[Bldg-sim] pro-rating fan power to supply/return fans per 90.1-2007?

James V Dirkes II, PE jvd2pe at tds.net
Wed Mar 16 17:51:40 PDT 2011


Dear Nick et al,

 

90.1 does not specify how to split up the fan heat when there is more than
one fan (although I haven't read the 2010 version yet).  My goal is to model
something close to reality, so I split supply and return fan power in rough
proportion to their total static pressure in the Proposed system.

If the (proposed) supply fan moves air against 4" TSP and the return fan
against 1" TSP, I allot 80% of the baseline power to the supply fan, 20% to
the return fan.  I don't try to be super precise, just reasonably close.
This requires a little more art than science if you have a VAV supply fan
and, say, a constant volume relief fan.

On a related note, I always model Baseline systems with draw-through fans
because:

.         ASHRAE is silent on the matter and 

.         The extra fan heat in the airstream results in a little more
airflow and makes it more favorable for the Proposed system J

.         Let me know if anyone thinks that's cheating!

 

I use EnergyPlus exclusively, but I suspect that it should work for any
software to do the following as a one-shot
get-the-baseline-fan-power-correct approach:

.         Knowing that fan power is always related to CFM

.         Also knowing (or assuming) fan and motor efficiency

.         And knowing any pressure correction

.         I calculate the system delta P that will result in the desired fan
power

.         Then I use that delta P, and those efficiencies as my baseline
input values

.         Fan power comes out right on the money every time (with
auto-sizing)

 

For example, using a System 3 or 4:

CFM * .00094 = allowable BHP (assuming no pressure correction)

 

Delta P = BHP *                     (fan eff'y * motor eff'y) * 6356 / CFM

                = (CFM * .00094) * (fan eff'y * motor eff'y) * 6356 / CFM

                = .00094 *                (fan eff'y * motor eff'y) * 6356

                = 5.9746 *                (fan eff'y * motor eff'y)

 

Note that CFM is not relevant to this calculation, making it very handy for
autosizing!  Also note that I combine the fan and motor efficiencies because
E+ uses their product as the single "fan efficiency" input, rather than
keeping fan mechanical and motor efficiencies separate.  Your software may
not do that.

 

 

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: Nick Caton [mailto:ncaton at smithboucher.com] 
Sent: Wednesday, March 16, 2011 4:23 PM
To: James V Dirkes II, PE; Paul Riemer; Paul Erickson; Andrew Craig; James
Hansen; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

James,

 

I do appreciate the heads up - I've been reading the ongoing discussion with
interest!

 

In the particular case I referenced, it didn't seem a big deal to me at the
time as to my understanding my baseline supply and return fans were
operating in tandem, but as I think it through now, I see this might skew
the results somewhat with regard to fan energies in the airstreams.  

 

This has got me wondering:  What, if anything, does 90.1 or similar have to
say regarding fan heat in the baseline airstreams?  

 

I've developed a practice in eQuest of intentionally specifying baseline fan
motors as outside the airstreams.  I've done this specifically to not have
their heat incident on the airstreams, because the procedure I follow for
calculating baseline efficiencies (backing out the calculated fan energy,
Pfan, from the prescribed efficiencies), can become a potentially
cumbersome/iterative process otherwise.  For example, a simulated baseline
sizing run gives an airflow/capacity, from which I calculate Pfan, which
once entered and simulated affects the calculated capacity, which affects
the airflow, so I have to recalculate Pfan. and so forth until the reported
figures match my calculations.  For one system this is cake, but when I've
got a bunch of baseline systems it's quite a pain.

 

I recognize some others follow a baseline efficiency procedure that don't
involve the Pfan for baseline heating/cooling efficiency calculations, for
whom baseline fan heat energy is a moot concern, but for the moment that's
really another can of worms I'd rather not open.  I'd specifically like to
hear if anyone knows of what guidance is or isn't out there for whether and
to what extent baseline fan heat should be incident on baseline airstreams.


 

Thanks again to everyone for providing such continuously supportive and
productive discussions - it makes me proud to be a part of this community!

 

~Nick

 

 

cid:489575314 at 22072009-0ABB

 

NICK CATON, E.I.T.

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com 

 

From: James V Dirkes II, PE [mailto:jvd2pe at tds.net] 
Sent: Tuesday, March 15, 2011 7:43 PM
To: Nick Caton
Subject: RE: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

Dear Nick,

 

It's not a big deal, but the physics are a little different if you put some
energy from the fan in the Return air stream and some in the supply air.
Some of the return air energy is normally exhausted and the coil never sees
it.  Probably preaching to the choir here, but can't help mentioning it.

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Tuesday, March 15, 2011 5:00 PM
To: James Hansen; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

Last time I did this, I entered 0.00 kW/CFM for the return fans, and all of
the calculated kW/CFM for the supply fans - better to keep it in one place
imho to avoid the headache of explaining it.  Just got news that project
wrapped up review with a silver rating this week =).

 

If you wanted to distribute the power regardless, I'd do it by weighting
with the airflow amounts.  Generally, I don't think it matters, so long as
you can backtrack your steps if asked to explain yourself ;).

 

~Nick

 

cid:489575314 at 22072009-0ABB

 

NICK CATON, E.I.T.

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Tuesday, March 15, 2011 2:05 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] pro-rating fan power to supply/return fans per
90.1-2007?

 

App G says that if the proposed building has return fans, the baseline
building is to be modeled with them as well, sized either at the minimum
required to ensure ventilation, or 90% of the supply volume, whichever is
greater.  And the fan power calculations show that the KW allowance is for
all supply, return, exhaust and relief fans combined.  

 

How do you generally pro-rate the fan power between supply and return fans
in the baseline model?  Ultimately it probably doesn't make too much
difference, but is there an actual ruling that tells you how to do this?

 

Thanks,

 

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

1010 N. Glebe Rd, Suite 200

Arlington, VA  22201-4749

703-338-5754 (Cell)

703-243-1200 (Office)

703-276-1376 (Fax)

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

 

 

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