Common practice of oil return design in refrigeration system

Common practice of oil return design in refrigeration system

When the condenser is installed higher than the compressor, a U-shaped bend is required at the intake pipe of the condenser to prevent oil from returning to the exhaust side of the compressor during shutdown and also help prevent liquid refrigerant from the condenser Flow back to the compressor; as for the liquid hammer problem, it is not very big because you are returning air from the motor, not from the cylinder head.

　　 One, suction piping design

　　1. The horizontal suction pipeline must have an inclination greater than 0.5% along the direction of refrigerant flow; the cross section of the horizontal suction pipeline must ensure that the gas flow rate is not less than 3.6m/s;

2. In the vertical suction pipeline, it is necessary to ensure that the gas flow rate is not less than 7.6～12m/s; the gas flow rate greater than 12m/s cannot significantly improve the oil return, it will produce high noise and lead to a higher suction pipe Road pressure drop

3. At the bottom of each vertical suction pipe, a U-shaped oil return bend must be established; if the height of the vertical suction pipe exceeds 5-6m, a U-shaped oil return bend must be established for every additional 5-6m; U The length of the oil return bend should be as short as possible to avoid excessive oil accumulation;

　　 2. Design of evaporator suction pipe

　　 1. When the system does not use a pump down cycle, a U-shaped interception bend should be set at the outlet of each evaporator to prevent the liquid refrigerant from flowing into the compressor under the action of gravity when the machine is stopped;

　　2. When the suction riser is connected to the evaporator, there should be a horizontal pipe and intercepting bend in the middle to install the temperature sensor; prevent the expansion valve from malfunctioning;

　　3. When the compressor is higher than other positions in the system, an oil return bend should be set on the rising riser about 3m, and the pipe diameter of the return gas riser should be reduced to increase the flow rate to bring the oil back to the compressor;

　　4. Generally, the U-shaped bend installed when the condenser is higher than the compressor is designed to prevent the condensation of the working fluid during shutdown and the return of oil during the next startup. In the case of reasonable system piping design, when the evaporator is lower than the compressor, an oil return bend should be considered, and the pipe diameter can be appropriately reduced to meet the oil speed (especially in low temperature conditions).

Generally, a hermetic compressor allows a lifting height of 4 meters, a semi-hermetic compressor allows a lifting height of 6 meters, and the horizontal pipeline slope is greater than 1:125; as long as the flow rate of the refrigerant is greater than 4m/s, the lubricating oil can be smoothly taken out ；

　　 The two-stage parallel return oil liquid level balance problem, the installation is also very important. The two compressors must be on the same horizontal position and have the same model (same power). The oil return header must be divided equally between the two units. The split port must be of a "Y" shape, and the branch pipes must be of the same height and length. , The pipe diameter is also the same to ensure the same oil distribution. Then there is the problem of operation adjustment. For example, the suction pressure of the two machines should be exactly the same, and there must be no flushing phenomenon in them, otherwise the oil will disappear immediately. If these cannot be guaranteed, the oil return will not be consistent.

　　 Three, heat pump system

　　 Maintain a higher return air temperature and pressure, increase a longer return air pipeline; add a regenerator, etc.

　　 Control the opening degree of the throttle valve (expansion valve). When the compressor is found to be frosted, the opening degree of the expansion valve should be adjusted in time to control the return steam pressure within a reasonable range.

About ten minutes before each shutdown, you must first close the liquid supply valve, store the refrigerant in the condenser, accumulator and other high-pressure parts, continue to run, reduce the liquid level in the evaporator and then shut down to ensure the next startup There is no liquid hammer phenomenon.

　　 Pay attention to the balance of air return and oil return when multi-stage parallel connection.