Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. Chapter 5 Measurement 79 measure of the intensity of heat energy, heat is a measurement of the energy content of a material. In the United States, the most common unit of measurement for heat is the Btu. 5.6.1 US Customary The British thermal unit (Btu) is the amount of heat energy required to change the temperature of 1 lb of pure water by 1°F. When the temperature of 1 lb of pure water is raised or lowered by 1°F, the quantity of heat moved equals 1 Btu. Since 1 Btu is a small amount of heat compared to the amounts of heat moved in some HVACR systems, a larger unit is used. The therm is defi as 100,000 Btu. fined In the HVACR trade, it is often necessary to relate the movement of the heat quantity to a time frame. Thus, the unit Btu per hour (Btu/hr) is commonly used. This term represents the number of Btu moved in an hour. The term ton of refrigeration is also used as a measurement of heat transfer. It is based on the melting of one ton (2,000 lb) of ice at 32°F in 24 hours. Since it requires 288,000 Btu of heat to melt one ton of ice, one ton of refrigeration equals 12,000 Btu/hr. 1 ton = 288,000 Btu = 288,000 Btu ___________ 24 hr = 12,000 Btu/hr Medium to large boiler systems are often rated in horsepower (hp). One horsepower is defi as the conversion of 34.5 lb of water into steam in one hour, when the water fined is starting from 212°F. Pounds of steam per hour, commonly called pounds/hour, is the rating used for very large boilers. It is the amount of water a boiler can turn into steam in one hour. 5.6.2 SI Heat measurement in SI units is based on the calorie or the joule. A calorie (cal) is the amount of heat energy required to change the temperature of one gram of water by 1°C. +1°C 1 gram of water 1 calorie of heat 1 kilogram of water 4.187 kJ of heat +1°C Goodheart-Willcox Publisher 1 Btu of heat +1°F 1 pound of water 5