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# 400 CFM per Ton

## 400 CFM per Ton

(OP)
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.

### RE: 400 CFM per Ton

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.

### RE: 400 CFM per Ton

You should use the equation

Q = 4.5 x cfm x Delta h

to get total heat

HVAC68

### RE: 400 CFM per Ton

(OP)
Hvac68 is delta h enthalpy.
Thank you, this really helps.

### RE: 400 CFM per Ton

Yes, Delta h is enthalpy difference.

HVAC68

### RE: 400 CFM per Ton

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

(OP)
Thank you all,

Hileg

### RE: 400 CFM per Ton

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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