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Hileg (Electrical) (OP)
19 Aug 05 15:54
A rule of thumb in air conditioning work is that 400 cfm of air is equal to 1 ton. If 400 cfm of air and a delta T of 20 degrees is achieved, this is equal to only 400X20X1.08=8640 btu/h.. This is not 12000 btu's. What am I missing here besides a college education.
trashcanman (Mechanical)
19 Aug 05 16:17
You are neglecting the latent heat portion of the cooling load.  The equation you quoted is for sensible heat.  The latent heat (3360 btu/hr) is absorbed at the coil by condensing moisture from the air.
HVAC68 (Mechanical)
20 Aug 05 0:15
You should use the equation

Q = 4.5 x cfm x Delta h

to get total heat

HVAC68

Hileg (Electrical) (OP)
20 Aug 05 18:48
Hvac68 is delta h enthalpy.
Thank you, this really helps.
HVAC68 (Mechanical)
20 Aug 05 22:21
Yes, Delta h is enthalpy difference.

HVAC68

25362 (Chemical)
21 Aug 05 10:38

The factor 1.08 reflects only sensible heat and is the product of

60 min/h * 0.075 lbm/CF * 0.24 Btu/(lbm*oF)

which when multiplied by X CFM results in

X * 1.08 Btu/h
  

Hileg (Electrical) (OP)
21 Aug 05 11:59
Thank you all,

Hileg
25362 (Chemical)
21 Aug 05 12:45

A small correction to my last paragraph, it should say: when multiplied by X (CFM) * ΔT (oF) results in

X*1.08*ΔT  Btu/h

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