1. Ejector oil return technology
The ejector is a fluid mechanical device that uses a high-pressure and high-speed driving flow (primary flow) to eject and suck another fluid (secondary flow). In the ejector oil return system, high-pressure refrigerant vapor is drawn from the exhaust side of the compressor into the ejector, and its special structure is used to suck the mixed liquid rich in lubricating oil and liquid refrigerant from the appropriate position of the evaporator, and then mixed into the compressor or suction pipe. The power source of the ejector oil return comes from the suction effect generated by the pressure difference between the exhaust pressure and the suction pressure, so there is no need to raise the evaporator position.
For chillers using ejector oil return, solenoid valves and angle valves can be set in the power source pipeline to adjust the required oil return volume by controlling the primary flow rate. At the same time, a dry filter should be set on the oil extraction pipeline of the evaporator to prevent impurities from entering the compressor, and a sight glass should be set to observe the oil return status. The power source of the ejector oil return is not limited to the high-pressure exhaust of the compressor. It can also use the high-pressure liquid refrigerant at the bottom of the condenser, the high-pressure lubricating oil at the bottom of the primary oil separator, and even the suction as the ejector power source. The specific connection method is slightly different.
2. Direct oil return technology
Direct oil return is a simpler oil return method. It does not require additional driving force, but allows the foam of the refrigerant and lubricating oil to be directly sucked into the compressor after treatment. Since the compressor will inhale too much foam, it will cause liquid compression problems, so the control of the return oil volume is very important.
The refrigerant flow control methods that match the direct oil return technology include throttling orifice plates and mixed throttling. Regardless of the method used, the refrigerant charge and the relative position of the unit condenser and evaporator are very important. Taking mixed throttling as an example, an electronic expansion valve is added in addition to the throttling orifice plate to directly detect the exhaust temperature of the compressor. When the compressor inhales too much liquid refrigerant, the exhaust temperature will drop, indicating that the liquid level is too high and the refrigerant is in oversupply; conversely, it indicates that the liquid level has dropped and the liquid supply of the evaporator should be increased. This monitoring system further enhances the reliability of the direct oil return system. Compared with the first two methods, the direct oil return method avoids the problem of wasting liquid refrigerant and consuming high-pressure refrigerant energy, and the exhaust gas of the compressor can be fully used for refrigeration. If supplemented with an intermediate air supply port and a good heat exchanger design, the performance of the unit will be significantly improved.
3. Gravity oil return
The core of gravity oil return is to lead the liquid refrigerant rich in lubricating oil from the appropriate position of the evaporator by lifting the position of the evaporator. With the help of the natural gravity formed by the height difference, the oil-rich refrigerant flows smoothly into the oil return heat exchanger and exchanges heat with the high-temperature liquid refrigerant from the condenser. This process not only improves the supercooling of the liquid refrigerant and enhances the refrigeration capacity of the unit, but also successfully evaporates the liquid part of the oil-rich liquid refrigerant into gaseous state, and then enters the compressor.
From the perspective of refrigerant flow control, the key to the success of the gravity oil return system lies in the selection of the oil extraction position in the evaporator and the liquid level control in actual operation. The oil extraction position directly affects the oil return effect, and whether the liquid level can adapt to the oil extraction position is the factor that determines the success or failure of the oil return. Therefore, accurate control of the liquid level (i.e., refrigerant flow) is particularly important. The main refrigerant flow control methods that match the gravity return oil system include high-pressure or low-pressure float valves, and electronic expansion valves that use condenser or evaporator liquid level sensors as control signals.
