Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles41Relative humidity is the ratio of the amount of moisture (water) in atmospheric air at a
given temperature to the maximum amount the air can hold at the same temperature. The
relative humidity can be expressed as the ratio of the vapor pressure to the saturation
pressure of water at that temperature.
Relative pressure Pr is defined as the quantity exp(s°/R) and is a dimensionless quantity
that is a function of temperature only since s° depends on temperature alone. Relative
pressure is used to relate the ratio of final to initial pressure in isentropic processes of
ideal gases where variable specific heats are required.
Relative specific volume vr is defined as the quantity T/Pr and is a function of
temperature only. Pr is the relative pressure. Relative specific volume is used to relate
the ratio of final to initial volume in isentropic processes of ideal gases where variable
specific heats are required.
Reversed Carnot cycle is a reversible cycle in which all four processes that comprise the
Carnot cycle are reversed during operation. Reversing the cycle will also reverse the
directions of any heat and work interactions. The result is a cycle that operates in the
counterclockwise direction. The reversed Carnot cycle is the Carnot refrigeration cycle.
Reversible adiabatic compression is the process in which a working fluid is compressed
(decreases in volume) reversibly and adiabatically.
Reversible adiabatic expansion is the process in which a working fluid expands
(increases in volume) reversibly and adiabatically.
Reversible isothermal compression is the process in which the temperature is held
constant while a working fluid is compressed (decreases in volume) reversibly.
Reversible isothermal expansion is the process in which the temperature is held
constant while a working fluid expands (increases in volume) reversibly.
Reversible process is defined as a process that can be reversed without leaving any trace
on the surroundings. Reversible processes are idealized processes, and they can be
approached but never reached in reality.
Reversible steady-flow work is defined as the negative of the integral of the specific
volume-pressure product. The larger the specific volume the larger the reversible work
produced or consumed by the steady-flow device. Therefore, every effort should be
made to keep the specific volume of a fluid as small as possible during a compression
process to minimize the work input and as large as possible during an expansion process
to maximize the work output.