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“In an air conditioning system, heat is transferred mechanically from room air to the outdoor air to establish human comfort [Miller and Miller, 2006]”.
Fig. 1: Air conditioning System and its Control by the use of AC drives. A) Principle of operation, B) Full Air Conditioning Cycle and C) General Control Layout of an Air conditioning System (A and B adapted from [Miller and Miller, 2006] and C adapted from [Mohan et al., 2003]).
1. Heat and Pressure
The state (solid, liquid or gas) of a substance depends on its amount of heat and pressure. The “heat content” of a substance is composed of “sensible heat” and “latent heat”. When sensible heat is added/subtracted to a substance, it affects its temperature without changing its state. The heat require to change the state of a substance, e.g. from solid to liquid, is called “latent heat”; this type of heat does not affect the temperature of the substance [Miller and Miller, 2006].
Additionally, a change in pressure affects the solidification, liquefaction and vaporization points of a substance. For example, water vaporizes at 100°C at a pressure of 14.7 psi (pounds per square inch). However, at a pressure of 10.1 psi, water vaporises at 89.4 °C. Hence, if the water were at a temperature of 89.48°C and a pressure of 14.7 psi, it would remain liquid, however, if its pressure were to be dropped to 10.1 psi, then the water would vaporize at this temperature [Miller and Miller, 2006].
2. Air Conditioning System
Air conditioning systems and refrigeration take advantage of the principles explained in the previous section (above). As an example, imagine a substance which is liquid at a pressure of 100 psi, but vaporizes at sea level atmospheric pressure (14.7 psi). This substance (liquid) contained in a tank (Fig. 1A), which is at 100 psi, is released into a long coil of tubing, via an expansion valve, to the atmosphere. According to the principle of thermal expansion, matter expands when heated. This implies an increase in its linear dimensions and volume. The opposite effect is obtained when heat is removed from a substance [Miller and Miller, 2006]. When the liquid enters the coil, its pressure is reduced, and consequently so is its vaporization point. Part of the liquid becomes gas by using its own heat; the remaining liquid takes heat from the coil and vaporizes as well. This implies reducing the temperature of the coil and consequently the temperature of the surrounding air. This process, of cooling the surrounding air, will continue as long as the tank releases the pressurised substance. In an air conditioning system, the cycle is closed by recovering the substance after it has done its job of cooling [Miller and Miller, 2006]. The full cycle in an air conditioning system is shown in Fig. 1B. A general control layout of an air conditioning system is shown in Fig. 1C.
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