Gas Turbine Handbook : Principles and Practices

Gas Turbine Inlet Treatment 127
ling water quality, controlling water flow rate is also important (too
little is ineffective and too much may be detrimental to machine life).
There are two types of evaporative coolers: pad and fogger.
The pad-type cooler consists of wettable porous media pads that
are kept moist by water sprayed or dripped continuously on their upper
edges. Air flows horizontally through the wetted media where the
water wicks through the media wall and into the airstream. Pad-type
coolers require low (less than 100 psig) pressure water flow and can
tolerate moderate levels of calcium carbonate (less than 150 ppm).
Calcium carbonate levels between 50 ppm and 150 ppm require additional
water flow to wash contaminants from the media. The media
pads have approximately the same capture efficiency as a pre-filter.
Media pad life is a function of clogging, sagging, decay, and bacterial
action. To maximize the life and effectiveness of the media pads, these
coolers should be installed downstream of the inlet air filter system.
The media pads should be sufficiently dried and removed when not
in use for prolonged periods of time.
A fogger-type cooler consists of water flow nozzles placed across
the face of the gas turbine inlet and a coalescer stage. These nozzles
distribute a fine mist of water into the air stream and the coalescer
stage eliminates non-evaporated water carry over. The quantity of
fogger nozzles is a function of nozzle orifice size, spray angle, cross sectional
area of the gas turbine inlet, and air flow velocity. Nozzle orifice
size is a function of the residence time the droplet has to evaporate and
the amount of contaminate in the water. Residence time is defined as
the water droplet size (measured in microns) and the duct length from
the plane of the fogger nozzles to the coalescer vanes. Typical nozzle
sizes range from 0.006 inches to 0.010 inches (producing droplets in the
25- to 75-micron size range) and water pressure ranges from 300 psig
to 1,000 psig. Due to the small nozzle sizes even moderate levels of calcium
carbonate in the water present a problem. Therefore, it is recommended
that water hardness (C a
CO 3
) be reduced to less than 10 ppm.
Operation between 10 ppm and 50 ppm is not impossible but will incur
increased maintenance with smaller nozzle sizes. Fogger-type evaporative
cooler nozzles can be placed either before or after the air filters.
(However, due to the “ foreign object” hazard created by the number and
size of the nozzles immediately upstream of the gas turbine compressor
inlet, it is recommended that this system only be installed upstream of
the air filters.)