Paxman made gas producers for their own gas engines and those of other manufacturers. Pictured right is a Paxman Type X gas engine complete with suction gas producer.
Because it provides such a full and clear description of the construction and operation of Paxman suction gas producers, the following is reproduced without amendment from Paxman Catalogue No. 650 of Gas Engines and Suction Gas Producers, dated March 1913.
The Paxman Suction Gas Plant consists essentially of the following three parts: (a) The Generator, where the gas is actually produced. (b) The Vaporiser, which makes the steam for enriching the gas. (c) The Scrubber, in which the gases are cleaned and cooled previous to being used in the engine. The GENERATOR consists of a cylindrical steel shell lined with firebricks; the latter are supported on a cast-iron dead plate which also carries the firegrate. The fuel, from which the gas is made, is admitted to the combustion chamber by means of a hopper, mounted on top of the generator. Gas is produced by drawing a mixture of air and steam through the incandescent fuel in the generator. The VAPORISER, a vessel into which water is fed, is heated by the gases leaving the generator, and supplies the necessary steam. The air for the plant being caused to pass through the Vaporiser is thus saturated with the requisite amount of steam before entering the generator. After heating the vaporiser, the gases pass into the coke scrubber, where they ascend through a column of coke, kept wet by a suitably arranged sprinkler. The gases then pass along the pipe main, and into the expansion box, which is in direct communication with the engine cylinder.
To start the apparatus, all that is required is to light a fire in the generator, gently blowing this with the hand fan, and in about five to ten minutes' time the gas can be lighted at the trial cock fixed between the scrubber and the engine. When the gas burns steadily, it is satisfactory, and the engine may then be started.
Thenceforward the whole process of gas production is automatic. At each suction stroke, the engine draws a sufficient quantity of gas for its requirements, and when the engine is stopped, the production of gas ceases. There is no gas generated under pressure, consequently there is no fear of explosion, in fact the whole apparatus works under a slight vacuum. Fresh fuel is only required to be fed through the hopper once every few hours, according to the size of the plant, so it will be seen that the amount of attention required is very small.
GENERATOR. The generator body is built up of mild steel plates, closely riveted to ensure gas-tight joints. The brick lining is a solid job from the cast-iron dead plate to the top of the generator, and the grate area is so proportioned as to ensure satisfactory working under the loads specified.
VAPORISER. The cast-iron vaporiser, which supplies the necessary steam to the plant, is so placed that it is heated by the gases leaving the producer. In this manner, heat is abstracted from the gases, and since they will thus be cooled considerably before entering the coke scrubber, much less cooling water is required in the scrubber than would otherwise be the case. The vaporiser is designed to offer a large heating surface to the gases, whilst at the same time such provision is made for expansion as to reduce to a minimum any tendency for the vaporiser to fracture due to varying temperatures of the producer. When hard water is used in the vaporisers of suction gas producers, it is found that calcareous matters are deposited, which, if not removed, act as very bad conductors of heat; as a result, the efficiency of the vaporiser is considerably impaired. Having regard to this fact, care has been taken to render the whole of the inside of the vaporiser readily accessible for inspection and cleaning purposes. In the smaller sizes of plants, namely from "A" to "D", the vaporiser is placed at the top of the generator, whilst in the larger sizes, "E" to "H" the vaporiser is arranged between the generator and the coke scrubber.
FIREGRATE. In the smaller sizes, a simple flat firegrate is provided, whilst in size "E" and upwards a stepped grate of improved design is arranged. With this latter type of grate, clinkering may be conveniently effected whilst the plant is at work.
FUEL HOPPER. The fuel feeding hopper is of substantial design, and easy of manipulation; it is so constructed as to prevent the admission of any air to the generator whilst charging is taking place.
GENERATOR DOORS. These doors, which close against machined faces, are of substantial design. There are no loose crossbars or wedges, but a malleable-iron hinge carries the door, which is kept tight by central pressure, and secured by means of a wing nut.
AIR AND STEAM CONNECTIONS. The air and steam supply to the producer is under absolute control, at the same time there is an absence of plug cocks or valves, which are often a source of considerable trouble, due to the tendency they have of corroding and so becoming difficult to operate.
INSPECTION COVERS, & c. Handhole and cleaning covers are provided on the gas pipes between the generator and the coke scrubber. Poking holes are suitably arranged on top of the producer, so that if occasion arises, the body of the fuel may be poked down in the generator .
HAND FAN. An efficient hand fan, fitted with machine-cut gears, is provided for blowing the fire when starting the plant.
COKE SCRUBBER. The coke scrubber is of very large dimensions, in order to ensure satisfactory cleaning and cooling of the gases, with a minimum consumption of water. The body of the scrubber is constructed of mild steel plates, riveted together, but the base is of cast iron, so as to obviate the corrosive action which is invariably set up in the bases of scrubbers which are made of mild steel plates throughout. The water sprinkler in the coke scrubber is of an improved type, in which effective distribution of water is obtained without the use of any fine spray holes; these latter are often a source of trouble due to the tendency which they have of becoming choked by rust or dirt. The scrubber is fitted with substantial cast-iron doors. having machined faces, through which cleaning or charging of the coke scrubber may be effected.
ARRANGEMENT OF THE PLANT. For plants of small and medium powers, it often happens that the space available is very limited; having this point in view, special attention has been given to the design of such plants, in order to render them easily adaptable to confined situations. An adjusting socket has been fitted in the gas pipe between the generator and the scrubber, with which arrangement it is an easy matter to vary considerably the relative positions of the generator and the coke scrubber.
FUEL. The standard type of producer plant illustrated is suitable for using either Welsh or Scotch anthracite, poor coal ("charbon maigre"), gas coke, charcoal, and certain kinds of foreign coal; but when fuel is used which is inferior to anthracite, it is advisable to have additional cleaning apparatus for the gas. For instance, when it is required to run on coke or charcoal, a static tar extractor should be fixed between the scrubber and expansion box; and in the case of certain foreign coals, an additional sawdust scrubber should be installed, in which the gas is further cleaned before reaching the engine. When inferior fuel is used in the producer, the power developed by the engine is reduced accordingly. We have plants installed in South Africa, driving stamps, which are running in some cases for a month, night and day, without any stoppage, using nothing but charcoal or the native coal procurable there.
It is of great importance, especially with enquiries from abroad, that clients should give us the fullest possible information regarding the fuel available, and the number of hours the plant will be required to work without shutting down.
The CONSUMPTION of anthracite coal with the Paxman plant and engine, working at full load, is less than 1 lb. per b.h.p. per hour; and taking fuel at £1 per ton, the cost is less than one-tenth of a penny per b.h.p.-hour. Numerous tests taken at our works show a consumption of less than 0.8 lb. per b.h.p. per hour. In the case of coke or charcoal, the consumption of fuel is slightly increased and ranges from 0.9 lb. to 1.2 lb. per b.h.p. per hour, according to the quality. The quantity burnt during a stand-by is very small, amounting in the case of a 20 b.h.p. plant to about ½ lb. of anthracite per hour.
At the present time we have producers working very successfully both at home and abroad with fuels of such varieties as wood refuse, tan bark refuse, cocoanut shells, chips, shavings, &c. Whilst these plants provide power without incurring any fuel costs, they afford at the same time a suitable means of destroying refuse, which in many trades so often proves an encumbrance. In the case of a plant for wood refuse, it is possible to obtain one brake-horse-power-hour for a consumption of 2½ to 3½ lbs. of fuel, depending upon the percentage of moisture present.
The main principles underlying the design of this plant are much the same as in the case of suction gas plants operating with anthracite as fuel. There are, however, the following principal points of difference: (1) No vaporiser or steam boiler is required; the necessary steam for enriching the gas, and at the same time keeping down the temperature of the producer, is derived from the moisture contained in the fuel itself. (2) A centrifugal washer is provided for the purpose of removing from the gases all tarry matters and impurities, which, if allowed to pass to the engine, would prevent its working satisfactorily.
Briefly stated, the action of the producer is as follows: On the suction stroke of the gas engine, a slight vacuum is created throughout the gas plant and in the pipes which connect the same with the engine. Air is then drawn through the fan slide into the generator, below the firegrate. As the air passes through the hot body of fuel, such chemical reactions take place that a combustible gas is produced. The gas so generated passes from the upper part of the producer into the dust collector box, thence to the base of the coke scrubber. It now passes upwards through a body of coke kept saturated with water, and afterwards enters the centrifugal washer, where it is freed from all tarry matters. Leaving the washer, the gas is drawn through a sawdust scrubber, where it is dried previous to passing along the gas pipes to the engine.
The starting up of the producer is quite a simple matter, and the attention required is little more than that demanded by an anthracite plant. Owing to the special design of the generator, a uniform draught is created throughout the entire body of fuel, and successful gasification is maintained without any necessity for the constant poking down of the fuel, which is essential to the working of some types of wood refuse producers. As there is practically no clinker formed in the generator, it will be seen that renewals of firebrick lining and firebars will only be necessary after considerably longer periods of running than would be possible with plants operating on anthracite.
It is found that with plants working on wood refuse, there is a great tendency for the pipes between the generator and the coke scrubber to become choked with tarry matters and sawdust drawn over from the producer. In the "Paxman" producer, special attention has been paid to this point, with the result that the above-mentioned trouble has been entirely eliminated. The dust collector box, placed between the generator and scrubber, is so arranged that the gases in passing through it are brought into intimate contact with jets of water which serve the dual purpose of cleaning and cooling the gas, and of keeping the gas passages free from all deposit. Any sawdust which may be drawn over is washed down the gas-pipe into the seal pit, whence it can be readily removed without in any way interfering with the working of the plant. The coke scrubber, fitted with cast-iron base, is of the same generous proportions as in our new design anthracite plants, and the same improved type of sprayer is fitted. The sawdust scrubber is also of ample size, to ensure that there will be no undue suction on the engine. The chimney pipe is fitted direct to the top of the producer, thus avoiding any bends or pockets for the accumulation of tarry deposits, at the same time there is no troublesome cock or valve in this pipe. The chimney is closed by means of an air-tight cover on top of the pipe, and is operated by means of a spindle and lever. Poking-holes are arranged in suitable positions round the generator body, and at various points in the gas connections.
The above type of plant may be adapted to use anthracite or coke, should this at any time become necessary due to a shortage in the supply of wood refuse.
Page updated: 04 MAR 2002