The variable-volume system compensates for variations in cooling requirement by regulating (throttling) the volume of air supplied to each zone. Air is supplied from a single-duct system and each zone has its own damper. Individual zone thermostats control the damper and the amount of air to each zone. Figure 2-10 is a schematic of a single-duct variable-air-volume (VAV) system with a throttling (damper only) ter- minal unit. Some VAV systems have fan-powered terminal units. In fan-powered units, as air flow is reduced from the main duct by damper action, more return air from the
Figure 2-10 Simplified control schematic of a single-duct VAV system.
DM MPS DM DM Outdoor air Return air Exhaust air DR T1
Supply fan with inlet vane damper From supply fan starter T3 Filter To other zones Zone volume damper Typical zone From selected zone thermostats NC High limit RA NO FS To supply fan starter T2 DA Low limit Cooling coil Heating coil HWR HWS DM F T SP DM NO V1 CHR CHS NC V2 Discriminator relay Highest Static pressure controller NC
room is drawn into the box by the fan and mixed with the primary cold air supply to give a constant air flow into the room (see Chapter 11).
A significant advantage of the variable-volume system is low initial and operat- ing costs. The first cost of the system is far lower than that of other systems that pro- vide individual space control because it requires only single runs of duct and a simple control at the air terminal. Where diversity of loading occurs, lower-capacity central equipment can be used, and operating costs are generally the lowest among all the air systems. Fan speed is controlled by maintaining a fixed static pressure at some appro- priate location in the ductwork. As cooling demand in individual zones drops and dampers close, the increasing static pressure in the main duct gives a signal that causes the fan speed to back off. Because the total volume of ducted air is reduced as the zone loads decrease, the refrigeration and fan horsepower closely follow the actual air- conditioning load of the building. There are significant fan power savings where fan speed is reduced in relation to the volume of air being circulated. This topic is dis- cussed in detail in Chapter 12.
During intermediate and cold seasons, the economizer arrangement discussed pre- viously can be used with outdoor air for cooling. In addition, the VAV system is vir- tually self-balancing, making the requirements of duct design less stringent. Improvements in damper and outlet diffuser design and variable speed drives for fan operation have allowed VAV systems to be throttled down to very low rates of flow without being noisy and inefficient.
Although some heating may be done with a variable-volume system, it is prima- rily a cooling system and should be applied only in locations where cooling is required for the major part of the year. Buildings with internal spaces having large internal loads are the best candidates. A secondary heating system, such as baseboard perime- ter or radiant panel heat, should be provided for exterior zones. During the heating season, VAV systems simply provide tempered ventilation air to these exterior spaces. Reheat may be used in conjunction with the VAV system. In this case reheat takes over to temper the air that has been throttled to some predetermined ratio.
Single-duct variable-volume systems should be considered in applications such as office buildings, hotels, hospitals, apartments, and schools, where full advantage can be taken of their low cost of installation and operation. Additional details of VAV sys- tems may be obtained from the ASHRAE Handbook, Systems and Equipment (1).
Dual-Duct System
In the dual-duct (double-duct) system, the central equipment supplies warm air through one duct run and cold air through the other. The temperature in an individual space is controlled by mixing the warm and cool air in proper proportions. Variations of the dual-duct system are possible; a simplified control schematic of one form is shown in Fig. 2-11.
For best performance, some form of regulation should be incorporated into the system to maintain a constant flow of air. Without this regulation the system is diffi- cult to control because of the wide variations in system static pressure that occur as load patterns change.
Many double-duct systems are installed in office buildings, hotels, hospitals, schools, and large laboratories. Where there are multiple, highly variable sensible heat loads this system provides great flexibility in satisfying the loads and in providing prompt and opposite temperature response as required.
Space or zone thermostats may be set once to control year-round temperature con- ditions. All outdoor air (an economizer) can be used when the outdoor temperature is low enough to handle the cooling load.
The mixing of hot and cold air in dual-duct systems generally causes them to be energy inefficient. Be sure to carefully consult Standard 90 or local building codes before adopting a dual-duct system. To save energy a dual-duct system should be pro- vided with control that will automatically reset the cold air supply to the highest tem- perature acceptable and the hot air supply to the lowest temperature acceptable. Using individual zone controls that supply either hot or cold air with a neutral or dead zone where only minimum outdoor air is supplied gives energy conservation that is better than with systems that mix hot and cold air.
Many dual-duct systems are in operation, but fewer are now being designed and installed. Improved performance can be attained when the dual-duct system is com- bined with the variable air-volume system. Two supply fans are usually used in this case, one for the hot deck and one for the cold deck, with each controlled by the static pressure downstream in each duct.