Lecture 5- Regenerative Flashcards
What is the basic idea behind regenerative cycles?
Transferal of heat to the working fluid REVERSIBLY from another part WITHIN the cycle
Between which points in the cycle should heat be transferred?
2 and 3- the boiler stage
Why is heat from the turbine not used from within the cycle?
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
What happens to steam in the 1st stage of the turbine?
Some is extracted and fed through the open feed water heater
The rest continues into the 2nd stage
What happens to steam in and after the 2nd stage of the turbine?
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
What temp should t3 be for maximum efficiency?
Half way between max and min temps for the cycle
What type of system is the feed heater?
An open adiabatic system
What should the bleed temp be for maximum efficiency?
The average of the boiler and condenser temps
What happens to the fluid streams in an open feed heater?
They freely mix
What is a closed feed heater?
Essentially a heat exchanger
What is the disadvantage of an open feed heater?
Pumps are required to raise the pressure of the bled working fluid.
This adds significantly to the cost
What do closed feed heaters work well for?
Streams at different pressures
What can be assumed about the output enthalpies of a closed feed heater (unless other info is provided)?
They are the same
What is a throttle used for?
Reducing the pressure from the bleed p to the p at which bled flow is sent to the condenser
What kind of process is the throttle process?
Isenthalpic (no enthalpy change)
What assumptions are used about feed lines?
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
Draw a block diagram of an open feedwater heater rankine cycle
Pump Boiler HP turbine LP turbine Condenser Feed Pump Open feedwater heater
How is the bleed mass flow rate calculated in an open feedwater cycle?
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
Draw a block diagram for a closed feedwater heater rankine cycle
Pump Closed feedwater heater Boiler HP turbine LP turbine Condenser Throttle
What is the heat from a closed feedwater heater used to do?
Reduce the heat input required after the pump
What is the mass balance of a closed feedwater heater?
h2 + yh5 - h3 - yh6 = 0
y = h3 - h2 / h5 - h6 where h3 = h6
mass * enthalpy in to feedwater = mass * enthalpy out of feedwater
What are the key points for a closed feedwater heater?
h6 = h8 = h3
How much waste heat is rejected for UK power stations but what is the issue with it
enough to supply all UK domestic heat requirements
It is at a very low temperature
What is combined heat and power generation CHP?
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)
What is the mathematical equation of the utilisation factor
Net work output + process heat used/Heat supplied
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
What are the different types of CHP plants?
Back pressure turbines
Pass out co generation plants
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
