SUMMER 13 EXAMINATION 12099 Model Answer …msbte.engg-info.website/sites/default/files/12099_Summer_2013...Answer: Reciprocating compressor is called as positive displacement compressor

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)

(ISO/IEC - 27001 - 2005 Certified)

SUMMER – 13 EXAMINATION Subject Code: 12099 Model Answer Page No: 1/25

Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme. 2) The model answer and the answer written by candidate may vary but the examiner may try to assess the understanding level of the candidate. 3) The language errors such as grammatical, spelling errors should not be given more Importance (Not applicable for subject English and Communication Skills). 4) While assessing figures, examiner may give credit for principal components indicated in the figure. The figures drawn by candidate and model answer may vary. The examiner may give credit for any equivalent figure drawn. 5) Credits may be given step wise for numerical problems. In some cases, the assumed constant values may vary and there may be some difference in the candidate’s answers and model answer. 6) In case of some questions credit may be given by judgement on part of examiner of relevant answer based on candidate’s understanding. 7) For programming language papers, credit may be given to any other program based on equivalent concept.

Q 1 a) Attempt any SIX of the following: 12

i) Define system and boundary. 2

Answer:

a) System: A thermodynamic system is defined as a definite quantity of matter or a region in

Space upon which attention is focused in the analysis of a problem.

b)Boundary: The system is distinguished from its surroundings by a specified boundary .

1

1

ii) Enlist Non- conventional energy sources. 2

Answer:

a) Solar energy b) Wind energy c)Tidal energy d) Biogas energy e) Geothermal energy

f) Ocean energy

½

for

each

iii) Define heat transfer and state mode of heat transfer. 2

Answer: Heat transfer: Transfer of energy from one region to another region because of

temperature

1




difference.

Mode of heat transfer: 1) conduction 2) convection 3) radiation

1

iv) Write the unit of thermal conductivity. 2

Answer: Watt/m-K Or Watt/m0 C 2

v) Enlist four uses of compressed air. 2

Answer:

1. Operating tools in factories

2. Operating drills and hammers in road building

3. Starting diesel engines

4. Operating brakes on buses, trucks and trains

5. Spray painting

6. Excavating

7. To clean the large workshops

½

for

each

vi) Define total heat of superheated stem. 2

Answer: Total heat of superheated steam is sum of sensible heat, latent heat and heat of

superheat.

2

vii) Enlist any four advantages of gaseous fuel over solid fuel. 2

Answer:

1. Higher calorific value

2. Lower storage capacity required

3. Better economy in handling

4. Better control of combustion

5. It is clean and free from dust

6. Practically no ashes

7. Higher combustion efficiencies

½

for

each




viii) Why reciprocating compressor is called positive displacement compressor. 2

Answer: Reciprocating compressor is called as positive displacement compressor because

compression of air takes place by reducing it’s volume with the help of reciprocating action of

piston.

2

Q1 b) Attempt any TWO of the following: 8

i) Calculate the dryness fraction of steam of 0.6 kg of water is in suspension with 24 kg

of dry steam.

4

Answer:

Data

Mass of water mf = 0.6kg

Mass of dry steam mg = 24kg

To find:

Dryness fraction of steam = x = ?

We have,

Dryness fraction = x =

Dryness friction fg

g

mm

mx

6.024

24

x

= 0.975

Dryness fraction is 0.975

( Formula 2 , Substitutions 1 & Answer 1 )




ii) give the classification of gas Turbine. 4

Answer:

According to the path of the working substance:

i) Open cycle gas turbine

ii) Close cycle gas turbine

iii) Semi-closed cycle gas turbine

According to process of combustion:

i) Constant pressure gas turbine

ii) Constant volume gas turbine

According to direction of flow:

i) Radial flow

ii) Axial flow

iii) Tangential flow

According to principle of action of expanding gases:

i) Impulse turbine

ii) Reaction turbine

According to their usage:

i) Constant speed

ii) Variable speed

1 for

each




iii) Draw a neat block diagram of a layout of a thermal power plant and its functions of

each component.

4

Answer:

Thermal power plant

The different components which are used in thermal power plant are listed as follows

i) Boiler iii) condenser v) Cooling Water pump vii) Feed water pump ix) Air blower ii)

Turbine iv) Cooling tower vi) Condensate pump viii) Fuel storage tank x) Chimney

1. Boiler: To generate the steam from water

2. Turbine : To generate the power by utilizing heat of steam

3. Condenser:

It reduces the backpressure upon the turbine by a considerable degree and therefore, the




work done per kg of steam during expansion is increased.

The exhaust steam condensate can be recycled as boiler feed water.

4. Cooling tower : To supply cooling water to condenser

5. Cooling water pump: It is a pump which circulates cooling water through condenser.

6. Condensate pump: It is a pump which removes condensate from the condenser to hot well.

7. Feed water pump: It is a pump which circulates water through boiler.

8. Fuel Storage tank: Function of fuel storage tank is to store the fuel for combustion.

9. Air blower: It is used to circulate the air through combustion chamber.

10. Chimney: Exhaust gases are exhausted to atmosphere through Chimney.

(Figure - 2 and function of any four components – 2)

Q 2) Attempt any TWO of the following: 16

a) One kg of air at 150 KPa and 300C underoes a constant pressure process until the

volume is tripled Determine;

i) Change in internal energy

ii) Change in entropy

iii) Change in enthalpy

8

Answer:

Given: Constant pressure process

Mass of air = m = 1kg

Initial volume = V1 = V

Final volume = V2 = 3V

Initial temperature = T1 = 300C = 30 + 273 = 303

0K

Initial pressure = P1 = 150KPa = 150 x 103 Pa = 150 x 10

3

Assume Cp = 1 kJ/kgK and Cv = 0.72 1 kJ/kgK or Cp = 1.005 kJ/kgK & Cv = 0.718 1kJ/kgK




To find:

1. Change in internal energy

2. Change in entropy

3. Change in enthalpy

We have, for constant pressure process,

∴

909K ( 2 marks )


Change in internal energy = ∆u = mCv (T2 – T1)

∴ Change in internal energy = ∆u = 1 x 0.72 (909 – 303)

∴ Change in internal energy = ∆u = 436.32kJ ( 2 marks )


Change in entropy = ∆S = mCp loge

∴ Change in entropy = ∆S = 1x1 loge

∴ Change in entropy = ∆S = 1.098kJ ( 2 marks )


Change in enthalpy = ∆h = mCp (T2 – T1)




∴ Change in enthalpy = ∆h = 1 x 1 (909 – 303)

∴ Change in enthalpy = ∆h = 606kJ ( 2 marks )

b) Classify the steam turbine on the basis of

i) Working principle

ii) Stages of expansion

iii) Position of shaft

iv) Nature of steam supply

v) Direction of steam flow

vi) Exhaust steam pressure

8

Answer:

Classification of steam turbines: Steam turbines may be classified as follows:-

1) On the basis of working principle/mode of steam action ( 2 marks )

a) Impulse turbine

b) Reaction turbine

c) Impulse-Reaction turbine

2) On the basis of stages of expansion ( 1 marks )

a) Single stage turbine

b) Multistage turbine

3) On the basis of position of shaft ( 1 marks )

a) Horizontal axis turbine

b) Vertical axis turbine

4) On the basis of nature of steam supply ( 2 marks )

a) High pressure steam turbine

b) Low pressure steam turbine

c) Medium pressure steam turbine

5) On the basis of direction of steam flow ( 1 marks )

a) Axial flow turbine

b) Radial flow turbine




6) On the basis of exhaust steam pressure ( 1 marks )

a) Condensing pressure turbine

b)Non condensing pressure turbine

c) Write the advantages working of Nonconventional Energy sources

i) Solar

ii) Bio-mass

iii) Wind energy

iv) Tidal energy

8

Answer:

Advantages

i ) Solar energy : ( 2 marks )

1. It is available in abundance

2. It is free from pollution

3. It has low operating and maintenance cost.

4. It can be directly converted into electricity using photovoltaic cell

ii) Bio-mass ( 2 marks )

1. Renewable energy source

2. Simple to use

iii) Wind energy ( 2 marks )

1. It is available free and is inexhaustible or renewable source of energy.

2. It is clean and non polluting

3. It has low maintenance cost

4. It has low cost of power generation of about Rs. 2.25/kWh

iv) Tidal Energy: ( 2 marks )

1.Independent on rainfall.

2.Free from pollution.

3.Valuable land not required.





a) Derive the equation for efficiency for Diesel cycle. 8

Answer:

Representation of Diesel cycle on P-V and T-S diagram is as follows

Fig. Representation of Diesel cycle on P-V and T-S diagram

Diesel cycle is the ideal model for the compression-ignition engine (Diesel engine). This air-

standard Diesel cycle is composed of four processes:

4-1: isentropic compression

1-2: Constant pressure heat addition

2-3: isentropic expansion

3-4: Constant volume heat rejection

Work done = Heat supplied – Heat rejected

= mCp(T2 – T1) – mCv(T3 – T4)

∴ Air standard efficiency,




= 1 - = 1- -------------------------------------------------- (i)

Now let compression ratio,

Compression ratio r =

Cutoff ratio

We know for constant pressure heating process 1-2

∴ ------------------------------------------------------- (ii)

Similarly for reversible adiabatic expansion process 2-3

∴ --------------------------------------------- (iii)

Similarly for reversible adiabatic compression process 4-1

------------------------------------ (iv)

Substituting value of in equation (ii) and (iii),

------------------------------------------------- (v)

and ---------------------------- (vi)

Now substituting the values of T1, T2, and T3 in equation (i)

= 1-

= 1- --- Equation for efficiency for diesel cycle

( P-V & T-S Diagram 4 marks & derivation 4 marks)




b) Compare centrifugal and axial flow compressor. 8

Answer:

Sr.

No.

Centrifugal compressor Axial Flow Compressor

1 Flow is perpendicular to axis of

compressor.

Flow of air is parallel to the axis of

compressor.

2 Low manufacturing and running cost. High manufacturing and running cost.

3 Requires low starting torque. Requires high starting torque.

4 Not suitable for multi-staging. Suitable for multi-staging.

5 Requires large frontal area for given

rate of flow.

Requires less frontal area for given rate of

flow.

6 Pressure ratio per stage is4:1. Pressure ratio is 1.1 to 1.2

7 Isentropic efficiency is 70% Isentropic efficiency is 80%

8 Used in supercharging I.C. engine and

for refrigerants and industrial gases.

Used universally with large gas turbine.

1 for

each




c) Classify atmospheric jet engine and explain with sketch turbo jet engine. 8

Answer:

Classification of atmospheric jet as follows:

According to steady combustion system: ( 2 marks )

Turbo-jet engine

Turbo-prop engine

Ram-jet engine

According to intermittent combustion system:

Pulse-jet engine

Figure Turbo-jet Engine (3marks)

Turbo-jet Engine:- (3marks)

Turbo-jet engine consists of diffuser, compressor, combustion chamber turbine and nozzle.

At entrance air diffuser causes rise in pressure in entering air by slowing it down.

A rotary compressor, which rises the pressure of air further to required value and delivers to the

combustion chamber. The compressor is axial or radial type driven by turbine.

In the combustion chamber, fuel is sprayed, as result of this combustion takes place at constant

pressure and the temperature of air is raised.

Then this product of combustion passes into the gas turbine gets expanded and provides

necessary power to drive the compressor.




The discharge nozzle in which expansion of gases is completed and thrust of propulsion is

produced. The velocity in the nozzle is grater then flight velocity.


a) Explain the working of and ideal single stage reciprocating compressor with neat

sketch. Draw theoretical indicator diagram for it.

8

Answer

Working principle of single stage reciprocating air compressor: A reciprocating compressor

consists of a cylinder, piston, inlet and outlet valves. The arrangement of compressor is shown in

figure.

During downward motion of piston, the pressure inside the cylinder falls below the atmospheric

pressure and inlet valve is opened due to the pressure difference. The air is taken into the cylinder

until the piston reaches bottom dead centre position.

As the piston starts moving upwards, the inlet valve closed and pressure starts increasing

continuously until the pressure inside the cylinder is above the pressure of the delivery side which

is connected to the receiver. At the end of delivery stroke small volume of high pressure

Figure: Single Stage Reciprocating Compressor




air is left in the clearance space. The high pressure air left in the clearance space expands as the

piston starts moving downwards and pressure of air falls until it is just below the atmospheric

pressure. The inlet valve opens as the pressure inside the cylinder falls below the atmospheric

pressure and the air from outside is taken in and the cycle is repeated. The P-V diagram for single

stage and single acting reciprocating air compressor with clearance and without clearance is as

follows:

Figure (a) With clearance Figure (b) Without clearance

(Figure 4 marks, indicated diagram 2 marks & explanation 2 marks)




b) A composite wall is formed of 10cm steel plate 6cm layer of asbestoes and 4cm layer of

fiber glass. The wall is subjected to temperature 10000C outer steel face and 250

0C at

outer fiber glass face. Calculate heat flow per square meter area of wall. The thermal

conductivity of steel, asbestoes and fiber glass ass 400, 100 and 25 w/m0c respectively.

Calculate the interface temperature.

8

Answer:

Data

Ksteel =400 W/m0c

Kasb=100 w/m0c

Kfg =25 w/m0c

Lsteel = 10 cm = 0.1 m

Lasb=6 cm =0.06 m

Lfg = 4 cm = 0.04 m




T1 = 10000C

T4 = 2500C

2

3

41

3

/ 4.306122

10 45.2

2501000

10 45.2

25

0.04

100

0.06

400

1.0

A

1

mWA

Q

A

xQ

R

TTQ

A

xR

AK

L

AK

L

AK

L

RRRR

Q

ST

Q

theq

theq

fg

fg

asb

asb

steel

steel

fgasbsteeltheq

Heat flow per square meter of area wall = 2/ 4.306122 mW ( 4 marks )

steelR

TTQ

21

AK

L

TTQ

steel

steel

21




400

1.0

1000 2T

A

Q

4.306122

400

1.0

1000 2T

T2 = 923.46 0C interface temperature between steel and asbestos ( 2 marks )

AK

L

TTQ

asb

asb

32

100

06.0

46.923 3T

A

Q

4.306122

100

06.0

46.923 3T

T3 = 739.72 0C interface temperature between asbestos and fiber glass. ( 2 marks )

c) A sample of coal has following composition by mass, carbon =80%, Hydrogen=5%

Oxygen=6% Nitrogen=2.5% Sulphur =1.5% and ash 5%. Find higher and lower

calorific value per kg of coal. Write dugong’s Formula.

8

Answer:

Data

Carbon C = 80% = 0.8

Hydrogen = H2 = 5% = 0.05

Oxygen = O2 = 6% = 0.06




Nitrogen = N = 2.5% = 0.025

Sulphur = S =1.5% = 0.015

Ash = 5% = 0.05

Dulong’s formula is H.C.V. of coal = 33800 C + 144500 ( H2 - O2/8 ) + 9300 S KJ / Kg

( 2 marks )

H.C.V. of coal = 33800 C + 144500 ( H2 - O2/8 ) + 9300 S KJ / Kg

=33800 x 0.8 + 144500 (0.05 - 0.06/8) + 9300 x 0.015

= 33320.75 KJ / Kg ( 3 marks )

L.C.V. of coal = H.C.V.- 9H2 x 2442 KJ / Kg

= 33320.75 – 9 x 0.05 x 2442

= 32221.85 KJ / Kg ( 3 marks )


a) i) Write steady flow energy equation for turbine and boiler.

ii) State Zeroth law and Kelvin plankes statement of second law of thermodynamics.

8

Answer:

General steady flow energy equation is

Steady flow energy equation for turbine:

w = (h1 – h2) +

Steady flow energy equation for boiler:

q = h2 – h1

Steady flow energy equation for compressor:

w = (h1 – h2)

Steady flow energy equation for Nozzle:

1 for

each




= h2 – h1

ii).Zeroth law:

When two systems are each in thermal equilibrium with third system, then two systems are also in

thermal equilibrium with one another. ( 2 marks )

Kelvin plankes statement: It is impossible to construct an engine working in a cyclic process,

whose sole purpose is to convert heat energy into equivalent amount of work. ( 2 marks)

b) Explain the factors which are considered for site selection of I.C. engine (Diesel)

power plant. Also state its application.

8

Answer:

Factors for considered for site selection of I.C. engine (Diesel) power plant ( 6

marks )

1. Availability of raw material

2. Nature of land

3. Cost of land

4. Availability of water

5. Transport facilities

6. Availability of labour

Application of Diesel Power Plant ( 2

marks )

The diesel power plants find wide applications in following plants

1) Peak load plants 2) Mobile plants 3) Stand by units

4) Emergency plant 5) Nursery station 6) Starting statio




c) Explain with a neat sketch working of Junker’s gas calorimeter. 8

Answer:

Junker’s gas calorimeter

This type of calorimeter is used to directlly determine the calorific value of gaseous fuels. Figuer

shows constructional features of the Junker’s gas calorimeter. it is constructed of thin sheet of

copper and consists of inner chamber known as combustion chamber which contains gas burner.

The combustion chamber is surrounded by a suitable annular space for the escapement of heated

gases after giving their heat to water passing through water jacket as shown in fig. A metered

quantity of gas is supplied at constant pressure to the burner through gas pressure regulator.

The burnt gases travel up through inner chamber down the annular space and out, at

approximately room temperature. The water enters as shown leaves at the top. The used to heat

definite quntity of water is accurately measured by gas meter and temperature of water is measured

with thermometers. From these readings heat content per m3 of gas is calculated.

( Figure - 4 and Explanation – 4 )




Q 6) Attempt any TWO of the following:

a) i) Compare liquid fuel and Gaseous fuel.

ii) Compare ultimate analysis and proximate analysis

16

i) Compare liquid fuel and Gaseous fuel. ( 4 points 1 marks for each)

Sr.No. Liquid fuels Gaseous fuels

01 Lower storage capacity required Large storage capacity required

02 They are not free from liquid impurities They are free from liquid impurities

03 Calorific value of liquid fuel is

expressed in kJ/kg

Calorific value of gaseous fuel is

expressed in kJ/m3

04 To determine calorific value Bomb

calorimeter is used

To determine calorific value Junker’s gas

calorimeter is used.

05 e.g. Petrol, Diesel, Kerosene etc. e.g. Natural gas, LPG, CNG etc.

ii) Compare ultimate analysis and proximate analysis ( 2 marks for each)

Sr.No. Ultimate analysis Proximate analysis

01 Ultimate analysis is coal is complete

breakdown of coal into chemical

constituents

Proximate analysis is coal is complete

breakdown of coal into physical

constituents

02 This analysis gives percentage of carbon,

hydrogen, oxygen, sulpher and ash.

This analysis gives percentage of moisture,

volatile matter, fixed carbon and ash.

8




b) Sketch and explain the air preheater. 8

Answer:

Air preheater is used to recover heat from exhaust flue gases. It is installed between economizer

and chimney. The air required for the purpose of combustion is drawn through air preheater where

its temperature is raised.

A schematic sketch of tubular type air preheater is as follows

After leaving the boiler or economizer the gaseous products of combustion travel through inside of

the tubes of air pre heater I the direction opposite to that of air travel and transfer some of their heat

to the air to be supplied to the furnace. Thus air gets initially heated before supplied to the furnace.

The gases reversed their direction near the bottom of air heater and suit hopper is fitted to the

bottom of air heater casing to collect soot.

( Figure - 4 and Explanation – 4 )




a) i) State the functions of following safety valve, fugible plug, blow off cock, water level

indicator.

ii) Sketch and explain working of screw compressor.

8

Answer:

i) Functions ( 1 marks for each )

Safety valve: To permit the steam in the boiler to escape to atmosphere when pressure in the steam

space exceeds a certain specified limit

Fusible plug: To extinguish fire in the event of water level in the boiler shell falling below a

certain specified limit to protect the fire tube.

Blow off cock: To drain out the water from the boiler for internal cleaning, inspection or other

purposes

Water level indicator: To ascertain constantly and exactly the level of water in the boiler shell or

to indicate water level in the boiler.

ii) ( Figure - 2 and Explanation – 2 )

Fig Screw Compressor

Screw Compressor-

It consists of two mutually engaged helical grooved rotors which are suitably housed in a




casing. Out of two rotors male rotor is driver and female rotor is a driven.

Male rotor has four lobes and female rotor as six flutes.

During rotation of rotor, air enters and takes space between male and female rotor. This air

traps and moves axially and radically with rotation of rotors and gets compressed due to

volume reduction.

Then this air discharged from upward direction. Speed of rotors is different due to different

number of lobes and flutes.

It handles 3.5 to 300 m3/min and maximum pressure ratio of 20. This system requires

lubrication. This compressor is noisy I operation. Used in refrigeration industry.

Documents

SUMMER 13 EXAMINATION 12099 Model Answer …msbte.engg-info.website/sites/default/files/12099_Summer_2013...Answer: Reciprocating compressor is called as positive displacement compressor