Baseline Fan power calculation

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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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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 answer #1). You can however account for adjustments present in separate 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.

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.

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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

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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

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FYI, the newest LEED EAp2 spreadsheet (attached) has embedded calculations
for the baseline fan power on Air-Side HVAC Details tab, which I think is
quite handy.

Maria

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Hi Robert!

Bill is quite correct (although I was not watching my inbox so closely!) - BHP is being determined in the 7group spreadsheet referencing the Fan efficiency table prescribed in 90.1. I am not an expert at de-constructing others' excel files, but observing the entries under the "fan motor efficiency" column, it's clear they're referencing an array which returns differing efficiencies, in turn dependent upon the calculated bhp. I follow a similar procedure in my personal spreadsheet, but I haven't figured out how they hid the efficiency array =).

~Nick

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NICK CATON, P.E.

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A couple kind folks immediately replied with the solution - in the off-chance I'm not the only one who didn't know this little trick:

You can hide worksheets just as you can hide rows/columns. The efficiency array is tucked into a hidden worksheet which can be reviewed by right clicking the worksheet tabs below and selecting "unhide" in the context menu.

Thanks... and remind me never to play hide and seek with you guys, haha =).

~Nick

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NICK CATON, P.E.

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Thanks Maria, and wow!

I remember first seeing the previous LEED v3 EAp2 spreadsheets for the first time and thinking "WOW, this is intense" ... I had no idea what was coming!

I appreciate the heads up - this will require some heavy review before my next new LEED project to adjust my workflow & supporting calculations. Playing around just a bit, this spreadsheet looks to have its own 'learning curve' to utilize, like some of the LEED Online templates over time, but being excel perhaps this is more easy to troubleshoot/work around...

Regards,

~Nick

[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.

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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

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It seems to just sum them up in cell M29. Is that what you meant,
Maria? I don't see any calculations, they just ask for you to input the
fan power.

Am I missing something?

Robert Wichert P.Eng. LEED AP BD&C

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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 ? 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

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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

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Robert,

Cell M30 (Allowed Fan Power) is auto-calculated by the template based on the
baseline system type, supply flow rate, pressure drop adjustments, and the
corresponding CFMd entered by user in rows 33-44. Cell M29 (System Fan
Power) which adds up user-entered supply, return/relief, and exhaust fan
power, can then be compared to the auto-calculated value in M30 to verify
compliance. I found the template to be pretty user friendly; the only catch
is that all HVAC-related tabs are inter-connected, so you have to fill out
the General HVAC tab in order for other HVAC tabs to behave properly.

Maria

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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 ??
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

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Dear Nick Caton,

Thank you very much for valuable inputs, from this it is clear that,

1. However there are different motors used in Proposed case for supply,
return and exhaust with different kW/cfm, we need to use one single motor
in with one kW/cfm in baseline case
2. In EAp2 Section 1 4 Tables of LEED under "Table 1.4.2", it asks
for separate wattage of supply, return and exhaust fans in baseline case,
So, there we need to enter as "Not Applicable"

Please let me know if my understanding is wrong.

Thanks,
Vamshi.

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All,

It seems there is much confusion on this issue. The Proposed fans are all
modeled explicitly (with the fan power as designed). In the Baseline case
each HVAC system fan power (Section G3.1.2.9) is calculated using the
supply airflow (Section G3.1.2.8) based on 20 F space to supply air
temperature difference. If the Proposed system uses multiple fans, then the
calculated system fan power is distributed among the fans in the Proposed
design. Appendix G does not provide any guidance for how this power should
be distributed among the system fans in the Baseline system, but usually
projects use the same proportion of the total system fan power as the
Proposed design. All the fan power can be modeled as a single supply fan,
but that will apply all the heat gain from the fan to the supply airstream
which may not be realistic for large systems. Note that exhaust fans in the
Proposed design that operate continuously during occupied periods are
considered part of the system and their power is included in the calculated
Baseline fan power. Exhaust fans that operate intermittently are considered
a process load and must be modeled identically in both cases. If the
Proposed process exhaust fan motor is larger than one HP, the motor
efficiency is regulated by Section 10 and savings may be claimed for using
high efficiency motors as compared to the minimum efficiency from Table
10.8.

Hope this helps!

Cam Fitzgerald

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Hi,

Thanks, but it is difficult to understand (some confusion). Let's say the
Proposed system has following specifications

Supply fan cfm = 40000
Supply fan Wattage = 20000
Return fan cfm = 35000
Return fan Wattage = 18000
Filter used = MERV 12
Exhaust fan cfm = 5000
Exhaust fan Wattage = 2500

And Baseline (0 deg orientation) has following specifications when
simulated with default fan kW/cfm and 20 deg F difference,

Supply fan cfm = 30000
Supply fan Wattage = 15000
Return fan cfm = 25000
Return fan Wattage = 13000

1. Please tell me how to distribute kW/cfm in basecase and from where will
I get baseline default exhaust specification?
2. Won't the change in baseline when we input calculated value in eQUEST
due to heat gain
3. Should we take all the 4 orientations average cfm for calculating
baseline cfms
4. Tell me how to calculate baseline fan power from above values

If, this example is confusing. I request you to give your own example for
understanding.
I have been having doubt related to fan power calculation. I am eager to
learn how to do it properly to avoid any GBCI comments.

Your valuable time put in resolving my doubt is appreciated.

Thanks,
Vamshi.

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So you are still confused. Hope this clarifies the process for you:

1. Example baseline fan power distribution calculations (other methods are
acceptable, but this is one of the most common approaches)

Proposed system:

Total fan kW (20+18+2.5=40.5 kW)

Design supply fan percent of total power (20/40.5=0.494)

Design return fan percent of total power (18/40.5=0.444)

Design exhaust fan percent of total power (2.5/40.5=0.062)

Baseline supply airflow calculated per Section G3.1.2.8 (Note: rarely a nice
round number, but I will use your number for this example) - 30,000 cfm

Assuming the baseline system is VAV (system types 5 - 8), and using a
pressure drop adjustment factor of 0.5 IWC (MERV 12 filter) , calculated
system fan power is 34.196 kW

Using the same ratios as the Proposed system, the Baseline fan power would
be: 16.893 kW supply; 15.183 kW return; and 2.120 kW exhaust.
(16.893+15.183+2.12=34.196)

You can use this power to determine the kW/cfm for each fan.

2. The supply fan may slightly increase the supply temperature, but this is
usually minimal and does not significantly impact the supply air
temperature.

3. The baseline case prior to rotation should be used.

4. See response to #1.

Good luck!

Cam

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Hi Cam,

This is so nice of you, I really appreciate your efforts in for clearing my
concepts and helping me out. This is really useful.

Here by I want to share some of the very useful links for energy modelers.

http://energy-models.com/tools/leed-fan-calculator
http://www.gbci.org/files/leedonline/Advanced_Energy_Modeling_for_LEED_V2_1c.pdf
http://www.rmi.org/ModelingTools
http://www.rmi.org/Content/Files/EMIT%20User's%20Guide.pdf

Thank you Mr. Nick, if initial clarification.

Thank you,
Vamshi

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With regards to LEED reviews, what we have done in the past, which has been successful, would be the following:

1. Calculate Total Proposed Design Fan Power --> in your case: 20,000 + 18,000 + 2500 = 40500

2. Calculate % Proposed Design Supply Fan Power --> 20,000 / 40,500 = 49.383%

3. Calculate % Proposed Design Return Fan Power --> 18,000 / 40,500 = 44.444%

4. Calculate % Proposed Exhaust Fan Power --> 2,500 / 40,500 = 6.173%

5. Calculate Total Baseline Fan Power allowances per Appendix G, G3.1.2.9 (another way to check your fan power allowance calcs is the newest version of COMcheck, which includes the fan power calcs, finally :) )

6. Apply the above percentages to the Total Baseline Fan Power allowance in order to breakout that allowance between Supply Fans, Return Fans, and Exhaust Fans.

7. The above percentages are fixed and therefore have nothing to do with Baseline rotations.

8. If you enter in the Baseline fan power as kW/CFM type input, the kW/CFM (per the G3.1.2.9 equations) is fixed and would not vary by rotation. The total kW does vary, based on calculated CFM, which does vary by rotation (though not a lot). eQuest is already taking this into account via the kW/CFM input.

Applying the percentages as described above is really the only assumption you can make for the Baseline model, other than to not break out the fan power at all, which I would argue is completely appropriate in many cases since they only include supply fans and some small exhaust fans. In such cases, taking the time to break out the fan power in the Baseline could be considered a waste of valuable time because this has an insignificant effect on the overall results. This is an area that I often simplify and ignore unless the project has significant return/exhaust fans (like a lab), or the GBCI reviewers make a fuss over this. My guess is, however, that the new 1.4 spreadsheet will give us no choice but to break out the fan power as desired by the GBCI reviewers.

Hope this helps.

Regards,

JAH

James A. Hess, PE, CEM, BEMP

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