38 



ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



fair market prices given, varies widely, 

 lying between 200,000 heat units per dollar 

 and 10,000,000 heat units per dollar. It 

 is possible to buy eight times as much 

 energy for a given amount of money in 

 the form of cheap coal as in the form of 

 low-priced gasoline, or 25 times as much 

 as in the form of high-priced gasoline or 

 kerosene. This being true, it might seem 

 to a casual observer as rather strange that 

 gasoline should be used at all, and the 

 fact that it is used in competition with 

 fuel of one-eighth to one twenty-fifth 

 its cost shows clearly that either the gaso- 

 line engine has some characteristics not 

 possessed by an engine or plant using 

 coal, which makes it able to do things 

 the other can not do, or that more of the 

 heat it contains can be transformed into 

 energy for useful work. Both of these 

 things are true. 



Thermal Efficienoy 



As was pointed out before, the different 

 kinds of machinery used to generate pow- 

 er render more or less of the fuel energy 

 into useful work; all systems do not give 

 equal returns for equal amounts of heat 

 supplied. If all the heat energy in fuel 

 were transformed into work with no losses 

 whatever in the mechanism, the machin- 

 ery would be said to have a thermal ef- 

 ficiency of 100 per cent, and it would re- 

 quire 2,545 heat units per hour to main- 

 tain an output of one horsepower. If half 

 of the energy in the fuel were lost in the 

 machinery, its thermal efficiency would be 

 said to be 50 per cent, and there would 

 be required 5.090 heat units per hour. If 

 only one per cent of the heat energy in 

 the fuel were transformed into useful 

 work, the efficiency of the machinery or 

 power plant would be said to be one per 

 cent and there would be required 254.500 

 heat units per hour to maintain one horse- 

 power. 



Steam plants in use represent a great 

 variety of styles or types, but in general 

 it may be said that the more complicated 

 and refined the plant the larger its size 

 the more efficient it is, because the com- 

 plication exists only as evidence of an 

 attempt to minimize the losses of heat in 

 the machinery. Similarly the more stead- 



ily the plant works at the output for which 

 it was designed the higher the efficiency 

 of the plant, and, conversely, the smaller 

 the plant the simpler the apparatus, or 

 the more intermittently it works, the 

 lower its efficiency. Steam-power plants 

 are built today to do every conceivable 

 sort of work, and range in size from one 

 horsepower to 100.000 horsepower. For 

 purposes of comparison neither the larg- 

 est nor the smallest should be used, nor 

 the best performance nor the worst per- 

 formance of these plants, but a figure rep- 

 resenting a fair average for the conditions 

 named should be taken. Large steam 

 plants in their daily work seldom use less 

 than two pounds of poor coal per hour 

 for each useful horsepower (known as a 

 brake horsepower), which is equivalent to 

 about 25,000 B. T. U. per hour, and which 

 corresponds to about 10 per cent thermal 

 efficiency. Small steam plants working 

 intermittently, such as hoisting engines, 

 may use as high as seven pounds of coal 

 per brake horsepower, which is equivalent 

 to about 100,000 heat units per brake 

 horsepower hour, or 2.5 per cent thermal 

 efficiency. Some plants will do better than 

 the above with proper conditions, and 

 some may do worse, but in general it may 

 be said that the performances of steam 

 plants lie between the limits of 2.5 and 10 

 per cent thermal efficiency. 



Plants consisting of gas producers for 

 transforming coal into gas for use in gas 

 engines have in general a much higher 

 thermal efficiency than steam plants do- 

 ing the same work. They are, however, 

 not built quite so small as steam plants, 

 the smallest being about 25 horsepower, 

 and in general they have not been built 

 so large, the largest being only a few 

 thousand horsepower. Their efficiency, 

 however, does not vary so much as is the 

 case with steam plants. It may be fair to 

 say that under the same conditions as 

 above outlined these plants will use I14 

 to 2 pounds of coal of fair or poor quality 

 per brake horsepower hour, which gives 

 a thermal efficiency ranging from IS to 10 

 per cent. These plants can be made to do 

 much better than this, and perhaps may 

 do worse, although the variation is not 

 nearly so great as for steam plants. 



