TRANSACTIONS OF SECTIOM G. 765 



7. On the Compound Principle in the Transmission of Power by Compressed 

 Air. By Professor A. C. Elliott, D.Sc.(Edin.). 



The heat dissipated in the compressors, passages, and supply-pipes of a com- 

 pressed-air power, transmission system is a waste product. Under the condition 

 that the compressed air must ultimately attain a temperature hut little ahove that 

 of the atmosphere, it is easy to show that the heat waste is a minimum when the 

 compression is performed isothermally (even were a sink temperature lower than 

 that of the atmosphere available). 



But the practical problem of effecting isothermal compression has never been 

 satisfactorily solved ; and, in fact, attempts in this direction have hitherto been 

 mainly based on mere adaptations of the old cylinder water-jacket and jet 

 appliances. But even in cases where both jets and water jackets have been applied, 

 the compression curve has been found to fall only to a very small extent below 

 the adiabatic. Amount of heat conducted is directly proportional, other things 

 being equal, to time and to difference of temperature. Now, the difference of 

 temperature between the air and the cooling water is zero at the beginning of the 

 stroke, and increases to a maximum at the point when the eduction valve opens. 

 We are, therefore, able to conclude in harmony with practical experience, first,, 

 that to effect anything like complete isothermal compression the piston speed must 

 be excessively small ; and, secondly, that the rate of flow of heat from the air to 

 the cooling water attains a maximum value just at completion of the compression. 

 At ordinary speeds, then, it appears that even with the jet a large proportion of 

 the total heat must be abstracted after the eduction valves have opened — that is 

 to say, after compression has been effected. 



The author stumbled some time ago on the principle of intermediate cooling-. 

 On this plan the compression is effected in two or more successive stages by a 

 compressor witli a corresponding number of properly proportioned cylinders con- 

 nected by receivers, forming a mechanism analogous, as the case may be, with a 

 compound, a triple, or a quadruple expansion steam-engine worked, as it were, in 

 the reverse direction. The outstanding point of difference is that each receiver is 

 provided with a jet or (preferably) surface cooling arrangement by which the 

 temperature of the air as it leaves the receiver is brought nearly to equality with 

 that of the atmosphere ; and there is practically no difficulty in effecting this 

 cooling, because the size and surfaces of the receivers are at our disposal. 



As compared with the ordinary simple system, the result to be expected is 

 either (a) with the same pressure a substantial gain in efficiency ; or (b) with a 

 higher pressure and the same efficiency a reduction in the sice of the supply-pipes 

 and the plant generally. Another point of very great importance is that if surface- 

 cooling be adopted in the receivers, trouble from the formation of ice in the exhaust 

 passages of the motors will almost certainly vanish. 



The author, however, soon learned that, at all events, the idea of the compound 

 (or two-stRge) compressor had been suggested some considerable time previously by 

 Professor Riedler in connection with the designs for the new plant to be put down 

 as an extension of the present Popp installation in Paris. But there is no doubt 

 whatever that he in turn has been anticipated by Mr. Morrison, the manager of 

 the Marquess of Lothian's colliery at Newbattle, near Edinburgh. The author 

 has just returned from a visit of inspection, and can vouch for the fact that Mr. 

 Morrison's claims are well grounded. 



It next occurred to the author that, just as the compression-line on the com- 

 bined diagram could be made up of discontinuous parts of adiabatics hugging the 

 ideal isothermal curve by the devices of a multiple-cylinder compressor and 

 intermediate cooling, so the expansion-line of a motor could be made to hug the' 

 ideal isothermal curve by very similar means. In fact, the compound motor is 

 simply the compound compressor, as it were, worked in the reverse direction ; but, 

 instead of intermediate cooling, as in the compressor, we have intermediate heat- 

 ing. We are thus enabled to recover from the atmosphere in the motor-cylinders 

 part of the energy dissipated at the compressors. 



The maximum economy is obtained in a compound, triple, or quadruple com- 



