Control strategy of oil return operation
The air source heat pump system should adopt a proper control strategy of oil return operation to bring the lubricant oil accumulated in system components such as heat exchangers and pipes back to the compressor.
1. Basic requirements for oil return operation
There is an amount of two-phase refrigerant or subcooled liquid in the condenser, evaporator and the connecting pipe between the condenser and evaporator; at this point the lubricant oil and liquid refrigerant are mutually soluble, so that the lubricant oil flows with the liquid refrigerant. However, in the refrigerant superheated region of the suction pipe, discharge pipe, condenser and evaporator, lubricant oil adheres to the inner wall of the pipe and has a high viscosity, making it difficult to flow with gas refrigerant.
When the interfacial friction force between the gas refrigerant and the lubricant oil is greater than the maximum static friction force between the lubricant oil and the inner wall of the pipe, the lubricant oil can flow with the gas refrigerant under the action of the gas-liquid shear stress, and the gas- liquid interfacial shear stress between the gas refrigerant and the lubricant oil is related to the flow speed of the gas refrigerant. It is generally believed that the lubricant oil can flow with the gas refrigerant when the flow speed of the gas refrigerant in the horizontal tube reaches 5m/s and the flow speed of the gas refrigerant in the vertical tube reaches lOm/s.
Reducing the superheat of the refrigerant at the evaporator outlet is beneficial to the oil return in the refrigerant superheated region in the evaporator.
2. Oil return cycle
The oil return cycle is related to the operating frequency of the compressor, the length of the connecting pipe, the height difference between indoor and outdoor units, the efficiency of the oil separator, the type of heat pump and other factors.
The connecting pipe of the minitype air source heat pump is generally short and oil return is relatively easy. However, for systems with large heating capacity and long connecting pipe, such as variable refrigerant flow (VRF) multi-split heat pump, the mass flow rate of the refrigerant at the lowest operating frequency of the compressor is small, and the flow speed of the refrigerant in the system is low, so it is almost impossible to return oil. On the other hand, the air source heat pump system has a low oil discharge rate at low frequency operation and the unit with large capacity is usually equipped with an oil separator, so that the oil discharge rate through the oil separator is even lower. Therefore, the oil return cycle of the heat pump system needs to be designed according to the operating frequency of the worst oil return situation.
The cycle of oil return operation depends on the reduction speed of the lubricant oil of the compressor. There are different oil return cycles for different systems.
During normal operation, when the compressor frequency is low, the flow speed of the refrigerant is low so that it is difficult to return oil. The lubricant oil is gradually accumulated to the low-pressure side until a balance between oil discharge and oil return of the compressor is formed. The lower the operating frequency of the compressor and the efficiency of the oil separator, the shorter the time to reach balance, and the easier it is to cause the compressor running with little lubricant oil. Therefore, it is necessary to start the oil return operation before the compressor is short of oil.
In summary, the oil return cycle is determined by the refrigerant flow speed, the oil discharge rate of the compressor, the efficiency of the oil separator and the system experiment.
3. Oil return runtime
The oil return runtime is the duration of the oil return process. The control system must ensure that the sufficient lubricant oil return to the compressor through the oil return operation. The cycle time of the refrigerant in the VRF multi-split heat pump along the system is related to the length of the connecting pipe and the flow speed of the refrigerant, while the cycle time of lubricant oil is relatively long, which is a very important parameter for designing a reasonable oil return runtime.
For the VRF multi-split heat pump, the oil return of the cooling operation is relatively difficult, so the cooling operation of the system is taken as an example. During the oil return process, lubricant oil stored in the indoor side needs to be brought back into the compressor through the flow of refrigerant . When performing oil return control, the electronic expansion valves of all indoor units will be turned on. For the indoor units that are not turned on by the user, the electronic expansion valves are also forcibly turned on, but the indoor fans are not turned on. The electronic expansion valves of the off-duty indoor units forcibly are turned on directly affects the oil return runt ime. If the oil return runtime is too short, it will be difficult for the lubricant oil to return to the compressor in time, but if the time is too long, the compressor will suck more liquid refrigerant, which will affect the reliability of the compressor.
The selection of the operating frequency of the compressor for oil return must ensure that the minimum flow speed of the refrigerant mentioned above is met. In practice, a reasonable margin should be considered to ensure the oil return effect, and the compressor operating frequency for oil return should be increased as much as possible. The higher the compressor operating frequency, the better the oil return effect is and the lower the suction pressure of the system is. For the VRF multi-split heat pump, the higher the compressor operating frequency for oil return, the more liquid refrigerant is sucked into the suction side when only some of indoor units operate. Therefore, in the process of product design, it is necessary to determine the amount of liquid refrigerant flooding back that can be taken in the system according to the effective volume of the gas-liquid separator. The compressor operating frequency for oil return should be considered according to the test conditions, and the oil return runtime should be appropriately extended.
- 4. Other problems in oil return process
- (1) Requirements for pressure difference
The pressure difference between high pressure and low pressure in the oil return process should be kept to a certain value to overcome the flow resistance and to meet the minimum flow speed of the refrigerant for oil return, which is considered from many aspects such as the compressor displacement, the compressor load and the size of the gas returning pipe of the unit. The larger the size of the gas returning pipe, the greater the required compressor load. However, if the displacement or load is too large, other problems will arise: first, too much liquid returned back into the compressor during the oil return period will have the lubricant diluted, so that the lubrication among the moving parts inside the compressor will be deteriorated, which easily leads to the compressor operation failure during the oil return period. Meanwhile, it is not conducive to leaving the returned lubricant oil inside the compressor; second, the suction pressure is too low and the pressure ratio increases during the system oil return. These problems can affect the normal operation of the compressor.
(2) Discharge temperature control
The phenomenon of too high a discharge temperature and too much liquid refrigerant flooding back should be avoided in the oil return process. If the discharge temperature is too high, the viscosity of the lubricant oil will decrease and the lubrication will be deteriorated, and the thickness of the oil film will also decrease. Too much liquid refrigerant flooding back leads to an increase in the oil discharge rate inside the compressor, and the liquid refrigerant boiling and vaporizing in the compressor will have more lubricant oil be discharged from the compressor.
If the discharge temperature is too high, the opening of the electronic expansion valve of the indoor unit will need to be adjusted, which easily leads to liquid refrigerant flooding back. Therefore, the compressor operating frequency for oil return and the opening of the electronic expansion valve need to be considered comprehensively in the design of the control logic.
