STEAM BOILERS 341 



the proper temperature has been reached. Use just steam enough 

 to keep the pressure at 70 pounds. Weigh the feed water supply be- 

 fore starting, then weigh again at the close of the run. If the run 

 has been of one hour's duration, divide the number of pounds of feed 

 water by 30, and this will give the horse power developed. If the 

 run has been only one-half hour, multiply by 2, then divide by 30. 



466. Power by heating surface. The heating surface 

 of a boiler consists of the entire area of those parts of the 

 surface which have fire on one side and water on the other. 

 In the horizontal tubular boiler it is all of the shell which 

 comes beneath the boiler arch, also the inside area of all 

 the tubes and about two-thirds the area of the tube sheets 

 less the area of the flues. In the vertical boilers it is the 

 total inside area of the fire box and as much of the tubes 

 as is below the water line. 



In the fire-box boilers it is the inside area of the water 

 legs, the crown sheet, and the flues and a portion of the 

 tube sheets. 



The common rating of boiler horse power by heating 

 surface is 14 square feet for each horse power. This 

 varies with the boiler, some styles requiring a little less 

 and some a little more. 



As an example, let it be desired to find the heating 

 surface of a horizontal tubular boiler. Find the total 

 area of the outside of shell and take about one-half of this. 

 The brickwork covers about one-half of the shell, hence, 

 one-half of it is all the heating surface there is in this 

 part. Now measure and compute the inside area of one 

 of the flues and multiply this by the number of flues. 

 Add this surface to the heating surface of the shell and 

 divide the sum by 14. This gives the horse power of the 

 boiler. 



467. Power by grate surface. This method is not very 

 often resorted to. In any case it can be only a rough 



