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How to protect the refrigeration compressor, you must master the following knowledge

How to protect the refrigeration compressor, you must master the following knowledge! Why should the compressor be protected? ? When the compressor is running, some abnormal conditions may occur, such as: the discharge 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. If an abnormal situation occurs, the compressor will be damaged if there are no protective measures. The protective measures taken on the compressor can be divided into four categories: 1. Prevent liquid strike; 2. Pressure protection; 3. Protection of built-in motor; 4.Temperature protection. Prevent liquid strike When too much liquid enters the cylinder and it is too late to discharge it from the exhaust valve, a liquid blow will occur in the cylinder. The high pressure generated during the liquid blow damages the cylinder, piston, connecting rod, etc. Series of protective measures. 1. Fake cover Press the air valve assembly 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 and the liquid is discharged, and the pressure in the cylinder quickly decreases. 2. Oil heater The lubricating oil in the crankcase dissolves refrigerant, and the dissolved amount increases when the ambient temperature is low. When the compressor starts, the pressure in the crankcase suddenly decreases, a large amount of refrigerant vaporizes, and the lubricating oil is foamy and is 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 strikes. 3. Gas-liquid separator The gas-liquid separator shown 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 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. The lubricating oil that cannot be vaporized flows into the outlet pipe from the return hole and then enters the compressor. Pressure protection 1. Control of suction and exhaust pressure When the compressor is running, due to the reasons of the system or the compressor itself, the discharge pressure may be too high or the suction pressure may be too low. For this reason, the suction and discharge pressures need to be controlled. The common controller is not high and low pressure controller, it is composed of high pressure control part and low pressure control part. When the exhaust pressure exceeds a given value, the high-pressure control part acts to cut off the power of the compressor to stop it; when the suction pressure is lower than the given value, the low-pressure control part acts to cut off the power of the compressor to stop it. The compressor shuts down; it also emits an audible and visual alarm signal. 2. Safety valve To prevent refrigerant from leaking to the atmosphere, a closed safety valve is used. The upper side of the valve disc bears the exhaust pressure, and the lower side receives the suction pressure and the spring force of the spring. When the exhaust pressure is too high, the valve disc moves downward to open the lateral hole at the lower part of the valve seat, and the high-pressure gas flows into the suction chamber through this lateral hole and the lateral hole on the valve body. When the exhaust pressure is lower than the specified value, the valve disc moves upward under the suction pressure and the elastic force of the spring to close the exhaust pipe. The opening pressure of the safety valve is adjusted with bolts. 3. Safety film Install a safety diaphragm between the suction and exhaust chambers. When the difference between the suction and discharge pressure exceeds the specified value, the diaphragm ruptures and the discharge pressure decreases. The filter screen is used to catch the broken diaphragm to protect the compressor. 4. Lubricating oil pressure difference controller The difference between the oil pressure at the outlet of the oil pump and the oil pressure in the crankcase. In order to protect the good lubrication of the moving parts of the compressor and to ensure the normal operation of some compressor air volume control mechanisms (such as hydraulic-tie rod control mechanisms), the low-pressure inlet and high-pressure outlet on the controller must be connected. 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 operated at an excessively high or low voltage, or when operating in a high-temperature environment, the internal temperature of the motor exceeds the allowable value and is frequently started When the starting current is too high, the temperature will be too high. In order to prevent the motor from overheating, in addition to proper use and maintenance, an overheating relay can also be installed. The overheat relay can be installed inside the winding, called the built-in temperature relay, or installed outside the motor, called the external temperature-current relay. When the internal temperature of the motor exceeds the specified value, the bimetal piece of the built-in temperature relay deforms to cause the contacts to trip, and the motor stops. When the temperature in the motor drops below the specified value, the contacts are reset and the circuit is reconnected. External temperature-current relay: a disc-shaped bimetal and a heater are connected in series between the two terminals. When the current in the motor is too large, the heating of the bimetallic sheet by the heater makes it in the dotted line position in the figure, the contact jumps, and the motor stops rotating. The external temperature-current relay protects the motor so that it will not overheat due to excessive current. However, in some occasions, the overheating of the motor is not due to excessive current, but due to insufficient low-temperature gas flowing through the motor. For example: because the refrigeration system is blocked, the amount of refrigerant circulation is small, the motor cooling is extremely poor, resulting in internal overheating, built-in The temperature relay plays the role of protecting the motor at this time. 2. Phase loss The 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 damage caused by lack of phase, an overload relay is used), which is composed of a mechanical moving part and an electromagnetic switch part. The mechanical movement part has four terminals, two on the top and two on the bottom. A heater is installed between the upper and lower terminals. When the phase is missing, the winding overload current of the other phases overheats the heater, the bimetallic disc is heated and deformed, and the pressure plate is pushed, so that the overload relay contact on the electromagnetic switch jumps, there is no current in the excitation coil, and the magnetic contactor is no longer closed The motor stops. The overload relay is also used for normal three-phase motors to protect the motor when the current is too large. 3. Unbalance between phases Unbalanced voltage between phases leads to unbalanced current in three phases. In the phase with the largest current, the rate of increase in temperature rise is about 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 unbalanced phases are the same as those taken in the absence of phases. Temperature protection The temperature mentioned here refers to the exhaust temperature and the case temperature. 1. Exhaust temperature Excessive exhaust gas temperature leads to decomposition of refrigerant, aging of insulating materials, carbonization of lubricating oil, damage to air valve, and clogging of capillary tube and filter drier. The 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 to cut off the circuit. If the exhaust gas temperature is too high due to hot gas bypass, the method of shutdown should not be used, and spray cooling should be used. 2. Chassis temperature The temperature of the casing will affect the life of the compressor. If the temperature of the cabinet is too high, it may be caused by the insufficient heat exchange capacity of the condenser. Therefore, you should check the landscape or water volume of the condenser. Overheating; the suction temperature is too high, the chassis is easy to overheat, in addition, the motor will overheat the chassis too hot. The fundamental way to avoid overheating of the casing and to protect the compressor is to properly deal with the above-mentioned problems, and at the same time install a temperature protector on the casing. The external temperature-current protector of the most commonly used case temperature protector. Place it in a suitable place on the casing. When the temperature of the casing is too high, the disc-shaped bimetallic sheet deforms due to the temperature, causing the contacts in the circuit to trip and the compressor to stop.

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