Portable Compressors for Coal-gas.
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Suggestions for Suitable Designs. Improvement of Existing Stationary Plant in the Direction of Weight Reduction Necessary.
The problem of arranging for the attachment of a small compressing plant to a heavy-vehicle chassis in order that it may compress gas taken direct iron' the mains and transfer it to a receiver carried by the lorry, at a convenient pressure, is an interesting one. The points to be kept in view are principally those of simplicity in construction and operation, and of low weight.
1000 Cubic Ft. Compressed in Ten Minutes.
We have assumed as a basis of calculation that it would be advisable to store 1000 cubic ft. of free coalgas and to compress it to 10 atmospheres or 150 lb. per sq. in. above that of the atmosphere. Allowing 23 per cent. on this figure for the various mechanical and volumetric inefficiencies of the compressor, we see that it is necessary -to calculate upon compressing 125 cubic ft. or 216,000 cubic inches of gas per minute; these figures represent the gross volumetric capacity of the compressor. In order to reduce the weight and volume of the outfit to a Minimum, a high speed of revolntion is necessary. We will take 1000 r.p.m. as the normal speed at which this compressor would run ; that this is unusually high for compressor work we are well aware. The difficulties of designing such a machine, however, should not be insurmountable; the problem is probably one of discovering a suitable design of outlet valve.
Two-stage Compressor Needed.
The compressor would have to be a two-stage one, that is to say, the compression would commence in a large or low-pressure cylinder and be completed in a second and smaller high-pressure cylinder. The size of the L.P. cylinder would have to be such that the displacement of its piston in 1000 compressing strokes would equal 216,000 cubic ins., the " -swept volume" of the cylinder would, therefore, need to be 216 cubic ins. It will be gathered that we are referring to engines of the single-acting type. It appears obvious that this is the design which would best lend itself to the special conditions of the problem we are considering.
A One-crank Trunk-piston Engine.
Alternative arrangements • of the two cylinders may be suggested. The first is a one-crank engine with two cylinders one above another, having a common piston of the trunk type, so that the actual working portion of the L. P. cylinder would be that space contained between the exterior of the H.P. piston and the walls of the L.P. cylinder proper. A short-stroke engine of 6 ins, is suggested. This is advisable in order that the over-all length of the engine may be kept -within reasonable limits, and also on account of keeping piston speeds low while maintaining high revolution speed. Practical considerations may possibly prevail, by reason of the fact that detrimental clearance at the end of the stroke is somewhat less on a percentage basis as the length of stroke increases, to increase the figure we suggest ; actual dimensions are not at the moment of great importance. In an engine designed on these lines the diameters of the trunk piston would be 8 ins. and 4i ins. respectively.
Cylinders Side-by-Side.
The other design suggested is that of a two-crank engine with side-by-side cylinders, the same stroke,
6 as already suggested, and cylinder bores of 6i ins, and 4 ins. respectively. These sizes are sea26
lected as much as anything so as to equalize the maximum thrust on the crankpin, thus keeping the sizes the same and minimizing first cost as far as possible. Whichever design is adopted, our suggestion is that the engine be put on the near side of the chassis, under the driver's seat, in the case of the one-crank engine, it is more than. likely that it will protrude above the level of the seat. In the ease of the sideby-side engine it might possibly be accommodated under. the seat. The drive w.nuld be by bevel or spur gearing from the front end of the gearbox.
Cooling.
The question. of cooling is interesting. One has to decide whether it would be necessary to cool the cylinders or the compressed gas during compression or after it leaves the cylinders. The pressure for which we ask is, as compared with the general run of compressing plant, high, and the temperatures, involved are, on that account, not insignificant. if we consider that the compression takes place entirely without loss of heat to the cylinder walls—that is to say, the compression is adiabatic—the gas, when compressed to ten atmospheres, would have attained a temperature of over 680 degrees Fahrenheit. This seems formidable ; on the other hand, it must_ not he forgotten that the whole period of compression is only to occupy ten minutes so that the considerations involved are of a class almost, we believe, entirely new to air-compressor makers, who have usually been accustomed to dealing with machines which might be set to run sometimes for days on end without stopping, but which, in any case, would probably be expected to run several hours and be compressing all the time. We except the as yet almost insignificant class of compressor used for tire-inflation. In the ten minutes compression, there would hardly be time for the cylinder to have itself risen to the temperature of the gas. On the other hand, still supposing for the rnomentlhat the compression takes place adiabatically, the volume of the compressed gas would be 182 cubic feet as compared with 90, which would be its volume when reduced to 60 degrees Fahrenheit, which is the temperature we have assumed for the incoming gas. Actually, of course, and even without any water-cooling, the compression would not take place under the theoretical conditions suggested, as heat would be given all the time in a greater or less degree to the cylinder walls, to the piping, and to the metal of the receiver. It would seem that in. order to get the greatest weight of gas —and therefore fuel for the longest mileage per charge—it would be advisable to cool as much as possible while compressing. It is, therefore, suggested that the cylinders be fitted with suitable jackets, that the outlet from these jackets should be turned on to the road, and that the inlet should be adapted for rapid coupling to the usual style of water tap met at gas-works--a push-on fit for rubber tubing is enough as a, joint. It would then only be necessary to. couple it to the water supply and turn on the tap at the same time as the connection was made with the gas mains. An ample supply of cooling ;water Would then be available, it remaining merely a matter of design to bring it into the closest possible contact with the walls of the compressing cylinders. It might even be found advisable so todesign the receiver that the water might also be turned over and about it or even through tubes inside it, with a view further to achieving the desirable end laid down at the commencement of this paragraph. This point should not be overlooked. Weights.
Lastly, as to weights, with a suitable engine designed on automobile lines with a view to eliminating, so far as possible, unnecessary weight—this object to be attained both by careful design and by the use of high-class materials—it should be possible to produce a compressor to weigh about 3 cwt. The weight of the receiver and fittings would he 151 cwt. : the total additional weight to be carried would therefore be 161 ewt. It must be pointed out that to drive the compressor as outlined and to compress 1000 ft. of gas in the short space of time prescribed, would necessitate the use of the full power of a 24 Lp. engine. The necessary gearing would, therefore, have to be designed with a view to that end. In the event of its being impracticable to design a compressor to run at the speed suggested, then it. will be necessary to allow for a, heavier compressor according as the speed of revolution of its shaft is reduced.