Premier Industries, Inc.
Gas Turbine Inlet Air Precooling
Frequently asked questions
What is Combustion Turbine Inlet Air Cooling?
Or "CTIAC "
& What does it mean to me?
Before the benefits of cooling the inlet air of combustion turbines can be fully understood, a basic understanding of how a turbine operates is necessary. The industrial heavy-duty gas turbine operates on the Brayton cycle. Without going through the entire thermodynamic analysis, two things can be said of this type of machine:
The output of the combustion turbine is mass flow dependent; meaning the work output increases as the mass flow through the turbine increases. The compressor however, is a constant "volume" machine when operated at a constant speed, such as the synchronizing speed in a turbine generator combination. To increase the mass flow, the density of that constant volume of air must therefore be increased.
The efficiency of the cycle is improved when either the inlet temperature is decreased, or the firing temperature is increased.
What is the mass flow through the turbine? Two constituents must always be present; fuel and air. The more air and fuel that can be combusted, and then expanded through the turbine section, the more power is produced. When the amount of air "mass" entering the turbine is increased, a proportional increase in fuel can be burned. Fuel cannot arbitrarily be increased, because the firing temperature would increase as well. Our goal should then be to maximize the air mass through the turbine. The only way to accomplish this is to increase the air density. This is where CTIAC comes in. Cooler, more humid air has a higher density. Fortunately, increasing the air density by lowering the inlet temperature also improves the efficiency of the gas turbine cycle.
The least expensive and most effective cooling method is "wetted media evaporative cooling". With uniform face velocities not exceeding 500 feet per minute "at any point", this type cooling will provide saturation efficiency of 95% or higher using 12" thick cooling media.
What information is needed to design air inlet cooling...?
The basic information required to design the air inlet cooling
system is:
1. Turbine operating characteristics at base
load, ISO conditions of 590 (f) ambient dry bulb and 600
(f) web bulb temperatures.
Operating characteristics are:
a. Megawatt output
b. Heat rate (BTU/KWH)
c. Gas cost per million BTU's
d. Treated water cost per
gallon (if known)
e. Energy value per kilowatt or
megawatt.
f. Climate data.
Climate data note: Total hours available
for evaporative cooling in 50 (f) increments is necessary for an
accurate prediction of savings to be realized and payback period. ASHRAE
design conditions provide a maximum that is exceeded only 1% (or scale used) of
the time.
Turbine output is inversely affected by
temperature. Unless total hours available for cooling are considered, the
results will be inaccurate and payback period undependable.
Structural & Mechanical considerations:
g. Existing air inlet
housing structure layout and dimensions of air inlet area.
h. Retain existing dry
filters...?
i. Replace entire
air inlet housing..?
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Given the operating characteristics above, Premier has programs available which will instantly provide the dollar amount of savings reported in increased output and earnings on sale of excess energy. This single dollar amount is the basis for determining payback.
An example of this report is taken from an installation at Nevada Power
in Las Vegas, NV., on an 83.5MW Westinghouse 501B6 Gas Turbine for the
temperature range of 100 to 1050 (f) for 491 available hours:
Fuel cost x Avg. H.R.
x Output
x Hours x Eff. Increase = Increased
earnings (Eff.)
$1.90 x
12,300BTU x 79,500KW x
491 x
(.95-.835) = $104,907
MBTU
KWH
(4718.5MW gain)
This report considers; Fuel cost, Heat Rate. Output with and without cooling, Number of hours available for cooling in that temperature range and dollars of increased earnings from Turbine efficiency increase.
Note: This calculation is for the hours available for cooling in only one temperature range of 100 to 105 degrees. The actual increased earnings for the Turbine for the entire year at all temperature ranges is $1,349,600 (prox).
Premier Industries, Inc., has compiled climate data for 91 U.S. Cities. This information is available in 5 degree increments and total hours available by month and/or year. You can contact them by email at premier@piec.com or call 800-254-8989 or fax 602-997-5998.
Evaporative cooling efficiency is a direct result of at least three (3)
major factors; (1) Wet bulb depression which is the difference between dry
bulb and wet bulb temperatures, (2) cooling media saturation efficiency which is
the percent of the wet bulb depression realized in temperature drop and (3) the
"uniform" face velocity of the air flow which is to say that the air
flow should be the same velocity "at any point" on the cooling media.
In order to accurately predict the outcome of adding wetted
media evaporative cooling of the inlet air, the wet bulb depression must be
known across the spectrum of temperature ranges. It is not sufficient
to use the "design condition" only to predict performance increase.
Click here to go to a sample climate report for Houston, Texas. You will note that there are 7685 hours available for evaporative cooling with discharge temperatures above 400(f). These hours are available provided that the saturation efficiency of 16" thick cooling media is a minimum of 95%. This is only achievable by using Glacier-Cor Super Saturation media.
The report generated below is similar to those provided by Premier Industries, Inc., on request (given the operating characteristics mentioned above). This report is for an 83.5MW turbine, Westinghouse model 501B6 in Las Vegas, Nevada.
The more humid climate in Houston, Texas obviously provides lower savings from evaporative cooling as compared to the hot dry climate of Las Vegas, Nevada. This illustrates the absolutely essential inclusion of detailed climate data in the prediction. This is also true of the Super Saturation cooling media performing at min 95% S.E. and preferred uniform air velocity of 500 feet per minute.
Click here to contact Premier for a similar report for your Turbine. (Climate data limited to the major cities of the USA)
Maximum climate condition (coincident dry bulb/wet bulb) is needed to
determine the total air flow volume required. The mass flow rate
is based on ISO conditions of 590(f) and 60% relative humidity.
The air volume in cubic feet per minute is less at ISO condition
than at the maximum condition. I.E. at 59/60 (ISO), the air volume may be
620,000 CFM. At 1150(f) dry bulb and 10% relative humidity, the
air flow required would have to be 667,000 CFM (prox). The designed air
flow would have to be 667,000 (in this example) with face area sufficient to
assure uniform velocity of selected feet per minute rather than 620,000 CFM.
This would require a face area of 1334 square feet vs 1240 square feet at ISO
conditions.
It is readily apparent that proper design based on outcome is
essential.
Properly designed to control air velocity uniformly across the face area of
the media and proper water delivery method, wetted media is the simplest yet
most effective method of evaporative cooling. Alternate versions of
evaporative cooling such as direct spray or fogging systems are not necessarily
true evaporative coolers.
The ideal evaporative cooler adds moisture to the air
molecule and is delivered in a gaseous or vapor form thereby cooling the
air. The direct spray or fogging systems appear to deliver the air in a
wet spray which operates in a slightly different process than the wetted media
type cooler.
"Specific humidity" increases the heat rate of the
turbine which is a negative effect. The colder air contains more mass and
therefore more oxygen density which increases the efficiency (output) of the
fuel burned by the turbine. The negative effect of increased humidity in
the air is offset by the positive effect of higher output due to the higher
oxygen density in the colder air.
No "raw" moisture is entrained in the air flow from
the wetted media type evaporative cooler. Too much moisture in the air stream
can cause blade deterioration to the turbine.
Where is it better to position the evaporative cooler, upstream or downstream of the dry filters ...?
The wetted media evaporative cooler is a very effective and efficient
filter. It will trap 87% (prox) of particulate down to 10 micron size.
The face area required to control air velocity is another important
consideration. For these 2 reasons, it is far better to position the
wetted media type evaporative cooler upstream of the dry filters.
Dry filter useful life is also greatly extended and maintenance
costs are likewise reduced when using the evaporative cooler as a pre-filter as
well as a cooler. Moisture carryover is not a consideration because there
will be no raw moisture entrained in the air stream from a properly designed and
sized wetted media cooler.
Mechanical installation of the system on retrofits is also
simplified.
Conversely there are disadvantages in downstream placement of the cooler. It is difficult to control air velocities uniformly across the face area of the media and the advantages of pre-filtering are lost.