LinkedIn Insight Jan 19 Solution - Grad Plus

Jan 19 Solution

January-2019

Elements of Mechanical Engineering (18ME15)

Time-3 Hours

Maximum Marks- 100

Note : 1. Answer Five full questions, chosing ONE full question from each module.

2. Use of Thermodynamics data hand book is permitted.

Module-I

Q.1. a) Explain briefly the principle of conversion of solar energy directly into electrical energy in a solar cell. [10 M]

Ans. In a solar cell, there is conversion of solar energy into electrical energy. Such an example of solar cell is Photovoltaic cell. Photovoltaics directly convert solar energy into electricity. These cells works on the principle of photovoltaic effect. When certain materials are exposed to light, they absorb photons and release free electrons. This phenomenon is called as the photoelectric effect.

The basic photovoltaic system for power generation is as follows.

  • Using semiconductor materials  the photovoltaic (PV) technology converts energy from solar radiation directly into electricity Since 1970’s.
  • As it does not contain any mechanical part so these system may lasts for decades. It requires less maintainance.
  • Photovoltaic projects may vary from small scale projects like lighting and pumping to the large scale projects like whole building and even utility-scale  photovoltaic farms.
  • We can see that in general solar energy is more expensive per KW as compared to other renewable sources , but it has no. of advantages.
  • As photovoltaic system can be installed at the user site, the full retail electricity rate rate of facility can be offset rather than wholesale power price.
  • It can also offset peak offset electricity rates in warmer climates. Because photovoltaic system matches peak demand efficiently.
  • It can be installed anywhere as it has modular structure and can be installed in any size. The only limitation is that for installation of PV system we need to have availability of sunny roofs or ground space.
  • Photovoltaic technology offers more incentives as compared to other renewable energy technologies.
  • For example, in states that have a solar set aside in their renewable energy source requirement, the renewable energy certificates can be much higher than those provided for other technologies.
  • Federal agencies are considering photovoltaic technologies as a part of new construction project or major renovation.
  • When semiconductor materials are exposed to light, the some of the photons of light ray are absorbed by the semiconductor crystal which causes a significant number of free electrons in the crystal. This is the basic reason for producing electricity due to photovoltaic effect. When light energy, or photons strikes a photovoltaic cell, electrons are knocked loose from a layer in the cell designed to give up electrons easily.
  • The difference between charges which is built into the cell pulls the electrons to other another cell layer before they recombine in their originating layer.
  • This migration of electrons create a charge between layers in the photovoltaic cell
  • Th4e positively and negatively charged layers of a photovoltaic cell through a load e.g light bulb will produce electricity  as the electrons flow through the circuit thus, the bulb will be lighted because the electrons are attracted back to the positive layer of cell.
  • These photovoltaic cell integrated into system create electricity
  • This energy can be used by electric machines, appliances, lights and so on by the conversion of energy through inverter.

So, this is the principle of conversion of solar energy directly into electrical energy in a solar cell.

b) Write a note on wind energy and its conversion. [10 M]

Ans. 1. The wind is a clean, free, and readily available renewable energy source. Each day, around the world, wind turbines are capturing the wind’s power and converting it to electricity. Wind power generation plays an increasingly important role in the way we power our world – in a clean, sustainable manner. Wind energy (or wind power) describes the process by which wind is used to generate electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. A generator can convert mechanical power into electricity. Mechanical power can also be utilized directly for specific tasks such as pumping water.

2. Wind energy is an indirect form of solar energy, because wind is induced by uneven heating of earth surface by the sun.

3. When wind mill is in operation, the wind approaches the blades moves the wind mill shaft thereby rotating the rotor of generator.

4. In this , the kinetic energy is converted into mechanical energy by the wind mill and then converted into electrical energy with the help of a generator.

5. The fig. shows the general layout of wind mill.

Wind power: How to make a windmill that generates electricity

6. Wind carries energy by virtue of its motion. The device which is capable of slowing down the mass of moving air, like a sail or propeller, can extract part of the energy and convert is into useful work.

There are three factors determine the output power generated from the wind mill, they are

(1)   The wind speed

(2)   The cross section of wind swept by rotor, and

(3)   The overall conversion efficiency of rotor, transmission system and generator or pump.

No device, however well-designed, can extract all of the wind’s energy because the wind would have to be brought to a halt and this would prevent the passage of more air through the rotor. The most that is possible is for the rotor to decelerate to whole horizontal column of intercepted air to about one-third of its free velocity.

6. A 100% efficient aerogenerator would therefore only be able to convert up to a maximum of around 60% of the available energy in wind into mechanical energy.

A well-designed blades will typically extract 70% of the theoretical maximum, but losses incurred in the gear box, transmission system and generator or pump could decrease overall wind turbine efficiency to 35% or less.

Advantages of wind energy system.- .

  • wind is available at no cost.
  • While generating power, it  does not cause pollution.
  •  It is available in many off-shore, on-shore and remote areas which is helpful in supplying the electric power in those areas.
  •  It has low maintainance cost and low power generation cost.

Disadvantages- 1. It has low energy density.

  • 2. It requires the favourable weather conditions and the locations is generally preferred outside the cities.
  • 3. The wind supply is variable, unsteady, intermittent and sometimes dangerous also.
  • The initial cost of set up is high.

Q.2 (a) Explain 1st law of thermodynamics. List the similarities and dissimilarities between work and heat.[10 M]

Ans. The first law of thermodynamics can be explained in different forms . They are as follows:

  1. When there is an change in the system, then the algebraic sum of work delivered to the surrounding is directly proportional to the algebraic sum of heat taken from the surrounding. i.e.    [latex] \oint dW\;\propto\oint dQ [/latex]
  2. Both heat and work are mutually convertible one into another  i.e. the energy can neither be created nor be destroyed but converted from one form to another.
  3. There is no any machine which is capable of producing work without expenditure of energy.

Similarities :- 1. Heat and work are both transient phenomenon i.e. When it undergoes a change both cross the boundary of the system.

2. Heat and work are both boundary phenomena i.e.   both are observed at the boundary of the system and they represent energy crossing the boundary of the system.

3. Both heat and work are path functions and do not represent the property of the system.

4. Both heat and work are inexact differentials.

Dissimilarities- 1. Heat can be transferred only when the temperature difference between the system and surroundings will exist. while work transfer can take place without the change in temperature between the system and surroundings.

2. In a stable system, there is no restriction for the transfer of heat. While in a stable system, there cannot be work transfer.

3. Heat cannot be converted into work completely. While work can be converted into the heat completely.

b) Define the following terms in relation to steam.   [10 M]

i) Dryness fraction- The term dryness fraction is related with wet steam. It is defined as the ratio of mass of steam of actual dry steam to the mass of steam containing it. It is usually expressed by the symbol ‘x’ or ‘q’.

If ms=mass of dry steam contained in steam considered. and

mw = weight of water particles in suspension in the steam considered.

Then ,  [latex] x=\frac{m_s}{m_s+m_w}[/latex]
For dry saturated steam, the dryness fraction (x) is equal to 1.

ii) latent heat- The amount of heat required to convert 1 kg of water at saturation temperature to dry saturated steam at constant temperature is called as latent heat.

iii) Degree of superheat- The difference between super-heated temperature (Tsup) and saturation temperature at same pressure is called degree of superheat.

iv) saturation temperature  – Saturation temperature means the boiling point. The temperature for corresponding saturation pressure at which liquid into its vapour phase.

Module-2

Q.3. a) Differentiate between water tube boiler and fire tube boiler. [04 M]

Ans.

Sr. No. Parameter Fire tube Boiler water tube Boiler
1 Principle In fire tube boilers the tube is surrounded by water and the flue gases flow through the tubes. 1. In water tube boilers the flue gases flow through the external surface of the tubes and water flow through the tubes.
2. Working pressure Due to large drum diameters, it works on low pressure upto 20 bar. Due to small drum diameters, it can work at high pressure upto 200 bar.
3. Construction It has simple and rigid construction. It has complex construction.
4. Initial cost The initial cost is low. The initial cost is high.
5. Operation and maintainance cost It has  high operation and maintainence cost. It has  low operation and maintainance cost.
6. Size Size is small and compact. Size is bulky.
7. Transportation and installation Difficult Easy
8. Firing system these are internally fired boilers. These are externally fired boilers.
9. Examples Cochran Lancashire and locomotive boilers. Babcock-Wilcox, La-mont, Benson boilers.

b) List the boiler mountings and accessories and also mention their uses. [06 M]

Ans. Boiler mountings : For the operation and safety of the boiler different fittings and devices are necessary. These devices are called as boiler mountings. For example. Safety valve, feed check valve, water level indicator, steam stop valve, etc.

The table given below depicts the location and functions of various boiler mountings:

Sr. No Boiler mounting Location Functions
1. Bourdon’s pressure gauge It is attached on the upper part of the front end plate. It is used to indicate the steam pressure in the boiler.
2. Safety valves It is attached on the top of front end plate. It is used to release the excess steam when the steam pressure inside the boiler exceeds.
3. Water level indicator It is attached to the lower part of front end plate. It is sued to indicate the water level inside the boiler.
4. Fusible plug It is fitted over the crown of the furnace or over the combustion chamber. It is used to put off the fire in the furnace of boiler when the water level falls below unsafe level.
5. Feed check valve It is fitted to the shell below the water level of the boiler. It is used to allow the supply of water at high pressure to the boiler and prevent the back flow of water.
6. Blow-off cock It is fitted to the lowest part of the boiler shell. It is used to empty the boiler for cleaning, repair and inspection. It is also used to discharge the mud and sediments carried with the feed water.
7. Steam stop valve it is fitted to the highest part of the boiler shell. It is used to regulate the flow of steam from he boiler to the engine and shut off the steam flow when not required.

Boiler accessories- the auxillary parts which are used to increase the overall efficiency o the plant are called as boiler accessories. For example. Economiser, air-preheater, water feeding equipment, superheater, etc.

The table given below depicts the function and location of various boiler accessories.

Sr. No Boiler accessory Location Function
1 Economiser It is fitted at the passage of flue gases from the boiler to chimney. It extracts the waste heat of the chimney gases to preheat the water before feeding into the boiler. This reduces fuel consumption.
2. Air preheater It is placed after the economiser and before the gases enter the chimney. It extracts the waste heat of the flue gases and pre-heat the air supplied to the combustion chamber. It reduces fuel consumption.
3. Superheater It is fitted in the path of flue gases flowing to the chimney. It is used to increase the temperature of the steam above its saturation temperature by passing the steam through a small set of tubes and hot gases over them.

c) With neat sketch explain the working of Babcock and Wilcox boiler. [10 M]

  • Ans. Babcock and Wilcox boiler is a water tube boiler as the water is inside the tubes and hot flue gases flows over the tubes.
  •  Fig. shows the diagram of Babcock and Wilcox boiler with its different functional parts.

Water Tube Boilers: Babcock and Wilcox boiler

  • The boiler shell consists of high quality of steel and is placed longitudinally. It is known as water and steam drum. Thw water level in the drum should be kept slightly above the centre.
  • The drum is connected by short tubes with uptake header and by long tubes with downtake header.
  •  A series of water tubes are connected to the uptake and downtake header at an angle of 15° to horizontal. This inclined position helps to make water flow.
  • The grate is arranged below the uptake header. The fuel is supplied to grate through fire door. The fuel is burnt and forms flue gases.
  • These flue gases are forced to move in a specific path over the water tube due to baffle arrangement.
  • The heat is transferred from hot flue gases to the water tubes and bottom cylindrical surface of the drum.
  • The water flow is set due to density difference in the water.
  •  The portion of water tubes at uptake header is subjected to the high temperature hot gases so the temperature of the water in this section rises due to decreased density and it flows to the drum via uptake header.
  • Simultaneously the water enters into tubes through downtake header and flow of water sets.
  •  The water and steam are seperated in the drum and as the steam is lighter it is collected in the upper portion of drum.
  • To improve the quality of steam the superheater is placed between water drum and tubes.
  • The steam formed in the drum is passed through the superheater and it becomes superheated.
  • This superheated steam is then supplied to the turbine for electricity generation.
  • The evaporative capacity for this boiler is upto 40000 kg/hr and operating pressure is 11 to 17 bar.

OR

Q.4 a) With a neat sketch explain the working of Pelton Wheel. [10 M]

Ans. 1. The Pelton turbine is a type of impulse turbine; These  turbines convert the pressure energy of water into kinetic energy entirely in the distributor. During the conversion, the water jet is accelerated in a nozzle and directed onto the blades of the Pelton wheel tangentially.

2.Components of the Pelton Wheel Turbine: The Components of Pelton Wheel Turbine are as follows.

  1. Penstock
  2. Nozzle and Spear
  3. Runner and buckets
  4. Casing
  5. Breaking Jet

1.Penstock- It is a channel or pipeline which controls the flow of water or it also acts as directing medium for the fluid flow.

2.Nozzle and Spear- The nozzle is used to increase the kinetic energy of water which is used to strike the buckets attached to the runner.

Spear is used to control the quantity of water striking the buckets. It is a conical needle installed inside the nozzle to regulate the water flow that is going to strike on the buckets or vanes of the runner. It is operated by a handwheel.

The rate of water flow increases and decreases when the spear is moved in a backward direction and forward direction respectively and that can be handled by means of a hand wheel(It is operated in the axial direction).

3. Runner and buckets-

  • The rotating part of the turbine is a runner which is a circular disc and on the periphery of which a number of buckets are evenly spaced.
  • The buckets are made of two hemispherical cups joined together. The splitter acts as a wall joining two hemispherical cups which can splits the water into two equal parts (i.e.on to the hemispherical cups.) deflected through an angle of 160 degrees to 170 degree.
  • The buckets of the Pelton turbine are made up of cast iron, cast steel bronze or stainless steel.

4.Casing- The case(outer cover) in which turbine is placed so that water can not splash outside(surroundings) called casing. The Pelton turbine with the casing is shown in the figure given below.

  • It also safeguard and helps the water to discharge to the trail race.
  • In order to make the casing, Cast iron or fabricated steel plates are used.

5. Breaking jet-

  • The spear is pushed in a forward direction into the nozzle so that there should be no water jet impinging onto the blades of the turbine and making the turbine to stop. but the runner keeps moving due to the inertia.
  • To stop the runner in the shortest period of time, a small nozzle is provided which directs a jet of water at the back of the vanes and that stops the runner of the turbine called as breaking jet.

Working of Pelton Wheel turbine-

Water flows from the nozzle with high kinetic energy along the tangent to the path of the runner and when the jet of water comes in contact with the bucket, it exerts a force on the bucket called as Impulse force.

In order to control the quantity of water striking the runner, the nozzle fitted at the end of penstock is provided with a spear or needle fixed to the end of a rod.

In a Pelton turbine, what does this mean: 'water is to leave bucket without any whirl'? - Quora

b) With a neat sketch explain the working of a Reciprocating pump, state the advantages and uses. [10 M]

Ans. Reciprocating pump is a positive displacement type pump in which liquid is displaced by a piston-cylinder arrangement which is driven by crank and connecting rod mechanism.

Reciprocating pumps are suitable for small capacities and high heads.

Reciprocating pumps can be classified as :-

i) Single acting and ii) double acting

Working of reciprocating pump- Fig. shows the arrangement of single acting reciprocating pump.

Reciprocating Pump – Working Principle, Types, Parts - Unbox Factory

i) Initially the crank is at IDC and starts rotating in clockwise direction. When the piston rotates, the piston moves towards right side and the piston  vaccum is created on the left side. This vaccum helps to open the suction valve and the liquid will be forced from sump to the left side of the piston. Now, when the crank reaches to ODC, the piston is on extreme right side the suction stroke is completed.

ii) At the end of suction stroke, the cylinder is full of liquid. When crank rotates from ODC to IDC in clockwise direction, the liquid will be compressed and high pressure will be created in the cylinder. Because of high pressure delivery valve opens and liquid is passed through the delivery pipe.

iii) At the end of the delivery stroke, the crank is at IDC and piston is on extreme left position.

Reciprocating pumps are used in following applications :-

i) It is used as a feed water pump in boilers, hydraulic jacks, kerosene pumps, hand operated pumps etc.

ii) It is used in industries and agriculture field.

iii) It is also used in service stations for pressure washing.

Advantages of Reciprocating pump-i) They has high delivery head and they can deliver fluid at high pressure.

ii) They are self-priming.

iii)They can provide continuous rate of discharge.

iv) Reciprocating pumps run at much lower operating speed and thus they are suitable for handling viscous fluid.

MODULE-III

Q.5. a) Differentiate between two-stroke and four stroke engine. [04 M]

Ans.

Sr. No Two stroke engine Four stroke engine
1 In one revolution of crank shaft the cycle is completed. In two revolutions of crankshaft cycle is completed.
2. In every revolution of crankshaft power stroke is obtained. In every two revolutions power stroke is obtained.
3. As there is one power stroke in one revolution, power produced for same size of engine is more. As there is power stroke for two revolutions, power produced for same size of engine is small.
4. .Ports are present in these engines. Instead of ports these engines contain valves.
5. As it’s construction is simple and it is light weight, its initial cost is low. As its construction is complicated and due to heavy weight , its initial cost is high.
6. It has dome shape or crown shape piston. It has flat crown.
7. It has low thermal efficiency. It has high thermal efficiency.
8. Consumption of lubricating oil is more. Consumption of lubricating oil is less.
9. It produces noise during operation. it produces less noise during operation.
10 These engines are used in scooters, mopeds, etc. These type of engines are used in cars, buses , trucks etc.

b) Explain with neat sketch construction and working of 4-stroke diesel engine with the help of theoretical P-V-diagram. [10 M]

Ans. i) Dr. R. Diesel introduced the Diesel cycle to analyse the performance of IC engine.

ii) Diesel cycle works at constant pressure when there is heat addition in it and this is the only difference between the Otto cycle and Diesel cycle.

iii) Therefore, Diesel engine is sometimes referred to as constant pressure cycle.

         

Fig. Diesel Cycle

  • Process 1-2 : This process represents reversible adiabatic i.e. isentropic compression process. The air is compressed through the compression ratio V1/V2.
  • Process 2-3: This process represents constant pressure heat addition process where air expands from volume V2 to V3. In actual engine, the heat addition takes place by injecting fuel in it. and that fuel  ignites due to high temperature of compressed air and combustion of fuel takes place at constant pressure.
  • Process 3-4 : Point 3 represents the cut off point of fuel injection. he process 3-4 represents the reversible adiabatic i.e. isentropic expansion.
  •  Process 4-1: This process represents heat ejection at constant volume process and hence the cycle gets completed.

Construction and working of four-stroke diesel engine- The four stroke diesel engine cycle also consists of suction, compression, power and exhaust strokes. .The basic construction of a four stroke diesel engine is same as that of four stroke petrol engine, expect instead of spark plug, a fuel injector is mounted in its place. Except these, there are only small difference between the four stroke petrol engine and four-stroke diesel engine.

i) In C.I engines the air is only drawn during the suction stroke instead of air-fuel mixture drawn in S.I engines.

ii) In C.I engines the fuel is injected by using injector into the cylinder and the high pressure and temperature mixture is burnt. Hence there is no need of spark plug.

What is a 4 stroke Diesel engine?

Construction:

  • A piston reciprocates inside the cylinder
  • The piston is connected to the crankshaft by means of a connecting rod and crank.
  • The inlet and exhaust valves are mounted on the cylinder head.
  • A fuel injector is provided on the cylinder head
  • The fuel used is diesel.

(a) Suction Stroke (First Stroke of the piston)

  • Piston moves from TDC to BDC
  • Inlet valve is opened and the exhaust valve is closed.
  • The pressure inside the cylinder is reduced below the atmospheric pressure.
  • Fresh air from the atmosphere is sucked into the engine cylinder through air cleaner and inlet valve.

(b)   Compression stroke (Second stroke of the piston)

  • Piston moves from BDC to TDC
  • Both inlet and exhaust valves are closed.
  • The air is drawn during suction stroke is compressed to a high pressure and temperature

(c)   Working or power or expansion stroke (Third stroke of the piston)

  • The burning gases (products of combustion) expand rapidly.
  • The burning gases push the piston move downward from TDC to BDC
  • This movement of piston is converted into rotary motion of the crank shaft through connecting rod.
  • Both inlet and exhaust valves are closed.

(d) Exhaust Stroke (Fourth stroke of the piston)

  • Piston moves from BDC to TDC
  • Exhaust valve is opened the inlet valve is closed.
  • The burnt gases are forced out to the atmosphere through the exhaust valve. (some of the burnt gases stay in the clearance volume of the cylinder)
  • The exhaust valve closes shortly after TDC
  • The inlet valve opens slightly before TDC and the cylinder is ready to receive fresh air to start a new cycle.

This is all about the construction and working of 4 stroke engine.

c) A four stroke single cylinder Diesel engine piston diameter 250 mm and stroke 400 mm. The mean effective pressure is 4 -bar and speed is 500 rpm. Diameter of the brake drum is 1000 mm. The effective brake load is 400 N. Find IP, BP and FP. [06 M]

Ans. Given data-d=250 mm=0.25 m, l=400 mm=0.4 m, imep=4 bar=4 × 105 N/m2, n=500 rpm,

db=1000 mm=1m, (W-S)=400 N, n=1

To find : i) I.P ii) B.P iii) F.P

Solution- i) Calculate the indicated power of diesel engine

[latex] I.P= \frac{n\times imep \times l\times A\times \left ( \frac{N}{2} \right )}{60\times 100}[/latex]

[latex] I.P=\frac{1\times 4\times 10^{5}\times 0.4 \left ( \frac{\pi }{4} \right )\times 0.25^{2}\times \left ( \frac{500}{2} \right )}{60\times 100}[/latex]

I.P= 32.7249 kW

ii) Calculate the brake power of diesel engine

[latex] B.P=\frac{(W-S)\pi d_{b}N}{60\times 100}=\frac{400\times \pi \times 1\times 500}{60\times 1000}[/latex]

   B.P   =10.4719 kW

iii) calculate the frictional power of diesel engine

F.P= I.P – B.P= 32.7249-10.4719

=   22.253 kW

F.P= 22.253 kW

OR

Q.6. a) What are the properties of good refrigerant? [04 M]

Ans. The desirable properties are as follows.

i) The refrigerant should have low boiling and freezing point.

ii) The evaporator pressure should be near to atmospheric pressure and condenser pressure should not too high as it would require stronger equipment and hence higher cost.

iii) The condenser and evaporator should maintain positive pressures to avoid entry of air and moisture in the system.

iv) For higher COP critical temperatures of the refrigerant should be very high and critical pressure should be low.

v) Latent heat of vapourization of a refrigerant should be as large as possible. Hence the amount refrigerant required as well as the initial cost of refrigerant reduces.

vi) Refrigerant should have high thermal conductivity.

vii) Refrigerant should not be toxic to produce health hazards.

viii) The refrigerant used should not be inflammable and explosive.

ix) Refrigerant should not be chemically reactive with material used for storage containers and piping. For example, ammonia reacts with the copper and its alloys in presence of water.

x) Refrigerant should have high COP, low power consumption per tonne of refrigeration.

xi) Refrigerant should be easily available at low cost.

b) Explain with neat sketch working principle of vapour compression refrigeration. [10 M]

Ans. i) The most commonly used method of refrigeration for refrigerators, air conditioners is Vapour Compression refrigeration.

ii) In this VCR system, the liquid refrigerant boils in the evaporator which at low pressure, by absorbing latent heat.

iii) The vapours formed gets condensed in  condenser which is at high pressure by rejecting latent heat.

iv) Thus, in this cycle heat is transferred in the form of sensible heat as well as latent heat which gives higher COP.

v) Fig. shows the simple vapour compression refrigeration system.

vi) The commonly used refrigerants are NH3, R-11, R-12, R-22. In modern refrigerators Freon-22 and 134-1 refrigerants are used.

Draw a neat block diagram of “vapour compression cycle”. | Mechanical Engg Diploma Topicwise Paper Solution

vii) Simple vapour compression refrigeration system consists of four different processes:

i) Compression

ii) Condensation

iii) Expansion

iv) Evaporation (vaporization)

  1. Compression- When there is suction stroke of the compressor, low pressure vapour in dry state is drawn from the evaporator. here, the temperature and pressure of vapour increases until the vapour temperatures is greater than the condenser temperature.
  2. Condensation- when there is condensation, high pressure refrigerant vapour enters the condenser where the cooling medium absorbs the heat and converts the vapour into liquid.
  3. Expansion- After condensation, the liquid refrigerant is stored in the receiver and from receiver it is passed to evaporator through expansion or throttle valve. this valve reduces the pressure by keeping the enthalpy constant( Throtting process).
  4. Evaporation( Vapourization) – After expansion, low pressure liquid refrigerant enters in evaporator where considerable amount of heat is absorbed by it and converted into vapour. This low pressure vapour is sucked by the compressor and the cycle repeats.

This is all about the Vapour Comparison refrigeration.

c) Explain the following. [06 M]

i) Refrigeration effect

Ans. Refrigeration is defined as the branch of science that deals with the process of reducing and maintaining the temperature of that space or material below the temperature of surroundings.

The system maintained at lower temperature is called as refrigerated system and equipment used to produce this is called refrigerator.

The cooling effect produced by refrigerator is termed as refrigerating effect. and the working substance used to produce this effect is know as refrigerant.

ii) Ton of refrigeration-  

Ans. The capacity of refrigeration system is given in Tons of Refrigeration. A ton of refrigeration is defined as the quantity of het required to be removed, to form one ton of ice at 0°C in 24 hours when initial conditions of water is 0°C  because some cooling effect will be obtained by melting the same ice.

S.I unit of 1 ton of refrigeration is

1 TR= 12660 kJ/hr= m211 kJ/min

or 1 TR= 3.517 kJ/sec = 3.517 kW

1TR= 3.517 kW

TR actually measures the rate of heat transfer.

iii) COP

Ans- COP of refrigerator is define as the amount of heat extracted from refrigerator to work supplied.

Let QA= Amount of heat extracted in refrigerator

Wnet= Net work supplied to system

COP=QA/Wnet

Module- IV

Q.7. write a note on application of ferrous and non-ferrous alloys. [06 M]

Ans. Ferrous alloys contain iron whereas non-ferrous do not contain iron in it.

Steel is one of the example of ferrous alloys.

steels are generally classified on the basis of percentage of carbon (C) as follows :

  1. Low carbon steel
  2. Medium carbon steel
  3. High carbon steel

As the carbon content in steel increases its strength, hardness and fatigue resistance increases whereas, ductility, malleability and toughness decreases.

a) Low carbon steel(0.008-0.30% C) : i) They are soft, ductile, malleable and can be easily formed in required shape.

ii) They can be easily cold worked for applications such as rolling, forging, sheet metal, pressing, drawing etc.

0.02 -0.10% C- stampings, fan blades, rivets, wires, bicycle tubes etc., 0.10- 0.20% C is used in bolts, structural steels for RCC work, grill, fabrication work, free cutting steel, welded structures. 0.20-0.30% C- Cams, crankshafts, gears, valves, carburizing steels.

b) Medium carbon steel (0.30-0.70% C) – They have properties in between low carbon and high carbon steel. They are difficult to cold work and hence are used in hardened condition after heat treatment. Based on the carbon content, they are used in the following applications: 0.30-0.40% C is used in wires, connecting rods, shaft, bolts and nuts, etc. 0.40-0.50% C- Axles, shafts, crankshafts, forgings, etc. and 0.50-0.60% C- It is used in gears, axles etc.

0.60-0.70% C is used in die blocks, valve springs, washers, set screws etc.

c) High carbon steel (0.70-2.00%C) : as there is higher content of carbon they are hard and brittle . They show better abrasive and wear resistance and can be further hardened by  heat treatment. Because of high hardness and wear resistance, they are used as tools and hence referred to as Tool steels. Based on the carbon content they are used in the following applications . If there is 0.70-0.90% C then it is used in hand operated tools, wrenches, chisels, agricultural applications, musical instruments etc. . When 0.90-1.10% C is used then it is used in  dies and tools such as drill bits, cutting blades, milling cutters, etc. When there is 1.30-1.50%C is used then it is used in heavy cutting tool etc.

Another type of alloy is Cast Iron. Cast iron are the alloys of iron and carbon. Generally, commercial cast iron are complex in composition and their carbon content is in the range of 2.3 to 3.7% with other elements like sulphur, manganese, phosphorus and silicon.  Cast iron are formed by melting a metal and casting  with or without machining to the desired final shape and size, hence called as cast iron.

Applications of cast iron- Cast iron are used in the following machine elements :

  • Machine beds, columns, hammers, road-rollers.
  • Pipe-fittings, valves, farm equipments, automotive parts.
  • Camshafts, crank shafts, gears, ordnance parts.
  • Motor covers, pump bodies, furnace parts.
  • Engine frames, piston and cylinder, cylinder blocks and bearing blocks etc.

ii) Non-ferrous alloys- Copper is one of the most widely used non-ferrous metal.

Various alloying elements are added to improve its properties. Major alloying elements are zinc, silicon, aluminium, lead, manganese, nickel, phosporous, tin, magnesium etc.

Applications of copper-  Copper is used in the manufacturing o following pats: electrical parts, heat exchangers, household utensils etc.

Brasses and Bronzes are types of non-ferrous alloys Brass is alloy of copper and zinc and Bronze is an alloy of copper and zinc.

Application of Brasses-It is used in manufacturing of coins, needles, jewellery, condenser tubes. Cartridge cases, headlights reflectors, springs, shafts, nuts, bolts welding rods, machine parts etc.

Bronze is an alloy of copper and tin.

It is used in the manufacturing of following pats: springs, gears, bearings, electrical appliances, bolts, rivets, pressure vessels, bells, marine containers, Valve bodies, ordnance parts, gun barrels, pipe fittings etc.

These are applications of ferrous and non-ferrous alloys.

b) Define composite material. State the advantages and applications of composite material. [05 M]

Ans.

  • Composite material is defined as a material formed by the combination o two or more chemically dissimilar materials with distinct boundaries between them.
  • Composite materials exhibit properties that are vastly different from those of the individual constituents and better suited for a particular applications.

Advantages of composite materials- i) It is light in weight and it has very high specific strength.

ii) It has low coefficient of thermal expansion.

iii) It has has high resistance to corrosion and chemicals like acids.

iv) It has high resistance to fatigue damage with good damping characteristics.

Disadvantage- i) Compression failure occurs at each reinforcing fiber in buckling.

ii) Some composites are brittle and have very low reserve strength.

iii) Composites have relatively low bearing, strength than metals.

c) Differentiate between soldering, brazing and welding. [09 M]

Ans.

Sr. No. parameter welding Soldering Brazing
1 Defination Welding is a joining process of metals by the application of heat and pressure. soldering is  process of joining two or more metal items by melting and flowing a filler metal into the joint below 427°C. Brazing is a metal joining process where a filler metal is heated above 427°C.and distributed between close fitting parts by capillary action.
2 source of heat By electric energy or by flame or chemical reaction. Electric soldering iron or flame. By flame or in furnace.
3 strength of joint strong weak Good
4 Surface finish poor Poor Good
5 Cost expensive cheap expensive
6 Applications
  • ships, aircrafts
  • Automobiles
  • ridge, towers, pressure veseles.
  • Storage tanks etc.
  • Similar and dissimilar metals
  • Electronic component
  • Sheet metal objects like food can, roof flashing etc.
  • wire joints
  • Jewellery components, etc
  • Non- metals to metals
  • Dissimilar metals
  • Porous metal component
  • Joining of carbide tool tips, turbine blades.
  • Pipes and heat exchangers etc.

OR

Q.8. a) Differentiate between open and crossed belt drive. [06 M]

Ans.

Sr. No. Types of belt drive Features
1. Open -belt drive

1. When both the driving and driven shaft are arranged in parallel and rotating direction then this belt is used.

2. Normally, the tight side of the belt should be the lower one, when the distance between the two shafts is large.

2. Crossed belt drive

1. When the driving and driven shaft are arranged in parallel, but rotating in opposite direction then we use crossed belt drive.

2. This type of arrangements provides increased frictional contact between the belt and pulley.

3. It is prone to excessive wear at a point where the belt crosses and rubs against each other.

4. It is used for low speed applications.

b) Enumerate the advantages and disadvantages of gear drive over belt drive. [06 M]

Ans.

Sr. No.  Parameter Gear Drive Belt Drive
1. Drive elements Gear and pinion Belt, driving pulley and driven pulley
2. Power transmission capacity It is high. It is low.
3. Space requirement It requires less space. It requires more space.
4. Lubrication It is required. It is not required.
5. operating noise It is moderate. It is less.
6. Precise alignment It is required It is not required
7. Manufacturing cost More Less
8. Type of drive Positive Non-positive

c) Derive an equation for length of belt in open belt drive. [08 M]

Ans. To designate a belt drive there are two main geometric terms required.

  1. Angle of contact (Angle of Lap)
  2. Length of the belt
  • Consider an open belt drive system as shown in fig.

θs– angle of contact (angle of lap) for driving (small ) pulley in rad

θB– angle of contact (angle of lap) for driving (big) pulley in rad

C= centre distance between driving and driven pulley in mm.

L-= lenth of belt in mm.

Angle of contact (Angle of Lap)

Refer fig. for Δ OsGOB, we have,

[latex] sin \alpha =\frac{O_{B}G}{O_{S}O_{B}}=\frac{(D-d)/2}{C}= \frac{D-d}{2C}[/latex]

[latex] \alpha =sin^{-1}\left ( \frac{D-d}{2C} \right )[/latex]——–(1)

Angle of Contact at driving (small) pulley,

θs=π-2α

or [latex] \theta _{s}=\pi -2sin^{-1}\left ( \frac{D-d}{2C} \right )[/latex]———-(2)

Angle of contact at driven (big)n pulley,

θB=π+2α

or [latex] \theta _{B}=\pi +2sin^{-1}\left ( \frac{D-d}{2C} \right )[/latex]————(3)

Length of Belt

Refer fig, for determining length of belt,

Length of belt,

L=l(arc ABC) + l(CD) +l(arc DEF) +l(FA)

=[latex] \left [ \frac{d}{2}.\theta _{s} \right ]+[C. cos\alpha ]+\left [ \frac{D}{2}.\theta _{B} \right ]+[C. cos \alpha ][/latex]

=[latex] \left [ \frac{d}{2}.(\pi -2\alpha )\right ]+[C. cos\alpha ]+\left [ \frac{D}{2}.(\pi +2\alpha ) \right ]+[C. cos \alpha ][/latex]

[latex] L= \frac{d}{2}.(\pi -2\alpha )+\frac{D}{2}.(\pi +2\alpha ) +2\; C. cos \alpha[/latex]———(4)

Rearranging equation (4) we get,

[latex] L= \pi \left ( \frac{d+D}{2} \right )+ 2\alpha \left ( \frac{D-d}{2}\right )+2\; C. cos \alpha[/latex]

[latex] L=\frac{\pi (D+d))}{2}+\alpha (D-d) +2C. cos\alpha[/latex]——–(a)

For small values of α,

[latex] \alpha =sin\alpha =\left [ \frac{D-d}{2C} \right ][/latex]————(b)

and,[latex]  cos \alpha = 1-2 sin^{2}\left ( \frac{\alpha }{2} \right )\approx 1-2\left ( \alpha /2 \right )^{2}[/latex]

[latex] cos \alpha = 1-2 \left ( \frac{D-d }{4C} \right )^{2}[/latex]

[latex] cos \alpha \approx 1-\frac{(D-d)^{2}}{8C^{2}}[/latex]     ——–(c)

[latex] L=\frac{\pi (D+d)}{2}+\left (\frac{D-d}{2C}\right )(D-d))+2C. \left [ 1-\frac{(D-d)^{2})}{8C^{2}} \right ][/latex]

[latex] L=\frac{\pi (D+d)}{2}+\frac{(D-d)^{2})}{2C}+2C-\frac{(D-d)^{2})}{4C}[/latex]

[latex] L= 2C+ \frac{\pi (D+d)}{2}+\frac{(D-d)^{2}}{4C}[/latex] ——(5)

Module-v

Q.9. a) Explain the following operation on lathe with suitable sketches : [10 M]

i) Turning-

Eccentric turning- i) If a cylindrical workpiece has two seperate axis of rotation one being out of centre to the other, then the workpiece is called as eccentric.

ii) The turning of different surfaces of the workpiece is known as eccentric turning.

iii) For eccentric turning, the workpiece is first mounted on its true centre and turned, hen it is remounted on the offset centre and the eccentric surfaces are machined.

Taper Turning- i) In taper turning, workpiece is rotated on the lathe axis and tool is fed at an angle to the axis of rotation of the workpiece.

ii) The tool is mounted on the compound rest which is attached to circular base.

iii) A circular base is graduated in degrees which can be swivelled and clamped at any required angle.

ii) Knurling

Ans. i) Rolled impressions are generally provided to the outer surfaces of some components such as dumbels, handles, measuring instruments and tools, gauges, etc.

ii) These depressions are provided for better grip as compared to smooth surface.

iii) These indentations are known as knurls and the corresponding surface is known as knurled surface.

iv) The operation performed for producing this knurled surface is called as knurling.

iii) Facing

Ans. i) The process in which the ends of a workpiece are machined to produce a flat surface with the axis.

ii) It involves feeding of the tool perpendicular to the axis of rotation of the workpiece.

iii) The tool used for facing is properly ground and mounted in a tool holder of the tool post.

iv) Thread cutting   

Ans- i) When cutting screws is threaded on a centre lathe is called as thread cutting.

ii) In thread cutting, helical grooves are produced on a cylindrical or conical surface by feeding the tool longitudinally when the workpiece is revolved between the centres  live centre and dead centre).

iii) For thread cutting, it is necessary that, for every revolution of the workpiece the tool should move parallel to the axis of workpiece by a distance equal to the lead of the screw to be cut.

iv) The motion is transmitted from the lathe spindle to the change gears ad finally to the lead screw.

b) Explain the following operation on milling machine with suitable sketches: [10 M]

i) Form milling

Ans. i) Form milling is the operation of machining irregular contours by means of form milling cutter.

ii) The irregular contour may be concave, convex or of any other shape.

iii) The cutting speed of form milling process is 20-30% less than that of the plain milling.

ii) Angular milling

Ans. i) Angular milling is used for machining a flat surface at an angle, other than right angle, to the axis of milling machine spindle. For ex. V-blocks.

ii) According to the type of surface ( single or two mutually inclined), the single or double angle cutters may be used.

iii) Gang milling.

Ans. i) Gang milling is the operation of machining several surfaces of the workpiece simultaneously.

ii) For this, table is fed against the number of cutters having same or different diameters mounted on a common arbor.

iii) This method saves machining time and is widely used in repetitive work.

OR

Q.10. a) Differentiate between open loop and closed loop systems. [06 M]

Ans.

Sr. No. Open loop system Closed loop system
1 This system involves tape feeding, interpretation of information with the help of tape reader, storing data in a buffer storage, converting it into electrical signal and sending this signal to the control unit. This system comprises of an additional feedback device along with transducer, which is accompanied by a comparator.
2. This system is cheaper and simpler. This system is complicated and costly as compared to open loop system.
3. Here, feedback device is absent. Here, feedback device is present.
4. As there is no feedback device, chances of error is always there. As comparator compares the obtained slide motion, chance of error is greatly reduced.

b) Define robot. write down industrial applications of robot. [04 M]

Ans.  i) A robot is a device which is designed in such a way that it can execute one or more tasks automatically with a good speed and precision.

ii) According to the different types of tasks there are many different types of robots to perform those tasks.

iii) Robots are guided can be with an external device like remote control or the control can be embedded within it.

iv) Robots are replacing humans in many ways. They can perform dangerous and risky tasks which humans cannot perform and generally they do not prefer to do because of the size limitations.

Applications of Robots – The areas of applications where robot technology is significantly used are as follows.

i) Heavy works- heavy parts or tools that can not be handled by human and can be easily handled by robots.

ii) Hazardous work conditions- Areas where conditions are unhealthy, unsafe and uncomfortable for humans such as foundry practices, welding practices, etc. robots reduce the risks of any mishap.

iii) Repetitive operation- For repetitive and time consuming operations robots are well suited than human operator.

iv) Material Handling- In manufacturing industries robots having gripper are used for moving parts from one position to another.

v) Assembly and inspection- This is highly repetitive and boring operation, hence the robots with small size and capacity ar used.

vi) High production applications- As the use of robots reduces non- productive time, higher production can be achieved.

c) Explain the components of CNC with a block diagram. [10 M]

Ans:-

  • CNC is an acronym for Computerized Numerical Control (CNC) is a Numerical Control system which uses micro-computer as the machine control unit.
  • Presence of microprocessor, RAM, ROM , Input/Output (I/O) devices has raised the automation level in the Numerical control system.
  • In CNC machines, part programs are used to control various functions which are generally entered through the keyboard.

The basic components of CNC are as follows:-

  1. Part Programming and Drawings- The part program is written by  observing the part drawing and the given cutting process parameters like feed, speed, depth of cut etc.
  2. Input/Output Devices -The written part of the program is entered into the micro-computer by using input devices like keyboard, Cd, DVD, etc.
  3. Memory, storage Devices- i) The entered part program is read by the micro-computer which is read by the machine control unit of CNC system.

ii) It controls all the movements of machine tool, actuation of all the drives, coolant supply, etc, of the machine tool.

5. CNC machine Tool- i) It is called the manufacturing arm of CNC machine tool system.

ii) it receives the raw materials and performs various operations which are needed.

iii) For performing these operations it should receive the information from the MCU.

iv) In CNC system, all the operations like spindle start and stop, tool positioning, speed control, etc, are fully automatic.

6.  Feedback Device : i) Feedback device receives feedback from the machine tool and gives it to the MCU.

ii) By using feedback device, the cause can be easily detected and rectified.

These are basic components of CNC. The block diagram of basic components of CNC system is as follows.


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