Constant air volume CAV
Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air. This helps to:
- Moderate internal temperatures.
- Replenish oxygen.
- Reduce the accumulation of moisture, odours, bacteria, dust, carbon dioxide, smoke and other contaminants that can build up during occupied periods.
- Create air movement, which improves the comfort of occupants.
Very broadly, ventilation in buildings can be classified as ‘natural’ or ‘mechanical’. Mechanical ventilation systems can also include heating, cooling, humidity control and air filtration. These functions are often described collectively as HVAC (Heating Ventilation and Air Conditioning).
- Variable air volume (VAV), in which the temperature of the supply air remains constant, but the volume varies (also known as variable volume, constant temperature VV-CT).
- Constant air volume (CAV) in which the volume of air supply remains constant, but the temperature varies (also known as constant volume, variable temperature CV-VT, or constant volume CV).
- Variable volume, variable temperature (VV-VT sometimes referred to as variable volume and temperature - VVT).
Constant air volume (CAV) systems are becoming less common in new buildings as VAV systems tend to provide closer control of air temperature and require lower fan speeds, as a result of which they can use less energy and generate less noise. However, CAV systems are still used in small and medium-sized premises with straight-forward HVAC requirements, as they can be relatively simple to install, can have a lower capital cost and tend to be reliable.
They are particularly common in simple systems where fan coil units are supplied with a constant volume of ‘fresh’ air.
Simple, single-duct CAV systems which supply air at a single temperature and constant volume might be suitable for a large space with simple, uniform thermal demand, such as a gymnasium.
This system can be enhanced by reheating the supply air in CAV terminal units to provide additional local control. This might be appropriate where there are some minor local variations to thermal demand. In this case, air is supplied to the system at the lowest temperature required and then its temperature is increased as necessary locally.
Alternatively, 'mixed-air' or 'dual-duct' systems can provide both heated and cooled air, the proportions of which are regulated locally in mixing boxes. This might be appropriate where there are significant differences in thermal demand. Air is supplied at the lowest and highest temperatures required and then mixed locally as necessary.
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