Installation Project and Usage Introduction of Parallel Compressor Units in Cold Storage

In recent years, with the massive investments of international logistics giants and retail giants in the Chinese market, as well as the renovation of domestic civil cold storage facilities and the rise of domestic retailers, China's low-temperature logistics industry and retail industry are undergoing a transition from traditional refrigeration equipment and systems to advanced foreign refrigeration systems and equipment. Now, let the editor tell you about the technical application of the parallel compressor system in cold storage.

The parallel compressor unit of a cold storage refers to a refrigeration unit composed of two or more compressors connected in parallel and sharing a set of refrigeration circuits. According to different refrigeration temperatures, refrigeration capacities, and matched condensers, the forms of parallel units can be diverse.
That is, the same set of units can be composed of compressors of the same model, or of different models. It can be composed of compressors of the same type (such as reciprocating compressors), or of different types (such as reciprocating compressors + screw compressors); it can carry a single evaporation temperature, or several different evaporation temperatures; it can be a single-stage system or a two-stage system; it can be a single-cycle system or a cascade system, etc. Commonly, it is a single-cycle parallel system of compressors of the same type.

For small and medium-sized cold storages, scroll compressors are too small, and parallel screw compressors are too expensive. Using reciprocating compressors is a more moderate and cost-effective choice.

II. What are the advantages of parallel units compared to single-unit units?

III. Compared with single units, what special attention should be paid to parallel units?
The refrigeration system of the parallel unit is composed of multiple refrigeration compressors, and the refrigeration pipeline is more complex than that of ordinary refrigeration units. Therefore, more caution and care are needed in the balance of pipeline connections. When the unit pipeline is connected to a group of multiple compressors, to ensure that the oil return is evenly distributed to each compressor, the interface of the main pipeline must be set in the middle position of multiple compressor heads, and then some branch pipelines are set on both sides respectively, allowing the oil return to flow evenly into the branch pipes of multiple compressors.

Moreover, valves should be installed on each branch pipe to adjust the amount of oil return. If not, and multiple downward branch pipes are successively led out from different parts of the main pipeline and connected to multiple compressors of the parallel unit, the oil return will be uneven. The oil return of the first compressor is always the fullest, and the oil return of the subsequent compressors gradually decreases in turn. In this way, it is possible to cause operational failures of the first compressor, such as huge vibrations, excessively high oil pressure, and overheating of the unit, resulting in accidents such as cylinder breakdown or seizure of the compressor, and damaging the equipment.
This is the problem caused by the uneven connection of the refrigeration pipelines of the parallel unit.

Four Steps in the Use Process of Parallel Compressors in Cold Storage
Four aspects need to be particularly noted in the use process of parallel compressors, as follows:
① Maintaining the Oil Level
② Countermeasures for Liquid Return
③ Installation of Protection Devices and Accessories
④ Confirmation of Pipeline Stress
Step 1 of Parallel Compressors:
When compressors are used in parallel, due to the difference between the oil supply amount of the compressor and the oil return amount of the system, long-term operation will cause the oil level of the compressor to change (the oil level of one side decreases, and the oil level of the other side rises).
When connecting compressors A and B of the same model, if the oil supply amount of compressor A is large and the oil return amount of the system is small, the oil level of A will continue to drop, and it will almost be in an oil-free state, resulting in poor lubrication and burning of the bearing. When the oil supply amount of compressor B is small and the oil return amount of the system is large, the oil level of B will continue to rise, and the motor rotor will almost be immersed in the oil, resulting in an increase in input power and a rise in the temperature of each part.
The phenomenon of oil level change (the oil quantity shifts to the other compressor) that occurs during the operation of parallel compressors is called [oil deviation]. In severe cases, the other compressor may be without oil within dozens of minutes.
Step 2:
Try to make the oil supply amount and oil return amount of each compressor equal in the suction pipeline, which can prolong the time to reach the oil-free state. However, these measures cannot completely eliminate the oil deviation of the compressor.
☆ Design a system that is not prone to oil deviation;
☆ After oil deviation occurs, it is necessary to adopt a design to restore the oil quantity of the compressor to the initial state.
Step 3: Countermeasures to Avoid Oil Deviation
① Connect compressors of the same model and the same capacity, and try to make the oil supply amount/oil return amount equal.
② For the exhaust pipeline of the compressor to the confluence part, and the branch part of the suction pipeline to the compressor, the components of each system should adopt the same shape, and the two compressors should try to maintain an equal flow rate of refrigerant + oil.
③ To further suppress the oil supply amount of the system, design an oil separator on the exhaust pipeline. It is best to adopt the measure that the oil separated from the two compressors returns to the system. The above three countermeasures ① and ② are necessary, and ③ should be implemented when necessary. The effect is very obvious.
Step 4:
① Carry out oil equalization operation after running for a certain period of time;
② For oil equalization operation, it is possible to carry out oil equalization operation through the exhaust pipes and suction pipes of both compressors, but even if it is possible, the required time is very long;
③ An effective method is to connect the oil storage positions of the two compressors through the oil equalization pipe and move the oil through the oil equalization pipe;
④ A representative oil equalization operation method is to turn on and off both compressors for a short time in the following order (Note 1) to correct the oil deviation.

Problems to Be Noted During the Operation of the Cold Storage
(1) During the operation of the cold storage, on the premise of ensuring the temperature of the cold storage, a relatively high evaporation temperature should be adopted as much as possible. When the evaporation temperature drops from -5°C to -30°C, under the same refrigeration capacity, the energy consumption will increase by more than 1 time. Some data also point out that for every 1°C increase in the evaporation temperature of the refrigeration device, 2% to 3% of energy can be saved. It can be seen that the change of the evaporation temperature has a great impact on the energy consumption of the refrigeration device, so the situation of too low evaporation temperature should be avoided as much as possible.
(2) Reasonably determine the condensation temperature. Since the condensation temperature corresponds to the condensation pressure, when the condensation temperature rises, the condensation pressure also rises, which will make the exhaust pressure of the refrigeration compressor rise and the compression ratio increase. This will reduce the volumetric efficiency of the compressor, increase the power consumption, and reduce the economy. On the other hand, an increase in the compression ratio will make the exhaust temperature of the compressor rise, increase the failure rate of the compressor, and reduce the safety and reliability of operation. Therefore, the refrigeration device should be operated at a relatively low condensation temperature as much as possible.

To achieve the operation of the refrigeration device at a lower condensation temperature, it is very important to arrange the operation of the refrigeration device at night.
(3) Reasonably select the ventilation and air exchange time of the cold storage. For some cold storages with ventilation and air exchange requirements, the load caused by ventilation and air exchange is relatively large, because the temperature and humidity difference between the inside and outside of the cold storage is large, and the heat and moisture exchange between the inside and outside of the room also has a great impact on the load of the cold storage. If it is arranged between 5:00 and 6:00 in the early morning, at this time, the temperature and humidity of the ambient air are relatively low, and the air is relatively fresh, so the heat and moisture load caused by ventilation and air exchange is relatively small. The operation experience of some high-temperature cold storages has proved this point.