Short Notes: Rankine Cycle & Concepts of Regeneration and Reheat | Short Notes for Mechanical Engineering PDF Download

 Page 1


Rankine Cycle & Concepts of Regeneration and Reheat
Rankine Cycle
The Rankine cycle is the fundamental operating cycle of all power plants where an 
operating fluid is continuously evaporated and condensed 2 3
2- 3 Isobaric Heat Transfer. High-pressure liquid enters the boiler from the 
feed pump (1-2) and is heated to the saturation temperature (2). Further 
addition of energy causes evaporation of the liquid until it is fully converted to 
saturated steam (3).
3- 4 Isentropic Expansion. The vapour is expanded in the turbine, thus 
producing work which may be converted to electricity. In practice, the 
expansion is limited by the temperature of the cooling medium and by the 
erosion of the turbine blades by liquid entrainment in the vapour stream as 
the process moves further into the two-phase region. Exit vapour qualities 
should be greater than 90%.
Page 2


Rankine Cycle & Concepts of Regeneration and Reheat
Rankine Cycle
The Rankine cycle is the fundamental operating cycle of all power plants where an 
operating fluid is continuously evaporated and condensed 2 3
2- 3 Isobaric Heat Transfer. High-pressure liquid enters the boiler from the 
feed pump (1-2) and is heated to the saturation temperature (2). Further 
addition of energy causes evaporation of the liquid until it is fully converted to 
saturated steam (3).
3- 4 Isentropic Expansion. The vapour is expanded in the turbine, thus 
producing work which may be converted to electricity. In practice, the 
expansion is limited by the temperature of the cooling medium and by the 
erosion of the turbine blades by liquid entrainment in the vapour stream as 
the process moves further into the two-phase region. Exit vapour qualities 
should be greater than 90%.
4-1 Isobaric Heat Rejection. The vapour-liquid mixture leaving the turbine (3­
4) is condensed at low pressure, usually in a surface condenser using cooling 
water. In well designed and maintained condensers, the pressure of the 
vapour is well below atmospheric pressure, approaching the saturation 
pressure of the operating fluid at the cooling water temperature.
1-2 Isentropic Compression. The pressure of the condensate is raised in the 
feed pump. Because of the low specific volume of liquids, the pump work is 
relatively small and often neglected in thermodynamic calculations.
Work done on pump, per kg of water, WP= h2-h1 
Energy added in steam generator, q-|= h3-h2 
Work delivered by turbine, WT= h3-h4 
Energy rejected in the condenser, q2= h4-h -|
The thermal efficiency of the Rankine cycle is given by
= = (fa - f a ) - ( f a ~ A ) = <fa - f a ) - ( f a ~fa)
q x h 3 - h 2 h 3 - h 2
Reheating of Steam
In the reheat cycle, the expansion of steam from the initial state 1 to the condenser 
pressure is carried out in two or more steps depending upon the number of reheats 
used.
Cycle efficiency improves with reheat, however the cycle efficiency in a single 
reheat plant is influenced by pressure at which steam is reheated. The efficiency 
increases as the reheat pressure is lowered and reaches a peak at a pressure ratio 
between 0.20 and 0.25
T * •
s
Reheating cycle
_ wt ~ rcy (h !-h :?+ hj-h^JCh^-hQ
Q \ ^i— k j— fU s
Reheating steam also increase the net work output of turbine.
Key Points
• Internal irreversibility of Rankine cycle (Real cycle) is caused by fluid frictions 
throttling and mixing.
• Externally, irreversibility of the Rankine cycle is caused due to the temperature 
difference between the combustion gases and the working fluid on the same 
side and the temperature difference between the condensing working fluid 
and the condenser cooling water on the sink side.
Page 3


Rankine Cycle & Concepts of Regeneration and Reheat
Rankine Cycle
The Rankine cycle is the fundamental operating cycle of all power plants where an 
operating fluid is continuously evaporated and condensed 2 3
2- 3 Isobaric Heat Transfer. High-pressure liquid enters the boiler from the 
feed pump (1-2) and is heated to the saturation temperature (2). Further 
addition of energy causes evaporation of the liquid until it is fully converted to 
saturated steam (3).
3- 4 Isentropic Expansion. The vapour is expanded in the turbine, thus 
producing work which may be converted to electricity. In practice, the 
expansion is limited by the temperature of the cooling medium and by the 
erosion of the turbine blades by liquid entrainment in the vapour stream as 
the process moves further into the two-phase region. Exit vapour qualities 
should be greater than 90%.
4-1 Isobaric Heat Rejection. The vapour-liquid mixture leaving the turbine (3­
4) is condensed at low pressure, usually in a surface condenser using cooling 
water. In well designed and maintained condensers, the pressure of the 
vapour is well below atmospheric pressure, approaching the saturation 
pressure of the operating fluid at the cooling water temperature.
1-2 Isentropic Compression. The pressure of the condensate is raised in the 
feed pump. Because of the low specific volume of liquids, the pump work is 
relatively small and often neglected in thermodynamic calculations.
Work done on pump, per kg of water, WP= h2-h1 
Energy added in steam generator, q-|= h3-h2 
Work delivered by turbine, WT= h3-h4 
Energy rejected in the condenser, q2= h4-h -|
The thermal efficiency of the Rankine cycle is given by
= = (fa - f a ) - ( f a ~ A ) = <fa - f a ) - ( f a ~fa)
q x h 3 - h 2 h 3 - h 2
Reheating of Steam
In the reheat cycle, the expansion of steam from the initial state 1 to the condenser 
pressure is carried out in two or more steps depending upon the number of reheats 
used.
Cycle efficiency improves with reheat, however the cycle efficiency in a single 
reheat plant is influenced by pressure at which steam is reheated. The efficiency 
increases as the reheat pressure is lowered and reaches a peak at a pressure ratio 
between 0.20 and 0.25
T * •
s
Reheating cycle
_ wt ~ rcy (h !-h :?+ hj-h^JCh^-hQ
Q \ ^i— k j— fU s
Reheating steam also increase the net work output of turbine.
Key Points
• Internal irreversibility of Rankine cycle (Real cycle) is caused by fluid frictions 
throttling and mixing.
• Externally, irreversibility of the Rankine cycle is caused due to the temperature 
difference between the combustion gases and the working fluid on the same 
side and the temperature difference between the condensing working fluid 
and the condenser cooling water on the sink side.
Advantages of Re-heating:
• Due to reheating, network done increases
• Heat supply increases
• Thermal efficiency increases
• Due to reheating, the turbine exit dryness fraction increases so moisture 
decreases - so blade erosion becomes minimum - so life of the turbine will be 
increased.
Regeneration
The mean temperature of heat addition (and so efficiency) can also be increased by 
reducing the amount of heat added at low temperatures in the economizer section 
of steam generator. In the regeneration process energy is exchanged internally 
between the expanding fluid in the turbine and the compressed fluid before heat 
addition.
Ideal regenerative cycle done not affect work output from turbine, it is more 
efficient with high steam rate.
Regeneration cycle with two direct contact 
feed water heaters
Efficiency of Steam Power Plant
Overall efficiency of the steam per plant is given by
tJo V K ill ^ ^ tftZ’ iv.i ' ^
Power available at theeenerator terminals
n*
Rate of energy released bv the combustion of fuel
Rate of energy absorption by water from steam 
Rate of energy released by combustion of fuel
_ Brake output of the turbine
1 nirbsii atztjz:zi
1 s u x flu ry
Internal output of the turbine
_ Net power transmitted by the generator 
Gross power produced bv plant
. Net work output
Heat supplied
_ Electrical output at generator terminal 
Brake output of turbine
• Reheating of steam improved the thermal efficiency of the plant, net work 
output of turbine, reduction in blade erosion (or quality of steam improve)
• By regeneration thermal efficiency of the plant can be increased but it does 
not affect work output from turbine.
Page 4


Rankine Cycle & Concepts of Regeneration and Reheat
Rankine Cycle
The Rankine cycle is the fundamental operating cycle of all power plants where an 
operating fluid is continuously evaporated and condensed 2 3
2- 3 Isobaric Heat Transfer. High-pressure liquid enters the boiler from the 
feed pump (1-2) and is heated to the saturation temperature (2). Further 
addition of energy causes evaporation of the liquid until it is fully converted to 
saturated steam (3).
3- 4 Isentropic Expansion. The vapour is expanded in the turbine, thus 
producing work which may be converted to electricity. In practice, the 
expansion is limited by the temperature of the cooling medium and by the 
erosion of the turbine blades by liquid entrainment in the vapour stream as 
the process moves further into the two-phase region. Exit vapour qualities 
should be greater than 90%.
4-1 Isobaric Heat Rejection. The vapour-liquid mixture leaving the turbine (3­
4) is condensed at low pressure, usually in a surface condenser using cooling 
water. In well designed and maintained condensers, the pressure of the 
vapour is well below atmospheric pressure, approaching the saturation 
pressure of the operating fluid at the cooling water temperature.
1-2 Isentropic Compression. The pressure of the condensate is raised in the 
feed pump. Because of the low specific volume of liquids, the pump work is 
relatively small and often neglected in thermodynamic calculations.
Work done on pump, per kg of water, WP= h2-h1 
Energy added in steam generator, q-|= h3-h2 
Work delivered by turbine, WT= h3-h4 
Energy rejected in the condenser, q2= h4-h -|
The thermal efficiency of the Rankine cycle is given by
= = (fa - f a ) - ( f a ~ A ) = <fa - f a ) - ( f a ~fa)
q x h 3 - h 2 h 3 - h 2
Reheating of Steam
In the reheat cycle, the expansion of steam from the initial state 1 to the condenser 
pressure is carried out in two or more steps depending upon the number of reheats 
used.
Cycle efficiency improves with reheat, however the cycle efficiency in a single 
reheat plant is influenced by pressure at which steam is reheated. The efficiency 
increases as the reheat pressure is lowered and reaches a peak at a pressure ratio 
between 0.20 and 0.25
T * •
s
Reheating cycle
_ wt ~ rcy (h !-h :?+ hj-h^JCh^-hQ
Q \ ^i— k j— fU s
Reheating steam also increase the net work output of turbine.
Key Points
• Internal irreversibility of Rankine cycle (Real cycle) is caused by fluid frictions 
throttling and mixing.
• Externally, irreversibility of the Rankine cycle is caused due to the temperature 
difference between the combustion gases and the working fluid on the same 
side and the temperature difference between the condensing working fluid 
and the condenser cooling water on the sink side.
Advantages of Re-heating:
• Due to reheating, network done increases
• Heat supply increases
• Thermal efficiency increases
• Due to reheating, the turbine exit dryness fraction increases so moisture 
decreases - so blade erosion becomes minimum - so life of the turbine will be 
increased.
Regeneration
The mean temperature of heat addition (and so efficiency) can also be increased by 
reducing the amount of heat added at low temperatures in the economizer section 
of steam generator. In the regeneration process energy is exchanged internally 
between the expanding fluid in the turbine and the compressed fluid before heat 
addition.
Ideal regenerative cycle done not affect work output from turbine, it is more 
efficient with high steam rate.
Regeneration cycle with two direct contact 
feed water heaters
Efficiency of Steam Power Plant
Overall efficiency of the steam per plant is given by
tJo V K ill ^ ^ tftZ’ iv.i ' ^
Power available at theeenerator terminals
n*
Rate of energy released bv the combustion of fuel
Rate of energy absorption by water from steam 
Rate of energy released by combustion of fuel
_ Brake output of the turbine
1 nirbsii atztjz:zi
1 s u x flu ry
Internal output of the turbine
_ Net power transmitted by the generator 
Gross power produced bv plant
. Net work output
Heat supplied
_ Electrical output at generator terminal 
Brake output of turbine
• Reheating of steam improved the thermal efficiency of the plant, net work 
output of turbine, reduction in blade erosion (or quality of steam improve)
• By regeneration thermal efficiency of the plant can be increased but it does 
not affect work output from turbine.
Combined Cycle Power Generation
It is an assembly of heat engines that works with the same source of heat. The 
principle of combined cycle power generation is that the exhaust of one heat 
engine is used as the used as the source of heat for another heat engine. This
• Heat supplied to boiler becomes reduced
• Thermal efficiency is increased since the average temperature of heat 
addition to the cycle is increased.
• Due to bleeding in the turbine, erosion of turbine due to moisture is reduced.
Characteristics of Ideal Working Fluid
• The fluid should have high critical temperature and the saturation pressure at 
the temperature of heat rejection should be above the atmospheric pressure.
• Specific heat of liquid should be small.
• The saturated vapour line of T-s diagram, very close to the turbine expansion 
process.
• The freezing point of the fluid should be below the room temperature.
• The fluid should be chemically stable, non-toxic, non-corrosive, not 
excessively viscous and low in cost.