How to Deal with Excessively High Suction Temperature of Refrigerated Air Dryer

During the operation of a refrigerated air dryer, if the suction temperature becomes excessively high and far exceeds the designed dew point temperature, 

the corresponding troubleshooting measures are as follows:

Malfunction of the expansion valve

When the cooling load (air processing capacity) rises, the refrigerant suction temperature increases, and the temperature of the thermostatic expansion valve’s sensing bulb rises accordingly. 

Through adjustment by the internal regulating spring, the valve opening enlarges to increase the flow rate of liquid refrigerant, so as to lower the suction temperature.

However, if impurities inside the expansion valve jam the regulating spring, the valve cannot open wider. The flow rate of liquid refrigerant remains unchanged. Under excessive cooling load, 

all liquid refrigerant vaporizes. The gaseous refrigerant absorbs heat, resulting in a sharp temperature rise and excessive superheat.

Solutions:
Dismantle the valve for maintenance or replace it.
Excessive heat transfer area or unreasonable design of the evaporator
Under normal operating conditions, liquid refrigerant occupies 70% of the total internal space of the evaporator to maintain appropriate suction superheat.

If the evaporator is designed with an overly large heat transfer area (large-size evaporators are often adopted in low-pressure refrigerated air dryers to control air pressure drop), 

the liquid refrigerant is fully vaporized prematurely inside the evaporator. Alternatively, due to structural defects, liquid refrigerant cannot fill the proper space inside the evaporator. 

The gaseous refrigerant absorbs substantial heat and temperature rises drastically, leading to excessively high suction temperature.

Solutions:
Upgrade to a larger model of thermostatic expansion valve and redesign the evaporator.
Unreasonable liquid pipeline design causing flash vaporization
When flash vaporization occurs in liquid refrigerant, the temperature of the liquid pipeline drops significantly, part of the liquid refrigerant turns into gas, and the refrigerant volume expands greatly.

The thermostatic expansion valve is designed for liquid refrigerant flow, with a fixed volumetric flow capacity for refrigerant (gas or liquid). Once flash vaporization takes place, 

the volumetric flow passing through the expansion valve stays constant, while the mass flow decreases considerably (gas occupies far larger volume than liquid of equal mass).

In other words, the thermostatic expansion valve delivers an insufficient mass of refrigerant to the evaporator, which causes excessive superheat of refrigerant vapor.
Solutions:
Redesign the liquid pipeline and eliminate factors that cause excessive pressure drop of liquid refrigerant inside the pipeline.