Mechanical springs, by A.M. Wahl

Mechanical springs, by A.M. Wahl


General considerations in mechanical spring design --
Spring materials: description and physical properties --
Fatigue properties of springs and spring materials --
Elevated-temperature properties of springs and spring materials --
General design considerations for helical compression or extension --
Aids in helical spring design: tables, charts, tolerances --
Practical selection and design of cold-wound helical compression springs --
Design of hot-wound compression or extension springs --
Helical extension spring design --
Design of square- or rectangular-bar helical springs --
Helical torsion springs --
Spiral, power, and neg'ator springs. --
Coned-disk or Belleville springs --
Flat and leaf springs --
Volute springs --
Ring springs --
Rubber springs and mountings --
Torsion-bar springs --
Theory for helical extension or compression springs --
Open-coiled helical springs with large deflections --
Theory for square- and rectangular-bar helical springs under axial loading --
Plastic-flow effects due to presetting in helical springs --
Rational method of analysis for fatigue or repeated loading of helical springs --
Buckling and lateral loading of helical compression springs --
Vibration and impact effects in helical springs --
Helical torsion spring theory --
Spiral spring theory --
Coned- or flat-disk spring theory --
Radially tapered disk spring theory.

download

fe-safe 2018 for Windows / Linux

company, and is one of the most powerful fusion analysis software in Finite Element. This software provides its users with the ability to define different models and methods for analyzing features, so that most industrial models can be analyzed using this software. 
Fe-safe is very important for many industries such as automotive, defense, marine, power plants, medical engineering, etc. 
Fe-safe is the first software developed and developed for fatigue analysis, and hence the best and most effective fatigue analysis methods can be developed.
One of the best features of this software is the analysis of thermomechanical fatigue, creep fatigue and fatigue in composite models that transforms it into a unique set of analyzes, and in fact, with the least complexity of software, the most difficult fatigue models for simulation users he does. 
What industry always calls for engineering is the use of minima to produce maximum production, and in the sense of maximum efficiency. But the need to guarantee their engineering designs is their continuing performance, and this can not be achieved except by simulating the long cycles of engineering mechanisms.
Many industrial complexes use limited finite element methods to design their products, and in particular stress analysis, but due to the lack of efficient analytical software, they continue to use manual calculations that make work greatly difficult. But fe-safe is best suited for industrial users by simplifying the fatigue definition.

download section 1

download section 2

size : 1.2 GB

password : www.downloadly.ir

Design Engineer’s Handbook Keith L. Richards

Design Engineer’s
Handbook
Keith L. Richards

Student design engineers often require a "cookbook" approach to solving certain problems in mechanical engineering. With this focus on providing simplified information that is easy to retrieve, retired mechanical design engineer Keith L. Richards has written Design Engineer's Handbook.

This book conveys the author's insights from his decades of experience in fields ranging from machine tools to aerospace. Sharing the vast knowledge and experience that has served him well in his own career, this book is specifically aimed at the student design engineer who has left full- or part-time academic studies and requires a handy reference handbook to use in practice. Full of material often left out of many academic references, this book includes important in-depth coverage of key topics, such as:

• Effects of fatigue and fracture in catastrophic failures
• Lugs and shear pins
• Helical compression springs
• Thick-walled or compound cylinder
• Cam and follower design
• Beams and torsion
• Limits and fits and gear systems
• Use of Mohr's circle in both analytical and experimental stress analysis

This guide has been written not to replace established primary reference books but to provide a secondary handbook that gives student designers additional guidance. Helping readers determine the most efficiently designed and cost-effective solutions to a variety of engineering problems, this book offers a wealth of tables, graphs, and detailed design examples that will benefit new mechanical engineers from all walks.

download

ADVANCES IN THE TECHNOLOGY OF STAINLESS STEELS AND RELATED ALLOYS ASTM Special Technical Publication No. 369

ADVANCES IN THE TECHNOLOGY
OF STAINLESS STEELS AND
RELATED ALLOYS
ASTM STP No. 369

During 1963 the American Society for Testing and Materials; the Metallurgical Society of the American Institute of Mining, Metallurgical, and Petroleum Engineers; and The Electrochemical Society and the National Association of Corrosion Engineers, organized symposia dealing with several aspects of the technology of stainless steels. Members of these organizations who were concerned with these activities decided that it would be advantageous to those interested in stainless steels if the papers presented at these meetings could be combined in a single pubhcation for convenient future reference. They formed an Intersociety Co-ordinating Committee to accomphsh this.

DOWNLOAD

Fatigue and Fracture Mechanics: 37th Volume STP 1526

Fatigue and Fracture Mechanics:
37th Volume
STP 1526

This special technical publication (STP1526) is a compilation of papers presented by several authors at the Ninth International ASTM/ESIS Symposium on Fatigue and Fracture Mechanics (37th National Symposium on Fatigue and Fracture Mechanics) and published in the Journal of ASTM
International (JAI) after successful peer reviews. The International Symposium was jointly sponsored by ASTM Committee E08 on Fatigue and Fracture and the European Structural Integrity Society. The Symposium was held during May 20–22, 2009 in Vancouver, British Columbia, Canada, in
conjunction with the May 18–19, 2009 standards development meetings of ASTM Committee E08.
The opening Jerold L. Swedlow memorial lecture was delivered at the Symposium by Professor Dr.-Ing. Karl-Heinz Schwalbe on analytical models for fatigue crack propagation and fracture. The symposium focused on three major tracks of fatigue and fracture of structures and materials under 1)
thermomechanical conditions, 2) multiaxial loading conditions, and 3) application of cohesive zone models to fracture problems. In addition, several papers were presented at the Symposium in the traditional areas of fatigue behavior, fracture mechanics and mechanisms, fatigue crack propagation,
and effects of residual stresses on fatigue and fracture.

DOWNLOAD

Theories of Failure | Strength of Materials

This video lecture will give you a good introduction to theories of failure in Strength of materials.Check http://www.learnengineering.org/2012/... to learn more about industrial applications of theories

TYPES OF FAILURE IN MATERIAL (FATIGUE, BRITTLE & DUCTILE FAILURE)

Modes of Material failure, Fracture , Creep , Fatigue And More

Modes of Material Failure, Fracture , Creep , Fatigue And  More

When the load on a ductile material exceeds the elastic limit, it becomes permanently deformed and elastic failure is said to have occurred. The material may still be intact but it is likely that the component from which it is made will no longer be fit for its intended purpose.

fracture
 Brittle materials such as cast iron, very often  fail in the elastic range with the brittle types of fracture shown in Figure 1 . Brittle fracture, which is also known as cleavage fracture, is more prevalent in materials with BCC and CPH crystal lattice structures. Under certain conditions, ductile materials can
also fail with a brittle type of fracture, as will be explained.

FIG 1

Both kinds of failure are to be avoided by incorporating a suitable factor of safety into the design of engineering components. As a general rule, factor of safety of at least 2 should be employed on static structures. With this in place, the working stress in the material should always be less than half of that which will cause failure. That is

In spite of the best intentions of design engineers, components sometimes fail in service. Static loads can be hard to predict and dynamic loads on the component parts of machinery, motor vehicles and aircraft are very difficult to analyse. Combinations of direct loading, shearing, bending and twisting are very often present. A complex stress system is then said to exist, the resultant of which may exceed the predicted working stress and lead to failure.

FIG 2

An additional danger is the presence of stress concentrations in a component. These can occur at sharp internal corners, holes, fixing points and welds. They are known as stress raisers, where
the stress may exceed that at which failure occurs. Under certain conditions, cracks can spread from these points, which eventually lead to failure. These kinds of failure are usually detected at the
prototype stage and the design modified to prevent them occurring.
Material faults such as the presence of cavities, impurities, large grain size and inappropriate heat treatment can also contribute to failure if not detected by quality control procedures.
Under certain circumstances, materials can fail at comparatively low stress levels that would normally be considered to be quite safe. The main reasons for this are changes in temperature, which can affect the properties of a material, and cyclic loading. Low temperatures can cause brittleness and loss of strength. High temperatures can cause the material to creep, and eventually fail,
under loads that are well below the normal elastic limit. A material is subjected to cyclic loading when it is repeatedly being loaded and unloaded. The loads may be well below that which would be
expected to cause failure, but over a period of time, failure can occur due to metal fatigue. Some of these failure modes will now be described
Materials can fail due to metal fatigue, creep and brittle fracture at stress levels which would normally be considered safe

Brittle fracture

The plastic deformation which precedes a ductile fracture takes a finite amount of time to take place. If a load in excess of that which will cause fracture is suddenly applied, as with an impact
load, there will be insufficient time for plastic deformation to take
place and a brittle form of fracture may occur. This can be observed during an Izod or Charpy impact test where an otherwise ductile material is suddenly fractured by an impact load. Brittle, or cleavage fractures usually have a granular appearance due to the reflection of light from the individual grains figure 3 . Too large a grain size can affect the strength of a material and make it brittle. Grain growth can occur when materials are operating at high temperatures for long periods of time. Here the grains feed off each other in cannibal fashion, reducing the strength of the material and increasing the likelihood of brittle fracture.

FIG 3

Some metals which exhibit ductile behaviour under normal conditions become very brittle at low temperatures. The temperature at which the change occurs is called the transition temperature.
Mild steel becomes brittle at around 0 ⁰C. As can be seen from Figure 4 the transition temperature is judged to be that at which the fracture surface of an Izod or Charpy specimen is 50% granular
and brittle and 50% smooth and ductile. It is generally the
metals with BCC and CPH crystal lattice structures that are affected in this way by low temperatures. The ferrite grains in steel, which are almost pure iron, have a BCC structure at normal temperatures. Chromium, tungsten and molybdenum are also BCC and suffer from low temperature brittleness in the same way as iron.

FIG 4

The cracking and sinking of the all-welded liberty ships in the Second World War was attributed to brittle fracture. The cold north Atlantic temperatures produced brittleness in the steel hulls.
Cracks appeared in places of stress concentration and these quickly spread with disastrous effects. The transition temperature in mild steel is raised by the presence of phosphorous and lowered
by the addition of manganese and nickel. A relatively high level of phosphorous was present in the steel of the liberty ships and all ship hulls are now made from steel which is low in phosphorous.

NOTE :At a transition temperatures of around 0⁰C mild steel with a high phosphorous content becomes brittle. The presence of nickel and manganese lowers the transition temperature.

FIG 5 DUCTILE FRACTURE 

Creep
Creep is a form of plastic deformation which takes place over a period of time at stress levels which may be well below the yield stress of a material. It is temperature related and as a general rule, there will be little or no creep at temperatures below

0.4×T

where
T = is the melting point of the material measured on the Kelvin scale.
For mild steel, T= 1500 ⁰C which is 1773 K and so there should be very little creep below 0.4×1773=709 K which is 436 ⁰C. It should be stressed that this is only a general rule and that some of the softer low-melting point metals such as lead, will creep under load at normal temperatures
With the more common engineering metals, creep is a problem encountered at sustained high temperatures such as those found in steam and gas turbine plant. Under extreme conditions it can
eventually lead to failure. A typical graph of creep deformation against time is shown in Figure 6

FIG 6 

Figure 6 shows the behaviour of a material which is above the threshold temperature at which creep is likely to occur. When it is initially loaded, the elastic extension OA is produced. If the stress in the material is below a level called the limiting creep stress or creep limit at that temperature, there will be no further extension.
If, however, the stress is above this level, primary creep AB, commences. This begins at a rapid rate, as indicated by the slope of the graph, and then decreases as work hardening sets in. It is followed by secondary creep BC, which takes place over a comparatively long period at a steady rate. The final stage is tertiary creep CD, where the deformation rate increases. Necking becomes
apparent at this stage, leading finally to fracture at D. Creep in polymer materials below the glass transition temperature is found to proceed in the same way

FIG 7

Increases in temperature and/or increases in stress have the effect shown in Figure 8. The rate at which the three stages of creep take place is increased and failure occurs in a shorter time.
Study of the nature of creep suggests that the plastic deformation is partly due to slipping of the planes of atoms in the grains and partly due to viscous flow at the grain boundaries. The atoms tend to pile up in an irregular fashion at the grain boundaries which would normally lead to work hardening. The high temperatures, however, have a relieving effect, and the smaller the grains the greater is the viscous flow at the grain boundaries

FIG 8

Creep resistance can be increased in two ways. The first is to introduce alloying elements which reduce slipping within the grains. The second is to have as course a grain structure as possible,
bearing in mind that this can lead to increased brittleness at normal temperatures. Many creep resistant materials have been developed over the last 50 years, in particular the nimonic series of
alloys which have been widely used in gas turbines
Failure due to material creep is likely to occur if a material is loaded for sustained periods above the creep limit stress and above the creep threshold temperature.

Fatigue
Fatigue failure is a phenomena which can occur in components which are subjected to cyclic loading. That is to say that they are repeatedly subjected to fluctuating or alternating stresses. Typical examples are the suspension units on motor vehicles and the connecting rods and crankshafts in internal combustion engines. The forces and vibrations set up by out-of-balance rotating parts can
also produce cyclic loading. The alternating stresses may be well below the elastic limit stress, and the material would be able to carry a static load of the same magnitude indefinitely. Failure
usually starts with a small crack which grows steadily with time. Eventually the remaining cross-sectional area of the component becomes too small to carry the repeated loads and the material
fractures.

FIG 9

It is found that in ferrous metals, there is a certain stress level below which fatigue failure will not occur no matter how many stress reversals take place. This is called the fatigue limit and is given the symbol SD. As a general rule for steels, the fatigue limit is about one half of the UTS of the material. The higher the stress above this value, the fewer will be the number of reversals or stress cycles before failure occurs.A typical graph of stress level against the number of cycles leading to failure for a material such as steel is shown in Figure  10. The graph is often referred to as an S–N curve

FIG 10

Fatigue cracks are observed to spread from points of stress concentration. Cyclic loading at stress levels above SD produces slip in the planes of atoms in the grains of a material. This results in the appearance of small extrusions and intrusions on the surface of an otherwise smooth material, as shown in Figure 11. Although the intrusions are very small, they act as stress raisers from which a fatigue crack can spread. If other stress raisers are present such as sharp internal corners, tool marks, and quench cracks from heat treatment, the process can be accelerated and these should be guarded against

FIG 11

The fracture surfaces of a fatigue failure have a characteristic appearance as shown in Figure 12. As the fatigue crack spreads, its two sides rub together under the action of the cyclic loading.
This gives them a burnished, mother-of-pearl appearance. Eventually, the material can no longer carry the load and fractures. The remainder of the surface, where fracture has occurred, has a
crystalline or granular appearance

FIG 12

There are many non-ferrous metals which do not have a fatigue limit and which will eventually fail even at very low levels of cyclic loading. Some steels, when operating in corrosive conditions,
exhibit these characteristics. The S–N graph for such materials is shown in Figure 13. Instead of a fatigue limit, they are quoted as having an endurance limit which is given the symbol SN. The
endurance limit is defined as the stress which can be sustained for a given number of loading cycles. Components made from these materials should be closely monitored, especially when used
in aircraft, and replaced at a safe time before the specified number of cycles N, has been reached.

FIG 13

Fatigue failure will eventually occur when components are subjected to cyclic loading at stress levels above the fatigue limit. The presence of stress concentrations accelerates the effect.


Degradation
Ferrous metals are affected by two kinds of corrosion. Low temperature or ‘wet’ corrosion is due to the presence of moisture and results in the formation of red rust. This, as you well know, is very
loose and porous. Red rust is an iron oxide formed by electrochemical action, in which the moisture acts as an electrolyte.
Adjacent areas of the metal, which have a different composition, such as the alternate layers of ferrite and cementite in the pearlite grains, become the anodes and cathodes. Corrosion occurs at the anode areas resulting in rust formation

FIG 14 

Figure 14 shows that the ferrite layers, which are almost pure iron, become anodes and corrode to form FeOH3 which is red rust. The same kind of electrolytic action can occur between adjacent
areas which have been cold worked to a different extent. Figure 15 shows a fold in a sheet of metal which is more highly stressed than the surrounding areas. In the presence of moisture, the region in
the fold becomes an anode and corrodes. This kind of electrolytic action is called stress corrosion. It is the form from which motor vehicle panels can suffer if they are not properly protected

FIG 15

High temperature or ‘dry’ corrosion occurs due to a direct chemical reaction between the metal and oxygen of the atmosphere. It results in the formation of black millscale when the metal is heated for forging or for heat treatment. Millscale is another form of loose and porous iron oxide whose chemical formula is FeO (Figure 16).

FIG 16

As has been mentioned, the oxide films that form on the surface of non-ferrous metals and alloys are generally quite dense. Polished copper, brass and silver very soon become tarnished but once a thin
oxide film has formed, it protects the metal from further attack. Sometimes the oxide film is artificially thickened by an electrolytic process known as anodising. Aluminium alloys for outside uses such as door and window frames, are treated in this way.

Key points
- Wet corrosion of ferrous metals is an electro-chemical process where moisture acts as an electrolyte and different regions of the material become anodes and cathodes.
- Dry corrosion is a direct chemical reaction between a material and oxygen in the atmosphere.

FIG 17

Solvent attack
Thermosetting plastics tend to have a high resistance to solvents and it is generally thermoplastics and rubbers which are most vulnerable. The action of the solvent is to break down the Van der Waal forces and take the polymers into solution. Industrial solvents used for degreasing and for paint thinners, petrol, fuel oil, lubricating oils and greases can have this effect on some polymers

FIG 18

Radiation damage and ageing
The ultra-violet radiation present in sunlight can have a degrading effect on some thermoplastics and rubbers. It progressively causes oxygen atom cross-links to form between the polymers. These cause the material to become brittle and can also lead to discolouration. Ultra-violet lamps and X-rays used in industrial processes can also cause this kind of degradation. Colouring pigments are often added during the polymer forming process, and this reduces the effect. The darker colours are the most effective, black being the best of all.

FIG 19

The degradation of thermoplastics due to ultra-violet radiation can be slowed down by the introduction of colouring pigments.

Deterioration of ceramics
The ceramic tiles, bricks, cements and natural stone used for building degrade with time due to moisture and pollutant gases in the atmosphere. The absorption of rain-water into the surface
pores can cause deterioration in winter. When the moisture freezes, it expands and over a period of time it can cause cracking and flaking. Sulphur from flue and exhaust gases combines with moisture in the atmosphere to form sulphurous acid which falls as acid rain. This attacks many types of ceramic building material and in particular, natural stone. The refractory ceramics used to
line furnaces, and the ladles for carrying molten metal, can suffer from thermal shock if heated too quickly. Because they are poor conductors of heat, there can be a very large temperature difference
between the heated surface and the material beneath it. As a result, the expansion of the surface layer can cause flaking or spalling.
Refractory linings can also be attacked at high temperatures by the slag which rises to the surface of molten metal. There are two types of slag which form, depending on the impurities present in the metal. One is acidic and the other is known as basic slag. Linings of silica brick are resistant to acidic slag whilst linings of dolomite, which contains calcium and magnesium carbonates, and magnesite, which contains magnesium oxide, are resistant to basic slag.

FIG 20

DAMAGE IN COMPOSITE MATERIALS: BASIC MECHANISMS, ACCUMULATION, TOLERANCE, AND CHARACTERIZATION STP 775

DAMAGE IN COMPOSITE
MATERIALS: BASIC
MECHANISMS,
ACCUMULATION,
TOLERANCE, AND
CHARACTERIZATION
STP 775

It is well established that the micro-events which reduce the strength apd stiffness, and determine the life of composite laminates (commonly referred to as "damage") are complex, various, and intricately related to a variety of failure modes under different circumstances. The study of individual details

of damage is certainly of academic interest. However, it was the objective of the symposium which formed the basis for this book to provide a forum for the general discussion of the specific nature of damage in composite materials as a collective condition, what might be called a "damage state." The

symposium was sponsored by Subcommittees E09.03 on Fatigue of Composite Materials and E09.01 on Research, in Committee E-9 on Fatigue. Committee E-7 on Nondestructive Testing also contributed in a formal way. The symposium material was chosen and organized to specifically serve

three groups of people.

1. Materials scientists and nondestructive evaluation practitioners: For this group, the symposium was intended to provide an opportunity to establish the mechanisms which create damage in composite materials, to discuss the experimental methods that can be used to investigate those mechanisms,

and to study the relationship of these mechanisms to loads, strains, and other environments.

2. Fatigue researchers in composite materials: For this group, whether they consider composites to be structural materials or models for studying microscopic damage of complex material systems such as metal alloys, ceramics, semi-crystalline polymers, etc., it was intended that the symposium

provide an attempt to establish the nature of damage accumulation, and to identify and characterize cumulative damage states as collective entities as an approach to anticipating the residual properties and response of such materials. This emphasis included an effort to develop modeling methods and

analytical techniques which can be used to represent damage states and to anticipate response in unfamiliar circumstances.

3. Designers and others primarily concerned with the application of composite materials to engineering structures and with the nondestructive testing of those structures: The symposium was intended to provide information to this group which could be used to assess the damage tolerance of various composite laminates and structures in terms of their subsequent strength, life

and stiffness following the formation of damage states.

DOWNLOAD

Fuels Specifications: What They Are, Why We Have Them, and How They Are Used

Fuels Specifications: What They
Are, Why We Have Them, and
How They Are Used

The publication of this manual was accomplished through the combined efforts of many
individuals. First and foremost, we would like to convey our sincerest appreciation to all of them—
particularly the thirteen authors, who are all experts in their particular fields and who bring a
broad spectrum of interests, experience, and knowledge of fuels specifications to this manual.
They have devoted considerable time, energy, and resources to support this endeavor. We also
appreciate the assistance of the ASTM publication staff—particularly Kathy Dernoga, Monica
Siperko, Sara Welliver, and Rebecca Edwards—who have given us much behind-the-scenes guidance
and assistance from the outset of this venture. In addition, we are grateful to the 20 experts
who have reviewed the various chapters and who, through their perusal of the chapters, made
suggestions that permitted good manuscripts to be made better. Finally, we would like to extend
our utmost appreciation to the industrial and governmental employers of all those involved in this
publication. They, ultimately, make it possible for us to produce manuals such as this for the
benefit of those who use petroleum standards worldwide.

DOWNLOAD

Standard Method for Estimating Kinetic Parameters by Differential Scanning Calorimeter Using the Borchardt and Daniels Method1

Standard Method for
Estimating Kinetic Parameters by Differential Scanning Calorimeter Using the Borchardt and Daniels Method1

Scope
1.1 This test method describes the determination of the kinetic parameters of activation energy, Arrhenius preexponential factor, and reaction order using the Borchardt and Daniels2 treatment of data obtained by differential scanning calorimetry. This test method is applicable to the temperature
range from 170 to 870 K (−100 to 600°C).
1.2 This treatment is applicable only to smooth exothermic reactions with no shoulders, discontinuous changes, or shifts in baseline. It is applicable only to reactions with reaction order
n # 2. It is not applicable to autocatalyzed reactions and, therefore, is not applicable to the determination of kinetic parameters for most thermoset curing reactions or to crystallization
reactions.
1.3 Electronic instrumentation or automated data analysis systems or treatments equivalent to this test method may be used.
NOTE 1—The user is advised that all electronic data treatment may not be equivalent. It is the responsibility of the user of such electronic data treatment to verify applicability to this test method.
1.4 SI values are the standard.
1.5 This test method is similar, but not equivalent to, ISO Method 11357, Part 5, which contains provisions for additional information not supplied by this test method.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

download

Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter (Precision Method)

Standard Test Method for
Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb
Calorimeter (Precision Method)

Scope
1.1 This test method covers the determination of the heat of combustion of hydrocarbon fuels. It is designed specifically for use with aviation turbine fuels when the permissible difference
between duplicate determinations is of the order of 0.2 %. It can be used for a wide range of volatile and nonvolatile materials where slightly greater differences in precision can be tolerated.
1.2 In order to attain this precision, strict adherence to all details of the procedure is essential since the error contributed by each individual measurement that affects the precision shall be kept below 0.04 %, insofar as possible.
1.3 Under normal conditions, the method is directly applicable to such fuels as gasolines, kerosines, Nos. 1 and 2 fuel oil, Nos. 1-D and 2-D diesel fuel and Nos. 0-GT, 1-GT, and
2-GT gas turbine fuels. 1.4 Through the improvement of the calorimeter controls and temperature measurements, the precision is improved over that of Test Method D 240.
1.5 The values stated in SI units are to be regarded as the standard.
1.6 This standard does not purport to address the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, see
Section 7, 10.6, A1.7.1 and Annex A3.

download

An Introduction To Calorimetry types And Uses , Bomb and Boy,s Gas Calorimeters

An Introduction  To Calorimetry types  And  Uses , Bomb  and  Boy,s Gas Calorimeters 

Introduction
Calorimetry is the field of science that deals with the measurement of the state of a body with respect to the thermal aspects in order to examine its physical and chemical changes. The changes could be physical such as melting, evaporation etc or could also be chemical such as burning, acid-base neutralization etc. Calorimeter is what is used to measure the thermal changes of a body. Calorimetry is applied extensively in the fields of thermochemistry in calculating the enthalpy, stability, heat capacity etc.

                                                 
Types of Calorimeter

The different types of calorimeters are
1- Adiabatic Calorimeters
2- Reaction Calorimeters
3- Bomb Calorimeters (Constant Volume Calorimeters)
4- Constant Pressure Calorimeters
5- Differential Scanning Calorimeters

Adiabatic calorimeters

An adiabatic calorimeter is a calorimeter used to examine a runaway reaction. Since the calorimeter runs in an adiabatic environment, any heat generated by the material sample under test causes the sample to increase in temperature, thus fueling the reaction.

No adiabatic calorimeter is fully adiabatic - some heat will be lost by the sample to the sample holder. A mathematical correction factor, known as the phi-factor, can be used to adjust the calorimetric result to account for these heat losses. The phi-factor is the ratio of the thermal mass of the sample and sample holder to the thermal mass of the sample alone.

FIG 1

Reaction calorimeters

A reaction calorimeter is a calorimeter in which a chemical reaction is initiated within a closed insulated container. Reaction heats are measured and the total heat is obtained by integrating heatflow versus time. This is the standard used in industry to measure heats since industrial processes are engineered to run at constant temperatures.[citation needed] Reaction calorimetry can also be used to determine maximum heat release rate for chemical process engineering and for tracking the global kinetics of reactions. There are four main methods for measuring the heat in reaction calorimeter:

FIG 2

Constant-pressure calorimeter
A constant-pressure calorimeter measures the change in enthalpy of a reaction occurring in solution during which the atmospheric pressure remains constant.

An example is a coffee-cup calorimeter, which is constructed from two nested Styrofoam cups and a lid with two holes, allowing insertion of a thermometer and a stirring rod. The inner cup holds a known amount of a solvent, usually water, that absorbs the heat from the reaction. When the reaction occurs, the outer cup provides insulation. Then

where

 = Specific heat at constant pressur

 = Enthalpy of solution

 = Change in temperature

 = mass of solvent

 = molecular mass of solvent

FIG 3

                                         
The measurement of heat using a simple calorimeter, like the coffee cup calorimeter, is an example of constant-pressure calorimetry, since the pressure (atmospheric pressure) remains constant during the process. Constant-pressure calorimetry is used in determining the changes in enthalpy occurring in solution. Under these conditions the change in enthalpy equals the heat.

Differential scanning calorimeter

In a differential scanning calorimeter (DSC), heat flow into a sample—usually contained in a small aluminium capsule or 'pan'—is measured differentially, i.e., by comparing it to the flow into an empty reference pan.
In a heat flux DSC, both pans sit on a small slab of material with a known (calibrated) heat resistance K. The temperature of the calorimeter is raised linearly with time (scanned), i.e., the heating rate dT/dt = β is kept constant. This time linearity requires good design and good (computerized) temperature control. Of course, controlled cooling and isothermal experiments are also possible.
Heat flows into the two pans by conduction. The flow of heat into the sample is larger because of its heat capacity Cp. The difference in flow dq/dt induces a small temperature difference ΔT across the slab. This temperature difference is measured using a thermocouple. The heat capacity can in principle be determined from this signal:

A modulated temperature differential scanning calorimeter (MTDSC) is a type of DSC in which a small oscillation is imposed upon the otherwise linear heating rate.

This has a number of advantages. It facilitates the direct measurement of the heat capacity in one measurement, even in (quasi-)isothermal conditions. It permits the simultaneous measurement of heat effects that respond to a changing heating rate (reversing) and that don't respond to the changing heating rate (non-reversing). It allows for the optimization of both sensitivity and resolution in a single test by allowing for a slow average heating rate (optimizing resolution) and a fast changing heating rate (optimizing sensitivity).

Safety Screening:- DSC may also be used as an initial safety screening tool. In this mode the sample will be housed in a non-reactive crucible (often Gold, or Gold plated steel), and which will be able to withstand pressure (typically up to 100 bar). The presence of an exothermic event can then be used to assess the stability of a substance to heat. However, due to a combination of relatively poor sensitivity, slower than normal scan rates (typically 2–3°/min - due to much heavier crucible) and unknown activation energy, it is necessary to deduct about 75–100 °C from the initial start of the observed exotherm to suggest a maximum temperature for the material. A much more accurate data set can be obtained from an adiabatic calorimeter, but such a test may take 2–3 days from ambient at a rate of 3 °C increment per half hour.

FIG 4

The bomb calorimeter

The bomb calorimeter is used to determine the calorific value of solid fuels and some of the less volatile liquid fuels such as fuel oil. It is also used by food technologists to determine the calorific
value of foods. As with the fuels which we use, the foods which we eat are chemical combinations of carbon and hydrogen, and when the moisture content has been removed, they can be burned
like a fuel to release their stored energy.

There are several different designs of bomb calorimeter but basically the apparatus consists of a screw-topped pressure vessel, the bomb, surrounded by water in a lagged copper calorimeter. The
screw top of the bomb contains insulated electrical connections and a pressurising valve. This enables it to be charged with oxygen to ensure that there is sufficient for complete combustion of the fuel. A small quantity of water, about 10 ml, is also placed inside the bomb.
Its purpose is to ensure condensation of the steam formed during combustion and absorb any acid products which are formed. Leads from the electrical connections extend inside the bomb and one of them supports a small porcelain crucible containing a measured sample of fuel mf. For solid fuels the fuse were connected between the leads is positioned in contact with the fuel sample. With liquid fuels a cotton thread is hung from the fuse wire into the fuel sample. The water equivalent of the bomb and
copper calorimeter mwc is supplied by the makers of the apparatus.
The arrangement is shown in Figure 5. The assembled and pressurised bomb is placed inside the insulated copper calorimeter and the electrical leads are connected to a low-voltage power supply. A measured quantity of water mw is added to the calorimeter to above the level of the bomb but slightly
below the level of the electrical connections. The stirrer, insulated lid and sensitive thermometer are then placed in position. The apparatus is allowed to stand for a period of time to allow the temperature inside the calorimeter to stabilise. 

FIG 5

The thermometer reading is then taken and the bomb is fired by switching on the power supply. Thermometer readings are then taken at 1-min intervals as the water temperature rises whilst all the time operating the stirrer. Eventually, after rising by one or two degrees, the temperature is observed to level-off and may then begin to fall as heat energy is lost to the atmosphere. A correction to the recorded temperature rise must be made for heat loss during the experiment.
This is done by plotting a graph of temperature against time, as shown in Figure 6.

FIG 6

The calorific value of the fuel can now be found by equating the heat released by the fuel to the heat received by the water, bomb and the copper calorimeter, that is

heat supplied  by fuel       -  heat received by the water bomb and calorimeter

mf × CV - (mw + mwc) ×cw(T2 -T1)

CV =  [(mw + mwc) ×cw(T2 -T1) ] / mf

Combustion of the hydrogen content of the fuel produces H₂O in the form of steam. This condenses inside the bomb, giving up its latent heat, which becomes a part of the total heat received by the
apparatus. Because this is included, the value of the calorific value obtained from the bomb calorimeter is known as the higher or gross calorific value of the fuel.
When a fuel is used in practical situations such as in an internal combustion engine, the steam from combustion leaves with the exhaust gases and condenses in the atmosphere or in the exhaust pipe. As a result its latent heat is not available for conversion into useful work and is subtracted from the higher calorific value of the fuel. This is then known as the lower or net calorific value. Fuel
technologists are able to calculate its value from a knowledge of the hydrogen content and it is the lower calorific value which is usually quoted by oil and gas suppliers.

FIG 7

Boy’s gas calorimeter
Boy’s gas calorimeter is used to determine the calorific value of gaseous fuels. It contains a burner from which the hot gases pass over a set of cooling tubes. A controlled flow of water through the
tubes cools the hot gases down to the gas supply temperature and thus extracts all the heat which has been released during combustion.
The burner receives a metered flow of gas at a steady supply pressure and a drain at the base of the calorimeter enables the condensed steam, formed from the combustion of hydrogen, to be
collected. The arrangement is shown in Figure 8 . Having adjusted the cooling water flow rate so that the exhaust gases emerge at approximately the ambient temperature, the cooling water inlet and outlet temperatures Ti and To are recorded. The gas supply pressure hg measured in metres of water on the U-tube manometer is also recorded together with the gas supply and exhaust temperature Ts. A gas meter reading is then taken and simultaneously
the measuring jars are placed in position to collect the cooling water and condensate.After a period of around 5 min the gas meter reading is again taken and the volume of gas Vs which has been
supplied is calculated. The mass of cooling water mw and the mass of condensate mc which have been collected are also recorded.

FIG 8

The calorific value of a gas is measured in MJm⁻³ at (NTP) and some preliminary calculations are required before its value can be determined. First, the gas supply pressure ps must be converted
from metres of water into pascals, that is,

pg=hg 𝜌 g

where
pg  = metres of water into pascals
hg =  gas supply pressure 
𝜌   = water density 
g   = gravity acceleration

This of course is the gauge pressure of the gas and it must now be converted to absolute supply pressure ps by adding to it the prevailing value of atmospheric pressure pa obtained from a Fortin or
precision aneroid barometer, that is,

ps=pg+ pa

The next task is to convert the volume of gas which has been used to the volume which it would occupy at NTP. You will recall that normal temperature, Tn - 15⁰C, and normal pressure,
pn -  101:325 kPa. This is done using the general gas equation

The higher or gross calorific value of the gas can now be calculated by equating the heat released by the gas to the heat received by the cooling water

heat released by gas -  heat received by cooling water

The lower or net calorific value of the gas can be found by subtracting the latent heat given up by the condensate from the heat received by the cooling water:

The value of the specific latent heat of vaporisation at NTP can be taken to be hfg -2453 kJ Kg⁻¹.
The cooling water flow rate through Boy’s gas calorimeter must be adjusted so that the exhaust gases emerge approximately at the ambient temperature