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124 Gas Turbine Handbook: Principles and Practices
ure 5-3). This power curve is influenced by four distinct factors: inlet
air temperature, turbine inlet temperature (i.e., combustor discharge
temperature), rotor speed(s), and compressor aerodynamics.
Turbine inlet temperature is a limiting factor in all gas turbines,
regardless of design configuration. This limit has been extended
through utilization of internal turbine airfoil cooling. Limitations due
to speed and compressor aerodynamics are distinguished as break
points or slope changes in the power curve. Often, these limitations
are of little consequence since they are small relative to the turbine
inlet temperature limitations. The effects of increasing ambient temperature
on gas turbine output clearly indicate the advantages of
cooling the compressor inlet, especially in hot climates. Lowering the
compressor inlet temperature can be accomplished by the installation
of an evaporative cooler, a fogger system, or a chiller in the inlet ducting.
Evaporative Cooling
The evaporative cooler is a cost-effective way to recover capacity
during periods of high temperature and low or moderate relative
humidity. The biggest gains are realized in hot, low humidity climates.
However, evaporative cooler effectiveness is limited to ambient
temperatures of 50°F (10°C) to 60°F (16°C) and above. Below these
temperatures, parameters other than turbine temperature will limit
gas turbine operation. Also, as the inlet air temperature drops the
potential for ice formation in the wet inlet increases.
Evaporative cooler effectiveness is a measure of how close the
cooler exit temperature approaches the ambient wet bulb temperature.
For most applications coolers having an effectiveness of 80%-
90% provide the most economic benefit. The actual temperature drop
realized is a function of both the equipment design and atmospheric
conditions. The design controls the effectiveness of the cooler, defined
as follows:
cooler effectiveness = (T 1DB
– T 2DB
)/(T 1DB
– T 2WB
) (8-1)
where:
T 1DB
is the dry bulb temperature upstream of the cooler
T 2DB
is the dry bulb temperature downstream of the cooler
T 2WB
is the wet bulb temperature downstream of the cooler.