Diagnosis 2

Correct Diagnosis!
[high side restriction]

The big give away was the liquid line temperature right after the liquid line filter/drier. The filter is so clogged that hardly any refrigerant is getting through. There is such a large pressure drop through the filter, it is acting like a metering device. Liquid refrigerant is flashing to a gas and removing heat from the surrounding area (the filter drier and it's casing). In real life, at that liquid line temperature of 31 °F the filter would be all frosted up and would certainly stand out like a sore thumb. However if the temperature happens to be above 32 °F there would be no visible frost, it would be sweating instead. This is an example of why it is common practice to place your hands on pipes and "feel" the temperature. A cold liquid line is not normal. Use common sense and refrain from touching discharge lines. They can be hot enough to cause severe burns. Since so little refrigerant is getting through to the evaporator all the other symptoms make sense.



The reduced amount of refrigerant getting through the filter/drier can evaporate before traveling very far through the evaporator coil. There is therefore a great deal of evaporator area left over for superheating. Hence, there is high superheat.


The reduced quantity of refrigerant on the low side of the system causes a lower than normal suction pressure. The compressor keeps trying to remove refrigerant from the evaporator at its normal pumping rate but the partially clogged filter/drier and the TX Valve cannot keep up with this rate so the low side pressure drops.


The lower than normal pressure suction vapours enter the compressor. The rarified vapours are compressed yet there are fewer molecules to share the heat of compression. It's like sharing $100 between 4 people instead of 50 people. With fewer people, each receives more money. With fewer gas molecules each receives more of the heat of compression. This results in higher temperatures. Hence the higher than normal discharge temperature.


Low pressure vapours are rarified compared to high pressure vapour. Low pressure vapours impose less of a load on the compressor. Since the compressor is performing less work, it draws less amperage. (The compressor motor draws less amperage)


The hotter than normal discharge gas now enters the condenser. However the partially clogged filter/drier is holding back the flow. Refrigerant is being kept in the condenser for a much longer time than normal. Therefore it keeps rejecting heat and cools down more than normal. The pressure and temperature drops lower than normal. Hence the lower than normal high side pressure.


The refrigerant is cooled more than normal while held back in the condenser by the high side restriction. The liquid line temperature therefore decreases and the amount of subcooling increases.

The above explanations should make it clear why all the test measurements varied from "normal" in such specific ways. If you had a tough time determining the solutions on the Causes-Effects page, you should return there now and solve the "Restriction (Liquid Drier)" section. If you understand the above explanations it should be easy to select the appropriate symptoms. Did you also notice the poor air temperature drop through the evaporator? A mere 2 °F temperature drop is not going to provide very effective refrigeration. That explains why the evaporator inlet air is so high. The refrigeration system is running so poorly, the box is gaining heat at a faster rate than the refrigeration system can get rid of it. The air temperature rise through the condenser is also very poor. That's because the refrigeration system is running so poorly it is not picking up enough heat to make a significant heat rejection.
It should not be necessary to make all of the above measurements in order to diagnose a clogged filter/drier. Once it was determined that the system was operating inadequately, checking for a temperature drop through a filter/drier should be done just to eliminate it as a possible cause. It takes no time at all to feel the inlet and outlet temperatures. If there is no difference you can continue on your pursuit for the fault. The above set of fault related operating parameters is however valuable in demonstrating the inter-relationship of all the operating parameters in the mechanical refrigeration process.