There are various types of compressors used in the heat pump unit and the fully enclosed piston compressor is the most commonly used type. However, when it’s in winter, there most probably are faults of this type of compressor due to frequent heating operations. We have found that the compressor faults could be three aspects as follows.
Figure 1: There are heat pump compressors making up the unit.
The oil level of the compressor is normal. The bearing, crankshaft, and connecting rod of the compressor are all in good condition, but the suction and discharge valve plates are broken.
The refrigerant oil of the compressor is dark and the upper and lower bearing sleeves fall off or are worn. The connecting rod is broken, the friction surface between the crankshaft, and the bearing and that between the crankshaft and the connecting rod have galling marks. There are wear marks on the motor rotor, and the suction and discharge valve plates are in good condition.
The insulation of the compressor to the ground is 0. But the bearing, crankshaft, and connecting rod of the compressor are normal.
Figure 2: The compressor burnout cause this part to be blackened.
It can be seen from the faults that the water-side system rupture of the unit causes the water to enter the compressor and forms a liquid strike, leading to the valve broken.
* Case of Water-Side System Rupture
When the unit operates in refrigeration, the water-side system is cut off. Because some users short-circuit the flow switch without permission, the unit can not protect it. The water in the heat exchanger (especially the liquid overfeed heat exchanger) freezes, which causes the heat exchange copper tube to freeze and crack so that the fluorine enters the water, and the water enters the compressor to form a liquid strike, causing damage.
In winter, when the unit does not operate, the user has discharged the frozen water inside the heat exchanger or fails to take the corresponding anti-freezing measures following the specifications. And the water enters the compressor to cause damage like in the first case.
It could be concluded that there was enough oil inside the compressor, but the poor lubrication caused the failure. The poor lubrication is due to the quality change of the lubricating oil. That is, the oil is diluted or the liquid refrigerant raises the oil level.
* Reasons for Liquid Return
In the refrigeration cycle, the refrigerant usually accumulates in the lowest-temperature part for condensation. If the unit has been shut down for a long time, the heat capacity of the compressor is larger than that of the condenser, evaporator, and liquid reservoir, so the compressor becomes the lowest-temperature part in the refrigeration cycle, making the refrigerant enter.
Because the lubricating oil can dissolve the refrigerant well, the refrigerant accumulated in the compressor dissolves in the lubricating oil, which is called dissolving. The amount of dissolved refrigerant varies according to the refrigerant charge, the structure of the refrigeration cycle, and the length of shutdown time. When saturated, it is about 30% to 100% of the lubricating oil. The diluted oil will cause poor lubrication and axle holding.
Figure 3: The refrigeration cycle, including condenser, expansion valve, evaporator, of a compressor.
Moreover, if the compressor stops for a long time, the lubricant will be separated according to the temperature of the compressor enclosure, the type of refrigerant, and lubricant. The lower part is refrigerant liquid (more refrigerant, less mixture of refrigerant and lubricant), and the upper part is a lubricant (more lubricant, less mixture of lubricant and refrigerant). If the compressor is started in such a state, the oil supplied to the bearing and other moving parts is almost the lubricant of only liquid refrigerant. Therefore, in a short time after starting, the bearing part, connecting rod, and other parts will be stuck and worn.
If the compressor is not fully preheated before starting, or the power of the electric heater of the crankcase is not enough, the above situation can not be avoided, resulting in the damage of the compressor.
When the unit operates as a heat engine, especially in high-humidity environments, it is easy to form frost. If the defrosting can not be timely or the defrosting is not complete, the low pressure will be lower, and the compressor will return a large amount of liquid, causing the compressor failure.
As mentioned above, the return fluid is the main factor of the shaft holding that leads to the bearing eccentricity, wearing the motor stator and causing the motor to short circuit and burnt. In fact, the cylinder structure of the fully enclosed piston compressor determines that it is not sensitive to the liquid strike. Even if some liquid refrigerant enters the compressor, it will not directly cause the valve plates to break or cause the motor to burn down.
At the same time, because the fully enclosed compressor mostly adopts centrifugal splash lubrication and there is no control of pressure difference, the compressor can also operate in the case of lack of oil lubrication.
At this time, the current keeps rising until the air switch (over-current protector) jumps. This process is due to the overload of the compressor, the current is large, and the temperature rise of the motor coil is very fast, until the embedded PTC acts.
The PTC temperature rise rate of the compressor is very sensitive under the condition of full load or overload and the air switch acts before PTC, so there is no basis that the lack of oil directly causes the motor to burn.
Figure 4: The compressor motor is burned.
* Reasons for the Pure Motor Burnout
The temperature of the motor is extremely high. The motor of the fully enclosed compressor is cooled by return air. The working condition is relatively bad in winter. Especially when the ambient temperature is low, the heat exchange is small, causing small refrigerant circulation, and low return pressure. And the defrosting on the electric control is not timely and incomplete, which will lead to insufficient motor cooling and heated coil.
This kind of continuous heating will be followed by high temperature, and PTC is not sensitive to the low current response under low load. Therefore, after the compressor starts several times, insulation damage will be caused by overheating and the motor will be burnt when the temperature is not reached.
There are impurities inside the refrigeration system, and the impurities corrode and wear the motor coil, causing short circuit burning.
There is supposed to be an anti-freezing device.
When it’s the shutdown state and the ambient temperature is lower than a certain value, the water pump and electric heating device should operate to avoid the freeze of the water system. At the same time, there should be a drainage device to drain the water from the water exchanger to prevent it from freezing if the unit is spare for a long time.
Figure 5: Antifreeze thermostat.
To ensure the normal operation of the unit, you cannot short-circuit the flow switch and various protection switches without permission. Observe the operation of the unit. When the temperature difference of the water between the inlet and outlet is becoming large, you should check the water pump, the water flow, and the water filter.
In the electric control program, ensure the heating time of the compressor crankcase heater before starting and that the compressor can be fully preheated to prevent damage.
Improve the defrosting process, ensure timely and thorough defrosting, improve the reliability of electric control, and prevent misoperation or inaction.
Perfect the system design, especially in the low-temperature heating condition. It should be reasonable to match the expansion valve and gas-liquid separator or take measures such as adding high and low-pressure bypass to prevent the liquid return problem of the unit.
Improve the process, strengthen management and enhance quality awareness. Ensure that the refrigeration system is clean, and there is no moisture. Manufacturing and processing quality is an important factor affecting the quality of the unit.
Strengthen the training of user's use, operation, and maintenance.