Unit6-refractories And Insulators

IB.Tech Engineering chemistry Question with answer complete material

Refractories & Insulators UNIT-VI Q.No.1 What are Refractories? Explain thermal-spalling, porosity, & strength of refractories (Pyrometric cone). Answer: Refractories are ceramic materials that can withstand high temperatures as well as abrasive and corrosive action of molten metals; slag’s 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 refractory materials:

The important properties are: 1. Porosity: Porosity of a refractory material is the ratio of its pore’s 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. All refractories contain pores, either due to manufacturing methods or deliberately by incorporating saw-dust or cork during manufacture. The pores may be open or closed; the latter are encountered in an oven-fired refractory. Porosity in the ratio of its pore’s volume to the bulk volume. Thus, porosity, P  [(W - D)/(W - A)] *100 W= Weight of saturated specimen D= Weight of dry specimen A = weight of saturated specimen submerged in water. 2. 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 high temperature, 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 by 1. 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. 3. Strength of refractories: Refractoriness is the ability of a refractory material to withstand the heat without appreciable softening or deformation under given service conditions. It is generally measured as the softening temperature. It is necessary that a refractory material should have a softening temperature higher than the operating temperature of the furnace in which it is to be used. Sometimes, it can be employed to withstand a temperature higher than its softening temperature since the outer part of refractory is at a lower temperature and still in solid state, providing strength. Thus, refractory material does not melt away although inner refractory lining in a furnace is at a much higher temperatures than the outer ones. Most of the commercial refractories do not exhibit sharp melting points and they soften gradually over a range of temperatures Measurement of refractoriness: The softening temperatures of refractories are generally determined by seger cones also called pyrometric cones test. In this test, behaviour of heat on cone of refractory and series of seger cones of standard dimensions are compared. These cones

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IB.Tech Engineering chemistry Question with answer complete material are small pyramid shaped with triangular base, 38 mm high with 19 mm long sides.

The test refractory in the form of a cone is kept along side similar sized standard cones and all are heated uniformly at 200C/hour or 1000C/hour or 1500C/hour or occasionally at 6000C/hour. As seger cones are made of a particular refractory of a definite softening temperature so they are assigned ascending seger cone numbers with increasing softening temperature. When the test cone softens and loses its shape, one of the standard seger cones also softens and loses its shape provided its refractoriness is close to that of the test cone. The serial number of this standard seger cone is noted and this number is the pyrometric cone equivalent (PCE) of the refractory under that test. When the test cone softens earlier than one standard cone, but later than the previous one, the PCE value of the test sample is approximated as average of the two. The temperature at which the softening or fusion of the test-cone occurs is indicated by its apex touching the base. Seger-cone number 1 5 10 15 20 30 35

Temperature (0C) 1110 1180 1300 1435 1530 1670 1770

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 refractories 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. Measurement: Seger cone test is not applicable for the measurement of strength. Because, some refractories soften gradually over a range of temperature, but under appreciable load, they collapse, far below their true fusion temperature. High alumina bricks and fire-clay are examples of such refractory materials. There are some other refractory materials like silica bricks which exert good load bearing characteristics up to their fusion temperatures as they soften over a relatively narrow range of temperature Thus, for good results, refractoriness-under-load test is performed by applying a load of (3.5 or 1.75 kg/cm2) to the refractory specimen (of size 5cm2 and 75cm high). The sample is then kept in carbon-resistance furnace and heating is stated at the rate of 100C/munute. The height of the specimen is plotted vs. temperature and RUL is expressed as the temperature at which 10% deformation takes place.

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IB.Tech Engineering chemistry Question with answer complete material 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 Insulators Sound 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 i. Pores: Most of the insulators are fibrous or granular bodies. They have pores. If the pore 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 has more 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 low b. It should be fire proof c. Should resist moisture absorption d. Chemically inert e. Low dense material f. Should be able to bear strong load g. Should be stable h. 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 naturally occurring compounds. Inorganic: Inorganic thermal insulators are asbestos, glass, calcium silicate etc. Thermal insulators Glass wool

Characteristic properties

Engg. Applications.

Soft and flexible, fire and Used as thermal insulating material in industrial heat proof, insect proof and domestic appliances like ovens, motors and vacuum cleaners. Cork Possesses good porosity, Used in the lining of cold storage, bottle stoppers, compressibility and water refrigerators. resistance a part from good thermal resistance. Asbestos Fire and weather proof, Used as heat insulator in boilers, for roof covering durable, light in weight. in the form of sheets. Cellular Possess large number of Used for heat insulation in cold storage and rubber cellular cavities. refrigerators. Vermiculites Mica like minerals Used as heat insulators for furnace and also as sound insulators. ______________________________________________________________________________ Engineering Chemistry (Refractories & Insulators)3 Prepared By B.SRINIVAS

IB.Tech Engineering chemistry Question with answer complete material 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 of heat or electricity or sound through them are known as insulators or Insulating materials. Insulators can be broadly classified into three categories. Thermal Insulators Sound Insulators Electrical 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. 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. Typical values 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 that higher 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 Nitrogen Electronegative like SF6 & CCl4 Mineral oils

It provides insulation between the overhead transmission lines. It is used in transformers to replace the harmful oxidising atmosphere. under high pressure, it is also used as dielectric in certain electrical capacitors. gases SF6 is used in electrical devices like capacitors, cables.

Silicone fluids

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 to impregnate the paper in solid type of cable. 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.

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IB.Tech Engineering chemistry Question with answer complete material 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 of electrical machines etc. Inorganic fibrous Below red heat it is used in woven cloth tape, paper and wire insulators (asbestos) covering. Insulating varnishes Used for impregnation coatings and adhesion impregnation of fibrous materials for improving the mechanical strength, and decreasing the hygroscopic of the fibrous material by replacing air in their pores. Synthetic rubbers Silicsone rubbers used in ship wiring, aircraft cables, high temperature arcs, motor winding insulations. Synthetic resins

Rigid insulators(Glass)

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. 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 & Write a short note on Silica bricks. Answer: Refractories are ceramic materials that can withstand high temperatures as well as abrasive and corrosive action of molten metals; slag’s and gases, without suffering a deformation in 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 refractories iii. Basic refractories i. Neutral refractories like graphite, zirconia and SiC refractories. These refractories are made from 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 resistant to acidic slag (like silica) and are often used as contaminant vessel for them. On the other hand, they are readily attacked by basic slag’s (like CaO, MgO etc.) and contact with these oxide materials should be avoided. iii. Basic refractories like Magnetite and Dolomite refractories. These refractories consist of basic materials like CaO, MgO etc. and are especially resistant to basic slags. That’s why they find extensive use in some steel making open hearth furnaces. The presence of acidic materials like 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 abrasion resistance; ii. Using refractory material of higher thermal expansion; iii. Using a refractory of refractoriness less than that of the operating temperature; ______________________________________________________________________________ Engineering Chemistry (Refractories & Insulators)5 Prepared By B.SRINIVAS

IB.Tech Engineering chemistry Question with answer complete material 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. Silica Bricks: Preparation: The raw material used for the manufacture of silica bricks are quartz, quartzite, sand, sandstone, ganister etc. siliceous rock is first crushed and ground with 2% lime & water. The resultant thick paste is then made into bricks by machine pressing. After drying, bricks are burnt in kilns. In about 24 hours, the temperature is slowly raised to about 15000C. This high temperature is maintained for about 12 hours. This step is essential since it allows quartzite to be converted into cristobalite careful cooling is then done and it takes about 1 to 2 weeks. Cristobalite is slowly changed into tridymite during cooling and in the final brick, the mixture of two results. Properties: a. Silica bricks are yellowish in colour with speaks throughout the body. b. These bricks are acidic and are therefore suitable for acidic furnace charges. c. Silica bricks are remarkable for their load bearing capacity. They can withstand a load of about 3.5 kg/cm2 up to about 16000C. d. Silica bricks are not susceptible to thermal spalling at temperature below 8000C. e. These bricks are light and possess high rigidity and mechanical strength. f. These bricks do not contract in use. Applications: Silica bricks are used in lining roof arches of open hearth furnaces and reverberatory furnaces, gas retort and walls of coke ovens.

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