Air Conditioning

The term “air conditioning” (AC) can be a source of confusion due to its various interpretations.

In its strictest sense, air conditioning refers to systems that regulate the moisture content, or humidity, of the air. This encompasses both humidification and dehumidification processes. Controlling humidity is crucial for maintaining the comfort of building occupants, preventing condensation, especially in specialised environments like swimming pools, and safeguarding sensitive items that require specific conditions.

However, dehumidification is typically achieved through cooling. As the air temperature decreases, its ability to retain moisture diminishes, leading to a rise in relative humidity. When relative humidity reaches 100%, the air becomes saturated, known as the dew point. Further cooling causes moisture to condense, effectively dehumidifying the air.

Therefore, humidity control and cooling are often grouped together under the term “air conditioning.” Cooling and dehumidification are vital for enhancing thermal comfort because high humidity limits the body’s ability to cool through perspiration. Consequently, achieving comfort relies on both temperature and humidity regulation, among other factors.

Cooling air alone, often referred to as “air conditioning,” is more accurately termed “comfort cooling.” However, comfort cooling may inadvertently include some dehumidification due to its cooling effect.

Alternatively, air conditioning is defined as the process of conditioning supply air to regulate humidity, quality (via filtration), and temperature. According to CIBSE Guide B, air conditioning entails precise control over humidity and temperature within the conditioned space. Close-control air conditioning, as suggested by CIBSE, involves maintaining temperature within 1°K and relative humidity within 10%, necessitating a complex process of dehumidification, cooling, reheating, and humidification.

According to the Department for Communities and Local Government (CLG) guide, an air conditioning system encompasses all components required to treat air, including temperature control and ventilation, humidity, and air cleanliness regulation. This includes fixed systems like split and centralised systems, as well as mechanical ventilation systems that do not provide cooling but serve cooled spaces through other means. Components intended solely for heating purposes are excluded from the definition of air conditioning systems.

Process

In mechanically ventilated commercial settings, air conditioning is commonly facilitated by air handling units (AHUs) linked to ductwork for distributing and extracting air within internal areas. Alternatively, AHUs can directly supply and extract air from a space.

Typically, air handling units consist of an insulated enclosure housing various components, including filter racks or chambers, a fan or blower, heating, cooling, and dehumidification systems, sound attenuators, and dampers. Units containing only a fan and a heating or cooling element, situated within the serviced space, are often referred to as fan coil units (FCUs).

Cooling can be achieved within the unit itself or through connections to central chillers.