CFM per ton

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Dear Sirs

I made a simulation of two one floor building for offices.
The results indicate the following:
Area / Sypply / Design Capacity
2810 sqft / 3974 cfm / 271cfm/ton
1017 swft / 1306 cfm / 302cfm/ton

Why the software gives these results so differente from the usual rule
of thumb 400cfm/ton?

Thank you very much.

Khristiam Alvarez

Khristiam Alvarez's picture
Joined: 2012-11-12
Reputation: 0


I don't know the exact situation of your building, but here are some things
to consider:

The CFM supplied to a space is driven by the sensible loads in the space.
The coil capacity, however, is driven by the sensible loads in the space,
the latent loads in the space (moisture from people breathing, for example),
the additional sensible loads on the coil (a fan, for example), and the
additional latent loads on the coil (outdoor air, for example).

If the only loads on your system were sensible space loads, your CFM/ton
would always be about 545 cfm, based on the sensible heat equation
(generally BTU/hr = 1.1*CFM*(deltaT), assuming the temperature difference
between your room air and your supply air is about 20F. As you add latent
loads and coil loads, the capacity increases while the CFM remains the same.
As a rule it often comes out to around 400 CFM/ton as you say. But if you
have a high percentage of latent loads or high coil loads, the capacity will
increase more and the CFM/ton will go down further. Additionally, if you
have a high deltaT between supply air and room air, you will require lower
CFM to meet the load, and your CFM/ton will again go down.

To summarize, the things I can think of that will give you low CFM/tons are:
--High percentage of latent loads
--High percentage of coil loads
--Lower than normal cooling supply air temperature
--Higher than normal room setpoint temperature

I hope this helps,

Christian Kaltreider's picture
Joined: 2011-09-30
Reputation: 1

Hi Khristiam:
You have orientation of windows differences, roof load vs no roof load, and quite possibly equipment load differences.
John Aulbach

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1