Enthalpy of superheated steam

From steam tables, enthalpy of superheated steam at 280°C and 28 bar... 

The density of superheated steam varies inversely with temperature and directly with pressure to make the mass flow calculation more complicated and less accurate. But if a steam flowmeter is used to indicate the actual delivery of thermal power, an interesting phenomenon appears temperature causes the enthalpy of superheated steam to vary in a way which offsets its effect upon density. Thus thermal power Q only varies with differential and pressure ... 

Cp = Speeific heat of the desuperheating water, Btu/lb-°F h, = Enthalpy of superheated system, Btu/lb hj = Enthalpy of saturated steam, Btu/lb Lh = latent heat of vaporization of water at t,j Btu/lb m = Mass of superheated steam into the system, Ib/hr mj = Mass of saturated steam out, Ib/hr n = Mass of desuperheating water used, Ib/hr t[ = Temperature of superheated steam, °F = Temperature of saturated steam, °F t,j, = Temperature of desuperheating water, °F... 

Therefore, hi > hi, which means that the steam leaves the turbine superheated. Although in the isentropic process condensation occms, friction in the turbine increases the steam temperature and therefore the enthalpy of the steam, preventing condensation. 

Use the saturated and superheated steam tables (Tables B.5, B.6, and B.7) to determine (a) whether water at a specified temperature and pressure is liquid, saturated vapor, or superheated vapor (b) the specific volume, specific internal energy, and specific enthalpy of liquid water or steam at a specified temperature and pressure (c) the vapor pressure of water at a specified temperature (d) the boiling point of water at a specified pressure and (e) the dew point of superheated steam at a specified pressure. 

If the working fluid is steam, check whether it is superheated or saturated at the first stage by comparing the inlet specific enthalpy, with the specific enthalpy of saturated steam appropriate to the inlet pressure, hg(po. ). If Aq.i dryness fraction and equation (15.112) to estimate the isentropic index, y. 

Evaporation of an aqueous calcium chloride solution results in the production of steam with x = 0. The enthalpy of this steam is composed of the contribution of liquid water and the heat of evaporation. Note that the vapor leaving a solution is superheated according to a boiling temperature rise. In the diagram tie lines between the enthalpies at X, = 0 valid for steam and the enthalpies of aqueous solutions are drawn. These lines indicate that a vapor phase is in equilibriirm with a boiling liquid solution. In chap. 7 and 8 it will be shown how problems encountered with the evaporation of a binary solution or the crystallization by cooling or evaporation can be solved. 

The most desirable reference conditions for internal energy and enthalpy in processes where chemical reactions take place are 0 K or 25°C, zero pressure, and standard chemical elements, such as C (graphite), H2 (gas), O2 (gas), N2 (gas), CI2 (gas), and S (rhombic sulfur), rather than the chemical species themselves that are in the mixture. With this reference condition, internal energy and enthalpy changes automatically take into account heat of reaction. Felder and Rousseau (2000) discuss this reference condition. As an example, the enthalpy of 1 kg of superheated steam at 300°C and 1 MPa relative to the elements H2 (gas) and O2 (gas) at 0 K and 0 Pa is determined to be -12,209.3 kJ. Alternatively, from the steam tables in van Wylen et al. (1994), for a reference condition of saturated liquid water at 0°C, the enthalpy is 3,051.2 kJ/kg. 

Reducing the steam pressure increases superheating at lower pressure but does very little to reduce the temperature. The letdown valve can be also viewed as a way of superheating steam. The major feature of the letdown process through a pressure reduction valve is that the enthalpy remains substantially the same. 

A.2-10 Properties of Superheated Steam (Steam Table), English Units (v, specific volume, enthalpy, btu/lb s, entropy, btn/lb °F)... 

Solution The conditions for the inlet steam are fixed, but the conditions of the outlet steam will depend on the performance of the turbine. First, estimate the steam flowrate from the process heating duty. A good approximation is that the sum of the heat content of the superheat and latent heat is constant from inlet to outlet. At the expander inlet, the heat content of the superheat is higher than that of the outlet, but the latent heat is lower in the inlet than in the outlet. The two trends tend to cancel each other out, with the total heat content of superheat and latent heat being approximately constant across the turbine. From steam properties, the enthalpy of the superheated steam HSup, enthalpy of the saturated steam HSat and enthalpy of the saturated condensate Hi at 100 barg are ... 

J1 The state of l(lbm) of steam is changed from saturated vapor at 10(psia) to superheated vapor at 30(psia) and 1,200(°F). What are the enthalpy and entropy changes of the steam What would the enthalpy and entropy changes be if steam were an ideal gas ... 

By interpolation in the tables for superheated steam at 300 kPa, we find that ste with this entropy has an enthalpy of... 

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