Cooling/Heating Effects on Duct Sizing 2

[Zeroman]

To calculate the fan cfm for my air handling unit should I use:

1. CFM=Qlatent/1.1/(Toutside-Tinside)

OR

2. CFM=(BuildingVolume)*(AirExchanges/hour)/60?

If I do use the first equation, then for different loads (coolong and heating) the cfm works out to be different. Should I size the ducts based on the larger cfm or the smaller? Thanks !

 

[Zeroman]

Sorry, A Correction to my 1st CFM Calculation, I meant Qsensible.

 

[quark]

You should go with larger cfm value. If you are increasing fresh air load accordingly, check for total load again.

You can check for cfm based on total load by the equation H_total = 4.5 x cfm x Enthalpy Difference.

Note: sensible heat = 1.08 (or 1.09) x cfm x Temp. Diff.
(either of these to values is sufficient in most of the cases)

 

[mt31]

A heating and cooling load calculation of the space to be conditioned will give required cfm for for your application. Always size to the larger duct when utilizing ducts for heating and cooling. Heating does not require as many cfms as cooling does.

 

[Zeroman]

Thanks quark and mt31.