HVAC Equations

[brattong]

Hi,

May I know when should I use q = 1.08 x cfm x (Ta-Tb), and when to use q = 4.45 x cfm x (enthalpy A - enthalpy B)?? I am really confused. Please advise.

 

[krb]

q=1.08 x cfm x (Ta-Tb) relates to sensible heat, q=4.5 x cfm x (delta h) relates to total heat, which is sensible plus latent heat.

 

[briand2]

Just use the first equation when it is certain that there will be no change in the moisture content (air passing through a heating coil, for example) but the second equation if the moisture content will be changing (due to condensation on a cooling coil, for example).

Regards,

Brian

 

[Koske]

You should be also careful with using volumetric flow instead of mass flow, because when air passes through cooling coil it changes density.

 

[brattong]

Koske, what do you mean ?? Can you explain more?

 

[ChrisConley]

I think that Koske is referring to the difference between scfm and acfm: standard cfm vs. actual cfm. When using a volumetric flowrate it is important to remember that the actual cfm will decrease as the temperature of the air decreases due to density increase. Likewise air volume will increase when heated.

One way to avoid this problem is to use mass flow rate (which of course can't change without violating a few physical laws), the other is to use scfm. Scfm refers to the flowrate of air at standard pressure and temperature. Universally scfm is used to the point where most people don't bother including the 's'.