This is taken from a past discussion in an HVAC-Talk thread about how to select a walk-in cooler evaporator so it will result in a suitable TD:
Could someone explain to me how you size an evaporator to get a specific TD? I tried to understand the explanation in the Heatcraft Engineering Manual, but I just can’t seem to get it.
1. Start with defining what your box temperature is.
2. Calculate the box load.
3. Determine what RH your product requires.
4. Find the proper coil TD range to achieve that RH from the Heatcraft Engineering Manual.
5. Select a condensing unit that will satisfy the box load at an evap temp equal to the box temp minus the TD.
6. Now…if you want to get real picky about it, you can plot out the system balance curve. This is a plot of Evap temp vs. Btuh which will show how the condensing unit capacity and evaporator capacity relate. Where the plots cross is where your system is balanced.
Clear as Mud?