Thermal expansion valve debugging method, working principle and selection reference

Thermal expansion valve is an important automatic control component of the refrigeration system, according to the balance method is divided into external balance type and internal balance type, generally installed between the liquid storage cylinder and the evaporator, it is the use of the temperature change of the temperature packet as a signal to adjust the valve opening, change the refrigerant flow, so that the refrigerant of medium temperature and high pressure through its throttling into low temperature and low pressure wet steam, The refrigerant then absorbs heat in the evaporator to achieve cooling.

Thermal expansion valve is mainly composed of induction mechanism, actuator, adjusting mechanism and valve body. The expansion valve is controlled by three forces to adjust the size of its opening degree, temperature sensing pressure, adjusting spring pressure evaporation pressure.

The difference is that the evaporation pressure collection sample point is different, the internal equilibrium collection point is the outlet position of the expansion valve, the external equilibrium evaporation pressure collection point is the evaporator outlet position, as we all know, the role of the thermal expansion valve is to control the superheat of the evaporator outlet, that is to say, the external equilibrium thermal expansion valve reacts correctly under any conditions. Below we focus on the internal balance thermal expansion valve will occur what problems.

Because there is a pressure drop in the evaporator, which leads to the internal balance expansion valve pressure collection value is larger than the external balance pressure collection value, of course, the expansion valve itself is only a mechanical part, this deviation will not know itself, will not make corrections, which will lead to the internal balance expansion valve to make the wrong decision.

For example, two identical refrigeration systems, expansion valve 1 is the internal balance, expansion valve 2 is the external balance, and the superheat is set to 8k. After the operation of the system, the expansion valve begins to adjust automatically, if the evaporator outlet is really 8k superheat, the external balance thermal expansion valve will remain in this state of operation, due to the existence of the evaporator pressure drop, the internal balance valve itself feels that the pressure is too high, but also need to close the valve, continue to the internal balance valve itself feels 8k, In fact, the real superheat of the evaporator outlet is greater than 8k, which will cause the evaporator to not be fully utilized.

The above is a theoretical analysis, of course, sometimes the pressure drop of the evaporator is very small, the impact on the expansion valve is relatively small, you can use the internal balance thermal expansion valve, after all, the internal balance thermal expansion valve does not need to connect the balance tube, reduce a welding process during the installation process, less later failure possibility. There will also be some advantages in price. So when you can use the internal balance thermal expansion valve, when you must use the external balance thermal expansion valve, we combined with some experience in the work, give some immature suggestions.

For different types of evaporators, under normal circumstances, plate heat exchangers and shell and tube evaporators use external balanced expansion valves. Reason: The pressure drop of the plate heat exchanger is relatively large, and the shell and tube evaporator is used in the refrigeration unit with large heat exchange.

For the refrigeration unit with liquid head, the external balanced thermal expansion valve is used, and the pressure drop of the liquid head is larger.

Under what circumstances to use the internal balanced thermal expansion valve: evaporator pressure drop is small, small, batch. The advantages of using internal balance are: there is no need to connect the balance pipe, reduce the installation process of the unit, reduce the failure point during the use of the unit, and the price is cheaper.

The external balanced thermal expansion valve accurately controls the outlet superheat of the evaporator, makes full use of the heat exchange area of the evaporator, and improves the energy efficiency of the unit.

Allowable value of evaporator resistance with internally balanced thermal expansion valve (kpa)

Summary: The pressure collected by the internal balance thermal expansion valve is the expansion valve outlet pressure, and the pressure collected by the external balance thermal expansion valve is the evaporator outlet pressure, so under the same conditions, the internal balance is larger than the external balance superheat, which reduces the utilization rate of the evaporator and reduces the energy efficiency of the system.