Open circuit gas turbine plant

Fig. I..3. Open circuit gas turbine plant (after Haywood [3 ).

In practical open circuit gas turbine plants with combustion, real gas effects are present (in particular the changes in specific heats, and their ratio, with temperature), together with combustion and duct pressure losses. We now develop some modifications of the a/s analyses and their graphical presentations for such open gas turbine plants, with and without heat exchangers, as an introduction to more complex computational approaches. 

Essentially, the analytical approach outlined above for the open circuit gas turbine plants is that used in modem computer codes. However, gas properties, taken from tables such as those of Keenan and Kaye [6], may be stored as data and then used directly in a cycle calculation. Enthalpy changes are then determined directly, rather than by mean specific heats over temperature ranges (and the estimation of n and n ), as outlined above. 

The discussion of the performance of gas turbine plants given in this chapter has developed through four steps reversible a/s cycle analysis irreversible a/s cycle analysis open circuit gas turbine plant analysis with approximations to real gas effects and open circuit gas turbine plant computations with real gas properties. The important conclusions are as follows ... 

This criteria of performance has less relevance to a combined heat and power plant which provides heat and generates electrical power. For an open circuit gas turbine plant, a more logical criterion is the energy utilisation factor (EUF) which can be calculated as... 

An important field of study for power plants is that of the combinedplant [ 1 ]. A broad definition of the combined power plant (Fig. 1.5) is one in which a higher (upper or topping) thermodynamic cycle produces power, but part or all of its heat rejection is used in supplying heat to a lower or bottoming cycle. The upper plant is frequently an open circuit gas turbine while the lower plant is a closed circuit steam turbine together they form a combined cycle gas turbine (CCGT) plant. 

A series of calculations for open circuit gas turbines, with realistic a.s.sumptions for various parameters, have been made using a code developed by Young [7], using real gas tables. These illustrate how the analysis developed in this chapter provides an understanding of, and guidance to, the performance of the real practical plants. The subscript G here indicates that the real gas effects have been included. 

The most developed and commonly used combined power plant involves a combination of open circuit gas turbine and a closed cycle (steam turbine), the so-called CCGT. Many different combinations of gas turbine and steam turbine plant have been proposed. Seippel and Bereuter [3] provided a wide-ranging review of possible propo.sed plants, but essentially there are two main types of CCGT. 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

Usually, a gas turbine plant operates on open circuit , with internal combustion (Fig. 1.3). Air and fuel pass across the single control surface into the compressor and combustion chamber, respectively, and the combustion products leave the control surface after expansion through the turbine. The open circuit plant cannot be said to operate on a thermodynamic cycle however, its performance is often assessed by treating it as equivalent to a closed cyclic power plant, but care must be taken in such an approach. 

For an open circuit (non-cyclic) gas turbine plant (Fig. 1.3) a different criterion of performance is sometimes used—the rational efficiency (tjr). This is defined as the ratio of the actual work output to the maximum (reversible) work output that can be achieved between the reactants, each at pressure (po) and temperature (To) of the environment, and products each at the same po. Tq. Thus... 

The Hawthorne and Davis analysis is first generalised for the [CBT]i open circuit plant, with fuel addition for combustion,/ per unit air flow, changing the working fluid from air in the compressor to gas products in the turbine, as indicated in Fig. 3.11. Real gas effects are present in this open gas turbine plant specific heats and their ratio are functions off and T, and allowance is also made for pressure losses. 

We first consider how the simple analysis of Section 7.3, for the combined doubly cyclic series plant, is modified for the open circuit/closed cycle plant. The work output from the gas-turbine plant of Fig. 7.3 is... 

Fig. 7.5. Open circuit ga.s turbine/closed steam cycle combined plant (CCGT). With supplementary firing (after...

Fig. 1.2 shows a gas turbine power plant operating on a closed circuit. The dotted chain control surface (F) surrounds a cyclic gas turbine power plant (or cyclic heat engine) through which air or gas circulates, and the combustion chamber is located within the second open control surface (Z). Heat (2b is transferred from Z to Y, and heat (2a is rejected from Y. The two control volumes form a complete power plant. 

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