Refrigeration & Equipment Engineering

1.7.Enthalpy and Entropy

Unit 1 - Fundamentals of Heat and Thermodynamics

1.7.Enthalpy
Enthalpy means heat content.It is denoted by the letter H.Enthalpy is a function of states and like the internal energy,it also depends on the initial and final states of the systems (anything that is under observation for experimental work is called a system. A gas placed in a cylinder for experimental work forms a system). The enthalpy of a refrigerant at a given temperature is the amount of heat that must be added to one pound of the refrigerant to bring it up to the required temperature of a liquid or vapour from the reference temperature. The total heat of the vapour includes the sensible heat of the liquid plus the latent heat needed to form the vapour at that same temperature.

Entropy
Entropy is the quantitative measure of disorder in a system. The concept comes out of thermodynamics which deals with the transfer of heat energy within a system. It is an important thermodynamic property of a working substance, which increases with the addition of heat and decreases with its removal over a small range of temperature, the increase or decrease of entropy, when multiplied by the absolute temperature gives the heat absorbed or rejected by the working substance.