for the building occupants, keep certain types of equipment rooms at a maximum temperature during the summer, or provide outdoor air makeup for exhaust air systems or equipment.
• Exhaust air systems operate under a negative air pressure to remove air from the spaces or equipment they serve and discharge this air to the outdoors.
Outdoor Air Systems
The two types of outdoor air systems we will discuss are ventilation air systems and makeup air systems.
Ventilation Air Systems Ventilation air systems are used to either meet the outdoor air ventilation requirements of the building occupants, or to use outdoor air to keep certain types of equipment rooms under a positive air pressurization and keep them at a maxi- mum temperature during the summer.
Dedicated Outdoor Air Systems Dedicated outdoor air systems (DOASs) are one part of what is known as a dual-path HVAC system. In a dual-path HVAC system, outdoor air for occupant ventilation and/or exhaust air makeup is handled by one air system while heating and cooling is handled by a separate air system (Fig. 5-6). In some situa- tions, the outdoor air is delivered by the DOAS directly to the spaces served at a tem- perature and relative humidity that are approximately equal to the space temperature
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and relative humidity. In other situations, the outdoor air is delivered by the DOAS to the return air duct for the heating and cooling equipment in a filtered but uncondi- tioned or partially conditioned state. Partial conditioning could include heating only or heating and cooling with no humidity control.
A DOAS unit normally has a heating coil, cooling coil, and reheat coil so that the 100% outdoor air that it delivers can be heated and cooled, or cooled and reheated (which is required to dehumidify the air). A humidifier may also be part of a DOAS unit, but humidification is not as often required as heating, cooling, and dehumidifica- tion. The heating and reheat coils can be hot water, steam, gas, or electric, and the cool- ing coil can be chilled water or DX refrigerant.
However, utilizing hot water, steam, gas, or electricity for reheat requires more energy than utilizing a form of energy recovery for reheat. An example of an energy recovery reheat coil is a hot gas reheat coil that is part of a self-contained (or packaged) refrigeration system. When dehumidification is required for this type of system, heat, which is normally rejected to the outdoors through an air- or water-cooled condenser, is rejected to the airstream by a hot gas reheat coil located downstream of the cooling coil. Hot gas reheat can be incorporated into air-cooled or water-cooled refrigeration Figure 5-6 Schematic diagram of a dual-path HVAC system where the DOAS unit utilizes a reheat coil.
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equipment, such as a packaged air-conditioning unit or a water-source heat pump unit. Packaged DOAS units with hot gas reheat still require an air- or water-cooled con- denser for heat rejection when cooling (but not reheat) is required. However, these units have the added piping, controls, and hot gas reheat coil that enable them to utilize the hot gas reheat coil in the dehumidifying mode of operation. The capabilities and con- figurations of each type of DOAS unit with hot gas reheat vary depending upon the equipment manufacturer. Normally, manufacturers that specialize in constructing dehumidification equipment for indoor swimming pools (natatoriums) have the best selection of DOAS equipment.
Another example of a DOAS unit is a 100% outdoor air, modular central station air handling unit with a heating coil and a heat pipe refrigerant coil wrapped around a chilled water cooling coil. In this type of system, precooling of the outdoor air upstream of the cooling coil and reheating of the outdoor air downstream of the cooling coil is accomplished by the wrap-around heat pipe refrigerant coil.
The wrap-around heat pipe refrigerant coil consists of two coils filled with refrigerant that are connected by refrigerant pipes. One refrigerant coil is mounted upstream of the chilled water cooling coil and the other refrigerant coil is mounted downstream of the refrigerant coil, and the interconnecting refrigerant pipes wrap around the end of the chilled water cooling coil. When the air entering the upstream refrigerant coil is above the boiling point of the refrigerant (approximately 45°F), the refrigerant is evaporated and, as a result, precools the air by up to 14°F (depending upon the entering air temperature) before it enters the chilled water cooling coil. This precooling of the air is a completely sensible cooling process (i.e., no moisture is condensed). The precooled air enters the chilled water coil, and the capacity of the chilled water coil is modulated to further cool and dehumidify the air to achieve a 55°F dew point,12 which corresponds to an approxi-
mately 55°F dry bulb temperature leaving the chilled water cooling coil. In the down- stream refrigerant coil, the refrigerant that was evaporated in the upstream coil is condensed and flows by gravity back to the upstream refrigerant coil. The heat rejected by the conden- sation of the refrigerant reheats the air the same amount that it was precooled by the upstream refrigerant coil (up to 14°F of reheat, depending upon the amount of refrigerant that was evaporated in the upstream coil). The result is that the outdoor air delivered to the building by the DOAS unit has a dry bulb temperature of 69°F and a dew point of 55°F.
Using a wrap-around heat pipe refrigerant coil for precooling and reheat in a DOAS unit requires very little energy input. The only energy required to accomplish this heat transfer is the added fan energy that is required to overcome the additional (air) static pressure losses through the upstream and downstream refrigerant coils. There is no compressor in the refrigerant circuit because it is not required to circulate the refrigerant through the coils. The refrigerant flows between coils in the liquid and gaseous phases strictly as a result of gravity and the heat that is absorbed by the refrigerant in the upstream refrigerant coil. Using a wrap-around heat pipe refrigerant coil has the added benefit of decreasing the required cooling capacity of the chilled water cooling coil.
When the entering air temperature is below the boiling point of the refrigerant (approximately 45°F), there is no refrigerant flow and no heat transfer associated with the wrap-around heat pipe refrigerant coil. The heat output of the heating coil must be mod- ulated to ensure that the air supplied by the DOAS unit does not drop below 69°F.
Figure 5-7 is schematic diagram of a DOAS unit that utilizes a wrap-around heat pipe refrigerant coil. Typical dry bulb and wet bulb air temperatures and air humidity ratios are given for the entering and leaving air conditions of the heat pipe and chilled
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water cooling coils to give an indication of the sensible and latent cooling and heating that occurs at each point within the process.
Positive-Pressure Equipment Room Ventilation Equipment rooms, including rooms that contain mechanical and/or electrical equipment, are commonly heated and ventilated only. Heat is provided during the winter typically through fan-forced unit heaters which recirculate room air to keep the rooms at a minimum of about 60°F (refer to Chap. 7 for a discussion of unit heaters). Ventilation is normally not provided in the winter. When the room temperature rises, normally above 85°F, unconditioned outdoor air is used to prevent the rooms from becoming excessively hot. This typically occurs during the summer but could occur at other times of the year depending upon the inter- nal heat gains to the room from the equipment. The maximum acceptable temperature within an equipment room is normally about 100°F. However, equipment rooms that contain electronic components, such as elevator controls or computer systems, cannot be warmer than 85°F. In this case, the room would require mechanical cooling either from the building HVAC system or from a separate HVAC system. For equipment rooms that can tolerate a space temperature as high as 100°F, an outdoor air ventilation system that utilizes unconditioned outdoor air to keep the space temperature below this maximum acceptable temperature should be designed. HVAC calculations need to be performed to determine both the heat gains to the room at the design summer out- door temperature and the outdoor airflow required to keep the room at 100°F, which is normally 5 to 10°F higher than the design summer outdoor temperature. However, the temperature within equipment room is not always this maximum temperature. The space temperature will vary between 85 and 100°F when ventilation is operating. When the space temperature within the equipment rooms is below 85°F and above 60°F, nei- ther ventilation nor heating will be provided by the heating and ventilating system.
Most equipment rooms are ventilated by an exhaust air system in order to place them under a negative air pressurization, which is necessary to prevent odors within the equipment rooms from migrating to adjacent spaces (refer to the Negative-Pressure Equipment Room Ventilation section later). However, equipment rooms containing gas-fired or fuel-burning appliances that do not have a direct connection to the out- doors for combustion air (i.e., the appliances utilize room air for combustion) should Figure 5-7 Schematic diagram of a DOAS unit with a wrap-around heat pipe refrigerant coil.
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not be placed under a negative air pressurization by the ventilation system. In these situations, it is necessary for the ventilation system to positively pressurize the equip- ment room by blowing outdoor air into the room and allowing the excess air that is not used for combustion, if any, to be relieved from the room. Figure 5-8 is schematic dia- gram of a positive-pressure equipment room ventilation system.
Makeup Air Systems Makeup air systems are utilized to provide the outdoor airflow required by certain types of equipment where it is not feasible or economical to condi- tion this outdoor airflow through the HVAC system. Makeup air systems for kitchen exhaust hoods and combustion air systems for gas-fired or fuel-burning appliances are two examples of makeup air systems.
Kitchen Exhaust Hoods NFPA Standard 96—Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations, the International Mechanical Code, and the authority having jurisdiction require certain kitchen appliances that produce grease- laden vapor, smoke, or steam to be installed under one or more commercial kitchen exhaust hoods to remove these contaminants from the kitchen environment (Fig. 5-9). The exhaust airflow required to effectively remove these contaminants is typically in the range of 50 to 60 cfm per square foot of exhaust hood face area, although airflows as high as 125 cfm per square foot of exhaust hood face area are possible (depending upon the type and heat output of the appliances installed under the hood). This exhaust air- flow must be replaced with outdoor airflow to prevent the negative air pressurization within the kitchen from exceeding 0.02 in. w.c. Because exhaust airflow from a commer- cial kitchen may be as much as double the supply airflow that is required to maintain the space temperature, special consideration must be given to conditioning the makeup outdoor airflow through a system that is separate from the HVAC system, particularly if cooling is provided for the kitchen in addition to heating.
If the kitchen is heated and ventilated only through an H&V air system, the outdoor airflow required by the exhaust hoods is commonly provided through the H&V air sys- tem. The H&V system will normally position the outdoor air and return air dampers to deliver the required outdoor airflow when the exhaust hoods are operating and reduce Figure 5-8 Schematic diagram of a positive-pressure equipment room ventilation system.
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the outdoor airflow when the exhaust hoods are not operating. However, if cooling is provided for the kitchen, it is usually infeasible to deliver the required outdoor airflow through the HVAC system. Delivering a high percentage of outdoor air through the HVAC system would require the use of a reheat system to adequately dehumidify the air during cooling operation to prevent unacceptably high relative humidity within the kitchen. The use of reheat is costly from an energy standpoint because of the simultane- ous cooling and heating that is required. In the case of a commercial kitchen, approxi- mately 70% of the outdoor airflow required by the kitchen exhaust hoods can be delivered by a makeup air system that is dedicated to this purpose.
Typically, the makeup air system will heat the outdoor airflow required by the exhaust hoods and deliver this air through a laminar flow diffuser located within the kitchen near the exhaust hoods or, if makeup-air-type hoods are utilized, deliver this air to the makeup air connection on each of the exhaust hoods.13 The goal is for the makeup air to be intro-
duced at a low velocity into the hood capture zone so that it does not affect the capture and containment capabilities of the hood. The heating coil in the makeup air unit is com- monly direct or indirect gas-fired or electric because the unit is often mounted on the roof of the building. If the heating coil is hot water or steam, and the makeup air unit is mounted on the roof, measures must be taken to protect the heating coil from freezing.
Combustion Air As mentioned in Chap. 4, a source of combustion air must be pro- vided for gas-fired or fuel-burning appliances. NFPA Standard 54 —National Fuel Gas
Code describes the combustion air requirements for gas-fired appliances and NFPA
Standard 31—Standard for the Installation of Oil-Burning Equipment describes the combus- tion air requirements for fuel-burning appliances.
If it is not feasible to obtain the required combustion air from the room in which the appliances are installed, transfer the combustion air from adjacent spaces, provide ade- quate openings to the outdoors, or provide a direct connection of outdoor combustion air to each appliance, it will be necessary to provide combustion air for the appliances through a mechanical forced-air system. We will refer to this as a combustion air makeup system. Figure 5-9 Schematic diagram of a kitchen exhaust hood makeup air system.
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A combustion air makeup system consists mainly of a fan that blows outdoor air into the equipment room in which the appliances are installed. The combustion air makeup system must be sized to deliver the amount of outdoor combustion air required by all of the appliances installed within the room. It is recommended that the combus- tion air be filtered and heated to at least 50°F to prevent freezing conditions within the equipment room. Furthermore, operation of the combustion air unit should be inter- locked with the operation of the appliances within the room so that combustion air is only provided when one or more of the appliances is firing.
The combustion air makeup system can also be used to ventilate the equipment room during the summertime in a manner similar to that which is discussed in the Positive-Pressure Equipment Room Ventilation section earlier. In this case, the combus- tion air makeup system would be controlled by a space thermostat in addition to being interlocked with the appliances. If there are multiple appliances within the equipment room and/or if the combustion air makeup system is used for summertime ventilation, provisions must be made in the room to relieve excess air to the outdoors that is not used for combustion.
Figure 5-10 includes a schematic diagram and sequence of operation for a combustion air makeup system that serves multiple gas-fired boilers and also provides positive- pressure ventilation of the equipment room in the summertime. The combustion air makeup system is designed to provide combustion air for six 1,800 thousand British thermal units per hour (MBH) input boilers.14