Heating Help

calculating btu capacity of an evaporator

does anyone know how to calculate the btu capacity of a dx evaporator coil? i know it has to do with taking a delta t reading across the coil, and some wet bulb readings but have yet to get the full, correct calculation

Here you go

In order to determine what the evaporator coil is doing at a particular time, you will need to know both the sensible and latent activity of the coil.

You can do this a number of ways.

You will need any two of the following pieces of information for both the supply and return air:

Dry bulb temperature, wet-bulb temperature, relative humidity. You will also need an accurate cfm value.

These values need to be obtained just before and just after the evaporator coil, not at the supply registers and return grills!

You will also need a psychrometric chart, on which you will plot the supply and return air points.

Once done, you can determine the enthalpy (heat content) of the supply and return air and then calculate the difference.

With all of these goodies in hand, plug those values into:

Qtotal = 4.5 x cfm x change in enthalpy

This will give you the evaporator capacity at that point in time (snapshot).

The 4.5 is simply a constant that changes the minute in cfm to the hour in Btuh, as well as accounts for the conversion between air volume (cubic feet) and the weight of air.

I hope this helps....

Have fun and let us know how you make out.

thank you

thanks for the reponse.i'll let you know