Figure 1-1.. Schematic of a compressor performance map.
wall."" When surge occurs， the flow is reversed and so are all the forces acting on the compressorespecially the thrust forces， which can lead to totaldestruction of the compressor. Thus， surge is a region that must be avoided. Choke conditions cause alarge drop in efficiency， but do not lead to destruction of the unit.
It is important to note that with the increase in pressure ratio and the number of stages the operating range is narrowed.
The turbo-compressors discussed in this section transfer energy by dynamic means from a rotating member to the continuously flowing fluid. The two types of compressors used in gas turbines are axial and centrifugal.Nearly all gas turbines producing over 5孔W have axial flow compressors. Some small gas turbines employ a combination of an axial compressor followed by a centrifugal unit. Figure 1-19 shows a schematic of an axial-flow compressor followed by a centrifugal compressor， an annular combus-tor， and an axial-flow turbine， very similar to the actual engine depicted in Figure 1-14.
.xial-Flow Compressors. An axial-flow compressor compresses its working fluid by first accelerating the fluid and then diffusing it to obtain a pressure increase. The fluid is accelerated by a row of rotating airfoils or blades (the rotor) and diffused by a row of stationary blades (the stator). The

Figure 1-19. A schematic of a cutaway of a small .as tur.ine used in helicopter or vehicular applications.
diffusion in the stator converts the velocity increase gained in the rotor to a pressure increase. 0ne rotor and one stator make up a stage in a compressor. A compressor usually consists of multiple stages. 0ne additional row of fixed blades (inlet guide vanes) is frequently used at the compressor inlet to ensure that air enters the first-stage rotors at the desired angle.In addition to thestators， an additional diffuser at the exit of the compressor further diffuses the fluid and controls its velocity when entering the combus-tors.
In an axial flow compressor air passes from one stage to the next with each stage raising the pressure slightly. By producing low-pressure increases onthe order of 1.1:1-1.4:1， very high efficiencies can be obtained. The use of multiple stages permits overall pressure increases up to 40:1. The rule of thumb for a multiple stage gas turbine compressor would be that the energy rise per stage would be constant rather than the pressure rise per stage.
Figure 1-20 shows multistage high-pressure axial flow turbine rotor. The turbine rotor depicted in this figure has a low-pressure compressor followedby a high-pressure compressor. There are also two turbine sections， and the

Figure 1-.0. A hi.h-pressure ratio tur.ine rotor. (Courtesy ALSTOM.)
reason there is a large space between the two turbine sections is that this is a reheat turbine and the second set of combustors are located between the high-pressure and the low-pressure turbine sections. The compressor pro-duces 30:1 pressure in 22 stages. The low-pressure increase per stage also simplifies calculations in the design of the compressor by justifying the air as incompressible in its flow through an individual stage.
Centrifugal Flow Compressors. Centrifugal compressors are used in small gas turbines and are the driven units in most gas turbine compressor trains. They are an integral partof the petrochemical industry， findingextensive use because of their smooth operation， large tolerance of processfluctuations， and their higher reliability compared to other types of com-pressors. Centrifugal compressors range in size from pressure ratios of 1:3:1 per stage to as high as 13:1 on experimental models. Discussions here are limited to the compressors used in small gas turbines. This means that the compressor pressure ratio must be between 3-7:1 per stage. This is con-sidered a highly loaded centrifugal compressor. With pressureratio"s，which exceed 5:1， flows entering the diffuser from the rotor are supersonic in their mach number (孔 > 1.0). This requires a special design of the diffuser.
　
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