Mollier diagram is chart drawn between enthalpy H (on ordinate) and entropy or S (on abscissa). It is also called as H-diagram. It depicts properties of water and steam for pressures up to 1000 bar and temperatures up to 800°C. In it the specific volume, specific entropy, specific enthalpy, and dryness fraction are given in incremental steps for different pressures and temperatures. A Mollier diagram is quite convenient in predicting the states of steam during compression and expansion, at the time heating and cooling, and during throttling and isentropic processes directly. It does not include any detailed calculations as is required while using the steam tables. Sample of a Mollier chart is shown in Figure given below for a better understanding.
There is thick saturation line which indicates 'dry and saturated state' of steam. The region below the saturation line signifies steam 'in wet conditions' and above saturation line, steam is in superheated state. The lines of constant temperature and of constant dryness fraction are drawn in wet and superheated regions. It should be kept in mind that the lines of constant pressure are straight in wet region but are curved in superheated region.