Big Baseline Boilers - COMBUSTION efficiencies!?

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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:image005.jpg at 01CE4BCD.1DA0F630]

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:image007.jpg at 01CE4BCD.1DA0F630] Here's a Fulton example [cid:image010.jpg at 01CE4BCD.1DA0F630] 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

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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.

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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

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I would like to issue a correction to my previous email.

The AHRI I-B-R ratings publication does include boilers with inputs > 2500 MBH. I've seen several entries in the April 2009 publication where the 2500 MBH input is exceeded, and values are listed for both combustion and thermal efficiency, an example of which is shown below. I'm not sure why then ASHRAE 90.1 doesn't specify Et for the larger hot water boilers.

[cid:image007.png at 01CE514E.EACC2770]

Thus, I go back to the point I previously made where I think we should model the thermal efficiency values, and they are available via AHRI.

In addition, there was another point made about additional losses and I agree. While not spelled out in Appendix G, the losses associated with boiler cycling can be significant and therefore should be modeled. These are the pre-purge and post-purge losses previously referenced by Mike in this thread. My understanding is that if you combine the flue losses, jacket losses, and cycling losses, you get the overall fuel-to-steam efficiency. If the boiler(s) are significantly oversized, the boilers will cycle a lot and fuel-to-steam efficiency can be quite a bit lower than either the combustion or thermal efficiency numbers. If this is not taken into account by energy modelers, then during the M&V phase, a big descrepancy will show up between modeled boiler efficiency and actual boiler efficiency. The M&V engineers will catch this. Ramya Shivkumar brought this up in separate email to me regarding M&V and I completely agree.

Several years ago, I performed detailed data collection and analysis on a pair of steam boilers for a hospital, as part of an energy audit/study. The boilers were oversized and cycling a lot, but even more so because the operator ran both even though he needed only one because he wanted both boilers hot and ready to go in case one went down. If you've ever been in his shoes, with the potential to lose your job if the doctors didn't have what they needed to run the hospital, you will understand why the operator was running the boilers the way he did. I simply pointed out through measurements that the boilers were cycling a lot because they only had a 4:1 turndown burner (i.e. cycles when load < 25%). The overall effect was that the boilers were operating at an overall fuel-to-steam efficiency of ~ 70 to 72 % versus the rated Ec value of 80 to 82 %. I was just showing how much they could save if they only ran one boiler. For what it's worth, the operator did not take my suggestion, but this does show that the difference between the combustion and fuel-to-steam efficiency can be significant. Thus minimum turndown and cycling should be taken into account in the energy models. eQuest does this via startup and standby times.

Regards,

JAH

James A. Hess, PE, CEM, BEMP

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