Lecture 4 - Rankine and Modified Rankine Flashcards Preview

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Flashcards in Lecture 4 - Rankine and Modified Rankine Deck (22)
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1
Q

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

A

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

2
Q

How is pump work calculated in the Rankine cycle?

A

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

3
Q

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

A

The specific volume at the initial pressure
Water is incompressible, so changes in vf are negligible

4
Q

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

A

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

5
Q

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

A

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)

6
Q

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

A

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

7
Q

How is the move to superheated vapour achieved?

A

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

8
Q

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

A

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

9
Q

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

A

The vapour will be wetter in the turbine

10
Q

What happens when a realistic turbine efficiency is added?

A

The dryness fraction (quality of vapour) tends to increase

11
Q

What is the purpose of the reheater cycle?

A

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

12
Q

What is added in the reheater cycle?

A

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

13
Q

What is the specific steam consumption?

A

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

14
Q

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

A

Carnot’s is higher than Rankine’s

15
Q

What is the effect on components when reducing ssc?

A

They are smaller

16
Q

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

A

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

17
Q

Why is carnot cycle not used in practice?

A

pumping from 4-1 to in mixture of liquid and vapour
liquid water droplet can badly damage the turbine blades

18
Q

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

A

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

19
Q

Draw a block diagram for a simple rankine cycle?

A

Pump
Boiler
Turbine
Condenser

20
Q

Draw a block diagram for a superheated rankine cycle?

A

Pump
Boiler
Superheater
Turbine
Condenser

21
Q

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

A

Pump
Boiler
Superheater
HP Turbine
Reheater
LP Turbine
Condenser

22
Q

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

A

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