GT Fundamentalscompressing the air to a higher pressure
adding and igniting fuel in the compressed air to increase its energy
directing the high temperature, high pressure air into an expansion turbine that converts the gas energy to the mechanical energy of a rotating shaft
the resulting lower temperature, lower pressure gasses are exhausted to atmosphere
Inlet
To ensure that the air supplied to the compressor is of sufficient quality, gas turbines have a filter house prior to the inlet bell mouth.
The filter house usually consists of several stages of filters of increasing efficiency to remove any particulate or debris that could damage the compressor blades.
Inlet
The air at the inlet may also be heated or cooled, depending upon the specific application and operating conditions.
Compressor
The compressor of a 61B gas turbine is an 17 stage, axial flow compressor.
Each stage consists of a set of rotor blades that increase the velocity of the air, raising its kinetic energy.
This is followed by a set of stator blades that diffuse the air to convert the kinetic energy into a pressure increase.
Inlet Guide Vanes
The compressor is equipped with variable stator blades at the inlet. These are referred to as variable inlet guide vanes (VIGV’s).
The angle of the VIGV’s is controlled to provide for smooth airflow through the compressor across the entire speed range. This prevents compressor stall or surge.
Inlet Guide Vanes
VIGV’s are also positioned to control firing and exhaust temperatures while the machine is carrying load.
Combustion
The 61B combustion system consists of 10 chambers arranged around the diameter of the compressor discharge casing.
Here, the compressor discharge air is directed into the chambers where it is mixed with fuel and ignited.
Transition pieces direct the hot gasses to the 1st stage nozzles of the turbine.
Combustion
The DLN 2.6 combustion system consist of six nozzles per combustion can, 5 located radially and 1 in the center.
Each can also has quaternary fuel pegs located upstream of the nozzles and arranged around circumference of the can.
This is a fully premixed combustion system which is capable of keeping NOx emissions in single digits.
Turbine
The 61B has a three stage turbine.
Each stage consists of a set of nozzles which increase the velocity of the gasses and direct them toward the rotating buckets. As the high energy gasses push against the buckets, their kinetic energy is converted into shaft power.
Exhaust
As the hot gasses leave the turbine, they are directed into the exhaust system.
Exhaust systems contain duct work to transport the gasses to a location desirable for release to the atmosphere (i.e. not at face level) and may also contain filters to reduce particulate emissions.
In combined cycle applications, the exhaust system will contain the HRSG.
Turbine Performance
Compressor discharge pressure (CPD), and
Firing temperature (TF)
Compressor discharge pressure is determined by compressor design and ambient conditions.
Pressure at the inlet and exhaust are approximately equal to atmospheric pressure.
As CPD increases, the differential pressure across the turbine stages will increase, therefore, more output from the turbine.
Firing Temperature
Firing temperature is defined as the highest temperature at which work is extracted.
In other words, it is the hot gas temperature at the trailing edge of the 1st stage nozzle.
If this temperature increases, the energy of the hot gas increases, resulting in more energy being transferred to the turbine.
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Gas Turbine Performance
Gas turbines are designed to produce a given output based on assumed ambient conditions.
Since the gas turbine is an air breathing machine, changes in ambient conditions will cause the output to vary from its designed value.
Performance numbers are based on an ISO standard conditions.
59 Degrees F (15 Degrees C)
14.7 PSIA (1 ATM)
Fuel Type
Gas turbine output will vary with the type of fuel used since some fuels have a higher heating value than others.
Natural gas is the fuel that will supply the highest output.
Evaporative Cooling
Since hotter air at the compressor inlet causes output to drop, some plants use evaporative coolers which lower inlet temperature by spraying a water fog at the inlet.
This is of limited value, especially in more humid locations, because the water fog will raise humidity while lowering temperature.
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