Treatment of Compressed Air - FRL Systems

Treatment of compressed air involves removing the moisture and impurities in the air after it leaves the compressor.  The air from the compressor can contain many impurities.  These impurities need to be filtered.  Special assemblies are available which can filter the air.

The FRL (Filter Regulator Lubricator) assemblies are examples of such air treatment systems.  The FRL unit consists of a filter which filters the air and removes particulate matter and dust.  The Regulator reduces the air pressure to the desired value while the Lubricator delivers a fine aerosolised spray of oil to the air.

The oil spray in the air may be desired for certain pneumatic tools such as drills and air motors.  The oil serves to lubricate the parts in these equipments.

Oil free Compressors

In compressors, oil is generally used to form a seal in the pockets formed by the rotating lobes and the casing.  The oil serves to prevent the leakage of air and improve efficiency.

However, this results in oil contamination of the air in the outlet.  This becomes an issue if the air is used for medical purposes, food manufacturing, etc.

Oil free compressors do not use oil in the compressing process.  Hence, there is no oil residue in the compressed air.  The casing and the rotor lobes are designed such that the sealing process does not require oil.

Oil will be used in other components of the machine such as the bearings and the gearbox.  However, this oil will not come in contact with the air.

Rotary Air Compressors

Rotary air compressors are a family of compressors in which air is drawn into spaces formed between the rotating lobes in the rotor and the casing of the compressor.  The lobes are designed that they form an air tight pocket with the casing.  The lobes are usually screw shaped.  The screw shaped rotating lobes mesh with each other to form a pocket in which the gas travels.    The air is pushed towards the outlet of the compressor.  The gas or the air is thus compressed.  

Rotary air compressors are used where large quantities of compressed air are required.  

Unlike the reciprocating compressors, where the compression occurs in cycles, the flow in a rotary compressor is steady and continuous.  This reduces pulsation in the gas.  

Function of an Intercooler in Multistage Compressors

A multistage compressor compresses gas or air over a series of stages.  Each stage increases the pressure by a certain amount till the final required pressure is reached.

When the gas is compressed, heat is produced.  This heat causes an increase in the volume.  The pressure thus obtained will be higher than the actual pressure due to the temperature increase.  Heat in the compressor can also damage the components of the cylinder, such as, the gaskets, o rings, etc.

This heat, therefore, has to be removed.  The intercooler is a cooling device which removes the heat as the gas passes from one stage to the next.

The intercooler consists of tubes which are cooled by a cooling medium, usually water.  As the gas passes through the tubes, it gets cooled.

Perfect or Complete Intercooling

If the temperature of the gas leaving the intercooler is equal to the room temperature, the cooling is said to be perfect.  This is known as perfect intercooling or complete intercooling

Imperfect or Incomplete Intercooling

If the temperature of the gas leaving the intercooler is greater than the room temperature, the cooling is said to be imperfect.  This is known as imperfect or incomplete intercooling.

Advantages of Multistage Compressors

Multistage compressors compress the gas or air in more than one stage.  The gas is progressively compressed over many stages to reach the final desired pressure.

The following are some of the advantages of multistage compressors over single stage compressors

1. The volumetric efficiency of a multistage compressor is more as compared to a single stage compressor for the same delivery pressure. 
2. Leakages are reduced
3. The torque required is uniform.  Consequently, the size of the flywheel is reduced.  
4. The cost is reduced for medium and large compressors.
5. Lubrication is easy as the operating temperatures are lesser.

Need for Multistage Compressors

A compresssor draws air into the chamber and compresses it.  The compressed air is then delivered into a storage vessel.  During the process of compression, the gas gets heated.  This heat has to be dissipated.  A single stage compressor is one, which has a single cylinder and a piston or a single centrifugal impeller.

When a large amount of gas needs to be compressed or when the compression ratio is large, it is not practical to use a single stage compressor.  This because of the following reasons

1. The heat developed during the compression process cannot be dissipated.  This can cause damage to the cylinder and components, such as, gaskets, seals, etc.
2. It is not practical to use a single stage compressor as the size of the piston and cylinder will become very large for large capacity compressors.

Volumetric Efficiency of Compressors

The volumetric efficiency is the volume of gas compressed for a specified displacement of the piston.

For a single stage compressor,

Volumetric efficiency = (Volume of gas entering the compressor per minute)/(piston                                                                  displacement per minute)


In a multistage compressor, there are many cylinders, the gas is progressively compressed across many stages.  In a multistage compressor, only the displacement of the low pressure cylinder is taken into account.  Therefore, for multistage compressors

Volumetric effficiency = (Volume of gas entering the compressor per minute)/(displacement of                                                low pressure piston per minute)

Classification of air compressors

Air compressors compress the air in the atmosphere to a high pressure and deliver it to a storage vessel.  This compressed air is used in a wide range of applications, such as, manufacturing, control systems, medicine, etc.

Compressors can be classified on the basis of different criteria.

They are

based on the mechanism of opertion.

1. Rotary compressor 
2. Reciprocating compressor

based on action

1. Single acting
2. Double acting

based on the number of stages

1. Single stage
2. Multi stage

based on pressure limit

1. Low pressure compressor (delivery pressure up to 1 bar)
2. Medium pressure compressor (delivery pressure from 1 to 8 bar)
3. High pressure compressor  (delivery pressure up to 10 bar)

based on capacity

1. Low capacity compressors
2. High pressure compressors

Classification of Circuit Breakers

Circuit breakers are used in a wide range of applications.  They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria.  Some of the classifications are below

Based on the interrupting medium

1. Air circuit breaker
2. Oil circuit breaker
3. SF6 circuit breaker

Based on type of action

1. Automatic circuit breakers
2. Non-automatic circuit breakers

Based on the method of control

1. Locally controlled circuit breakers
2. Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)

Based on the type of mounting

1. Panel mounted circuit breakers
2. Remote from panel mounted circuit breakers
3. Rear of panel mounted circuit breakers

Based on location

1. Outdoor circuit breakers
2. Indooor circuit breakers

Based on voltage

1. Low voltage circuit breakers
2. Medium voltage circuit breakers
3. High voltage circuit breakers

Sensitivity of a Relay

The relay in a protection system should be sensitive enough to operate when a fault occurs.  A sensitive relay improves the reliability of the system.

When the parameter exceeds the set value, the relay should start operating.

The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation.

The sensitivity is indicated by a sensitivity factor Ks

where

Is is the minimum short circuit current in the zone and
Io is the minimum operating current for the relay.

The sensitivity of a relay is also related to the VA of the input to the relay.  Lesser the VA of the input, greater will be the sensitivity and vice versa. For instance, a relay which has 1 VA as its measuring input will be more sensitive than a relay, which has 5 VA as its measuring input.

Features of a Protection System

A reliable and effective protection system is a crucial part of any power system.  The protection system protects the equipment in the power system, such as generators, motors, transformers, etc from damage to faults.

The protection system also ensures reliability by localizing and isolating the fault and minimizing its impact on the rest of the power system.

The requirements of a power system are as follows

1. To isolate the equipment or component in which the fault has occurred.  The isolation should be quick enough to prevent damage to the component itself.  For instance, a short circuit inside can severely damage a transformer.  The differential relay, in such a situation, should immediately act and trip the transformer.

2. To isolate the smallest possible section of the power system to minimize the interruption.

3. To prevent disturbance or disruption to other parts of the power system.  The fault, if not isolated in time, can cause the upstream breakers to trip.

A well-designed protection system will greatly increase the reliability and performance of a power system.

Modelling the Components for a Load Flow Analysis

The Load Flow Analysis is done to determine the flow of real and reactive between different buses in a power system.  It also helps in determining the voltage and current at different locations. 

To conduct a Load Flow Analysis, the components in a power system need to be modelled.  The modelling is done by developing equivalent circuits of the components, such as the generator, transmission lines and line capacitances.


The Generator equivalent circuit is shown below.

The Thevenin equivalent circuit.

This consists of a voltage source and a resistance and an inductance in series with the load.



E = V + IZ

where Z is the steady stage impedance

The Norton equivalent circuit consists of a power source and an admittance in parallel.

INorton = V/Z

INorton = YV

Load

The load is modelled as a resistnce and inductance in a series circuit that is earth

Transmission lines

Transmission lines are modelled as

Short (less than 80 km)
Medium (80 to 250 km) or
Long lines ( 250 km and above)

Causes of faults in a power system.

The faults in a power system can be caused by a wide range of causes.  Below is a list of some of the most probable causes for electrical faults.

Overvoltage is caused due to surges such as lightning or due to switching loads on and off.  In generators, it can be caused due to the failure of the field controller.

Heavy winds which can cause lines to snap or trees to fall on them. This can cause open circuits, earth faults and short circuits.

Ageing which can result in weakening of insulation and failure. This can result in short circuits and earth faults.

Chemical pollution and deposition on the insulators that can result in flash overs

Faults due to wildlife, such as birds, snakes, mice, etc which can cause short circuits and earth faults.

Collision of vehicles on towers and transmission poles can cause the towers to fall.

The table below will give an idea of the percentage of the types of faults in the different equipment in a power system

Overhead lines                      50%
Transformers                         12%
Switch gear                            15%
Cables                                   10%
Miscellaneous                         8%
Instrument Transformers        2%
Control Equipment                 3%

Silicon Controlled Rectifiers - An Overview

Silicon Controlled Rectifier

A silicon controlled rectifier is a three terminal electronic component.  It consists of an anode, a cathode and a gate.

The device is similar to a diode, except that it needs to be switched on with an external voltage applied to the gate during the positive half cycle.  Once, the SCR has been trigerred, it "fires" and conducts as long as it is positively biased.

In AC circuits, during the negative half cycle, the SCR switches off automatically.

SCRS find application in applications where the current needs to be controlled.  They are used in many electronic equipments, such as inverters, converters, speed controller, etc.

Gate Triggering methods SCRs

There are different methods of trigerring the gate of a Silicon Controlled Rectifiers (SCRs).  They are

Using DC Voltage
By applying a positive voltage to the gate with respect to the cathode, the junction J2 can be forward biased.  This will switch on the SCR.  This process requires a constant dc voltage to be applied between the Gate and the Cathode, which is a disadvantage.  Besides, there is no isolation between this triggering dc voltage and the main dc supply

Using AC voltage
In AC applications, the trigger voltage can be obtained from the AC voltage suitable reduced.  The phase of the AC voltage is modified and applied to trigger the SCR at the desired instant.  The SCR will continue to conduct till the negative half cycle.

A separate transformer is required for the trigger circuit which increases the cost

Pulse triggering
This is the most widely used form of triggering.  In this method, a pulse of a small duration is applied to the gate to switch it ON.  Sometimes, a series of pulses are applied.  The pulse need not be continuous.  This reduces the losses in the gate.

Natural and Forced Commutation in SCRs

Commutation in dc machines refers to the changing of current flow from one circuit to another.  In SCRs ( Silicon controlled Rectifiers) and thyristors, it refers switching off a conducting electronic component.

In AC circuits, SCRs are commutated by the negative half cycle which reverse biases the anode and cathode terminals.  This is known as natural commutation.

However, in DC circuits, special circuits should be designed to switch off the SCRs once, they have been switched on.  The current is reduced to zero by means of external circuits.  This is known as Forced Commutation

Methods of turning on an SCR

The SCR cannot switch on on its own once its anode and cathode are connected to the positive and negative terminals respectively.

The following are some of the methods.

Gate Triggering

This is the most popular method.  A single pulse or a train or pulses are applied to the gate terminal of the SCR.  This creates a forward bias across junction J2 and switches on the SCR.

Thermal Triggering
When the SCR is heated above a certain value, more hole-electron pairs are produced this increases the charge carriers and can cause the SCR to switch on.

Light Triggering

When light is made to fall on the junction in reverse bias, hole-electron pairs are created due to the energy of the incident light.  This can cause the SCR to fire.  Special components such as LASCR (Light activated Silicon controlled Rectifiers) and LASCS ( Light activated Silicon controlled Rectifiers) work on this principle.  This method of triggering is cheaper when designing components of higher ratings.  The light is conducted to the junction by means of optical cables.

dv/dt triggering
In this method of triggering, a rapid change in the voltage current to flow through the junction J2 which acts as a dielectric between two conductive junctions (J1 and J3).  The SCR will switch on even if the voltage is low provided the rate of change of the voltage is high.

Advantages and disadvantages of grounding the neutral

The advantages of grounding (earthing) the neutral are as follows

1. Sensitive current protection schemes can be used to quickly identify an earth fault.
2. The external surges and overvoltages caused by lightning or switching are discharged to the ground.  If the neutral is not grounded, these waves will get reflected and cause overvoltages in the system.
3. The phase voltages are within limit and the value is the voltage between the phase to ground.
4. Arcing grounds, which occur at the location of an earth fault are avoided.

The disadvantages of grounding the neutral are

1. The system will trip even for a minor earth fault.  This affects the reliability of the power supply.
2. The zero sequence currents which flow through the neutral can cause interference to telecommunication lines.

Generator Neutral Breaker

The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance).  In such systems, the fault current in the line due to an earth fault will be high.

The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral.  This breaker is opened simultaneously with the armature and the field breaker.  This will bring the fault current to zero quickly.

Pre Insertion Resistors in Circuit Breakers

Circuit breakers used in switching of long transmission lines have a resistors which is pre-inserted between the contacts before the contacts are closed. This resistor is called the Pre-insertion resistor. The function of this resistor is to limit the initial charging current of the line. The resistance of the line is around 500 ohms.

Once the closing command is given to the breaker, the resistor is first connected across the contacts. This resistance in series limits the line current. A few milliseconds later, the contacts are closed. 

While opening the breaker, the pre-insertion resistor is first disconnected before the contacts are opened by the circuit breaker. Pre-insertion resistors are also used in lines which have transformers to limit the high inrush current.

Capacitance Switching of Equipment

Capacitance is the phenomenon of holding electrostatic charge. In electrical systems, long transmission lines, power cables and capacitor banks can have large amounts of capacitance. In a circuit containing capacitance, the current will lead the voltage by 90 degrees. This means that at the instant of the current zero crossing, the voltage across the breaker contacts will be the maximum. If a circuit is isolated at this instant, the high system voltage will be retained by the line capacitances. If the breaker is opened when the current is zero and the voltage is maximum, half a cycle later when the supply voltage reaches maximum in the opposite direction, the voltage across the breaker contacts will be 2V. This can result in a restriking voltage being developed and a flashover occurring across the circuit breaker. Once the flashover due to the restrike occurs, oscillations are set up in the line between the system inductance and the capacitance. These oscillations and the restrikes they cause can result in the line voltage reaching up to 4 times the voltage (4V). Hence, in lines with high capacitances, air blast circuit breakers or multi break circuit breakers are used for isolation.

Resistor switching of an SCR

A Silicon controlled Rectifier (SCR) is a semiconductor device which conducts in only one direction.  It has three terminals.  An anode, a cathode and a gate.  Unlike a diode, however, it needs to be switched on by a pulse applied to the gate.  

The circuit below shows the method of switching on an SCR using a resistor.  The power source is connected across the SCR.  The gate voltage is provided by the voltage divider circuit.  The variable resistor, R4 is used to control the firing angle.  

The Diode D1 prevents negative voltage from reaching the gate during the negative half cycle.  The SCR will be switched off during the negative half cycle by the supply voltage

Circuit Diagram - Resistor switching of an SCR

Transient Stability Analysis

The Transient Stability Analysis is done to determine the behaviour of a system during transients or sudden changes in a power system.

Transients occur when there are power flows from one source to another.  They also occur during faults when a load or a generating point is cut off.  This can cause oscillations in the voltage or power.  Many of these oscillations will quickly get resolved and the system will return to its steady state operation.

However, in certain situations, the oscillations may can increase in severity and can cause fluctuations in the voltage or power which can affect the system and can cause trippings.  Hence, it is necessary to ensure that the system will be stable in the event of a transient.

The stability of a system is classified into

Steady State Stability and
Transient Stability

Steady state stability is the ability of the system to respond to small oscillations in the voltage or slow changes in the load.

Transient stability is the ability of the system to respond to sudden, unexpected changes such as the tripping of a power source or a fault in a tranmission line.

Transient stability analysis is used in relay setting and in determining the clearing time of breakers.  They are also used to determine the voltage level of a power system and the power transfer capability between different systems.

Fault Analysis

A Fault Analysis is a study which describes the fault currents and the behaviour of a power system during an electric fault.  Faults may be line to line faults or line to ground faults. The fault analysis provides information about the  The Fault Analysis is used to determine the ratings of fuses and circuit breakers.

Using the fault analysis, we can determine the maximum current which will be developed during a fault.  The bus bars, breakers and other transmission equipment should be equipped to withstand the heavy current which flow during a fault.

The protective relays are set based on the current calculated during the fault analysis.

Load Flow Analysis

The Load Flow Analysis is done to determine the voltage, current, real and reactive power in a particular point in a power system as well as the flow from one point to another. 

The Load Flow Analysis helps understand the operation and behaviour of the system when a generator trips or when a big load is suddenly cut off.  Load Flow Analysis also helps identify routes to transfer power when a transmission line has to be isolated due to a fault.  This ensures reliable power supply and ensures quick restoration in the event of blackouts. 

Load Flow Analysis is done during the design of the power system.  It should also be done before any modification of the power system such as the addition of loads or generating units.

Components and Structure of a Power System

A Power system has three main components
They are

• The Generating System
• The Transmission System 
• The Distribution System

Generating System
The Generating System is the source of the power.   The generation can be from generators, solar panels, etc.  Power can be generated from different sources such as hydropower, wind turbines, nuclear plants,etc.

Components: Synchronous Generators, induction generators, solar panels,

Transmission System
The transmission system transmits the generated power over large distances to the distribution centres such as industries and cities.  The distribution areas can be thousands of kilometres away from the generating stations.  The voltage is stepped up to high values to minimize the losses using transformers.  The power is then transmitted through the power lines to the distribution areas.

Transmission systems can be categorized into

Primary Transmission Systems, which transfer power at voltage of 110 kV and above.  These lines are hundreds of miles long.  They are connected to secondary receiving substations

Secondary Tranmission Systems, which receive the power from the primary transmission system send it to the distribution systems.  The voltage levels in the secondary transmission systems are about 33kv to 66kV

Components: Transformers, Circuit Breakers, Overhead Transmission Lines, Underground Cables.

Distribution Systems
The distribution system receives power from the transmission system and distributes the power to the individual customers at the required voltage.  The industrial supply voltage can be 33kV or 11kV.  The domestic supply voltage is 440 or 220V

Components: Transformers, underground and overhead transmission lines.

Power System Analysis

A Power System Analysis is a very important exercise in the design and operation of a power system.  The Power System Analysis is used to evaluate the performance of a power system. 

Power System Analysis deals, chiefly, with three important parts

They are

1. Load Flow Analysis
2. Short Circuit Analysis and 
3. Stability Analysis

A Power System Analysis helps the following aspects.

1. Study the ability of the system to respond to small disturbances caused by the application/removal of small or large loads.
2. Design of the breakers and isolating equipments.
3. Plan for future expansion of the power system
4. Study the response of the system to different fault conditions.
5. Observe and monitor the voltage, real and reactive power betwee different buses.
6. Calculate the setting of the relays and the design of the protection system.

Power Balance Equation

The power balance equation describes the relation between Power Demand and Power Generation in a power system.

The equation is

Where

P_D is the Total Power Demand
P_G is the output of individual generating stations

The sum of the power generated should equal the demand for power.

Welding Slag

Welding flux is used to protect the weld area from contamination.  The flux forms a protective layer over the surface.  In certain cases, it is left as a residue over the weld area.  This is called welding slag.

Welding Slag has to be removed in order to be able to view the welded area for inspection as well for aesthetics and visual appearance.  Welding slag can be chipped away with a pointed hammer.  Sometimes, it is also removed by grinding.

Welding Flux

Welding flux is a substance used during the welding process to shield the weld area from atmospheric contamination.  In metal arc welding, the flux is part of the welding rods.

When the arc is initiated, the high temperature melts the work piece and the welding rod.  The flux coating on the rod also melts and forms a protective shield over the weld poor, protecting it from the atmospheric gases.

Shielded metal arc welding

Shielded metal arc welding is one of most popular methods of welding.  Shielded metal arc is simpler than other forms of welding.  It requires an welding transformer kit and an electrode.  Shielded metal arc welding works by creating an electric arc between the work piece and the electrode.

The welding transformer can produce high currents.  Current from the welding transformer passes through a cable and then through the electrode into the work piece which is earthed or connected directly to the transformer.

An arc is generated by touching the work piece with the electrode and then breaking the contact.  The high current which is interrupted now forms an arc as it tries to jump across the gap between the electrode and the work piece.

Shielded metal arc welding is widely used in almost all industries.  It is also used in construction.  Shielded metal arc welding can be used to weld both ferrous and non-ferrous metals.

Porosity in Welding

Porosity in welding is the formation of air pockets or bubbles in the weld area.  Porosity makes the weld weak and can cause failure of the weld area. 

Some of the causes of porosity are

• Improper shielding
• Presence of water or moisture on the weld surface
• Presence of paint, which can get vaporised during the weld process.
• Improper gas flow setting in gas welding

Shielding Gas

Shielding gases are gases used in gas arc welding.  Shielding gases serve to insulate the weld are from gases such as oxygen and water vapour.  This is necessary to prevent oxidation at the weld area at high area and porosity(formation of bubbles in the weld). 

Inert and semi-inert gases are used for shielding.  Examples are Helium, Argon, Carbondioxide, etc.  Shielding gases are generally denser than air (an exception is helium).  They also have good thermal conductivity. 

Rockwell Scale

The Rockwell scale is a scale to measure hardness.  The scale is based on the depth of the indentation produced on a material.  The test works by using an indentor to makes an indentation.

The Rockwell test is a non-destructive testing.  Its setup is easy to install.

The indentation hardness is linearly related to tensile strength.  Thiss permits the quick and reliable testing of bulk materials. 

The test
A minor load is placed on the specimen.  The depth of the indentation formed is noted.  This is the zero point.  A bigger load, called the major load, is now placed upon the minor load.  The depth of the indentation with reference to the zeropoint is noted. 

The depth of indentation and the hardness are inversely related.  A hard material will have lesser depth of indentation while a softer material will have a greater depth of indentation. 

The hardness of a material can be directly calculated from the formula

HR= N- d/s

where N is the Rockwell scale used and s is the scale factor

High speed steels

Hi Speed steels are steels which have high hardness even at high temperatures.  They also have good wear resistance. These steels have molybdenum, vanadium, chromium and tunsten as their constituents.  These elements generally constitute about 7% of the material.  These elements form carbides.

The name High Speed Steels was used as they cut faster than other types of steels.

High speed steels are categorized after the name of their constituents, such as Molybdenum high speed steels, Vanadium high speed steels, Chromium high speed steels and so on.

High Speed steels are used in cutting equipments, such as blades and saws. They are also used in tool bits and in dies. 

Quenching

Quenching is process of strengthening and hardening steels and alloys.  Quenching is the quick cooling of a hot metal, such that the phase transformation do not occur. The hardness is increased as the crystal grain size is increased. 

Quenching is done in oil or in water.  The medium is chosen depending on the level of quenching desired.  Quenching in water can sometimes cause too much hardness that the material may crack.  In such situations, oil may be chosen.  Salt baths and special polymers are also used in special applications. 

Quenching Rate
Quenching rate is the rate at which the drop in temperature has to occur.  Oil has a quenching rate lesser than water.  It means that the temperature will fall less faster in oil than in water. 

Tempering

Tempering is a heat treatment used to reduce the hardness of a metal and to increase its toughness and ductility. The internal stresses in the material are relieved during the process. 

It is used particularly in iron alloys, such as steel.  It is generally used after quenching.  Quenching causes the material to become hard and brittle.  The metal, in that state, may not be suitable for many applications.

Tempering is done by heating the material and then gradually cooling it.  The heating should be very gradual to prevent the formation of cracks.  The temperature to which the metal must be heated is based on the desired toughness and hardness.   

Galvanizing

Galvanizing is the process of applying a coat of zinc on to a steel surface.  Galvanizing protects the steel from corrosion.  Galvanizing is a very popular method of protection. 

The layer of zinc protects the steel from the environment.  In addition, galvanizing also provides galvanic protection. 

Galvanic Corrosion
Galvanic corrosion when two different metals are joined together.  When placed in a conductive environment, one of the metals becomes the anode while the other metal becomes the cathode.  The anode corrodes and is deposited in the cathode.  The anode thus wears off.

In galvanized steel, the zinc becomes the anode and wears off.  The steel is thus protected.

The coating can be of different thickness depending on the application.  Different surface finishes can also be made.

There are different methods of galvanizing

Hot Dip Galvanizing
Hot dip Galavanizing involves immersing a steel component in a molten bath of zinc.  A coat of zinc is applied to the component.

Electrogalvanizing
Electrogalvanizing involves electroplating a layer of zinc on a steel component. Zinc is connected to a positive terminal of a DC power source while Steel is connected to the negative terminal of the source.  Zinc gets deposited on the cathode. 

Tool Steels

Tools steels are steels which have high hardness and resistance to abrasion. They are alloys with significant amounts of tungsten, molybdenum, vanadium and chromium.  The hardness comes due to the presence of carbides. 

They are used to make tools, dies, molds and hammers using in metalworking.  Knives are also made from tool steels. 

Colour coated steels

Colour coated Steels are steels which have a colored protective coating applied during the manufacturing process.  The color of the coating is based on the requirement.  The coating also serves to protect the steel from corrosion.

Paints, laminates and vinyl dispersions can be used for the coatings.

Color coated steels can be directly used in the applications.  This saves time during the manufacturing  or construction process.  These steels are used in the construction sector, the automotive industry and in making home furniture. 

TMT bars

TMT bars or Thermo Mechanically treated bars are metallic rods widely used in the construction sector as reinforcements for concrete structures. These rods have high strength and ductility. TMT bars have a tough outer region and a soft inner core that impart high tensile strength and elongation point.  TMT bars also have good weldability.

TMT bars are made by rolling steel wires and passing them through water whose pressure at an optimal pressure.  The cooling creates a hard outer surface while the inner core is soft.

They are used in the constructions of houses, bridges, dams, etc. They have high thermal resistance and can withstand high temperatures. 

Galvalume

Galvalume is an alloy of aluminium and zinc (45% zinc and 55% aluminium).  Galvalume is a registered trademark. 

Galvalume has high thermal resistance. It is also resistant to corrosion.  Galvalume can also be easily formed.  It has smooth appearance and is preferred for roofing applications in the construction industry. 

Galvalume is a good reflector and can reflect thermal radiation.  This reduces the cooling costs and increases comfort. 

Drill bits

Drill bits are rod shaped cutting tools which are used to drill holes in metals, wood, plastic and even bone.  Drill bits are fitted to a drilling machine.  The bits are rotated at high speeds and cut by removing material. The material that is cut is removed during the circular motion of the drill. 

Drill bits come in various shapes and sizes.  There are standardized size charts based on which the bits can be chosen.

Drill bits are made using different types of steel.  Drill bits made with low carbon steels are softer and may require frequent sharpening or replacement.  Drill bits can be made with high carbon steel, which is harder.  High speed steels are also used to make drill bits.

Diamond coated drill bits can be used to drill holes in glass and other hard materials.

Hole Saws

A hole saw is a blade which is used for sawing.  It consists of an annular ring.  It makes a hole in the material.  The cut portion is in a circular shape.  The holesaw has saw teeth on the periphery of the ring.  Sometimes, industrial diamonds are mounted on the periphery. 

Holes saws are more efficient than drill bits as all the material need not be removed by drilling.  Once the outer periphery of the material is cut, the material to be removed in pieces. Less power is therefore required. 

The downside is that more torque is required to use the hole saw.  This requires more powerful drills as compared to the drill bit.  Holesaws need to be guided carefully by the operator in the desired direction.  They can easily drift away from the intended direction. 

Industrial Diamonds

Industrial diamonds are used to cut materials.  They are used in cutting, drilling and polishing.    A diamond is a very hard substance.    Hence, it can cut hard substance such as glass and hardened steel which cannot be cut by other materials.  Diamonds can also resist very high pressure

Natural diamonds as well as synthetic diamonds can be used as industrial diamonds.  High grade diamonds are used for jewellery while low grade diamonds can be used for industrial applications. 

Aerosols

An Aerosol is a solution of a liquid in gas.  Aerosols consist of minute particles of liquids which are floating in gas.  Clouds, fog and mist are examples of natural aerosols. 

Other examples of aerosols, we come across in daily life, spray painting, perfume sprays, pesticide spray guns, etc. 

Aerosols find wide applications in the field of medicine.  Many medicines, such as those for asthma, are delivered as aerosols.  Aerosols are effective in delivered precise amounts of medications to the correct location using metered dose inhalers. 

Compressed air is used as the propellant in many applications.  In medicine, aersols are delivered using hydrofluoroalkanes. 

Colloids

A Colloid is a solution that contains a substance which is uniformly distributed in a liquid.  Milk is an example of a colloidal solution.  Other examples are jelly, plaster, muddy water, etc.

A colloidal solution has two main components. They are the colloidal particles and the dispersing medium.  A key feature, which distinguishes colloids from suspensions is that a colloidal solution never settles.  The particles will always remain suspended. 

Colloidal solution exhibits a property called the Brownian movement.  The particles have a random, zig zag motion, when observed under the microscope.  This is due to collisions between the particles in the dispersing medium. 

When a beam of light is shone on to a colloidal solution, the particles on the path of the light will be illuminated.  For examples, on a foggy night, the path of light from a car head light can be seen. 

Foam

A foam is a solution of gas in a solid or a liquid.  An example of foam would be a sponge.  The froth on the surface of mug of soapy water is also foam.  In a foam, bubbles of gas are separated by a solid or a liquid medium. 

Foams can be open celled, in which the gas can enter or leave the foam or close celled, such as bubble wrap, where the gas is trapped and cannot move. 

Foams find application in fire extinction.  Many fire extinction methods use foams, particularly for fires caused by oils and other flammable materials.  Mattresses and cushions are also examples of foams. 

Foams are used for thermal insulation and for shock protection.

Emulsion

An emulsion is a solution of one liquid in another liquid.  When two liquids, which normally do not mix with each other are added together, an emulsion is obtained.  An example is a mixture of oil and water.  The size of the droplets can change over the liquid and they are not static. 

If there is more water than oil in the solution, the solution is called water in oil solution.  If there is more oil than water, the solution is called oil in water solution.

The yolk of an egg is also an emulsion. 

Stabilization of an emulsion
An Emulsion is an unstable solution.  In a solution of oil in water, over time, the oil and water will start to separate.  This will eventually result in two separate constituents.    Emulsifiers are substances which will stabilize an emulsion. Emulsifiers work by making the droplets in an emulsion remain separate.  This ensures that one constituent remains suspended in the other constituents.

Emulsions paints are used to paint homes.  They have a rich matte finish. The emulsions consist of small droplets, such as vinyl, which are suspended in water.  They can be applied on walls and ceilings.

Boronizing

Boronizing or Boriding is a method of hardening, in which steel is heated in a bath containing compounds of boron, called boriding mixture. 

Boron carbide is commonly used in the boriding mixture.  The atoms of boron diffuse into the surface of the steel material, hardening it. The boron reacts with the constituents elements in the steel such as iron, nickel and cobalt, forming Iron borides, nickel borides and cobalt borides. 

Boronized metals have high wear resistances and can be used in applications such as drilling and cutting.  Boronizing also makes metals resistant to corrosion.

Knoop Hardness Test

The Knoop hardness test is a test for micro-hardness test.  It is used to test materials which are extremely brittle.  The indentor in the Knoop hardness test is a pyramidal diamond point.  A known load of 100 grams is pressed against the specimen. 

The hardness test is given by

HK = Load/ (Correction Factor x Area of the impression in sq.mm)

The correction factor is unique to the shape of the indentor and is provided by the manufacturer of the test apparatus.

The advantages of this test are that only a small specimen is required.  A microscope is required to measure the dimensions of the indentation.

Silicon Carbide

Silicon Carbide or Carborundum is a widely used abrasive.  Silicon Carbide has the formula SiC.  It occurs naturally as moissanite, though it is quite rare.  Most Silicon Carbide is synthetically made.    It is very hard, which makes it useful in cutting tools.

Silicon Carbide is also used in hard ceramics.  These ceramics are formed by sintering grains of Silicon Carbide.  Such ceramics are used as in car brakes and in clutches.

In Electronics, Silicon Carbide is used as semiconductors and in LEDs (Light Emitting Diodes). Silicon Carbide is used in the making of  bullet proof vests. It is also used as a refractory material in furnaces as it can with stand high temperatures.

Boron Carbide

Boron Carbide is an extremely hard material.  It is artificially produced and is used as an abrasive and in cutting tools.  It is also used in composite materials. 

Boron Carbide is made by heating carbon with boron oxide in an electric furnace.  The boron oxide is reduced, forming boron carbide.  The resulting powder is pressed at high temperature to form the material. 

Boron Carbide is has a tendency to absorb neutrons.  Hence, it is also used as control rods in nuclear reactors to control the chain reaction.

Boron Nitride

Boron Nitride is a very hard material, second only to diamond.  It is formed by heating boric oxide with ammonia.  The powder obtained is purified to remove metallic residues before commercial use. 

Boron Nitride is used in electric insulators and in cutting tools.  It also has good chemical resistance.  Its resistance to abrasion makes it suitable for use in nozzles which handle slurry and in high pressure water jet cutters. 

Mohs' Scale of Hardness

Mohs' scale of hardness is a scale of grading materials based on their hardness.  It is based on the ability of one material to scratch another material.  The scale was developed by the German geologist, Friedrich Mohs.

The scale is from 1 to 10, 1 being the hardest and 10, the softest. Talc, a soft mineral used for making powders, is at scale 10.  All minerals will be able to scratch talc.  Talc cannot scratch any other mineral.  Diamond, being the hardest mineral, is at position 1.  Diamond can scratch all materials.  No mineral will be able to scratch diamond.

To find the Mohs' value for an unknown mineral, the mineral is used to scratch a mineral of known value, say glass.  It is able to scratch glasss, its value is more than that of the glass, which is 5.  The mineral can be made to scratch minerals upper in the ranking.  The Mohs' value can thus be found.

Heat Deflection Temperature

Heat Deflection Temperature (HDT) is the temperature at which a plastic material deforms under a specific load.  This is an important parameter for in the design an manufacture of components using thermoplastic materials. 

Some polymers can withstand light loads at high temperatures while other will soften and lose rigidity at those temperatures.

Heat Aging

Heat Aging in polymers is used to observe the function of a polymer over a period of time at high temperatures.  Many plastics will be used at high temperatures over extended periods of time.  The heat aging test gives an idea of the behavior in such conditions.

The samples of the polymers are placed in an oven and exposed to high temperatures for a specified period of time.  The specimens are then taken out and their mechanical properties are tested.  Heat aging tests are sometimes combined with UV exposure tests as well.

Softening in Plastics

Many polymers will not have a specific melting point, which marks the transition from solid to liquid.  Instead, they will gradually soften as the temperature is increased.  This is known as softening. 

The temperature at which a certain extent of softening occurs is called the softening point for that polymer.

The softening temperature for a particular material is determined by the Vicat test.

In this test, a specimen of the polymer to be tested is placed in a Vicat apparatus.  A needle with a cross section area of 1 sq.mm is placed on the specimen with a load of 10 N for the Vicat A test and 50 N for the Vicat B test.

The temperature at which the needle makes an impression 1mm deep is noted as the softening temperature.

Brittleness Temperature

As the temperature drops, polymers become brittle.  When a plastic is chosen for operation at low temperatures, its reliability should be checked.  The brittleness temperature should be lower than the normal operating temperature of the application. 

Brittleness Test
The brittleness test is done by placing a number of specimens of a polymer in a bath.  The temperature is gradually lowered.  The specimens are continually impacted with a hammer at a speed of 2000 mm/s. 

At a specific temperature, the specimens will start cracking.  The temperature at which half of the specimens crack is called the Brittleness Temperature of the polymer.

Liquid Nitrogen

Liquid nitrogen finds wide application in the fields of industry, medicine, agriculture, etc.  Liquid nitrogen has a boiling point of -193 degrees.  It is stored in vacuum flasks. 

In industries, it is used for shrink fitting, in superconducting systems, and in machining.  It also finds application in medicine, where it is used in removing warts and in cryogenic surgery. 

Liquid Nitrogen is commercially produced by the cryogenic distillation of liquid air.  Pure Nitrogen can also be liquified to produce liquid nitrogen.

Liquid Nitrogen should be handled with care as it may cause cold burns if it comes in contact with skin. 

Throttling

Throttling is the process of restricting the flow of a liquid or a gas in a pipeline.  Throttling is done using a valve, a porous plug or an orifice.  A flexible hose can be throttled by pressing it.

Throttling is used to limit the flow in industrial systems.  For example, the flow of air or fuel to an engine can be throttled.  This would vary the power output of the engine.  The speed control in a vehicle is called the throttling system.

The throttling process can result in a drop in temperature due to the Joule Thomson effect. 

Triple Point

The Triple Point is the temperature at which a substance exists in all three phases, solid, liquid and gas in thermodynamic equilibrium.  For instance, the triple point of water is 273.16 K or .016 deg. C

Triple points are useful in calibrating thermometers.  The temperature of a pure substance is adjusted such that all three phases are in equilibrium.  This temperature, the triple point, is then used to calibrate the thermometers.

Liquid Air

Liquid Air is air that has been liquified.  Air is pressurized and then cooled. The cool air is allowed to escape through a small opening.  This reduces the temperature further. due to a phenomenon called the Joule Thomson effect.  The cold air is again compressed, cooled and allowed to escape through the opening.  This process is continued until the temperature drops further.  Droplets of liquid air are formed. 

Other gases such as liquid nitrogen and oxygen can be distilled from liquid air at specific temperatures.  For example, liquid nitrogen will separate from the mixture and evaporate at -195.79 deg.C.  The vaporized gas can be distilled separately to produce liquid nitrogen. 

Liquid air can also be used for cooling in many applications.

The Joule Thomson Effect

The Joule Thomson effect is the phenomenon in which a gas, when allowed to expand through a hole or a porous plug will experience a drop in temperature.  The Joule Thomson effect is an important property used in the liquefaction of gases and in refrigeration.  The process is adiabatic, that is, no heat is exchanged with the surroundings. 

Joule Thomson Coefficient
The ratio of the rate of change of temperature to the rate of change of pressure is known as the Joule Thomson Coefficient of the gas at a given pressure. 

The Joule Thomson effect also occurs in piping when the pipe diameter changes suddenly.   This has to be factored during the design.

Transient Stability and Steady State Stability

Transient Stability

Transient Stability is the ability of a power system to return to its normal state after a major disturbance, such as a fault or a disconnection or connection of a large load.

When there is a disturbance in the system, there are oscillations.  These oscillations are called swings.  Transient stability analysis is concerned with the response of the power system to such oscillations.    A power system with proper response will bring the system back to steady state operations within a short period of time.

Steady State Stability

Steady State Stability is the ability of a power system to respond to slow or gradual changes in its operating parameters.  When a number of power sources and loads are connected to a system, there will be gradual shifting of loads from one generator to another.  These oscillations, if not properly controlled, can develop into large oscillations which can cause bigger disturbances.

Diamond based Thermal Compounds for IC Heat Sinks

Diamond based thermal compounds are used in ICs to transfer heat to the heat sink for dissipation.  Diamond has very high thermal conductivity even greater than that of silver.  This makes this ideal for heat conducting pastes.

The heat generated should be conducted and safely dissipated to the outside environment. This is done using heat sinks.  The heat sink is mounted on the IC.  The  heat conducting compound is applied between the heat sink and the IC. 

Diamond based thermal compounds are used in circuits where a large amoung of heat is generated.  24 carat micronized diamonds are added to thermal grease along with diamond particle loadings.  This makes it an extremely efficient thermal conducting material.

Integrated Circuits developed large amounts of heat during operation.  This is particularly high in large scale integrated circuits, where thousands of components are crowded within a small area.

BH Analyzer

The B-H Analyzer is an instrument which can measure and plot the B-H curve of a given material.  They can also be used to determine the core loss at high frequency.

BH analyzers are used to analyze the behavior of circuit components at different frequencies.  The B-H curves are plotted across a wide frequency spectrum.

BH Analyzers are also used to determine the permeability of materials used in the construction of electric machines.

Coercivity

Coercivity is the strength of the magnetic field required to demagnetise a ferromagnetic material.  It is also described as the ability of a material to resist demagnetization.

Materials with high coercivity are made into permanent magnets, such as Alnico.  The unit of coercivity is ampere/meter.

Materials with low coercivity are made into electromagnets, such as soft iron.

The coercivity can be calculated from the B-H curve of a material.  The horizontal distance between points b and a in the BH curve in the right is the coercivity

The unit of coercivity is Ampere/metre

Sandpaper

Sandpaper is used to remove materials, such as dirt, paint, etc from the surface of metals or ceramics.  Sandpapers consist of sheets of paper, one side of which is coated with an adhesive.  The paper can be held and rubbed against the surface, which is to be cleaned. 

Originally, Sand, glass or seashells were used as abrasives. Today, however, the abrasives used are aluminium oxide and silicon carbide. 

Sand papers are also known as emery papers.  The sandpapers are standardized according the size of particles they contain. The sand paper number specifies the finenes of the abrasive used.  A higher number indicates greater fineness. A number 6 will have coarser particle than a number 20 sand paper.

Backing refers to the paper or fabric to which the abrasive crystals are bonded.  The backing can be cloth, polyester, rayons. 

The abrasive is bound to the backing by means of a adhesive substance called the bonding.  The bonding should be strong enough to hold the abrasive while the rubbing action takes place. 

Clogging is the accumulation of dirt and removed material between the abrasive crystals.  This can affect the effectiveness of sandpaper action.  Wet Sandpapers can be used with a liquid, such as water to remove the clogged impurities.

Shock Absorber

A Shock absorber is a device which absorbs shock and vibration.  The energy of the shock is converted to heat and dissipated.  Shock absorbers can be mechanical or hydraulic. 

In automobiles, shock absorbers serve to absorbs the bumps and irregularities on the road surface.  They also make sure that the tyres are in contact with the roads at all times.  This is essential for reliable braking and steering action. 

Shock absorbers are designed in many ways.  The simplest shock absorber is a spring based one. The load of the chassis is transmitted through a spring to the axle.  Both coiled springs and leaf springs are used in shock absorbers. 

Leaf springs consist of strips of metals placed on top of one another and rivetted together.  The energy of the shock is dissipated as heat as the strips rub against each other.

Hydraulic shock absorbers consist of a piston and a cylinder containing hydraulic oil.  The piston pushes against the oil which is forced trough a small opening in the piston.  The oil takes time to enter the opening due to its viscosity.  The resistance offered by the oil serves to retard the movement of the piston and absorbs the vibration.

Bumpers in Automobiles

A bumper in an automobile is the plastic component which runs along the length of the vehicle in the front.  It is made of a flexible and tough polymer, fibreglass or a composite material.

The bumper serves to absorbs the energy of low velocity collisions.  The bumper deforms and absorbs the energy of a collision.  Bumpers are designed to regain their original dimension after the energy of the collision has been dissipated.

Besides absorbing energy of collisions, they also serve to protect pedestrians by minimizing the impact.  Some bumpers are designed to guide the people to topple over the vehicle rather than under it.

Air Conditioning in Vehicles

The Air conditioning system is a vital aspect of today's automobile.  It is difficult to imagine a car without it.  The Air Conditioning unit, as the name suggests, "conditions" air.  Air is conditioned in terms of temperature and humidity.  The temperature of the air is reduced and its humidity is removed.

The Air conditioning cycle or the refrigeration cycle works by compressing a gas, cooling it, allowing it to expand and absorb heat from the air to be cooled and the compressing it to a liquid again.  The refrigerant is the gas which is used as the working fluid.  In earlier times, ammonia was used as the refrigera 

The air conditioner consists of the following components.

Compressor:  The compressor is the central part of the air conditioning system. The compressor compresses the refrigerant (the gas used for refrigeration).  It is driven by the engine of the automobile through a belt. 

Condenser: The compressed air from the condenser is cooled by the condenser by means of a cooling fan. 

Evaporator: The Evaporator is place where the liquid refrigerant transforms into a gas.  As this happens, heat is absorbed from the surrounding regions.  There is a expansion valve which regulates the evaporation of the liquid and consequently the temperature.  Outside, air is passed through the tubes.  The air loses heat and becomes cool and is blown into the car as cool air. 

Accumulator.  The accumulator is used as reservoir for the liquid has not still evaporated.  This can cause damage to the compressor.  The Accumulator serves to hold the liquid refrigerant.

Aerodynamic Drag in Automobiles

Aerodynamic drag is the resistance caused by air to the motion of the automobile.  Drag is created in the direction the automobile is moving in.

The power developed by the engine of the automobile is used to overcome the drag.  This lost power affects the mileage.    Aerodynamic drag also creates noise.   Therefore, aerodynamic drag is not desirable.

The aerodynamic drag increases with the speed of the vehicle.  Automobiles are designed to overcome drag and air resistance.  This is done by making the vehicle streamlined with a smooth surface, avoiding sharp edges.  In many vehicles, the lights and the wheel arcs are designed as part of the vehicle.

Air fuel ratio

Air Fuel ratio is the ratio of air supplied to the combustion chamber to the fuel injected.  It is a critical parameter of engine performance.  A proper air fuel mixture will result in proper and efficient combustion. 

If the air is more relative to the fuel, the mixture is called a lean mixture.  This results in lesser power generation.  The required torque will not be developed. 

If the air is less compared to the fuel, the mixture is called a rich mixture.  This results in incomplete combustion.  The smoke emitted is dark in color due to the presence of carbon particles which have not been oxidised.  The engine will not develop the required power.

Stoichiometric combustion is the ideal combustion in which all the fuel is combusted with no excess air.  It is not possible to mix the ideal amount of fuel with the ideal amount of gas in practice. 

The ratio of the mass of air to the mass of fuel is called the Air-Fuel ratio or AFR.

λ the air-fuel equivalence ratio

The air fuel equivalence ratio is the ratio of the air-fuel ratio to the stoichiometric air-fuel ratio for a given fuel.   It is denoted by the Greek letter λ.

Using the air equivalence ratio, λ is convenient in situation where engines use different fuels.

When the engine is running in rich air-fuel mixture i.e when there is excess fuel, λ is less than one.  When the engine is running with lean air fuel mixture, i.e. when air is more than required, λ is greater than one.

Afterburners for Pollution Control

An afterburner is a device which reduces pollution by oxidizing the harmful constituents of the exhaust gas such as VOCs (Volatile Organic Compounds) at high temperature to form carbondioxide and water.  Afterburners are used in industries and in equipments such as incinerators.

In an afterburner, the harmful gases of an industrial process are driven into a burner by means of a powerful blower.  The high temperature of the burner causes the molecules of the pollutants to breakdown into simpler compounds like carbondioxide and water.

The gases are then led to the chimney where they are released into the atmosphere.

Regenerative Thermal Oxidizer

The Regenerative Thermal Oxidizer is another variant of the afterburner in which the heat used to decompose the molecules of the Volatile Organic Compounds is retrieved from the exhaust gas itself. 

Cupola Furnaces

A Cupola furnace is a furnace used in foundries for the purpose of melting metals.  The cupola is, generally, made of steel. It is tall and cylindrical.   It has a lining of refractory bricks.

The molten metal is extracted through taps which are, usually, in the bottom of the cupola.  Cupolas have provisions called tuyeres in the side to feed fresh air to the coke to obtain higher temperatures.

The cupola furnace is filled with coke.  At the bottom, a layer of wood is placed and ignited.  Air is blown through the side channels.  The metal to be melted is placed on top of the cupola.  The coke burns in the air to form carbon monoxide, which on further oxidation becomes carbon dioxide.

As the molten metal comes down the cupola, it reacts with the oxides of carbon.  This produces metal with a high carbon content.

Crucible Furnace

A crucible furnace is used where the requirement of metals is not continuous.  It is used, when molten metal may be required intermittently. 

The crucible is a container made of graphite and clay.  It can withstand very high temperatures.  Crucible furnaces are used to melt non-ferrous metals which have relatively lower melting points.  These furnaces are powered by gas, electricity or coke.

Crucible furnaces are classified into

Tilting furnaces where the crucible is tilted and the molten metal spills out.
Ladle furnaces where the molten metal is removed by means of a ladle.
Bale out furnacez where the crucible is lifted and the molten metal is poured into the mould. 

Brazing

Brazing is the process in which two metals are joined by a filler metal, which is melted and poured into the joint.  Brazing is similar to soldering but is done at a higher temperature.  It differs from welding in that the work pieces are not melted. 

Due to capillary action, the molten metal is able to flow into even minute gaps in the joint.  Brazing can result in joints, which have good strength. 

To prevent the formation of oxides when the material is heated, a layer of flux, such as borax is applied over the metal which is heated.  The type of flux depends on the base metals and the filler metals used for the joint.

Kiln

Kilns are chambers used for heating.  They are used in processes such as baking bricks, curing, hardening, etc.  The kiln is a thermally insulated chamber which is heated.  Kilns are heated by an in-built furnace or by an external source, such as electric furnaces. 

In some kilns such as lime kilns and brick kilns, the material to be processed are exposed to the flames while in others the material is kept in a separate chamber which is heated. 

The kiln is usually lined with refractory bricks which can withstand high temperatures.

Curing

Curing is a process in which the molecules of the polymers are crosslinked with one another.  This creates a material which is harder, more stable and tougher.

Curing is usually done by heating.  It can also be done by exposing the material to be cured to ultraviolet radiation.  The curing process in rubber is called vulcanization. 

Refractory bricks

Refractory bricks are used to line the sides of furnaces and kilns.  These bricks are made of materials, which are chemically inert and can withstand high temperatures. They also have very low thermal conductivity.  This prevents the loss of heat.

These bricks are fired at high temperatures.  The bricks vitrify and are then glazed.  Refractory bricks are made of the oxides of aluminium, magnesium and silicon.

The type of refractory brick depends on the material the furnace will handle and the temperatures it will operate in.

The refractory bricks are attacked to the furnace walls by means of a special metallic support called an anchorage.  The anchorage is a support structure made of metals which can withstand the high temperatures.

Anchorage in Furnaces

An Anchorage is the supportive network in refractory furnaces, on which the refractory bricks or tiles are mounted.  The anchorage is made of steel or other alloys which can withstand the high temperature.

Thermal Shock

When a material is subjected to sudden changes in temperature, its dimensions and shape undergo rapid changes.  These changes in temperature can cause stress in the material.  This stress can cause the material to crack and fail. 

The stress caused by such sudden change in temperature is called Thermal Shock. 

The intensity of the thermal shock is dependent on the temperature coefficient of expansion of the material.  Materials with lower temperature coefficient of expansion can withstand temperature differences better than those with higher temperature coefficients. 

Borosilicate glass is an example of a material which can withstand sudden temperature differences. 

Thermal shock resistance is the property of a material to resist thermal shock. Thermal Shock Testing involves subjecting the test material alternatively to high and low temperatures. 

Stress

Stress refers to the forces exerted by particles of a material exert on one another. 

For example, consider, a loaded truck standing on a bridge.  The weight of the truck exerts a force on the material in the bridge.  The bridge is thus stressed by the weight of the truck.

Stress can be due to an external force or due to internal flaws and defects in the material. Examples of materials having internal stresses are prestressed concrete, tempered glass, etc. 

Stress is defined as the force per unit cross sectional area. 

Stress = Force/ Area

Stress can be expressed in many units such as kg/sq.m, pascals, pounds/sq. inch, etc. 

Normal Stress and Shear Stress
The stress on an object can be resolved into two types stresses  They are normal stress and shear stress.

Normal Stress is the stress  that is normal to the plane of the material. Shear Stress is the stress that is parallel to the plane of the material.

The deformation suffered by the body as a result of stress is called strain.

Strain

The deformation produced in a material by stress is called strain.  When stress is applied on a material, there is a change in dimensions.  For instance, if a steel rod is stretched, its length increases.  This change in dimensions is called strain.

True Strain
True Strain is the natural logarithm of the ratio of the final length to the original length.

True Strain = Ln (Final Length/Original Length)

Engineering Strain
Engineering Strain expresses strain as the ratio of the change in length to the original length.

Engineering Strain = Change in Length/ Original Length

Strain Hardening

Strain Hardening or Work hardening is the process of increasing the strength of a material using plastic deformation.  The metal to be strain-hardened is stretched beyond its yield to a point just before it will fracture.  At this point, the metal becomes stronger and will resist deformation.  More stress will be required to deform the material.

For instance, low carbon steel is stretched beyond the yield point and aged (left for a few days).  The material will have a higher yield stress.  Steel can be hardened by rolling it between a pair of rollers.  The dimension of the steel sheet is reduced and the metal is hardened. 

Strain hardening occurs as the dislocations cause the atoms to move to other locations and are anchored there.  Strain hardening is a cold working process.  It is done below the recrystallization temperature of the material. 

A simple example would be to bend a piece of wire or a paper clip in opposite directions for a few times.  Fatigue sets in the material.  The point of the bending becomes hardened.  Further bending will occur at a point above or below the earlier point.

Composite materials

Composite materials are made out of two or more materials.  The constituent materials may have different chemical and physical properties.  The composite material formed will have properties, which differ from the individual materials. 

Composite materials are generally lighter, stronger and less expensive than ordinary materials. They can be easily moulded in a the desired shapes. Examples of composite materials are concrete. plywood and fibre-reinforced plastics. 

Each of the constitutent materials will give a specific property to the composite.  One material may give strength while another will give rigidity or resistance to corrosion.  In Fibre reinforced plastic, the fibre gives strength to the material while the plastic resin holds the fibre together and gives shape to the material. 

Composite materials find wide applications in automobiles, buildings, medicine and in space technology.

SuperAlloys - An Overiew

Superalloys are alloys which can withstand very high temperatures.  They have good mechanical strength even at high temperatures and can resist creep.

The basemetals used in superalloys are usually iron, nickel or cobalt.  Aluminium or Titanium are added to the alloys. Metals lose their strength as the temperature increases.  However, superalloys retain their mechanical properties even at temperatures up 70 percent of their melting points.  Superalloys are used in spacecrafts and in aircraft jet engines. 

Superalloys can also be developed as single crystals. 

Oxidation of the superalloysalloys is a concern at high temperatures. To prevent this, a layer of oxide is allowed to form at the surface.  This layer of oxide prevents further oxidation. 

Superalloys can be processed in a number of methods such as investment casting, sintering, directional solidification, single crystal growth, etc.

Creep deformation

Creep is the tendency of materials to deform permanently.  Creep can occur at ambient temperature or at temperatures below the melting point.  The deformation can happen when the material is not loaded.  For instance, a pipe left lying can deform even without any load. 

Creep increases as the temperature increases.  Hence, the study of creep is important for components which will operate at high temperatures.

Soft metals such as lead, aluminium or solder can creep even at room temperatures.  Material such as tungsten can resist creep deformation even at high temperatures.  As a rule of thumb, creep can occur at around 35 percent of the melting point for metals. 

Creep deformation can be classified into three stages

Stage 1
This is the primary stage.  At this stage, the strain rate or the deformation rate is very high.  The strain rate slows gradually and stabilizes.

Stage 2
At this stage, the strain rate stabilizes and is almost constant. 

Stage 3
In this stage, the strain rate again increases.  This is due to failure processes such as necking or the formation of cracks.  The material is permanently deformed at this stage. 


Creep can be prevented by choosing materials which have higher melting point.  Special alloys which resist creep can be chosen for critical applications.  Materials with bigger grain size will also have lesser creep.

Viscous and Elastic Deformation

Viscous and Elastic Deformation

Viscous deformation is the deformation in which the deformation varies linearly with time.  The material does not come back to its original position.  Examples of viscous deformation are honey, wax, etc.

Elastic Deformation is the deformation in which the material returns to its original shape after the stress is removed.  In elastic deformation, the deformation is proportional to the stress.   Eg. Metals.

Viscoelastic deformation.

Certain materials exhibit both viscous and elastic deformation.  In these materials, some deformation is elastic while some deformation is viscous.  Examples are rubber, plastics, etc.  Wood is also a viscoelastic material.

Applications
Viscoelastic materials are used in applications where vibration damping is required.  Viscoelastic materials are also used where impact and shock absorption are required, such as the bumper of a car.

Bingham Plastic

A Bingham plastic is a viscoplastic material which flows when the stress is high but behaves as a solid body when the stress is low.  Toothpaste is a Bingham plastic.  When no pressure is applied, there is no flow of plastic through the tube.  When pressure is applied, the toothpaste is extruded through the tube. Slurry is another example of a Bingham plastic

The mathematical model for a Bingham Plastic is

Shear Stress = minimum yield stress + plastic viscosity * shear rate

In a Bingham plastic, flow does not occur until the stress reaches a specific value called the minimum yield stress. 

Viscoplasticity

Viscous deformation refers to the deformation which varies linearly with time. The deformation is viscous up to a certain point.  Once the yield stress is exceeded, the deformation becomes plastic, where the deformation is not proportional to the stress.

Creep deformation is an example of viscoplasticity.

Viscoplasticity is studied in the analysis of crash tests in automobiles.  It is an important characteristic when choosing materials, which can withstand high loads. 

Hooke's law

Hooke's law states that the force developed in a spring is linearly proportional to the distance travelled.  The law was proposed by Robert Hooke, the 17th century physicist.

Mathematically, it can be expressed as

F = kX

where
F is the force in newtons
k is the spring constant
X is the distance travelled in metres

Hooke's law for stress and  strain

Hooke's law can be generalized elasticity to say that "the strain of an elastic object is proportional to the stress".  Once the elastic limit is exceeded, Hooke's law will not hold true.

For plastic materials
Strain = Proportionality constant x Stress

The proportionality constant will be unique for different materials. 

Inconel

Inconel is a superalloy containing iron, nickel chromium.  It is generally used in high temperature applications.  Inconel can resist oxidation and corrosion, which makes it suitable for applications in extreme environments.  At high temperatures, a passivating layer of oxide is formed by initial oxidation.  This layer prevents further oxidation.  Inconel is non magnetic.

Inconel is a difficult alloy to shape.  It gets hardened during the working process itself (work hardening).  Special manufacturing techniques are thus required. 

Inconel is a trademark of Special Metals Corporation. 

Inconel finds application in cryogenic tanks, jet engines, gas turbines, etc.

Nimonic

Nimonic is a superalloy used in high temperature applications.  It is used to produce valves in IC engines, gas turbines, etc.

It is an alloy of Nickel and Chromium. It also has cobalt, titanium and aluminium.   It is available in different grades, such as nimonic 75, nimonic 80A and nimonic 90.

Passivation

Passivation is the process of making a material "passive".  The material is made inert or with reduced ability to react with the surroundings.  This is necessary to prevent rust formation and corrosion.

Passivation involves the creation of a protective layer on the surface of the material.  This is usually an oxide.  Iron can be passivated by heating it in an atmosphere of oxygen.  A layer of oxide is formed.  This layer prevents further attack by oxygen.

Nickel pipes can be used to handle fluorine by passivation.  A layer of fluoride is allowed to form on the surface.  This prevents further reaction.

Rolling

Rolling is a metal working procedure, in which, a metal is rolled between two rollers.  Compressive force is applied on the material and the thickness is reduced along with an alteration of the grain structure. 

Rolling results in a reduction of the thickness and a hardening of the material.  Cold rolling is done below the recrystallization temperature while hot rolling is done above the recrystallization temperature. 

Hot Rolling improves the grain structure and makes it more uniform.  It also removes porosity and other casting defects from the metal. 

Rolling is used to produce metal sheets.  Metal can be passed through consecutive sets of rollers to progressively reduce the thickness.  Metal foils used for wrappings are also produced by rolling. 

Rolling can also produce shapes such as T and L shaped profiles

Sheet metal

Sheet metal is metal that has been formed into sheets.  It is available as sheets or as coiled strips.  Sheet metals are usually made of steel or aluminium. Other metals, such as gold and platinum can also be made into sheet metals. 

Sheet metals are used in a variety of applications like creating ducts and pipes for an HVAC system, in the building of the automobile body, to create an enclosure for panels, etc.

Sheet metal is usually obtained by rolling.  Sheet metal is cut by special tools and bent to form the required shapes. 

Malleability is the property of a material to be made into sheets.

Cryogenic Machining

Cryogenic machining is the machining of components in the presence of a cryogenic fluid.  Cryogenic fluid is used instead of the cooling liquid at the machining surface.  Liquid nitrogen is delivered to the cutting edge of the tool.  This increases the speed of the machining.  More parts can be machined in the same time.

Conventional machining causes high temperatures.  These high temperatures create thermal stress on the tool and can decrease its life.  Cryogenic machining extends the life of the tool as high temperatures are avoided.

Cryogenic machining is cleaner than conventional machining.  Burr formation is greatly reduced.  Residual stress is also lessened.

Cryogenic machining can be retrofitted in normal machining equipment.

Cryogenic Treatment - An Overview

Cryogenic treatment is the process of subjecting materials to cryogenic (very low) temperatures up to -190 degrees C.

Cryogenic treatment is used to relieve residual stresses and to increase wear resistance.  The component to be treated is immersed in a bath of liquid nitrogen or any other refrigerant and slowly cooled till the target temperature.  It is then gradually brought back to room temperature. 

Corrosion resistance is also improved as a result of cryogenic treatment.  Electrical and mechanical properties are also improved.

Induction Hardening

Induction hardening is a process in which a material is heated by electromagnetic heating.  The heating is done until the temperature exceeds the transformation temperature.  The induction is done using a copper coil that carries an electric current. 

Due to the alternating electromagnetic field, eddy currents are induced in the surface of the material.  The depth of the hardening can be controlled by controlling the current through the induction coil.

After the heating, the material is quenched in a cooling medium, such as oil.  Carbon alloy steels are generally hardened in this manner. 

Gears, pinion shafts, bearing races are some of the components which can be hardened by induction hardening. 

Flame hardening

In Flame hardening, the material to be hardened is heated with an oxy-acetylene torch to a high temperature till the surface has become austenitic.  The material is then quenched in a spray of water.  The austenite is converted to martensite and the surface is hardened. 

It is difficult to control depth of hardening in flame hardening. Along with the surface getting hardened, other properties such as bending and torsional strength and fatigue resistance are also improved.

Flame hardening can be used to harden the surface of any metal. 

Case hardening

Case hardening is a method of hardening low carbon steels.  Low carbon steels have poor hardenability. 

In case hardening, a "case" of hard material is created around the relatively soft body. 

The case hardening process involves heating the component to a high temperature .  The hot steel component is immersed in a case hardening compound.  The case hardening compound  is high in carbon. 

The component is again heated and then quenched in water.  The process can be repeated till the desired depth of hardness is obtained.  In case hardening, the outer surface of the metal is hardened while the inner surface is kept soft. 

Nitriding

Nitriding is a process of hardening steels.  The material to be hardened is heated in an atmosphere of ammonia at a high temperature below the final tempering temperature of steel.    Nitrides are formed in the surface. 

The formation of nitrides hardens the surface.  Nitriding can be done only in steels that contain elements, which can form nitrides, such as molybdenum, aluminium and chromium.

Unlike other methods of hardening, nitriding does not require quenching.   After nitriding, the components are subject to stress relieving processes.  Sometimes, the surface formed is ground to remove the outermost layer which may be brittle. 

Abrasive blasting

Abrasive blasting is a process used to remove foreign materials and clean the surface of a material.  Blasting involves firing materials such as abrasives, sand, glass beads using compresssed air through a specially designed spray nozzle at high velocity.

The abrasives which impinge on the surface at high velocity carry away the undesirable materials.  Different textures, from matte to shiny, can be obtained depending on the abrasive used.

Workers should were proper protective devices such as masks and suits during the procedure.

In addition to cleaning, abrasive blasting is also used as a finishing process.  It is used to give  a specific desired texture to material.  In certain situations, it is used to roughen the surface for processes such as thermal spraying.

Annealing

Annealing is a method of softening steel and relieving the internal stresses.  The steel is heated to about 50 degrees C above the austenitic temperature.  It is held at that temperature for sometime.  Then, it is gradually cooled to the ambient temperature. 

Annealing increases the ductility and increases its workability of steel.  The hardness of the steel is reduced by annealing.  Metals like copper, brass and gold can also be annealed. 

At high temperatures, the metal is susceptible to oxidation.  This can be preventing by conducting the heating in an inert atmosphere of special gases such as forming gas ( a mixture of hydrogen and nitrogen) or  endothermic gas ( a mixture of carbonmonoxide, nitrogen and hydrogen).

Glass is also annealed to remove internal stresses and increase the strength.

Machining Coolants

Coolant are liquids applied to the machining contact surface during the process.  There are a wide range of coolants, such as oils, emulsions, pastes, gels, etc. A coolant is an emulsion of oil suspended in water. 

Coolant liquids are broadly classified into
     Oil based coolants (straight and soluble oils)  and
     Chemical machine coolants (synthetic and semisynthetic)

The functions of the coolant are

To maintain a low temperature. 
The tolerances specified during the machining process may become distorted due to expansion at high temperature.  Hence, the temperature should be maintained at specified range to ensure correct measurement. 

To lubricate the cutting surface
Excessive friction at the cutting surface can cause hardening of the material, making cutting difficult. 

Clean the surface
The coolant keeps the surface by removing material like burrs, dust, etc

Prevent rust
oxidation can occur at the high temperature caused by cutting.  Coolants prevent rust formation by forming a protective surface. 

The coolant is delivered to the desired location by different methods.  Some machines use spraying or flooding.  In some machines, a jet of coolant liquid is focussed on the work area.

The coolant liquid is also a fertile medium for bacterial growth.  This can cause infections over a period of time.  Sometimes, antiseptics are added to prevent bacterial growth. 

Crankcase relief valves

Crankcase explosion valves are special valves which are mounted on the doors of the crankcase. These valves are spring loaded and operate when the pressure inside the crankcase increases due to an explosion.

Under normal conditions, they are mounted on a seat and are in the closed conditions. When they are pushed open from the inside, the release the high pressure gases in the crankcase.

They are usually provided with a flame trap which prevents flames, if any, from escaping outside. Once the pressure has been relieved, the valves will return to their closed position.

Flame Traps

Flame traps or Flame Arrestors are protective devices which stops fire from passing through it but allows gas to flow freely. Flame traps are useful in many applications where inflammable gases are used. Flame traps consist of a network or mesh of metal.

 The trap works by absorbing the heat from the fire and preventing it from reaching the other side. A flame trap can limit a fire accident by limiting the flame from spreading.

Flame traps are located at specific intervals in the pipeline. Flame industries are used in many industries such as petrochemical, refining, paper manufacturing, etc. Flame traps should only be used for applications they have been designed for. They also need to be periodically checked for corrosion and insect infestation.