Steam cycles Flashcards
Efficiency improvement strats
Increase the temperature of heat addition
Decrease the temperature of heat rejection
Increased temperature and pressure of live steam
Steel barrier
600 C 300 bar
Reheat efficiency?
Increased area enclosed in T-s cycle diagram => increased efficiency
But, costly to exceed 600C and 30 MPa (steel barrier)
Condenser temp fixed. We need to move upwards (increase T) or to the right (increase P) in T-s diagram to increase efficiceny
Feed water heating (FWH) – regenerative cycle
• Increases the mean temperature of heat addition (the system moves towards a Carnot cycle) • Reduces the irreversibilities (or heat losses) related to steam generation → increased thermal efficiency Similar to reheating FWH increases enclosed area in T-s diagram => increased efficiency
Laupichler method
- Determine position of Turbine extractions
- Divide equally the enthalpy drop along expansion line
Steam turbine components
• Stationary blade directs flow and turns potential
energy (P/T) into kinetic energy(velocity)
• Rotating bladesconvert kinetic energy
into impulse => rotation of turbine rotor
Steam turbine losses
• Turbine blades - frictional losses • Slots between rotating and static parts: steam leakage • Turbine outlet: kinetic loss due pressure loss • Losses due to steam wetness • If expansion passes saturation line • Water stays on the blades => kinetic loss • Droplets cause blade erosion => lowered efficiceny
Cooling towers (wet/dry)
A. Wet system - evaporative (figure) - water cooled by evaporation - More efficient than air: high heat capacity - Condenser pressure: 0.03-0.05bar B. Dry system - Used in dry areas - No direct contact between water and air - Condenser pressure up to: 0.06-0.07bar
Cooling towers (draft)
Natural draft: by chimney effect - Only economic in large sizes, due to large concrete structure - No fan work -> low operating cost Mechanical draft: by fans - Large axial fans force air through the tower (1MWe/200MWe) - Used in regions with large Temperature variations to control flow situation/cooling capacity
Condensers mixing type +surface type
• condensing the exhaust steam by rejecting the
heat of vaporisation to the cooling
water
• Provide short-term storage of condensate
• Provide a low-pressure collection point for
condensate drains from other systems in the
plant
• Provide for de-aeration of the collected
condensate.
Boiler walls
Modern boilers are - radiant boilers - water tube based - Membrane walls absorb heat and cool the walls - Temperature difference between gas and water in the range: 500-800ºC
Boiler types
Natural draft boiler Water tube boilers A Natural circulation • Density gradient = driving force • Restricted to subcritical cond. B Forced circulation • Pump = driving force • Subcritical C Once though or univ. pressure • No dome/circulation • Subcritical/supercritical
Boilers – heat exchanger (HEX) arrangements
A. Single pass boiler (+) minimized accumulation of ashes in the boiler: avoiding erosion (+) One frame work/body in which HEX tubes can move freely (-) Boiler height: 20-30 m higher than double pass B. Double pass boiler (+) Flexible position of tube banks in pass 2 (+) Lower investment cost/construction time (-) increased thermal stress: T-gradient in cross section between 1st and 2nd pass
Air preheating
- Preheats air against hot flue gas exiting boilers
- Decreases boiler heat loss and increases efficicency
- Improves ignitions stability of flames by faster heating of combusiton
reactants
Boiler types mech
fluidised bed (pulverised or circulating) grate fired
