Capacity control of a condenser means reducing its capacity. This understanding of capacity control raises the question of why the condenser capacity should ever be reduced. When the condenser operates at full capacity, the condensing temperature will follow the wet-bulb temperature as it drops, and thus the compressor power will be reduced. In general the recommended strategy is to operate the condenser with full capacity, dropping the condensing temperature until limited by one or more of the following conditions:
– the condensing pressure is too low to adequately feed level-control valves and expansion valves
– the pressure of defrost gas is too low to achieve a satisfactory defrost,
– if the plant uses screw compressors with their oil cooled by direct injection of refrigerant, the pressure of the liquid must be high enough to force an adequate flow rate of liquid into the compressor
– savings in compressor power by further lowering of the condensing temperature are less than savings that would be possible in pump and fan motors of the condensers
If the pressure of liquid is too low, expansion valves and liquid-level controllers will not be able to pass enough refrigerant and capacity of evaporator coils will drop. In the case of a level controller to an intermediate-pressure sub-cooler/desuperheater, subcooling of the liquid will suffer and desuperheating of discharge gas from the low-stage compressor will degrade. It would be possible to use valves with larger ports, but this change could cause unstable feeding when the liquid pressure is high.
In order to defrost a coil the saturation temperature of the defrost gas must be well above 0°C (32°F). Tests12 on an ammonia coil showed that successful defrosts could be achieved with a saturation temperature of the defrost gas of 15° C (59°F) resulting in saturation temperature inside the coil of approximately 10° C (50°F). Before a plant operator would set the minimum condensing temperature at 15°C (59°F) for unattended operation, defrosts of coils should be monitored for a period of time to be confident that no residual frost or ice remains on the coil following this defrost.
Manufacturers of compressors who apply direct injection for oil cooling recommend13 minimum condensing temperatures of approximately 21°C (70°F) in R-22 and ammonia systems to provide adequate pressure for injection of the liquid.
The question of reducing air or water flow to save power required by the fan and pump motors will be addressed further in Section 7.15.
Still another situation which may influence the minimum condensing pressure occurs if the plant recovers heat from the high side of the system for uses elsewhere in the facility. A simple economic analysis may be able to establish whether it is cheaper to pay for the recovered heat from other sources and reap the saving on compressor power with a lower condensing pressure.
The two principal methods of reducing the condenser capacity are to reduce or cycle the flow of spray water or the air flow. Adjusting the flow rate of spray water will be be dismissed rather quickly in the next section, and the regulation of air flow evaluated in Section 7.14.