Gas Turbine Handbook : Principles and Practices

128 Gas Turbine Handbook: Principles and Practices
WET COMPRESSION
As an extension of the fogger-type cooling approach, water
is allowed to enter the compressor and evaporation takes place
within the compressor. When water droplets enter the compressor
the process is referred to as “overspray” or “wet compression.”
Evaporation of the water droplets inside the compressor provides
continuous cooling of the air thus leading to a reduction in the
compressor work and compressor discharge temperature for a
given pressure ratio, and a change in the stage work distribution.
Since 65% to 75% of the turbine work is used to drive the compressor,
a reduction in compressor work results in an increase in
shaft output power. While more fuel is required to bring the vaporized
water/air mixture up to a given turbine inlet temperature, the
percentage increase in power output is greater than the percentage
increase in fuel consumed resulting in a net decrease in overall
heat rate.
Wet compression results in higher compressor airflow at a
given speed and pressure ratio, which tends to unload the first few
compressor stages and to increase loading on the last few stages.
The maximum desirable ratio of water-to-air flow is limited by
compressor surge or stall and combustor efficiency. A coating of
liquid on the airfoil surfaces will change the blade path geometry
and the related position of the surge line. The injection of untreated
water will result in contaminants being deposited on the airfoil
surfaces. This, in turn, leads to a change in airfoil geometry and
the position of the surge line. A more general impact is a rapid
decrease in compressor performance. Therefore, it is essential that
the water be treated as discussed earlier in this chapter. The combustor
and turbine performance is also affected by wet compression
but the affect is small for the range of water-to-air flow ratios
encountered.
The mass flow capability of some compressors is limited as
evidenced by the decrease in available power on cold days (Figure
5-3). Therefore, when increasing the mass flow care must be taken
not to exceed the structural capability of the compressor.
A second concern is the formation of ice in the gas turbine
inlet if the wet compression system is operated at or below 50 degrees
F. Without the protection of the inlet air filters ice could be