Book - School of Science and Technology

Choice of refrigeration plant 543of the order of an additional 20% power absorbed by the compressor motor. Since acooling tower may be located at a considerable distance from the refrigeration plant,horizontally or vertically, such a combination has obvious attractions as far as flexibilityis concerned and it is common practice for the machinery to be sited in abasement and the cooling tower at roof level. It is of course necessary that an opencircuit cooling tower should be positioned sufficiently above the refrigeration plant toprevent any drainage or priming problems. Where this cannot be achieved, an intermediateheat exchanger may be installed.Storage systemsAn annual cooling load profile, plotted for almost any air-conditioned building, wouldshow that the maximum load only occurs on a few occasions. This is due to the fact thatmany of the components of the total are not only seasonal but transitory hour-by-hour.Thus, where the central plant is sized to match the peakload, it will operate at considerablyless than its full capacity for the majority of the time. An alternative to sizing arefrigeration plant to meet the peakcooling load would, therefore, be to provide somemeans to store a cooled medium (water, ethylene glycol solution, ice, etc.) and thus allowa somewhat smaller plant to run for longer periods at full output. The stored energywould then be available, as required, to make up the deficit between the plant capacityand the peakrequirement. The advantages of such an arrangement are:. reduced size of refrigeration plant. reduced maximum demand on power supply. reduced unit energy charge, if run off-peak. inherent standby capacity in the energy store. stability available for control.The parallel disadvantages are:. higher capital cost. increased space requirement. possibility of lower coefficient of performance with low temperature or ice storagesystems.The storage system concept may be applied in varying degrees. It is not uncommon inthe British Isles for a limited quantity of chilled water to be provided for peak lopping and,in this case, capacity would be provided to supplement a water chilling plant for one ortwo hours in the day, at the time of maximum load. At the other extreme, it would not beimpossible to provide sufficient storage to meet a total peak-day cooling load, the plantrunning during night hours only during the period when an off-peaktariff applies. Afurther option would be to run the plant continuously over 24 hours and to provide alimited level of storage such that during the whole period of demand for cooling, both theplant output and the stored energy would be used in parallel. Figure 19.11 illustrates thesethree basic operating modes and indicates the relationship between plant size and storedwater quantity.Whereas the design of a hot water storage system (Chapter 5, p. 138) is able to limit therequired volume of the vessels by applying pressure and storing water at an elevatedtemperature, parallel action (i.e. storage at a temperature much lower than that of usage)is not really practical in the case of chilled water. A temperature of about 4 C is thatmost usually chosen as the practical minimum but, even at this level, design problems