Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. 86 Math for HVACR Conversions may be within the same measurement system, such as converting Btu/hr to horsepower, which are both units in the US Customary system. However, it may be necessary to convert from US Customary units to the SI system of units as well. Conversion Factors for Heat Measurement of the US Customary System Multiply: by To obtain: boiler hp 34.5 pounds of steam/hour boiler hp 33,475 Btu/hour output boiler hp 33.5 Mbh (1,000 Btu/hr output) pounds of steam (@ 212°F) 970 Btu/hr therm 100,000 Btu/hr tons of refrigeration 12,000 Btu/hr Divide: by To obtain: pounds of steam/hour 34.5 boiler hp Btu/hour output 33,475 boiler hp Mbh (1,000 Btu/hr output) 33.5 boiler hp Btu/hr 970 pounds of steam (@ 212°F) Btu/hr 100,000 therm Btu/hr 12,000 tons of refrigeration Goodheart-Willcox Publisher As an example of the need for calculating conversions, consider a boiler that is rated at a capacity of 300 horsepower (hp). All boilers require a safety valve, and the ASME Boiler and Pressure Vessel Code requires the capacity of the safety valve to equal or exceed the capacity of the boiler. Though boilers may be rated in hp, valves are not. Safety valves are rated in pounds of steam per hour for large valves and Btu/hr for smaller valves. To determine the capacity of a valve (in pounds of steam per hour) required for a 300 hp boiler, use a multiplier of 34.5 as shown on the chart. This will convert hp into pounds of steam per hour. 300 hp × 34.5 = 10,350 pounds of steam per hour Conversion Factors for Heat Measurement of the SI System Multiply: by To obtain: calories 4.184 joules calories 0.004184 kilojoules joules 0.23901 calories joules 0.001 kilojoules kilojoules 1,000 joules kilojoules 239.00574 calories Goodheart-Willcox Publisher