Constant volume cooling is a simple system often used to provide tempered fresh air in multi-zone buildings containing partially centralised air/water systems (such as a fan coil installation). Constant volume systems can also be used to cool single zones, such as clean rooms, and operating theatres in hospitals.
As the name suggests, constant volume systems provide a fixed volume of air at a temperature and humidity determined by the conditions of the space being served. They are therefore not generally suitable as the primary cooling system for buildings with multiple zones, as each zone will have varying cooling requirements. The exception to this is a constant volume system that incorporates re-heaters in each zone: if the supply air is too cold for the conditions in a zone, the re-heater will raise its temperature slightly, thereby providing local control. However, reheaters are no longer widely used since this approach is generally wasteful of energy.
A major application for constant volume systems is to provide fresh air in partially centralised air/water systems, such as fan coil or chilled beam installations. For these applications, a constant volume system would provide tempered ventilation air, and may meet a small proportion of the heating or cooling load. The primary heating and cooling requirement would be met by the room units, such as fan coils or chilled beams.
Benefits
- Suited to certain types of public spaces and foyers; commonly used to provide fresh air in air/water systems such as fan coil and chilled beam installations
- Can be used for single zone applications, where only one set of internal conditions need to be satisfied
- Simple and relatively easy to maintain
Limitations
- Not generally suitable for multiple-zone applications (such as offices), as each zone will have varying cooling requirements.
FREQUENTLY ASKED QUESTIONS
Constant volume systems are not suitable for buildings with multiple zones because they provide a fixed volume of air at a fixed temperature and humidity, which cannot accommodate the varying cooling requirements of different zones. This limitation can lead to overcooling or undercooling in certain areas, resulting in energy waste and discomfort.
Re-heaters in constant volume systems allow for local control by raising the temperature of the supply air in each zone if it is too cold for the local conditions. This approach enables zone-level temperature control, but it is generally considered energy-inefficient and has largely fallen out of favor. Modern systems often employ more efficient and flexible control strategies.
In partially centralized air-water systems, such as fan coil or chilled beam installations, constant volume systems provide tempered ventilation air to maintain good indoor air quality. They work in conjunction with the air-water system to provide a complete HVAC solution, offering a simple and efficient way to manage ventilation and cooling requirements.
Yes, constant volume systems can be used in conjunction with other HVAC systems, such as variable air volume (VAV) systems or dedicated outdoor air systems (DOAS). This hybrid approach allows designers to leverage the strengths of each system, providing a more comprehensive and efficient HVAC solution for complex buildings.
Constant volume systems are commonly used in healthcare facilities, particularly in operating theaters, clean rooms, and laboratories, where precise temperature and humidity control are critical. They are also used in patient rooms, corridors, and other areas where a fixed volume of tempered fresh air is required to maintain good indoor air quality and prevent the spread of infections.
