n Refrigeration

the condenser, the refrigerant is condensed from vapour to liquid. The process is essentially the reverse of what is happening in the evaporator but it’s happening at a warmer temperature because we’ve increased the pressure.

the air outside.

Continued from page “43” the limitations associated with where you hooked them up. In a perfect world, you would be able to measure the superheat at the evaporator and eliminate the error caused by pressure drop and temperature rise. Some tools use Bluetooth to be able to do a remote temperature measurement, but a pressure measurement is not possible unless there is an access valve added at the evaporator outlet.

Subcooling

Subcooling is the temperature below the condensing temperature of a substance. The condenser in an air conditioner is designed to reject the heat absorbed in the evaporator and added by the compressor. In

An R410A air conditioner condensing at 110 F would have a condensing pressure of 365.5 psig. Figure 5 shows this relationship with a screenshot of the PT app and a schematic of the air conditioning system. One of the trip-ups that I see regularly is caused by the fact that subcooling is happening in the warm part of the system where superheat is usually discussed in relation to the cold part of the system. One way that sometimes helps get these straight is to realize that your hot cup of coffee is subcooled since it is below the boiling point of coffee — hot things can be subcooled.

Once the refrigerant in the condenser has completely condensed, it is still warmer than

If there is enough refrigerant in the system for liquid to back up at the condenser outlet, then the refrigerant will have a chance to cool off more. This additional change in temperature is the subcooling. Figure 6 shows the relationship between the pressure measured at the condensing unit outlet being used to determine the condensing temperature and the liquid outlet temperature. The difference between these temperatures, 10 F in our case, is the subcooling.

Subcooling measurements on residential air conditioning systems are not prone to the same errors as superheat measurements are because they are being done right at the condenser outlet.

Measure accurately

Both adding refrigerant and troubleshooting air conditioning systems properly is highly dependent on being able to accurately measure and understand both superheat and subcooling. This article has outlined the process for obtaining superheat and subcooling measurements and explained some of the errors we can see when doing them.

However, it has so far ignored the additional challenge of blended refrigerants. In the case of 400 series refrigerants (R449A, R410A, etc.), it is important to remember to use the dew point temperature from the PT chart or app to determine superheat and the bubble point temperature when calculating subcooling. If you take a look at Figure 6, you will see that the PT app I used has a slider to switch between the two. Happy troubleshooting! :

Greg Scrivener is the lead refrigeration engineer and a partner at Laporte Consultants, Calgary, and works throughout Canada and the U.S. He is a professional engineer and journeyperson refrigeration mechanic. He can be reached at GScrivener@laporteconsultants.com

INTERNALBATTERY