Most plants operate at the combination of maximum refrigerating capacity and design ambient temperature only a small percentage of the total time. In most cases, the refrigeration capacity is less and/or the ambient temperature is lower than design, offering the opportunity to reduce the condensing pressure. Up to a certain point, the plant operator should exploit the reduced condensing temperature to conserve compressor power. The catalog data for reciprocating and screw compressors in Chapters 4 and 5, respectively, indicate a power saving of the order of 3% per °C reduction in condensing temperature (1.7% per °F) in the range of 35°C (95°F) and 0°C (32°F) evaporating temperatures.
In operating regimes of perhaps 50% or more refrigerating capacity and wetbulb temperatures above 15°C (59°F), the evaporative condenser would be operated with full fan and spray-water pump capacities. For lower refrigerating capacities and wet-bulb temperatures, the designer of the system may have specified controls that limit how low the condensing temperature (pressure) can drop. The setting of that minimum pressure is at the operator’s discretion.
The conclusion has been reached that regulating the rate of air flow is the preferable method of reducing condenser capacity. An additional method in a multiple-condenser installation is to shut down completely one condenser and bring it back on line when necessary. With the exception of variable-speed drives, all of the methods of capacity control are a form of cycling wherein the condensing pressure drops low enough to reduce the capacity. When the condensing pressure passes through the control dead band, it rises to a point where full fan operation resumes. No good purpose is served by rapidly cycling the control, because it causes excessive wear on the affected motors and possibly erratic feeding of the evaporators serviced by expansion valves and pressure fluctuations downstream of level-control valves.
The next consideration is how to operate the fans to minimize combined compressor and fan power. The power required by the fans is small, relative to that required by the compressor, at least at full load. Figure 7.17 shows relative power requirements for the compressor and condenser fans as a function of the refrigeration load.
At full refrigeration capacity, the fan motors draw about 5%–8% of the power drawn by the compressor motor. As the refrigeration load decreases, the compressor power drops, but the fan power remains constant if the air-flow rate is not reduced.
Three curves are shown for the compressor power—the top one applies if the condensing temperature remains constant through the entire load range. The condensing temperature progressively drops as the load decreases because, as Fig. 7.13 shows, the condensing temperature falls as the load on the condenser drops. The intermediate compressor power curve in Fig. 7.17 represents the compressor power at reduced air flow rate, and suggests a trade-off between savings in fan power and compressor power.
The optimum conditions to shift from full fan operation to partial operation may be different for each plant. However, to conserve energy, many plant operators are overly influenced by the visibility of all the condenser fans operating and are not as conscious of the extra compressor power required.