Ideal cycles Flashcards
What causes the ideal rankine heat engine cycle to differ from the actual cycle
Fluid friction causes pressure drops in boiler, condenser and pipings
Heat loss to surroundings
Pump requires greater work input and turbine produces smaller work output
Steam leakage into surroundings and air leaking into condenser
Ideal cycle
Cycle closely resembles actual cycle, but totally internally reversible
What are the idealizations of ideal power cycles
No friction
Quasi-equilibrium
Adiabatic
Main elements of thermodynamics cycles
Working fluid: gas/liquid that converts thermal E <-> mech by phase change or heat of compression/expansion
Boilers/evap: injects energy from heat source into working fluid
Condensers: condense while releasing heat (decrease enthalpy)
Turbine: convert superheated vapour into sat vap while doing shaft work
Pumps for liquid: increase fluid KE and P, using shaft work
Reversible cycle
Totally reversible, highest thermal efficiency of all heat engines operating within the same temp. levels
Reciprocating engine
Stroke: largest distance piston can travel 1 dir
Bore: diameter of piston
Intake-valve: air-fuel mixture enters into a piston
Exhaust valve: combustion products expelled out
Clearance volume: vol formed in cylinder when piston at TDC (top dead center)
Max vol: vol at bdc
Displacement vol: vol displacement from bdc to tdc
Compression ratio: ratio of max vol to min vol formed in the cylinder
Mean effective pressure
Theoretical constant pressure that is acted on piston during power stroke gives same net work thats actually developed in 1 cycle (compares performances)
Carnot impracticalities
Compression:
Not easy to control condensation
2 phase compressors hard to design
Limit max temp
Heat transfer:
max temp below crit temp = limitation, less thermal efficiency
High moisture content erodes turbine
How does ideal rankine eliminate carnot impracticalities
Completely condense working fluid in compressor: 1 -> 2 subcooled
Superheat steam in boiler: avoid condensation
Steps of ideal rankine cycle
1->2 Isentropic compression in pump (liquid)
2->3 Isobaric heat addition in boiler
3->4 Isentropic expansion in a turbine
4-> 1 Isobaric heat rejection in condenser
Differences between actual cycle and carnot cycle
Fluid friction drops pressure in boiler, pipes, condenser for actual
Heat loss surroundings
More work in less work out
steam leakage
How to increase thermal efficiency
Increase average temp for heat transfer or decrease temp fo heat rejection
1) lower condenser pressure
2) Superheating steam
3) Increasing boiler pressure
Cogeneration
plant produces electricity and meets process heat requirements for another industrial process
ie prod of more than one useful source of energy from same source
Difference between ideal and actual refrigeration cycle
Actual differs because it has irreversibilities (fluid friction + surr heat transfer lower COP)
Non isentropic compression
Superheated vapor at evap exit
Subcooled at condenser exit
P drops in condenser + evap
Heat sources for heat pump systems
Atm air
Water
Ground source (geothermal)
