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Chilled water is commonly used in buildings to provide cooling.
Typically, water is cooled in chiller units, and is then distributed by pipework to air handling units where it is used to cool air that is ducted through the building for ventilation. It can also be used for the dehumidification of ventilation air. As the temperature of air falls, it is less able to 'hold' moisture, that is, its saturation water vapour density falls, and moisture will begin to condense, dehumidifying the air. See air conditioning for more information.
Chilled water may also be used to provide cooling to integrated service modules, chilled beams, chilled ceilings, underfloor cooling, for industrial processes and so on. The use of chilled water to cool the building fabric (rather than ventilation air) is sometimes described as 'active thermal mass'.
- In compression systems, a liquid refrigerant with a low boiling point absorbs heat from the return water and boils in an evaporator to form a gas. The resulting gas is then compressed, which increases its temperature further. The gas is then condensed, releasing its latent heat which is rejected. The process then repeats.
- Absorption refrigeration works on a similar basis, however, in this case, the refrigerant gas is absorbed in a solution which is then heated in a ‘generator’ so that the refrigerant evaporates again, but this time at a higher pressure and temperature. The gas is then condensed, releasing its latent heat which is rejected. The process then repeats.
The rejection of heat from chiller units can be achieved by:
- Air cooling, which rejects heat to the outside air by circulating it through the condenser.
- Evaporative cooing, which uses the addition of water mist to the air to enhance the cooing effect.
- Water cooling, which is generally suited to large systems and requires connection to cooling towers.
The exact opposite of the refrigeration process can be achieved by a heat pump, which reverses the cycle so that heat is supplied to the building rather than cooling. Some systems are reversible, able to supple either heat or cooling. See Heat pump for more information.
The temperature of chilled water will depend on the purpose for which it is being used. Chilled water supplying air handling units might be in the range of 4°C to 10°C, with the return temperature perhaps 5°C higher than the supply temperature. However, where a surface is being cooled, for example a chilled beam, a higher temperature might be sufficient because of the relatively larger cooling surface available. Typically 14°C to 17°C can be adequate. This higher temperature (no lower than 13°C) also helps prevent condensation forming on the cool surface.
Chilled water storage (CWS), can be used when chiller units have spare capacity, such as during the night, with chilled water being stored in insulated tanks, to be used during peak load periods.
NB Refrigerants (rather than chilled water) can be used to provide cooling directly to spaces in variable refrigerant flow (VRF) systems. This is based on the flow of refrigerant between an external condensing unit and multiple internal evaporators (typically fan coil units). See variable refrigerant flow for more information.
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