Energy and Power Equivalences

ENERGY, WORK, AND HEAT Thermodynamics There is one additional unit of energy encountered in engineering applications. It is the horsepower-hour (hp-hr). It is a mechanical unit of energy defined by the following relationship: 1 hp-hr = 1.980 x 10⁶ ft-lbf These relationships can be used to convert between the various English system units for the various forms of energy. Most computations involving the energy of the working fluid in an energy transfer system are performed in Btu’s. Forms of mechanical energy (such as potential energy, kinetic energy, and mechanical work) and other forms of energy (such as P-V energy) are usually given in foot-pounds-force. These are converted to Btu’s by using 1 Btu = 778.3 ft-lbf. This conversion factor is often used. In fact, a constant called the mechanical equivalent of heat, usually denoted by the symbol J and sometimes referred to as Joule’s constant, is defined as: . J 778^ft lbf Btu Power is defined as the time rate of doing work. It is equivalent to the rate of the energy transfer. Power has units of energy per unit time. As with energy, power may be measured in numerous basic units, but the units are equivalent. In the English system, the mechanical units of power are foot-pounds-force per second or per hour (ft-lbf/sec or ft-lbf/hr) and horsepower (hp). The thermal units of power are British thermal units per hour (Btu/hr), and the electrical units of power are watts (W) or kilowatts (kW). For engineering applications, the equivalence of these units is expressed by the following relationships. 1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10^-3 kW 1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10^-4 kW 1 kW = 3.413 x 10³ Btu/hr = 737.6 ft-lbf/sec Horsepower is related to foot-pounds-force per second (ft-lbf/sec) by the following relationship: 1 hp = 550.0 ft-lbf/sec These relationships can be used to convert the English system units for power. HT-01 Page 24 Rev. 0