ENERGY, WORK, AND HEAT ThermodynamicsThere is one additional unit of energy encountered in engineering applications. It is thehorsepower-hour (hp-hr). It is a mechanical unit of energy defined by the following relationship:1 hp-hr = 1.980 x 10^{6} ft-lbfThese relationships can be used to convert between the various English system units for thevarious forms of energy.Most computations involving the energy of the working fluid in an energy transfer system areperformed in Btu’s. Forms of mechanical energy (such as potential energy, kinetic energy, andmechanical work) and other forms of energy (such as P-V energy) are usually given infoot-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:.J778^{ft lbf}BtuPower is defined as the time rate of doing work. It is equivalent to the rate of the energytransfer. Power has units of energy per unit time. As with energy, power may be measured innumerous basic units, but the units are equivalent. In the English system, the mechanical unitsof 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 electricalunits of power are watts (W) or kilowatts (kW). For engineering applications, the equivalenceof these units is expressed by the following relationships.1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10^{-3} kW1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10^{-4} kW1 kW = 3.413 x 10^{3} Btu/hr = 737.6 ft-lbf/secHorsepower is related to foot-pounds-force per second (ft-lbf/sec) by the following relationship:1 hp = 550.0 ft-lbf/secThese relationships can be used to convert the English system units for power.HT-01 Page 24 Rev. 0

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