Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles1Glossary
Absolute entropy is entropy calculated relative to the absolute reference point
determined by the third law of thermodynamics.
Absolute humidity (specific humidity or humidity ratio) is the mass of water vapor
present in a unit mass of dry air; that is, it is the ratio of the mass of water vapor to the
mass of dry air in atmospheric air.
Absolute pressure is the actual pressure at a given position and it is measured relative to
absolute vacuum (i.e., absolute zero pressure). Throughout this text, the pressure P will
denote absolute pressure unless specified otherwise.
Absolute temperatures are temperatures measured on the Kelvin scale or Rankine scale,
and these temperatures vary between zero and infinity.
Absorption chillers are air-conditioning systems based on absorption refrigeration, and
they perform best when the heat source can supply heat at a high temperature with little
temperature drop.
Absorption refrigeration systems involve the absorption of a refrigerant by a transport
medium. The most widely used absorption refrigeration system is the ammonia–water
system, where ammonia (NH 3 ) serves as the refrigerant and water (H 2 O) as the transport
medium. Absorption refrigeration systems are economically attractive when there is a
source of inexpensive heat energy at a temperature of 100 to 200°C. Some examples of
inexpensive heat energy sources include geothermal energy, solar energy, and waste heat
from cogeneration or process steam plants, and even natural gas when it is available at a
relatively low price.
Absorptivity is the fraction of the radiation energy incident on a surface that is absorbed
by the surface.
Acid rain is defined as rain or snow that washes acid-laden droplets from the air on to
the soil.
Adiabatic combustion temperature (see adiabatic flame temperature)
Adiabatic flame temperature is the maximum temperature the products of combustion
will reach in the limiting case of no heat loss to the surroundings during the combustion
process. The adiabatic flame temperature attains its maximum value when complete
combustion occurs with the theoretical amount of air.
Adiabatic process is a process during which there is no heat transfer. The word
adiabatic comes from the Greek word adiabatos, which means not to be passed.