TROUBLESHOOTING HEAT PUMP REFRIGERANT SYSTEMS…UNDERSTANDING THE NATURE OF FAILURES (continued)
The engineering explanations of superheat and subcooling are…well they’re probably above my level of intellect. But for those of us who calculate the numbers for analysis and diagnostic purposes only, they simply tell us how much liquid refrigerant is in the evaporator and condenser coils. And there is an engineered amount of liquid in each coil for a given system. Since there’s no way to directly measure the amount of liquid, we have to use the temperature method. In order to understand how this works you have to visualize what’s going on inside either coil.
Inside the condenser coil, the hot gas enters and begins moving through the tubing, all the while giving up heat to the cooler condenser air. At some point the vapor temperature equals the saturated (condensing point) temperature of the refrigerant at whatever value the head pressure is running. When the vapor reaches that temperature, it condenses to the liquid phase. At some location near the exit point in the coil, the refrigerant volume is 100% liquid. Since it is still exposed to the condenser air, it will give up additional heat and cool down below the condensing temperature. We measure the liquid line tubing temperature and take that value as the liquid temperature. The difference in the liquid and condensing temperatures is what we call subcooling (the definition of “sub” here is “below”). If an R-22 system head pressure is 225 psi, the saturated (condensing) temperature is 110F. If the liquid line temp is 95F, the subcooling is 15F.
Inside the evaporator coil, the liquid enters and begins moving through the tubing, taking in heat from the warmer evaporator air. For the particular operating suction pressure, the saturated temperature of the refrigerant equals the boiling point temperature of the liquid. Whatever heat is absorbed by the liquid is used in changing its phase from liquid to vapor, rather than increasing the liquid temperature. At some location before the exit point of the coil, all the liquid will have changed to the vapor phase, but will continue to take in heat from the evaporator air. So the temperature of the vapor will increase above the saturated refrigerant temperature. We measure the suction line tubing temperature and take that value as the temperature of the vapor. The difference in the vapor temperature and the boiling point (saturated) temperature is the superheat (the definition of “super” here is “above”). So, if our R-22 system suction pressure is 70 psi, the boiling point is about 40F. If the suction line temperature measures 55F, the superheat is 15F.
Contrary to what we sometimes get accustomed to working with, refrigeration systems aren’t designed to operate at certain pressures. They are designed to operate with certain evaporator and condenser coil temperatures. The resulting system pressures are dependent on the design coil temperatures and the particular refrigerant chosen for the particular application. But we can’t easily or accurately measure the refrigerant temperatures inside the coils, so we measure the pressures and convert those values to saturated refrigerant temperatures, using the temperature scales on the gauges or a refrigerant P-T chart.
And as said earlier, refrigeration systems also have design values for superheat and subcooling. If we know the design coil temperatures, the design superheat / subcooling values, the type refrigerant, have a set of gauges and a thermometer suitable for measuring tubing temperatures, we can begin to analyze a system.
To be continued…
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