As the temperature drops outside and we move into the heating season, many A/C and refrigeration systems will still be expected to operate during the cold weather months.
Let's look at the low ambient operation of an A/C or refrigeration system. The metering device requires a pressure drop for proper operation. If the head pressure falls too low, the pressure drop across the metering device may not be large enough, resulting in the A/C or refrigeration system no longer operating properly. The system can suffer from such things as evaporator freeze-ups, compressor slugging, and capacity loss. It is crucial to prevent the head pressure from falling too low.
Fan cycling is the easiest and cheapest way to maintain a minimum head pressure. By using a close on rise pressure control we can successfully cycle the condenser fan on or off to maintain head pressure. Fan cycling controls can be adjustable or non-adjustable. The non-adjustable fan cycling controls are selected by the pressure settings required for the installation. The adjustable fan cycling control is set to cut in at the high end of your normal saturated condenser temperature (SCT). This may typically be between 120 and 130 degree SCT. The cut out is adjusted for the lowest end of normal operating SCT. This will likely be 90 to 100 degree SCT. Some of the disadvantages to fan cycling are wear and tear on the fan motor from starting and stopping and wear and tear on the compressor from the continuous head pressure fluctuations as the fan cycles.
A fan speed controller is a better option and is easy to install. Instead of slamming the condenser fan off and on, the fan speed control will slow the fan down. The fan speed control utilizes a liquid line temperature sensor or a high-pressure transducer to control the fan speed. As the condenser pressure or liquid line temperature starts to fall, the fan motor will slow down. As the temperature or pressure begins to rise, the fan motor will speed up. By modulating the fan speed we can keep a more consistent head pressure, which gives better system low ambient performance while limiting wear and tear on the compressor and fan motor. Fan speed controls from ICM are available for use with both ball and sleeve bearing motors. They are adjustable with minimum speed settings and full speed start options.
Condenser flooding is the best type of low ambient control. Condenser flooding utilizes a special low ambient refrigerant control valve, which floods the condenser with liquid effectively reducing the condensing surface to maintain a minimum condenser pressure. This method will maintain a constant minimum head pressure while allowing the refrigeration system to operate at temperatures well below that of a system using fan cycling or fan speed controls. Condenser flooding is a much more involved installation as condenser re-piping will be required. A head pressure refrigerant control valve, a properly sized receiver and additional refrigerant will be needed.