1 INTRODUCTION

1.1 SCOPE

This technology evaluation summarizes available data on the performance, reliability, maintenance, and owning and operating costs of gas turbine prime movers. Various thermodynamic cycles will be considered including simple regenerative, intercooled, reheat, and compound cycles. Each gas turbine cycle, with exception of the regenerative cycle, will be treated as a unit to preclude any discussion of design details of the individual components. Closed-and combined-cycle gas turbines are considered in separate ICES Technology Evaluation reports.

This evaluation covers the generic characteristics of currently available gas turbines, ranging in size from 80 shaft horse power (SHP) for a portable type, simple-cycle machine to 134,000 SHP for regenerative units. Included for discussion are units manufactured world-wide.

1.2 GENERAL DESCRIPTION

1.2.1 Simple-Cycle Gas Turbine

Figure 1.1 shows the general layout of a typical simple-cycle gas turbine. Ambient air enters the compressor intake, is compressed and

mixed with fuel for burning in the combustion chamber. The hot gases are expanded in the turbine and exhausted to the atmosphere.

Normally, a packaged gas turbine unit includes, as standard equipment, the items shown in Fig. 1.1 along with necessary controls for prime-mover operation, oil sump, and oil cooler - all skid mounted. Optional equipment, such as insulation, enclosure for noise attenuation and/or outside use, exhaust silencer, and inlet filters, also is available.

1.2.2 Regenerative-Cycle Gas Turbine

The regenerative-cycle gas turbine is very similar to a simple-cycle unit. The only difference is that the low-pressure hot exhaust gases are used to heat the high-pressure compressor discharge air in a regenerator. This may be accomplished through the use of either a stationary regenerator or a rotating-type regenerator. The stationary regenerator is used with a large industrial gas turbine; whereas, the rotating regenerator is used with smaller, vehicular-type gas turbines.

1.2.3 Intercooled, Reheat, and Compound Cycles

The intercooled, reheat, and compound cycles are discussed only on a theoretical basis, because they are not standard, available cycles. The intercooled cycle consists of a two-stage compressor with a heat exchanger between stages. The reheat cycle consists of a two-stage turbine with a second combustion chamber between stages. The compound cycle uses a combination of intercooling, reheating, and regeneration.

1.3 SELECTION CRITERIA

Selection criteria for ICES applications of gas turbines include:

(1) thermal efficiency,*
(2) part-load performance,
(3) recoverable heat efficiency,**
(4) starting and loading characteristics,
(5) reliability and maintainability, and
(6) costs.

 Figure 1.2 shows the useful variables for estimating the gas turbine cycle full- and part-load performance.

Throughout this evaluation, the relationships of turbine performance characteristics versus control variables are displayed graphically and, as an aid to computer simulation, are represented by an empirical equation of the form:

For each figure, X is the independent control variable (the abscissa), Y is the dependent variable (the ordinate), and coefficients of the polynomial are as noted. The coefficients have been determined by a computerized version of the method of least squares; however, the quadratic polynomial is not the "best" model in some cases. An exponential or logarithmic function may be necessary to describe more accurately the trends in data. In general, the polynomial equations should not be used with values of the independent variable (X) outside the indicated range.

1.4 STANDARD PRACTICE

1.4.1 Applicable Codes and Standards

In general, ASME Power Test Code PTC 22-1966, Gas Turbine Power Plants, covers all types of gas turbines, test methods, and auxiliary equipment. However, members of the world-wide gas turbine community have almost completely accepted the use of the International Standards Organization (ISO) standard day, 59°F (15°C) at sea level and 50% relative humidity, over the ASME standard condition of 80°F at 1,000 ft elevation.

A new ASME standard² currently is being developed to solve some rating problems and to adopt the standard day as given by the ISO. For cycles that use cooling water, the new standard includes a cooling-water inlet temperature of 59°F (15°C).

 1.4.2 Standard Ratings

As mentioned above, the standard rating conditions are 59°F (15°C) and 50% relative humidity at sea level (1 atm). Units normally are given two ratings: a peak rating and continuous rating. The peak rating is given for a unit that will run less than 1000 h/yr or three hours out of a 24-hr day. Continuous duty is considered to be more than 1,000 h/yr, and this rating generally is about 10% less than peak rating. Ratings are assigned with zero inlet and exit pressure losses to allow comparison of units that will be installed in differing applications or environments. The purchaser must calculate the effect of particular inlet and exit losses on unit output. 

Ratings also are assigned with "standard" fuel conditions, even though units may operate on various fuels. The fuel used normally is liquid distillate fuel with lower heating value (LHV) of 18,400 Btu/lb and a density of 7 lb/gal. All fuel data in this evaluation have to be adjusted to the "standard" fuel. 

1.4.3 Derating Factors

 Two atmospheric parameters significantly affect the performance of gas turbines. Figure 1.3 shows the effect of both the compressor inlet temperature (°F) and the altitude (ft) on the rated capacity of the gas turbine.

The data shown in Fig. 1.3 can be expressed by the following expressions of rated capacity versus compressor inlet temperature (T) or altitude (H): 

For every degree (°F) drop in the compressor inlet temperature, the unit output capacity increases by about 0.4%. An increase in siting altitude above the nominal rated condition at sea level reduces the capacity by 3.33%/1000 ft.

Relative humidity and inlet and exit pressure losses do not strongly affect unit output. For continuous-duty applications, peak capacity generally is aerated by about 10%.