In the first section of this chapter, we discussed the various types of air systems com- monly used in commercial buildings, focusing on the overall systems and the ways in which the various systems meet the needs of the areas served. In this section, we will discuss the physical characteristics, connections, and design considerations common to air system equipment.
Physical Characteristics
HVAC Air Systems
As discussed earlier in this chapter, HVAC air systems circulate air primarily to meet the heating and/or cooling needs of the spaces they serve.
Centralized Equipment An example of a centralized air system is a modular central station air handling unit18 that contains a hot water heating coil and a chilled water cooling coil
that receive their heating and cooling energy from a remote central plant. Figure 5-13 is a graphical representation of an air handling unit on a floor plan drawing.
Decentralized Equipment An example of a decentralized air system is a packaged19 roof-
top unit (RTU) that contains an indirect gas-fired heater and a complete refrigeration system for cooling (Figs. 5-14 and 5-15).
Another example of a decentralized air system is an air handling unit with a split refrigeration system that provides DX cooling or heat pump operation. This type of system is called a split system. In a split system, the compressor and condenser are contained within an outdoor air-cooled unit called a condensing unit. The outdoor air- cooled condensing unit is connected to the evaporator (DX cooling coil) in the air han- dling unit by refrigerant liquid and refrigerant suction piping. The refrigerant liquid piping carries the high-pressure refrigerant liquid from the outlet of the condenser to the thermostatic expansion valve on the inlet of the DX cooling coil. The refrigerant suction piping carries the low-pressure refrigerant gas from the outlet of the DX cooling coil to the suction of the compressor.
In order to increase the unloading capabilities of the refrigeration system, a hot gas bypass line can be connected to the high-pressure (hot gas) side of the compressor, which carries a portion of the hot gas to the DX cooling coil, bypassing the condenser.
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The hot gas adds load to the DX cooling coil during low-load conditions and keeps the compressor running.
One limitation of a split system is the distance between the air handling unit and the air-cooled condensing unit. Special sizing of the refrigerant lines must be performed by the equipment manufacturer if the distance between the two units exceeds the allow- able separation distance, which is usually 75 to 100 ft. Furthermore, the allowable sepa- ration distance between the air handling unit and the condensing unit may be decreased if the condensing unit is installed at a higher elevation than the air handling unit.
Figure 5-16 illustrates a rooftop, cooling-only air handling unit with a split-system DX cooling coil. The refrigeration system has two separate refrigeration circuits and a hot gas bypass line for each circuit. This explains why there are six refrigerant lines con- necting the two units.
Another example of a decentralized air system is a computer room air-conditioning (CRAC) unit with an electric heating coil, self-contained steam humidifier, and outdoor air-cooled condenser (Figs. 5-17 through 5-19).
Figure 5-13 Floor plan representation of a modular central station air handling unit with hot
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C h a p t e r F i v eFigure 5-14 Roof plan representation of a packaged RTU with indirect gas-fired heater and DX
cooling coil.
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C hap ter Fi veFigure 5-16 Roof plan representation of a rooftop, cooling-only air handling unit with a split-system DX cooling coil.
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C h a p t e r F i v eFigure 5-17 Floor plan representation of two CRAC units with electric heating coils and self-
contained steam humidifiers.
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C hap ter Fi veFigure 5-19 CRAC unit connection detail.
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C h a p t e r F i v eVentilation Air Systems
Ventilation air systems provide ventilation only and do not provide any heating or cool- ing to the spaces they serve.
Outdoor Air Equipment (Positive Pressure) Figures 5-20 and 5-21 show the section view and photograph of a modular central station DOAS unit with a hot water heating coil, wrap-around heat pipe refrigerant coil, and a chilled water cooling coil.
Figure 5-22 is a photograph of a kitchen exhaust hood makeup air unit with an indirect gas-fired heater.
Exhaust Air Equipment (Negative Pressure) Figures 5-23 through 5-31 are examples of dif- ferent types of exhaust fans that are commonly used in exhaust air systems.
Figure 5-23 is a photograph of an upblast grease exhaust fan. Figure 5-24 is a photograph of a downflow fan.
Figures 5-25 and 5-26 illustrate a roof plan representation and photograph of two kitchen exhaust hood makeup air units with indirect gas-fired heaters, two upblast grease exhaust fans, and a downflow exhaust fan.
Figures 5-27 and 5-28 illustrate a floor plan representation and detail of an in-line centrifugal fan used in a garage ventilation system.
Figures 5-29 through 5-31 illustrate a floor plan representation, detail, and photo- graph of a propeller fan.
Connections
Air system equipment will have some of the following connections, depending upon the type of air system and components within the system:
• Heating water supply and return piping connections to the hot water heating coil
• Chilled water supply and return piping connections to the chilled water cooling coil
• Refrigerant suction, refrigerant liquid, and possibly hot gas piping connections to the DX refrigerant cooling coil
• Condensate drain piping connection to the condensate drain pan under the cooling coil
• Steam supply and steam condensate return piping connections to the steam heating coil
• Steam supply and steam condensate drain piping connections to the centralized steam humidifier
• Domestic cold water supply and steam condensate drain piping connections to the self-contained gas-fired or electric steam humidifier
• Gas piping connection to the gas-fired heater and/or self-contained gas-fired steam humidifier
• Vent and possibly combustion air connection to the gas-fired heater and/or self-contained gas-fired steam humidifier
• Heat pump water supply and heat pump water return piping connections to water-source heat pump units
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C hap ter Fi veFigure 5-20 Section view of a modular central station DOAS with a hot water heating coil, wrap-around heat pipe refrigerant coil, and a chilled
water coil.
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C h a p t e r F i v eFigure 5-21 Photograph of a modular central station DOAS with a hot water heating coil, wrap-
around heat pipe refrigerant coil, and a chilled water coil.
Figure 5-22 Photograph of a kitchen exhaust hood makeup air unit with an indirect gas-fired
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C h a p t e r F i v eFigure 5-23 Photograph of an upblast grease exhaust fan.
Figure 5-24 Photograph of a downflow exhaust fan.