![]() ![]() Rudolf Clausius gave the first definition of entropy in 1865. Now, let’s have a small glance at the history of enthalpy vs entropy and find out where these concepts came from. unit of entropy is Joule per Kelvin (J/K). If \(dQ\) heat is added to a heat reservoir at an instantaneous temperature \(T\), then their ratio represents the infinitesimal change in entropy \(dS\). In thermodynamics, Rudolf Clausius defined entropy as: What is EntropyĮntropy, represented by \(S\), can be defined in a handful of ways. Hence, enthalpy represents the heat of an isobaric reaction. \(\Delta H = \Delta U P \Delta V = q\) (Heat) If a reaction takes place at constant pressure, then \(\Delta P = 0\) and, \(\Delta H = \Delta U P \Delta V V \Delta P\) The change in enthalpy is a more helpful quantity for practical purposes. unit of enthalpy is the same as that of energy, i.e Joules(J). The product of pressure and volume also represents the work done on the system that produces such system dimensions. Similarities Between Enthalpy and EntropyĮnthalpy, represented as \(\)H\(\) for a thermodynamic system, is the sum of the internal energy and the product of pressure and volume of the system.The effect ofpressure on the enthalpy of natural gases is dependent upon the pressure-volume-temperature relationships. 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 best available datafor natural and petroleum refinery gases indicates the relationship shown. The effect of temperature upon the enthalpy at consistent pressure is expressedas the ?heat capacity? or ‘specific heat’ of the gas. A careful study ofthe known properties of natural gas indicates that this is possible. 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. The enthalpy-entropy diagram for natural gases is to the gas engineer what thesteam diagram is to the steam-power engineer. The power requiredfor the compression of the gas may be readily determined by converting theincrease in enthalpy into the desired units. 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. 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. 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. 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. 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. ![]()
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