Elements of Gas Turbine

Introduction:
                There are basically four elements of a gas turbine power plant. These are compressor, intercooler and heat exchangers, combustion chambers and gas turbines. These are explained in this section.
 

1. COMPRESSORS

• The high flow rates of turbines and relatively moderate pressure ratios necessitate the use of rotary compressors.
• The types of compressors, which are commonly used, are of two types, centrifugal and axial flow types.
• The centrifugal compressor consists of an impeller (rotating component) and a diffuser (stationary component). The impeller imparts the high kinetic energy to the air and diffuser converts the kinetic energy into the pressure energy.
• The compressors may have single or double inlet.
• The efficiency of multistage compressor is lower than a single stage due to the losses.  
• The axial flow compressor consists of a series of rotor and stator stages with decreasing diameters along the flow of air. The blades are fixed on the rotor and rotors are fixed on the shaft. The stator blades are fixed on the stator casing. The stator blades guide the air flow to the next rotor stage coming from the previous rotor stage.
• The advantages of axial flow compressor over centrifugal compressor are high isentropic efficiency (90-95%), high flow rate and small weight for the same flow quantity.

Fig: Axial flow compressor

2. INTERCOOLERS AND HEAT EXCHANGERS   
   

• The intercooler is generally used in gas turbine plant when the pressure ratio used is sufficiently large and the compression is completed with two or more stages. A cross-flow type intercooler is generally preferred for effective heat transfer. The regenerators, which are commonly used in gas turbine plant, are of two types, recuperator and regenerator.
• In a recuperative type of heat exchanger, the air and hot gases are made to flow in counter direction as the effect of counter flow gives high average temperature difference causing the higher heat flow.
• The performance of the heat exchanger is determined by a factor known as effectiveness. The effectiveness of the heat exchanger is defined as:

 Fig: Ritz Regenerative Heat Exchanger

                                                                                                                                                                                                                                                                                                 

3. COMBUSTION CHAMBERS:       The gas turbine combustion system has to function under certain different operating conditions which are not usually met with the combustion systems of diesel engines:

• The chemical reaction takes place relatively slowly thus requiring large residence time in the combustion chamber in order to achieve complete combustion.
• It is impossible to ignite and maintain a continuous combustion with such weak mixture.
• A pilot or recirculated zone should be created in the main flow to establish a stable flame that helps to ignite the combustible mixture continuously.