The air-cooled condensing set
The heat-rejection capacity of an air-cooled condenser is almost directly proportional to the difference between the condensing temperature and the dry-bulb of the air entering, as Figure 12.6 shows. Enough extra surface is sometimes provided to sub-cool the liquid by 8° or 9°C but to see how a condenser actually performs we must look at its behaviour when piped to a compressor to form a condensing unit, or set.
Air temperature onto condensor °C
Condensing temperature °C Fig. 12.6 Typical condenser and compressor performance, assuming 8.3° sub-cooling.
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Compressor capacity can be described in terms of: cooling effect (refrigeration) against saturated suction temperature or pressure (Figure 12.5) for a given condensing temperature, cooling effect against condensing temperature for a given saturated suction temperature, or as heat rejected (cooling effect plus compressor power) against condensing temperature for a given saturated suction temperature. The upper part of Figure 12.6 shows compressor performance as heat rejection rate and the lower part as cooling effect, both against condensing temperature.
The intersections of the compressor and condenser curves in the upper part of Figure
12.6 indicate the condensing temperature obtaining for given suction and entering air temperatures and these points of intersection can then be transferred to the lower part of the figure to give the corresponding refrigeration effects. Thus the point R’ (28°C on to the condenser, 4.4°C suction temperature) represents a heat rejection rate of 63.4 kW at a condensing temperature of 40.6°C and transforms to the point R (28°C on to the condenser, 4.4°C suction temperature) and a refrigeration effect of 52.2 kW at the same condensing temperature. Similarly, S’ transforms to S, and so on. In this way it is possible to plot characteristic curves representing cooling capacity of a condensing set at a given dry-bulb on to the condenser against saturated suction temperature. From Figure 12.6 we can deduce that when air at 28°C enters the condenser the refrigeration duties are 62.9 kW, 52.2 kW and 43.1 kW for suction temperatures of 10°C, 4.4°C and -1.1°C, respectively. This is shown in Figure 12.7 by a curve for the condensing set passing through the point R, with air on at 28°C. An alternative approach is to express the performance of the condensing set for a given condensing temperature instead of a given air temperature on to it. For example, the refrigeration duties would then be quoted as 64.5 kW, 52.2 kW and 42 kW for respective suction temperatures of 10°C, 4.4°C and -1.1 °C when the condensing temperature was 40.6°C.
Posted in Engineering Fifth Edition