The rise of inlet
fogging and wet
Gas turbines are now being subjected to heavy cycling as well as a far greater number of starts than ever, driven by the growing presence of
renewables on power grids around the world.
In some areas, gas turbines are increasingly
required to take a load following role when
production from renewables increases or
Cycling and rapid starts impose heavy
stresses on turbomachinery, leading to excess
maintenance costs and unplanned outages.
In essence, some of the cost of renewable
energy is being shifted to gas turbines in
increased operating and maintenance costs.
However, it is possible to use fogging and
wet compression to augment the amount of
power available. This can be accomplished
in such a way as to lessen the toll exerted by
rapid cycling, improve effciency, and reduce
emissions from gas turbine power plants.
Inlet fogging consists of spraying a fog of
demineralized water into the gas turbine inlet
air. With evaporative fogging, the amount
of water sprayed is controlled to reach
100 per cent relative humidity, or less, at the
gas turbine inlet. Cooling the inlet air to a
heavy-frame GT by 20°F (- 6. 6°C) causes the
output to increase by about 8 per cent and
the heat rate improves by almost 2 per cent.
Wet compression fogging consists of
spraying more water than can evaporate in
the inlet airfow. This ‘overspray’ is carried into
the compressor where it evaporates and gives
an intercooling effect.
Wet compression increases the mass fow
slightly and reduces compressor work, both
of which result in an additional power boost.
Spraying wet compression water at the rate of
1 per cent of the air-mass fow will produce a
power boost of about 8 per cent while heat
rate is improved by about 1. 5 per cent.
Wet compression produces about the
same power boost whether the ambient
humidity is low or high. If there is no upstream
evaporative cooler or chiller, the air will not
be fully humidifed. In that case, some of the
wet compression spray will evaporate before
entering the compressor. Consequently, the
power boost will be slightly higher on a hot, dry
day as opposed to a cool, wet day.
More than one thousand fog systems have
been installed on gas turbines around the
world. This includes several hundred systems
that provide both evaporative cooling and wet
compression. Systems designed specifcally
for wet compression, with water spray rates of
1–2 per cent of the air mass fow, have been in
use for decades.
Some early wet compression systems
caused compressor blade erosion as the
nozzles used produced droplets that were too
large. Most have since been retroftted with
Wet compression seems to be gaining
popularity. Systems are currently being
installed on seven GE 7FA.05 machines in the
US, two Alstom GT26 units in Turkey and four
MHI 501F machines in Mexico.
In a market where gas turbine plants are
increasingly being required to take a load
following role for renewables, fogging and
wet compression can play a vital role if they
are properly controlled. This can equate to
higher revenue, reduced fuel costs and lower
Coal and nuclear plants have historically
been relied upon for baseload grid power.
But stringent environmental regulations have
led to many of these plants being shuttered.
With low natural gas prices and improved
gas turbine effciency, combined-cycle plants
were looked upon as an alternative form
of baseload power – and, in some regions,
In a market where gas turbine plants are increasingly being required to take a
load following role for renewables, fogging and wet compression can play a vital
role if they are properly controlled, writes Thomas Mee III
Fogging can lessen the toll exerted by rapid cycling
Credit: Mee Industries