INDUSTRIAL GAS TURBINE Turbine Section

50-How can turbines in general are classified?
Turbines can be classified by:
The action of flow on the turbine blades:

1. Impulse. 
2. Reaction. 
3. Impulse and reaction combined.

The number of step reductions involved:

1. Single stage. 
2. Multi-stage.

The direction of flow:

1. Axial
2. Radial.

• Reheat or non reheat.
• Tandem compound or cross compound (this configuration is for steam turbine only)

What are the main design considerations that used in turbine design?
1. Degree of Reaction (D.O.R)
DOR = (the enthalpy drop across the moving blade) / (enthalpy drop across stage)

• If D.O.R= 0 %, its pure impulse
• If D.O.R= 10 %, its impulse stage with small reaction
• If D.O.R= > 10 %, its reaction stage

Note that pure impulse is occur at the root of the blade & as we go further along the blade, degree of reaction will increase.

2. Velocity Ratio (VR)

VR = (blade speed /Flow speed)

• Impulse turbine has highest efficiency at VR = 0.5 VR & Reaction Turbine at VR = 0.9
• Note that VR will increase as blade wheel become larger that's why reaction blades are used in low pressure turbines (such as steam turbines) because it develops its highest efficiency at higher velocity ratio.

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