).
In most of the engineering applications, vibration is signifying to and fro motion , which is undesirable. Galileo discovered the relationship between the length of a pendulum and its frequency and observed the resonance of two bodies that were connected by some energy transfer medium and tuned to the same natural frequency. Vibration may results in the failure of machines or their critical components. The effect of vibration depends on the magnitude in terms of displacement, velocity or accelerations, exciting frequency and the total duration of the vibration.
Free Vibration– In Free vibration, the object is not under the influence of any kind of outside force.
In free vibration the body at first is given an initial displacement and the force is withdrawn. The body starts vibrating and continues the motion of its own accord. No external force acts on the body further to keep it in motion. The frequency of free vibration is known as free or natural frequency.
The free vibration of an elastic body can further be of three types:
a)Longitudinal vibration: when the particles of the body move parallel to the axis of the body, the vibration is known as longitudinal vibration.
b)Transverse vibration: when the particles of the body move nearly perpendicular to the axis of the body, the vibration is known as transverse vibration.
c)Torsional vibration: When the particles of the body move in a circle about the axis of the body, the vibration is known as torsional vibration.
Forced Vibration– In forced vibration, the object is under the influence of an outside force.
This can be understood more clearly by the following example:-
When a pendulum vibrates it is free vibration because it does not depend on any outside force to vibrate whereas when a drilling machine vibrates, it depends on a force from outside. Therefore, it is an example of forced vibration.
A linear system is defined as one in which the relationship between the input and output signals can be described by a linear differential equation.
Often in Vibrations and Acoustics, the calculation of the effect of a certain physical quantity termed as the input signal on another physical quantity, called the output signal.
An example is that of calculating vibration velocity v(t), which is obtained in a structure when it is excited by a given force F(t). That problem can be solved by making use of the theory of linear time- invariant systems. A linear time-invariant system describes the relationship between an input signal and an output signal. For example, the input signal could be a velocity v(t), and the output signal a force F(t), or the input signal an acoustic pressure p(t) and the output signal an acoustic particle velocity u’(t). If the coefficients are, moreover, independent of time, i.e., constant, then the system is also time invariant.
Discrete System Components A system is defined as an aggregation of components acting together as one entity. The components of a vibratory mechanical system are of three different types, and they relate forces to displacements, velocities, and accelerations. The component relating forces to displacements is known as a spring. For a linear spring the force Fs is proportional to the elongation or
where k represents the spring constant, or the spring stiffness, and x2 and x1 are the displacements of the end points.
Viscous damper or a dashpot The component relating forces to velocities is called a viscous damper or a dashpot. It consists of a piston fitting loosely in a cylinder filled with liquid so that the liquid can flow around the piston when it moves relative to the cylinder. The relation between the damper force and the velocity of the
piston relative to the cylinder is
in which c is the coefficient of viscous damping; note that dots denote derivatives with respect to time. Finally, the relation between forces and accelerations is given by Newton’s second law of motion:
where m is the mass.
The spring constant k, coefficient of viscous damping c, and mass m represent physical properties of the components and are the system parameters.
Note that springs and dampers are assumed to be massless and masses are assumed to be rigid.
Equivalent spring constant Springs can be arranged in parallel and in series. Then, the proportionality constant between the forces and the end points is known as an equivalent spring constant and is denoted by keq, as shown in Table below:
Certain elastic components, although distributed over a given line segment, can be regarded as lumped with an equivalent spring constant given by keq = F/δ, where δ is the deflection at the point of application of the force F. A similar relation can be given for springs in torsion. Table given above lists the equivalent spring constants for a variety of components.
Equation of Motion The dynamic behavior of many engineering systems can be approximated with good accuracy by the mass-damper spring model. Using Newton’s second law in conjunction with equations for Fs, Fd and Fm given above and measuring the displacement x(t) from the static equilibrium position, we obtain the differential equation of motion as below:
which is subject to the initial conditions x(0)=x0, ẋ(0)=v0, where x0 and v0 are the initial displacement and initial velocity, respectively.
Equation given above is in terms of a single coordinate. namely x(t) is therefore said to be a single-degree-of-freedom system.
Free Vibration of Undamped Systems Assuming zero damping and external forces and dividing above equation through by m, we obtain
In this case, the vibration is caused by the initial excitations alone. The solution of above equation is
which represents simple sinusoidal, or simple harmonic oscillation with amplitude A, phase angle ф, and frequency .
The time necessary to complete one cycle of motion defines the period.
The reciprocal of the period provides another definition of the natural frequency,namely,
where Hz denotes hertz[1 Hz = 1 cycle per second (cps)].
Free Vibration of Damped Systems Let F(t)=0 and divide through by m.Then, Equation of motion reduces to
ξ is the damping factor, a non-dimensional quantity. The nature of the motion depends on ξ. The most important case is that in which 0<ξ<1.
In this case, the system is said to be underdamped and the solution of above equation is
ωd is the frequency of damped free vibration and is the period of damped oscillation.
The case ξ=1, represents critical damping, and Cc is the critical damping coefficient,although there is nothing critical about it. It merely represents the borderline between oscillatory decay and aperiodic decay. In fact, Cc is the smallest damping coefficient for which the motion is aperiodic. When ξ>1, the system is said to be overdamped.
Logarithmic Decrement Quite often the damping factor is not known and must be determined experimentally. In the case in which the system is underdamped, this can be done conveniently by plotting x(t)
versus t, and measuring the response at two different times separated by a complete period.
Whirling of Rotating Shafts Many mechanical systems involve rotating shafts carrying disks. If the disk has some eccentricity, then the centrifugal forces cause the shaft to bend, as shown in Figure (a) below. The rotation of the plane containing the bent shaft about the bearing axis is called whirling. Figure (b) below shows a disk with the body axes x, y rotating about the origin O with the angular velocity ω.
The geometrical center of the disk is denoted by S and the mass center by C.The distance between the two points is the eccentricity e. The shaft is massless and of stiffness keq and the disk is rigid and of mass m. The x and y components of the displacement of S relative to O are independent from one another and, for no damping, satisfy the equations of motion
Resonance occurs when the whirling angular velocity coincides with the natural frequency. In terms of rotations per minute, it has the value
where fc is called the critical speed.
- Marks’ Standard Handbook for Mechanical Engineers Eleventh Edition.
- Fundamentals of Sound and Vibrations by KTH Sweden.
The Cooling Load
The cooling load on refrigerating equipment seldom results from any one single source of heat. Rather, it is the summation of the heat usually evolves from several different sources.
Cooling load calculations may be used to accomplish one or more of the following objectives:
- Provide information for equipment selection, system sizing and system design.
- Provide data for evaluating the optimum possibilities for load reduction.
- Permit analysis of partial loads as required for system design, operation and control.
Commonly used terms relative to heat transmission and load calculations are defined below in accordance with ASHRAE Standard 12-75, Refrigeration Terms and Definitions.
Space – is either a volume or a site without a partition or a partitioned room or group of rooms.
Room – is an enclosed or partitioned space that is usually treated as single load.
Zone – is a space or group of spaces within a building with heating and/or cooling requirements sufficiently similar so that comfort conditions can be maintained throughout by a single controlling device.
British thermal unit (Btu) – is the approximate heat required to raise 1 lb. of water 1 deg Fahrenheit, from 59°F to 60°F. Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the “ton,” which is 12,000 Btu per hour and Watts. Some countries utilize one unit, more than the others and therefore it is good if you can remember the relationship between BTU/hr, Ton, and Watts.
- 1 ton is equivalent to 12,000 BTU/hr. and
- 12,000 BTU/hr is equivalent to 3,516 Watts – or 3.516 kW (kilo-Watts).
Cooling Load Temperature Difference (CLTD) – an equivalent temperature difference used for calculating the instantaneous external cooling load across a wall or roof.
Sensible Heat Gain – is the energy added to the space by conduction, convection and/or radiation.
Latent Heat Gain – is the energy added to the space when moisture is added to the space by means of vapor emitted by the occupants, generated by a process or through air infiltration from outside or adjacent areas.
Radiant Heat Gain – the rate at which heat absorbed is by the surfaces enclosing the space and the objects within the space.
Space Heat Gain – is the rate at which heat enters into and/or is generated within the conditioned space during a given time interval.
Space Cooling Load – is the rate at which energy must be removed from a space to maintain a constant space air temperature.
Space Heat Extraction Rate – the rate at which heat is removed from the conditioned space and is equal to the space cooling load if the room temperature remains constant.
Dry Bulb Temperature – is the temperature of air indicated by a regular thermometer.
Wet Bulb Temperature – is the temperature measured by a thermometer that has a bulb wrapped in wet cloth. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air. Evaporation is slower when the air is already full of water vapor. For this reason, the difference in the temperatures indicated by ordinary dry bulb and wet bulb thermometers gives a measure of atmospheric humidity.
Dew point Temperature – is the temperature to which air must be cooled in order to reach saturation or at which the condensation of water vapor in a space begins for a given state of humidity and pressure.
Relative humidity – describes how far the air is from saturation. It is a useful term for expressing the amount of water vapor when discussing the amount and rate of evaporation. One way to approach saturation, a relative humidity of 100%, is to cool the air. It is therefore useful to know how much the air needs to be cooled to reach saturation.
Thermal Transmittance or Heat Transfer Coefficient (U-factor) – is the rate of heat flow through a unit area of building envelope material or assembly, including its boundary films, per unit of temperature difference between the inside and outside air. The U-factor is expressed in Btu/ (hr °F ft²).
Thermal Resistance(R) – is the reciprocal of a heat transfer coefficient and is expressed in (hr °F ft²)/Btu. For example, a wall with a U-value of 0.25 would have a resistance value of R = 1/U = 1/0.25=4.0. The value of R is also used to represent Thermal Resistivity, the reciprocal of the thermal conductivity.
Sizing Of Air-Conditioning System
Concepts and fundamentals of air conditioner sizing is based on heat gain, and/or losses in a building. It is obvious that you will need to remove the amount of heat gain – if it is hot outside. Similarly, you’ll need to add in the heat loss from your space – if outside temperature is cold. In short, heat gain and loss, must be equally balanced by heat removal, and addition, to get the desired room comfort that we want.
The heat gain or heat loss through a building depends on:
- The temperature difference between outside temperature and our desired temperature.
- The type of construction and the amount of insulation is in your ceiling and walls. Let’s say, that you have two identical buildings, one is build out of glass, and the other out of brick. Of course the one built with glass would require much more heat addition, or removal, compared to the other – given a same day. This is because the glass has a high thermal conductivity (U-value) as compared to the brick and also because it is transparent, it allows direct transmission of solar heat.
- How much shade is on your building’s windows, walls, and roof? Two identical buildings with different orientation with respect to the direction of sun rise and fall will also influence the air conditioner sizing.
- How large is your room? The surface area of the walls. The larger the surface area – the more heat can lose, or gain through it.
- How much air leaks into indoor space from the outside? Infiltration plays a part in determining our air conditioner sizing. Door gaps, cracked windows, chimneys – are the “doorways” for air to enter from outside, into your living space.
- The occupants. It takes a lot to cool a town hall full of people.
- Activities and other equipment within a building. Cooking? Hot bath? Gymnasium?
- Amount of lighting in the room. High efficiency lighting fixtures generate less heat.
- How much heat the appliances generate. Number of power equipments such as oven, washing machine, computers, TV inside the space; all contribute to heat.
The air conditioner’s efficiency, performance, durability, and cost depend on matching its size to the above factors. Many designers use a simple square foot method for sizing the air-conditioners. The most common rule of thumb is to use “1 ton for every 500 square feet of floor area”. Such a method is useful in preliminary estimation of the equipment size. The main drawback of rules-of-thumb methods is the presumption that the building design will not make any difference. Thus the rules for a badly designed building are typically the same as for a good design.
It is important to use the correct procedure for estimating heat gain or heat loss. Two groups—the Air Conditioning Contractors of America (ACCA) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE)—publish calculation procedures for sizing central air conditioners.
Reputable air conditioning contractors will use one of these procedures, often performed with the aid of a computer, to size your new central air conditioner.
Heating Load V/s Cooling Load Calculations
As the name implies, heating load calculations are carried out to estimate the heat loss from the building in winter so as to arrive at required heating capacities. Normally during winter months the peak heating load occurs before sunrise and the outdoor conditions do not vary significantly throughout the winter season. In addition, internal heat sources such as occupants or appliances are beneficial as they compensate some of the heat losses. As a result, normally, the heat load calculations are carried out assuming steady state conditions (no solar radiation and steady outdoor conditions) and neglecting internal heat sources. This is a simple but conservative approach that leads to slight over estimation of the heating capacity. For more accurate estimation of heating loads, one has to take into account the thermal capacity of the walls and internal heat sources, which makes the problem more complicated.
For estimating cooling loads, one has to consider the unsteady state processes, as the peak cooling load occurs during the day time and the outside conditions also vary significantly throughout the day due to solar radiation. In addition, all internal sources add on to the cooling loads and neglecting them would lead to under estimation of the required cooling capacity and the possibility of not being able to maintain the required indoor conditions. Thus cooling load calculations are inherently more complicated.
In determining the heating load, credit for solar heat gain or internal heat gains is usually NOT included and the thermal storage effects of building structure are generally ignored. Whereas in cooling load calculations, the thermal storage characteristics of the building play a vital role because the time at which the space may realize the heat gain as a cooling load will be considerably offset from the time the heat started to flow.
Heat Flow Rates
In air-conditioning design, four related heat flow rates, each of which varies with time, must be differentiated:
- Space heat gain —————-How much heat (energy) is entering the space?
- Space cooling load ————-How much energy must be removed from the space to keep temperature and relative humidity constant?
- Space heat extraction———–How much energy is the HVAC removing from the space?
- Cooling load (coil)—————How much energy is removed by the cooling coil serving various spaces plus any loads external to the spaces such as duct heat gain, duct leakage, fan heat and outdoor makeup air?
Space Heat Gain
This instantaneous rate of heat gain is the rate at which heat enters into and/or is generated within a space at a given instant. Heat gain is classified by:
The manner in which it enters the space –
- Solar radiation through transparent surfaces such as windows
- Heat conduction through exterior walls and roofs
- Heat conduction through interior partitions, ceilings and floors
- Heat generated within the space by occupants, lights, appliances, equipment and processes
- Loads as a result of ventilation and infiltration of outdoor air
- Other miscellaneous heat gain
Whether it is a sensible or latent gain –
Sensible heat – Heat which a substance absorbs, and while its temperature goes up, the substance does not change state. Sensible heat gain is directly added to the conditioned space by conduction, convection, and/or radiation. Note that the sensible heat gain entering the conditioned space does not equal the sensible cooling load during the same time interval because of the stored heat in the building envelope. Only the convective heat becomes cooling load instantaneously. Sensible heat load is total of
- Heat transmitted thru floors, ceilings, walls
- Occupant’s body heat
- Appliance & Light heat
- Solar Heat gain thru glass
- Infiltration of outside air
- Air introduced by Ventilation
Latent Heat Loads – Latent heat gain occurs when moisture is added to the space either from internal sources (e.g. vapor emitted by occupants and equipment) or from outdoor air as a result of infiltration or ventilation to maintain proper indoor air quality. Latent heat load is total of
- Moisture-laden outside air form Infiltration& Ventilation
- Occupant Respiration & Activities
- Moisture from Equipment & Appliances
To maintain a constant humidity ratio, water vapor must condense on cooling apparatus at a rate equal to its rate of addition into the space. This process is called dehumidification and is very energy intensive, for instance, removing1 kg of humidity requires approximately 0.7 kWh of energy.
Space Heat Gain V/s Cooling Load (Heat Storage Effect)
Space Heat Gain is ≠ to Space Cooling Load
The heat received from the heat sources (conduction, convection, solar radiation, lightning, people, equipment, etc…) does not go immediately to heating the room air. Only some portion of it is absorbed by the air in the conditioned space instantaneously leading to a minute change in its temperature. Most of the radiation heat especially from sun, lighting, people is first absorbed by the internal surfaces, which include ceiling, floor, internal walls, furniture etc. Due to the large but finite thermal capacity of the roof, floor, walls etc., their temperature increases slowly due to absorption of radiant heat. The radiant portion introduces a time lag and also a decrement factor depending upon the dynamic characteristics of the surfaces. Due to the time lag, the effect of radiation will be felt even when the source of radiation, in this case the sun is removed.
Differences between Space Heat Gain and Space Cooling Load
Differences between instantaneous heat gain and cooling load is due to heat storage affect.
The relation between heat gain and cooling load and the effect of the mass of the structure (light, medium & heavy) is shown below. From the figure it is evident that, there is a delay in the peak heat, especially for heavy construction.
Actual cooling load and solar heat gain for light, medium and heavy Construction
Space Cooling V/s Cooling Load (Coil)
Space cooling is the rate at which heat must be removed from the spaces to maintain air temperature at a constant value. Cooling load, on the other hand, is the rate at which energy is removed at the cooling coil that serves one or more conditioned spaces in any central air conditioning system. It is equal to the instantaneous sum of the space cooling loads for all spaces served by the system plus any additional load imposed on the system external to the conditioned spaces items such as fan energy, fan location, duct heat gain, duct leakage, heat extraction lighting systems and type of return air systems all affect component sizing.
- Principles of Refrigeration, Second edition by Roy J. Dossat
- Cooling load calculations and principles by A. Bhatia
An engineering device or process can be studied either experimentally (testing and taking measurements) or analytically (by analysis or calculations). The experimental approach has the advantage that we deal with the actual physical system, and the desired quantity is determined by measurement, within the limits of experimental error. However, this approach is expensive, time consuming, and often impractical. Besides, the system we are analyzing may not even exist. For example, the entire heating and plumbing systems of a building must usually be sized before the building is actually built on the basis of the specifications given. The analytical approach (including the numerical approach) has the advantage that it is fast and inexpensive, but the results obtained are subject to the accuracy of the assumptions, approximations, and idealizations made in the analysis. In engineering studies, often a good compromise is reached by reducing the choices to just a few by analysis, and then verifying the findings experimentally.
Modeling in Engineering
The descriptions of most scientific problems involve equations that relate the changes in some key variables to each other. Usually the smaller the increment chosen in the changing variables, the more general and accurate the description. In the limiting case of infinitesimal or differential changes in variables, we obtain differential equations that provide precise mathematical formulations for the physical principles and laws by representing the rates of change as derivatives. Therefore, differential equations are used to investigate a wide variety of problems in sciences and engineering (Fig. l–16). However, many problems encountered in practice can be solved without resorting to differential equations and the complications associated with them.
The study of physical phenomena involves two important steps.
In the first step, all the variables that affect the phenomena are identified, reasonable assumptions and approximations are made, and the interdependence of these variables is studied. The relevant physical laws and principles are invoked, and the problem is formulated mathematically. The equation itself is very instructive as it shows the degree of dependence of some variables on others, and the relative importance of various terms.
In the second step, the problem is solved using an appropriate approach, and the results are interpreted.
Many processes that seem to occur in nature randomly and without any order are, in fact, being governed by some visible or not-so-visible physical laws. Whether we notice them or not, these laws are there, governing consistently and predictably what seem to be ordinary events.
Preparing very accurate but complex models is usually not so difficult. But such models are not much use to an analyst if they are very difficult and time consuming to solve. At the minimum, the model should reflect the essential features of the physical problem it represents. There are many significant real world problems that can be analyzed with a simple model. But it should always be kept in mind that the results obtained from an analysis are at best as accurate as the assumptions made in simplifying the problem. Therefore, the solution obtained should not be applied to situations for which the original assumptions do not hold.
A solution that is not quite consistent with the observed nature of the problem indicates that the mathematical model used is too crude. In that case, a more realistic model should be prepared by eliminating one or more of the
questionable assumptions. This will result in a more complex problem that, of course, is more difficult to solve. Thus any solution to a problem should be interpreted within the context of its formulation.
Problem Solving Technique
The first step in learning any science is to grasp the fundamentals, and to gain a sound knowledge of it. The next step is to master the fundamentals by putting this knowledge to test. This is done by solving significant real-world
problems. Solving such problems, especially complicated ones, require a systematic approach. By using a step-by-step approach, an engineer can reduce the solution of a complicated problem into the solution of a series of simple problems (Fig. 1–17). When solving a problem, we recommend that you use the following steps zealously as applicable. This will help you avoid some of the common pitfalls associated with problem solving.
Step 1: Problem Statement
In your own words, briefly state the problem, the key information given, and the quantities to be found. This is to make sure that you understand the problem and the objectives before you attempt to solve the problem.
Step 2: Schematic
Draw a realistic sketch of the physical system involved, and list the relevant information on the figure. The sketch does not have to be something elaborate, but it should resemble the actual system and show the key features. Indicate any energy and mass interactions with the surroundings. Listing the given information on the sketch helps one to see the entire problem at once. Also, check for properties that remain constant during a process (such as temperature during an isothermal process), and indicate them on the sketch.
Step 3: Assumptions and Approximations
State any appropriate assumptions and approximations made to simplify the problem to make it possible to obtain a solution. Justify the questionable assumptions. Assume reasonable values for missing quantities that are necessary. For example, in the absence of specific data for atmospheric pressure, it can be taken to be 1 atm. However, it should be noted in the analysis that the atmospheric pressure decreases with increasing elevation. For example, it drops to 0.83 atm in Denver (elevation 1610 m).
Step 4: Physical Laws
Apply all the relevant basic physical laws and principles (such as the conservation of mass), and reduce them to their simplest form by utilizing the assumptions made. However, the region to which a physical law is applied must be clearly identified first. For example, the heating or cooling of a canned drink is usually analyzed by applying the conservation of energy principle to the entire can.
Step 5: Properties
Determine the unknown properties at known states necessary to solve the problem from property relations or tables. List the properties separately, and indicate their source, if applicable.
Step 6: Calculations
Substitute the known quantities into the simplified relations and perform the calculations to determine the unknowns. Pay particular attention to the units and unit cancellations, and remember that a dimensional quantity without a unit is meaningless. Also, don’t give a false implication of high precision by copying all the digits from the screen of the calculator—round the results to an appropriate number of significant digits.
Step 7: Reasoning, Verification, and Discussion
Check to make sure that the results obtained are reasonable and intuitive, and verify the validity of the questionable assumptions. Repeat the calculations that resulted in unreasonable values. For example, insulating a water heater that uses $80 worth of natural gas a year cannot result in savings of $200 a year.
Also, point out the significance of the results, and discuss their implications. State the conclusions that can be drawn from the results, and any recommendations that can be made from them. Emphasize the limitations under which the results are applicable, and caution against any possible misunderstandings and using the results in situations where the underlying assumptions do not apply. For example, if you determined that wrapping a water heater with a $20 insulation jacket will reduce the energy cost by $30 a year, indicate that the insulation will pay for itself from the energy it saves in less than a year. However, also indicate that the analysis does not consider labor costs, and that this will be the case if you install the insulation yourself.
Keep in mind that you present the solutions to your instructors, and any engineering analysis presented to others, is a form of communication. Therefore neatness, organization, completeness, and visual appearance are of utmost importance for maximum effectiveness. Besides, neatness also serves as a great checking tool since it is very easy to spot errors and inconsistencies in a neat work. Carelessness and skipping steps to save time often ends up costing more time and unnecessary anxiety.
Fundamentals of Thermal-Fluid Sciences by Yunus A. Çengel and Robert H. Turner.
There is no universal definition of leadership and indeed many books have been devoted to the topic of leadership. James MacGregor Burns describes a leader as one who instills purpose, not one who controls by brute force. A leader strengthens and inspires the followers to accomplish shared goals. Leaders shape the organization’s values, promote the organization’s values, protect the organization’s values and exemplify the organization’s values. Ultimately, Burns says, “Leaders and followers can raise one another to higher levels of motivation and morality… leadership becomes moral in that it raises the level of human conduct and ethical aspiration of both the leader and the led, and thus has a transforming effect on both.” Similarly, Daimler Chrysler’s CEO Bob Eaton defines a leader as “… someone who can take a group of people to a place they don’t think they can go.” “Leadership is we, not me; mission, not my show; vision, not division; and community, not domicile.” As the above illustrates, leadership is difficult to define in anything other than lofty words.
The Malcolm Baldrige National Quality Award has a more grounded definition of leadership in its core values. As stated in its core values and concepts, visionary leadership is:
“An organization’s senior leaders should set directions and create a customer focus, clear and visible values, and high expectations. The directions, values and expectations should balance the needs of all your stakeholders. Your leaders should ensure the creation of strategies, systems, and methods for achieving excellence, stimulating innovation, and building knowledge and capabilities. The values and strategies should help guide all activities and decisions of your organization. Senior leaders should inspire and motivate your entire workforce and should encourage all employees to contribute, to develop and learn, to be innovative, and to be creative.
Senior leaders should serve as role models through their ethical behavior and their personal involvement in planning, communications, coaching, development of future leaders, review of organizational performance, and employee recognition. As role models, they can reinforce values and expectations while building leadership, commitment, and initiative throughout your organization.”
Leadership can be difficult to define, However, successful quality leaders tend to have certain characteristics.
Characteristics of Quality Leaders
There are 12 behaviors or characteristics that successful quality leaders demonstrate.
- They give priority attention to external and internal Customers and their needs. Leaders place themselves in the customers’ shoes and service their needs from that perspective. They continually evaluate the customers’ changing requirements.
- They empower, rather than control, subordinates. Leaders have trust and confidence in the performance of their subordinates. They provide the resources, training, and work environment to help subordinates do their jobs. However, the decision to accept responsibility lies with the individual.
- They emphasize improvement rather than maintenance. Leaders use the phrase “if it isn’t perfect, improve it” rather than “if it ain’t broke, don’t fix it.” There is always room for improvement, even if the improvement is small. Major breakthroughs sometimes happen, but it’s the little ones that keep the continuous process improvement on a positive track.
- They emphasize prevention. “An ounce of prevention is worth a pound of cure” is certainly true. It is also true that perfection can the enemy of creativity. We can’t always wait until we have created the perfect process or product. There must be a balance between preventing problems and developing better, but not perfect, processes.
- They encourage collaboration rather than competition. When functional areas, departments, or work groups are in competition, they may find subtle ways of working against each other or withholding information. Instead, there must be collaboration among and within units.
- They train and coach, rather than direct and supervise. Leaders know that the development of the human resource is a necessity. As coaches, they help their subordinates learn to do a better job.
- They learn from problems. When a problem exists, it is treated as an opportunity rather than something to be minimized or covered up. “What caused it?” and “How can we prevent it in the future?” are the questions quality leaders ask.
- They continually try to improve communications. Leaders continually disseminate information about the TQM effort. They make it evident that TQM is not just a slogan. Communication is two way–ideas will be generated by people when leaders encourage them and act upon them. For example, on the eve of Desert Storm, General Colin Powell solicited enlisted men and women for advice on winning the war. Communication is the glue that holds a TQM organization together.
- They continually demonstrate their commitment to quality. Leaders walk their talk– their actions, rather than their words, communicate their level of commitment. They let the quality statements be their decision-making guide.
- They choose suppliers on the basis of quality, not price. Suppliers are encouraged to participate on project teams and become involved. Leaders know that quality begins with quality materials and the true measure is the life-cycle cost.
- They establish organizational systems to support the quality effort. At the senior management level a quality council is provided, and at the first line supervisor level, work groups and project teams are organized to improve the process.
- They encourage and recognize team effort. They encourage, provide recognition and reward individuals and teams. Leaders know that people like to know that their contributions are appreciated and important. This action is one of the leader’s most powerful tools.
In order to become successful, leadership requires an intuitive understanding of human nature– the basic needs, wants, and abilities of people. To be effective, a leader understands that:
- People, paradoxically, need security and independence at the same time.
- People are sensitive to external rewards and punishments and yet are also strongly self-motivated.
- People like to hear a kind word of praise. Catch people doing something right, so you can pat them on the back.
- People can process only a few facts at a time; thus, a leader needs to keep things simple.
- People trust their gut reaction more than statistical data.
- People distrust a leader’s rhetoric if the words are inconsistent with the leader’s actions.
Leaders need to give their employees independence and yet provide a secure working environment–one that encourages and rewards successes. A working environment must be provided that fosters employee creativity and risk taking by not penalizing mistakes.
A leader will focus on a few key values and objectives. Focusing on a few values or objectives gives the employees the ability to discern on a daily basis what is important and what is not. Employees, upon understanding the objectives, must be given personal control over the task in order to make the task their own and, thereby, something to which they can commit. A leader, by giving the employee a measure of control over an important task, will tap into the employee’s inner drive. Employees, led by the manager can become excited participants in the organization.
Having a worthwhile cause such as total quality management is not always enough to get employees to participate. People, (and, in turn, employees) follow a leader, not a cause. Indeed, when people like the leader but not the vision, they will try to change the vision or reconcile their vision to the leader’s vision. If the leader is liked, people will not look for another leader. This is especially evident in politics. If the leader is trusted and liked, then the employees will participate in the total quality management cause. Therefore, it is particularly important that a leader’s character anti competence, which is developed by good habits and ethics, be above reproach. Effective leadership begins on the inside and moves out.
Total Quality Management, 3rd edition by D.H Besterfield
This guide aims to raise awareness of your legal rights and responsibilities, as an employee, so that you can enjoy a safe and healthy workplace.
Employers have legal obligations to ensure a safe and healthy workplace for their employees in the first instance – and also for anyone else who may visit the workplace such as customers, contractors and members of the public.
All employers, whatever the size of the business, must:
- design, provide and maintain workplaces which are safe and without risk to health;
- identify any hazards (actual or potential) and take measures to control the risks, preferably by eliminating them – but if that is not possible, by reducing them as far as possible;
- ensure that safe working practices are developed and implemented,
- implement measures to reduce the risk of bullying and harassment;
- provide adequate first aid facilities;
- provide employees with information, instructions, and training set up contingency plans to deal with accidents and emergencies (including the evacuation of the workplace);
- ensure that ventilation, temperature, lighting, toilet, washing and rest facilities meet the standard of health, safety and welfare sought by the statutory bodies;
- ensure that appropriate work equipment is provided and is properly used and regularly maintained;
- take necessary precautions against the risks caused by flammable or explosive hazards, electrical equipment, noise, dust and radiation;
- take reasonable steps to avoid potentially dangerous work involving manual handling and provide manual handling training where required;
- provide health supervision, as needed;
- provide protective clothing, where required and appropriate warning signs;
- report specific accidents, injuries, diseases and dangerous occurrences to the appropriate authorities; and maintain records of accidents and injuries as appropriate.
Employees’ Rights & Responsibilities
The law establishes significant rights for employees – as well as responsibilities to co-operate in appropriate behavior to protect their own well-being.
The right of employees to work in a safe and healthy environment is enshrined in law. As such, it cannot be withdrawn or diluted by your employer. The most important rights and responsibilities are set out below.
- to have any risks to your health and safety properly controlled, as far as possible;
- to be provided, free of charge, with any personal protective and safety equipment;
- to stop work and leave your work area, if you have reasonable concerns about your safety, without being disciplined;
- to tell your employer about any concerns about your health and safety at work;
- to get in touch with the appropriate authority, without being disciplined, if your employer refuses to address to your concerns;
- to be consulted by your employer about safety, health and welfare at work and to be provided with specific information on these issues; and
- to select Safety Representatives, as part of this consultation with your employer.
Your responsibilities to take reasonable care of your health and safety; to take reasonable care not to put other people at risk;
- to participate in appropriate training;
- to adhere to the employer’s health and safety policies;
- to make proper use of any personal protective equipment;
- to report any injuries, strains or illnesses you may have suffered as a result of your work; and
- to tell your employer of any health-related issue that may affect your work performance (for example, becoming pregnant, taking prescribed medication or suffering an injury) so that the employer can make appropriate adjustments in your working arrangements.
Common Workplace Problems
Every room where people work should have sufficient floor area, height and unoccupied space for purposes of health, safety and welfare. While additional accommodation may be necessary if there is a need for wheelchair access, generally in offices 4.65 square metres is the minimum amount of floor space required for each person working in a room (This includes the area occupied by an office desk and chair but excludes filing cabinets and other office furniture).
The regulations do not specify a maximum temperature but, as a guide, a minimum comfortable working temperature for indoor sedentary workers is reckoned to be 16º Centigrade within one hour from the start of work with the maximum comfortable working temperature at 27º Centigrade (when undertaking light duties).
Workplaces need to be adequately ventilated. Windows or other openings may provide sufficient ventilation but, where air conditioning is provided this should be regularly maintained.
Lighting should be sufficient to enable people to work and move about safely. If necessary, local lighting should be provided at individual workstations and at places of particular risk such as corridors and stairs. Lighting and light fittings should not create any hazard. Automatic emergency lighting, powered by an independent source, should be provided where sudden loss of light would create a risk.
Workstations and breaks away from the screen
Employers must plan work at visual display units (VDUs) so that it is interrupted periodically by breaks or changes in activities to reduce exposure to the VDU. Although regulations set no required breaks, no single continuous period of work at a screen should not exceed one hour. If you use a VDU as a significant part of your daily work, you have a right to seek appropriate eye tests which must be made available and paid for by your employer.
Where manual handling is required, clear guidelines should be followed. For details, see the websites of the Republic’s Health and Safety Authority – www.hsa.ie – or the UK’s Health and Safety Executive – www.hse.co.uk.
Every workplace must have clear evacuation procedures in place and carry out regular fire drills to ensure employees are aware of the procedures.
Slips, trips and falls
The main causes of slips, trips and falls in the workplace are:
- uneven floor surfaces;
- unsuitable floor coverings;
- wet floors;
- changes in levels;
- trailing cables;
- poor lighting; and
- poor housekeeping.
If you fall, seek medical assistance if required; notify your employer about the incident; ensure a report is filed if necessary; and demand that the hazard is removed.
Workplace stress occurs when the demands of the job and/or the working environment exceeds a worker’s capacity to meet them. The symptoms of stress may be physical, mental and/or behavioural. You should familiarise yourself with your employer’s policy on stress.
Bullying and Harassment
Bullying in the workplace is a health and safety issue. It can lead to health problems and give rise to further safety issues. It is also an industrial relations matter – and may have legal consequences. Employers have a duty of care to all employees, to ensure they are both mentally and physically safe at work and that their health is not adversely affected by anything or anyone in the working environment. This duty of care means employers must behave and respond reasonably in such matters.
The information outlined in this document is intended for guidance only – and should not be regarded as a definitive legal statement.
Sources of information
Useful sources of information on health and safety are: