[Equest-users] Boiler Curves in LibrarySubject: [Bldg-sim] [Equest-users] Boiler Curves in Library

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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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Well, being a glass-half-empty kind of guy, I wouldn?t say the DOE2 curve is ?valid.? But yes, it (whatever curve you choose or create) is used in the boiler simulation to determine the HIR every hour whenever the/each boiler?s PLR is greater than MIN-RATIO. As Erik suggests, you should think about custom/other curves. The default penalty from the standby time seems reasonable for an older boiler.

In a lot of ASHRAE 90.1 PRM models, boilers turn on and off many days, particularly for office and school buildings that go into setback mode in the evening. The PRM models are required to ignore piping heat losses and other sources suggest we utilize the ?ON-DEMAND? loop control, so if all coil heating loads disappear, the HW pumps and boilers turn off. In the real world most hot water plants are either continuous, turned on and off seasonally as you note, or turned on and off based on OAT. And when the pumps are on, there are loop loss loads.

Fred

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Anonymous

Thanks Fred, that is very helpful and gives almost a complete picture of boiler efficiency.

To confirm, regardless of whether the boiler modulates or is single stage, the DOE2 part load HIR curve is valid only down to the specified MIN-RATIO. For values less than that, it cycles between the consumption determined by the HIR at MIN-RATIO and the Standby consumption. So if Standby consumption is ignored, the MIN-RATIO variable should actually improve the apparent efficiency because it eliminates the low efficiencies at extremely low PLRs (e.g. from 0 to 5%). Is that your understanding?

I was thinking that Start-up Time, for an actual boiler, would only occur seasonally if the boiler is shut off. That seems to agree with my data in that there is some type of load every single hour, so the Start-up Time should be negligible. That leaves only the ?Standby Time? variable as additional consumption during cycling. I am using the default value of 0.027 hours (which I understand means 2.7% of full-load consumption), which is resulting in a 1-2% drop in annual efficiency when compared to only using the PLR HIR curve. Does that seem reasonable?

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Joined: 2016-07-15
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Well, being a glass-half-empty kind of guy, I wouldn?t say the DOE2 curve is ?valid.? But yes, it (whatever curve you choose or create) is used in the boiler simulation to determine the HIR every hour whenever the/each boiler?s PLR is greater than MIN-RATIO. As Erik suggests, you should think about custom/other curves. The default penalty from the standby time seems reasonable for an older boiler.

In a lot of ASHRAE 90.1 PRM models, boilers turn on and off many days, particularly for office and school buildings that go into setback mode in the evening. The PRM models are required to ignore piping heat losses and other sources suggest we utilize the ?ON-DEMAND? loop control, so if all coil heating loads disappear, the HW pumps and boilers turn off. In the real world most hot water plants are either continuous, turned on and off seasonally as you note, or turned on and off based on OAT. And when the pumps are on, there are loop loss loads.

Fred

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