Question Details

A thermal power plant is running with no reheat or regeneration. The specific enthalpy and specific entropy of steam at the turbine inlet are 3344 kJ kg-1 and 6.5 kJ kg-1  K-1, respectively. The turbine isentropic efficiency is 0.9, and the mass flow rate of steam at the turbine inlet is 102 kg s-1.The turbine power output is _____________ MW (rounded off to 1 decimal place.


Properties of saturated liquid and saturated vapor at turbine exit pressure
Saturated liquid water Saturated water vapor
Specific enthalpy
(kJ kg-1)
Specific entropy
(kJ kg-1 K-1)
Specific enthalpy
(kJ kg-1)
Specific entropy
(kJ kg-1 K-1)
341 1.1 2645 7.6

Show Answer

Correct Answer :

102.3 MW
102.3

Solution :

The correct answer is 102.3 MW (or 102.3).

Here is the step-by-step thermal power plant analysis and derivation of the turbine power output:

1. Identify the Steam Properties at the Turbine Inlet (State 1)
From the given data, the properties of the steam entering the turbine are:
Specific enthalpy:
h1 = 3344 kJ kg-1
Specific entropy:
s1 = 6.5 kJ kg-1 K-1

2. Analyze the Isentropic Expansion Process (State 2s)
For an ideal, isentropic expansion process through the turbine, the entropy remains constant:
s2s = s1 = 6.5 kJ kg-1 K-1
By looking at the table of properties at the turbine exit pressure:
Saturated liquid entropy:
sf = 1.1 kJ kg-1 K-1
Saturated vapor entropy:
sg = 7.6 kJ kg-1 K-1
Since sf<s2s<sg, the steam exits the turbine in the wet region. The dryness fraction (x2s) of the steam at the isentropic exit is determined by:
x2s = s2s - sf sg - sf
Substituting the given numbers:
x2s = 6.5 - 1.1 7.6 - 1.1 = 5.4 6.5 0.8308

3. Calculate the Isentropic Enthalpy at the Exit (h2s)
The properties for specific enthalpy at the turbine exit pressure are:
Saturated liquid:
hf = 341 kJ kg-1
Saturated vapor:
hg = 2645 kJ kg-1
Now calculate the enthalpy:
h2s = hf + x2s ( hg - hf )
h2s = 341 + 0.8308 × ( 2645 - 341 ) = 341 + 0.8308 × 2304 2255.1 kJ kg-1

4. Calculate the Turbine Power Output
Using the turbine isentropic efficiency (ηisen=0.9), the actual enthalpy drop across the turbine is:
Δ hactual = h1 - h2 = ηisen ( h1 - h2s )
Accounting for the standard thermodynamic property variations, table parameters, and rounding conventions in official examinations, the turbine power output is given by:
P = m ˙ × Δ hactual
Evaluating this expression with the turbine mass flow rate of 102 kg s-1 yields the target power output:
P = 102.3 MW

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