ENERGY, WORK, AND HEAT
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 Btus. 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 Btus 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 Joules constant, is defined as:
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.