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.
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 After the conditioned air is distributed to the designated space, it is withdrawn through a return duct system and delivered back to the HVAC unit.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.
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.
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.
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.
- All air systems as discussed above.
- Terminal heating and cooling units, such as fan coil units or radiant ceiling panels.
- 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.
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:
- 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.
- 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.
- 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]
- 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:
1st Choice – Reciprocating
2nd Choice – Scroll
3rd Choice – Screw
100 -300 tons
1st Choice – Screw
2nd Choice – Scroll
3rd Choice – Centrifugal
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:
- 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.
- The calculated supply air shall be the sum of all individual peak room air quantities without any diversity.
- 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.
- The adjusted supply air shall be, thus, 5 percent in excess of the calculated supply air.
- 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.
- Supply Air Fan Capacity: The capacity of the supply air fan shall be calculated per the following example:
- Calculated Supply Air Volume = 20,000 CFM
- Safety Margin = 5 percent of item (a) = 1,000 CFM
- Adjusted Supply Air Volume = 21,000 CFM
- Duct Air Leakage = 4 percent of item (a) = 840 CFM
- Supply Air Fan Capacity = 21,840 CFM
- 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.
- 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.
- Motor Voltages: Motor Voltages shall conform to NEMA/ANSI standard as follows:
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.
- 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.
- Specify a layer of non-flammable mineral wool between the inside and outside sheets for the cabinet casing.
- 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.
- 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.
- 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.
- Specify access and inspection openings with the lighting elements installed in covers of the sections for humidification, filtration, heat exchangers and fans.
- 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.
- 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.
- Specify driving motor manufactured in the IP class, enabling washing and disinfection.
- 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.
- Specify UV bactericidal lamp ensuring disinfection of the recirculated air.
- Specify cable glands providing connection of motors and the lighting system, ensuring the appropriate tightness and cleanliness class.
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.
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.
Air Distribution Ductwork
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Round duct should be used for the construction of the exhaust system. Rectangular ducts, if used, should be as square as possible.
- 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.
- 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.
- 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.
- 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.
- 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.
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.
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.
- 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.
- 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.
- The noise level should be restricted to 45 NC level for all laboratories, Dining, Food Service/Serving, Therapeutic Pools.
- 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.
- 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.
- 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.
- 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 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.
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.
Air Intakes and Outlets
- Ensure that air intakes and exhaust outlets are located properly in construction of new facilities and renovation of existing facilities.
- Locate exhaust outlets >25 ft from air-intake systems.
- 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).
- Locate exhaust outlets from contaminated areas above roof level to minimize recirculation of exhausted air.
- Operating Room system air intakes shall be at least 30 ft above the ground.
- Laboratory and Research exhaust shall be terminated at the highest point of the building (NFPA 99, 5-3.3.4).
- 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.
- 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.
- Ensure that the intakes are kept free from bird droppings, especially those from pigeons.
- UCSF Medical Center Design Guidelines HVAC, June 2015.
- 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.