Lecture 5- Regenerative

Question 1

What is the basic idea behind regenerative cycles?

Answer 1

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

Question 2

Between which points in the cycle should heat be transferred?

Answer 2

2 and 3- the boiler stage

Question 3

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

Answer 3

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

Question 4

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

Answer 4

Some is extracted and fed through the open feed water heater

The rest continues into the 2nd stage

Question 5

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

Answer 5

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

Question 6

What temp should t3 be for maximum efficiency?

Answer 6

Half way between max and min temps for the cycle

Question 7

What type of system is the feed heater?

Answer 7

An open adiabatic system

Question 8

What should the bleed temp be for maximum efficiency?

Answer 8

The average of the boiler and condenser temps

Question 9

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

Answer 9

They freely mix

Question 10

What is a closed feed heater?

Answer 10

Essentially a heat exchanger

Question 11

What is the disadvantage of an open feed heater?

Answer 11

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

Question 12

What do closed feed heaters work well for?

Answer 12

Streams at different pressures

Question 13

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

Answer 13

They are the same

Question 14

What is a throttle used for?

Answer 14

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

Question 15

What kind of process is the throttle process?

Answer 15

Isenthalpic (no enthalpy change)

Question 16

What assumptions are used about feed lines?

Answer 16

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

Question 17

Draw a block diagram of an open feedwater heater rankine cycle

Answer 17

Pump
Boiler
HP turbine
LP turbine
Condenser
Feed Pump
Open feedwater heater

Question 18

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

Answer 18

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

Question 19

Draw a block diagram for a closed feedwater heater rankine cycle

Answer 19

Pump
Closed feedwater heater
Boiler
HP turbine
LP turbine 
Condenser
Throttle

Question 20

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

Answer 20

Reduce the heat input required after the pump

Question 21

What is the mass balance of a closed feedwater heater?

Answer 21

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

Question 22

What are the key points for a closed feedwater heater?

Answer 22

h6 = h8 = h3

Question 23

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

Answer 23

enough to supply all UK domestic heat requirements

It is at a very low temperature

Question 24

What is combined heat and power generation CHP?

Answer 24

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)

Question 25

What is the mathematical equation of the utilisation factor

Answer 25

Net work output + process heat used/Heat supplied

Question 26

What does a back pressure turbine allow you to do?

Answer 26

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

Question 27

What are the different types of CHP plants?

Answer 27

Back pressure turbines

Pass out co generation plants

Question 28

What happens in a pass out cogeneration plant?

Answer 28

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