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Simplified Method of Combustion Turbine Supercharging

  1. Aug 2, 2014 #1
    I have been interested in (the concept of) large frame industrial combustion turbine supercharging since the early 90's. However, although I believe the concept has merit and has been done successfully in at least one location nearly 3 or 4 decades ago, the concept has not evolved since.

    Perhaps this is the free market at work, or survival of the fittest (in CT uprate concepts,) but nonetheless I feel if the concept as demonstrated 40 years ago is improved upon it may offer a lower cost method (than typical methods) to offset the power derate of an industrial combustion turbine generator due to warm ambient air temperatures.

    I recently came up with a new idea on how to possibly increase the air density of the air being drawn into the 1st stage of the axial compressor, but do not have the necessary fluid mechanic skills to determine if the concept is feasible or not. Thus, I am out looking for answers or trying to vet the idea to see if it has merit or if it does not, and even possibly violates laws of energy or thermodynamics.

    The concept of inlet pressurization to increase density and mass flow of the combustion turbine in warm weather has always had to compete against both simpler and more complex methods that are more typical and well accepted. These include inlet spray cooling, evaporative cooling, inlet cooling with refrigeration cycles, and others. Please let me know if you are interested in vetting this idea and I will explain further.
     
    Last edited: Aug 2, 2014
  2. jcsd
  3. Aug 2, 2014 #2
    When first considering the upgrade of an industrial large frame combustion turbine generator in early 1990's, a GE Frame 7B was considered for inlet pressurization (supercharging,) as well as for installation of an inlet fogging system, conventional evaporative cooler, inlet cooling using chilled water or ice storage, and inlet cooling with a much larger refrigeration cycle. A study done by a well known engineering firm estimated cost, and as expected the inlet spay cooling or fogging system was the lowest cost for added incremental MW output, followed by addition of a conventional evaporative cooler. However, these two methods offered few if any incremental KW's on humid days, unlike the concept of inlet pressurization or direct sensible cooling.

    Without referencing the 20 - 25 year old report for exact numbers, the assumptions and results of the inlet pressurization case were as follows:

    Inlet temperature of ambient air: 95 degrees F
    Inlet air flow to compressor, nominal: about 1.1 million pounds per hour
    Net combustion turbine generator output at base (maximum) load: 46.0 MW
    Size of industrial FD fan required to boost air pressure and density: 4,000 KW motor, 4 MW parasitic incr power
    Fan output: approximately 1.25 million pounds per hour at 60 inches water column pressure boost
    Special requirements: Inlet duct reinforcement to withstand the increase in duct pressure, and evaporative cooler or spray cooling to cool heated inlet air to increase its density. The estimated (counterproductive) temperature rise of the inlet air going through the fan was approximately 15 degrees F.

    Estimated gross output from combustion turbine generator: 56 MW at base load
    Net incremental power gain: 6 MW, or 13%
    Estimated cost incl fan, aux transformer, evap cooler: 225 to 250 $/incr KW
    Pros: Large power boost, way more than evaporative cooling and independant of RH
    Cons: Large capital outlay, more complex than evaporative cooling, only one operating unit in USA (about 20 MW, much smaller,) may be more expensive if air cooled generator requires supplemental cooling at high amb.

    Even though the concept does not appear to change the compressor surge margin, the controls would have to be designed to avoid compressor surge or stall, or turbine overfiring in the event of a fan trip.
     
  4. Aug 2, 2014 #3
    The new concept is as follows: Is it theoretically possible? Is there a net gain if designed correctly? Could it be practical?
     
  5. Aug 2, 2014 #4
    Here is the idea: Is it theoretically possible? Is there a net gain in output of the combustion turbine cycle? If designed correctly or optimized, could it be economical?

    Air at 90 degrees F is drawn into the filter housing of a 7FA combustion turbine at approximately 2.8 million pounds per hour and at 10 feet per second. After the transition duct, the horizontal duct is ¼ the original cross sectional area at 30 ft wide and 14 feet high, and thus the air velocity increases to 40 feet per second.
    The air then accelerates to 80 feet per second as it passes through the silencer section of the duct. Baffles 8” wide on 16” centerlines are parallel to the flow and thus force the air to pass through 8” wide openings 14 feet tall.

    Proposal: Instead of increasing air density and mass flow into the combustion turbine by using a fan that handles the entire mass flow of air, a ring header for compressed air admission parallel to the air flow is placed around each 8” X 14 ft tall opening, and additional compressed air is blown into the duct at a velocity several times higher than the CT compressor induced flow, such that additional air velocity is the result. Think of air movers that use compressed air, or Dyson’s bladeless fan.

    Question: Is it possible to induce more airflow into the 1st stage of the combustion turbine compressor in this manner, or after the air de-accelerates past the silencers and “venturi” section, will the density be unaffected and mass flow unchanged?
     
    Last edited: Aug 2, 2014
  6. Aug 8, 2014 #5
    inlet duct of combustion turbine - silencer section

    Fluid Mechanics People, please help. Reference the attached diagram.

    Will the air flow in #/hr in the downstream duct entering the axial flow compressor increase due to the addition of high velocity compressed air (small arrows) at several times the velocity of the air being drawn in by the compressor (large arrows?) This is the main question I am hoping others can help answer. It doesn’t matter what the source of the compressed air is, or what its exact velocity is or pressure in the headers added at the front of the silencer baffles. Will this in effect do the same thing as a large fan being used to boost the static pressure downstream by pressurizing the entire flow? If so, assuming the compressed air is cooled to slightly below ambient, will it raise the temperature of the air downstream of the silencer section (like the fan will,) and thus reduce density and offset some of the mass flow increase thus requiring downstream cooling to result in an increased mass flow to the (constant volume) axial compressor?
     

    Attached Files:

  7. Jan 26, 2015 #6
    This looks like a very interesting concept, but I see out of over 800 views, nobody even expressed an opinion in the last 6 months.
     
  8. Jan 27, 2015 #7
    I don't understand you, you start to explain your idea, but you continue with a bunch of questions
     
  9. Feb 8, 2015 #8
    Sorry, you are right, I didn't explain it well. The concept is pretty straight forward:
    A combustion turbine draws air in through filters and restrictive area of intake silencer baffles. At the restricted area of the baffles where air flow is approximately 60 fps through the duct, could compressed air being blown into the duct at a very high velocity tangent to the in drawn air be used to increase mass flow into the compressor of the turbine to increase potential turbine output?

    Thus, can a venturi technique be used, and will air flow into the turbine increase? I believe in order to work, the density of the downstream air entering the compressor has to see a sinificant increase.
     
  10. Feb 8, 2015 #9
    When turbo charging has been looked at previously for large industrial combustion turbines, the concept would involve a very large fan to boost the inlet duct pressure by approximately 1 - 2 psig. Then, because all the combustion air has been heated by 10 or 20 degrees F, it must be cooled with direct spray cooling or an evaporative cooler. Otherwise, the density does not increase adequately for a large net gain. The problem is the large aux parasitic load, and the cost of a more robust evap cooler, or extra long intake duct to evap the water for cooling. On a 7B combustion turbine rated about 50 MW, the FD fan would be about 4 MW, but the net power output about 6 MW incremental.
     
  11. Feb 8, 2015 #10
    I am trying to find a simpler way to turbo charge the inlet. Is the concept used by an industrial air mover or a Dyson bladeless fan adaptable? A new company called Turbophase is turbo charging large industrial turbines using a gas engine driven compressor with intercooling &. recouperator. The hot compressed air is added to the compressor discharge. It works, but very large footprint, hard to permit, and complex. Is there a better way?
     
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