When the compressor is running, some abnormal situations may occur, 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. When an abnormal situation occurs, if there is no protective measure, the compressor will be damaged.
The protective measures taken for the compressor can be divided into four categories:
Preventing liquid hammer;
Pressure protection;
Protection of built-in motor;
Temperature protection.
1. Preventing liquid hammer
When too much liquid enters the cylinder and there is no time to discharge it from the exhaust valve, liquid hammer will occur in the cylinder. The high pressure generated by liquid hammer will damage the cylinder, piston, connecting rod, etc., so a series of protective measures need to be taken.
1. False cover: The air valve assembly is pressed against the end of the cylinder with a spring to form a false cover. When the pressure in the cylinder is too high, the exhaust valve is lifted up, the liquid leaks out, and the pressure in the cylinder drops rapidly. The false cover is used for fully enclosed compressors. The lift limiter of the strip exhaust reed valve is pressed on the valve plate with a pressure relief spring. When the pressure in the cylinder increases due to liquid hammer, the lift limiter is lifted up, causing the pressure in the cylinder to drop. After the liquid hammer disappears, the pressure relief spring presses the lift limiter on the valve plate, and the compressor continues to operate.
2. Oil heater: The lubricating oil in the crankcase contains refrigerant, and the amount dissolved increases when the ambient temperature is low. When the compressor starts, the pressure in the crankcase suddenly decreases, a large amount of refrigerant vaporizes, the lubricating oil is foamy and sucked into the cylinder, causing liquid hammer. Using an oil heater to heat the lubricating oil before starting and reducing the amount of refrigerant dissolved in the lubricating oil is an effective measure to avoid liquid hammer.
3. Gas-liquid separator: The gas-liquid separator is also called a 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, and the liquid refrigerant in it is heated and vaporized and enters the upper part of the outlet pipe. The lubricating oil that cannot be vaporized flows from the reflux hole into the outlet pipe and then enters the compressor.
2. Pressure protection
1. Suction and exhaust pressure controller: The suction and exhaust pressure control in the cold storage unit mainly relies on pressure controllers (also called pressure switches). These controllers monitor and adjust the system pressure to ensure the efficient and safe operation of the refrigeration system. When the exhaust pressure exceeds the preset maximum safety value, the controller activates a switch, which usually cuts off the power supply of the compressor and stops the compressor to prevent equipment damage or safety accidents caused by excessive pressure. When the pressure drops to a safe range, the controller restarts the compressor.
2. Oil pressure differential controller: The cold storage oil pressure differential controller is an important device specially used to monitor and control the oil pressure of the compressor in the refrigeration system. It ensures that the lubricating oil circulation of the compressor is in a normal state and avoids equipment damage caused by insufficient oil pressure. The oil pressure differential controller mainly focuses on the pressure difference between the oil pump supply pressure and the suction pressure in the crankcase to determine whether the oil circulation is normal.
III. Built-in motor protection
1. Overheating: The internal temperature of a well-designed motor operating under specified conditions will not exceed the allowable value. However, when the motor is operated at too high or too low voltage, or in a high temperature environment, the internal temperature of the motor exceeds the allowable value. When the motor is started frequently, the temperature will be too high due to excessive starting current.
In order to prevent the motor from overheating, in addition to correct use and maintenance, an overheating relay can also be installed. The overheating relay can be installed inside the winding, called a built-in temperature relay, or installed outside the motor, called an external temperature protector. When the internal temperature of the motor exceeds the specified value, the bimetallic strip of the built-in temperature relay causes the contact to jump due to deformation, and the motor stops running. When the temperature inside the motor drops below the specified value, the contact is reset and the circuit is reconnected.
A disc-shaped bimetallic strip and a heater are connected in series between the two terminals of the external temperature current relay. When the current in the motor is too large, the heating of the bimetallic strip by the heater causes it to be in the dotted position in the figure, the contact jumps, and the motor stops rotating.
2. Phase loss: A phase loss in a three-phase motor will cause the motor to fail to start or be overloaded. To protect the motor from damage caused by phase loss, an overload relay is used. The overload relay is 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 temperature rise increases by about twice the square of the voltage imbalance ratio. For example: a 3% voltage imbalance produces a temperature rise of about 18%. The measures to protect the motor from phase imbalance are the same as those taken when there is a phase loss.
4. Temperature protection
1. Exhaust temperature: The exhaust temperature protection of the cold storage compressor is an important safety measure to prevent equipment damage and reduced system efficiency caused by excessive exhaust temperature. The exhaust temperature is usually monitored by a thermostat or temperature sensor installed near the exhaust port. These devices can sense temperature changes in real time and take action when the preset high temperature threshold is reached. : When the exhaust temperature exceeds the safety setting value, the thermostat will immediately act to shut down the compressor by cutting off the power supply to the compressor or sending a signal to the control system, thereby preventing the temperature from rising further.
2. Casing temperature: The casing temperature will affect the life of the compressor. Excessive casing temperature may be caused by insufficient heat exchange capacity of the condenser, so the scenery or water volume of the condenser should be checked, whether the water temperature is appropriate, and air or other non-condensable gases are mixed in the refrigeration system. The condensation pressure will rise and the casing will overheat; if the suction temperature is too high, the casing is easy to overheat. In addition, overheating of the motor will also cause the casing to overheat.
The fundamental way to avoid casing 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 in a suitable place on the casing. When the casing temperature is too high, the disc-shaped bimetallic strip senses the temperature and deforms, causing the contacts in the circuit to jump open and the compressor to stop.
3. Oil temperature protection: The oil temperature protection of the cold storage compressor is an important measure to ensure the stable operation of the system and extend the life of the compressor. In order to prevent the oil temperature from being too high, the system is usually equipped with an oil temperature protection device. When the oil temperature is detected to be abnormally high, this device will trigger an alarm or directly put the compressor into a shutdown protection state to avoid lubrication failure and equipment damage caused by excessive oil temperature. : The high temperature of the lubricating oil will cause its viscosity to decrease, the lubrication effect to weaken, and increase the wear of the moving parts inside the compressor such as bearings. In severe cases, it can cause bearing damage and compressor failure.
4. Cooling water cut-off protection: If the cooling water of the compressor water jacket is cut off, it will cause the compressor exhaust temperature to rise, and in severe cases, the cylinder will be deformed. A pair of electrical contacts are installed on the water outlet pipe of the compressor cooling water jacket. When water flows through, the electrical contacts are connected by water, and the relay sends a signal to make the compressor in a state where it can be started or operate normally. If the water flow is interrupted, the relay contacts are disconnected, and the compressor cannot start or shuts down due to an accident. However, there are often bubbles in the water flow, which can cause false operation, and the water jacket water cut-off will not cause an accident immediately, so the relay should be delayed, generally for 15~30s.
The above-mentioned protection devices are not all protection control contents. In addition, there are liquid level protection control, medium pressure protection control (to prevent the low-pressure stage exhaust pressure of the two-stage compression system from being too high), etc. These protection measures may not be fully adopted in actual projects, and should be determined according to specific circumstances.
