GATE ME 2019 Set 1 — Question 59
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Thermodynamics → Power Cycles → Rankine, Reheat & Regenerative
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Question
A steam power cycle with regeneration as shown below on the diagram employs a single open feedwater heater for efficiency improvement. The fluids mix with each other in an open feedwater heater. The turbine is isentropic and the input (bleed) to the feedwater heater from the turbine is at state 2 as shown in the figure. Process 3-4 occurs in the condenser. The pump work is negligible. The input to the boiler is at state 5. The following information is available from the steam tables:
The mass flow rate of steam bled from the turbine as a percentage of the total mass flow rate at the inlet to the turbine at state 1 is _______.
| State | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Enthalpy (kJ/kg) | 3350 | 2800 | 2300 | 175 | 700 | 1000 |
Correct answer
19 to 21
Solution
To find the percentage of steam bled from the turbine, we perform an energy balance on the open feedwater heater (OFWH).Let be the total mass flow rate at the turbine inlet (state 1).
Let be the mass flow rate of steam bled from the turbine at state 2.
Let be the fraction of steam bled.In the OFWH, the bled steam at state 2 mixes with the condensate from the condenser at state 4 to produce saturated liquid at state 5. Since pump work is negligible, the state after the condenser pump is assumed to be state 4.Energy balance for the OFWH:Dividing by :From the given table:
kJ/kg
kJ/kg
kJ/kgSubstituting the values:As a percentage:The mass flow rate of steam bled is of the total mass flow rate at the inlet.
Let be the mass flow rate of steam bled from the turbine at state 2.
Let be the fraction of steam bled.In the OFWH, the bled steam at state 2 mixes with the condensate from the condenser at state 4 to produce saturated liquid at state 5. Since pump work is negligible, the state after the condenser pump is assumed to be state 4.Energy balance for the OFWH:Dividing by :From the given table:
kJ/kg
kJ/kg
kJ/kgSubstituting the values:As a percentage:The mass flow rate of steam bled is of the total mass flow rate at the inlet.
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