Parallel Installation and Precautions for Scroll Compressors

2024-09-25

I. Advantages of using compressors in parallel:

Effective capacity control can be achieved by starting/stopping the compressors in the unit to adjust the capacity.
Improved reliability. Compared with a single compressor, it has fewer starts. According to different control logics, two compressors can be controlled at a relatively appropriate speed to ensure the service life of the compressor.
Reduced starting load. The starting times of each compressor can be separated by time delay methods.
Standby function.
If one compressor is damaged, there is still some capacity, which can ensure the normal operation of the air conditioning system and reduce the pressure on the user.

II. Precautions for parallel connection of compressors:
The system parameter matching requirements and basic performance requirements are the same as those of a single-unit system. In a parallel system, special attention should be paid to checking the reliability of system oil return, especially for multi-connected units.
The parallel system also needs to pay attention to the following two points:

Gas balance between compressors in the parallel system: It is necessary to ensure that the suction pressure difference between parallel compressors is about 0.02-0.03.
Oil balance between compressors: In the oil return process, there should be no partial flow that causes too much or too little oil in the compressor. In addition, a filter screen, temperature and pressure protection switch should be set at the oil return port to ensure that the oil level is in an appropriate position.
When compressors are used in parallel, due to the difference between the oil discharge amount of the compressor and the system oil return amount, long-term operation will cause changes in the oil level of the compressor.
When connecting compressors A and B of the same model, if the oil discharge amount of compressor A is large and the system oil return amount is small, the oil level of A will continue to drop and finally almost no oil will be present, which can seriously lead to the burnout of the compressor. When the displacement of compressor B is small and the system oil return is large, the oil level of B will continue to rise, and eventually the stator and rotor will be immersed in oil, resulting in increased power consumption. In addition, it may suck into the compressor scroll plate and cause overcurrent and oil hammer phenomena.
By using suction pipelines where the oil discharge amount and oil return amount of each compressor are as equal as possible, the time of no-oil state can be reduced. However, these measures cannot fundamentally eliminate the oil imbalance of the compressor. When designing a system that is not prone to oil imbalance, if oil imbalance occurs, a design must be adopted to restore the oil amount of the compressor to the initial state.
Countermeasures to avoid oil imbalance:
①. Connect compressors of the same model and the same displacement, and try to equalize the oil discharge amount/oil return amount as much as possible.
②. At the confluence of the exhaust pipelines of the compressors and at the branch of the suction pipelines to the compressors, each component of the system should have the same shape, and the two compressors should maintain as equal refrigerant flow and oil flow as possible.
③. To further suppress the oil discharge amount of the system, an oil separator is designed in the exhaust pipeline to make the separated refrigeration oil return evenly to the compressor (the design of the oil return pipeline should be consistent).
Perform oil equalization operation after a certain period of operation:
A representative oil equalization operation method is to correct the oil imbalance by briefly turning ON and OFF both compressors in the following sequence.
①. Stop one compressor (OFF) for 2 minutes: Oil will move to the operating (ON) compressor through the oil equalization pipe.
②. Reverse the ON-OFF relationship of ① for 2 minutes: Oil will move to the compressor on the operating (ON) side through the oil equalization pipe.
③. Restore to the original operating state (both return to the operating state).
④. Verify the oil level balance of each compressor under various working conditions (long piping, high drop, low-temperature heating, maximum cooling, etc.) to ensure that the oil level in each compressor is above 400 ml (the oil level observed through the sight glass is above 1/3).
⑤. The current protection value of a conventional parallel compressor is set at 1.2 to 1.4 times the rated current.

III. Installation requirements for compressors:

It is required to be installed on the same horizontal plane as much as possible. The selection of the compressor center distance should be convenient for loading and unloading the compressor. The pipeline layout should be as symmetrical as possible, and a gas-liquid separator should be designed.
An oil separator can be equipped for each compressor or installed on the exhaust pipe.