# [Equest-users] Baseline Fan power calculation

Maria Karpman maria.karpman at karpmanconsulting.net
Mon Mar 25 11:00:08 PDT 2013

```Robert,

The fan mechanical efficiency implied in 90.1 may also be deduced from the
formula given for A in the footnote to Table 6.5.3.1.1A: A = sum of (PD ¡Á
CFMD/4131). ¡®A¡¯ is expressed in units of BHP, so by comparing this
equation to the theoretical equation  Fan BHP = (cfm x static press "w.c.) /
(6356 x fan efficiency) we get mechanical efficiency of 4131/6356~65%

Maria

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Bishop,
Bill
Sent: Monday, March 25, 2013 12:40 PM
To: RobertWichert
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Fan power calculation

Hi Robert,

I apologize for confusing motor efficiency with fan mechanical efficiency.
Since section 6.5.3.1 describes the maximum allowable fan power, we can
expect it to use conservative allowances for mechanical efficiency and
static pressure. Using the allowance for bhp (bhp ¡Ü CFMS*0.00094 + A) and
neglecting the pressure drop adjustments (A), we can determine static
pressures (SP) for a range of mechanical efficiencies by setting 0.00094 =
SP/(6356xefficiency). Using a range of mechanical efficiencies from 0.4 to
0.6 (ASHRAE Pocket Guide, 7th ed. p. 246), we get a range of static
pressures from 2.4¡± to 3.6¡± w.g. That¡¯s higher than your 0.5¡±, but seems
reasonable to me for determining max allowable fan power.

Regards,

Bill

William Bishop, PE, BEMP, LEED AP | Pathfinder Engineers & Architects LLP

Senior Energy Engineer

134 South Fitzhugh Street                 Rochester, NY 14608

T: (585) 325-6004 Ext. 114            F: (585) 325-6005

<mailto:wbishop at pathfinder-ea.com> wbishop at pathfinder-ea.com
<http://www.pathfinder-ea.com/> www.pathfinder-ea.com

P   Sustainability ¨C the forest AND the trees. P

From: RobertWichert [mailto:robert at wichert.org]
Sent: Monday, March 25, 2013 12:17 PM
To: Bishop, Bill
Cc: Nick Caton; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Fan power calculation

Thanks Bill,

I verified that the references you gave lead to the result of the
calculation in the spreadsheet, but if the theoretical calculation below has
any validity at all, the assumed fan efficiency in 90.1 is about 8%.  I used
a fan static pressure rise of .5 in. H2O.

So, this fan efficiency seems a little low to me.

You said "Fan efficiency is from ASHRAE 90.1, Table 10.8 as described in G3.
1.2.9." but Table 10.8 is Fan Motor efficiency, which is about 85%.  I have
no problem with that, but to use the theoretical fan power equation I need
the Fan efficiency.  Do you have any idea what fan efficiency is assumed in
90.1?

I also notice that the fan power calculation in 90.1 does not include a
pressure rise component, which I find odd.  Is the pressure rise across the
fan assumed somewhere?  From the checks described above, it appears to be
about .5 in. H2O.  Does this seem reasonable?

Robert Wichert P.Eng. LEED AP BD&C
+1 916 966 9060
FAX +1 916 966 9068

===============================================

On 3/25/2013 8:01 AM, Bishop, Bill wrote:

Robert,

I¡¯m sure Nick is typing furiously as I write this, but I¡¯ll probably
finish before him.

Taking a quick look at the spreadsheet that Nick attached to his email, it
calculates baseline fan power using the method described in Appendix G and
6.5.3.1.1. Fan efficiency is included in the Pfan calculation per G3.1.2.9.

Fan efficiency is from ASHRAE 90.1, Table 10.8 as described in G3.1.2.9.

Regards,

Bill

William Bishop, PE, BEMP, LEED AP | Pathfinder Engineers & Architects LLP

Senior Energy Engineer

134 South Fitzhugh Street                 Rochester, NY 14608

T: (585) 325-6004 Ext. 114            F: (585) 325-6005

<mailto:wbishop at pathfinder-ea.com> wbishop at pathfinder-ea.com
<http://www.pathfinder-ea.com/> www.pathfinder-ea.com

P   Sustainability ¨C the forest AND the trees. P

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of
RobertWichert
Sent: Monday, March 25, 2013 10:52 AM
To: Nick Caton; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline Fan power calculation

on-topic, I'll bring it up again.

The theoretical fan power equation, Fan BHP = (cfm x static press "w.c.) /
(6356 x fan efficiency) can be used to calculate the theoretical fan BHP,
but you need the fan efficiency.  This seems to be NOT the way the BHP is
being calculated by the spreadsheet that was attached previously.

Is there a reason for that?

Was there an assumed fan efficiency used in the spreadsheet, and was it
based on anything particular, or just a conservative assumption?

Thanks!

Robert Wichert P.Eng. LEED AP BD&C
+1 916 966 9060
FAX +1 916 966 9068

===============================================

On 3/25/2013 7:34 AM, Nick Caton wrote:

Hi Vamshi,

I advise carefully reviewing the footnotes under Table 6.5.3.1.1A, which
include procedure for determining baseline fan power adjustments.

1.      Pfan represents the sum power of all fans in a given baseline system
(with exception to baseline systems #6/#8, fan powered VAV boxes).

2.      Again, there is not a separate supply/return power calculation (see
airstreams from the proposed design.

3.      Probably not.  Review the difference between CFMS and CFMD carefully
at the cited Table.  A CFM quantity representing the airflow each element
encounters (CFMD) should be applied against each pressure drop adjustment,
not an ¡®aggregate¡¯ of supply+return.

cement your procedural understanding of how to arrive at the sum total for
¡°A.¡±

Regards,

~Nick

cid:489575314 at 22072009-0ABB

NICK CATON, P.E.

SENIOR ENGINEER

Smith & Boucher Engineers

25501 west valley parkway, suite 200

olathe, ks 66061

direct 913.344.0036

fax 913.345.0617

www.smithboucher.com

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of vamshi
ranga
Sent: Monday, March 25, 2013 8:17 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Baseline Fan power calculation

Hi All,

I have a doubt regarding calculating fan power as per ASHRAE 90.1-2007.

My proposed system has, supply return and exhaust fans separately with kW
and cfm mentioned for each fan.

So, for calculating baseline fan power,

1. should I calculate it separately for each fan type?

2. can I take pressure drop adjustment both in supply and return fan power
calculation?

3. should I sum all the cfms of supply, return and exhaust and than apply
pressure drop adjustments to aggregated cfm.

Kindly help me out in solving the doubt. If you have any example fan power
calculations, or web links those could guide me regarding this, plz do share
them.

Regards,

Vamshi.

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