Evaporating and the TP Chart

​The refrigerant leaving the metering device is at a much lower pressure than the liquid line delivering it to the metering device. The liquid is pushing forward into the evaporator coil and rapidly expanding (hence the term “expansion-valve”) into droplets and vaporized refrigerant.
The droplets shed down the tube walls picking up the heat energy from the external load (heat) being applied to the exterior of the evaporator coil. As the droplets absorb enough energy (latent heat of vaporization) they transform into vapor (boil). The refrigerant will be in a state of “saturation” (equal parts of vapor and liquid) until the last drop of liquid is boiled away.

​The pressure on the low-side is dictated from the vaporization process and will remain constant while the continuous boiling occurs. This is the saturation pressure and it has an associated saturation temperature that goes with it. If the low side, (suction) pressure is reading 92.7 (~93) psig on your manifold, and the refrigerant is 404A, then you know the evaporator is operating at 44 F per your TP (Temperature Pressure Chart).

But now you put your thermometer on the suction line leaving the coil (because you usually can’t check the coil directly) and the temperature is 54 F, what up with that?
Coils are designed with a function called “super-heat” built in. It is a section of coil exposed to the load that allows the refrigerant gas to be further heated up by the heat available in the load. This function, the super-heat function, protects the compressor from liquid flood back, a dangerous condition that will severely damage the compressor.
As a note, here, expansion valve manufacturers usually pre-calibrate new thermostatic expansion valves to operate at 10 degrees Fahrenheit of super-heat, I suggest you don’t adjust it unless you REALLY, REALLY, know what you are doing.
If you could measure the evaporator it still wouldn’t read 44 degree directly since you must account for all the interference from the blower/heat-load interacting with the coil surface, a likely reading would be between 46 F and 54 depending on where you take it and how well the reading is insulated from the process. Manufacturers use a device called a “thermos-well” to obtain an approximation of the coil temperature when needed. The thermos-well is a tube inserted into the refrigerant line which is partially protected from the load and can readily sense the coil temperature with a thermocouple or temperature gauge. (See below.)