SECOND LAW OF THERMODYNAMICS 245 



does not condense. Thus, S. LeRoy Brown has found that heat is 

 imparted to a metal surface about twenty-four times as fast by con- 

 densing steam than by a gas at the same temperature. 



(e) Effect of high piston velocity. If the piston speed is too great, 

 the pressure of the expanding steam becomes ineffective because the 

 portions of the steam near the moving piston are expanded and cooled 

 before the remote parts of the steam are affected. This effect is 

 negligible at the highest piston velocities which are mechanically 

 feasible. 



(/) Puffing. When the steam at the end of a stroke is still at a 

 pressure which exceeds the pressure in the condenser (or which exceeds 

 the pressure of the outside air when no condenser is used), it rushes 

 through the exhaust port as a sharp puff. Puffing is to be avoided by 

 sufficiently reducing the steam pressure by expansion in the cylinder. 



The greatest items of waste in the ordinary sense of actual loss of 

 heat are (a) the incomplete combustion of the fuel and (b) the carry- 

 ing away of great quantities of heat in the flue gases. The economic 

 use of fuel for the production of mechanical power requires, therefore, 

 a properly designed furnace and intelligent and careful stoking to 

 insure complete combustion, and it requires a sufficient exposure of 

 boiler surface and frequent cleaning of the same to facilitate the flow 

 of heat from the hot gases into the boiler. 



The most pronounced sweeping process which intervenes between 

 the completed combustion and the final exhaust of the steam is the flow 

 of heat from the very high temperature of the fire in the furnace to' 

 the moderately low temperature of the water in the boiler, and the 

 greatest waste in the operation of the steam engine in the sense of 

 loss of availability of heat for conversion into work is involved in this 

 sweeping process, and it can hardly be avoided in the steam engine 

 because of the danger involved in the generation of steam at very high 

 pressure in a large boiler. 



The best gas engines convert about 30 per cent, of the heat of the 

 fuel into mechanical work. The best steam engines convert about 10 

 or 12 per cent, of the heat of the fuel into mechanical work. The 

 ordinary run of steam engines convert only 4 or 5 per cent, of the heat 

 of the fuel into mechanical work. 



(This article is to be followed by a second article on entropy.) 



