Air Conditioning (AC) System
AC systems can be classified by the methods of achieving the within-the-space cooling (and heating):
1.1 Unitary unit
It is a compact, self-contained unit located within or next to the air-conditined space. Such a system includes the direct expansion (DX) system.
1.2 Split Unit
It is also a compact, self-contained DX system with the condensing unit (compressor and condenser) located at some distance from the evaporating unit (evaporator and expansion valve). Long refrigerant pipings connect the condensing and evaporating unit.
1.3 Central Systems
Central systems include: all-water system, all-air system and air-water system.
1.3.1 All-water system
In this system, a cooling medium (chilled water or brine – but not refrigerant) is supplied from a remote source and circulated through the terminal unit within the conditioned space.
1.3.2 All-air system
In an all-air system, the final cooling medium (air) is brought into the conditioned space through ducts and distributed within the space through outlets.
1.3.3 Air-water system
Such a system uses chilled water and cold air from a central station. The major part of thermal load is met by chilled water which is circulated through a coil.
1.4 All-air central centralised systems
1.4.1 Constant air volume (CAV) system
In a CAV system, the volume flow rate of air is constant. During part-load operation (eg on rainy days), terminal reheaters can be used to increase the supply air temperature to ensure conditioned space air dry bulb temperature is maintained at comfort level. Without reheat, indoor air temperature cannot be controlled when zone sensible loads are lower than design-day values; resulting in uncomfortable conditions.
1.4.2 Variable air Volume (VAV) system
In a VAV system, the volume flow rate of air is varied to ensure comfortabale indoor air dry bulb temperatures. An indoor air thermostat senses indoor air temperature variation and sends a signal to a VAV box control unit which increases the volume flow rate of supply air when the sensible load increases and vice-versa.
1.4.3 Dual-Duct/Multizone (DDMZ) system
In this system, the supply air temperature is varied to meet changing zone sensible loads. When zone sensible load decreases, zone air temperature will decrease and the indoor air thermostat sends a signal to the mix box damper to mix more bypassed air with less cool air, such that the higher supply air temperature will ensure that the indoor air temperature will rise to the set point value.
In the system, the supply air temperature is variable, but the volume flow rate of supply air is constant.
In dual-duct system, the mixing is done at the terminal unit, while in multizone system the mixing is accomplished at the air handling unit (AHU).
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