2nd Year Vapor-Power Cycles Flashcards
What to the Carnot Cycle Graphs look like?
What is the Carnot Cycle?
Ideal cycle using 4 ideal processes (2 isothermal, 2 adiabatic)
Calculates theoretical maximum efficiency os a heat engine
All processes are reverable
What are the four processes in the Carnot Cycle?
1-2 isothermal expansion - T(H)=const - Gas does Work
2-3 adiabatic expansion - T(H) to T(L) - Q=0 - Gass does work
3-4 isothermal compression - T(L)=const - Work on Gass
4-1 adiabatic compression - T(L) to T(H) - Q=0 - Work on Gass
What are T(L) and T(H)?
T(L) = lower sink temperature
T(H) = higher thermal source
Isentropic efficiency for a work consuming device?
COMPRESSOR
=Ws/Wa
=h2s - h1/ h2a - h1
(actual work in will be more than ideal)
Isentropic efficiency for a work producing device?
TURBINE
=Wa/Ws
= h2a-h1 / h2s-h1
(actual work produced will always be less than ideal)
Calculating work in?
All parts of T-s diagram which make the line “go upwards” is qin.
Caculates by h2-h1 for each individual “rises”
Calculate work out?
all “drops” in T-S diagram.
calculated by h2-h1 for each individual drop
how do you calculate h at the end of the turbine?
use s at the beginning of the turbine,
assume isentropic so s2-s1
use this value to calculate X
then use X to calculate h.
is isentropic efficiency given, use:
efficiency = h2a-h1 / h2s-h1
to find h2a
Describe the rankine Cycle:
Similar to the Carnot cycle except the condensor condenses the fluid to a saturate iquid. It then uses a pump before expanding again.
In a Rankine Cycle, how do you calculate the pump work?
w = h2-h1
this can be simplified in this case only to:
vf(P2-P1)
Most of the time the pump work will be ignored
What is the Work ratio?
Rw = Net work out/Positive work out
the closer Rw is to 1 then the less sensitive to inequalities it becomes
In a “Rankine plus superheat” cycle, what is the difference?
After expansion to a saturated gas, the fluid is then superheated to a higher s using a heat exchanger so that the dryness fraction at the exit is larger.
Use the same pressure as was used before it was superheated.
Adding a Reheat does this again to magnify the benefits.
What is regeneration?
When a fraction of the steam is extracted from the turbine at a higher temperature than the rest to help reheat the process.
Fraction Etracted = y = m1/m2
Fraction left = 1-y = m1/m3
OPEN = mixing process used
CLOSED = heat exchange process used
What is the best approach for these questions?
Draw the diagram
Find s and h for all points necessary
take care with steam tables and units
