Lecture 4 - Rankine and Modified Rankine

Question 1

Why is the Rankine cycle efficiency lower than that of the Carnot’s?

Answer 1

The condenser removes more heat from the fluid to bring it to the saturation line, so the head addition will be at a lower average temperature

Question 2

How is pump work calculated in the Rankine cycle?

Answer 2

W5-4 = -(h4-h5) = vf4(p5-p4)

Question 3

What does vf represent in the pump work calculation, and how does is apply to water?

Answer 3

The specific volume at the initial pressure

Water is incompressible, so changes in vf are negligible

Question 4

What happens to the working fluid as it moves through the turbine in a rankine cycle?

Answer 4

It becomes a wet vapour. As expansion continues towards the end of the turbine, the fluid contains more liquid

Question 5

How can one increase dryness by moving points on the T-s diagram?

Answer 5

Point 3 to the right, ideally such that it intersects with the saturated vapour line
Point 2 must be moved right too for an isentropic turbine (to follow isotrope line)

Question 6

When increasing dryness, what do we move up the constant pressure line from what to what?

Answer 6

From the saturation line to the super-heated vapour region of the T-s diagram

Question 7

How is the move to superheated vapour achieved?

Answer 7

The working fluid (now a dry vapour) is sent to a super-heater

Question 8

What are advantages of adding more heat with a super-heater?

Answer 8

The cycle makes better use of the material limit
The average temp is higher so the cycle is more
Dryer turbine exit flow

Question 9

What is the effect of increasing boiler pressure when a super-heater is used?

Answer 9

The vapour will be wetter in the turbine

Question 10

What happens when a realistic turbine efficiency is added?

Answer 10

The dryness fraction (quality of vapour) tends to increase

Question 11

What is the purpose of the reheater cycle?

Answer 11

To increase the dryness of the vapour further and increase the average temperature of the cycle

Question 12

What is added in the reheater cycle?

Answer 12

An additional turbine and a reheater (can be multiple sets)

Question 13

What is the specific steam consumption?

Answer 13

The s.s.c. is the amount of steam required to generate a unit work output (usually kW) per hour

Question 14

How do Rankine and Carnot cycles’ s.s.cs compare?

Answer 14

Carnot’s is higher than Rankine’s

Question 15

What is the effect on components when reducing ssc?

Answer 15

They are smaller

Question 16

How the efficiencies of the modified Rankine cycles compare to that of the Carnot’s?

Answer 16

They are less efficient, but the more complicated, the closer to the ideal Carnot

Question 17

Why is carnot cycle not used in practice?

Answer 17

pumping from 4-1 to in mixture of liquid and vapour

liquid water droplet can badly damage the turbine blades

Question 18

How are the issues of carnot cycle overcome in the rankine cycle?

Answer 18

State 4 is moved to the saturation line - fully condense fluid therefore compressing just liquid

Question 19

Draw a block diagram for a simple rankine cycle?

Answer 19

Pump
Boiler
Turbine
Condenser

Question 20

Draw a block diagram for a superheated rankine cycle?

Answer 20

Pump
Boiler
Superheater
Turbine
Condenser

Question 21

Draw a block digram for a superheated rankine cycle with reheat?

Answer 21

Pump
Boiler
Superheater
HP Turbine
Reheater
LP Turbine
Condenser

Question 22

What is the key point to remember when calculating the fluid state in the intermediate stage in a modified rankine?

Answer 22

You cannot assume that the outlet of the HP turbine will be on the saturation line - could still be superheated steam