[Equest-users] Big Baseline Boilers - COMBUSTION efficiencies!?

[Busman, Michael R]

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] On Behalf Of Nick Caton
Sent: Friday, April 26, 2013 10:55 AM
To: 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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