Lecture 4 - Rankine and Modified Rankine Flashcards
Why is the Rankine cycle efficiency lower than that of the Carnot’s?
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
How is pump work calculated in the Rankine cycle?
W5-4 = -(h4-h5) = vf4(p5-p4)
What does vf represent in the pump work calculation, and how does is apply to water?
The specific volume at the initial pressure
Water is incompressible, so changes in vf are negligible
What happens to the working fluid as it moves through the turbine in a rankine cycle?
It becomes a wet vapour. As expansion continues towards the end of the turbine, the fluid contains more liquid
How can one increase dryness by moving points on the T-s diagram?
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)
When increasing dryness, what do we move up the constant pressure line from what to what?
From the saturation line to the super-heated vapour region of the T-s diagram
How is the move to superheated vapour achieved?
The working fluid (now a dry vapour) is sent to a super-heater
What are advantages of adding more heat with a super-heater?
The cycle makes better use of the material limit
The average temp is higher so the cycle is more
Dryer turbine exit flow
What is the effect of increasing boiler pressure when a super-heater is used?
The vapour will be wetter in the turbine
What happens when a realistic turbine efficiency is added?
The dryness fraction (quality of vapour) tends to increase
What is the purpose of the reheater cycle?
To increase the dryness of the vapour further and increase the average temperature of the cycle
What is added in the reheater cycle?
An additional turbine and a reheater (can be multiple sets)
What is the specific steam consumption?
The s.s.c. is the amount of steam required to generate a unit work output (usually kW) per hour
How do Rankine and Carnot cycles’ s.s.cs compare?
Carnot’s is higher than Rankine’s
What is the effect on components when reducing ssc?
They are smaller
How the efficiencies of the modified Rankine cycles compare to that of the Carnot’s?
They are less efficient, but the more complicated, the closer to the ideal Carnot
Why is carnot cycle not used in practice?
pumping from 4-1 to in mixture of liquid and vapour
liquid water droplet can badly damage the turbine blades
How are the issues of carnot cycle overcome in the rankine cycle?
State 4 is moved to the saturation line - fully condense fluid therefore compressing just liquid
Draw a block diagram for a simple rankine cycle?
Pump
Boiler
Turbine
Condenser
Draw a block diagram for a superheated rankine cycle?
Pump Boiler Superheater Turbine Condenser
Draw a block digram for a superheated rankine cycle with reheat?
Pump Boiler Superheater HP Turbine Reheater LP Turbine Condenser
What is the key point to remember when calculating the fluid state in the intermediate stage in a modified rankine?
You cannot assume that the outlet of the HP turbine will be on the saturation line - could still be superheated steam
