January 9, lyos.J 



SCIENCE. 



55 



eently, been accepted as siiffieient, and 

 greater economies have not been sought. 



In the matter of compression we still 

 occasionally find very inefficient pumps in 

 use, but manufacturers generally have 

 found that it pays to use high-grade eco- 

 nomical compressors. The greatest loss 

 is that in the air motor itself. In a large 

 number of cases it is impracticable or, at 

 most, inconvenient to employ reheaters, 

 and we find very generally that the air is 

 used at normal temperature for the various 

 purposes to which it is applied. 



To obtain the most satisfactory results 

 the air -must be used expansively, but usu- 

 ally where the demand for power is inter- 

 mittent no attempt has been made to re- 

 heat the air, and as a resiilt the combined 

 efficiency of compressor and motor is quite 

 low, varying in general from 20 to 50 per 

 cent. While low working pressures are 

 more efficient than high, the use of such 

 pressures would demand larger and heavier 

 motors and other apparatus which is un- 

 desirable. 



The advantages of higher pressures in 

 reducing cost of transmission are also well 

 recognized, and the present tendency is to 

 use air at 100 to 150 pounds instead of 

 the 60 or 70 pounds of a few years ago. 



By reheating the air to a temperature 

 of about 300° F., which may often be ac- 

 complished at small expense, the efficiency 

 is greatly increased ; in some cases this has 

 been shown to be as high as 80 per cent. 

 While the lower pressures are yet -more 

 efficient, the loss due to higher compression 

 is not serious. 



If air be used without expansion it will 

 be seen that there is a material loss in 

 efficiency; but, on the other hand, if it 

 be used expansively without reheating, 

 trouble may be experienced, due to the 

 drop in temperature below the freezing 

 point. If moisture be present this will 

 cause the formation of ice, which may clog 



the passages if proper precautions are not 

 taken to prevent it. The low temperature 

 will not in itself cause trouble; if, there- 

 fore, the moisture which the compressed 

 air holds in suspension be allowed to settle 

 in a receiving tank, placed near the motor 

 or other air apparatus and frequently 

 drained, less trouble will be experienced 

 from this cause. 



'VATiile it may be impracticable to reheat 

 the air in certain cases, yet there are many 

 situations where a study of means to over- 

 come the losses referred to would result in 

 marked economies. 



The greater adaptability of compressed 

 air to various purposes causes its use to in- 

 crease along with that of the electric motor, 

 for it has a different field of usefulness, 

 independent of power transmission ; at the 

 same time when the requirements are 

 properly observed in its production and 

 use, its economy as a motive power in 

 special cases compares favorably with other 

 systems. With a better knowledge of the 

 principles involved we may expect much 

 better results than have yet been attained. 



But compressed air possesses so many ad- 

 vantages that, however inefficient it may be 

 as a motive power, its application to shop 

 processes will be continually extended as 

 its usefulness becomes better known. 



Mention has been made of the use of 

 hydraulic motors as a factor in the sub- 

 division of power, but these are being used 

 to such a limited extent for this purpose 

 that we shall not consider them at the pres- 

 ent time. 



There is, however, a growing field of use- 

 fulness for hydraulic power in manufactur- 

 ing operations which is peculiar to this 

 agent alone, namely, its use in forging and 

 similar work. Where hydraulic power ex- 

 ists for this purpose it is also generally used 

 for a variety of purposes which could be 

 accomplished just as well, and often more 



