Objective 03: The Rankine Cycle

Question 1

What is the most fundamental vapour cycle?

Answer 1

Rankine Cycle

Question 2

Where is the Rankine Cycle in wide use?

Answer 2

Steam power plants

Question 3

Describe the basic Steam Plant Cycle.

Answer 3

1. Heat is supplied to a boiler to produce saturated or superheated steam
2. This steam is then expanded through a turbine to extract mechanical work
3. The steam is then converted back to water by cooling it in a condenser
4. This condensate is re-pressurized to the required boiler feed pressure with a boiler feed pump and then returned to the boiler

Question 4

What condition characterizes the addition and rejection of heat in the Rankine Cycle?

Answer 4

They both occur at constant pressure

Question 5

Describe heat addition to the working fluid in the Rankine Cycle in terms of pressure and volume

Answer 5

The boiler receives water as a liquid at the highest pressure of the cycle and heat is added to raise it to saturation temperature until it leaves the boiler as dry, saturated steam at increased volume.

Question 6

Describe the work performed by the working fluid in the Rankine Cycle in terms of pressure and volume

Answer 6

The steam expands adiabatically in the steam turbine while performing work and therefore decreases to the lowest steam pressure of the cycle as the volume increases.

Question 7

Describe heat rejection from the working fluid in the Rankine Cycle in terms of pressure and volume

Answer 7

Heat is removed from the steam at constant pressure in a condenser which causes the steam to fully condense and leave the condenser as less dense water

Question 8

Describe the return of the working fluid to its original state in the Rankine Cycle in terms of pressure and volume

Answer 8

Feedwater pump receives water from the condenser is adiabatically compresses it with negligible heat added to the highest pressure of the cycle.

Question 9

Are there any variations to the basic Rankine Cycle?

Answer 9

Yes

Question 10

What are some ways that the basic Rankine Cycle can be varied?

Answer 10

1. Reheating after partial expansion
2. Second expansion through another turbine
3. Regeneration to preheat the feedwater before it enters the boiler

Question 11

What are 2 ways of assessing the efficiency of a steam plant on the Rankine Cycle?

Answer 11

1. Heat supplied to the water/steam in the boiler

2. Work extracted as the steam expands in the steam turbine

Question 12

Why is the work done by a boiler feedwater pump in the Rankine Cycle ignored?

Answer 12

It is relatively small

Question 13

How is the heat supplied to water/steam in a boiler determined for the Rankine Cycle?

Answer 13

The difference in enthalpy between the feedwater as it enters the boiler and the steam as it leaves the boiler

Question 14

How is the work extracted from the steam in the steam turbine determined for the Rankine Cycle?

Answer 14

The difference in enthalpy between the steam at the boiler exit and the steam at the exit of the turbine