Steam enthalpy difference

AH = Enthalpy difference between source and receiver, Btu/lb. For a typical condensing steam turbine, it would be the difference between the inlet steam and the liquid condensate. 

Equation 23.7 is based on the actual change in steam enthalpy across the turbine. Although both Equations 23.6 and 23.7 have the same form, their coefficients have completely different meanings. Comparing Equations 23.6 and 23.7, it becomes apparent that the slope of the linear Willans Line (Equation 23.6) is related to the isentropic enthalpy change and turbine isentropic efficiency9. 

Step 4. Now measure the enthalpy difference in British thermal units per pound of steam between points A and B on the vertical (y axis) scale. 

The enthalpy difference between 150-psig saturated steam, at 360°F, and steam condensed under a good vacuum, is roughly 1000 Btu/lb. 

AH = enthalpy difference between incoming water and steam produced for export (GJ) e p = exported electricity (GJ equivalents) off= offgas produced (kg) - reference case only... 

Their thermal efficiency is not very different and in a top-fired furnace can be as high as 95 %. The enthalpy difference between inlet and exit, often referred to as reformer duty, is made up of the heat required to raise the temperature to the level at the tube exit and the enthalpy of the reforming reaction. In a typical tubular steam reforming furnace, about 50% of the heat generated by combustion of fuel in the burners is transferred through the reformer tube walls and absorbed by the process gas (in a conventional ammonia plant primary reformer 60 % for reaction, 40% for temperature increase). 

Here AH ° = -155 kJ mol". By suitable combination of (2.41) and (2.43) the overall enthalpy difference may become approximately zero. There are still problems in controlling the temperature across the reactor, because the oxidation reaction (2.43) is considerably faster than the steam reforming (2.41). Proposed solutions include the use of a catalyst filament wire design leading to near-laminar flow through the reactor (Horny et ah, 2004). 

AH = enthalpy difference between incoming water and steam produced for export (GJ)... 

The temperature of the steam-cooled reactor shown is 285°F. The heat that must be transferred from the reactor into the steam generation system is 2.5 X 10 Btu/hr. The overall heat transfer coefficient for the cooling coils is 300 Btu/lir ft- F. The steam discharges into a 25-psia steam header. The enthalpy difference between saturated steam and liquid condensate is 1000 Btu/lb, . The vapor pressure of water can be approximated over this range of pressure by a straight line. 

The thermochemical data for auxiliary substances at 298.15 K have been taken from the CODATA Key Values for Thermodynamics [4], the reference base recommended for use in accurate work. Values at other temperatures are derived using enthalpy differences as indicated above, except for water, for which the enthalpy differences are consistent with the new NBS/NRC Steam Tables [16]. These are shown in Table 2. Molar masses are calculated from the lUPAC 1981 Atomic Weights [17]. The value for the gas constant, R=8.31448 J/(mol.K)-l, which is needed in the calculation of volumes, is from the set that is currently being recommended to CODATA by its Task Group on Fundamental Constants [35]. Thus all of these data are up-to-date and internationally recognized. 

It is noted that initially, no energy is assumed to enter this interval from a hot utility, such as steam ati higher temperature that is, Ssteam = 0. Hence, 30 x 10 Btu/hr arc available and flow down as aieskl ual, R, into the next lowest interval 2 that is, /J = 30 X 10 Btu/hr. Interval 2 involves streams HI, H2, and C2 between 235°F and 240°F AT = 5 F), and hence, the enthalpy difference is... 

There can be confusion and misunderstanding from plant personnel on steam prices. Are we talking about steam price based on the point of use (marginal steam price) or total steam from the boiler house (average steam price) Should the steam be priced at the point of generation or point of use Does the steam price include both fixed and variable costs Does the average steam price vary for different production rates Why is the steam price obtained based on steam enthalpy instead of costs These are the questions that will become the focus of this chapter. 

Solution. If BFW conditions are used as the basis to measure the heat available in steam, the heat available with a steam is equal to the enthalpy difference between the steam and BFW. Thus, the costs for MP and LP can be calculated based on the enthalpy ratio as... 

Both enthalpy and work-based steam pricing methods rely on thermodynamic laws as the basis. Cooper (1989) argued that the steam pricing should reflect economic reality. Since the operating cost for a steam system mainly consists of fuel burned for steam generation. Cooper (1989) proposed to use the concept of fuel equivalent (FE) as the basis for steam pricing. In this method, the ratio of FE for steam at different pressures is used to derive the steam prices in placement of the ratios of enthalpy and availability. 

An ovei all unit energy balance. The energy content of the steam, electricity, and fuel used should equal the total BTUs rejected to air and water coolers plus the enthalpy difference between feeds and products. 

This is not always an easy question to resolve. The feedback controller may be asked to perform a number of different services. In the heat-exchanger application it can be useful in correcting for heat loss, in which employment it should add an increment of heat to the process at all loads this would amount to a zero adjustment. Or its principal function might be to correct for variable steam enthalpy, in which case it should apply a span adjustment by setting the coefficient K. In another process, linearity could be the largest unknown factor. But a single feedback controller can hardly be called upon to do all these things. 

The portion of the potential energy that can be used to produce power is called the available energy and is represented by the isentropic enthalpy difference between the initial steam condition hi and the final condition corresponding to the exhaust pressure h. If the condensate enthalpy is hu, the ideal Rankine cycle efficiency, or the thermal efficiency, is... 

Estimate the enthalpy difference between saturated steam at 10 bar and at 100 bar. Compare your value with the experimental one. 

Energy balances differ from mass balances in that the total mass is known but the total energy of a component is difficult to express. Consequently, the heat energy of a material is usually expressed relative to its standard state at a given temperature. For example, the heat content, or enthalpy, of steam is expressed relative to liquid water at 273 K (0°C) at a pressure equal to its own vapor pressure. 

Figure 8 shows that increasing the heat flux at constant mass velocity causes the peak in wall temperature to increase and to move towards lower enthalpy or steam quality values. The increase in peak temperature is thus due not only to a higher heat flux, which demands a higher temperature difference across the vapor film at the wall, but to a lower flow velocity in the tube as the peaks move into regions of reduced quality. The latter effect of lower flow velocity is probably the dominant factor in giving fast burn-out its characteristically rapid and often destructive temperature rise, for, as stated earlier, fast burn-out is usually observed at conditions of subcooled or low quality boiling. 

---