Special Maintenance Considerations - Effective Building Maintenance: Protection of Capital Assets - page 93

— Replace Burner and/or Boiler with Higher Efficiency Unit:
Even if combustion efficiency is maximized, the overall boiler
efficiency of smaller cast iron and steel boilers will be lower due
to poorer heat transfer efficiency in the boiler sections. Thus,
when these boilers reach the end of their service life, alterna-
tives should be considered. Generally, steel boilers are more ef-
ficient than cast iron boilers. For boilers up to about 1,500,000
Btu/h capacity, the use of a condensing boiler, providing overall
boiler efficiency as high as 96%, should be considered.
For large firetube boilers, the overall boiler efficiency is dic-
tated by the number of “passes” that the flue gas makes through
the boiler. A 2-pass boiler has a maximum overall efficiency of
only about 80%, while a 3-pass boiler has an efficiency of 83-84%
and a 4-pass boiler can achieve 85-86%.
Whenaboilermustbereplaced,itisalwayscost-effectivetore-
placeitwiththehighesteficiencyunitavailable.
• Isolate Off-Line Boilers : It is important to prevent boiler cycling
to maintain overall efficiency. For most facilities, multiple smaller
boilers will result in more efficient operation than a single larger
boiler.
With multiple boilers, when heating loads are low, the imposed
load may be met by one boiler while the other(s) remain idling on
standby. Idling boilers, however, consume energy to meet standby
losses, which can be increased by continuous induced flow of air
through them and out the stack. To reduce these losses, each boiler
should be equipped with an automatically controlled stack damp-
er. Thus, instead of idling, the off-line boilers can be shut down
and their stack dampers closed to eliminate losses.
• Add Stack Gas Economizer : In even a well-tuned boiler system,
typical stack gas exit temperature will be 350-450°F, depending on
the type of fuel. To prevent condensation in the stack, the flue gas
temperature cannot be reduced to below about 250°F for natural gas
and 325°F for oil, but that limitation still allows anywhere from 100
to 125°F of heat recovery from the flue gases. A heat exchanger can
be installed in the stack leaving the boiler and this wasted heat can
be used to preheat make-up water or feedwater or to heat combus-
tion air.