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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
COP HP, and expressed as desired output divided by required input or COP HP =Q H/W net, in

Heat transfer (heat) is defined as the form of energy that is transferred between two
systems (or a system and its surroundings) by virtue of a temperature difference.

Ideal gas is a gas that obeys the ideal-gas equation of state.

Ideal-gas equation of state (or ideal-gas relation) predicts the P-v-T behavior of a gas
quite accurately within some properly selected region where Pv = RT.

Ideal gas specific heat relation is Cp = Cv + R.

Internal energy U of a system is the sum of all the microscopic forms of energy.

Internal energy change of an ideal gas is given as

Irreversible processes are processes which, once having taken place in a system, cannot
spontaneously reverse themselves and restore the system to its initial state.

Irreversibility is the factors that cause a process to be irreversible. They include friction,
unrestrained expansion, mixing of two gases, heat transfer across a finite temperature
difference, electric resistance, inelastic deformation of solids, and chemical reactions.

Isentropic process is an internally reversible and adiabatic process. In such a process
the entropy remains constant.

Iso- prefix is often used to designate a process for which a particular property remains
constant.

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