Steam available-energy

The method used in the previous example, called the extraction method, assumes that the sole purpose of the turbine is to produce shaft power. Therefore, the shaft work is charged for the capital cost of the turbine and for the steam available energy used by the turbine to produce the work. With this rationale, the additional equation is obtained by equating the unit costs of high- and low-pressure steam available-energy, cLp = cHp. The result is that the shaft work bears the entire burden of the costs associated with the turbine process and capital expense. 

In this paper, the property called available energy will be used to assess operating costs. Because available energy does define a substance s potential value as a fuel, no corrections are needed when the consumption of one fuel is compared to that of another. For example, one unit of steam available energy serving to "fuel" a turbine is fully equivalent to one unit of electrical available energy delivered to an electric motor. 

An inerease in ambient air temperature will deerease the available energy for the generator. This assumes that the fresh feed and eoke burn remains eonstant. The expander horsepower does not ehange, but the air blower horsepower inereases with inereased air temperature, eausing the exeess energy to deerease. Steam and water may need to be added to the flue gas flow at various points in the system to eontrol afterburning. In Figure 4-64, the solid eurves are for a normal flow of steam. The dotted eurves are for inereases in the steam rate by 3.05 times, 4.85 times, and 6.05 times the normal flowrate. 

To compare the values of steam at various pressures for design studies or accounting once the plant is built, the AB method is useful. The maximum available energy in a working fluid can be determined from... 

AB = Maximum available energy in Btu/lb AH = Enthalpy difference between the source and receiver, Btu/lb. For a typical condensing steam turbine it would be the difference between the inlet steam and the liquid condensate To = Receiver temperature, °R... 

Figure 14-27A. Available energy in steam-theoretical steam rates, 9 to 35 Ib/kw-hr, for single-stage general-purpose turbine. (Used by permission Westinghouse Electric Corp., Steam Division.)...

Determine available energy in steam from Figures 14-27Aand 14-27B. 

A few degrees of superheat are recommended (5-15°F), but if superheated steam is to be used, its effect must be considered in the ejector design. A high degree of superheat is of no advantage because the increase in available energy is offset by the decrease in steam density [16],... 

Most of our available energy is obtained indirectly from chemical energy. In the steam turbine, the generation of mechanical work proceeds through pathway I (Figure 9.1) and includes heat and electrical energies, whereas that... 

If the demand for LPS exceeds the amount of exhaust steam available, the PC opens the pressure let-down bypass. FC-1 serves to minimize the bypass flow. If the pressure controller (PC) opens the bypass and the let-down flow rate exceeds the set point of FC-1, the previously inactive (saturated) FC-3 becomes active and starts cutting back the LPS flow to the boiler feedwater preheater and thereby reduces the plant s demand for LPS. This is an energy-efficient response, because the energy recovered from the LPS supplied to the feedwater preheater is less than the energy content of the HP steam that is needed to produce that LPS. 

A simple thermodynamic analysis provides considerably more data to work with. The required task is to separate propylene from propane. On a theoretical basis the ideal work (the minimum availability change) required for this separation is about 400 k BTU s/hr, of which an appreciable fraction is needed to raise the temperature of the products to the final values shown. The available energy (availability, exergy) supplied to this process from the condensing low pressure (20 psig) steam is 18.6 M BTU s/hr. 

This proposal reduces available energy input (7.6 M BTU/hr work vs. 18.6 M BTU/hr steam), but does not reduce the total energy input (7.6 + 65.3 = 72.9 M BTU/hr). Thus the idea does not save energy from a first law viewpoint. From the second law viewpoint the heat supplied to the reboiler at 77°F (TQ) contains no useful energy. 

The available energy flou through five major sections of sulphuric acid plant is given in figure 2. The major inputs to this system are sulphur and pouer, with demineralised (DM) water uet air, process water and cooling water from environment. The useful outputs from the system are sulphuric acid and steam. Losses to environment include heat losses from various equipments blowdown water steam from deaerator vent warm water and stack gas. 

The problem must be set up such that the objective function and the Lagrange constraint equations are functions of the state and decision variables (Equations 4 and 5). A major deviation from the procedure outlined by Tribus and El-Sayed (5) is in the selection of the Lagrange constraint equations and state variables. The added complexity of having steam as the working fluid (compared to an ideal gas in the gas turbine optimization performed by Tribus and El-Sayed) makes it impractical to select state variables that correspond to available-energy flows. Consequently, this requirement was relaxed entirely. This gives the designer the opportunity to use any variable as a state variable,... 

The system s Second Law efficiency rises as the work/heat ratio increases (Figure 7). This is partially due to improved performance of the turbine, pump, and condenser and the higher temperature steam from the boiler. This considerably decreases the available-energy destruction due to heat transfer in the boiler. Thus, the turbine can take advantage of this for the production of shaft work. 

In the application of this method to a Rankine cycle cogeneration system, generalized costing equations for the major components have been developed. Also, the utility of the method was extended by relaxing the rule that each state variable (and hence each Lagrange constraint) must correspond to an available-energy flow. The applicability was further extended by the introduction of numerical techniques necessary for the purpose of evaluating partial derivatives of steam table data. 

In the system of Interest the reboiler transfers some of the available-energy In steam to the bottoms stream. The column uses the Increase In available-energy of the bottoms to transform feed Into products. The condenser processes the reflux for subsequent re-use In the column. In the reboller and column available-energy Is supplied In one form and converted Into a product. However the condenser exists for the sole purpose of decreasing the available-energy of the reflux. Or, in other terms, the condenser serves to eliminate entropy from the system. 

Heat Transfer Area, A 2 m Available Energy Dissipation,(T a) kcal Pressure in the Evaporator, P atm Fraction of the Generated Steam Reused,... 

The energy saving measures described in Section 5.1.2 have considerably reduced the demand side (e.g., C02 removal, higher reforming pressure, lower steam to carbon ratio, etc.). On the supply side, the available energy has been increased by greater heat... 

Table 14.1. The net calorific value of wood depends on its moisture content here the values are for a typical hardwood. The reduction in available energy with moisture content is due to the need to vapourize and superheat the steam to the same temperature as the flue gases. Typically the net calorific value for a hardwood is 18.2 MJ kg that for a softwood is 19.2 MJ kg (due to its higher lignin content) and that for bark is 19.7 MJ kg (due to extractives).

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