It looks like you are mostly on the right track. Try doing it again with a more realistic supply air WB temp. With no duct loss, the supply air will be saturated, so the DB and WB temperatures will be the same. There is always some duct loss, but the WB temperature will still be very close to the DB temperature, unless duct losses are very bad.Originally posted by shophound
Let's take the OP's numbers of 80 degrees dry bulb, and add 67 degrees wet bulb for the return air temperature (52% RH). Supply will be OP's 60 degrees, let's plug in 50 for wet bulb (49 % RH). 160 CFM supply air to room @ 600 fpm.
Enthalpy difference = 11.2 Btu/lb of dry air
Specific volume of supply air = 13.0 cubic feet/lb of dry air
160 CFM divided by 13.0 cubic feet/lb = 12.3 lbs/min
12.3 lbs/min x 11.2 Btu/lb = 137.8 137.8 x 60 = 8268.00 Btuh
So, is this near the mark? The main question I had (I need to actually hold my sling psychrometer up to a supply air duct sometime) was what to enter for wet bulb for supply air.
Thanks for the brain stretcher.
Try refiguring it with supply air conditions of 60º DB and WB to represent more realistic supply air conditions.
Also look at the specific volume again. With the supply air conditions of 60º DB and 50º WB you figured it with, the specific volume should have been close to 13.2 cubic feet per pound. At 60º DB and 60º WB, the specific volume will be about 13.26 cubic feet per pound.
With return air conditions of 80º DB and 67º WB, supply air conditions of 60º DB and 60º WB, and 160 CFM, you would have about 3675 btuh of cooling. That is a much more realistic number.
Actually, now that I look at it, it looks like you figured it with supply air conditions of 50º DB and 50º WB instead of the 60º DB and 50º WB you stated, wich is why I was getting different numbers than you with the conditions you stated.
I don't know what Psychrometric chart you are using, but if you don't have one already, you should get the big laminated one Trane sells. It is 11"x17", so it is easy to read everything. You can draw on it with a soft pencil and erase the pencil marks when you are done.
It is "Form Number 1-43.191 Jan. 1983"