Lecture 5- Regenerative Flashcards

1
Q

What is the basic idea behind regenerative cycles?

A

Transferal of heat to the working fluid REVERSIBLY from another part WITHIN the cycle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Between which points in the cycle should heat be transferred?

A

2 and 3- the boiler stage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why is heat from the turbine not used from within the cycle?

A

The turbine will no longer be adiabatic
It is impossible to make an efficient turbine which also acts as a heat exchanger
The fluid in the turbine would have a high wetness fraction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What happens to steam in the 1st stage of the turbine?

A

Some is extracted and fed through the open feed water heater

The rest continues into the 2nd stage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What happens to steam in and after the 2nd stage of the turbine?

A

It expands then goes through the condenser as usual

It is fed through a feed pump to bring it up to the right pressure then added to the feed heater

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What temp should t3 be for maximum efficiency?

A

Half way between max and min temps for the cycle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What type of system is the feed heater?

A

An open adiabatic system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What should the bleed temp be for maximum efficiency?

A

The average of the boiler and condenser temps

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What happens to the fluid streams in an open feed heater?

A

They freely mix

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is a closed feed heater?

A

Essentially a heat exchanger

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the disadvantage of an open feed heater?

A

Pumps are required to raise the pressure of the bled working fluid.
This adds significantly to the cost

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What do closed feed heaters work well for?

A

Streams at different pressures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What can be assumed about the output enthalpies of a closed feed heater (unless other info is provided)?

A

They are the same

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is a throttle used for?

A

Reducing the pressure from the bleed p to the p at which bled flow is sent to the condenser

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What kind of process is the throttle process?

A

Isenthalpic (no enthalpy change)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What assumptions are used about feed lines?

A

Pump work is negligible
Enthalpies are the same at saturation enthalpy at that pressure
Enthalpy of compressed liquid in feed line is ~= to that of saturated liquid at the same temperature

17
Q

Draw a block diagram of an open feedwater heater rankine cycle

A
Pump
Boiler
HP turbine
LP turbine
Condenser
Feed Pump
Open feedwater heater
18
Q

How is the bleed mass flow rate calculated in an open feedwater cycle?

A

SFEE balance
enthalpy balance the mass flow rate * enthalpy in = mass flow rate * enthalpy at the mixing point
0 = 1h7 - yh3 - (1-y)h6
y = h7 - h6 / h3 - h6
where y is the mass flow rate bled off after the first turbine stage

19
Q

Draw a block diagram for a closed feedwater heater rankine cycle

A
Pump
Closed feedwater heater
Boiler
HP turbine
LP turbine 
Condenser
Throttle
20
Q

What is the heat from a closed feedwater heater used to do?

A

Reduce the heat input required after the pump

21
Q

What is the mass balance of a closed feedwater heater?

A

h2 + yh5 - h3 - yh6 = 0
y = h3 - h2 / h5 - h6 where h3 = h6
mass * enthalpy in to feedwater = mass * enthalpy out of feedwater

22
Q

What are the key points for a closed feedwater heater?

A

h6 = h8 = h3

23
Q

How much waste heat is rejected for UK power stations but what is the issue with it

A

enough to supply all UK domestic heat requirements

It is at a very low temperature

24
Q

What is combined heat and power generation CHP?

A

A power plant where waste heat is also supplied for heating, both heat and power are generated from a single supply of primary fuel (a total energy scheme)

25
What is the mathematical equation of the utilisation factor
Net work output + process heat used/Heat supplied
26
What does a back pressure turbine allow you to do?
Steam from the back pressure turbine is not fully condensed back to atmospheric pressure therefore the steam can be used in other processes across the plant
27
What are the different types of CHP plants?
Back pressure turbines | Pass out co generation plants
28
What happens in a pass out cogeneration plant?
Provides wider range of possible heat/power ratios Allows bleeding out of steam part way through turbine, rest of the steam expands to condenser pressure and is then cooled