not necessary for the system specified will be
deleted. Stokers and stoking equipment selected
will be based on the following:
Boilers having output capacities of 3.5 Megawatts
(12,000,000 Btuh) or more will be equipped with
mechanically-driven grates operating continuously or
intermittently. Dump grates will not be permitted
in boilers in this size range. Spreader stokers
will be specified for watertube boilers using
bituminous coal with ash content on a dry basis in
excess of 8 percent or ash fusion temperature lower
than 1204 degrees C (2200 degrees F).
Underfeed or pulsating grate stokers may be
specified for firebox packaged boilers when
bituminous coal of any composition is used.
Underfeed stokers will be the screw-feed type for
boilers with capacity of less than 1635 kg (3600
pounds) of steam per hour, and the ram type for
larger boilers. Pulsating grate units will be
water-cooled and complete with automatic coal feed
and continuous ash removal. Conveyor stokers may be
specified for watertube boilers if suitable for the
type of coal available. Chain or traveling grate
may be specified by deletion of the inapplicable
type of grate, or the choice between the two types
may be left to the Contractor by including both
types in the description. If steam drives are
specified, plant must have an auxiliary boiler or an
alternate source of steam for startup. The
following is a general guide in determining which
type of grates to investigate:
Size
Steam rate of
boiler kg/hr (lb/hr)
Type of Grate and Stoker
1,135 - 9,070
Single retort, stationary grate,
(2,500 - 20,000)
underfeed stoker
9,070 - 13,600
Single retort, moving grate,
(20,000 - 30,000)
underfeed stoker
2,270 - 34,020
Reciprocating grate, front
(5,000 - 75,000)
continuous ash discharge stoker
2,270 - 45,360
Vibrating conveyor grate, front
(5,000 - 100,000)
continuous ash discharge stoker
**************************************************************************
Spreader stokers shall be the overfeed self-feeding type suitable for
burning a portion of the coal in suspension, but sized assuming 100 percent
combustion on the grate. [Coal shall be evenly distributed across the full
width of the grate by not less than [_____] feeder units. Unit shall be
designed for operation of any feeder independently of the others, or it
shall be possible to operate all feeders simultaneously.] Feeders shall be
capable of handling and uniformly distributing coal over the grate area.
Feeders shall be the mechanical-rotating type, shall have no moving parts
SECTION 15559A
Page 29
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