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HVAC Forums => HVAC Help => Topic started by: piru on February 11, 2016, 08:36:16 PM

Poll
Question: air at 90 db and 70 dp is cooled to 65db and 58 dp.how much heat is removed in btu/lb in the process?
Option 1: 40.0 Btu/lb votes: 1
Option 2: 28.0Btu/lb votes: 1
Option 3: 14.0Btu/lb votes: 0
Option 4: 12.75Btu/lb votes: 4
Title: AC question
Post by: piru on February 11, 2016, 08:36:16 PM
let me know how to find the answers
Title: Re: question
Post by: Admin on February 17, 2016, 09:32:00 PM
I'm not 100% sure I'm right.

We use a Psychrometric Chart to solve this problem.

A Dry Bulb of 90F and a Dew Point of 70F would give us a Wet Bulb of 75.6F, a Relative Humidity of 52% and Enthalpy of 39.55 BTU/lb.

A Dry Bulb of 65F and a Dew Point of 58F would give us a Wet Bulb of 60.6F, a Relative Humidity of 78.1% and Enthalpy of 26.8 BTU/lb.

39.55 BTU/lb - 26.8 BTU/lb = 12.75 BTU/lb.

I attached a copy of the Psychrometric Chart I used.  Hopefully it makes sense, it's been 20 years since I used one  ;D
Title: Re: AC question
Post by: Porcupinepuffer on February 19, 2016, 11:07:15 AM
The psychrometric chart I used is different, but I got 12.6 btu/lb. Mine has the wb and dp temperatures on the left side and uses the horizontal lines for dp numbers and wb lines are angled downward.
Title: Re: AC question
Post by: Admin on February 19, 2016, 12:06:37 PM
Yes I believe all his available answers are wrong.  The enthalpy for a DB of 90F and DP of 70F is actually 38.95 BTU/lb.

38.95 - 26.87 = 12.08 BTU/lb.

We all seem to be in the same ballpark.  It might be one of those, pick the most correct answers.
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