Analysis of The Mollier Diagram to Simplify The Calculations of Thermodynamic Magnitudes

The enthalpy-entropy chart, also known as the Mollier diagram, has different technical applications. It is widely used to estimate the enthalpy of pure substances and the mixtures of substances that are most frequently found in Engineering. Mollier's chart serves to simplify calculations of enthalpy, entropy, pressure, temperature, specific volume and the quality of steam and water. The Mollier diagram is the representation of the properties of water and water vapor. A main H-S coordinate system (Enthalpy-Entropy) is used.

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The diagram was created in 1904 when Richard Mollier plotted total heat against entropy. At the 1923 Thermodynamics Conference held in Los Angeles, it was decided to name, in his honor, as a "Mollier diagram" any thermodynamic diagram that he used the enthalpy h as one of its axes.

Constant Pressure Lines and Enthalpy Lines. The horizontal lines are the lines of constant pressure, and the vertical lines are the lines of constant "enthalpy", that is the amount of heat present in a kilo of refrigerant. Note that the pressures are absolute pressures and that the scale is logarithmic. Enthalpy. Although enthalpy is sometimes defined as "total heat", it is more correctly and specifically defined as the sum of all the energy supplied by a given mass of matter in any thermodynamic condition. The formula for calculating the enthalpy is indicated below.h = u + p v / jWhere:H: enthalpy (kcal / kg)U: internal energy (kcal // kg)P: absolute pressure (kgf / cm2)V: specific volume (m3 / kg)J: equivalent mechanical energy. Saturated liquid line and saturated steam line.

As the following figure shows, the diagram is divided into three main parts separated by the saturated liquid line and the saturated steam line. The part to the left of the saturated liquid line is called the "sub-cooled zone". At any point in the sub-cooled zone, the refrigerant is in the liquid state and its temperature is below the saturation temperature corresponding to its pressure.

The part to the right of the saturated steam line is called the "recharged zone". In this part, the refrigerant is in the form of superheated steam. The central part of the table, between the lines of saturated liquid and saturated steam, is called "phase change zone", which represents the phase change of the refrigerant between the liquid and vapor states. At any point between the two lines, the refrigerant has the form of a mixture of liquid and vapor. As seen in the previous figure, the point of union between the saturated liquid line and the saturated steam line is called "critical point". The temperature and pressure at this point are referred to respectively as "critical temperature" and "critical pressure". Critical Temperature. The critical temperature of a gas is the highest temperature at which said gas can be condensed by application of pressure. The critical temperature differs according to the types of gases.

Dry Steam Lines. The phase change from liquid to vapor occurs progressively from the left to the right while the phase change from vapor to liquid occurs from right to left. The mixture of liquid and vapor near the saturated liquid line is almost pure liquid. On the contrary, the mixture of liquid and vapor near the saturated vapor line is almost pure vapor.

The "dry steam" lines that extend from the critical point to the bottom through the central section of the table and approximately parallel to the saturated liquid and vapor lines indicate the percentage of vapor in the mixture. with increases of 10%.For example, at any point in the dry steam line closest to the saturated liquid line, the dry vapor of the liquid and vapor mixture (X) is 0.1, which means that 10% (by weight) of the mixture is vapor, and 90% is liquid.