Central station system. All rooms
The centralization of all the plant from the conditioned space occurring for safety reasons, but also provides better access for operation and maintenance, and as the transmission of noise. Since all the air passes through the plantroom, you can arrange for any proportion of outside air to 100%. It may be necessary for some applications, and the external air for other duties will reduce the cooling load in cold weather. For example, in systems is considered in Section 28.1, there may still be a cooling load is required whenever the ambient temperature up to 12C dry bulb, but this design supply air temperature, so all of cooling can be done with the surrounding air, not mechanical cooling.
Air distribution the zone assumes that the sensible and latent heat loads are more or less constant zone (see Fig. 28.2). As soon as there are vast differences, it is necessary to ensure that cold air is enough to ensure the greatest burden, and re-heating the air in the other areas. Where the Central plant to serve a number of separate rooms and floors, are converted into the system with reheating coils in each zone of the branch duct (see Fig.
28.3). It is recognised that this is wasteful of energy and can, in extreme cases, will require changes in heat load, almost as high as the load on the cooling system.
To make the system of air-conditioning, a more economical for multi installations, the number of cooled air to the individual zones can be made variable, and decrease when cooling load is less. This will also reduce the number of re-required heat. This re-heat can be by means of a coil, like before or by mixing with a variable number of warm air, supplied via the second channel of the system (see Fig. 28.6 28.4).
In the first of these methods, the reduction of the air mass flow is limited by considerations of propagation velocity in premises, and on a lighter load of air can be used together with more re-heat to keep the air accelerates. Within this constraint, any proportion of sensible and latent heat can be met to achieve the correct room conditions. However, the full control of the humidity would be very wasteful of energy and a simple thermostatic control is preferred.
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Air-Conditioning 
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