Question

Consider a steam power plant that operates on an ideal regenerative rankine cycle. Steam enters turbine at 6 MPa and 450 deg and is condensed in the condenser at 20 kPa. Bleed Steam is extracted from the turbine at 0.4 MPa to heat the boiler feed-water in an open feed-water heater, water leaves the feed water heater as a saturated liquid. Construct a property table giving the pressure, enthalpy and phase for all the state points identified in the cycle and hence determine the thermal efficiency for the power plant

Answer 1

Concepts and reason

Ideal Regenerative Rankine Cycle:

Regenerative Rankine cycle is an idealized thermodynamic steam cycle. This cycle uses water as a working fluid. The ideal steam cycle consists of a constant pressure boiler that utilizes part of heat to produce steam. The steam produced is used to produce work by using turbine.

From the turbine, the exhaust steam is flow into the open feed water heater and condenser. The working fluid is carried to condenser from the turbine where the heat is rejected from the steam to convert it into saturated liquid.

The condensed liquid is again pumped into feed water heater by using pump 1. Finally Pump 2 is used to pass the fluid from feed water heater to boiler.

The schematic diagram of Ideal Regenerative Rankine cycle is shown in figure (1).

Turbine (Isentropic expansion process 5-6&5-7):

The superheated steam from the boiler has an elevated temperature and pressure; it expands isentropically in turbine to produce work and then discharged to the condenser with relatively low pressure. Neglect heat transfers with the surroundings.

Condenser (Constant pressure heat rejection process 7-1):

In condenser, the steam from the turbine is condensed to saturated liquid water in the condenser.

Pump (Isentropic compression process 1-2&3-4):

In Isentropic compression process, the working fluid (water) from the condenser via open feed water heater at low pressure is pumped into the boiler at high pressure. No heat is transferred with the surroundings and pump.

Boiler (Constant pressure heat addition process 4-5):

The saturated liquid water enters the boiler and is converted into steam at constant pressure by the addition of heat into boiler.

Open feed water heater (Process 6-3):

Open feed water heater contains deaerator used to remove the non-condensable gases which present in feed water. It preheats the feed water and passes to the boiler for greater efficiency.

Ratio of work done:

Ratio of work done of the Rankine cycle is defined as the ratio of turbine work output from the cycle to the work supplied to the pump.

Enthalpy:

Enthalpy is a measure of total energy in the system. Enthalpy is the summation of internal energy and work done by or on the fluid. It is denoted by the letter H.

Answer 2

Answer

Part a

The net work output per kilogram of steam flowing through the boiler 

Part b

The thermal efficiency of the cycle .