While maintaining constant airflow, single-duct constant volume systems change the supply air temperature in response to the space load (Figure 9). Single-Zone Systems. The simplest all-air system is a supply unit serving a single-temperature control zone. The unit can be installed either in or remote from the space it serves, and it may operate with or without distribution ductwork. Ideally, this system responds completely to the space needs, and well-designed control systems maintain temperature and humidity closely and efficiently. Single zone systems often involve short ductwork with low pressure drop and thus low fan energy, and single-zone systems can be shut down when not required without affecting the operation of adjacent areas, offering further energy savings. A return or relief fan may be needed, depending on the capacity of the system and whether 100% outdoor air is used for cooling as part of an economizer cycle. Relief fans can be eliminated if provisions are made to relieve over pressurization by other means, such as gravity dampers.
Multiple-Zone Reheat. The multiple-zone reheat system is a modification of the single-zone system. It provides (1) zone or space control for areas of unequal loading; (2) simultaneous heating or cooling of perimeter areas with different exposures; and (3) close tolerance of control for process or comfort applications. As the word reheat implies, heat is added as a secondary simultaneous process to either preconditioned (cooled, humidified, etc.) primary air or recirculated room air. Relatively small low-pressure systems place reheat coils in the ductwork at each zone. More complex designs include high-pressure primary distribution ducts to reduce their size and cost and pressure reduction devices to maintain a constant volume for each reheat zone.
The system uses conditioned air from a central unit generally at a fixed cold air temperature that is low enough to meet the maximum cooling load. Thus, all supply air is always cooled the maximum amount, regardless of the current load. Heat is added to the airstream in each zone to match the cooling capacity to the current load in that zone. The result is very high energy use. However, the supply air temperature from the unit can be varied, with proper control, to reduce the amount of reheat required and the associated energy consumption. Care must be taken to avoid high internal humidity when the temperature of air leaving the cooling coil is permitted to rise during the spring and fall.
When a reheat system heats a space with an exterior exposure in cold weather, the reheat coil must not only replace the heat lost from the space, but also must offset the cooling of the supply air (enough cooling to meet the peak load for the space), further increasing energy consumption compared to other systems. If a constant volume system is oversized, the reheat cost becomes excessive.
Bypass. A variation of the constant volume reheat system is the use of a bypass terminal unit instead of reheat. This system is essentially a constant volume primary system with a VAV secondary system. The quantity of room supply air is varied to match the space load by dumping excess supply air into the return ceiling plenum or return air duct, i.e., by bypassing the room. When bypass terminal units dump to a return air plenum, the return air plenum temperature is reduced and the plenum must be kept at a lower pressure than the room so that cooler air does not spill into the room through the return air grilles. A return fan is often used to create that negative pressure. While this reduces the air volume supplied to the space, the system air volume and fan energy remains constant. Refrigeration or heating at the air-handling unit is reduced due to the lower return air temperature. A bypass system is generally restricted to small installations where a simple method of temperature control is desired, a modest initial cost is desired, and energy conservation is less important.