Superheated steam enthalpy

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... 

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

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 ... 

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

Again by interpolation, we find that superheated steam with this enthalpy has additional properties ... 

A turbine operates adiabatically with superheated steam entering at T, and P, with a mass flow rate m. The exhaust pressure is P2 and the turbine efficiency is i). For one of the following sets of operating conditions, determine the power output of the turbine and the enthalpy and entropy of the exhaust steam. 

Consider the first section of the turbine and the first feedwater heater, as shown by Fig. 8.6. The enthalpy and entropy of the steam entering the turbine are found from the tables for superheated steam. The assumption of isentropic expansion of steam in section I of the turbine to 2,900 kPa leads to the result ... 

Use the same procedures for each property—enthalpy, specific volume, and entropy—as given in step 2, but change the sign between the lower volume and entropy and the proportional factor (temperature in this instance), because for superheated steam, the volume and entropy increase as the steam temperature increases. Thus,... 

To determine the final temperature, enter the superheated-steam table at 14.7 psia (101.4 kPa), the final pressure, and project across to an enthalpy value equal to or less than the known enthalpy, 1231.3 Btu/lb (2864.0 kJ/kg). [The superheated steam table is used because the T-S diagram (Fig. 19.8) shows that the steam is superheated in the final state.] At 14.7 psia (101.4 kPa) there is no tabulated enthalpy value that exactly equals 1231.3 Btu/lb (2864.0 kJ/kg). The next lower value is 1230 Btu/lb (2861.0 kJ/kg) at T = 380°F (193.3°C). The next higher value at 14.7 psia (101.4kPa) is 1239.9 Btu/lb (2884.0 kJ/kg) at T = 400°F (204.4°C). Interpolate between these enthalpy values to find the final... 

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. 

Show that water at 400°C and 10 bar is superheated steam and determine its specific volume, specific internal energy, and specific enthalpy relative to liquid water at the triple point, and its dew point. 

The superheated steam table. Table B.7, lists values for both liquid water and steam. If you wish to determine H for liquid water at a temperature T and pressure P that cannot easily be found in this table, you may calculate it in the following manner (1) look up U and V for sam-rated liquid at the specified temperature in Table B.5 (2) assume these values are independent of pressure, and calculate H P,T) = (7 + PV. Furthermore, if the pressure is not excessive (say, less than 10 bar) or if it is unknown, neglect the PV correction and use the saturated liquid enthalpy H(T) given in Table B.5. 

The change of enthalpy for the cooling of the superheated steam may now be determined. Since the mass flowrate of one stream is unknown, its AT/ must be expressed in terms of this mass flowrate, which is represented in Figure 9 asm Ib/h. 

Analytic functions linking entropy and enthalpy for saturated and superheated steam... 

Table 16.3 Specific enthalpy and entropy for superheated steam comparison of approximating function results with steam tables...

Specific heat at constant pressure, J/g-°C or Btu/Ib- F of feed Enthalpy of thick liquor, J/g or Btu/lb H, of condensate of feed Hj, of saturated steam H , of vapor or superheated steam Mass flow rate, kg/h or Ib/h of liquor leaving single-effect evaporator rhj, of feed ifi, of steam and steam condensate Number of evaporator effects... 

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