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IB.Tech Engineering chemistry Question with answer complete material

Refractories & Insulators UNIT-VIQ.No.1 What are Refractories? Explain thermal-spalling, porosity, & strength of

refractories.

Answer: Refractories are ceramic materials that can withstand high temperatures as well asabrasive and corrosive action of molten metals; slags and gases, without suffering a deformation

in shape. The main objective of a refractory is to confine heat.Criteria of good refractory material or essential properties of good refractorymaterials:

The important properties are:

a. Thermal spalling: Rapid changes in temperature, cause uneven expansion and contraction of

refractory material, thereby leading to development of internal stresses and strains. This is in turn

are responsible for cracking, breaking or fracturing of a refractory brick or block under hightemperature, collectively known as thermal spalling. Thermal spalling can also caused by the

variation in the coefficient of expansion due to slag penetration in the refractory brick. A good

refractory must show a good resistance to thermal spalling. Spalling can be decreased by1. Avoiding sudden temperature changes.

2. Over firing the refractories.

3. Modifying the furnace design.4. Using high porosity, low coefficient of expansion, and good thermal conductivity refractory

bricks.

b. Porosity: Porosity of a refractory material is the ratio of its pores volume to the bulk volume.Porosity can also increase the thermal shock resistance. The least porous bricks have the highest

thermal conductivity, strength, resistance to abrasion and corrosion.100*A)]-D)/(W-[(WP =

c. Strength of Refractories:

a. Refractoriness: is the ability of a refractory material to withstand the heat without

appreciable softening or deformation under given service conditions. The refractory material

should have a softening temperature higher than the operating temperature of the furnace in

which it is to be used.b. Refractoriness-under-load: Temperature resistance and load bearing capacity are the two

essential qualities of a refractory. This is due to the fact that commercial refractory which are

used for lining high temperature furnace are expected to withstand varying loads of the charge.Hence they should possess high strength and excellent temperature resistance.

Q.No.2: Define Insulators? What are thermal insulators? Discuss their engineering

applications and characteristics

Answer: Insulators: The substances which are capable of retarding or prohibiting the flow of

heat or electricity or sound through them are known as insulators or Insulating materials.

Insulators can be broadly classified into three categories.

Thermal InsulatorsSound Insulators

Electrical Insulators

Thermal insulators: Thermal insulators are those materials with very low thermal

conductivities. These essentially prevent heat loss. Insulation capacity is inversely proportional

to conductivity. The properties of a thermal insulator depends on

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_Engineering Chemistry (Refractories & Insulators) Prepared By B.SRINIVAS1


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i. Pores: Most of the insulators are fibrous or granular bodies. They have pores. If thepore size is big, then heat transfer by convection is possible. So low pore size is

preferred.

ii. Presence of moisture: Moisture enhance thermal conductivity because, water hasmore thermal conductivity than air. If the pores are closed type then water can water

cannot enter. Thermal insulator should not react with water.

An ideal thermal insulator should have the following characteristics:a. Thermal conductivity should be lowb. It should be fire proof

c. Should resist moisture absorption

d. Chemically inerte. Low dense material

f. Should be able to bear strong load

g. Should be stableh. Should be odorless

i. Should be inexpensive

Thermal insulators are two types:

Organic: An organic thermal insulator poses large number of small pores. These are naturallyoccurring compounds.

Inorganic: Inorganic thermal insulators are asbestos, glass, calcium silicate etc.

Q.No.3: Define insulators? What are electrical insulators? Discuss their characteristics &

engineering applications.Answer: Insulators: The substances which are capable of retarding or prohibiting the flow ofheat or electricity or sound through them are known as insulators or Insulating materials.

Insulators can be broadly classified into three categories.

Thermal Insulators

Sound InsulatorsElectrical Insulators

Electrical insulators: The materials which are used to prevent the loss of electricity through

certain parts in an electrical system are known as electrical insulating materials or Dielectricals.

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_Engineering Chemistry (Refractories & Insulators) Prepared By B.SRINIVAS

Thermal

insulators

Characteristic properties Engg. Applications.

Glass wool Soft and flexible, fire and

heat proof, insect proof

Used as thermal insulating material in industrial

and domestic appliances like ovens, motors and

vacuum cleaners.

Cork Possesses good porosity,

compressibility and waterresistance a part from good

thermal resistance.

Used in the lining of cold storage, bottle stoppers,

refrigerators.

Asbestos Fire and weather proof,

durable, light in weight.

Used as heat insulator in boilers, for roof covering

in the form of sheets.

Cellular

rubber

Possess large number of

cellular cavities.

Used for heat insulation in cold storage and

refrigerators.

Vermiculites Mica like minerals Used as heat insulators for furnace and also assound insulators.

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These materials apart from acting as electrical insulators can also store electric charge. Thus,these materials have two functions, Insulation and Storage of charge.

Characteristics:

1. A good insulator possesses low electrical conductivity or high resistivity. Typicalvalues being 109 to 1020 ohm-cm at room temperature.

2. Dialectics used in capacitor should have high dielectric constant so that grater

amount of energy can be stored in relatively thin insulation.3. In an insulating material dielectric loss are caused by a. the absorption of

electrical energy and leakage of current through the material.

This due to the fact that the absorption of electrical energy, under an alternating

field gives rise to dissipation of the electrical energy in the insulating material.The leakage of current through the insulating material takes place as a result of

conduction, especially at higher temperature.

4. Good dielectric materials should have low porosity. this is due to the fact thathigher porosity increases the moisture holding capacity and moisture adversely

affects the electrical properties.

5. An ideal insulator should have least thermal expansion and contraction.

6. Good insulating materials should be chemically inert to acids, alkalies, oils,solvents, moisture, gas fumes etc.

Important electrical insulating materials and their engineering applications:

Insulator Engg. Applications.

Air It provides insulation between the overhead transmission lines.

Nitrogen It is used in transformers to replace the harmful oxidising

atmosphere. under high pressure, it is also used as dielectric in

certain electrical capacitors.

Electronegative gases

like SF6 & CCl4

SF6 is used in electrical devices like capacitors, cables.

Mineral oils In transformers, a light fraction oil like transil oil is used to allow

converting cooling. under pressure, light oils are also used in oil

filled high voltage cables. more viscous or talky oils are used toimpregnate the paper in solid type of cable.

Silicone fluids Used as coolants in radio, pulse and aircraft transformers.

Fluorinated liquids Used in transformers and small electric and radio devices.

Drying oils Used in transformers and motor coils.

Non drying oils Used in insulating resin compositions

Paper & press boards Used for winding, cable coil insulation, primary dielectrics in

capacitors, windings and mica insulations, slot insulations ofelectrical machines etc.

Inorganic fibrous

insulators (asbestos)

Below red heat it is used in woven cloth tape, paper and wire

covering.

Insulating varnishes Used for impregnation coatings and adhesion impregnation of

fibrous materials for improving the mechanical strength, anddecreasing the hygroscopic of the fibrous material by replacing air

in their pores.

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Synthetic rubbers Silicsone rubbers used in ship wiring, aircraft cables, high

temperature arcs, motor winding insulations.

Synthetic resins PVC used in flexible wire coverings, cable sheathings, insulating

electric wires and low voltage cables.

Teflon used as capacitor dielectrics and insulating material for

almost all kinds of windings.Rigid insulators(Glass) Used for wire coverings, line insulations, glass to metal seals and

other electrical engineering applications.

Q.No.4: Define Refractories, classify it, to explain Conditional for failure of a refractory

material

Answer: Refractories are ceramic materials that can withstand high temperatures as well as

abrasive and corrosive action of molten metals; slags and gases, without suffering a deformationin shape. The main objective of a refractory is to confine heat.

On the basis of the chemical properties of their constituent substances, refractories are

classified into three categories:

i. Neutral refractories

ii. Acid refractoriesiii. Basic refractories

i. Neutral refractories like graphite, zirconia and SiC refractories. These refractories are madefrom weakly basic/acidic materials like carbon, zirconia (ZiO2) and chromite (FeO.CrO2)

ii. Acid refractories like alumina, silica and fire clay refractories. These refractories consist of

acidic materials like alumina (Al2O3) and silica (SiO2). These refractory materials are resistantto acidic slag (like silica) and are often used as contaminant vessel for them. On the other hand,

they are readily attacked by basic slags (like CaO, MgO etc.) and contact with these oxide

materials should be avoided.

iii. Basic refractories like Magnetite and Dolomite refractories. These refractories consist ofbasic materials like CaO, MgO etc. and are especially resistant to basic slags. Thats why they

find extensive use in some steel making open hearth furnaces. The presence of acidic materialslike silica is deleterious to their high-temperature performance.

Conditional for failure of a refractory material:

i. Using a refractory material which does not have required heat, corrosion and abrasionresistance;

ii. Using refractory material of higher thermal expansion;

iii. Using a refractory of refractoriness less than that of the operating temperature;iv. Using basic refractory in a furnace in which acidic reactants and/or products are being

processed and vice-versa;

v. Using lower-duty refractory bricks in a furnace than the actual load of raw materials in

products;vi. Using refractories which undergo considerable volume changes during their use at high

temperature.

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