How to Address Excessively High Suction Temperature of Refrigerated Air Dryer

During the operation of a refrigerated air dryer, if the suction temperature is excessively high and far higher than the designed dew point temperature, 

adopt the following handling methods:

Malfunction of Expansion Valve

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

Driven by the internal regulating spring, the valve opening increases to raise the flow rate of liquid refrigerant and lower the suction temperature.

However, impurities trapped inside the expansion valve may jam the regulating spring and prevent the valve from opening further. The flow rate of liquid refrigerant remains unchanged. 

Under heavy cooling load, all liquid refrigerant vaporizes. The gaseous refrigerant absorbs heat, causing a sharp temperature rise and excessive superheat.

Solutions:
Dismantle the valve for maintenance or replace it with a new one.
Excessive Heat Transfer Area or Improper Design of Evaporator
Under normal operating conditions, liquid refrigerant occupies 70% of the internal volume of the evaporator to maintain proper suction superheat.

If the evaporator is designed with an oversize heat transfer area (large evaporators are commonly used in low-pressure refrigerated air dryers to control air pressure drop), 

liquid refrigerant will be completely vaporized prematurely inside the evaporator. Alternatively, due to structural defects, liquid refrigerant cannot fill the designated space in the evaporator. 

The gaseous refrigerant absorbs heat and experiences a dramatic temperature increase, resulting in excessively high suction temperature.

Solutions:
Upgrade to a larger-sized thermostatic expansion valve and redesign the evaporator.
Improper Liquid Pipeline Design Causing Flash Vaporization

When flash vaporization occurs to liquid refrigerant, the temperature of liquid pipelines drops drastically, part of the liquid refrigerant turns into gas, 

and the refrigerant volume expands significantly.

The selected thermostatic expansion valve is designed for liquid refrigerant passage, with a fixed volumetric flow capacity for refrigerant (whether gaseous or liquid). 

Once flash vaporization occurs, the volumetric flow through the expansion valve stays constant, while the mass flow decreases greatly (gas occupies much larger volume than liquid with identical mass).

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