[Equest-users] Baseline Fan power calculation
RobertWichert
robert at wichert.org
Mon Mar 25 09:55:19 PDT 2013
You're right, Bill, my 0.5 in. H2O was too low, but we use very small
fans all the time in residential installations that run lower than
that. So when I do a LEED or 90.1 or CA T-24, I know the fan power is
lower than the motor nameplate, but I have trouble getting the small fan
manufacturers to even know what fan efficiency is... Alas...
I really appreciate the reference to mechanical efficiency in the ASHRAE
Pocket Guide. I'm going to go after that now.
Thanks again for your thoughts and insight.
Robert Wichert P.Eng. LEED AP BD&C
+1 916 966 9060
FAX +1 916 966 9068
===============================================
On 3/25/2013 9:40 AM, Bishop, Bill wrote:
>
> 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 ≤
> CFM_S *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, 7^th
> 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 StreetRochester, NY 14608
>
> T: (585) 325-6004 Ext. 114F: (585) 325-6005
>
> wbishop at pathfinder-ea.com
> <mailto:wbishop at pathfinder-ea.com>www.pathfinder-ea.com
> <http://www.pathfinder-ea.com/>
>
> PSustainability -- 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?
>
>
>
> Your thoughts?
>
>
>
>
> 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
> P_fan 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 StreetRochester, NY 14608
>
> T: (585) 325-6004 Ext. 114F: (585) 325-6005
>
> wbishop at pathfinder-ea.com
> <mailto:wbishop at pathfinder-ea.com>www.pathfinder-ea.com
> <http://www.pathfinder-ea.com/>
>
> PSustainability -- the forest AND the trees.P
>
> *From:*equest-users-bounces at lists.onebuilding.org
> <mailto: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
> <mailto:equest-users at lists.onebuilding.org>
> *Subject:* Re: [Equest-users] Baseline Fan power calculation
>
> I believe I have written about this in the past, but since this
> seems to be 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.P_fan 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 answer #1). You can however account for
> adjustments present in separate airstreams from the proposed
> design.
>
> 3.Probably not. Review the difference between CFM_S and CFM_D
> carefully at the cited Table. A CFM quantity representing the
> airflow each element encounters (CFM_D ) should be applied
> against each pressure drop adjustment, not an 'aggregate' of
> supply+return.
>
> You may also find the attached resource published by 7group
> helpful to 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>
> [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
> <mailto: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.
>
> Thanks in advance.
>
> Regards,
>
> Vamshi.
>
>
>
>
>
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