The following estimated steam consumptions are given for what they are worth; they are of comparative value in relation with the power consumption on the “suction” scheme: One firm claims, in an actual proposal for a plant to be erected in this country, a consumption of 30 lbs. of steam per min. to deal with 150 lbs. of ashes per min., or 4 tons per hr. This is approximately equivalent to 72 electrical h.p. for dealing with 4 tons of ashes per hr. A second firm states that a steam jet plant dealing with 12 tons per hr. will require 3,466 lbs. of steam per hr. at 130 lbs. pressure: this, if passed into a modern steam driven generator, would produce over 130 h.p. hours. These figures indicate how variable are the estimates of power required. Note.—The “suction” schemes for wheat actually work out at slightly more than 1 h.p. per ton per hr. in single-nozzle plants, and 1½ h.p. per ton per hr. in twin nozzle plants.
Lay-out of Plant. The plant is usually designed on the following lines: Immediately under the ash hoppers are funnel-shaped tee-pieces fitted to a cast iron pipe laid on the floor, or preferably in a small trench just below the ground level. These funnel inlets are usually covered with a cap when not in use, a tight joint being established by the “suction” in the pipe line. When used on Lancashire boilers having no ash basement the ashes are raked from under the furnaces on to the floor, and swept into the inlets mentioned (see Fig. 25). Large pieces of clinker are broken by hand until they enter the intake pipe, when they are immediately conveyed through the rest of the system.
Fig. 25.—Brady Steam Jet Ash Conveyor.
In all large boiler houses with a proper ash basement it is usual to have a travelling clinker breaker, motor driven, which can be moved on light rails under each ash hopper and over each intake. The breaker receives all the ashes when released by the hopper valve, crushes them to a suitable size and discharges them by gravity over the intake funnels, whence they are transported to the ash tank or hopper.
The method of creating the moving air currents is by passing steam through specially designed nozzles which are placed at the extreme end of the intake pipe, and force the air out of the pipe, thus inducing a stream of air to enter at the intake openings, and carry forward the ashes which have been fed into the pipe with the air. When the underground pipe has to rise vertically to cross roads, etc., or to reach an overhead tank, it is usually found necessary to insert “booster” jets to impart additional velocity to the ashes, which are naturally retarded seriously in changing their direction at the bend or elbow. Should circumstances necessitate many bends being employed in the pipe line the number of “booster” jets has to be increased, and the total cost of steam for operation is increased seriously.
The capacity of the conveyor depends upon the volume of air passed through the pipe in a given time, and the ashes must not be slacked before handling, but must be handled either straight from the furnaces or allowed to cool and then conveyed to the ash hopper.
An 8 in. pipe is the largest used, and this will handle approximately 8 tons of ash per hour. Any increase over this size of pipe necessitates a consumption of steam which makes the scheme impracticable.
The conveyor pipe may be run at any angle, elevation or level, and therefore is not handicapped by the rigid straight line, point-to-point, requirements of bucket elevators, skips, etc.
The abrasive action of ashes is well known, and when they are travelling at the high speed necessary with this form of conveyor they cause considerable wear at the bends and elbows in the pipe line. To overcome this a special mixture of iron has been obtained, which is extremely hard and wear-resisting. Steel is quite unsuitable and ordinary cast iron is too soft for these conditions.