Boiler Curves in Library

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Can someone direct me to where I can find an explanation of how the part-load boiler curves were developed? I can find the formula coefficients for the heat-input ratio in the BDLLIB.dat file and an explanation of how to apply them in the DOE2 Engineer's Manual. But how were the formulas and coefficients developed? Are they based on some study results described somewhere in the documentation? I am seeing very little part-load degradation in performance using the forced draft curve and I was wondering why. Thanks.

Brad Painting
Facility Strategies Group, LLC
1012 Market Street, Suite 307
Fort Mill, SC 29708
Phone: (803) 493-9706

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Hi Brad,
I don't know how the default boiler curves were developed, but...
It is easy enough to create custom curves based on manufacturer performance data, which is available for many boiler models.
There are other inputs that affect simulated fuel consumption in eQUEST besides the boiler HIR-FPLR curve and full-load HIR, including MIN-RATIO and STANDBY-TIME. If the boiler part-load ratio is below the MIN-RATIO, there are cycling losses based on the STANDBY-TIME. The MIN-RATIO is usually known based on the boiler model but I don't think I've seen published numbers on cycling losses sufficient to calculate STANDBY-TIME. But I have adjusted the defaults when calibrating a model to known fuel consumption.
The ASHRAE Handbook has some reference material. S32.6 is on boiler efficiency and includes an efficiency curve for a modulating boiler, showing efficiency improving slightly at part loads (instead of degrading) "from the increase in the ratio of heat exchanger surface area to heat input as the firing rate is reduced". F19.12 Boiler Model provides some fundamental research and lists references if you want to dig into it further.

Regards,
~Bill

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Although this old paper from 1990 dealt with PLR curves for residential equipment,
the concept is the same for commercial equipment.

Joe

Joe Huang
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Although this old paper from 1990 dealt with PLR curves for residential equipment,
the concept is the same for commercial equipment.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

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Doesn't directly answer your question, but this report by Taylor
Engineering and PG&E discusses the DOE2.2 boiler curves and compares to
measured boiler performance:
http://www.etcc-ca.com/sites/default/files/OLD/images/boiler_research_project_-_ats-te_final_report_pcb_05092012.pdf
.

-Erik

*Erik Kolderup, PE, LEED AP*
erik at kolderupconsulting.com | 415.531.5198 | www.kolderupconsulting.com

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Thanks for the feedback everyone. I am comparing some of the information in the PG&E study with the algorithms described in the DOE2 Engineer?s manual. I am confused about how the MIN-RATIO variable is used. It seems like the engineer?s manual states that the equipment only cycles on/off when the load drops below MIN-RATIO, but it also states that the part load performance curves account for cycling (pg. v.10):

[cid:image004.png at 01D26B1B.F18A56F0]
For a single stage boiler then, what is the MIN-RATIO doing if all of the part load performance is already attributed to cycling?

The graph of ?Unit 1? performance in the PG&E study (pg. 40) closely matches the performance curve from the Atmospheric-Blr-HIR-fPLR curve in the library. ?Unit 1? is a single stage boiler, so all part load performance would occur from cycling. What would the best curve be to simulate the part load performance of a modulating non-condensing boiler?

[cid:image005.jpg at 01D26B1D.45019180]

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First,
THANK YOU Erik for pointing out that study. I haven?t had time to look at it but it?s something that could be useful.

Brad,
You are spot on with your comment. Despite its name, I currently consider the ?MIN-RATIO? for the DOE2/eQ boiler models to really be the minimum PLR value at which the HIR f(PLR) curve is valid, and simulated hourly efficiency is held steady below that value (w/some adjustments if using DOE-2.2). If ?MIN-RATIO? was part of a mechanistic model of a boiler (instead of a curve fit model), it would certainly be set to 100% for a single stage boiler. Also note the eQUEST/DOE-2.2 standard boiler representation includes ?Start-up Time? and ?Standby Time? which affect hourly efficiency below the ?MIN-RATIO? and attempt to account for losses and mass of the boiler when in cycling mode. See the Misc tab in the Boiler forms.

The ?default? eQ curve, also cited in T24 ACM/COMNET, results in extremely low hourly efficiencies below 20% part load. The default curve for EnergyPlus (cited in the follow-on PNNL guide) uses a different representation Efficiency f(PLR) instead of HIR f(PLR) and it results in less degradation at very low loads. T24 ACM/COMNET/PNNL cites a 25% ?default? for the baseline input for MIN-RATIO, but it?s not clear exactly why. In my limited experience with older, atmospheric commercial boilers, most stage 33%, 66%, 100% (small burner, large burner, both burners).

Note that real, in-place boiler efficiency at low loads will depend on a great number of factors, including the boiler water temperature (which is not considered in these representations) and whether the water is pumped through the boiler continuously or only when the boiler cycles on. FORTUNATELY, all the nasty ?traditional? standby losses are minimized, but not eliminated, in decent modern lightweight forced-draft boiler and HW system design.

In the chart below, note the non-linear horizontal axis scale, for clarity of comparison below 30% PLR.
[cid:image001.png at 01D26B24.EA04E630]

Fred

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