Let’s take a trip back to the basics to understand subcooling in the first place. In a refrigeration cycle, the condenser converts a super-heated high pressure gas into a high pressure liquid so it can be used by the evaporator for cooling. It does this by removing enough heat from the gas to cause it to condense to a liquid at a prevailing pressure and temperature (saturated liquid point). Any additional heat removed in the condenser is called “subcooling”.The amount of subcooling that can be measured at the condenser outlet is a good indicator of condenser performance.
If the condenser is operating efficiently, the conversion from refrigerant vapor to liquid is complete BEFORE the refrigerant reaches the last several passes in the condenser coil. These last several passes should be filled with liquid only. Because the coil is still transferring heat, now from the liquid refrigerant in these last few passages, it’s temperature continues to drop and we are subcooling the refrigerant.
Subcooling is a good measure of how long the refrigerant takes to pass through the condenser which is called “stay time”. The right amount of subcooling for any particular unit can vary with the type of system and its application. At a proper charge, with clean coils, and design air flow, subcooling can fall in the 10 -20 F range.
So, knowing this information, how can subcooling help with troubleshooting? Let’s look a some different scenarios. If a system has an insufficient refrigerant charge (under charged unit), the refrigerant flows very rapidly through the condenser trying to satisfy the load conditions. This does not allow enough “stay time” for the refrigerant to condense and be properly subcooled resulting in little or no subcooling.
On the other hand, a system with an improperly adjusted or plugged expansion valve will exhibit extremely high subcooling (i.e., maybe 30 F) due to excessive amount of liquid being “backed up” into the condenser. The curious part of both of these situations is that they have an identical effect on the evaporator side of the system: inadequate cooling, low head pressure, and high super-heat.
So, here we have 2 different situations causing the same problem on the evaporator side of the system. So, how do we know what the problem is and diagnose it properly? ONLY by taking a subcooling measurement can the exact cause of the problem be pinpointed without relying on “trial and error” procedures. By making a subcooling measurement, the tech could possibly prevent an unneeded expansion valve change-out. If the subcooling is ZERO, the expansion valve could not be closed due to lost bulb charge or plugged orifice.
If the subcooling is low and the super-heat is high, you can probably assume a low refrigerant charge. To double-check this conclusion, measure the compressor amp draw. the reading should be low. The temperature difference across the coil should also be low.
This is just one example of how subcooling can help in troubleshooting. Below is a chart that combines subcooling, superheat, pressures and amp draws. It is a very good diagnostic chart to keep with you in the field.
Properly used, subcooling is a valuable tool that can dramatically reduce the time required to diagnose a system malfunction. It, ALONG WITH suction super-heat, play increasingly more important roles in servicing and maintaining a refrigerant system or air conditioner. I just gave you a couple of scenarios above — can you think of any other area this would have been invaluable to you in making a diagnostic?
We always need to remember, air conditioners are SYSTEMS. We always need to look at all “sides” of that system — condenser, evaporator, air flow, etc.. They all contribute or deter from a system operating properly. MAKE USE OF THE CHART ABOVE — take ALL of the necessary measurements — You will solve problems if you do a complete diagnostic!
(chart above was taken from ProficienTECH training manual. Thanks to York Training Department for permission to use the chart.)