Expert Decryption: Analysis of the Causes of High Superheat and Low Suction Pressure in the Refrigeration System!

2025-04-15

The causes of this kind of failure can be summed up in one word: blockage!
I. System-related Causes:

Low inlet pressure of the expansion valve:

The liquid supply rising pipeline is too high;
The liquid pipeline is too thin;
The condensing pressure is too low.
The above three reasons will all lead to a low inlet pressure of the expansion valve, resulting in insufficient pressure difference across the expansion valve.
Solution:
Increase the inlet pressure of the expansion valve and replace the diameter of the liquid pipeline.

Flashing before the expansion valve:
Due to the pressure drop, insufficient liquid supply, or the presence of non-condensable gases in the refrigeration system, gases will be generated in the liquid pipeline; bubbles can be observed in the liquid pipeline through the liquid sight glass, or a specific whistling sound can be heard before the expansion valve.
Solution:

Increase the liquid supply of the system;
Re-evacuate the system to remove the non-condensable gases;
Clean the filter screen and the drying filter;
Check the diameter of the liquid pipeline of the refrigeration system or increase the head (increase the subcooling degree) to ensure the continuity of the liquid before the valve.
II. Causes Related to the Expansion Valve:
The opening degree of the valve may be limited by the pressure drop across the evaporator;
Solution:
Change to a thermal expansion valve with an external equalizer tube;
(External equalizer thermal expansion valve)
The valve hole may be blocked;
Solution:
If it is ice blockage or dirt blockage caused by moisture in the system, when the system is heated up or the expansion valve body is artificially heated, the pressure of the suction pipeline will suddenly increase, and the moisture or dirt will be absorbed and removed by the drying filter of the liquid pipeline.
The formation of wax-like substances is usually caused by the use of low-quality refrigeration oil. The special refrigeration oil recommended by the compressor manufacturer should be used.
(Expansion valve core)
The superheat degree is set too high during debugging.
Adjust the opening degree of the valve again according to the valve's instruction manual and the actual situation;
The power component of the valve may be damaged or leaking. Replace the power component or use a new thermal expansion valve;
The thermostatic bulb is not functioning;
Solution:
Add an insulation layer or provide artificial heat to the power component;
Use a thermal expansion valve with liquid cross-charging.
III.
Other Causes:
In addition to the blockage of the expansion valve, there are other reasons that restrict the flow of the working medium. If it is on the liquid pipeline, you can find a frost layer at the blockage point or observe a situation of too low temperature at that point; if it is on the suction pipeline, you will find a relatively high pressure drop. The following aspects are involved:
The filter or the drying filter is too small or blocked. Solution: Clean the filters in all systems, and sometimes add a drying filter to the liquid pipeline;
The solenoid valve fails or is damaged. Solution: Replace it with a suitable solenoid valve;
There is a ball valve on the liquid pipeline or the suction pipeline, and the opening of this ball valve (manual valve) is insufficient. Solution: Replace the ball valve or repair it;
Check the diameter of the liquid pipeline or the suction pipeline. If the diameters of both are too small, it will cause a too large pressure drop.