11 kinds of common protection devices for refrigeration compressors.

Date:Dec 31, 2019

When the compressor is running, there may be some abnormal situations, such as: the exhaust pressure is too high, the suction pressure is too low, the oil pressure is insufficient, the motor is overheated, and excessive liquid enters the cylinder.  In case of abnormal conditions, the compressor will be damaged without protective measures.


Protection measures for compressors can be divided into four categories:

Prevent liquid strike;

Pressure protection;

Protection of built-in motor;

Temperature protection.

First, prevent liquid strike

When too much liquid enters the cylinder to be discharged from the exhaust valve, there will be liquid impact in the cylinder. The high pressure generated during the liquid impact will damage the cylinder, piston, connecting rod, etc. Therefore, a series of protective measures should be taken.

1. False cover: The air valve assembly is tightly pressed against the end of the cylinder with a spring to form a false cover.  When the in-cylinder pressure is too high, the exhaust valve will be jacked up and the liquid will be discharged, thus the in-cylinder pressure will decrease rapidly.  False cover is used for fully enclosed compressor.  The lift limiter of the strip-shaped discharge reed valve is pressed on the valve plate by a pressure release spring.  When the in-cylinder pressure rises due to hydraulic stroke, the lift limiter is jacked up, causing the in-cylinder pressure to drop.  After the liquid impact disappears, the pressure release spring presses the lift limiter against the valve plate, and the compressor continues to operate.

2. Oil heater: The lubricating oil of crankcase is dissolved with refrigerant, and the dissolved amount increases when the ambient temperature is low.  When the compressor is started, the pressure in the crankcase suddenly decreases, a large amount of refrigerant vaporizes, and lubricating oil is foamed and sucked into the cylinder, causing liquid impact.  Using oil heater to heat lubricating oil before starting and reducing the amount of refrigerant dissolved in lubricating oil are effective measures to avoid liquid strike.

3. Gas-liquid separator: The gas-liquid separator is also called liquid reservoir.  The gas-liquid mixture from the evaporator is separated in the gas-liquid separator, and the gas enters from the upper part of the outlet pipe and flows out from the lower part.  The separated liquid is stored at the bottom of the separator, the liquid refrigerant therein is heated and gasified to enter the upper part of the outlet pipe, and the lubricating oil that cannot be gasified flows into the outlet pipe from the return hole and then enters the compressor.

Second,Pressure Protection

1. Control of suction and discharge pressure: when the compressor is running, due to system reasons or the compressor itself, the discharge pressure may be too high or the suction pressure may be too low. Therefore, the suction and discharge pressures need to be controlled.  Common controllers are high and low pressure controllers, which consist of a high pressure control part and a low pressure control part.  When the exhaust pressure exceeds a given value, the high-pressure control part acts to cut off the power supply of the compressor and stop it.  When the suction pressure is lower than the given value, the low-pressure control part acts to cut off the power supply of the compressor and stop it.

2. Oil pressure difference controller: During the operation of the oil pressure protection refrigeration compressor, its moving parts need lubricating oil with constant pressure for lubrication and cooling. In order to ensure the safe operation of the compressor, the oil pressure must be controlled. When the oil pressure drops to a certain value, a signal shall be sent to stop the compressor.  When the pressure difference between the inlet and outlet of the hydraulic pump is too high or too low, the controller acts to cut off the compressor power supply and the motor stops rotating.

Third, built-in motor protection

1. Overheating: The internal temperature of a well-designed motor operating under specified conditions will not exceed the allowable value, but the internal temperature of the motor will exceed the allowable value when the motor is operating at too high or too low a voltage, or in a high-temperature environment. In frequent starting, the temperature will be too high due to too large starting current.

In order to prevent the motor from overheating, in addition to proper use and maintenance, overheating relays can also be installed.  Overheating relay can be installed inside the winding, called internal temperature relay, or installed outside the motor, called external temperature protector.  When the internal temperature of the motor exceeds the specified value, the bimetal with the built-in temperature relay causes the contact to jump off due to deformation, and the motor stops running.  When the temperature in the motor falls below the specified value, the contacts are reset and the circuit is switched on again.

A dish-shaped bimetallic strip and a heater are connected in series between the two terminals of the external temperature current relay.  When the electric current in the motor is too large, the heating of the bimetal by the heater makes it in the dotted line position in the figure, the contacts jump off, and the motor stops rotating.

2. Lack of phase: A lack of phase in a three-phase motor will cause the motor to fail to start or overload.  In order to protect the motor from phase loss, overload relays are used. Overload relays are also used for normal three-phase motors to protect the motor when the current is too large.

Unbalanced voltage between phases leads to unbalanced current in three phases.  In the phase with the largest current, the proportion of temperature rise increase is about twice the square of the voltage imbalance proportion.  For example, a 3% voltage imbalance produces a temperature rise of about 18%.  The measures taken to protect the motor from phase-to-phase imbalance are the same as those taken when the phase is missing.

Fourth. Temperature Protection

1. Exhaust temperature: Excessive exhaust temperature will lead to refrigerant decomposition, aging of insulation materials, carbon deposition of lubricating oil, damage of air valves, and clogging of capillaries and dry filters.  The main protection method is to use temperature controller to sense the exhaust temperature. The temperature controller should be placed near the exhaust port. When the exhaust temperature is too high, the temperature controller will act to cut off the circuit.  If the exhaust temperature is too high due to hot gas bypass, the shutdown method should not be adopted, and spray cooling should be adopted.

2. Shell temperature: The shell temperature will affect the service life of the compressor.  Excessive casing temperature may be caused by insufficient heat exchange capacity of the condenser, so check whether the scenery or water quantity of the condenser and the water temperature are appropriate. If air or other non-condensable gases are mixed into the refrigeration system, the condensation pressure will rise and the casing will overheat.  If the suction temperature is too high, the casing will easily overheat. In addition, overheating of the motor will also overheat the casing.

The fundamental way to prevent the casing from overheating and protect the compressor is to correctly handle the above-mentioned problems and install a temperature protector on the casing.  The most commonly used casing temperature protector is placed at a suitable place on the casing. When the casing temperature is too high, the dish-shaped bimetallic strip will feel the temperature and deform, causing the contacts in the circuit to jump off and the compressor to stop.

3. Oil temperature protection: to prevent the excessive temperature of lubricating oil. During the operation of the compressor, sometimes even though the oil pressure difference is normal, the oil temperature may be too high and the lubricating performance may decline, causing friction parts such as bearing shells to burn out. Therefore, oil temperature protection should be controlled.

4. Water cut-off protection of cooling water: If the cooling water in the compressor jacket is cut off, it will cause the exhaust temperature of the compressor to rise and, in serious cases, the cylinder will deform.  A pair of electrical contacts are installed on the water outlet pipe of the compressor cooling water jacket. When there is water flow, the electrical contacts are conducted by water, and the relay sends out a signal to enable the compressor to be in a starting or normal operation state.  If the water flow is interrupted and the electrical contact of the relay is disconnected, the compressor cannot be started or shut down accidentally.  However, there are often bubbles in the water flow, which will cause misoperation. moreover, water jacket water cut-off will not cause an accident immediately. therefore, the relay should be delayed for 15~30s.

The above-mentioned protection devices are not all protection and control contents. In addition, there are liquid level protection control and medium pressure protection control (to prevent the exhaust pressure of the low pressure stage of the two-stage compression system from being too high), etc. These protection measures may not be all adopted in actual projects, and shall be determined according to specific conditions.

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