[Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
James Hess
JHess at tmecorp.com
Mon May 13 15:39:20 PDT 2013
Excellent post Nick! :)
Seems like every time I read some part of ASHRAE 90.1 in detail, I come away with learning something new.
Table 6.8.1.F does specify Et for the larger (> 2500 MBH) steam boilers after 3/2/2010 (see screen shot below)
I wonder then if the values for the larger capacity hot water boilers is either a misprint, or since this size boiler is not under the AHRI I-B-R testing document, the 90.1 committee didn't know what to put so they left it as Ec (i.e. seems like there's a hole here).
This is a bit confusing, but my overall take away is that Et is specified enough times in the table that the intent is to account for both the flue losses and the other major losses included in the Et values (i.e. skin/jacket/convection heat losses). Therefore, I think we should be using Et values whenever possible.
For smaller capacity hot water boilers, those values are available from AHRI. For larger, well, we don't run into a lot of those so this may not be a big deal (i.e. usually, you put in more smaller boilers versus fewer larger boilers, because your "N+1" redundant capacity costs much less). For the steam boilers, I believe we just have to get that data from the manufacturer, as available. For example, this info is available from Cleaver Brooks and Hurst. I've seen it.
Regarding the last item of your post, I've usually just used the default natural draft boiler and its associated curve in eQuest. I would then compare to the better condensing curve for the Proposed Design, since just about all of our projects utilize the high efficiency condensing hot water boiler type. The natural draft curve is not that good, but I interpret section G.3.1.3.2 literally, and therefore use the natural draft boiler curve. I would agree that what you are doing is more realistic because I don't think many manufacturers make or projects utilize the natural draft boilers anymore. The more demanding emissions and combustion controls associated with modern boilers require (I believe) a powered inlet air system. I've never seen a natural draft boiler on any of our projects and I've been in this business since 1999, or ~ 14 years, unless we were removing them, in which case they were replaced with new boilers that had the combustion inlet air fans. In short, I don't know why the Appendix G boiler is a natural draft boiler, I've always been curious about this, but using the natural draft boiler helps to show some savings versus more modern boilers, so I'm cool with it.
I hope this is somehow helpful.
Thanks! :)
[cid:image009.jpg at 01CE5000.CBBA9D70]
Regards,
JAH
James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR
Mobile: (501) 351-4667
From: Nick Caton [mailto:ncaton at smithboucher.com]
Sent: Monday, May 13, 2013 5:09 PM
To: James Hess; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
So, I see a number of logical paths forward for large 90.1 baseline boiler modeling. These scale in difficulty from a documentation standpoint, but each degree of effort can net additional potential for a better performance rating:
a) [EASIEST] Conscientiously take a step back from reality and choose to not model any losses beyond flue losses. You can justify this decision by pointing at the fact 90.1 does not regulate nor set any bar for non-flue losses in this highest capacity range, therefore none should be modeled (LEED CIR response language often follows this format). Procedure: Enter boiler HIR as 1/Ec for both baseline and proposed models.
b) [LESS EASY] Assert your proposed equipment's documented thermal efficiency, which includes non-flue losses and any means of mitigating/recovering those losses (i.e. jacket insulation, flue heat recovery means) may be modeled as nothing in 90.1 precludes this degree of detail. In turn, opt to model the baseline without any such non-flue heat losses (or gains), as the means by which to do so aren't prescribed. Procedure: Enter baseline boiler HIR = 1/Ec, and proposed boiler HIR = 1/Et
c) [NOT AS EASY] Maintain the same difference in thermal vs. combustion efficiency between the proposed and baseline models, in keeping with Jim's suggestion below. Example (using made up numbers): Proposed boiler has a combustion efficiency of 88% and a thermal efficiency of 86%. Baseline boiler is prescribed to have an 80% combustion efficiency, therefore the same boiler equipment thermal efficiency should be 78% (2% non-flue losses). Conversely, if your system's thermal efficiency is an improvement over its combustion efficiency, the same logic holds your baseline should appreciate the same net efficiency gains. Procedure: Proposed HIR = 1/Et PROPOSED, Baseline HIR = 1/(EcBASELINE-(Ec-Et)PROPOSED)
d) [LEAST EASY] To permit the proposed system's efficiency measures to be fully realized, identify the features NOT required by 90.1 or its referenced testing standards (haven't done the legwork, but this list may include stuff like flue heat recovery means, jacket insulation, and similar). Then, identify/document a real-world packaged boiler system of identical/similar capacity to the proposed equipment, excluding those features, and still matching the prescribed minimum combustion efficiency. Document and cite this equipment's thermal efficiency for use in the baseline model. Procedure: Baseline HIR = 1 divided by that thermal efficiency, Proposed HIR = 1/Et.
It's possible any or none of these approaches would be acceptable to a LEED reviewer for a given project, but anecdotally I have successfully used each mode of logic in other areas.
As long as we're on the topic, I'd like to hear other's thoughts on a closely related point - I've always matched my baseline boiler part load curve to my proposed equipment's curve (whether it is sourced from the library or "custom-rolled" to match real world equipment). Has anyone successfully used (and documented) different curves between baseline& proposed models for boiler equipment without a LEED reviewer taking issue? If so, on what did you base the baseline curve?
Thanks as always for the great discussion!
~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 James Hess
Sent: Wednesday, May 08, 2013 9:19 AM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
This is an interesting discussion.
Regarding the email below, I think that's a reasonable approach. I also agree with the previous posts in this thread.
To add some more to this particular discussion ... we know the two efficiencies are not the same, but I agree, getting good data on jacket/skin/radiation/convection (whatever you want to call them) losses or thermal efficiency values is sometimes difficult to get.
Sometimes, we get lucky and it's available. For example, Cleaver Brooks publishes a value of 0.25% of full load for the CBLE fire tube boiler product line.
However, I can see those losses being much greater than that for different boilers, depending on the specific boiler design.
Also, for hot water boilers, you may get lucky and find both the Combustion Efficiency and the Thermal Efficiency in the following AHRI publication (there may be a more recent version available, not sure):
[cid:image001.png at 01CE4B3C.1B9EDC80]
[cid:image003.jpg at 01CE4FFD.6ED5AB20]
I think you can see from that publication that the difference between thermal and combustion efficiency is sometimes significant, depending on the specific make and model of the boiler.
Also, you can check the AHRI online product directory
http://www.ahridirectory.org/ahridirectory/pages/cblr/defaultSearch.aspx
Here's an AERCO example ... these seem reasonable; the Thermal Efficiency values are <= Combustion Efficiency values
[cid:image004.jpg at 01CE4FFD.6ED5AB20]
Here's a Fulton example
[cid:image005.jpg at 01CE4FFD.6ED5AB20]
Some of these are not making sense, since Thermal Efficiency is > Combustion Efficiency for several of the entries
We know (or at least I thought we knew) that thermal efficiency should always be less than combustion efficiency, never greater, since thermal efficiency includes the flue losses plus the jacket losses. If that's true, then why does both the online directory and the publication show thermal efficiency values that exceed the combustion efficiency values ???
Perhaps AHRI has a different definition of thermal efficiency versus combustion efficiency ? Or there are misprints in the AHRI online directory and publication ?
Interesting ...
It also doesn't make sense to me that ASHRAE 90.1 calls for Et for small boilers, but Ec for larger boilers. So we ignore the jacket losses for larger boilers, even though they are much greater? I think the issue is that the AHRI IBR testing (i.e. ANSI Z21.13) only covers boilers up to 2,500,000 Btu/hr, then it's the wild west after that, unless the larger boilers are covered by an ASME standard (not yet reference by ASHRAE) ?
One last interesting piece of info ... the AHRI IBR ratings per ANSI Z21.13 are based on entering water temp conditions of 80 degrees, even for non-condensing boilers, even though operation at that EWT condition would void the boiler warranty and destroy the boiler (i.e. condensing liquids dropping out in stack gas, then rusting out the stack and boiler). I've always been a bit curious as to why boilers are rated at conditions that they cannot operate at.
Any feedback on this would be appreciated.
Thanks! :)
Regards,
JAH
James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR
Mobile: (501) 351-4667
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 Jim Fowler
Sent: Monday, April 29, 2013 11:39 PM
To: 'Nick Caton'; 'Busman, Michael R'; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Cc: 'The Watt Doctors - Dave Weigel'
Subject: Re: [Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
I smile at this quandary (sorry...), having asked it myself, but not in the context of eQuest... I've rationalized that the combustion and thermal efficiencies are identical when the boiler is in a conditioned space, because the jacket losses heat the space. I typically use a 1-2% downrate of the combustion efficiency to get to thermal, if the boiler is in an unconditioned space.
Jim Fowler, PE, CBCP, CEA
++206-954-3614
[GEA logo cropped]
Audits and Commissioning of Industrial
Municipal and Commercial Buildings
www.globalenergyaudits.com<http://www.globalenergyaudits.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 Nick Caton
Sent: Monday, April 29, 2013 7:55 AM
To: Busman, Michael R; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Cc: The Watt Doctors - Dave Weigel (Dave.Weigel at thewattdoctors.com<mailto:Dave.Weigel at thewattdoctors.com>)
Subject: Re: [Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
Mike - thanks a bunch for your response!
I can appreciate and relate to your experience with the boiler industry marketing folks... it's very easy to get the run-around, and is often difficult/impossible to get information needed for modeling from product literature. I've come to the same conclusion as you advised, and here's my accounting/reasoning for those interested:
The footnote in 90.1 Table 6.8.1-F defines combustion efficiency as "100% less flue losses," so I that was the basis of this query. There's additionally reference to this procedure: 10 CFR part 431<http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&tpl=/ecfrbrowse/Title10/10cfr431_main_02.tpl>
That procedure defines combustion and thermal efficiency as follows<http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&SID=bceba9fa026032bc4b393f9c462988bf&rgn=div8&view=text&node=10:3.0.1.4.19.5.54.2&idno=10>:
Combustion efficiency for a commercial packaged boiler is determined using test procedures prescribed under § 431.86 and is equal to 100 percent minus percent flue loss (percent flue loss is based on input fuel energy)
Thermal efficiency for a commercial packaged boiler is determined using test procedures prescribed under § 431.86 and is the ratio of the heat absorbed by the water or the water and steam to the higher heating value in the fuel burned.
Clearly they're not quite the same thing, where combustion efficiency does not account for jacket/standby/other losses affecting net output.
I haven't seen numbers to verify this, but perhaps ultimately jacket losses are such a small fraction (at these largest capacities) that it's accepted to be irrelevant. Put another way, the actual values for Et and Ec for boilers larger than 2.5MBtuh/h just aren't very different.
Running with that assumption, and unless anyone can answer further to this, I'll treat baseline thermal and combustion efficiencies identically moving forward for large boilers: HIR = 1/Ec = 1/Et. If nothing else, it's a "safe/conservative" approach for LEED, in the sense that if there is an error of procedure here it is only artificially helping the baseline and would in turn prove a beneficial correction for the performance rating down the road.
Thanks again!
~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: Busman, Michael R [mailto:MBusman at chevron.com]
Sent: Friday, April 26, 2013 1:47 PM
To: Nick Caton; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: RE: Big Baseline Boilers - COMBUSTION efficiencies!?
Nick,
It has always been my understanding that combustion efficiency includes flue losses as it is calculated based on CO2% or O2% in the stack gas and net stack temperature (stack temp.-boiler room ambient temp entering the burner). The only other thing I can think of that would affect the thermal efficiency or useful Btu's output as steam or hot water/fuel Btu's input would be radiant and convective jacket losses from the boiler shell at 100% firing rate. Although manufacturers are reluctant to publish data on jacket losses, they could typically range from about 1%-4% based on the design and mfg. This will be a fixed quantity of Btu's no matter what the firing rate is, so this loss as a % of boiler load increases as % load decreases.
As an example, look at an Ajax or Rite Boiler. They look pretty similar and are both manufactured within a fairly close proximity of each other. The front and back ends have uninsulated steel head plates about 1" thick to gain access to the tube bundle. Below the head plates are uninsulated thin sheet metal panels to gain access to the burners. With the boilers firing, I've measured temperatures of the upper plates in the 200-300 deg. range and the lower sheet metal panels in the area of 450 deg. This is kind of an extreme scenario, but the jacket losses are probably a significant number even though the rated or measured combustion efficiency might be 82%.
The other piece of the puzzle when it comes to seasonal boiler efficiency I don't know how DOE-2 handles would be cycling losses. If you take that Rite or Ajax boiler equipped with an atmospheric burner, the seasonal efficiency could easily approach 50%. During every off cycle, boiler room air will flow up through the boiler cooling the firebrick and mass of the boiler. The next firing cycling, those draft cooling losses need to be replaced by additional Btu's. A cycling boiler with a power burner could also have a low seasonal efficiency as the result of cooling losses from pre-purge and post-purge operation to get rid of unburned fuel.
I don't know if that helps or confuses the matter. After that long dissertation, I would suggest using the rated combustion efficiency.
My best,
Mike Busman
Michael R. Busman, CEM
Lead Project Engineer II
Chevron Energy Solutions
A Division of Chevron U.S.A., Inc.
145 S. State College Blvd.
Brea, CA 92821
Direct 714-671-3561
Fax 714-671-3438
eFax 866-420-0335 (Include my Full Name followed by "CAI:MHTZ" on Cover Sheet)
Mobile 310-387-2083
mbusman at chevron.com<mailto:mbusman at chevron.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 Nick Caton
Sent: Friday, April 26, 2013 10:55 AM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?
Hi everyone!
90.1-2007 Table 6.8.1F for boilers prescribes a minimum combustion efficiency (EC) of 82% for gas boilers larger than 2.5MBtu/h. This is the burner's efficiency before flue losses.
The same boiler's thermal efficiency, inclusive of flue losses, would be a lesser figure.
When modeling such boilers for an Appendix G baseline, what is actually appropriate to enter for the boiler's HIR input? As I understand it, this HIR input in eQuest is the inverse of thermal efficiency (net input vs. net output), and would be inclusive of flue losses.
This is for a LEED project, so I'd like to cite something solid to demonstrate what thermal efficiency is appropriate to assume for a baseline boiler of this size. Has anyone gone down this path before?
Thanks!
~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
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