KEH Energy Engineering

Cycling Efficiency

What is it, How do you determine it?

Whenever a boiler operates at a fixed firing rate, or the load for the boiler is less than the boiler's output at minimum firing rate, the boiler serves the load by cycling (starting and stopping). Cycling Efficiency determines the heat loss efficiency (using methods described by ASME) at all firing rates and accounts for the additional losses that exist when the boiler is cycling. KEH Energy Engineering developed this concept and makes the two spreadsheets that determine Cycling Efficiency available for you to use to justify replacement of an outsized boiler.

You're invited to try it out. A spreadsheet can be downloaded by clicking on the selection below. By downloading and using one of these, you agree to reward KEH with one half of one percent of the first year's savings derived from replacing your outsized boiler.

Download Cycling Efficiency spreadsheet
for hot water boilers, Lotus Version
Download Cycling Efficiency spreadsheet
for steam boilers, Lotus Version
Download Cycling Efficiency spreadsheet
for hot water boilers, Excel Version
Download Cycling Efficiency spreadsheet
for steam boilers, Excel Version

How it came about - outsized boilers

Should I replace that boiler? In late 1997 KEH was asked that seemingly simple question and realized that something was wrong. The customer's boiler was firing for about thirty seconds then shutting down for periods as long as ten minutes. Since the outside temperature was slightly below freezing Ken decided there was an opportunity to save energy there and spent some extra time analyzing the building and boiler. The outcome of the analysis was the boiler had a capacity almost six times greater than required.

Further investigation revealed the building had been converted from a warehouse use to office space. The conversion included re-glazing to dramatically reduce the load because of the high fenestration factor. Other energy conserving activities during the renovation included adding insulation and caulking to reduce infiltration. That's how a boiler is "outsized."

In preparing the analysis of the building KEH developed a Lotus(tm) spreadsheet that facilitated comparing an existing and proposed boiler for determining the actual efficiency of each under the varying load conditions that a heating boiler is exposed to. The spreadsheet was further developed on a subsequent project to account for cycling operation of modulating boilers (which cycle at loads below minimum fire).

Several applications of the spreadsheet to existing operations revealed that many heating boilers that may be rated at 80% or even 84% efficiency were operating at efficiencies as low as 30%. It also proved that a smaller, less efficient (manufacturer's rating) boiler could serve a load more efficiently if it is "rightsized."

The Spreadsheet

The spreadsheet calculates heat losses based on the assumption that the boiler and refractory will heat air forced through the boiler during purges and drifting through while the burner is shut off about the same as the products of combustion heat the boiler and refractory. Operating time is calculated as a function of the actual load on the boiler and how quickly that heat is lost. That way a cycle can be determined and the efficiency of the boiler under a low load (where the burner is cycling on and off) can be calculated. A macro in the spreadsheet repeats the calculation for outdoor air temperatures between 60 degrees and design, filling a table in the process (you may have to modify the table to extend to your design low temperature). The table provides a means of producing a graph of the boiler efficiency from minimum to maximum load. Two sets of data are calculated permitting a quick comparison of the cycling efficiency of two boilers.

Using average tempeatures (extracted from degree day data), or bin data, you can determine the cost of operation for the two boilers compared in the spreadsheet to estimate the savings possible by replacing an outsized boiler.

Several applications of the spreadsheet to existing operations revealed that many heating boilers that may be rated at 80% or even 84% efficiency were operating at efficiencies as low as 30%. It also revealed that a smaller, less efficient (manufacturer's rating) boiler could serve a load more efficiently if it is "rightsized."

Samples of the spreadsheet results are described with some examples in a publication of the Association of Energy Engineers, ENERGY ENGINEERING, Volume 95, Number 4,1998.

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