The hot gas bypass technology of low temperature chiller plays an important role in improving system efficiency and preventing the compressor from overheating. However, there are also certain risks at the same time, especially in terms of the liquid return of the compressor.

I. Basic Principles and Functions of Hot Gas Bypass Technology

Hot gas bypass technology refers to directly bypassing the high-temperature and high-pressure gas discharged by the compressor to the low-pressure side to maintain the return gas volume of the evaporator, thereby avoiding frequent starts and stops or damage to the compressor due to excessively low suction pressure. This technology optimizes the operating state of the compressor by adjusting the opening degree of the bypass valve to control the gas flow entering the suction pipe. Its main functions include:

- Enhance compressor efficiency: By reducing the temperature at the return gas end, the burden on the compressor is alleviated, thereby improving its working efficiency.

- Improve system performance: By mixing a certain proportion of cold air at the return air end, the cooling performance of the refrigeration system can be enhanced.

- Reduce compressor overheating: Hot gas bypass can effectively lower the operating temperature of the compressor and prevent overheating from occurring.

Ii. The Impact of Hot Gas Bypass Technology on the Risk of Liquid Return in Compressors

Although hot gas bypass technology has significant advantages in improving system efficiency, its potential problems in terms of the risk of liquid return in the compressor cannot be ignored. The following is the relevant analysis:

1. Hot gas bypass may cause liquid to be carried in the intake

During the hot gas bypass process, some high-temperature and high-pressure gas directly enters the low-pressure side and may form liquid refrigerant after throttling. If these liquid refrigerants directly enter the compressor, it may cause the compressor to draw in liquid, which in turn may lead to liquid hammer. Liquid hammer not only damages the mechanical components of the compressor, but also leads to the dilution and wear of the lubricating oil, ultimately affecting the service life of the compressor.

2. Hot gas bypass may lead to the accumulation of liquid refrigerant in the evaporator

During the hot gas bypass defrosting process, a large amount of liquid refrigerant may accumulate in the evaporator. When the system switches back to the normal cooling mode, these liquid refrigerants may flow into the compressor, causing liquid return or liquid hammer. This phenomenon is particularly common in air source heat pump systems because when hot gas bypass defrosting occurs, the liquid refrigerant in the evaporator may not be discharged in time when switching back to the cooling mode.

3. Hot gas bypass may cause the intake temperature to be too high

During the hot gas bypass process, high-temperature gas directly enters the suction pipe, which may cause the suction temperature to be too high. Excessively high suction temperature not only increases the load on the compressor but may also cause it to overheat and even get damaged. In addition, excessively high suction temperature may also affect the evaporation efficiency of the refrigerant and reduce the overall performance of the system.

Iii. Measures to Prevent the Risk of Liquid Return in Compressors

To effectively prevent the risk of compressor liquid return caused by hot gas bypass technology, the following measures can be taken:

1. Add a gas-liquid separator

In the hot gas bypass system, adding a gas-liquid separator can effectively separate the liquid refrigerant entering the suction pipe and prevent it from entering the compressor. The gas-liquid separator can be installed in the bypass pipeline to ensure that only the gaseous refrigerant enters the compressor, thereby avoiding liquid hammer. In addition, the gas-liquid separator can also be installed at the outlet of the evaporator to prevent the liquid refrigerant from entering the suction pipe.

2. Adopt liquid spray cooling technology

Liquid spray cooling technology is an effective measure to prevent the risk of liquid return in compressors. By installing a liquid injection valve on the bypass pipeline, the liquid refrigerant can be injected into the suction pipe, thereby reducing the suction temperature and preventing the compressor from overheating. Meanwhile, liquid spray cooling can also reduce the accumulation of liquid refrigerant and lower the risk of liquid return. In addition, liquid spray cooling can also enhance the refrigeration efficiency of the system and reduce energy consumption.

3. Optimize the control strategy of the bypass valve

The control strategy of the bypass valve has an important influence on the operating state of the compressor. By optimizing the opening pressure value and response threshold of the bypass valve, it can be ensured that the bypass valve accurately opens under the appropriate pressure, avoiding system instability caused by too low or too high pressure. In addition, dynamically adjusting the response threshold of the bypass valve to perfectly match the system requirements is also a key measure to improve system stability and extend the service life of the compressor.

4. Adopt heat storage devices

In the hot gas bypass system, adding a heat storage device can effectively absorb the waste heat of the compressor and prevent the formation of liquid refrigerant. The heat storage device is filled with ethylene glycol solution, which can absorb the heat discharged by the compressor, thereby reducing the temperature of the bypass gas and preventing the formation of liquid refrigerant. In addition, the heat storage device can also prevent the liquid refrigerant from entering the compressor by heating the bypass gas and causing it to evaporate into a gaseous state.

5. Design the bypass pipeline reasonably

The design of the bypass pipeline is crucial to the stability and safety of the system. The bypass pipeline should avoid being directly connected to the suction pipe but should be connected to the inlet or outlet of the evaporator to reduce the accumulation of liquid refrigerant. In addition, steam tracing pipes should be installed on the bypass pipeline to prevent the gas temperature in the bypass pipe from being too low and condensing into liquid. Meanwhile, the installation position of the bypass pipeline should avoid the backflow of liquid refrigerant, especially in systems that are operating at low load for a long time.

Iv. Conclusion

The hot gas bypass technology has significant application value in low temperature chillers, but its potential problems in terms of the risk of liquid return in the compressor cannot be ignored. By adding gas-liquid separators, adopting liquid spray cooling technology, optimizing the control strategy of bypass valves, using heat storage devices and rationally designing bypass pipelines and other measures, the risk of liquid return in the compressor can be effectively prevented, ensuring the stability and safety of the system. These measures not only help to extend the service life of the compressor, but also improve the overall performance and energy efficiency of the system.