Condensing and the TP Chart
First, let’s hit condensing. In a refrigeration machine, the condenser, has three jobs, de-super-heating the hot gas from the compressor, condensing the refrigerant back into a liquid, and sub-cooling the refrigerant to maintain a solid flow of liquid for the metering device (expansion device, TXV, capillary tube, EXV, etc.)
De-super-heating: The gas leaves the compressor at the highest temperature in the system. The excess heat from friction and compression must be removed to approach the saturated (condensing) temperature. This is usually done in the first section of the condenser. In the illustration below, from point 2 to point 3, the fan, forcing air across the condenser, lowers the refrigerant temperature to 100 Fahrenheit from a discharge temperature of 160 Fahrenheit.
The lowering of the temperature by removing the heat from the refrigerant is a ‘sensible’ cooling process where no change in state of the refrigerant occurs (it’s still 100% vapor).
Condensing: Once the refrigerant reaches its saturation temperature, the refrigerant condenses, during which, the energy being removed from the refrigerant is spent turning the refrigerant into liquid, the temperature and pressure do not change during the condensation phase. The condensation phase is a latent cooling process. The refrigerant exists in the condenser in varying states of liquid and vapor until only liquid remains.
The resulting condenser pressure is the saturation pressure and dictates the pressure of the ‘high side’ of the system.
Sub-Cooling: When only liquid remains, another sensible process can begin, it is called sub-cooling. Sub-cooling is further lowering the temperature of the liquid refrigerant below its saturation temperature, the pressure, does not change.
The TP table, temperature pressure table available free from most any refrigerant manufacturer website, is a list of the saturation pressures and temperatures for individual refrigerants. (There are also apps for that!).
In the above example, the refrigerant is 404a, and the condensing temperature was 100 F, the pressure would be 235 psig (pounds per square inch gauge or what your manifold would read). See below:
However, if the gauge was reading, 218 psig, then out condensing temperature would be 95 F.
Say the pressure is 218 psig, and you read the line temperature leaving the condenser and it is 100 F?
The higher than saturation temperature at the liquid line leaving the compressor indicates a problem, could be under-charge, air in the system, or some reason the unit isn’t condensing the refrigerant (double check your temperature reading device against boiling water to make sure it reads correctly should read 212 F in the water).
So, now what if the temperature is reading below 95 when the pressure is 218 psig? That would mean there is sub-cooling occurring, when the liquid is being cooled below its saturation temperature.
If we see about 90 F on the liquid line, the condenser is probably doing its job, marginally anyhow.
Condenser sub-cooling range varies based on design but it is required to have enough sub-cooling to maintain a liquid full liquid line regardless of the pressure drop between the condenser and metering device (expansion valve).
The typical range will be from 5 to 15 degrees depending on the type of metering device, with 9-12 being common for TXV and EXV systems (always check with manufacturer data) and higher for capillary tube systems (which vary with load considerably)
Low sub-cooling ranges are usually reserved for close couple units where there is no appreciable pressure drop.
The sub-cooling is always taken just after the condenser, if there is a receiver or suction heat exchanger the temperature after those devices indicates how they are working not the condenser.
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I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.