400 CFM per Ton

[Hileg]

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]

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]

You should use the equation

Q = 4.5 x cfm x Delta h

to get total heat

HVAC68

 

[Hileg]

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

 

[HVAC68]

Yes, Delta h is enthalpy difference.

HVAC68

 

[25362]

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]

Thank you all,

Hileg

 

[25362]

A small correction to my last paragraph, it should say: when multiplied by X (CFM) * [Δ]T (^oF) results in

X*1.08*[Δ]T Btu/h​