Entropy steam

Steam turbine integration. Figure 6.32 shows a steam turbine expansion on an enthalpy-entropy plot. In an ideal turbine, steam... 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

The theoretical steam rate (sometimes referred to as the water rate) for stream turbines can be determined from Keenan and Keyes or Mollier charts following a constant entropy path. The theoretical steam rate is given as Ib/hr/kw which is easily converted to Ib/hr/hp. One word of caution—in using Keenan and Keyes, steam pressures are given in PSIG. Sea level is the basis. For low steam pressures at high altitudes appropriate coirections must be made. See the section on Pressure Drop Air-Cooled Air Side Heat Exchangers, in this handbook, for the equation to correct atmospheric pressure for altitude. 

Note lhal in Fig. 7.. the. steam entropy is scaled by a factor /x = obtained from tlie heat balance,... 

The theoretical values for enthalpy may be read from a Mollier chart, where hj is the enthalpy of the steam at the turbine inlet, and hg is the enthalpy of the steam at the exhaust pressure and at the inlet entropy. The expansion of steam through the turbine is theoretically at constant entropy. These theoretical rates must be corrected for performance inefficiencies of the particular turbine. The calculations presented here are good for the average design, but exact values for a particular make and model turbine must be quoted by... 

Figure 2-38. Mollier (enthalpy-entropy) diagram for steam.

Conventional Partial Molal Entropy of (H30)+ and (OH)-. Let us now consider the partial molal entropy for the (1I30)+ ion and the (OH)- ion. If we wish to add an (HsO)+ ion to water, this may be done in two steps we first add an H2O molecule to the liquid, and then add a proton to this molecule. The entropy of liquid water at 25°C is 16.75 cal/deg/mole. This value may be obtained (1) from the low temperature calorimetric data of Giauque and Stout,1 combined with the zero point entropy predicted by Pauling, or (2) from the spectroscopic entropy of steam loss the entropy of vaporization. 2 Values obtained by the two methods agree within 0.01 cal/deg. 

The entropy of water vapour is usually referred to that of liquid at 0° C. as zero, and the entropy of unit mass of steam at 0° C. is therefore ... 

The entropy and intrinsic energy of steam (or other vapours) may be determined by the following method, due to Clausius. 

Steam tables indicate an arbitrary zero internal energy and entropy for water in its liquid state, at the triple point of water. 

Figure 1 IB. Temperature-entropy diagram for water and steam 814...

Enthalpy-pressure diagram for water-steam 813 Enthalpy term, energy balance equation 74 Entropy 3,28... 

Temperature-entropy diagram, water and steam 814 Temperature gradient, flow over plane surface 688... 

C14-0048. Calculate the entropy change of 15.5 g of steam that condenses to liquid water at 373.15 K. Without doing additional calculations, what can you say about the entropy change of the surroundings ... 

Hin = specific enthalpy of the inlet steam Hout = specific enthalpy of the outlet steam His = enthalpy of steam at the outlet pressure having the same entropy as the inlet steam... 

Next, determine the isentropic enthalpy change A His- From steam properties, the entropy of the inlet steam is ... 

The slope of the line allows for the determination of the enthalpy of vaporization of water, A//Vap, and the y intercept yields the entropy of vaporization, A. S vap As both the enthalpy and the entropy of water increase as the phase change liquid — vapor occurs, the slope and y intercept of the Clausius-Clapeyron equation are negative and positive, respectively. At 373 K these thermodynamic quantities have values of AHvap = 40.657 kJ mol-1 and ASvap = 109.0 J K-1 mol-1. The leavening action due to water vapor or steam arises from the increased amount of water vapor that forms as pastry temperatures initially rise in the oven and then from the increased volume of the water vapor as temperatures continue... 

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. 

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