Firetube Boilers and Accessories

ThERMAL EFFICIENCY
The effectiveness of the boiler as a heat exchanger. It is the ability of the boiler to exchange heat through tubes and
furnace, by radiation, conduction and convection, to the transfer medium (water). A few of the factors affecting thermal
efficiency are heating surface, tube number and diameter, furnace tube length and diameter.
COMBuSTION EFFICIENCY
This is a measure of the ability of the burner to effectively and completely burn the fuel, coupled with the thermal
efficiency of the boiler. Burners requiring high amounts of excess air to provide flame stability will be less efficient.
Combustion efficiency does not take into account heat loss to the surrounding air through the boiler jacket and piping.
FuEL TO STEAM EFFICIENCY
Sometimes referred to as overall efficiency. This is a ratio of heat output to heat input. This includes boiler jacket and
piping losses to the surrounding environment. It is the percent of useable heat in the steam (or hot water) compared to
the heat input supplied by the burner. It is also defined as the combustion efficiency less boiler jacket and piping loss
(radiation and convection losses). Since fuel-to-steam efficiency reflects the portion of actual usable heat supplied to
the system, it is most useful when comparing performance of similar equipment, or when doing fuel savings analysis.
BOILER RATINGS
Ratings may be expressed in the following units:
BHP = Boiler Horsepower. One BHP is the evaporation of . lbs of 1 °F water per hour into dry saturated
steam at the same temperature.
BHP =
BTUh
, 7 BTUh/BHP
BTU = British Thermal Units: that quantity of heat required to raise one (1) lb. of water one
(1) degree on the Fahrenheit scale. BTUh = BTU/hr.
MBH = 1000 BTUh
PPH = Pounds of steam per hour
PPH =
MBH
0.970 MBH/PPH
Gross Rating – The full output of a boiler actually available to the heating or process system at the outlet nozzle.
The Commercial Steel Boiler Industry catalogs gross output as 80% of input.
Net Ratings – The net connected design load that can be supplied with heat by a boiler of given output, allowing
for normal system piping losses and pickup from a cold start. Since steam system piping losses may be
expected to be larger than for water systems, and steam boilers require greater heat input from a cold start
than water boilers before heat flows to the system, steam net ratings for automatic fired boilers are slightly
lower in relation to gross output than water net ratings. Hand fired net water and steam ratings are equal
and are lower in relation to gross output in order to provide ample allowance for rapid pickup from a
banked fire.
MBH net water = Gross MBH x 0.87
MBH net steam = Gross MBH x 0.776
Square Foot Ratings – Net ratings based on a heat emission rate of 0 BTUh per square foot of radiation for
steam, and 1 0 BTUh per square foot for water.
Steam Net MBH Water net MBH
Square Feet Net Steam = Square Feet Net Water =
0. MBH / sq. ft. 0.1 MBH / sq. ft.
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