Specific work

The reader is advised to start with this book and to then delve further into the computational literature pertaining to his or her specific work. It is impossible to reference all relevant works in a book such as this. The bibliography included at the end of each chapter primarily lists textbooks and review articles. These are some of the best sources from which to begin a serious search of the literature. It is always advisable to run several tests to determine which techniques work best for a given project. 

Food Applications. A number of features make en2ymes ideal catalysts for the food industry. They are all natural, efficient, and specific work under mild conditions have a high degree of purity and are available as standardi2ed preparations. Because en2ymatic reactions can be conducted at moderate temperatures and pH values, simple equipment can be used, and only few by-products are formed. Furthermore, en2ymatic reactions are easily controUed and can be stopped when the desired degree of conversion is reached. 

If we are going to follow HAZWOPER principles, why should we determine if the operation falls under these requirements The answer is simple. If we follow these principles it will help to make sure that a job is done safely. If the specific work falls in a gray area, using HAZWOPER principles will help to eliminate controversy over any compliance issues. 

The prime contractor s SSAHPs for Sites D, E, and G and the subcontractor s SSAHP for Site I did not contain detailed discussions of specific work tasks to be performed by employees or the hazards associated with those tasks. As a result, SOPs associated with each job or task had not been prepared. 

A reversible recuperative a/s cycle, with the maximum possible heat transfer from the exhaust gas, qj = Cp(74 — 7y), is illustrated in the T,s diagram of Fig. 3.2, where 7y = 72. This heat is transferred to the compressor delivery air, raising its temperature to 7x = 74, before entering the heater. The net specific work output is the same as that... 

The isentropic temperature rise for maximum specific work (J , ) is obtained by differentiating Eq. (3.11) with respect to x and equating the differential to zero, giving... 

For the closed recuperative cycle [CHTXJi, with states 1,2,X,3,4, Y as in the T,s diagram of Fig. 3.10, the net specific work is unchanged but the heat supplied has to be reassessed as heat qj is transferred from the turbine exhaust to the compressor delivery air. Using the heat exchanger effectiveness, e = (Tx — — Ti) the heat supplied... 

The Hawthorne and Davis approach thus aids considerably our understanding of a/s plant performance. The main point brought out by their graphical construction is that the maximum efficiency for the simple [CHT]i cycle occurs at high pressure ratio (above that for maximum specific work) whereas the maximum efficiency for the recuperative cycle [CHTX]i occurs at low pressure ratio (below that for maximum specific work). This is a fundamental point in gas turbine design. 

Calculation of the specific work and the arbitrary overall efficiency may now be made parallel to the method used for the a/s cycle. The maximum and minimum temperatures are specified, together with compressor and turbine efficiencies. A compressor pressure ratio (r) is selected, and with the pressure loss coefficients specified, the corresponding turbine pressure ratio is obtained. With the compressor exit temperature T2 known and Tt, specified, the temperature change in combustion is also known, and the fuel-air ratio / may then be obtained. Approximate mean values of specific heats are then obtained from Fig. 3.12. Either they may be employed directly, or n and n may be obtained and used. 

Finally, carpet plots of efficiency against specific work are shown in Fig. 3.16, for all these plants. The increase in efficiency due to the introduction of heat exchange, coupled with reheating and intercooling, is clear. Further the substantial increases in specific work associated with reheating and intercooling are also evident. 

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

The (arbitrary) overall efficiency and specific work quantities obtained from these calculations are illustrated as carpet plots in Fig. 4.11. It is seen that the specific work is reduced by the turbine cooling, which leads to a drop in the rotor inlet temperature and the turbine work output. Again this conclusion is consistent with the preliminary analysis and calculations made earlier in this chapter. 

Fig. 4.11. Calculation of efficiency of simple CBT plants—single-.step cooled ICBTlica uncooled [CBT ]ii—a.s a function of specific work with pressure ratio (r) and maximum temperature as parameters and with r)p< = t), = 0.9. 7hi = 1073 K (after Ref. 5 ).

Fig. 5.4 shows a carpet plot of overall efficiency against specific work for the cooled [CBTJici plant (single step) with pre.ssure ratio and combustion temperature as parameters. As shown earlier, by the preliminary air standard analysis and the subsequent calculations in Chapter 4, there are relatively minor changes of thermal efficiency compared with the uncooled plant [CBT]iuc, but there is a major effect in the reduction of specific work. 

Fig. 5,4. Overall efficiency and specific work for [CBTli<-i plant with single-step cooling of NGVs, with combustion temperature and pressure ratio as parameters (after Ref. [5], Chapter 4).

In the second development, the emphasis is on taking advantage of the increa.sed specific work associated with evaporative intercooling and of the increased mass flow and work output of the turbine. Any gain on the dry efficiency is likely to be marginal, depending on the split in pressure ratio. 

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

Fig, 6.17. Overall efficiency and specific work of dry and wet cycles compared. 

The CHAT cycle may be seen as a low loss evaporative development of the dry intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic advantage—increase in turbine work (and heat supplied ) with little or no change in the compressor work, leading to an increased thermal efficiency and specific work output. 

In summary, all these wet cycles may be expected to deliver higher thermal efficiencies than their original dry equivalents, at higher optimum pressure ratios. The specific work quantities will also increase, depending on the amount of water injected. 

For the ISTIG cycle, Fig. 6.18 shows thermal efficiency plotted against specific work for varying overall pressure ratios and two maximum temperatures of 1250 and 1500°C. Peak efficiency is obtained at high pressure ratios (about 36 and 45, respectively), before the specific work begins to drop sharply. Note that the pressure ratios of the LP and HP compressors were optimised within these calculations. 

Macchi et al. provided a similar comprehensive study of the more complex RWI cycles as illustrated in Fig. 6.19, which shows similar carpet plots of thermal efficiency against specific work for maximum temperatures of 1250 and 1500°C, for surface intercoolers. The division of pressure ratio between LP and HP compressors is again optimised within these calculations, leading to an LP pressure ratio less than that in the HP. For the RWI cycle at 1250°C the optimisation appears to lead to a higher optimum overall pressure ratio (about 20) than that obtained by Horlock [5], who assumed LP and HP pressure ratios to be same in his study of the simplest RWI (EGT) cycle. His estimate of optimum pressure ratio... 

Rufli s calculations (Fig. 7.7a, b), indicated that the optimum pressure ratio for a CCGT plant is relatively low compared with that of a simple gas turbine (CBT) plant. In both cases, the optimum pressure ratio increa.ses with maximum temperature. Davidson and Keeley [6] have given a comparative plot of the efficiencies of the two plants (Fig. 7.9), showing that the optimum pressure ratio for a CCGT plant is about the same as that giving maximum specific work for a CBT plant. 

Fig. 7.9. Overall efficiency of CCGT plant compared with overall efficiency and specific work of CBT plant...

The result of subcontractor qualification is a list of capable subcontractors that will be invited to bid for specific work. 

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