Pneumatic conveying

A method of removing ashes from boiler furnaces which has been developed extensively in America is essentially a pneumatic system, although steam is the conveying medium instead of moving air. Steam is used because the apparatus is always in use on boiler plants, from which steam can be taken as conveying medium. No air compressor or other special plant is required. On the other hand, the simple use of a connection from the steam main is a matter of very little importance, and no check is ever made as to the amount of steam so used, hence 99 per cent. of the users consider that the steam jet costs practically nothing for “power” compared with a compressor which would have a certain sized motor connected, and could not escape attention as an additional power consumer.

Steam Consumption. Investigation into the actual consumption of steam jets would often give very startling results, especially after the plant had been in operation for some time and the nozzles had begun to cut and wear. As proof of the waste of steam possible in such a plant, it is interesting to note that Mr. David Brownlie, in a paper on Automatic Stokers,[1] gave results of actual tests made on steam jets as used in certain classes of stokers in which steam jets are allowed to blow down the hollow furnace bars. These tests showed that, whereas the makers estimated the steam consumption of the jets to be about 2 per cent. of the boiler output, the tests on 80 plants showed a consumption varying from 0·5 per cent. up to as much as 21·4 per cent. of the total output of boiler.

As further evidence of the waste of steam that can occur due to neglect of the cutting effect on the nozzles, one American firm has designed an ingenious warning or “tell-tale.” A small hole is drilled almost, but not quite, through the nozzle. While the nozzle retains its initial shape and size the apparatus acts normally, but as soon as the small amount of metal covering the end of the hole has worn away, the hole is exposed, and a certain amount of steam passes through it to a steam whistle which blows continuously until a new nozzle has been inserted in place of the one which is now worn so much as to make it uneconomical in steam consumption.

Provided that means are taken to prevent waste of steam due to worn nozzles, the steam jet conveyor is very serviceable and, being flexible and convenient, it is very useful for the purpose for which it has been developed.

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.

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.

Steam Jets. The James Brady Foundry Co. (Chicago, U.S.A.) state, in their Bulletin on this subject, that the special steam jet elbows are usually placed at the top and bottom of a vertical riser. The jet of steam from the nozzle enters the elbow directly in front of, and parallel to, the face of a special wearing liner. This prevents or reduces the wear on the liners, as the jet protects the liners from the pounding action of the ash. A renewable sectional liner is provided of specially hard metal at all points in which the material makes actual contact with the pipe or fittings. These liners are interchangeable in all elbows, and each individual liner can be turned end for end when affected by wear.

In cases where the length of horizontal run exceeds 125 ft. it is necessary to supplement the primary nozzles by “booster” steam jets to maintain the velocity of the air current.

Buffer Boxes. At the discharge end of the pipe line it is necessary to insert a baffle or buffer box to take the impact of the ashes, and thus prevent wear on certain parts that are not designed to stand up to the destructive effects of the impact. The function of the box is to bring the ashes to rest, so that they may fall by gravity into the ash tank or on to the storage pile. When delivering into a tank it is very essential to install a buffer box, as otherwise the velocity with which the ashes enter the tank will pack them so tightly that they will not discharge automatically through the valve or gate. The location of the ash hopper can be wherever most convenient for loading the vans, railway trucks, or barges, etc., but preference should be given to a site which makes possible a pipe run with a minimum number of bends.

One American firm of engineers, the Vacuum Ash and Soot Conveyor Co., New York, have done away with the numerous steam jets and the blowing effect produced thereby, and rely entirely on suction by using a sealed ash tank and exhausting the container and pipe system by means of a single steam jet injector built into the roof of the ash tank, and discharging its steam directly into the air.

By this means it is claimed that the following advantages are obtained: (1) No sand-blast effect, such as is inevitable when blowing at high velocity. (2) No steam enters the ash tank and consequently there are no condensation troubles. (3) Much less dust is blown into the atmosphere as the steam is never in contact with the dust. (4) Conveyor pipes are cleaner since no steam enters them, as in “blowing,” and there is therefore no condensation, caking and corrosion.