Handbook of Turbomachinery Second Edition Revised - Ventech!

(1,300 8F). But modern gas turbines operate at turbine inlet temperatures
that are well above these limits. There is also a significant difference in the
operating temperature between advanced aircraft turbines and industrial
turbines. This is the result of fundamental differences in the required life,
weight, fuel/air quality, and restrictions on emissions.
For advanced aero engines turbine rotor inlet temperatures (TRIT)
close to 1,650 8C (3,000 8F) and compressor pressure ratios around 40:1 are
becoming a reality. A high specific power, which is a primary objective for
this type of engine, is usually obtained along with high efficiency. Such
extreme operating conditions inherently require frequent engine inspections
and continuous health monitoring. For industrial engines the leading
requirement is long-term durability without frequent inspections and
overhauls. Typically, major industrial turbine components are expected to
endure at least 30,000 hours between overhauls, with potential to be
repairable such that the engine life can be extended to 100,000 hours. This
compares to aircraft turbine component lives of only a few thousand hours.
This factor, as well as the usual requirement of a compressor discharge
pressure to be below the available gas pipeline fuel supply pressure, leads to
a moderately high turbine rotor inlet temperature. The TRIT limit for a
modern industrial gas turbine is established presently in the range of 1,260–
1,3708C (2,300–2,5008F). Figure 1 illustrates the recent history of
continuously rising TRIT and compressor pressure ratios. This rising
temperature trend results in the operating gas temperatures significantly
exceeding acceptable material capability limits and requires the application
of cooling in various engine hot section components, particularly in the
components exposed to the high-temperature environment. Relatively cold
air from the compressor discharge and, in some cases, from intermediate
compressor stages is the traditional source to cool turbine components.
After performing the cooling task, this air is discharged into the main
stream. The cooling air discharged within each particular cooled stage
practically does not perform any work on this stage before it is accelerated
to the mainstream velocity. This leads to significant losses in engine
performance. In summary, disadvantages of the open air cooling system
include a relatively low cooling effectiveness, significant losses of work
required to compress cooling air, and mixing losses that reduce aerodynamic
efficiency of the turbine. The main advantage of the open air cooling system
making it the most common for gas turbines is its simplicity, compared to a
close loop cooling system.
With compressed air pressure ratios for modern aero engines
exceeding 30:1 and reaching 40:1, the temperature of the compressor
discharge air approaches 650 8C (1,200 8F). This creates a significant
difficulty in using this air to cool the turbine discs, stators, and vicinity of
Copyright © 2003 Marcel Dekker, Inc.