A Series of Enthalpy-entropy Charts for Natural Gases

Abstract

Abstract Enthalpy-entropy diagrams are presented for natural gasses of 0.6, 0.7, 0.8,0.9, and 1.0 gravity over the pressure range of 5 to 10,000 lb. per sq. in. andtemperature range of 32 degrees to 700 degrees Fahrenheit. The charts indicatedirectly the work requirement and temperature rise for adiabatic compression ortemperature change for free expansion of natural gases. Computation and Uses of Charts The Mollier diagram in which the enthalpy (heat content) is plotted against theentropy with lines of constant temperature, pressure, and in some cases volume, has been found most convenient when dealing with the compression, expansion, and flow of fluids. In dealing with the flow of fluids, the sum of the increasein the enthalpy plus the increase in kinetic energy plus the increase inpotential energy of the fluid in flow, is equal to the sum of the heat and workadded to the fluid while flowing between the entrance and exit of the flowsystem. In cases where there is no significant change in potential energy or in kineticenergy (velocity), it follows that the increase in enthalpy is equal to thetotal energy supplied to the fluid. Under such conditions the changes in theproperty of the fluid as it flows through a throttling valve, choke, or anyother similar arrangement, may be read directly from the enthalpy-entropydiagram by following a horizontal line between the known pressures. When compression or expanding a gas by which means of a compressor or engine inwhich no heat is added to or subtracted from the gas, but only work done, thechanges in the properties of the gas may be determined along a vertical line ofconstant entropy between the entering and exit pressures. The power requiredfor the compression of the gas may be readily determined by converting theincrease in enthalpy into the desired units. The enthalpy-entropy diagram for natural gases is to the gas engineer what thesteam diagram is to the steam-power engineer. For this reason it would beextremely convenient of a reasonably satisfactory enthalpy-entropy diagramcould be prepared as a function of the gravity of the gas. A careful study ofthe known properties of natural gas indicates that this is possible. The effect of temperature upon the enthalpy at consistent pressure is expressedas the ?heat capacity? or ‘specific heat’ of the gas. The best available datafor natural and petroleum refinery gases indicates the relationship shown. Fromthis it is clear that the specific heat of natural gases is a function only ofthe gas gravity and the temperature at atmospheric pressure. The effect ofpressure on the enthalpy of natural gases is dependent upon the pressure-volume-temperature relationships. T.P. 1747