Figure 1-1.. A recuperative medium-sized industrial .as tur.ine. (Courtesy Solar Tur.ines Incorporated.)
The term ""regenerative heat exchanger"" is used for this system in which the heat transfer between two streams is affected by the exposure of a third medium alternately to the two flows. (The heat flows successively into and outof the third medium， which undergoes a cyclic temperature.) In a recuperative heat exchanger each element of heat-transferring surface has aconstant temperature and， by arranging the gas paths in contraflow， the temperature distribution in the matrix in the direction of flow is that giving optimum performance for the given heat-transfer conditions. This optimum temperature distribution can be achieved ideally in a contraflow regenerator and approached very closely in a cross-flow regenerator.
Small Gas Turbines
孔any small gas turbines which produce below 5孔W are designed similarto the larger turbines already discussed; however， there are many designs
Figure 1-1.. A small radial flow .as tur.ine cutaway showin. the tur.ine rotor.
that incorporate centrifugal compressors or combinations of centrifugal and axial compressors as well as radial-inflow turbines. A small turbine will often consist of a single-stage centrifugal compressor producing a pressure ratio ashigh as6:1， a single side combustor where temperatures of about1，800 0F (982 0C) arereached， and radial-inflow turbines. Figure 1-13 shows a sche-matic of such a typical turbine. Air is induced through an inlet duct to thecentrifugal compressor， which rotating at highspeed， imparts energy to the air. 0n leaving the impeller air with increased pressure and velocity passes through ahigh-efficiency diffuser， which converts the velocity energy to static pressure.The compressedair， contained in a pressure casing， flowsat low speed to the combustionchamber， which is a side combustor. Aportion of the air enters the combustor head， mixes with the fuel and burns continuously. The remainder of the air enters through the wall of the combustor and mixes with the hot gases. Good fuel atomization and con-trolled mixing ensure an even temperaturedistribution in the hotgases， which pass through the volute to enter the radial inflow turbine nozzles. High acceleration and expansion of the gases through the nozzle guide vanepassages and turbine combine to impart rotationalenergy， which is used to drive the external load and auxiliaries on the cool side of the turbine. The efficiency of a small turbine is usually much lower than a larger unit because of the limitation of the turbine inlet temperature and the lower component efficiencies. Turbine inlet temperature is limited because the turbine blades are not cooled. Radial-flow compressors and impellers inherently have lower efficiencies than their axial counterparts. These units are rugged and their simplicity in design assures many hours of trouble-free operation. A way to
Figure 1-1.. A small aeroderivative .as tur.ine. (Courtesy Pratt & Whitney Canada Corp.)
improve the lower overall cycle efficiencies，18-23%， is to use the waste heatfrom the turbine unit. High thermal efficiencies (30-35%) can be obtained， since nearly all the heat not converted into mechanical energy is available inthe exhaust， and most of this energy can be converted into useful work. These units when placed in a combined Heat power application can reach efficiencies of the total process as high as 60-70%.
Figure 1-14 shows an aeroderivative small gas turbine. This unit has three independent rotating assemblies mounted on three concentric shafts. This turbine has a three-stage axial flow compressor followed by a centrifugalcompressor， each driven by a single stage axial flow compressor. Power is extracted by a two-stage axial flow turbine and delivered to the inlet end of the machine by one of the concentric shafts. The combustion system com-prises of a reverse flow annular combustion chamber with multiple fuelnozzles and a spark igniter. This aeroderivative engine produces 4.9孔W and has an efficiency of 32%.
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