VAV systems are the most widely used types of HVAC air systems for medium- and large-sized commercial building projects (projects larger than 10,000 ft2) because VAV
systems are flexible, energy efficient, and provide a comfortable indoor environment. As mentioned previously, VAV systems deliver variable supply airflow at a constant temperature (typically 55°F) through the primary air duct5 to multiple VAV
terminal units, each of which serves a separate temperature zone. Each VAV terminal unit contains a motor-operated damper that modulates the primary airflow to the zone, an inlet airflow sensor, and, in some instances, a heating coil and a small recir- culating air fan.6
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The supply airflow from the VAV air handling unit is normally modulated to main- tain a constant static pressure within the primary air duct system. This is measured by a duct static pressure sensor, which is typically located two-thirds the way down the primary air duct system. The modulation of the supply airflow tracks with the needs of the VAV terminal units; that is, as more primary air dampers in the VAV terminal units open to supply more air to the zones, the static pressure in the primary air duct system decreases and more supply airflow is delivered by the VAV air handling unit to restore the static pressure in the duct system. Conversely, as the primary air dampers close, the primary air duct static pressure increases and less supply airflow is delivered by the VAV air handling unit to compensate.
The air handling unit for a VAV system is the same as would be required for a CAV system with the exception that there is a means of modulating the supply airflow deliv- ered by the unit. The most common way of modulating the supply airflow of the unit is by controlling the frequency of the signal sent to the supply fan motor through a Figure 5-4 Schematic diagram of a multizone CAV system.
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variable frequency drive (VFD).7 The speed of an alternating current (ac) motor is
directly proportional to the frequency of the input signal to the motor. Therefore, as the frequency of the VFD output signal to the motor is reduced, motor speed is reduced, and supply airflow is also reduced. The converse is true as the frequency of the VFD output signal to the motor is increased. The maximum frequency of the VFD output signal is that of the VFD line input frequency, or 60 Hz. The supply fan motor will oper- ate at full speed when it receives a VFD output signal of 60 Hz.
Care should be taken when designing VAV systems that utilize DX refrigerant coils for cooling. Unless the refrigeration system is equipped with adequate capability to unload the refrigeration system capacity, freeze-up of the DX refrigerant cooling coil could occur under low airflow conditions.8 Also, refrigeration systems having a capac-
ity that is less than about 25 tons generally do not have the capability to adequately accommodate VAV operation. Unloading of the refrigeration system, adequate control of the discharge air temperature, and incorporation of a VFD into the unit cabinet are all issues that pose problems for these smaller-sized pieces of DX equipment. Therefore, equipment manufacturers do not currently offer VAV operation for air systems that utilize DX refrigerant cooling coils in sizes less than about 25 tons. However, recent energy-efficiency requirements in the industry are causing equipment manufacturers to develop technologies that will enable VAV capability for smaller-sized pieces of DX equipment. In the future, this lower limit of 25 tons for VAV operation in DX equipment may drop to as low as 10 tons or less.
The most common use of a VAV system is in serving multiple temperature zones. Therefore, we will discuss multiple-zone VAV systems first and then discuss the use of a VAV system for a single-zone application.
Multiple-Zone
VAV Terminal Units Variable primary airflow is delivered to the zones through the modulation of the primary air damper in the VAV terminal units. As the zone tempera- ture decreases, the primary air damper is modulated closed to supply less (55°F) pri- mary air to the zone. Once the primary air damper reaches its predetermined minimum position (usually about 25% of maximum airflow9), upon a further drop in the zone
temperature,VAV terminal units that have heating capabilities will position the primary air damper to the heating airflow and modulate the output of the heating coil to main- tain the heating setpoint of the zone temperature sensor.10 Fan-powered VAV terminal
units are also equipped with a small fan that recirculates air (normally from the ceiling return air plenum) through the heating coil of the VAV terminal units. The various types of VAV terminal units are discussed in more detail in Chap. 7. Figure 5-5 is a sche- matic diagram of a VAV system serving multiple VAV terminal units.
Dual-Duct Dual-duct VAV air systems are about as uncommon as dual-duct CAV air systems. The most likely time an HVAC system designer would encounter a dual-duct VAV air system would be in the case of a system that was originally designed as a dual- duct CAV system but was later renovated to function as a VAV system. The HVAC system designer may also have the task of designing the modifications to a dual-duct CAV system to convert it to a dual-duct VAV system.
Dual-duct VAV systems function much the same way as dual-duct CAV systems except the supply airflow to the zones is variable, not constant. The supply fan in the
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dual-duct air handling unit has to be equipped with a means to modulate its airflow in response to static pressure in both the main hot and cold ducts. The energy efficiency of a dual-duct VAV system would be about the same as that of a conventional VAV system utilizing VAV terminal units. Care should be taken in converting a dual-duct CAV sys- tem to a VAV system to ensure that the zones do not require constant supply airflow to serve as makeup for constant exhaust airflow.
Single-Zone Operation of a single-zone VAV system is similar to the operation of a VAV system serving multiple zones, except the supply airflow is modulated to maintain the cooling setpoint of the (single) zone temperature sensor rather than to maintain a con- stant primary air duct static pressure. The supply air temperature is maintained at 55°F as long as the zone temperature sensor is calling for cooling.
Once the zone temperature drops below the cooling setpoint (typically 75°F), the air handling unit will operate in the heating mode: cooling will be disabled, the supply fan will operate at the predetermined heating airflow, and the output of the heating coil Figure 5-5 Schematic diagram of a variable air volume system.
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within the unit will be modulated as required to maintain the heating setpoint of the zone temperature sensor (typically 70°F).11
Once the zone temperature rises above the cooling setpoint, the air handling unit will operate in the cooling mode: heating will be disabled, the output of the cooling coil will be modulated as required to maintain the supply air temperature at 55°F, and the supply air- flow will be modulated to maintain the cooling setpoint of the zone temperature sensor.
The advantage of a single-zone VAV over a single-zone CAV system is that during cooling operation, the supply air temperature will remain constant at approximately 55°F. This consistently cool supply air temperature will result in a lower space relative humidity than the same area served by a CAV system where the supply air temperature can vary anywhere between 55°F (full cooling load) and 75°F (no cooling load). The higher space relative humidity resulting from the use of a CAV system is exacerbated by outdoor air ventilation in moist climate zones and by a high density of occupants in the areas served by the unit.
Single-zone VAV systems that utilize DX refrigerant cooling coils have the same limitations on the minimum unit size as VAV systems serving multiple zones, that is, a minimum unit size of about 25 tons. In most cases, this results in an area that is too large to be practically served by one zone of temperature control. However, there are excep- tions, such as an auditorium, gymnasium, or warehouse, where a single zone could require as much as 25 tons of cooling.