[Bldg-sim] Baseline Fan Power vs. Proposed Fan Power

Robby Oylear robbyo at rushingco.com
Mon Jan 7 10:17:53 PST 2008

So I updated the kW/CFM using a conversion of kW = bhp * .7457 / motor
efficiency, but the fan heat delta t is still greater than 3 deg F, which
takes my SAT of 51 deg F down below the 6 deg F minimum between the air
and the coil.  

I'm not sure if this is a result of the calculation or the actual air
handler selection cutting it too close on the 6 deg F separation.  I'm
wondering if it would be alright to manually set the fan delta t to 3 deg
F? The calculated value ranges from 3.06 to 3.11 deg F, meaning I'm only
six thousandths of a degree off to the point of fixing the error. I assume
there isn't a way to get eQUEST to accept a coil delta T of less than 6
deg F, so this seems like the only way to fix this error when I'm using
actual submittal data.

Robby Oylear
Project Engineer
direct: 206.788.4571
cell: 206.354.2721

-----Original Message-----
From: Michael Rosenberg [mailto:Michael.Rosenberg at state.or.us] 
Sent: Friday, January 04, 2008 2:24 PM
To: bldg-sim at lists.onebuilding.org; Robby Oylear
Subject: Re: [Bldg-sim] Baseline Fan Power vs. Proposed Fan Power


Either way the kW should be somewhat close unless you are performing the
calculation incorrectly. I know this is quite possible, since a while
back, as a test, I gave the nightmare formula in the 2004 version of
Appendix G and a bhp to three engineers in my office and got three
different answers! That said, the correct way to do it is to convert bhp
directly to kW. Don't forget to include the efficiency of the motor in the
calculation. By the way, that formula will be gone as soon as the 2007
version of Appendix G is published.  


Michael Rosenberg
Oregon Department of Energy
625 Marion St. N.E.
Salem, OR 97301-3742
Phone : (503) 373-7809
Fax: (503) 373-7806

>>> "Robby Oylear" <robbyo at rushingco.com> 1/4/2008 12:51:12 PM >>>
In calculating the fan power for the baseline system I used the equation
in section G3.1.2.9.  For my proposed building I took the bhp from the
equipment submittals and used the same equation in G3.1.2.9 to convert to
kW in order to calculate my kW/CFM input in eQUEST.  However, after doing
this I get an error when running the simulation due to the air passing
over my chilled water coils being less than 6 deg F more than my chilled
water temperature.  After looking into the problem I found that eQUEST is
calculating a higher fan heat gain than what was shown in the submittal,
based on the kW/CFM input.  I then noticed that if I use a standard
conversion from bhp to kW (1 bhp = 0.7457 kW), I get a lower kW.  Am I
correctly using equation G3.1.2.9 for the proposed fan power or should I
just be doing a standard conversion?  It doesn't seem possible to get the
right temperature across the coil without calculating my proposed fan kW


Robby Oylear

Project Engineer

direct: 206.788.4571

cell: 206.354.2721

 <http://www.rushingco.com/> www.rushingco.com 


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