Air conditioning 3: Large systems
The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system described above. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil. A humidifier and various filters have also been added in figure 3.
Figure 3. A large air conditioning system functions much like that described in figure 2, except that chilled water is used to cool the air. Outside air is drawn in, filtered and heated before it passes through the main air conditioning devices. The coloured lines in the lower part of the diagram show the changes of temperature and of water vapour concentration (not RH) as the air flows through the system.
There are many variations on the basic design sketched in this article. Most refinements are to save energy. The ratio of recycled air to fresh air can be increased when the outside air has, by chance, the right water content and temperature to push the room climate in the right direction. This is called enthalpy control, to make common sense seem scientific. The heat generated in the coolant condenser can be re-used in the heater, fed by recirculating hot water.
The amount of air circulating in the system is vastly more than that required for the inhabitants to breath, to avoid carbon dioxide narcosis and to avoid smelling each other. Air is mainly used to carry heat and moisture to all corners of the building. Since the heat capacity of air is rather small, quite large amounts are needed to ensure the uniformity of temperature that is required by the specification. Here is another example of the danger of specifying without adequate understanding. If the building is old and uninsulated the heat loss through the outer walls will force a large air circulation, which will in turn require large ducts or make a lot of noise. A leaky building requires considerable moisture added in winter. The RH of the air leaving the humidifier will then be rather high, with a risk that pathological microorganisms will thrive in remote corners of the ducts. Fitting air conditioning into existing buildings is not easy.
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