New Technology: A New Control Method for Throttle Valves in Refrigeration Systems

Two Valve Control Methods
1.1 Control by the Suction Superheat Degree of the Compressor
The suction superheat degree refers to the difference between the suction temperature of the compressor and the saturated gas temperature corresponding to the suction pressure. It is one of the indicators reflecting the heat exchange efficiency of the evaporator. By controlling the set superheat degree, the required refrigeration operating conditions can be obtained. Its working process is that the throttle valve senses the change in the refrigerant superheat degree at the outlet of the evaporator through the temperature sensing bulb to control the opening degree of the valve and adjust the flow rate of the refrigerant entering the evaporator so that its flow rate matches the heat load of the evaporator. When the heat load of the evaporator increases, the valve opening degree also increases, and the refrigerant flow rate increases accordingly; conversely, the refrigerant flow rate decreases. This is the traditional control method that is currently the most widely used and has relatively mature technology.
1.2 Control by the Discharge Superheat Degree of the Compressor
The discharge superheat degree refers to the difference between the discharge temperature of the compressor and the saturated gas temperature corresponding to the discharge pressure. On the premise of ensuring that the compressor does not carry liquid and does not lose oil, by controlling the discharge superheat degree, the unit can always adapt to changes in the external environment and load usage, ensuring the reliability and optimal performance of the unit. Currently, this control method is still controversial and is still in the exploration and discussion stage.

Comprehensive Comparison of the Two Control Methods
The refrigeration system is a complete system, and the four major components need to be coordinated during the operation of the unit. Among them, the evaporator and the condenser are affected by the external environment. When the external environmental conditions change, it will cause an impact between the evaporator and the condenser. This mutual influence is the coupling factor. Therefore, when controlling the throttle valve, the parameters affected by the evaporator and the condenser should be considered simultaneously. Correspondingly, it cannot be simply controlled according to the suction superheat degree of the compressor. If the discharge superheat degree is introduced to control the throttle valve, it can simultaneously reflect the impact of external conditions on the evaporator and the condenser. Therefore, controlling the throttle valve by the suction superheat degree has many advantages, but also has disadvantages and deficiencies.
If the two control methods are organically combined, the control will be more precise and the control of the throttle valve will be more effective. The suction superheat degree control can directly reflect the refrigeration operating conditions of the evaporator. This method is simple and effective. However, its biggest drawback is that it can only operate and control according to the change in the cooling load of the evaporator, is not sensitive to changes in the external environment, and is insufficient for the control of the entire system. So it is a simple control method that does not consider the coupling factor.
2.1 Advantages of Discharge Superheat Degree Control
(1) The bandwidth of the suction superheat degree is small, such as 10 degrees in a dry evaporator and less than 2 degrees in a flooded evaporator. Through the amplification effect of the compressor, the bandwidth of the discharge superheat degree is greatly increased, which greatly reduces the requirement for the detection accuracy of the temperature sensor, reduces the impact of the adjustment amount on the change in the valve opening degree, and prevents the occurrence of adjustment oscillation.
(2) When the pressure drop in the suction pipe is relatively large, and when the defrosting of the air-cooled heat pump starts and ends, there may be zero or negative suction superheat degree, which may lead to liquid compression and oil loss. The discharge superheat degree can completely avoid this problem.
(3) Since the suction superheat degree is only related to the evaporator and there is a lack of timeliness in temperature detection, it is very easy for the adjustment cycle of the throttle valve and the loading and unloading cycle of the compressor to be out of phase or in opposite phase, often resulting in low-pressure alarms, high discharge temperature protection, or low discharge temperature protection. The discharge superheat degree control reflects the actual operating conditions of the other three major components except the throttle valve and can naturally achieve synchronization with the loading and unloading direction of the compressor.
(4) High discharge temperature is one of the potential hidden dangers of compressor burnout, and low discharge temperature protection is one of the precursors of possible oil loss. The suction superheat degree control cannot control these two points. However, the discharge superheat degree control can not only improve the control efficiency but also predict the trends of the aforementioned high discharge temperature protection or low discharge temperature protection, and then implement preventive control.
2.2 Disadvantages of Discharge Superheat Degree Control
(1) The control of the discharge superheat degree changes with the operating conditions of the compressor and the outdoor environmental temperature and is an uncertain value. It is necessary to establish a multi-parameter target optimization model, and the difficulty of control implementation is relatively large.
(2) The leakage of lubricating oil and the situation of oil return will all have an impact on the discharge superheat degree.
(3) In a two-stage or multi-stage compression system, there are many factors affecting the discharge temperature of the final high-pressure compressor, and it is difficult to achieve stable operation of the evaporator at the set operating conditions by controlling the throttle valve through the discharge superheat degree.

New Control Method
Based on the above analysis, considering that both methods have their own advantages, it is conceived that the suction superheat degree control can be used as the main control, and the discharge superheat degree control can be used as the auxiliary control, and the two can be organically combined to control the throttle valve. Specifically: on the premise of achieving the purpose of suction superheat degree control (that is, reasonably adjusting the flow rate, keeping the heat transfer area of the evaporator fully utilized, and preventing accidents such as the compressor being damaged by the suction of liquid), and fully considering the impact of changes in the external environmental conditions on the condensation temperature, that is, reflected in the change in the discharge temperature, comprehensively considering and controlling multiple parameters affecting the system. The preliminary idea is to detect multiple parameters affecting the suction superheat degree and the discharge superheat degree, and then calculate the optimal target values of the suction and discharge superheat degrees according to the model, and then fit the two to control the throttle valve, realizing multi-parameter input and single-parameter output control. It is expected that this control method can play a role in improving the COP of the entire refrigeration system and reducing the energy consumption of the system, but further theoretical analysis and experimental verification are still needed.