11 common protection devices for refrigeration compressors When the compressor is running, some abnormal conditions may occur, such as: exhaust pressure is too high, suction pressure is too low, oil pressure is insufficient, motor is overheated, excess liquid enters the cylinder, etc. In the event of an abnormal situation, the compressor will be damaged if there is no protective measure. The protective measures taken on compressors can be divided into four categories: Prevent liquid shock: Pressure protection: Built-in motor protection: Temperature protection. 1. Prevent liquid shock When too much liquid enters the cylinder to be discharged from the exhaust valve, liquid hammer will appear in the cylinder. The high pressure generated during the liquid hammer will cause the cylinder, piston, connecting rod..., etc. Damage, so a series of protective measures need to be taken. 1. Dummy cover: The air valve assembly is pressed tightly on the end of the cylinder with a spring to form a dummy cover. When the pressure in the cylinder is too high, the exhaust valve is lifted, the liquid is discharged, and the pressure in the cylinder drops rapidly. Dummy cover is used for hermetic compressor. The lift limiter of the strip exhaust reed valve is pressed on the valve plate by a pressure release spring. When the pressure in the cylinder rises due to the hydraulic shock, the lift limiter is lifted, 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 run.
2. Oil heater: The lubricating oil in the crankcase has refrigerant dissolved in it, and the dissolved amount increases when the ambient temperature is low. When the compressor starts, the pressure in the crankcase suddenly drops, a large amount of refrigerant vaporizes, and the lubricating oil is foamed and sucked into the cylinder, causing liquid shock. Using an oil heater to heat the lubricating oil before starting to reduce 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 the 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 volume is stored at the bottom of the separator, the liquid refrigerant in it is heated and vaporized into the upper part of the outlet pipe, and the lubricating oil that cannot be vaporized flows into the outlet pipe from the return hole and then enters the compressor. 2. Pressure protection 1.Control of suction and discharge pressure: When the compressor is running, due to the system or the compressor itself, the discharge pressure may be too high or the suction pressure may be too low. For this reason, it is necessary to control the suction and discharge pressure. The common controller high and low pressure pressure controller consists of a high pressure control part and a low pressure control part. When the discharge pressure exceeds a given value, the high-pressure control part will act to cut off the compressor power supply and stop it; when the suction pressure is lower than the given value, the low-pressure control part will act and cut off the compressor power supply and stop it.
2. Oil pressure difference controller: During the operation of the oil pressure protection refrigeration compressor, its moving parts need to be lubricated and cooled with a certain pressure of lubricating oil. 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 should 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 will act to cut off the compressor power and the motor will stop rotating. Three, built-in motor protection 1. Overheating: The internal temperature of a well-designed motor that runs under specified conditions will not exceed the allowable value, but when the motor is running at too high or too low a voltage, or in a high-temperature environment, the internal temperature of the motor exceeds the allowable value , During frequent starting, the temperature will be too high due to excessive starting current. In order to overheat the motor, in addition to correct use and maintenance, an overheating relay can also be installed. The overheat 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 bimetal with the built-in temperature relay deforms and causes the contacts to trip and the motor stops. When the temperature in the motor drops below the specified value, the contact is reset and the circuit is reconnected.
A disc-shaped bimetallic piece and a heater are connected in series between the two terminals of the external temperature and current relay. When the current in the motor is too large, the heating of the bimetallic sheet by the heater makes it at the position of the dotted line in the figure, the contact jumps and the motor stops rotating. 2.Phase loss: Phase loss of a three-phase motor will cause the motor to fail to start or overload. In order to protect the motor from being damaged by the lack of phase, an overload relay is used. The overload relay is also used in a normal three-phase motor to protect the motor when the current is too large.
Unbalanced voltage between phases causes unbalanced current in three phases. In the phase with the largest current, the increase in temperature rise is approximately twice the square of the voltage imbalance ratio. For example: 3% voltage imbalance produces about 18% temperature rise. The measures to protect the motor from being unbalanced between phases are the same as those taken when the phase is missing. Four, temperature protection 1.Exhaust temperature: Excessive exhaust temperature leads to decomposition of refrigerant, aging of insulating materials, carbonization of lubricating oil, damage to air valves, and clogging of capillary tubes and drying filters. The main protection method is to use a thermostat to sense the exhaust temperature. The thermostat should be placed close to the exhaust port. When the exhaust temperature is too high, the thermostat will act and cut off the circuit. If the exhaust temperature is too high due to the hot gas bypass, the shutdown method should not be used, and the spray cooling should be used.
2. Housing temperature: The housing temperature will affect the life of the compressor. Excessive temperature of the cabinet may be caused by insufficient heat exchange capacity of the condenser. Therefore, check the scenery or water volume of the condenser and whether the water temperature is appropriate. If air or other non-condensable gases are mixed in the refrigeration system, the condensation pressure will increase. Overheating; if the suction temperature is too high, the casing is prone to overheating. In addition, overheating of the motor will also overheat the casing. To avoid overheating of the cabinet, the fundamental way to protect the compressor is to correctly handle the above-mentioned problems and install a temperature protector on the cabinet. The most commonly used case temperature protector, place it in a suitable place on the case. When the case temperature is too high, the disc-shaped bimetallic plate will feel the temperature and deform, causing the contacts in the circuit to open and the compressor to stop. . 3. Oil temperature protection: prevent the lubricating oil temperature from being too high. During the compressor operation process, sometimes although the oil pressure difference is normal, the lubrication performance may be reduced due to the high oil temperature, which may cause the friction parts such as the bearing bush to be burned. To control the oil temperature protection. 4. Cooling water cut-off protection: If the cooling water of the compressor water jacket is cut off, it will cause the compressor discharge temperature to rise, and in severe cases, the cylinder will deform. A pair of electrical contacts are installed on the outlet pipe of the compressor cooling water jacket. When water flows through, the electrical contacts are conducted by the water, and the relay sends a signal to make the compressor in a starting or normal operation state. If the water flow is interrupted, the electrical contact of the relay is disconnected, and the compressor cannot be started or stopped in an accident. But there are often bubbles in the water flow, which will cause malfunctions, and the water jacket will not cause an accident immediately, so the relay should be delayed to operate, generally the delay is 15~30s. The aforementioned protection devices are not all of the protection and control content. In addition, there are liquid level protection control, medium pressure protection control (to prevent the low pressure stage of the two-stage compression system from excessively high exhaust pressure), etc. These protection measures are not necessarily all in the actual project. The adoption should depend on the specific circumstances.
