Enthalpy - pressure - temperature - entropy diagrams

If a Mollier diagram (enthalpy-pressure-temperature-entropy) is available for the working fluid the isentropic work can be easily calculated. 

FIG. 2-8 Temperature-entropy diagram for carbon monoxide. Pressure F, in atmospheres density p, in grams per cubic centimeter enthalpy H, in joules per gram. From Must and Stewart, NBS Tech. Note 202, 1963.)... 

Din was the editor of a series of books designed to provide reliable thermodynamic data for industrially important gases. Temperature-entropy diagrams were chosen as the most generally useful graphical presentations and these are supplemented by tables of entropy, enthalpy, volume, heat capacity at constant pressure and at constant volume, and Joule-Thomson coefficients. Unfortunately, there is no consistency in the choice of units, although the thermochemical calorie is employed. The report on each substance (i.e. ammonia, carbon dioxide, carbon monoxide, air, argon, acetylene, ethylene, and propane) consists of a brief introduction, a survey of experimental data, a description of methods used for the thermodynamic calculations, and a set of tables. 

Fifteen percent of the air compressed from 0.101 MPa to 20.2 MPa in a Lindle precooled liquefier is liquefied. With the aid of a temperature-entropy diagram calculate values of rhhhh, the enthalpy of the high-pressure stream, and rh h, the enthalpy of the low-presure stream. Take the cold end of the exchanger just above the expansion valve as the reference zero for the changes in enthalpy. Plot values ol rh Ah and rhc Ah versus the temperature. Does the temperature at which the A T becomes zero agree with the temperature to which the air must be precooled to achieve 15% liquefaction ... 

When the gas does not behave as an ideal gas, the change in its condition can be followed on a temperature-entropy or an enthalpy-entropy diagram. The intermediate pressures P,i. P 2,. .., are then selected so that the enthalpy change (A//) is the same in each cylinder. 

The thermodynamic properties of a number of compounds are shown in Appendix D as pressure-enthalpy diagrams with lines of constant temperature, entropy, and specific volume. The vapor, liquid, and two-phase regions are clearly evident on these plots. The conditions under which each compound may exhibit ideal gas properties are identified by the region on the plot where the enthalpy is independent of pressure at a given temperature (i.e., the lower the pressure and the higher the temperature relative to the critical conditions, the more nearly the properties can be described by the ideal gas law). 

A thermodynamic diagram represents the temperature, pressure, volume, enthalpy, and entropy of a substance on a single plot. (Sometimes data for all these variables are not included, but the term still applies.) The most common diagrams are temperature/entropy, pressure/enthalpy (usually In P vs. FT), and enthalpy/entropy (called a Moltier diagram). The designations refer to the variables chosen for the coordinates. Other diagrams are possible, but are seldom used. 

A schematic of the entropy diagram and the thermodynamic cycle is shown on the right side of the diagram. Temperature is shown on the x-axis and entropy is on the y-axis. The lines identified as h, and hz represent constant enthalpy. Lines P, and P2 are constant pressure. The parabolic curve is the locus of points representing the vapor-liquid equilibrium. The thermodynamic cycle is traced by the points labeled 1 through 7. 

Figures 55.3 and 55.4 show the temperature-entropy and pressure-enthalpy diagrams of the heat pump cycle, respectively. The heat pump cycle works as follows ...

The PVand ZP diagrams are examples of such charts that we encountered already. There is no limitation as to the properties that can be chosen to represent the axes and this freedom leads to various possible combinations. Three charts that find widespread use the pressure-enthalpy chart, the temperature-entropy chart, and the enthalpy-entropy chart, also known as Mollier chart. These are explained below. 

Available data on the thermodynamic and transport properties of carbon dioxide have been reviewed and tables compiled giving specific volume, enthalpy, and entropy values for carbon dioxide at temperatures from 255 K to 1088 K and at pressures from atmospheric to 27,600 kPa (4,000 psia). Diagrams of compressibiHty factor, specific heat at constant pressure, specific heat at constant volume, specific heat ratio, velocity of sound in carbon dioxide, viscosity, and thermal conductivity have also been prepared (5). 

Figure D-4 Enthalpy-log pressure diagram for Refrigerant 22. Temperature in °F, volume in ft3/lb, entropy in Btu/(lb °R), quality in weight percent. (Reprinted by permission of El du Pont de Nemours Company, 1967.)...

Mollier diagram A chart relating enthalpy, entropy, temperature, and pressure of steam. 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

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