Importance of On Coil and Off Coil Temperatures for AC Unit

For cooling, you cannot know total load of the coil if you do not know latent heat it would remove.

Latent heat is read off a psychrometric chart, or Mollier diagramm (which are similar, however, not the same) if you know the dew points (humidity) of the air before and after the coil.

Lujo,

You refer to three rating parameters that have relevance in cooling mode:

1. DB/WB temperatures of air entering the coil

2. Air DB/WB temperatures of air leaving the coil

3. Sensible and latent heat removal produced

Any two of these together are sufficient to specify the performance of the AC unit since the third parameter can calculated from them.

BTW sometimes specs state all three parameters. Since any two determine the third, this can result in a contradiction. Here the MC has to request a clarification.

The amount of water vapor in the air will affect the load on the coil.It requires heat (load) to evaporate the water,and regains the same amount when it condenses into liquid.The total heat minus the sensible heat will give the latent heat for heating applications,likewise the opposite for cooling , so all other things being equal, more energy will be required to heat a space that has high humidity than a space that is dry.

By specifying the WB/DB temps they are covering this variable to give more exact figures for all environmental conditions(Compare Phoenix Arizona to Tampa Florida).