Deck 9: Thermodynamics and Rankine Cycle

Question 1

Considerfollowing options for a simple Rankine cycle
i. increasing pressure at exist of turbine
ii. increasing average temperature of heat addition
iii. decreasing pressure at exit of turbine
iv. superheating of steam
v. decreasing pressure during superheating.
Which combination of above will increase Rankine efficiency?

A)i, ii & iv
B)i, ii & v
C)iii, iv & v
D)ii, iii & iv

Answer 1

ii, iii & iv

Question 2

Higher dryness fraction of steam at turbine exit in a Rankine cycle

A)will increase cycle efficiency
B)will increase turbine work
C)will protect turbine blading
D)will reduce pump work

Answer 2

will protect turbine blading

Question 3

The main purpose of reheat Rankine cycle is

A)to reduce moisture in steam & thus to protect turbine blading
B)to obtain significant gain in efficiency
C)to obtain significant gain in net work
D)to reduce heat supplied & yet maintain turbine work

Answer 3

to reduce heat supplied & yet maintain turbine work

Question 4

The temperature of feed water can be increased in following two ways in a regenerative Rankine cycle
i. by passing feed water around turbine casing in direction opposite to flow of steam
ii. by bleeding certain portion of steam at the end of h.p expansion & mixing with condensate to make feed water at the temperature of boiler

A)i is preferred over ii for higher efficiency
B)ii is preferred over i for reduced moisture content of steam in l.p side
C)only i is implemented in actual practice
D)by preferring ii over i significant gain in efficiency results

Question 5

In a steam power plant which cycle requires two or more feed pumps

A)regenerative rankine cycle
B)reheat rankine cycle
C)simple rankine cycle
D)idealized regenerative rankine cycle

Question 6

In a regenerative Rankine cycle as compared to simple Rankine cycle

A)the pump work is smaller
B)the addition of heat is smaller
C)the superheat is to lesser extent
D)the heat rejected in condenser is smaller

Question 7

A Rankine cycle is less efficient than Carnot cycle because

A)the working fluid is steam
B)heat is not added athighest temperature of cycle
C)condensation processis extended into liquid phase
D)superheating occurs at constant pressure

Question 8

A regenerative Rankine cycle may approach efficiency of a Carnot cycle between same temperature by

A)reducing heat to condenser
B)reducing volume of steam passed into condenser
C)increasing turbine work
D)adding heat a highest temp.

Question 9

Average temp.at which heat is supplied is increased in

A)simple rankine cycle
B)reheat rankine cycle
C)regenerative rankine cycle
D)modified rankine cycle

Question 10

A power cycle using steam as working fluid consist of
i. constant pressure heat addition
ii.adiabatic expansion
iii. constant pressure heat rejection &
iv. adiabatic compression. The cycle is

A)simple rankine cycle
B)modified rankine cycle
C)carnot cycle
D)regenerative rankine cycle

Question 11

Steam enters the turbine at 5.44 bar with dryness fraction of 0.9 & is expanded adiabatically to a pressure of 0.68 bar. It will enter the condenser with a quality very close to

A)0.79
B)0.815
C)0.84
D)0.86

Question 12

A Rankine cycle has efficiency of 19.4%. The steam enters the turbine with enthalpy of 32.85 kJ/Kg while heat of water entering the boiler is 423.6 kJ/Kg.the work of the turbine is 560 kJ/ Kg. the pump work is

A)3.5 kj/kg
B)5 kj/kg
C)6.5 kj/kg
D)7 kj/kg

Question 13

A steam power plant working on Rankine cycle produces steam in the boiler at a rate of 100 kg/s with enthalpy of 3280 kJ/kg from water whose total heat content is 420 kJ/kg. The plant has Rankine efficiency of 24.65 while pump requires 5 kJ/kg of work. The turbine generates power of

A)71 mw
B)71.5 mw
C)72 mw
D)72.5 mw

Question 14

In a simple Rankine cycle steam was expanded from 100 bar & 380°C to 1 bar 0.8 dry state. The pump motor consumes 10 kJ/kg of energy. The efficiency of cycle is

A)31%
B)34%
C)29.8%
D)30.6%

Question 15

In a reheat Rankine cycle steam with enthalpy of 3300 kJ/kg is expanded in h.p turbine to a state where enthalpy is 2650 kJ/kg. The steam is further expnaded from a state where enthalpy is 2950 kJ/kg to enthalpy of 2525 kJ/kg & latent heat of 2201 kJ.kg. The work of the cycle & amount of reheat respectively are

A)1075 kj/kg,300 kj.kg
B)1399 kj/kg, 300 kj/kg
C)1075 kj/kg, 324 kj/kg
D)1399 kj/kg, 324 kj/kg

Question 16

In above question the heat supplied & efficiency respectively are

A)3276 kj/kg, 42.7%
B)2976 kj/kg, 36.12%
C)300 kj/kg, 32.6%
D)330 kj/kg, 42.4%

Question 17

Due to which of the following reasons the Carnot cycle cannot be realised in actual practice?

A)high volume ratios in the cylinder
B)very high pressure developed in the cylinder
C)both a & b
D)none of the above

Question 18

The flow through turbine on a Mollier chart is represented by

A)vertical line
B)horizontal line
C)curved line convex up
D)curved line convex down

Question 19

The latent enthalpy of vapourisation at critical point is

A)zero
B)minimum
C)maximum
D)none of the above

Question 20

Above critical point, a substance exists as

A)vapour
B)liquid
C)gas
D)supersaturated fluid

Question 21

Triple point of a pure substance is a point at which

A)solid & vapour exist together
B)solid & liquid exist together
C)liquid & vapour exist together
D)solid, liquid & vapour phases existtogether

Question 22

Which of the following cycles is used in thermal power plants?

A)brayton
B)carnot
C)ericsson
D)rankine

Question 23

In a Carnot engine, when the working substance rejects its heat to sink, the temperature of sink

A)remains same
B)decrease
C)increases
D)becomes 0k

Question 24

Which of the following is used as a working substance for a Carnot cycle?

A)ideal gas
B)real gas
C)steam
D)atmospheric air

Question 25

……… is the ratio of actual cycle efficiency to that of the ideal cycle efficiency

A)efficiency ratio
B)work ratio
C)effectiveness
D)none of the above