The damage of air-conditioning refrigeration compressor and its preventive measures
An 82000DWT bulk carrier of a shipping company broke down on the 7th day after leaving the factory. The crew described that when it started, the air-conditioning refrigeration compressor could not run continuously, and a large current occurred and an alarm was triggered. After opening the inspection, it was found that the piston and oil pump were all Has been damaged. See Figure 1 below.
After receiving the insurance policy, the shipyard contacted the manufacturer and immediately adjusted a new compressor to the port of destination far away in South Africa to solve the shipowner’s difficulties in time. However, there is a big dispute about the responsibility of the accident. The shipowner thinks it is a product quality problem, while the manufacturer’s service engineer thinks that the crew has made a mistake in operation, there is a large current and the crankcase is short of oil, and the shipowner thinks that if it is short of oil, why not Low fuel level alarm and stop?
The author has dealt with five or six such similar problems. Figures 2 and 3 show the wear and tear of parts and components of another large container ship's refrigeration compressor. Why does it fail so frequently? A refrigeration compressor costs five or six dollars, plus freight and tariffs and other costs are even higher. The author thinks it is necessary to attract everyone's attention.
nvestigation and Analysis
1.1 Analysis on the phenomenon of air-conditioning compressor liquid hammer
To analyze the damage, start with the working principle of refrigeration. The gaseous refrigerant is compressed by the refrigeration compressor to be a high-temperature and high-pressure gas. It becomes liquid after being cooled by the cooler, and then is throttled by the thermal expansion valve to evaporate and absorb heat in the evaporator (cold storage), so that the food in the cold storage is cooled to achieve For the purpose of food refrigeration, the refrigerant evaporates and absorbs heat and becomes a gas, which is sucked into the compressor for compression again. However, if the liquid refrigerant does not completely evaporate into gas, some of the liquid refrigerant will be sucked into the compressor cylinder, which will flush out the lubricating oil on the surface of the piston and cylinder, which will cause the cylinder to strain and overheat. Some small metal particles will contaminate the entire interior of the machine. These liquid refrigerants also dilute the lubricating oil in the crankcase of the compressor. As the lubricating oil in the compressor is diluted by more and more liquid refrigerants, the lubrication performance gradually decreases, and the moving parts such as crankshaft bearings and pistons cannot be lubricated normally. , Until these moving parts dry friction and bite. The bearing’s financialization is on the crankshaft (as shown in Figure 4), while the motor continues to run, and finally the main bearing, connecting rod, and piston may be broken (as shown in Figure 4 and Figure 4), and a large amount of debris falls on Inside the compressor oil pan. Due to the vaporization of the liquid refrigerant in the lubricating oil, the crankcase oil of the compressor has an artificially high oil level due to foam (as shown in Figure 7). The liquid level of the oil tank has not fallen below the alarm value, so the lubricating oil has not been triggered. Low-pressure alarm, the compressor continues to run in the absence of effective lubrication until it is blocked and damaged, and then develops to the motor generates a large current alarm and shuts down, and causes the motor to be overloaded and damaged or burned. If too much liquid enters the compressor directly, it may even cause damage to the compressor, because the liquid is incompressible. This situation is the liquid hammer phenomenon of the refrigeration compressor.
1.2 Adjustment of thermal expansion valve of air conditioning system
The air-conditioning compressor liquid hammer phenomenon usually occurs at night. At this time, the external environment temperature drops and the refrigeration task of the entire refrigeration device is relatively low, that is, when so much refrigeration capacity is not required, the equipment has an excess refrigeration capacity. When the thermal expansion valve is opened too large, or whenever the compressor is started, or the equipment is subjected to a large amount of load, the thermal expansion valve will open, which will input more liquid refrigerant to the evaporator. In addition, the superheat setting If the setting is too low, the pressure drop on the suction side causes the pressure under the thermal expansion valve diaphragm to drop. The pressure drop on the suction side reacts faster than the pressure drop in the expansion valve bulb, so the opening of the thermal expansion valve is increased at first. Until the temperature sensor feels the reaction and the pressure begins to drop to prompt the thermal expansion valve to close (as shown in Figure 8), but at this time, part of the refrigerant liquid has entered the evaporator (the greater the valve opening, the greater the amount of liquid) Once the liquid enters the suction pipe, it then enters the compressor. Therefore, under various operating conditions, it is necessary to monitor the superheat setting of the thermal expansion valve and maintain a stable superheat setting to avoid such problems. Selecting a valve with a smaller orifice can play a stable control role.
caused
Picture 3: Thermal expansion valve
The pressure below the diaphragm decreases → the valve opens up → the liquid volume increases → the pressure of the temperature sensing bulb decreases → the valve closes
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Induction and summary
There are many reasons for liquid shock, such as: 1. Low evaporator load; 2. Overcapacity of refrigeration equipment; 3. Arrangement in cold storage; 4. Poor air circulation; 5. Unreasonable food stacking; 6. Failure of evaporator fan 7. Oil pollution in the evaporator tube; 8. Poor evaporator defrosting (the evaporator is frozen, there is no air convection, and the heat conduction is poor; 9. The thermal expansion valve: the size is too large, or the model is wrong, the balance pipe is blocked, and the temperature is sensed. The package position is wrong, the superheat setting is too low, etc.
Measures to prevent liquid shock: 1. The correct size of the thermal expansion valve must be selected; 2. The superheat must be set at least 6-8 K; 3. If necessary, check and set the defrost control
The main reasons for compressor damage: 1. The refrigerant liquid returns; 2. The liquid starts; 3. The liquid is blocked; 4. The compressor is overheated; 5. The lack of lubrication.
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Management considerations
To prevent damage to the compressor, it is necessary to strengthen daily maintenance, the setting temperature should not be too low, and the superheat setting must be properly stable to prevent the occurrence of the above-mentioned liquid hammer problem. Once a fault alarm or shutdown occurs, the cause must be found and the fault removed The root cause of the problem, do not start again blindly.
