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These gas turbines were designed shortly after World War II and intro-duced to the market in the early 1950s. The early heavy-duty gas turbine design was largely an extension of steam turbine design. Restrictions ofweight and space were not important factors for these ground-basedunits， and so the design characteristics included heavy-wall casings split on hor-izontal centerlines， sleeve bearings， large-diameter combustors， thick airfoilsections for blades andstators， and large frontal areas. The overall pressure ratio of these units varied from 5:1 for the earlier units to 35:1 for the units in present-day service. Turbine inlet temperatures have been increased and run as high as 2500 0F (1371 0C) on someof theseunits， this makes the gas turbine one of the most efficient prime mover on the market today reaching efficiencies of 50%. Projected temperatures approach 3000 0F (1649 0C)and， if achieved， would make the gas turbine even a more efficient unit. The Advanced Gas Turbine Programs sponsored by the U.S. Department of Energy has these high temperatures as one of its goals. To achieve thesehigh temperatures， cooling is being used in the latest designs to
achievethegoalsofmainsteamtaining blade metal temperatures below 1300 0F (704 0C)， and prevent hot corrosion problems.
The industrial heavy-duty gas turbines employ axial-flow compressors and turbines. The industrial turbine consists of a 15-17 stage axial flow compressor; with multiple can-annular combustors each connected to the other by cross-over tubes. The cross-over tubes help propagate the flames from one combustor can to all the other chambers and also assures an equalization of the pressure between each combustor chamber. The earlier industrial European designs have single-stage side combustors. The new European designs do not use the side combustor in most of their newer designs. The newer European designs have can-annular or annular combus-tors since side (silo type) combustors had a tendency to distort the casing. Figure 1-8 is a cross-sectional representation of the GE Industrial Type GasTurbine， with can-annular combustors， and Figure 1-9 is a cross-sectional representation of the Siemens Silo Type Combustor Gas Turbine. Theturbine expander consists of a 2-4-stage axial flow turbine， which drives both the axial flow compressor and the generator.
The large frontal areas of these units reducethe inlet velocities， thusreducing air noise. The pressure rise in each compressor stage isreduced，creating a large， stable operating zone.
Figure 1-.. A frame-type .as tur.ine with can-annular com.ustors. (Courtesy .. Power Systems.)
Figure 1-9. Frame-type .as tur.ine with silo type com.ustors. (Courtesy Siemens Power .eneration.)
The auxiliary modules used on most of these units have gone through considerable hours of testing and are heavy-duty pumps and motors.
The advantages of the heavy-duty gas turbines are their longlife， highavailability， and slightly higher overall efficiencies. The noise level from this type of turbine is considerably less than an aircraft-type turbine. The heavy-duty gas turbine"s largest customers are the electrical utilities， and independ-ent power producers. Since the 1990s the industrial turbines have been the bulwarks of most combined cycle power plants.
The latest frame type units introduced are480孔W units using steamcooling in the combined cyclemode， enabling the firing temperatures to reach 2600 0F (1427 0C). This enables efficiency in the combined cycle mode to reach 60% plus.
.ircraft-Derivative Gas Turbines
Aeroderivative gas turbines consist of two basic components: an aircraft-derivative gas generator， and a free-power turbine. The gas generator serves as a producer of gas energy or gas horsepower. The gas generator is derived from an aircraft engine modified to burn industrial fuels. Design innovations are usually incorporated to ensure the required long-life characteristics in theground-based environment. In case of fan jet designs， the fan is removed and a couple of stages of compression are added in front of the existing low-pressure compressor. The axial flow compressor in many cases is divided into two sections a low-pressure compressor followed by a high-pressurecompressor. In thosecases， there are usually a high-pressure turbine and alow-pressureturbine， which drives the corresponding sections of the com-pressor. The shafts are usually concentric thus the speeds of the high pres-sure and low-pressure sections can be optimized. In thiscase， the power turbine is separate and is not mechanically coupled; the only connection isvia an aerodynamic coupling. In thesecases， the turbines have threeshafts， all operating at independent speeds. The gas generator serves to raise com-bustion gas products to conditions of around 45-75 psi (3-5 Bar) and temperatures of 1300-1700 0F (704-927 0C) at the exhaust flange. Figure 1-10 shows a cross section of an aeroderivative engine.
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