Machine efficiency energy output

Figure 16.33 shows a schematic of a simple gas turbine. The machine is essentially a rotary compressor mounted on the same shaft as a turbine. Air enters the compressor where it is compressed before entering a combustion chamber. Here the combustion of fuel increases its temperature. The mixture of air and combustion gases is expanded in the turbine. The input of energy to the combustion chamber allows enough power to be developed in the turbine to both drive the compressor and provide useful power. The performance of the machine is specified in terms of the power output, airflow rate through the machine, efficiency of conversion of heat to power and the temperature of the exhaust. Gas turbines are normally used only for relatively large-scale applications, and will be dealt with in more detail in Chapter 23. 

For such a machine, there are three important dependent variables required horsepower (P), specific output energy (g/z), and efficiency e). The quantity (g/z) is the energy output per unit mass of fluid outflow. Each of these is a function of the following independent variables ... 

In any process such as the cycle of material the conversion of energy is to work, useful constructs is limited by thermodynamic reasoning to a maximum amount (not 100%). This maximum thermodynamic efficiency cannot be achieved by any machine working at a real speed and which operates under constraints. The resultant work output, we shall refer to as optimal insofar that waste is avoided. As the constraints in the ecosystem are often ill-defined the reader will observe a certain looseness in the use of the words efficiency and effectiveness (fitness) throughout this book (see Section 4.7 and Appendix 4C). 

Frequently it will be appropriate to oversize the condensation stages of machines which have to be started up at a time when steam generated from process heat is not yet available, i.e. when the entire steam output has to be provided by fired steam boilers. This reduces the steam rate required to start up the plant and ensures that the steam boiler plant can be dimensioned so that it meets the normal continuous steam demand of the plant. However, it will be desirable as a rule to use bleeding and backpressure turbines which are not only much more cost-effective than condensation turbines, but which also improve the overall energy efficiency considerably owing to the combination of energy, process steam and heating steam. 

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