The pressure in the condenser side of the system must always be limited to a maximum allowable value, and a pressure control is used to stop the compressor if necessary.
High-pressure cut-outs, Figure 9.3, are fitted to all but the smallest of systems. Where a compressor is fitted with a shut-off valve, the pressure connection should be upstream of the valve. The compressor outlet pressure is brought to one side of a bellows or diaphragm, and balanced by an adjustable spring. A scale on the control indicates the pressure setting to commercial accuracy and is checked on commissioning the system.
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Figure 9.3 Pressure switch (Danfoss)
If the spring pressure is overcome, the switch will open and stop the compressor. The cut-out can also operate a warning. The cut-out point only needs to be some 2 bar higher than the expected summer operating pressure but there is a tendency to set such controls much higher — sometimes as much as 8 bar above summer pressures. At this setting, the user will not get a warning of abnormal running until the fault has reached serious proportions.
Since excess pressure indicates malfunction of part of the system — usually a condenser fault or incorrect closure of a valve — the high-pressure switch should be reset manually, not automatically.
Where the refrigeration system is providing an essential service which should not be interrupted, one high-pressure switch may be set at a warning level and operate an alarm, without stopping the compressor. A second switch, set somewhat higher, will stop the equipment if this warning is ignored and if excessive pressures are reached. All high-pressure cut-outs should be checked at least once a year, for correct setting and operation.
Abnormally low suction pressures will lead to high discharge temperatures, owing to the high compression ratio, and possible malfunction of other components. Air cooling coils may frost excessively, or water chillers freeze.
A low-pressure cut-out switch is usually fitted to stop the compressor under these circumstances. Settings may be 0.6-1.0 bar below the design evaporator pressures, but depend very much on the type of system. The cut-out setting should be above atmospheric pressure if possible to avoid the ingress of air through any leaks.
Abnormally low pressure may not be an unsafe condition and the low- pressure switch may be automatic reset, closing again at a pressure corresponding to a temperature just below that of the load.
If a plant has been shut down long enough for all pressures to equalize and is then restarted, the suction pressure will pull down below normal until the liquid refrigerant has begun to circulate. Under such circumstances the low — pressure switch may operate. This is a normal occurrence, but may require the addition of a delay timer to prevent frequent starting of the compressor motor.
A low-pressure switch can also be used in conjunction with a thermostat and a solenoid valve in the pump-down circuit. In this method of control, the thermostat does not stop the compressor but de-energizes the liquid line solenoid valve to stop the supply of refrigerant to the evaporator. The compressor continues to run and pumps down the evaporator until stopped by the low — pressure switch. When the thermostat again calls for cooling, it opens the solenoid valve, liquid enters the evaporator and the low-pressure switch will close again to restart the compressor. This method is used to ensure that the evaporator is kept clear of liquid when the plant is off. If there is any leak at the solenoid valve, it will cause the compressor to restart periodically to remove the surplus liquid from the coil (see Figure 9.4). Pressure switches are also made in miniature encapsulated versions, mainly pre-set for use in integrated control circuits.
Posted in Refrigeration and Air Conditioning