Question

Intermediate Thermodynamics Question.

Even though it would increase efficiency, why is it expensive to operate with reduced condenser pressures in Rankine cycles?

Answer 1

Condensed Pressure plays an important role in Thermal Poutes plant operation. It wink's to the amount of work done by the turfine which affects the output and hence the efficiency of overall cycle. By lowering the condenser pressure, The Steam is Condensed at a lower pressure. so this results in more expansion through the turbine which increases the work done by the turlune. This can be visualised using Tos diagram of a Rankine cycle P2z=c 1 P4 = constant T-S Duagliom ga Rankine yde

This is the simplest form of rankine cycle on Tus plot. Now if you lower the line 4-1 in the above fugure ie; If you louer the pressure at which the steam condenses. The work done by the Turbine e lire 3-4 will increase resulting in higher work done. by the Turluine. The other parameters remaining the same, le same heat input with more work output. Hence, the efficiency ш алидор . Summilarly, ich the condensed pressure increases iei Less vaccum say due to higher Cooling water temperature. The steam will condense at a higher pressure resulting in less expansion through the turbine resulting in decreased work output and less efficiency. Even though ich condenser pressure is very low there is an increase in chance of phase change in turhuine.

The turbine efficiency directly reduces the work produced in the turbine and therefore the overall efficiency. The inefficiency of pump increases the enthalpy of the liquid leaving the pump and therefore reduces the amount of energy required to evaporate the liquid. However the energy to drive the pump is usually more expensive than the energy to feed the boiler.