In the modern world, the construction sector demands comfort and energy efficiency. The design of heating, ventilation, and air conditioning (HVAC) systems has a significant impact on both, and working with a qualified engineering firm has a positive impact on the end result.
A fundamental decision in HVAC design is selecting an appropriate air handling configuration: constant air volume (CAV) or variable air volume (VAV). Each option has advantages and disadvantages, and using the correct setting increases comfort and efficiency.
This article provides an overview of CAV and VAC systems, describing the principles and calculations used by HVAC engineers during the design process.
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Constant Air Volume Systems
CAV systems maintain a constant airflow, as their name implies, and desired internal conditions are achieved by adjusting the air supply temperature. For example, when a building needs higher cooling power on a hot summer day, the CAV system delivers cooler air. In chilled water systems, the cooling effect depends on the flow of cold water supplied to the fan coil unit. In turn, the water flow controller operates based on thermostat settings.
CAV systems can be single-zone or multi-zone types, although they are best suited for single-zone applications where the load undergoes minimal changes over time. Some examples are auditoriums, theaters and museums. Multi-zone CAV systems require duct heating to deliver different air temperatures to individual zones, which reduces their energy efficiency .
The operation of a multi-zone CAV system can best be described with an example. Consider the following design conditions:
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An indoor air handling unit (AHU) serves three zones (A, B, and C).
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Each of the three zones has a dedicated thermostat.
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The AHU obtains cold water from a cooling tower chiller.
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Air duct heaters are attached to main branches serving individual zones.
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Thermostats are set to 55°F (A), 68°F (B), and 72°F (C).
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The AHU provides cool air at 55°F.
Once the supplied air enters the duct system, it can only be heated and not cooled. Therefore, it must be supplied at the lowest of the three required temperatures – 55°F for zone A in this case. After zone A is served, the airflow can be heated to the temperatures required for other zones: 68°F for zone B and 72°F for zone C.
Although the operating principle is simple, it has an efficiency limitation. Some cooling output is wasted when air is reheated in zones B and C, and the duct heaters themselves also consume energy.
Variable Air Volume Systems
As you can imagine, VAV systems maintain constant air temperature and instead adjust airflow depending on load. VAV systems can have single-zone, multi-zone, or dual-duct configurations. Just as CAV is the preferred option in single-zone systems, VAV is recommended for multi-zone systems.
Dual-duct VAV systems have separate ducts for hot and cold air, and each zone has a plenum where its airflow is mixed. The proportion of hot and cold air depends on the desired temperature for each specific zone. This HVAC configuration is the most expensive in terms of installation, operation and maintenance.
Multi-zone systems have VAV boxes that control the airflow delivered to individual zones. Unlike CAV, the cold water flow supplied to the fan coil unit is kept constant. VAV systems reach their full potential in applications where there are multiple zones with variable load. Some examples are shopping malls, hotels and office buildings.
VAV boxes serve a similar function to the duct heaters used in a CAV system. Instead of reheating the air according to the needs of each individual zone, the airflow is controlled by keeping the temperature constant.
Basic calculations in CAV and VAV design
The HVAC design process involves complex calculations and energy modeling. However, the equations that describe the system's operations are simple. This section describes how temperature and airflow are adjusted according to load.
The starting point is the sensible heat gain equation, which is fundamental in HVAC systems:
- Q = 1.08 x CFM x ΔT
Where:
- Q = Load of the room or zone served (BTU/hour)
- CFM = Airflow in cubic feet per minute
- ΔT = Difference between ambient temperature and supplied air temperature
Example 1 – VAV System
Let's assume a room has a load (Q) of 10,000 BTU/h, with an internal temperature of 75°F and a supply air temperature of 55°F. In this case, the difference is 20°F. To calculate the required airflow, the sensible heat equation can be rearranged:
- Q = 1.08 x CFM x ΔT
- CFM = Q ÷ (1.08 x ΔT)
- CFM = 10,000 BTU/h ÷ (1.08 x 20°F) = 463 cfm
In this case, the VAV system would have to adjust the airflow to 463 cubic feet per minute for the corresponding zone. Let's observe the effect when the load is increased to 12,000 BTU/h.
- CFM = 12,000 BTU/h ÷ (1.08 x 20°F) = 555 cfm
The 20°F temperature difference is maintained constant under a load of 12,000 BTU/h, while increasing airflow to 555 cfm.
Example 2 – CAV System
In this case, the cooling load and ambient temperature are the same as those used in the example above: 10,000 BTU/h and 75°F. However, the airflow is fixed at 500 cfm and the air supply temperature is adjusted. The sensible heat equation would be rearranged as follows:
- ΔT = Q ÷ (1.08 x CFM)
- ΔT = 10,000 BTU/h ÷ (1.08 x 500 cfm)
- ΔT = 18.52°F
The air supply must be 18.52°F below ambient temperature, which is equivalent to 56.48°F. Repeating the calculation for 12,000 BTU/h, the following result is obtained:
- ΔT = Q ÷ (1.08 x CFM)
- ΔT = 12,000 BTU/h ÷ (1.08 x 500 cfm)
- ΔT = 22.22°F
In this case, the required supply air temperature is 52.78°F.
Conclusion
HVAC engineers can choose between CAV and VAV systems depending on the application. While a CAV system uses variable air temperature and constant airflow, a VAV system maintains constant temperature and variable airflow. CAV systems are best suited for single-zone applications where the load changes little, while VAV systems are the best option for multi-zone applications with a constantly changing load.
