Gas Turbine Types

Gas turbines are classified as single-shaft or two-shaft and simple or regenerative cycle. A two-shaft configuration is distinguished by the lack of a mechanical connection between the gas generator (power-producing) system and the power turbine (power-utilizing) system. This arrangement allows greater speed-horsepower flexibility than the single-shaft in which all compressors and turbines are mechanically connected. Also, the starting requirements and controls are simplified, because the load compressor does not require acceleration during the start cycle.

A variation of the two-shaft gas turbine is the aircraft derivative machine. The gas generator is an aircraft jet engine or an adaptation thereof. The power turbine may be of a custom design. Alternately, aircraft based designs may be used. They are covered in API Standard 616 [12].

Simple and regenerative cycle turbines differ in configuration, efficiency, and equipment cost (see Figures 7-19, 7-20, and 7-21). The regenerative turbine costs more because of the heat exchanger and extra ducting required to preheat the combustion air using the turbine exhaust gas. An increase in efficiency of about 6-7% results. Simple-cycle turbines normally have thermal efficiencies of 20-26% and regenerative

Figure 7-19. Performance map showing the effect of pressure ratio and turbine inlet temperature on a simple cycle [13].

Figure 7-19. Performance map showing the effect of pressure ratio and turbine inlet temperature on a simple cycle [13].

regenerator effectiveness = 0.8

regenerator effectiveness = 0.8

Figure 7-20. Performance map showing the effect of pressure ratio and turbine inlet temperature on a regenerative cycle [13].

Figure 7-20. Performance map showing the effect of pressure ratio and turbine inlet temperature on a regenerative cycle [13].

types 28-32%. However, because most process applications of gas turbines use the exhaust gas heat in steam-producing boilers or other process uses, simple-cycle units are more common. With exhaust heat recovery, simple-cycle gas turbine efficiency meets or exceeds regenerative cycle efficiency.

Another reason for the increased use of gas turbines as prime movers in the process industry is the high thermodynamic cycle efficiencies and subsequent low operating cost.

Drix fr s 295

9,000

9,000

Figure 7-21. Approximate gas turbine fuel rates [7].

Figure 7-21. Approximate gas turbine fuel rates [7].

Simple-cycle efficiency does not usually mean as much to process users as total-cycle efficiency, because the gas turbine is not usually economic in process applications without some type of heat recovery. Total-cycle efficiency is most important in any economic evaluation. In a cycle with heat recovery, the only major loss that is charged to the cycle is the heat exhausting from the boiler stack. With the good comes the bad. Gas turbine maintenance is generally somewhat higher in cost and should be included in the total evaluation.

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