Fuels for Furnaces.

FUELS

The selection of the best fuel should be based upon a study of the
comparative prepared costs, cleanliness of operation, adaptability to
temperature control, labor required, and the effects of each fuel upon the
material to be heated and upon the furnace lining. Attention must be
paid to the quantity to be burned in each burner, the atmosphere (fuel /
air ratio) desired in the furnace, and the uniformity of temperature distribution
required, which determines the number and the location of the
burners. Common methods of burning furnace fuels are as follows:

Solid Fuels (Almost Entirely Bituminous Coals)

Coal was once a common fuel for industrial furnaces, either hand-fired,
stoker-fired, or with powdered coal burners. With the increasing necessity
for accurate control of temperature and atmosphere in industrial
heating, coal has been almost entirely replaced by liquid and gaseous
fuels. It can be expected that methods will be developed for the production
of a synthetic gas (natural-gas equivalent) from coal.

Liquid Fuels (Fuel Oil and Tar)

To burn liquid fuels effectively, first it is necessary to atomize the oil
into tiny droplets which then vaporize and burn. Atomization can be
accomplished mechanically or with the aid of steam or air. With heavy
oils and tar, it is important to maintain the proper viscosity of the oil at
the atomizer by preheating the fuel.
For larger industrial burners, combustion air is supplied by fans of
appropriate capacity and pressure. Combustion air is induced with some
smaller burner designs.

Gaseous Fuels

Burners for refined gases (natural gas, synthetic gas, coke-oven gas, clean
producer gas, propane, butane):
Two-pipe systems: Include blast burners (open or closed setting), nozzle
mixing, luminous flame, excess air (tempered flame), baffle, and
radiant-tube burners, all for low-pressure gas and air.
Premix systems: Air and gas mixed in a blower and supplied through
one pipe.
Proportioning low-pressure mixers: Air and gas supplied under pressure
and proportioned automatically (air aspirating gas or gas inspirating
air). The resulting mixture is burned in tunnel burners, radiant-cup,
baffle, radiant-tube, ribbon, and line burners.
Pilot flames are generally used to ensure ignition for gas and oil
burners. Insurance frequently requires additional safety provision in two
main categories: an interconnected pressure system to prevent lighting
if any burner in a zone is open, and burner monitors using heat or light
to permit ignition.
Burners for crude gas (raw producer-gas, blast-furnace gas, or cokeoven
gas):
Simple mixing systems with large orifices and simple mechanisms
which cannot become clogged by tar and dirt contained in these gases.
Separate gas and air supplies to the furnace, with all mixture taking
place within the furnace.

Coal and its Types

Coal is a black or brownish-black combustible solid formed by the 
decomposition of vegetation in the absence of air. Microscopy can
identify plant tissues, resins, spores, etc. that existed in the original
structure. It is composed principally of carbon, hydrogen, oxygen, and
small amounts of sulfur and nitrogen. Associated with the organic matrix
are water and as many as 65 other chemical elements. Many trace
elements can be determined by spectrometric method D-3683. Coal is
used directly as a fuel, a chemical reactant, and a source of organic
chemicals. It can also be converted to liquid and gaseous fuels.



Meta-anthracite is a high-carbon coal that approaches graphite in
structure and composition. It usually is slow to ignite and difficult to
burn. It has little commercial importance.
Anthracite, sometimes called hard coal, is hard, compact, and shiny
black, with a generally conchoidal fracture. It ignites with some difficulty
and burns with a short, smokeless, blue flame. Anthracite is used
primarily for space heating and as a source of carbon. It is also used in

electric power generating plants in or close to the anthracite-producing
area. The iron and steel industry uses some anthracite in blends with
bituminous coal to make coke, for sintering iron-ore fines, for lining
pots and molds, for heating, and as a substitute for coke in foundries.
Semian thracite is dense, but softer than anthracite. It burns with a
short, clean, bluish flame and is somewhat more easily ignited than
anthracite. The uses are about the same as for anthracite.
Low-volatile bituminous coal is grayish black, granular in structure and
friable on handling. It cakes in a fire and burns with a short flame that is
usually considered smokeless under all burning conditions. It is used for
space heating and steam raising and as a constituent of blends for improving
the coke strength of higher-volatile bituminous coals. Low volatile
bituminous coals cannot be carbonized alone in slot-type ovens
because they expand on coking and damage the walls of the ovens.
Medium-volatile bituminous coal is an intermediate stage between
high-volatile and low-volatile bituminous coal and therefore has some
of the characteristics of both. Some are fairly soft and friable, but others
are hard and do not disintegrate on handling. They cake in a fuel bed and
smoke when improperly fired. These coals make cokes of excellent
strength and are either carbonized alone or blended with other bituminous
coals. When carbonized alone, only those coals that do not expand
appreciably can be used without damaging oven walls.
High-volatile A bituminous coal has distinct bands of varying luster. It
is hard and handles well with little breakage. It includes some of the best
steam and coking coal. On burning in a fuel bed, it cakes and gives off
smoke if improperly fired. The coking property is often improved by
blending with more strongly coking medium- and low-volatile bituminous
coal.
High-volatile B bituminous coal is similar to high-volatile A bituminous
coal but has slightly higher bed moisture and oxygen content and is less
strongly coking. It is good coal for steam raising and space heating.

Some of it is blended with more strongly coking coals for making
metallurgical coke.
High-volatile C bituminous coal is a stage lower in rank than the B
bituminous coal and therefore has a progressively higher bed moisture
and oxygen content. It is used primarily for steam raising and space
heating.
Subbituminous coals usually show less evidence of banding than bituminous
coals. They have a high moisture content, and on exposure to
air, they disintegrate or ‘‘slack’’ because of shrinkage from loss of
moisture. They are noncaking and noncoking, and their primary use is
for steam raising and space heating.
Lignites are brown to black in color and have a bed moisture content
of 30 to 45 percent with a resulting lower heating value than higher-rank
coals. Like subbituminous coals, they have a tendency to ‘‘slack’’ or
disintegrate during air drying. They are noncaking and noncoking. Lignite
can be burned on traveling or spreader stokers and in pulverized
form.
The principal ranks of coal mined in the major coal-producing states
are shown in Table 7.1.3. Their analyses depend on several factors, e.g.,
source, size of coal, and method of preparation. Periodic reports are
issued by the U.S. Department of Energy, Energy Information Agency.
They provide statistics on production, distribution, end use, and analytical
data

APPLICATIONS OF POWDER METALLURGY

APPLICATIONS OF POWDER METALLURGY

The powder metallurgy process has provided a practical solution to the problem of

producing refractory metals, which have now become the basis of making heat-resistant

materials and cutting tools of extreme hardness. Another very important and useful item

of the products made from powdered metals is porous self-lubricating bearing. In short,

modern technology is inconceivable without powder metallurgy products, the various fields

of application of which expand every year. Some of the powder metal products are given as

under.

1. Porous products such as bearings and filters.

2. Tungsten carbide, gauges, wire drawing dies, wire-guides, stamping and blanking

tools, stones, hammers, rock drilling bits, etc.

3. Various machine parts are produced from tungsten powder. Highly heat and wear

resistant cutting tools from tungsten carbide powders with titanium carbide, powders

are used for and die manufacturing.

4. Refractory parts such as components made out of tungsten, tantalum and

molybdenum are used in electric bulbs, radio valves, oscillator valves, X-ray tubes

in the form of filament, cathode, anode, control grids, electric contact points etc.

5. Products of complex shapes that require considerable machining when made by

other processes namely toothed components such as gears.

6. Components used in automotive part assembly such as electrical contacts, crankshaft

drive or camshaft sprocket, piston rings and rocker shaft brackets, door, mechanisms,

connecting rods and brake linings, clutch facings, welding rods, etc.

7. Products where the combined properties of two metals or metals and non-metals

are desired such as non-porous bearings, electric motor brushes, etc.

8. Porous metal bearings made which are later impregnated with lubricants. Copper

and graphite powders are used for manufacturing automobile parts and brushes.

9. The combinations of metals and ceramics, which are bonded by similar process as

metal powders, are called cermets. They combine in them useful properties of high

refractoriness of ceramics and toughness of metals. They are produced in two forms

namely oxides based and carbide based.

LIMITATIONS OF POWDER METALLURGY

LIMITATIONS OF POWDER METALLURGY
1. Powder metallurgy process is not economical for small-scale production.
2. The cost of tool and die of powder metallurgical set-up is relatively high
3. The size of products as compared to casting is limited because of the requirement
of large presses and expensive tools which would be required for compacting.
4. Metal powders are expensive and in some cases difficult to store without some
deterioration.
5. Intricate or complex shapes produced by casting cannot be made by powder
metallurgy because metallic powders lack the ability to flow to the extent of molten
metals.
6. Articles made by powder metallurgy in most cases do not have as good physical
properties as wrought or cast parts.
7. It may be difficult sometimes to obtain particular alloy powders
8. Parts pressed from the top tend to be less dense at the bottom.
9. A completely deep structure cannot be produced through this process.
10. The process is not found economical for small-scale production.
11. It is not easy to convert brass, bronze and a numbers of steels into powdered form.

ADVANTAGES OF POWDER METALLURGY

 ADVANTAGES OF POWDER METALLURGY

1. The processes of powder metallurgy are quite and clean.

2. Articles of any intricate or complicated shape can be manufactured.

3. The dimensional accuracy and surface finish obtainable are much better for many

applications and hence machining can be eliminated.

4. Unlike casting, press forming machining, no material is being wasted as scrap and

the process makes utilizes full raw material

5. Hard to process materials such as diamond can be converted into usable components

and tools through this process.

6. High production rates can be easily achieved.

7. The phase diagram constraints, which do not allow an alloy formation between

mutually insoluble constituents in liquid state, such as in case of copper and lead

are removed in this process and mixtures of such metal powders can be easily

processed and shaped through this process.

8. This process facilitates production of many such parts, which cannot be produced

through other methods, such as sintered carbides and self-lubricating bearings.

9. The process enables an effective control over several properties such as purity,

density, porosity, particle size, etc., in the parts produced through this process.

10. The components produced by this process are highly pure and bears longer life.

11. It enables production of parts from such alloys, which possess poor cast ability.

12. It is possible to ensure uniformity of composition, since exact proportions of

constituent metal powders can be used.

13. The preparation and processing of powdered iron and nonferrous parts made in this

way exhibit good properties, which cannot be produced in any other way.

14. Simple shaped parts can be made to size with 100 micron accuracy without waste

15. Porous parts can be produced that could not be made in any other way.

16. Parts with wide variations in compositions and materials can be produced.

17. Structure and properties can be controlled more closely than in other fabricating

processes.

18. Highly qualified or skilled labor is not required. in powder metallurgy process

19. Super-hard cutting tool bits, which are impossible to produce by other manufacturing

processes, can be easily manufactured using this process.

20. Components shapes obtained possess excellent reproducibility.

21. Control of grain size, relatively much uniform structure and defect such voids and

blowholes in structure can be eliminated.

Production of Metal Powders

 Production of Metal Powders

Metallic powders possessing different properties can be produced easily. The most

commonly used powders are copper-base and iron-base materials. But titanium, chromium,

nickel, and stainless steel metal powders are also used. In the majority of powders, the size

of the particle varies from several microns to 0.5 mm. The most common particle size of

powders falls into a range of 10 to 40 microns. The chemical and physical properties of metals

depend upon the size and shape of the powder particles. There are various methods of

manufacturing powders.The commonly used powder making processes are given as under.

1. Atomization

2. Chemical reduction

3. Electrolytic process

4. Crushing

5. Milling

6. Condensation of metal vapors

7. Hydride and carbonyl processes.

The above mentioned metallic powder making techniques are discussed briefly as under.

1. Atomization

In this process, the molten metal is forced through an orifice and as it emerges, a high

pressure stream of gas or liquid impinges on it causing it to atomize into fine particles. The

inert gas is then employed in order to improve the purity of the powder. It is used mostly

for low melting point metals such as tin, zinc, lead, aluminium, cadmium etc., because of the

corrosive action of the metal on the orifice (or nozzle) at high temperatures. Alloy powders

are also produced by this method.

2. Chemical Reduction Process

In this process, the compounds of metals such as iron oxides are reduced with CO or H2

at temperatures below the melting point of the metal in an atmosphere controlled furnace.

The reduced product is then crushed and ground. Iron powder is produced in this way

Fe3O4 + 4C = 3Fe + 4CO

Fe3O4 + 4CO = 3Fe + 4CO2

Copper powder is also produced by the same procedure by heating copper oxide in a

stream of hydrogen.

Cu2 + H2 = 2Cu + H2O

Powders of W, Mo, Ni and CO can easily be produced or manufactured by reduction

process because it is convenient, economical and flexible technique and perhaps the largest

volume of metallurgy powders is made by the process of oxide reduction.

3. Electrolytic Process

Electrolysis process is quite similar to electroplating and is principally employed for the

production of extremely pure, powders of copper and iron. For making copper powder, copper

plates are placed as anodes in a tank of electrolyte, whereas, aluminium plates are placed in

to the electrolyte to act as cathodes. High amperage produces a powdery deposit of anode

metal on the cathodes. After a definite time period, the cathode plates are taken out from the

tank, rinsed to remove electrolyte and are then dried. The copper deposited on the cathode

plates is then scraped off and pulverized to produce copper powder of the desired grain size.

The electrolytic powder is quite resistant to oxidation.

4. Crushing Process

The crushing process requires equipments such as stamps, crushers or gyratory crushes.

Various ferrous and non-ferrous alloys can be heat-treated in order to obtain a sufficiently

brittle material which can be easily crushed into powder form.

5. Milling Process

The milling process is commonly used for production of metallic powder. It is carried out

by using equipments such as ball mill, impact mill, eddy mill, disk mill, vortex mill, etc.

Milling and grinding process can easily be employed for brittle, tougher, malleable, ductile and

harder metals to pulverize them. A ball mill is a horizontal barrel shaped container holding

a quantity of balls, which, being free to tumble about as the container rotates, crush and

abrade any powder particles that are introduced into the container. Generally, a large mass

to be powdered, first of all, goes through heavy crushing machines, then through crushing

rolls and finally through a ball mill to produce successively finer grades of powder.

6. Condensation of Metal Powders

This process can be applied in case of metals, such as Zn, Cd and Mg, which can be boiled

and the vapors are condensed in a powder form. Generally a rod of metal say Zn is fed into

a high temperature flame and vaporized droplets of metal are then allowed to condense on

to a cool surface of a material to which they will not adhere. This method is not highly

suitable for large scale production of powder.

7. Hydride and Carbonyl Processes

High hardness oriented metals such as tantalum, niobium and zirconium are made to

combine with hydrogen form hydrides that are stable at room temperature, but to begin to

dissociate into hydrogen and the pure metal when heated to about 350°C. Similarly nickel and

iron can be made to combine with CO to form volatile carbonyls. The carbonyl vapor is then

decomposed in a cooled chamber so that almost spherical particles of very pure metals are

deposited.

Safety Recommendations for Gas Welding

Safety Recommendations for Gas Welding

Welding and cutting of metals involve the application of intense heat to the objects being

welded or cut. This intense heat in welding is obtained from the use of inflammable gases,

(e.g. acetylene, hydrogen, etc.) or electricity. The intense welding heat and the sources

employed to produce it can be potentially hazardous. Therefore, to protect persons from

injury and to protect building and equipment against fire, etc., a set of recommendations

concerning safety and health measures for the welders and those concerned with the safety

of the equipments etc., have been published by BIS and many other similar but International

organizations. By keeping in mind these recommendations or precautions, the risks associated

with welding can be largely reduced. Therefore, it is suggested that the beginner in the field

of gas welding must go through and become familiar with these general safety

recommendations, which are given below.

1. Never hang a torch with its hose on regulators or cylinder valves.

2. During working, if the welding tip becomes overheated it may be cooled by plunging

the torch into water; close the acetylene valve but leave a little oxygen flowing.

3. Always use the correct pressure regulators for a gas. Acetylene pressure regulator

should never be used with any other gas.

4. Do not move the cylinder by holding the pressure regulator and also handle pressure

regulators carefully.

5. Use pressure regulator only at pressures for which it is intended.

6. Open cylinder valves slowly to avoid straining the mechanism of pressure regulator.

7. Never use oil, grease or lubricant of any kind on regulator connections.

8. For repairs, calibrations and adjustments purposes, the pressure regulators should

be sent to the supplier.

9. Do cracking before connecting pressure regulator to the gas cylinder.

10. Inspect union nuts and connections on regulators before use to detect faulty seats

which may cause leakage of gas when the regulators are attached to the cylinder

valves.

11. Hose connections shall be well fittings and clamped properly otherwise securely

fastened to these connections in such a manner as to withstand without leakage a

pressure twice as great as the maximum delivery pressure of the pressure regulators

provided on the system.

12. Protect the hose from flying sparks, hot slag, hot workpiece and open flame. If dirt

goes into hose, blow through (with oxygen, not acetylene) before coupling to torch

or regulator.

13. Store hose on a reel (an automobile wheel) when not in use.

14. Never allow the hose to come into contact with oil or grease; these deteriorate the

rubber and constitute a hazard with oxygen.

15. Use the correct color hose for oxygen (green/black) and acetylene (red) and never

use oxygen hose for acetylene or vice versa.

16. Always protect hose from being trampled on or run over. Avoid tangle and kinks.

Never leave the hose so that it can be tripped over.

Hazards of fumes, gases and dusts can be minimized by (i) improving general ventilation

of the place where welding is carried out (ii) using local exhaust units, and (iii) wearing

individual respiratory protective equipment.