Test: Steam Turbines - 3 - Mechanical Engineering MCQ

Expansion in turbine is approximated as isentropic expansion.

A stage of a reaction turbine consists of each row of blade i.e., one is fixed and other is movable.

Curtis stage is a velocity compounded impulse turbine, Rateau stage is pressure compounded impulse turbine.

Compounding is done to operate the turbine under manageable speed so that turbine can be synchronize with generator. Hence to reduce the speed of turbine compounding is done.

For Parson’s reaction turbine or 50% reaction turbine
α₁ = β₂
α₂ = β1

Since in the Parson's reaction turbine, the blade of turbine acts as nozzle and hence the relative velocity as exit will be more than that at inlet.

Degree of reaction can be defined as

Since in impulse turbine all the expansion takes place in nozzle.
Hence enthalpy drop in moving blade = 0

Willan’s line shows the steam consumption plotted against the turbine load show a linear relationship with throttle governing
w_s = a + bL
where a = no load steam consumption (kg/s)
b = steam rate (kg/kWs)
L = load (kW)

For 50% reaction turbine

Power developed = m(V_w1 + V_w2)U
Input energy = 

Since, V_f2 = V₁








For maximum efficiency 
By differentiating ρ = u/V₁

Power developed by the Runner

Blade efficiency =
 

For maximum efficiency of impulse turbine.
It is assumed that friction on the turbine blade is neglected i.e.,   and blade is symmetrical i.e., β₁ = β₂ from the velocity triangle
V_w1 =V₁cosα₁

Reheat factor

Since constant pressure lines are diverging in forward direction, cumulative enthalpy drop is greater than isentropic enthalpy drop. Hence the reheat factor is greater than unity. Its value lies between 1.04 to 1.08.

If at the exit of turbine, steam is wet i.e., some water particles are suspended in steam, the impact of these water particle leads to blade erosion.

Compounding is done to reduce the speed of impulse turbine to manageable speed.

At the diverging section of converging diverging nozzle flow velocities are very-very high (M > 1) due to this shock is observed at the diverging portion.

P₂ = critical pressure at throat
P₁ = inlet pressure

Critical pressure ratio

For superheated steam
n = 1.3

Critical pressure ratio

For wet steam
n = 1.035 + 0.1x₁
Where x₁ = initial dryness fraction of steam
Assuming, x = 0.92
n = 1.127

For dry and saturated steam
n = 1.135

At critical pressure ratio the discharge becomes maximum and after that the discharge remains constant irrespective of any decrease in pressure ratio.

Air in the condenser decreases the condensing efficiency of condenser.