for water tube boilers are based on the following:
a) Bare, metal-covered, or metallic-ore-covered tubes and headers-
projected area (external diameter times length of tube) of the tubes or
b) Extended surfaces (metal and metallic surfaces extending from
the tubes or headers): sixty percent of the flat projected area, except that
metal blocks not integral with tubes or headers, extended surfaces less than
1/4 inch (6 mm) thick or more than 1-1/4 inch (31 mm) in length, and the part
of the extended surface which is more than one tube or header radius from the
tube or header from which it extends are not included.
c) Furnace exit tubes -- the projected areas of those portions of
the first two rows of exit tubes receiving radiant heat from the fire.
d) Grate heat release rate -- This burning rate is the higher
heating value in the coal used per hour at rated boiler capacity divided by
the total active burning area of the stoker grate. Maximum values are
provided in Section 5, Table 12. These values are based on the assumption
that furnace walls are water cooled, that there is adequate furnace volume,
and that the most desirable type of coal for the unit is used. In the absence
of these conditions, values should be reduced to ensure satisfactory
combustion. A high grate heat release rate will cause high carbon loss and
increased particulate emissions.
e) Flue gas velocities through tube banks -- Gas and light oil
flue gas velocities are typically determined by the need to limit draft
losses. Coal, wood or solid waste boilers need to limit flue gas velocities
to prevent undue erosion of boiler convection tubes. The gas velocities
through the convection section of these boilers shall not exceed velocities
shown in Table 4.
f) Tube Spacing -- Tube spacing is governed by the amount of
deposits which will accumulate. Spacing is not critical for boilers firing
gas or distillate oil. Boilers firing solid fuels or residual oil need proper
spacing and proper soot blowing arrangement to limit deposit buildup on tubes,
poor heat distribution, poor efficiencies and premature tube failures.