Arkansas HVACR NewsMagazine March 2025

HVACR NewsMagazine March 2025

Tech News

Here, we will primarily talk about the indoor coil in cooling mode. We have a very basic representation of an indoor coil in the illustration below. I have cut away sections of the copper piping to get a sense of the refrigerant's progression as it travels along the piping, absorbing heat and changing from a saturated state to vapor. You will also notice in the image that this indoor coil has a TXV attached to it.

to note that these numbers are highly variable from system to system. At this point, the refrigerant is considered saturated. As stated earlier saturation is a state where both liquid and vapor exist at the same time and place. As the refrigerant continues its journey through the indoor coil, it begins to boil and absorb heat from the inside air as it passes over the coil. Most of the heat is absorbed when the refrigerant changes state, but we can't measure the quantity because the temperature stays the same. The illustration should not be thought of as an exact replication of the refrigeration changing state, but it shows the incremental change from liquid to vapor as the refrigerant boils. The desired outcome in an efficient system is to have the refrigerant in its fully vapor state before it leaves the evaporator coil. Once the refrigerant has fully transitioned into vapor, it will continue to absorb heat. This temperature increase is considered superheat. Theoretically, the lower the superheat, the more efficient the system would be. However, there are risks associated with very low superheat, such as undesirable liquid reaching the compressor.

The TXV is a mechanical metering device that requires a full column of liquid refrigerant to be fed to it. This full column of liquid is pushed through the TXV's modulating valve, where it experiences a sudden pressure drop as it is sprayed into the expansion line. (Note: this simple illustration does not show distributor tubes for reasons of simplicity.) During this sudden pressure drop, the refrigerant flashes into a percentage of vapor and liquid. For the purpose of this article, we will say 75% liquid and 25% vapor, but it's important