Enthalpy charts

Figure B-4. Ammonia pressure-enthalpy diagram. (From Short, Kent and Walls, Pressure-Enthalpy Charts for Selected Fn i neering Substances, Gulf Publishing Company, Houston, TX, 1970.)...

Frequently, some plants use mixtures of some of the hydrocarbon refrigerants because of local convenience. In such cases it is important to develop the appropriate mixture s physical property and enthalpy charts for design, because the properties of only one of the components cannot define the mixture. 

A more informative diagram is the pressure-enthalpy chart which shows the liquid and vapour states of the fluid (Figure 2.6). In this diagram, a fluid being heated passes from the suhcooled state (a), reaches hoiling point (h) and is finally completely evaporated (c) and then superheated (d). The distance along the sector h-c shows the proportion which has heen evaporated at any enthalpy value. 

Figure D-3 Pressure-enthalpy chart, carbon dioxide. (From ASHRAE Handbook of Fundamentals, 1967.)...

With the nomenclature used in Volume 2, Example 14.3, H = 2765 kJ/kg and, assuming isentropic expansion to 3.5 kN/m2, from the entropy—enthalpy chart ... 

The high-pressure steam is saturated at 1135 kN/m2 at which Hx = 2780 kJ/kg. If this is allowed to expand isentropically to 101.3 kN/m2, then from the entropy-enthalpy chart, given in the Appendix, H2 = 2375 kJ/kg and the dryness faction is 0.882. 

Finding Work of Compression with a Thermodynamic Chart Hydrogen sulfide is to be compressed from 100°F and atmospheric pressure to SOpsig. The isentropic efficiency is 0.70. A pressure-enthalpy chart is taken from Starling (Fluid Thermodynamic Properties for Light Petroleum Systems, Gulf, Houston, TX, 1973). The work and the complete thermodynamic conditions for the process will be found. 

Figure 1.5 Typical enthalpy chart (not to scale). (Data from J. B. Maxwell, Thermal Properties, Data Book on Hydrocarbons, D. Van Nostrand, Princeton, NJ, 1957, pp. 98-127.)...

Figure 1.5 reveals the data curves that Table 1.10 would shape if plotted. Please note the x and y axes are linear. This is a typical replica of the enthalpy charts in Maxwell s Data Book on Hydrocarbons [19], As old as Maxwell s book is, it is still one of the more sought out and technically notable books of our time. Compare Fig. 1.5 to Table 1.10. Observe how Table 1.10 may be used in easy interpolations for any enthalpy reading, liquid or gas state, and pressures from vacuum to 100 atm. Please note in Fig. 1.5 also how the 1.0-atm curve joins the saturation vapor line same for the 100-atm curve, how it joins the sat-... 

If you followed the quest of the previous paragraphs, realize that these referenced Maxwell enthalpy charts and Table 1.10 are near directly and totally governed by temperature alone. Observe how Fig. 1.5 lays all the enthalpies to display two curves plotted as enthalpy vs. temperature. Both curves start at 0°F and end at 1000°F. With the two pressure curves as shown in Fig. 1.5, one can determine any enthalpy value, gas or liquid. You simply need one temperature. Pressure-based interpolation may then be made linearly between the two temperature intercept points of these two curves as shown in Fig. 1.5. Please note that the dashed temperature lines are the same as the column temperatures given in Table 1.10. Thus, Table 1.10 may be used just as if one were using the curve types of Fig. 1.5 to derive enthalpy values. Table 1.10 is proposed as an improved, easier-to-read resource as compared to a curve-plotted chart. The table gives an advanced get-ahead step, giving you the curve points to read to make your interpolation. 

The easiest way to solve this problem is to use the pressure-enthalpy chart for n-butane found in the text. Basis 10 lb n-butane. 

Figure J.2 Pressure-enthalpy chart for carbon dioxide.

Since enthalpies are being taken from more than one source, we should take care that the reference state is the same for all tables and charts used in a given problem. The steam tables in Geankoplis use saturated liquid water at 32°F and 1 atm as the reference state. The enthalpy chart for NaOH solutions in Geankoplis also use 0 wt% NaOH at 32°F and 1 atm as the reference state. 

A throttle does not change the enthalpy of the fluid throttling is an isenthalpic process. For a given input state and a specified outlet pressure, one finds the outlet temperature by conducting a one-dimensional search for a temperature at which the enthalpy is equal to the input enthalpy. An enthalpy chart provides a convenient means for the search. Another method of solution is to apply the Joule-Thompson coefficient, defined by... 

Thermal Properties of Nitration Acids. Heals of Solution. To determine the heat evolved during the actual process of nitration of a hydrocarbon by mixed acids, it is necessary to consider not only the heat of nitration but also various heats of solution. These may be obtained from the enthalpy chart developed by McKinley and Brown (Fig. 4-5). For each of the three components, the enthalpy is taken as zero at the standard state, consisting of the pure liquid component at a temperature of 32 F and a pressure of 1 atm. Plotted against the same abscissa but against different ordinates are the specific heat data for the system. From this figure, containing both 32 F relative enthalpies and specific heats, the enthalpy of any liquid mixtures of these components at ordinary temperatures can be readily calculated by reading the desired relative enthalpy at 32 F and the specific heat from the chart. 

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. 

The pressure-enthalpy chart of Fig. 9.26 shows the course of a typical process. Line AC represents the boiling of the refrigerant under the influence of the process load. This takes place in the first exchanger referred to above. From the viewpoint of one following the chlorine flow, this is a liquefier. From the viewpoint of one following the refrigerant flow, and in common refrigeration industry parlance, it is an evaporator. 

It is usefiil to view this process on an hP generali2Bd enthalpy chart, as shown in Figure E5.10. The lever rule is illustrated. The quality can be found by ... 

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