In a simple air-cooled compressor, outside air is forced across the refrigerant coil by a fan. The compressed refrigerant gas is cooled and liquefied. The gas returns to the cooling coil in the conditioned space, where it absorbs heat, boils, and returns to the condenser as a gas. The fan operates whenever the system is in use.
The head pressure, a measurement on the gas side of the coil, is a critical factor in the operation of the unit. If the ambient air is not cool enough to condense the liquid, the system pressure builds up, to the point where the pressure switch disables the system before permanent damage can occur. Low-temperature outside air can cause operating problems as well, if the inside coils don’t impart enough heat to the refrigerant to boil it. Often systems will have inlet dampers to choke off a portion of the airflow, so the refrigerant is not supercooled. Alternately, a variable-speed drive can modulate the airflow to maintain head pressure within ideal operating limits. This is the most energy-efficient means of head-pressure control.
Another control strategy for air-cooled condensers is a throttling valve in the liquid-refrigerant line. Partially closing this valve causes the coil to fill up part way with refrigerant, thereby reducing the capacity of the coil. This technique, called flooding, doesn’t require control of the airflow by any means, so it’s effective for small systems that cannot easily damper the airflow or control fan speed with a VFD or by use of a two-speed fan motor.
Sometimes a unit or system with many air compressors has several compressors start at the same time. Automatic controls will enable the compressors to be staged so that only one unit will run initially; then a second will cut in as the load increases, and so forth. This strategy not only reduces the energy needed to operate the cooling equipment but also reduces the peak electrical demand use of the system.
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Excerpt from Energy Conservation Projects.