Abhishek

Hi ....I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.

Hygroscopic vs Hydroscopic: 7 Important Comparison

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Hygroscopic vs Hydroscopic

The terms hygroscopic and hydroscopic may sound similar but their meanings completely differ from one another.

Hygroscopic substance refers to the substance that can take and hold moisture from the surroundings. Hydroscope is an instrument used to see objects deep underwater. This article discusses about hygroscopic vs hydroscopic substances in detail.

Hygroscopic vs Hydroscopic

Hygroscopic vs Hydroscopic:

AspectHygroscopyHydroscopy
DefinitionThe ability of a substance to absorb moisture from the surrounding environment.The practice of observing objects underwater.
Scientific RelevanceSignificant in chemistry, physics, material science, meteorology, and various industries.Relevant in marine biology, underwater archaeology, and maritime activities.
Key ExamplesSubstances like salt, sugar, honey, and certain chemicals.Instruments like traditional and modern hydroscopes.
ApplicationsUsed to control moisture in food and pharmaceuticals, in humidity sensors, and in maritime cargo management.Used for studying marine life, archaeological underwater exploration, and inspecting underwater structures.
Instruments/ToolsHygrometers and other moisture measuring devices.Hydroscope and various underwater viewing devices.
Historical BackgroundLong-standing concept in the scientific study of moisture absorption.Originates from the invention attributed to Hypatia of Alexandria for underwater observation.
Physical Process InvolvedInvolves absorption or adsorption of water molecules.Involves visual observation through a medium (like water) using specialized equipment.
Industries ImpactedFood processing, pharmaceuticals, construction, and maritime industries.Marine biology, maritime exploration, and underwater archaeology.

Hygroscopic substances

Hygroscope refers to the phenomenon of attracting water molecules via absorption or adsorption. Hygroscopic substances are capable of taking away moisture from the surroundings and holding it. This decreases the relative humidity of the surrounding. The relative humidity of substance is directly proportional to the amount of moisture the substance can hold.

Engineering materials like ABS, Cellulose , Nylon etc are hygroscopic in nature.In some composites, due to difference in hygroscopic properties of two materials, there can be detrimental effects such as stress concentration. The amount of moisture taken by a substance is a function of temperature and humidity of the surrounding.

The rate of transfer of moisture decreases as it approaches equilibrium. This happens because of two reasons- the driving force behind moisture transfer decreases and the diffusional resistance to mass transfer increases as the surface taking up moisture nears to equilibrium.

Hygroscopic vs hydroscopic
Image: Apparatus for measuring hygroscopicity of fertiliser

Image credits: Wikipedia

Storage of hygroscopic materials

Hygroscopic materials are usually stored in sealed bags. These bags are simply kept in those places where the moisture content has to be regulated. A common example is silica gel which is used to take away moisture content from the products such as water bottles, lunchboxes, water filters etc.

If these materials are not properly stored, the desired moisture content will not be achieved. Moisture content is an essential factor for determining a machine’s life. If it is not regulated properly then simply because of improper moisture content, life of machines will be altered.

Hygroscopic materials in different pressure conditions

The partial pressure of hygroscopic materials and the ambient pressure can affect the moisture of the system directly.

When the material is subjected to high pressure (isothermally beyond saturation point), then the specific humidity will decrease and relative humidity will keep on increasing. The added moisture will affect material’s quality. An example of such pressure fed system is pneumatic system wherein the hygroscopic material is conveyed through air.

When the material is subjected to negative pressure, the specific humidity remains constant and relative humidity decreases as pressure of the air in conveyer decreases.

Applications of deliquescent materials

The phenomenon of absorbing moisture up to such an extent that the substance dissolves completely in the water to make a solution. Liquids like Sulfuric acid and and salts like Sodium Chloride are examples of deliquescent substances.

In chemical industries, deliquescent materials are used for absorbing water content from chemical reactions. These materials are also known as desiccants. Desiccants like silica gel are used for absorbing moisture from the surrounding environment.

Hydroscopy

Hydroscopy is completely different from hygroscopy. Hydroscopy is the practice of looking and observing things underwater. This can be done by using the instrument called hydroscope. The original hydroscope was invented by Greek scholar and scientist philosopher, Hypatia of Alexandria.

Hydroscope itself is not any instrument. Hydroscope refers to the type of any instrument that is used to measure properties related to water. The hydroscope is generally made out of tubes and a transparent cap at the end made of plastic or glass for viewing.

It is difficult for humans to see underwater without using hydroscope. When we try looking underwater with naked eye, water rushes on the surface of eyeball and distorts the light coming to the pupil. Hydroscope prevents this distortion by providing a transparent material which allows light to enter the eye and avoiding contact with water. If required, we can also achieve magnification underwater.  

Examples of hydroscopy

Complexity of hydroscope varies from application to application. It can be as simple as a tube with two lens and as complex as a computer controlled lens with variable magnification.

Some examples of hydroscopy are as follows-

  • For viewing objects near the surface of oceans, a long tube is fitted with lenses so as to see the objects that can’t be seen otherwise.
  • In defence practices, subsurface water is detected by the use of surface nuclear magnetic resonance technique.

Applications of hydroscope

Hydroscopy is an important technique that allows us to study aquatic life and perform underwater tasks. Everything that requires deep water excursions is achieved by using hydroscope.

Following are the applications of hydroscope-

  • Scientists use hydroscopes for looking at marine life which dwells deep inside the ocean. Many marine animals and plants have been discovered with the use of hydroscopes.
  • Archaeologists use hydroscopes to search for ancient remains which might have submerged deep underwater.
  • Hydroscopy is used for inspection of ship hulls and underwater pipelines to check for corrosion.
  • Rescue missions in caves which are flooded by seawater.

Chiller Work: 7 Important Facts You Should Know

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Chillers are machines used to dehumidify or cool fluids. There are various types of chillers classified on the basis of working fluid used, working mechanism used etc.

This article explains how does a chiller work, different types of chillers used in industry and general information about compressors used in air cooled chillers.

How does an air-cooled chiller work?

Ever seen multiple fans installed on the top of a building? They are used for cooling purposes inside the building. These fans are a part of a bigger system known as air cooled chiller.

Chiller is a machine that absorbs heat using vapour compression cycle, vapour absorption cycle or vapour adsorption cycle. The cool fluid can be passed through a heat exchanger for further applications. Concepts of thermodynamics are used in air cooled chillers to cool the fluid or dehumidify air.

Chillers collects heat from water and sends it back to air handling unit which uses cool water for its operation. After AHU’s operation, the water temperature rises and is brought back to the air chiller.

How does an industrial chiller work?

The main purpose of industrial air chiller is to cool the water and send it back to the AHU (Air Handling Unit). After AHU does its specified task, the water inside the AHU becomes warm. This warm water is sent back to the inlet of chiller. This cycle continues till the end of AHU’s operation.

The air chiller absorbs heat from the processed water that comes into the inlet of the chiller. Heat is absorbed with the help of chiller’s evaporator.

After the liquid refrigerant passes through evaporator, its phase changes to gas and pressure decreases in this process. After compression, the refrigerant that leaves has high pressure and high temperature.

This gas enters the condenser where it is cooled by condensing fans. The cooling fans blow away the heat into ambient hence it is suggested to install air chillers outside the room or at a place where dumping heat is not an issue.

An industrial air chiller has following components- Evaporator, condenser, compressor, pump and cooling fans.

  • Evaporator-It takes away heat from the water to change the phase from liquid to gas.
  • Compressor-Temperature and pressure of the gas is increased by compressing the gas in compressor.
  • Condensing fans/cooling fans-The cooling fans blow away the heat from the refrigerant reducing the temperature of gas.
  • Condenser-The phase changes back to liquid inside the condenser.

What are industrial chillers used for?

Industrial chillers are used for cooling mechanisms, products and a wide range of machinery. It can be centralized where one chiller can be used for multiple applications or decentralized where each and every application has one dedicated chiller.

Chillers are used in plastic industries, metal cutting work oils, injection and blow moulding, cement processing. They are also used in gas turbine cooling system, high heat applications such as MRI and lasers in hospitals.

Liquid cooled chillers are used for indoor operations due to as liquid absorbs the rejected heat. Air cooled chillers are meant for outdoor installations because the heat is rejected in the ambient. Hence, most air cooled chillers are installed at the top of buildings.

Types of compressors used in air cooled chillers

There are various types of compressors that can be used in chillers depending on the load requirements in the application. Following are the compressors that can be used in chillers-

  • Reciprocating compressor-A simple positive displacement pump which used a piston to deliver gas at high pressure. The gas enters the cylinder in the suction stroke when the piston is at bottom dead centre. The gas is compressed in the next stroke when the piston move towards the top dead centre. Compressed gas leaves through the delivery valve. This type of compressors deliver compressed gas in pulsations.
  • Rotary screw compressor-Rotary compressors are used in large sized refrigeration applications such as chillers. These have rotary type positive displacement mechanism and provide continuous delivery of compressed gas unlike reciprocating compressors which have pulsations. Rotary compressors are more quiet in operation.  
  • Vane compressor-Most common type of compressor is the vane compressor. It uses centrifugal force to compress the gas. These compressors uses vanes instead of helical screws to generate compressed air.
  • Scroll compressor-A scroll compressor uses two spiraled scrolls for compressing the gas or refrigerant. Usually one scroll is fixed and other orbits with a little offset without rotating. The tapped gas between the scrolls get compressed due to the relative motion between scrolls. Its efficiency is slightly higher than reciprocating compressors.
how does a chiller work
Image: Reciprocating compressor
Image credit: No machine-Kompresors, CC BY-SA 3.0

Water cooled chillers

As the name suggests, water cooled chillers use water instead of air for cooling. It uses latent heat for cooling purposes.

External cooling towers supply water that is used to cool the gaseous refrigerant in the condenser. Inside the condenser, refrigerant’s phase changes. The gaseous refrigerant turns into liquid refrigerant and is then re-circulated in the system.

Advantages and disadvantages of water cooled chillers

Every mechanical component has its own pros and cons. Designers have to make a trade off between pros and cons to make the best design suitable for the particular application. Following are the advantages and disadvantages of water cooled chillers

Advantages of water cooled chillers-

  • They are more efficient than air chilled coolers.
  • They don’t create much noise while operating.
  • They can be used in both small scale and commercial scale applications.

Disadvantages of water cooled chillers-

  • Due to continuous requirement of water, water cooled chillers are not feasible to use in areas having water shortage problems.
  • As the number of components are increased (cooling tower and pumps), installation cost of water cooled chillers is more.

Vapour compressed chillers vs vapour absorbed chillers

Vapour compressed and vapour absorbed chillers are both air cooled chillers. The principle difference between vapour compressed chiller and vapour absorbed air chiller is the way of cooling.

Vapour compressed chillers Vapour absorbed chillers
Vapour compressor chillers use following components- evaporator, condenser, compressor and an expansion unit. Refrigerant extracts unwanted heat, this refrigerant is pumped by the action of compressor. Vapour absorption chillers use same components as vapour compressed chillers except compressor. Instead of compressor, there is an absorber, generator and a pump. Heat source itself is used to pump refrigerant around the system for cooling purposes.
Table: Difference between vapour compressed chillers and vapour absorbed chillers

It is clear that vapour absorbed chiller has more parts but it is cheaper to operate as it does not need any compressed air for operation.

Gas Turbine Efficiency: 5 Interesting Facts To Know

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Gas turbine efficiency formula

Turbines are machines that harness kinetic energy of any fluid and help converting it to another form of energy (mostly electrical).

The turbines which use gas as working fluid are called as gas turbines. Gas turbines normally work on Brayton cycle to achieve desirable output.

For an ideal Brayton cycle (shown in figure below), efficiency is calculated as-

Gas turbine 1
Image: Gas Turbine cycle (Process 3-4(s) represents turbine)
gif

Where, h represents the enthalpy and subscript represents the state in the Brayton cycle.

Turbine efficiency is given by-

gif

Where,
Subscript s denotes actual state.

Gas turbine efficiency curve

Gas turbine cycle efficiency rises exponentially till an optimum value of pressure ratio is reached, after that there is no significant change in the efficiency. The factors on which the efficiency of gas turbine depends are inlet temperature, pressure ratio and specific heat ratio of the working fluid.

Gas turbine efficiency curve on the other hand increases slowly. With higher inlet temperature, the efficiency of gas turbine increases. The graph below shows the relation between inlet temperature and turbine efficiency-

gif
Gas turbine efficiency 1
Image: Gas turbine efficiency Vs Inlet temperature

Hydrogen gas turbine efficiency

The need for Hydrogen turbine arises due to environmental concerns. Hydrogen as a fuel is very environment friendly. These turbines reduce CO2 emissions.

Hydrogen is mixed with the working fluid and this combination of Hydrogen-fuel mixture gives a better efficiency than using fuel alone. Using Hydrogen in large amounts is a problem because of its storage. Governments and private companies are working a way out for safer transport and storage of Hydrogen fuel.

How to calculate gas turbine efficiency

Mechanical losses lead to certain drop in performance of machines. According to second law of thermodynamics, no machine can give 100% efficiency.

The efficiency of gas turbines can be calculated using following steps-

  1. Calculate enthalpy at all points in the gas turbine cycle.
  2. Calculate actual work done by turbine using the formula-

    Work done= h4-h3
  3. Calculate actual work done by turbine using the actual values of enthalpy after mechanical losses.
  4. Calculate efficiency using the relation–
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Open cycle gas turbine efficiency

An open cycle is a cycle where the working fluid is not brought back to its initial conditions. Rather, it is discarded into sink. The efficiency formula of such cycles don’t change but the values change due to change in value of variables that is temperature and pressures.

An example of gas turbine open cycle is shown below-

openc 1
Image: Open cycle gas turbine

Practice questions

What affects gas turbine efficiency?

Gas turbine efficiency depends mainly on three factors-

  • Inlet Temperature-
    Increasing the inlet temperature of the turbine increases its efficiency. Adding to which, decreasing sink temperature also increases the efficiency of gas turbines but it can be decreased upto ambient conditions only so it does not create much effect on efficiency.
  • Pressure ratio-
    The pressure ratio P2/P1 is an important characteristic that affects the efficiency of gas turbine.
  • Specific heat ratio-
    Specific heat ratio for ideal gases is around 1.4, real gases have values around 1.2-1.3. A good working fluid should have specific heat ratio value closer to isentropic value that is 1.4.

Why gas turbines have low efficiency?

Gas turbines work on constant volume cycles. As gases have lower density, they need extra work to be compressed hence increasing compressor work.

The formula for efficiency is given as efficiency = work done/heat added

As work done by compressor increases, the net work done decreases so the overall efficiency decreases. The efficiency of gas turbines can be increased by number of ways. Most common ways of improving efficiencies of gas turbines are regenerative cooling, intercooling, reheating.

How to increase efficiency of gas turbine?

There are number of ways by which the efficiency of a gas turbine can be increased. The factors that affect the efficiency directly are temperature, pressure ratio and specific ratio. Altering these values can directly affect the efficiency.

 Hence, the ways that are proposed to increase efficiency include altering these values. Various methods used to increase the efficiency of gas turbines are-

  • Regeneration-

    In this method, exhaust gas is used to heat the working fluid at point 2. This results in decrease of exhaust gas temperature and increase in efficiency. The diagram of regenerative gas turbine cycle and efficiency formula is given below-
Regenrative HE 1
Image: Regenerative gas turbine cycle
  • Intercooling-
    In this method, the compressor work is decreased by compressing the air in two stages. The air is cooled before going to the second compressor. This cooling of air between two stages is called intercooling. Decreasing the compressor work is directly associated with increase in efficiency.
  • Reheating-
    In this method, two turbines are used instead of one. One turbine is used to produce work and other turbine drives the compressor. More heat is added in this process. Due to decrease in compressor work and high inlet temperature, efficiency increases. The diagram of reheat gas turbine cycle is shown below-
Reheat 1
Image: Reheat gas turbine cycle
  • Reheating, intercooling and regeneration combined-
    In this method, all three methods are combined. The set up costs may soar up but overall efficiency increases by combining above three methods.

Combined gas turbine cycle efficiency

Combined gas turbine cycle uses multiple gas turbines working in tandem to provide more output.

The exhaust from single gas turbine cycle is still hot enough that it can run another cycle. Usually a heat exchanger is used between exhaust of first engine and inlet of second engine so as to use different working fluids. The output of second cycle is lesser than the first cycle but the overall efficiency of combined gas turbine cycle increases.

The first cycle is called as topping cycle and produces greater efficiency. The next cycle is called as bottoming cycle and may have different fuel (depending upon exhaust temperature of first cycle) and produces lesser efficiency than the first one. Overall the combined cycle can produce 50% more efficiency.

The formula to calculate overall efficiency of combined gas turbine cycle is given below

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Common Bolt: 22 Types You Should Know

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Fasteners are used to join two parts or lock one part into another. Bolt is one such type of mechanical fastener.

Bolts have external threads that use tightening torque to fulfill the purpose. Unlike screws, most bolts require a nut to achieve a tight joint. This article discusses about different bolt types with pictures.

Types of bolt with pictures

Bolts come in different materials and sizes. Their selection depends on the application.

Various types of bolts are-

Anchor bolt

Anchor bolts are used on concrete surfaces. It acts as a fastener between a concrete surface and part which is to be fastened or joined.

Generally a concrete epoxy is applied inside the hole (made in concrete slab). Anchor bolt is then twisted inside this epoxy. After epoxy dries, a firm and strong joint is produced. This type of bolt transfers both tension and shear loads.

types of bolts with pictures
Image: Workers securing structure using anchor bolt
Image credits: “CH053 – Catenary B-914E North Rebar and Anchor Bolts (03-29-2012)” by MTA C&D – EAST SIDE ACCESS is licensed under CC BY 2.0

Types of anchor bolts

Anchor bolts are of two types depending upon when they are used-

  • Cast in place: These are installed before pouring of concrete. Examples of cast in place anchor bolts are hex head bolt, L bolt, J bolt etc.
  • Post installed: These are installed after pouring of concrete. Examples of post installed anchor bolts are Undercut anchors, sleeve type anchors, stud type anchor etc.

Carriage bolt

Carriage bolts are typically used to fasten a metal and wood or fasten two woods together. Some are specially designed such that they are used for fastening two different metals.

This type of bolt is distinguished from other bolts by its mushroom shaped head. It has a square cross section just below the head and then circular throughout. This makes the bolt self-locking when put inside a hole with square cross section.

Carriage bolt
Image: Carriage bolts
Image credits: “Threaded” by Thad Zajdowicz is licensed under CC BY 2.0

Elevator bolt

Elevator bolts, as the name suggests, are used in conveyer belt systems.

They have a large flat head and are designed to hold canvas belts. They are very strong and can be used in home toolkit. They are also called as bucket bolts and used a hex nut.

Flange bolt

  It has a flange like feature just below the head which allows the bolt to carry high amount of loads and becomes easier for production.

 A flange bolt finds its application in automotive and plumbing industries.

Hanger bolt

Hanger bolts are headless bolts that are typically used to join wood to metal.

These bolts do not have heads. They have wood or slow threading on one side and machine threading on other. They may require a nut depending upon the load applied.

Hexagonal bolt

Hexagonal or hex bolts have a distinct hexagonal head. These bolts come in variety of finishes and find applications in automotive and construction industry.

The hexagonal shape of its head makes the grip stronger and easier for the worker. It can be loosen or tighten by hand also. They are available in both partially threaded and fully threaded types. They can carry high tensile loads.

Huck bolt

Huck bolts have different locking mechanism than other bolts. Where most bolts use a nut for locking, huck bolts use a cylindrical collar which has a smooth internal diameter. This collar is placed on a pin with locking groves.

These bolts provide direct meta to metal contact which reduces the effect of transverse vibration.

Lag bolt

Lag bolts are one of the toughest fasteners. They are thick and usually have a hexagonal head to account for force needed to install them.

These bolts are used in construction industry to fasten pieces of lumber together. It provides a durable connection due to its high strength. These bolts can bear heavy loads.

Plow bolt

Plow bolt has a unique feature, that is, a non-protruding head. This bolt has a square or hexagonal cross section below the head which is used to tighten or loosen the bolt.

This type of bolts are usually used in farm and road construction industry.

Square head bolt

As the name suggests, this type of bolt has a square head. These type of bolts were used when hexagonal bolts didn’t enter the market.

These bolts are used mostly for aesthetic purposes i.e. for giving a rusty old fashion look.

Stud bolt

Stud refers to a threaded bar. Stud bolt is simply a cylindrical bar having threads at both ends. Two nuts at each end in locking the system.

Usually one end of the stud is kept fixed and other moving. So it finds its application in machines where one end is mobile for example in lathes.

Timber bolt

Timber bolts have flat heads and are used to join two pieces of wood. It has two fins below the head which avoids the bolt to move inside the wood.

T bolt

T bolts have a T shaped head. These are widely used in CNC machines.

U bolt

U bolt have the shape of letter U. These bolts have threads on both the ends and are locked with the help of nut. These bolts have threads on both the ends.

Tower bolt

Tower bolts are used in doors. It is used as a stopper for doors that is it locks the door.

Eye bolt types

Appears as a hook, eye bolt is used for lifting applications. Eye bolts can be used in pushing, pulling, hoisting, anchoring applications etc. The main parts of eye bolt are- eye, shoulder and shank.

Eye bolt has two types, they are-

  • Shouldered eye bolt- It has a shoulder on top of the shank. This allows the bolt to bear angular stresses as well. If locked properly, this bolt allows side stresses as well.
  • Non-shouldered eye bolt-Shoulder is absent in non-shouldered bolts. Due to this, only vertical (in-line) stresses can be are allowed else the bolt will fail.

Types of nuts

Nuts are used for locking the bolt in its place. They have a circular cross section and have inner threads that lock with external threads of bolt.

According to the type bolt used, the type of nut also varies. Various types of nuts are-

  • Axle hat nuts
  • Hex nuts
  • Jam nuts
  • Lock nuts
  • Push nuts
  • Rod coupling nuts
  • Speed nuts
  • Square nuts
  • Tee nuts
  • Wing nuts

Types of failures in bolts

Every mechanical component fails after certain point of time.

Excessive stresses can also cause failure in bolts. According to type of stress, bolt failure can occur in following ways-

  • Shear failure
  • Tensile failure
  • Bearing failure of bolt
  • Bearing failure of plates
  • Tensile failure of plates
  • Block shear failure

Bolt finish types

Surface finish is an essential feature of any mechanical component. Frictional forces acting on a smooth surface are lesser than those acting on a rough surface.

Common ways of finishing in bolts are-

  • Zinc electroplating
  • Galvanising
  • Other techniques include using of black oxide, blue phosphate.

Practice questions

1. What are types of foundation bolts?

Foundation bolts are used for securing a machine to the base or support. The base acts as the foundation here.

Most commonly used foundation bolts are anchor bolts. These bolts bear the weight of the structure. These bolts transmit stresses to the foundation that incurred from the structure.

2. What is bolt pretension and is it different from bolt preload?

Bolt pretension or bolt preload is the tension created in the joint due to compressive force exerted by the nut.

This is very essential for any joint because it gives a tight sealed joint. A lose joint may lead to bolt failure or simply failure of whole system. Preloading ensures a proper joint.

3. What is the difference between a bolt and a machine screw?

Although both screws and bolts are fasteners having external threads, they both have certain features that vary.

Bolt is a mechanical fastener that uses a washer or nut to tighten the joint. A screw is a tapered mechanical fastener that doesn’t use washer or nut. They are simply locked with the internal thread of hole. Sometimes, screws create their own threads to get locked inside the hole.

4.  What is the difference between bearing bolt and friction bolt?

The principle difference between bearing bolt and friction bolt lies in the name itself.

In bearing bolt, the stresses are generated from the sides of hole that is bolt contacts with the sides of hole. In friction bolt, the stress is transferred across the plates with the help of friction. The bolt is tightened to clamps which creates high pressure among the parts.

5. What is the difference between a wedge anchor bolt and a sleeve anchor bolt?

Wedge anchor and sleeve anchor are both used for supporting the structure. The difference lies in the type of support and material on which it is to be used.

Wedge anchors are used in heavy duty applications are mostly used for poured concrete. On the other hand, sleeve anchor is used in light duty applications and can be used in brick and mortar where wedge anchor can’t be used.

6. What are screws without heads called?

Headless screws or screws without heads are also called as set screws.

These screws have thread at both the ends and are designed to join two parts together at both the ends.

Magneto Ignition System: 11 Important Facts

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Have you wondered what happens to petrol when it reaches the fuel tank? Well the answer is simple, the fuel is ignited to produce a certain amount of thermal energy which then gets converted into mechanical energy (rotary motion of wheels). 

There are two ways by which fuel can be ignited- with the help of electric spark or by applying high pressure. Now the question arises, how to create a spark inside the engine? This is the situation where magneto ignition system comes into play.

In spark ignition engines (petrol engines), a spark is required to ignite the fuel. The source of electricity to create a spark may vary according to engine requirements. Read this article further to get a deep insight on how does a magneto work.

What is magneto ignition system?

Spark ignition engines create a spark to ignite air-fuel mixture. This spark is created with the help of an ignition engine.


An ignition system that uses a rotating magnet (magneto) for generating electricity is known as magneto ignition system. This electricity is used to power spark plugs.

Magneto ignition system diagram

How does magneto work
Image: Magneto ignition system
Image source

Parts of magneto ignition system

Magneto ignition system uses following parts-

Many parts are employed which work in harmony to give desired output. The basic parts of magneto are discussed below-

  • Magneto
    Magneto refers to a group of rotating magnets used for producing high voltage. The rotational speed of engine (rpm) is directly proportional to the voltage produced by rotating magnets. Based on the rotation of parts, magneto is of three types-  

    -Armature rotating type
    -Magnet rotating type
    -Polar inductor type
  • Distributor
    As the name suggests, distributor invites ignition surges and then distributes it among individual spark plugs. Distributor has a rotor in the center and metallic electrode on the periphery.
  • Primary and secondary winding
    Primary winding act as the input that is draws the power from source and secondary winding having more number of turns acts as output. Secondary winding is connected to distributor.
  • Cam
    Cam facilitates the motion of magnet. It is connected to the poles of magnet.
  • Circuit breaker
    Cam motion is designed in such a way that it breaks the circuit at certain intervals. When the circuit is breaks, the capacitor starts charging by primary current.
  • Capacitor
    A capacitor is an assembly of two metallic plates placed at a small distance from each other.  Capacitor stores charge.
  • Spark plug
    Spark plug is used for igniting air-fuel mixture inside the engine cylinder. Spark plug has two metallic electrodes separated by a small distance.

How does a magneto work?

Magneto system employs a rotating magnet as the source of electricity, rest of the working is similar to the battery ignition system. Working of magneto ignition system is explained briefly below-


As the engine rotates magnet inside the coil, an EMF is generated and so a current starts flowing through the coils. As the poles of magnet start moving away from the coil, the magnetic flux begins to decrease. At this point, the cam breaks the circuit (cam-type contact breaker).

As the contact breaker breaks the circuit, the flow of current disrupts. As a result, capacitor starts charging and voltage on the secondary winding increases rapidly. The voltage increases up to such an extent that it is able to jump small gaps. When this happens, spark is created and fuel-air mixture is ignited.

Types of magneto ignition systems

Based on the engine rotation, magneto ignition system can be of following type-

  • Magnet rotating type- In this type, magnet rotates and armature is kept fixed. As a result there is a relative motion between magnet and the windings. In modern days, this type of magneto ignition system is commonly used.
  • Polar inductor type- In this type, both the coil and magnet is kept fixed. The moving part here is a soft iron core having projections at fixed intervals.
  • Armature rotating type- In this type, magnet is fixed and the armature rotates.

Dual magneto ignition system

Usually a single magnet is used in small engines like in that of two wheelers. Big engines like that of aircrafts need an extra magnet for safety. In dual magneto ignition system, two magnets are used instead of one. This increases the safety factor of the engine.

Dual magneto ignition system is used in aircraft engines where each engine cylinder has two spark plugs and each spark plug is fired by its individual magneto. In case where failure of one magneto takes place, other magneto keeps the engine running with a slight decrease in efficiency.

High tension magneto| Low tension magneto

There are two types of magneto- high tension and low tension magneto. Their working principle being same in ignition system. Both of these magnetos have a minute difference between them.

High tension magneto produces pulses of high voltage that are sufficient enough to jump across the length between two electrodes of spark plug. This type of magneto works when the circuit breaks, only then the voltage rises up to desired level. The main disadvantage of this type of magneto is that it deals with very high voltage.

Low tension magneto produces a low voltage that is distributed in the transformer coil which is again connected to spark plug. Using a low tension magneto eliminates the need of dealing with high voltages. This type of magneto is generally used in spark ignitors and not in spark plugs.

Battery ignition system| Difference between battery and magneto ignition system

Battery ignition system serves the same purpose as magneto ignition system. It acts as the source of electricity that is used to produce sparks in spark plug.  

Battery ignition system was commonly used in four wheelers but now it is being used in two wheelers as well. A 6V or 12V battery is used to produce a spark unlike magneto ignition system where magneto was the source of electricity.

Battery takes more space hence it was not suggested to use it in two wheelers where space constraint is more. Nowadays compact battery systems are available that can be used in two wheelers also.

The major difference between a battery and magneto ignition system is the source of electricity. In battery ignition system, as the name suggests, battery is used as the source of electricity whereas magneto ignition systems use magneto for generating electricity.

Electronic ignition systems

Electronic ignition systems use electrical circuits having transistors that are controlled by sensors to produce spark. This type of system can ignite even a lean mixture and provides better economy.

Electronic system is divided into two types- Transistor and distributorless ignition system. Electronic ignition system in general, doesn’t use breaker points like those used in magneto ignition system. Hence, this type of system provides breakerless ignition.

Advantages and disadvantages of magneto ignition system

Not every system is ideal, every system has its own pros and cons. It is a design trade off which decides which type of system needs to be used. Following are the advantages of magneto ignition system-

  • It generates electricity on its own hence no need of battery.
  • It occupies less space.
  • No problem of charging or discharging of battery as it doesn’t use one.
  • High efficiency/reliability due to high voltage spark.

Disadvantages of magneto ignition system are-

  • Costlier than other ignition systems.
  • During start, quality of spark is low due to low engine speed. It gets higher with high engine speed.

Practice questions

How does a magneto ignition system work?

Ans: Magneto ignition system works on the principle of Faraday’s first law of electromagnetic induction.

The relative motion between magnet and transformer coils induce an electromotive force (EMF). Due to this, a varying electric current is produced. As rotation of the magnet progresses and poles start moving farther from the coil, a circuit breaker breaks the circuit and disrupts the flow of current.

Due to this, a high voltage is produced at secondary coil which is then distributed to the spark plugs. The voltage is high enough for it to jump across the length between two electrodes of spark plug.

What are the main advantages and disadvantages of magneto ignition system?

Ans: The magneto ignition system has its own pros and cons. Advantages of magneto ignition system are as follows-

  • No batteries are requires as magneto itself generates electricity.
  • Takes up less space than other ignition systems.
  • No problem of discharge as no batteries are used.

The following are disadvantages of magneto ignition system-

  • Expensive as compared to other ignition systems.
  • The voltage produced is directly proportional to the engine speed. So low voltage is produced at start due to low engine speed.

What are the three types of ignition systems?

Ans: To ignite the air-fuel mixture, an ignition system is required. For industrial applications, three types of ignition systems are commonly used-

  • Battery ignition system
  • Magneto ignition system
  • Electronic ignition system

What is the purpose of magneto in an ignition system?

Ans: Magneto is a rotating magnet whose rotation speed is equal to the engine speed.
        
      Pulses of high voltage are required to produce a spark in spark plugs. These pulses are produced by a magneto. The spark produced ignites the air-fuel mixture.

Why magneto ignition system is not used however it has higher efficiency and low maintenance?

Ans:  Magneto system works solely on mechanics of engine rotation hence the voltage keeps varying at different speeds. Electronic ignition system is more efficient overall as it can also ignite lean air-fuel mixture. With use of transistors and sensors, the precision of producing sparks improved. Also, mechanical components are ought to wear after certain period of time.

Because of above reasons, magneto systems are not used these days. However, they were best suitable at the time of their invention.

What route is followed by current in magneto ignition system?

Ans: Current in magneto ignition system is induced by varying magnetic flux around the coil.

The induced current flows through primary winding. A circuit breaker breaks the circuit at certain intervals. Current flow disrupts when the circuit is broken. This results in increase of voltage at secondary winding which is connected to spark plug. As the poles reverse, the flow of current reverses.

What is a more efficient ignition system coil and battery or magneto?

Ans: The answer to this question depends on the basis of comparison.

        If we compare on the basis of space and discharge rate, then magneto is more efficient as it takes up less space and has no issue of discharging.

       If we compare on the basis of ignition timing, then battery ignition system is more efficient as it doesn’t have fixed ignition timing. Magneto ignition system is designed mechanically so, it has a fixed ignition timing.

This becomes a problem at low speeds because of low voltage produced. Hence, an ignition system with variable ignition timing is more efficient than one with fixed ignition timing.

Hydraulic Diameter : Calculation of Pipe, Rectangle, Ellipse, FAQs

by
CodeCogsEqn

Table of Contents

Hydraulic diameter definition

Circle being the simplest shape, easiest form of calculations come around while dealing with circular cross sections. When fluid flows through a non-circular duct, we convert the cross section to circular for convenient calculations. This newly derived diameter of circular cross section is called as hydraulic diameter. It is denoted as Dh. Hence, we can find the same results for a non-circular duct as circular duct by using the concept of hydraulic diameter.

Hydraulic diameter equation

Hydraulic diameter can be found using the formula given below-

Dh = 4A/P

Where,
Dh is hydraulic diameter
A is area of non-circular cross section
P is the wetted perimeter of non-circular cross section

Hydraulic diameter is a function of hydraulic radius Rh, which can be found by dividing area of cross section, A by wetted perimeter, P.

CodeCogsEqn

Note that Dh = 4Rh

This relation is different from the conventional relation between diameter and radius (i.e. D = 2R). This difference arises only while converting non-circular cross sections to circular.

Note- Law of conservation of momentum is satisfied while calculating the hydraulic diameter. Also, hydraulic diameter is not same as normal diameter. Dh is same only for circular conduits.

hydraulic diameter
Simple representation of hydraulic diameter

Hydraulic diameter and Reynold’s number

Reynold’s number is used in fluid mechanics and heat transfer to find the type of flow, laminar or turbulent. Hydraulic diameter is used in the formula to calculate Reynold’s number.
Reynold’s number is the ratio inertia forces to viscous forces. It is a dimensionless number named after Irish scientist Osborne Reynolds who popularized this concept in 1883.

This number shows the effect of viscosity in controlling the velocity of flowing fluid. A linear profile of viscosity is developed when the flow is laminar. In Laminar flow, the fluid flows in such a way that it appears as if it was flowing in parallel layers. These layers do not intersect each other and move without any disruption in between them. This type of flow usually occurs at slow speeds. At slow speeds, mixing of two layers doesn’t take place and fluid flows in layers stacked above one another.

Laminar flow helps us to measure the flow of highly viscous fluids as this type of flow gives a linear relationship between flow rate and pressure drop. Favorable conditions for laminar flow is high viscosity and low velocity. At greater speeds, the fluid particles start behaving in a different manner resulting in mixing of fluid layers. Such mixing gives rise to turbulence and hence the name turbulent flow. Turbulent flow is desirable when proper mixing of fluid is required. One such example is mixing of fuel with oxidizer in rocket engines. Turbulence helps in thorough mixing of fluid.
Reynold’s number can be calculated from the equation given below-

                                                            CodeCogsEqn 3

Where,
Re is Reynold’s number
u is mean speed velocity (in m/s)
ν is kinematic viscosity (in m2/s)
Dh is hydraulic diameter (in m)

In a circular pipe,
Laminar flow, Re < 2000
Transient flow, 2000 < Re <4000
Turbulent flow, Re > 4000

For a flat plate,
Laminar flow, Re <5,00,000
Turbulent flow, Re > 5,00,000

Laminar flow and turbulent flow

Hydraulic diameter of circular pipe | hydraulic diameter of cylinder

Circular pipes are most commonly used pipes for transporting fluid/gas from one place to other (even for large distances). Water pipelines are real life example of circular ducts that are used for transporting fluid. These pipes can carry large distances such as from water filter stations to homes as well as short distances such as ground water tank to terrace water tank. The hydraulic diameter of circular pipe is given by-

Dh = 4πR2/2πR = 2R

                                                                      
Where,
R is the radius of circular cross section.

Circle

Hydraulic diameter of rectangular duct

Rectangular ducts are used when spacing is an issue. Moreover, rectangular ducts are easy to fabricate and reduce pressure loss. Air conditioners use rectangular ducts to avoid pressure losses. The hydraulic diameter of rectangular duct is given by-

Dh = 4ab/2(a+b) = 2ab/ a+b

                                                                         
Where,

a and b are the lengths of larger and shorter sides.

Rectangle
For square cross section,

a = b

Dh = 2a2/2a = a

Where,
a is the length of each side of square.

Hydraulic diameter of annulus

Sometimes, to increase/decrease the rate of heat transfer, two fluids are passed through an annular tube such that one fluid flows outside the other. heat transfer rate is affected by the action of two fluids. Hydraulic diameter of annulus is given by-    

gif

Where D and d are diameters of outer circle and inner circle respectively.

Annulus

                                                                           

Hydraulic diameter of triangle

gif

Where,
l is the length of each side.

Triangular cross section
                                                   

Hydraulic diameter of ellipse

Dh = 4wh(64-16e2)/w+h(64-3e4)

Where, e= w-h/w+h

Hydraulic diameter of plate heat exchanger | hydraulic diameter of shell and tube heat exchanger

Heat exchangers are thermal devices used for transferring heat from one fluid to other in order to decrease/increase the temperature of fluid as desired. Many types of heat exchangers exist out of which most commonly used are plate and shell tube heat exchangers. Fluids can be passed through the heat exchanger in two ways. In first type, both hot and cold fluids are injected in the same direction hence, it is called as parallel flow heat exchanger. In second type, fluids are passed through the tube in opposite directions hence it is called as a counter flow heat exchanger.

Based on this, evaporator and condenser are designed. In evaporator, the hot fluid’s temperature remains same while the cold fluid gets warmer. In condenser, the temperature of cold fluid remains same and hotter fluid’s temperature decreases.

The rate of transfer in heat exchanger is given by following relation-

For hot fluid: Qh = mh Cph (Thi – Tho )
For cold fluid: Qc = mc Cpc (Tco – Tci )

By conservation of energy,
Heat lost by hot fluid = heat gained by cold fluid.
=> Qh = Qc

Where,
Qh denotes heat lost by hot fluid
Qc denotes the heat gained by cold fluid
Thi is the temperature of hot fluid at inlet
Tho is the temperature of hot fluid at outlet
Tci is the temperature of cold fluid at inlet
Tco is the temperature of cold fluid at outlet
mh is the mass of hot fluid (in Kg)
mc is the mass of cold fluid (in Kg)
Cph is the specific heat of hot fluid (in J/K-Kg)
Cpc is the specific heat of cold fluid  (in J/K-Kg)

In plate heat exchangers, heat cuts through the section and separates hot and cold fluids. This type of heat exchanger is used in many industrial applications. They are used in heat pump, oil cooling systems, engine cooling system, thermal storage systems etc.
Plate heat exchanger has a rectangular/square cross section hence, hydraulic diameter is given by-

                                                                        Dh = 2ab/a+b            

Where,
a and b are lengths of shorter side and longer side respectively.

Plate heat exchanger 2
Plate heat exchanger
Image credits: https://commons.wikimedia.org/wiki/File:Plate_frame_1.svg

In shell and tube type heat exchanger, tubes are installed in a cylindrical shell. Both hot and cold fluids are passed through these tubes in such a way that one fluid flows outside the other fluid. Due to this, heat is transferred from one fluid to another. Shell type heat exchanger is widely used in industries mainly in chemical processes and applications where high pressure is needed.
Shell tube heat exchanger has annular cross section hence, hydraulic diameter is given by

                                                                               Dh = D-d
shell tube
Shell and tube heat exchanger
Image credits: Straight-tube heat exchanger 2-pass

Equivalent diameter vs Hydraulic diameter

Equivalent diameter and hydraulic diameter differ in values. The diameter of circular duct which gives same pressure loss as rectangular duct for equal flow is called as equivalent diameter. Even though circular ducts have least surface area for given pressure loss, they are not suitable for fabrication. Rectangular ducts are easy to fabricate hence they are used in practical cases. When flow rate and pressure drop is known, then to design a rectangular duct, we use friction chart to find the equivalent diameter and then required dimensions by fixing certain parameters like aspect ratio or length of any one side.

The ratio of length of shorter side to longer side is called as aspect ratio.

AR = a/b
                                                               

We can find equivalent diameter by Huebscher equivalent diameter equation. It is shown below-
                   De = 1.30 (ab)0.625/(a+b)0.25

Where,

a and b are length of shorter side and longer side respectively.

Recent studies have concluded that equivalent diameter being derived from empirical relations, is not reliable while calculating pressure losses in pipes. Hence, we use hydraulic diameter in all cases.

What is the difference between hydraulic diameter, equivalent diameter and characteristic length in fluid mechanics and heat transfer?

Hydraulic diameter, as discussed earlier, is the newly derived diameter from a non-circular duct such that the flow characteristics remain same. Hydraulic diameter is used for calculating Reynold’s number which helps us to understand whether the flow is laminar, transient or turbulent.

The diameter of circular duct which gives same pressure loss as rectangular duct for equal flow is called as equivalent diameter.

Pressure loss in a pipe is given by Darcy-Weisbach equation-  

gif

Where,

ρ is the density of the fluid (kg/m^3)
D is the hydraulic diameter of pipe (in m)
l is the length of pipe (in m)
v is the mean flow velocity (in m/s)Characteristic length is basically volume of a system divided by its surface area.
It can be equal to hydraulic diameter in some cases.

Mathematically,

Lc = Vsurface/Asurface

For square duct-
Lc = a

For rectangular duct-

Lc = 2ab/a+b

In heat transfer, characteristic length is used for calculating Nusselt number.The ratio of convective heat transfer to conductive heat transfer is called as Nusselt number. It shows what type of heat transfer dominates.
Nusselt number, Nu is given by-

Nu = hLc/k

What is the difference between hydraulic radius and hydraulic depth / hydraulic mean depth?

There is a misconception that hydraulic radius and hydraulic depth are same. They both have different meanings and hold individual significance while measuring fluid properties. The concept of hydraulic radius and hydraulic depth is discussed in detail below.

The ratio of cross sectional area of flow to the wetted perimeter is called as hydraulic radius.
Rh = A/P

The ratio of cross sectional area of flow to free water surface or top surface width is called as hydraulic depth.

Hd = A/T

where,

A is the cross sectional area of flow
T is the width up to top surface or free surface.

Mathematically, hydraulic mean depth and hydraulic radius are same.

What is the physical significance of hydraulic diameter in fluid and thermal sciences?

Practically, Reynold’s number is used to check the behaviour or nature of the fluid flow. This in turn helps us in finding Nusselt number which is then used to find the rate of heat transfer from the closed conduit.
Hence, Reynold’s number is a very important dimensionless number which plays a vital role in both fluid and thermal sciences. But to find Reynold’s number, first we need to find hydraulic diameter of the closed conduit. For non-circular cross sections, hydraulic diameter provides a value of diameter such that its flow characteristics are equivalent to that of a circular cross section.

The ratio of convective heat transfer to conductive heat transfer is called as Nusselt number.

Nusselt number is given by following relation-

For laminar flow: Nu = 0.332 Re0.5 Pr0.33
For turbulent flow: Nu = 0.039 Re0.8 Pr0.33

Where,
Re denotes Reynold’s number
Pr denotes Prandtl number

The ratio of momentum diffusivity to thermal diffusivity is called as Prandtl number. It is named after German scientist Ludwig Prandtl. This dimensionless number helps us in calculations related to forced and natural heat convection. Its significance is that it helps us to study the relation between momentum transport and thermal transport capacity of fluid.

Prandtl number is calculated by the formula given below-

Pr = μCp/k

Where,
Pr is Prandtl number
µ is dynamic viscosity
Cp is specific heat

Note that Nusselt number can also be found using the relation: Nu = hLc/k, when we know the values of convective and conductive heat resistances.

In simple words, hydraulic diameter forms the basis for finding the behaviour of flow and rate of heat transfer from the fluid that is flowing in a closed conduit. With that, it also brings us easy calculations by converting a non-circular conduit to a circular one.