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Steam Math Lesson #2this thread has 4 replies and has been viewed 6294 times


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#1




Steam Math Lesson #2
Well, the other one finally died down a bit so I thought I would throw on another, I don't know about the rest of you, but I am doing some cutting, pasting and printing on some of this stuff!
Boiler Rating Factors In the early days of steam power, boilers and engines were coordinated in size through knowledge that the typical steam engine required about 34 lbs. of steam per horsepower per hour. (34 lbs./hp/hr.) A typical 100 horsepower steam engine required 3,450 lbs. of steam per hour when it was operated at rated load. A terminology was created in the boiler industry to fit these facts. A 3,450 lbs. per hour steam boiler was called a 100 horsepower boiler. It was sized appropriately for a 100 horsepower engine. Thirtyfour and one half pounds of boiler steam per hour became known as a developed boiler horsepower. 1 Boiler horsepower  Nominally 34.5 lbs. of steam per hour. 1 Rated boiler horsepower  10 square feet of effective heating surface (in firebox type boilers) (6, 8, and 12 square feet also have been used). 1 Developed boiler horsepower  33,475 B.t.u. absorbed per hour. (34.5 lbs. water x 970.3 Btu/lb.) Per cent rated capacity  (Developed horsepower) / (Rated horsepower) Factor of evaporation  B.t.u. absorbed per lb. of steam / 970.3 Actual evaporation  Pounds steam generated per hour. Equivalent evaporation  (Actual evaporation) x (Factor of evaporation). Kilo B.t.u. per hour, kB  Total B.t.u. absorbed per hour / 1,000 Mega B.t.u. per hour, MB  Total B.t.u. absorbed per hour / 1,000.000 Boiler horsepower, A.S.M.E. A boiler horsepower as defined by the rules of the American Society of Mechanical Engineers, which is now generally accepted as standard in this country, is the equivalent to evaporation of 34.5 lb. of water per hour, from and at 212 deg. F. Expressed in heat units this is 970.3 B.t.u. x 34.5 = 33,475 B.t.u. Boilers ordinarily do not evaporate at 212 deg. F., therefore, it is necessary to consider the factor of evaporation in calculating boiler horsepower. The factor of evaporation is the total heat in one pound of steam as it leaves the boiler, less the heat in one pound of feedwater, divided by 970.3. Data for calculating boiler performance. The following data will be found valuable in calculating boiler performance. 778 ft. lb. = 1 B.t.u. 1 American gallon of fresh water = 8.336 lb. and 231 cu. in. 1 cu. ft. of fresh water = 62.4 lb. 1 cu. ft. of fresh water = 7.4805 American gallons. 1 Atmosphere = 14.696 lb. per sq. in. (Usually rounded off to 14.7lb.) 1 Atmosphere = 33.8 ft. of fresh water. 1 Atmosphere = 29.921" mercury (Hg.) head. 1 lb. of air at 60 deg. F., and atmospheric pressure = 13.1 cu. ft. 1 lb. gauge pressure = 1 atmosphere. 
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#2




Re: Steam Math Lesson #2
Mike,
What is meant by "effective heating surface", and how is this calculated? The "other" Mike 
#3




Re: Steam Math Lesson #2
[QUOTE=MYaeger]
1 Boiler horsepower  Nominally 34.5 lbs. of steam per hour. 1 Rated boiler horsepower  10 square feet of effective heating surface (in firebox type boilers) (6, 8, and 12 square feet also have been used). So where did 34.5 lbs come from? What does heating surface have to do with an boiler efficiency calculation? 
#4




Re: Steam Math Lesson #2
Effective heating surface would be any heating surfaces exposed to combustion gases . . . waterwall tubes, water lined fireboxes, tubes, etc.
Years ago they found that the average steam engine consumed 34.5 lbs. of water per horsepower per hour, so that's where that rating came from. Here's an example on how to calculate boiler horsepower: Q. What is the horsepower of a boiler evaporating 12,000 lbs. of feed water per hour from a feedwater temperature of 170 deg. F. into steam at 150 lbs. gauge pressure? (Total heat of 1 lb. of steam at 150 lbs. gauge is 1195 B.t.u.) A. 170  32 = 138 B.t.u. required to raise 1 lb. from 32 to 170 deg. F. 1195  138 = 1057 B.t.u. added to each pound of steam generated. 1057 x 12,000 = 12,684,000 B.t.u. added to 12,000 lbs. of steam per hour. One (1) boiler horsepower = 34.5 lbs. of water at 212 deg. F. converted to steam at 212 deg. F. 970.3 = B.t.u. required to convert 1 lb. of water at 212 deg. F. to steam at 212 deg. F. 34.5 x 970.3 = 33,475 B.t.u. (one boiler horsepower) 12,684,000 / 33,475 = 379 horsepower (ans.) Note: If the steam flow remains constant, such as in this case, 12,000 lbs./hr., and the feedwater temperature rise's, the boiler horsepower will calculate to a lower figure . . . because . . . the boiler doesn't have to fire as hard! 
#5




Re: Steam Math Lesson #2
Years ago they found that the average steam engine consumed 34.5 lbs. of water per horsepower per hour, so that's where that rating came from.
So Then a steam engine rated for 60hp would consume 60 x 34.5 = 2070 lbs of water and hour or 248.50 gallons if we used 8.33 lbs per gallon of water, let me see if I have this correctly if the engine was operating at the full brake horse rating of 60hp I would need to bring the engineer 248.5 gallons every hour just to keep up with the consumption. 
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