Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles23Heat-driven systems are refrigeration systems whose energy input is based on heat
transfer from an external source. Absorption refrigeration systems are often classified as
heat-driven systems.
Heat engines are devices designed for the purpose of converting other forms of energy
(usually in the form of heat) to work. Heat engines differ considerably from one another,
but all can be characterized by the following:
- They receive heat from a high-temperature source (solar energy, oil furnace,
nuclear reactor, etc.). - They convert part of this heat to work (usually in the form of a rotating shaft).
- They reject the remaining waste heat to a low-temperature sink (the atmosphere,
rivers, etc.). - They operate on a cycle.
Heat exchangers are devices where two moving fluid streams exchange heat without
mixing. Heat exchangers are widely used in various industries, and they come in various
designs. The simplest form of a heat exchanger is a double-tube (also called tube-and-
shell) heat exchanger composed of two concentric pipes of different diameters. One fluid
flows in the inner pipe, and the other in the annular space between the two pipes. Heat is
transferred from the hot fluid to the cold one through the wall separating them.
Sometimes the inner tube makes a couple of turns inside the shell to increase the heat
transfer area, and thus the rate of heat transfer.
Heat pump is a cyclic device which operates on the refrigeration cycle and discharges
energy to a heated space to maintain the heated space at a high temperature. It is a cyclic
device which causes the transfer of heat from a low-temperature region to a high-
temperature region.
Heat pump coefficient of performance is the efficiency of a heat pump, denoted by
COPHP, and expressed as desired output divided by required input or COPHP = QH/Wnet,
in.
Heat rate is the expression of the conversion efficiency of power plants in the United
States and is the amount of heat supplied, in Btu’s, to generate 1 kWh of electricity. The
smaller the heat rate, the greater the efficiency.
Heat reservoir is a thermal energy reservoir since it can supply or absorb energy in the
form of heat.
Heat sink is a heat reservoir that absorbs energy in the form of heat.
Heat source is a heat reservoir that supplies energy in the form of heat.