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Boiler Capacities

Steam boilers output can be expressed in Boiler Horsepower, MBTU or in Pounds of Steam delivered per hour.

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Boiler load - the capacity of a steam boiler - is often rated as boiler horse powers, lbs of steam delivered per hour, or BTU.

Lbs Steam delivered per Hour

Large boiler capacities are often given in lbs of steam evaporated per hour under specified steam conditions.

BTU - British Thermal Units

Since the amount of steam delivered varies with temperature and pressure, a common expression of the boiler capacity is the heat transferred over time expressed as British Thermal Units per hour. A boilers capacity is usually expressed as kBtu/hour (1000 Btu/hour) and can be calculated as

W = (hg - hf) m                                         (1)


W = boiler capacity (Btu/h, kW)

hg = enthalpy steam (Btu/lb, kJ/kg)

hf = enthalpy condensate (Btu/lb, kJ/kg)

m = steam evaporated (lb/h, kg/s)

Boiler Horsepower - BHP

The Boiler Horsepower (BHP) is

  • the amount of energy required to produce 34.5 pounds of steam per hour at a pressure and temperature of 0 Psig and 212 oF, with feedwater at 0 Psig and 212 oF.

A BHP is equivalent to 33,475 BTU/Hr or 8430 Kcal/Hr and it should be noted that a boiler horsepower is 13.1547 times a normal horsepower.

  • 1 horsepower (boiler) = 33445.6 Btu (mean)/hr = 140671.6 calorie/min (thermo) = 140469.4 calorie (mean)/min = 140742.3 calorie (20oC)/min 9.8095x1010 erg/sec = 434107 foot-pound-force/min = 13.1548 horsepower (mech) = 13.1495 horsepower (electric) = 13.3372 horsepower (metric) = 13.1487 horsepower (water) = 9809.5 joule/sec = 9.8095 kilowatt

Horsepower (hp) can be converted into lbs of steam by multiplying hp with 34.5.

Example - Boiler Horsepower to lbs of Steam Conversion

200 hp x 34.5 = 6900 lbs of steam per hour

Lbs of steam can be converted to hp by dividing lbs steam per hour by 34.5

Example - Lbs of Steam to Boiler Horsepower Conversion

5000 lbs of steam / 34.5 = 145 hp boiler

Converting Boiler Horse Power (BHP) to kBtu/hr

Boiler Horse Power
4.5 151
6 201
9 301
15 502
20 670
30 1004
45 1507
50 1674
60 2009
70 2344
80 2678
100 3348
  • 1 BHP = 33479 Btu/hr
  • 1 Btu/hr = 0.293 Watts
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