Portable Compressors for Coal-gas.
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In an article on page 200 of our issue of the 2nd inst., we made suggestions for the design of suitable gas-compressing plant to be carried permanently on heavy-vehicle chassis. We give below the results of further investigations of the problem, particularly in regard to a conversa, tion we have had with a director and general manager of Peter Brotherhood, Ltd., a notable maker of high-speed compressors, When, in our previous article on portable compressors for coal-gas, we suggested for the compressing engine a speed of 1000 revolutions per minute, it was with the conviction that we were asking the makers of compressing engines to meet an unusual and difficult requirement. We pointed this fact out at the time, and, while strongly of opinion, based on our experience both of air-compressor and internalcombustion-engine design, that such a speed was not only feasible but the best for the project in view, considering all the circumstances, we were nevertheless prepared to be told that little more than half the number of revolutions per minute was advisable or even possible. We were, however, quite willing to join issue on the point, and ready to prove our case. In any event, and supposing a 500-r.p.in. machine was advocated, a portable compressing plant on the lines laid down is practicable, although the gross weight of the outfit would have to be a hundredweight or so more than our estimate owing to the increase of cylinder capacity necessary as a set-off to the lower speed.
The Exception to the Rule.
An extended inquiry tended to show that we had good grounds for our expectations, and compressor manufacturers in general appear to incline to 500 revolutions per minute as a maximum. The exigencies of war requirements, however, have operated already in other regards than motoring to produce a stritable machine. It runs at 1500 revolutions per minute, and works at a high pressure. It may almost be said to be the exception that proves the rule laid down above, but it is, nevertheless, a good and useful exception. The maker of this machine is well known in the general engineering industry and to theAdmiralty, particularly for the class of work in question.
Double-barrelled Experience Requisite.
Peter Brotherhood and Co.; Ltd., makes compressors of all kinds and sizes, but chiefly small, highspeed, high-pressure units, running at 500 r.p.m. and working at pressures of 3000, 3500 and, in some cases, S6 high as 6000 lb. per square inch. This is only one branch 9f its manifold activities, two others which should be mentioned, as they have, indirectly, a bearing on our subject, are torpedo engines and aircraft engines. It will be seen that this company has the requisite double-barrelled experience called for in connection with the design of the plant we are proposing—of compressors, and of high-speed lightweight petrol-engine construction.
We recently had the pleasure of an interview with Mr. C. W. Bryant, a director and the general manager of the company, who was good enough to give us the benefit of-his views on the matter.
Higher Working Pressure Suggested.
The first question discussed was that of the working pressure, or, in other words, the pressure at which the gas should be stored. In our article we suggested 150 lb. per square inch. Mr. Bryant is of opinion that 300 would be better. The problem is one of weight. On the one hand, with a low pressure, the thickness of metal in the cylinders and gas-container can be re+iced ; on the other, with increased pressure, the a verall dimensions of these important parts are decreased in inverse ratio to the pressure employed. The company's experience in other branches of in
036 dusky, however, has gone to show that, on the whole, the net result of adopting a higher pressure is a reduction in the gross weight of the plant.
300 lb. Compression by Two Stages Practicable.
Another consideration which we had kept in view in our earlier deliberations was that, at 150 lb., two stages of compression would be sufficient. No objections, we are told, exist to prevent the same principle being followed when operating at the higher pressure.
A Simple Method of Thoroughly Cooling the Gas.
With regard to cooling, a very simple and correspondingly thorough system was suggested. It assumes the presence of a fairly copious supply of cold water from the mains, such as will, in all probability, be available wherever supplies of gas can he obtained. It comprises cooling of both cylinder-walls, cooling of the gas in its passage from the L.P. or first-stage cylinder to the H.P. or second-stage cylinder, and after-cooling of the fully-compressed gas as it leaves the compressor.
Compressor Works in a Cistern of Water.
The method is merely to enclose both the working cylinders by a cistern, large enough to contain them and coils of pipe disposed round them. The gas, compressed through the first stage, is taken from the L.P. cylinder outlet through a coil of pipe which may, for convenience be one or two turns taken round the cylinder itself, leaving, however, water-space between pipe and cylinder. The delivery end of this pipe ia applied to the inlet passage of the H.P. compressing cylinder. Similarly from this cylinder the delivery pipe takes the form of a coil round the cylinder, afterwards leaving the cistern and being directed to the gas-container. The inlet for the water is preferably near the delivery pipe from the compressor, so that the water flows in a direction contrary to that of the gas, the coldest water meeting the hottest gas.
With regard to suitable valve gear for high-speed compressors, the alternative systems of mechanicallyoperated or automatic valves are available. [The motorist should not confuse the issues here by applying his experience of these two systems for the inlet valves of petrol engines. The cases are not parallel, and automatic valves are used on the majority of compressors.—En.} In the 'case of the former, either sliding valves of the piston type, rotating valves similar to those used on certain Darracq engines, poppet valves like• the usual petrol-engine components, or others similar in principle may be employed. This type of valve gear is more expensive than that one embodying automatic valves. With automatics, the main object of the designer is to reduce the weight, so that inertia of the valves, which affects largely the speed at which they may be operated, is a minimum. This ia more readily attained in the case of the delivery valve, which may be comparatively limited in port area, than with the inlet valve, which, for the best results in regard to volumetric efficiency, which expresses the ratio of induced gas to cylinder capacity, should have as large, free, and rapid an opening as possible, thus allowing easy entrance for the gas. It must also close quickly, so that no part of the contents is expelled when the piston commences its return stroke. These requirements call for a, large vale of considerable inertia.
Best System a Compromise.
In actual operation, the compressor with mechanimlly-operated valves compresses the gas up to a predetermined pressure ; when the delivery valve opens, this pressure is immediately modified to agree with 1st of the gas in the delivery pipe ; for our pur3ose the latter would vary from atmospheric pressure k•300 lb. per square in. above that of the atmosphere during each 10 minutes of use. With an automatic autlet valve this disability would not occur. It may, be obviated to some extent by placing a retaining valve in the delivery pipe, a further expense and complication which must be endured in connection with the system of positive operation. These considerations would appear to point to the best system being a compromise of mechanically-operated inlet valve, to open and close at the correct periods, on similar lines maybe to the petrol-engine system, with an automatic outlet or delivery valve. Alternatively, in the two-stage machine, all the valves might be mechanically operated except the final delivery valve. The latter arrangement is, we understand, carried out in the ultra-high-pressure compressors made by Peter Brotherhood, Ltd., which operate in four, five, or sometimes six stages, and work satisfactorily..
Gas for Commercial Vehicles, Benzole for Touring Cars, the Solution of the Home-produced Fuel Problem.
Mr. Bryant, of course, could not offer immediately to take active steps in the direction of producing suitable portable compression plant to meet the require ments of commercial vehicles. We gathered that he was favourably impressed with the possibilities of coal-gas-operated road vehicles, and particularly when engine modifications were made so as to conform more exactly with best conditions for its use. He thought that engines designed for gas might be applied to road vehicles, and that by an arrangement of throttle-valve control limiting the volume of the charge, petrol could occasionally be used to overcome difficulties due to temporarily running short of gas. The limitation of the charge would reduce the compression and obviate pre-ignition, which would, with petrol as fuel in gas engines, occur. Given a specific inquiry, his company would be pleased to quote for a suitable plant, and he had no doubt whatever as to the practicability of a compressor to meet the requirements. It is probable that at the moment, owing to the nature and importance' from a national point of view, of the work in hand by the company, some difficulty would be experienced in obtaining permission to proceed with the manufacture of such compressors. A recognition of the possibilities of coal-gas for transport-wagon use, coupled with a failure to augment the supplies of petrol to a degree necessary for the countries' internal needs, may make this matter one of paramount importance. In that event, Brotherhoods, Ltd., would either make the compressors or place its experience at the disposal of others. After the war, of course, with no such limitation of its activities, the company would be willing to make such plant commercially. In the latter regard, Mr. Bryant seemed to be wholeheartedly in favour of a strong endeavour being made for an all-British fuel being rendered obtainable in sufficient quantities for all purposes. He concurred with our suggestion that in this regard bensole for touring cars and light vans, and coal-gas for heavy vehicles, offered a practical solution.