An air cooler coil works at partial load either when the enthalpy of the air entering the coil is less than the summer design value, or when the capacity of the coil is deliberately reduced by the use of motorised valves in the chilled water piping, or by face and by-pass dampers (see sections 10.7 and 8.6).
The majority of air conditioning systems operate at partial load for most of the time. As an example, an office block in London is likely to be at full cooling duty for a total of only 30 hours, in July and August, out of a total annual working year of about 2300 hours.
It is good practice to design systems to take advantage of the natural cooling capacity of the outside air. Figures 13.21(a) and (b) illustrate how this may be done. The dry-bulb temperature in London exceeds 10°C for about 1300 hours of the total working time. Since 10°C or 11°C is typical of the design dry-bulb temperature commonly required from a cooler coil, the refrigeration plant could be switched off for the remaining 1000 hours, in the case of an all-air system, such as variable air volume.
On the other hand, with air-water systems (such as fan coil), chilled water must be available for the terminal units all the time they are working and, to provide this, natural cooling is consequently related to the outside wet-bulb. In London, this exceeds a sling value of 5°C for about 1600 hours in a working year. It is possible to obtain a chilled water flow temperature of 11°C or 12°C, when the wet-bulb is less than about 5°C, by pumping primary chilled water through the cooler coil in the air handling plant, without running the refrigeration compressor, for the remaining 700 hours.
The behaviour of an air cooler coil at partial load is not easily predicted in a simple way, but section 10.7 deals with this and it is illustrated in Figure 10.7.
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