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Chapter 2.5: Insulation & Refractories
Part-I: Objective type questions and answers
- A thermal insulator is
a) good conductor of heat and has high thermal conductivity
b) poor conductor of heat and has high thermal conductivity
c) good conductor of heat and has low thermal conductivity
d) poor conductor of heat and has low thermal conductivity
- The insulation material suitable for low temperature application is
a) Mineral fibre b) Fibre glass c) Silica d) polyurethane
- The insulation materials used for medium temperature application is
a) Asbestos b) Calcium silicate c) Mineral fibre d) All of the above
- ‘Ceramic fibre insulation’ is suitable up to temperature of
a) 540 °C b) 1050 °C c) 1430 °C d) 1850 °C
- Match the following:
Insulating material Suitable temperature a) Rockwool i) - 178 °C to 4 °C b) Fibre glass ii) -167 °C to 82 °C c) Polystyrene iii) Up to 820 °C d) Polyurethane iv) Up to 540 °C
a-iii); b-iv); c-ii); d-i)
- Select the suitable cost effective insulation for steam pipelines with temperature of 540 °C.
a) calcium silicate b) fibre glass c) rock wool d) Alumina
- The unit for thermal coefficient of insulation and refractories is SI system is ____.
a) K.cal/m-hr-°C b) K.cal/ m²-hr-°C c) K.cal/m²-°C d) K.cal/ m-°C
- The coefficient of thermal expansion of refractory material should be ___.
a) low b) High c) Medium d) very high
- Magnesite, chrome-magnesite, dolomite are examples of --------- type of refractory
a) acid b) basic c) neutral d) none of the above
- Alumina is a ____ type of refractory.
a) acid b) basic c) Neutral d) None of the above
The heat conductivity of refractory material should be ___.
a) low b) High c) Medium c) None
- The most common form of refractory material extensively used in the construction of small furnaces is-----
a) silica bricks b) Fire brick
c) chrome magnesite d) Calcium silicate blocks
- The thermal conductivity of a ideal refractory material should be ____.
a) low b) high c) medium d) None of the above
- Mark the best choice of insulation material for electric heat treatment furnace among the following group.
a) glass wool b) calcium silicate c) fire bricks d) ceramic fibre
- The refractory which can resist most of the chemical attack and is unaffected by hydrocarbons, water and steam present in flue gas is -------
a) Alumina b) Ceramic Fibre c) Fire Brick d) Mineral Fibre
- High emissivity coatings are applied on:
a) Outer surface of furnace b) Refrigeration pipings
c) Inner surface of furnace d) None of the above
- The content on which ceramic fibres will be produced in two temperature grades is ___.
a) Al 2 O 3 b) SiO 2 c) ZrO 2 d) All the above
- The component required to reduce shrinkage levels in alumino silicate fibre is ___.
a) Al 2 O 3 b) ZrO 2 c) SiO 2 d) All the above
- The effect of thermal conductivity on thermal resistance of an insulation is ___.
a) Increases with increased thermal conductivity
b) Decreases with decreased thermal conductivity
c) Decreases with increased thermal conductivity
d) Increases with decreases thermal conductivity
- The insulation which can be used for lining furnaces operating up to 1850oC is:
- Name any two insulating material used for insulating steam pipelines
Insulating materials used for steam pipelines are rock wool (mineral fibre) and glass fibre.
- List some of the applications of fibre glass and calcium silicate insulation materials?
Fibre glass: It is mainly used to insulate industrial ovens, heat exchangers, driers, boilers and pipe work.
Calcium silicate: Mainly used to insulate furnace walls, fire boxes, back up refractory, flue lining and boilers
- Describe how ceramic fibre is made. List some of the applications of it?
Ceramic fibre is made from high purity alumina and silica grains, melted in an electric furnace and blasted by high velocity gases into light fluffy fibres. It is mainly used to insulate furnace and kiln back up refractory, fire boxes, glass feeder bowls, furnace repair, induction coil insulation, high temperature gaskets and wrapping material
- What is economic thickness of insulation?
The effectiveness of insulation follows the law of decreasing returns. Hence, there is a definite economic limit to the amount of insulation, which is justified. An increased thickness is uneconomical and cannot be recovered through small heat savings. This limiting value is termed as economic thickness of insulation.
- What are the parameters required to evaluate economic thickness of insulation.
For determination of economic thickness following parameters are required:
i) Cost of fuel ii) Annual hours of operation iii) Heat content of fuel iv) Boiler efficiency v) Operating surface temperature vi) Pipe diameter / thickness of surface vii) Estimated cost of insulation viii) Average exposure ambient still air temperature
- Write the simple equation for heat loss calculation useful for up to 200 °C temperature.
The surface heat loss can be computed with the help of a simple relation up to 200 °C surface temperature.
S = [10 + (Ts-Ta) / 20] x [Ts-Ta]
Where,
S = Surface heat loss in kcal/hr m^2
Ts = Hot surface temperature in °C
Ta = Ambient temperature in °C
- Describe the term ‘refractory material’.
Any material, if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures are called ‘refractory’ material.
In furnaces, apart from fuel savings, what are the other advantages by use of insulation? The advantages by use of insulation in furnaces, apart from fuel savings are: i) Offers better process control by maintaining process temperatures ii) Prevents corrosion by keeping higher flue gas temperatures above acid dew point. iii) Provides fire protection iv) Absorbs vibration
- What are the general requirements of a refractory material?
The general requirements of a refractory material are
i) Ability to withstand action of molten metal slag, glass, hot gases etc.
ii) Ability to withstand load and abrasive forces
iii) Low coefficient of thermal expansion
iv) Should be able to conserve heat
v) Should not contaminate the material with which it comes into contact
- What are the characteristics of acidic, basic and neutral type refractories?
Acidic: it readily combines with base
Basic: consists mainly of metallic oxides which resist the action of bases
Neutral: does not combine neither with acids nor bases
- Describe in brief how selection of refractories is done? Describe with an example?
The selection of refractories for any particular application is made with a view to achieve the best performance of the furnace, and depends on certain properties of the refractories. Firebrick is the most common form of refractory material used extensively in the construction of small furnaces. Other refractories which are used in furnace construction are high alumina brick, silica brick, magnesite brick, chromite brick, zirconia brick, monolithics etc.
- List important properties of ceramic fibre insulation?
The properties are
¾ Low thermal conductivity
¾ Light Weight
¾ Lower heat storage
¾ Thermal shock resistant
ii. Pipe diameter/thickness of surface
iii. Estimated cost of insulation.
Average exposure ambient still air temperature.
- A furnace wall is made of 75 mm thick fire clay brick and 6.4 mm thick mid steel plate. The inside surface of brick temperature is 647 °C and outside air temperature is 27 °C.
Determine:
i) The heat loss per m^2 area of the furnace wall.
Consider K1 (brick) = 1.1 W/m-°C
K2 (steel) = 39 W/m-°C
h 0 (outside heat transfer coefficient) = 68 W/m^2 -°C
Considering unit surface area
R (^) brick =
K
L
R steel =
K
L
= x
R Conv. = 0. 0147
0
h
RTotal = Rbrick + Rsteel + Rcov = 0.
ii) Q (heat loss per m^2 area) = Total
i a
R
T − T
= 7460 /^2
= W m
7 .5 mm L
6.4 mm L 2
Ti = 647 °C
Ta = 27 °C
- Steam pipeline 100 mm diameter is not insulated for 100 metre length supplying steam at 10 kg/cm^2 to the equipment. Find out the fuel savings if it is properly insulated with 65 mm insulating material.
Given:
Boiler efficiency – 80%
Fuel Oil cost - Rs.15000/tonne
Surface temperature without insulation – 170oC
Surface temperature after insulation – 65oC
Ambient temperature – 25oC
Existing Heat Loss
S = [10+ (Ts-Ta)/20] x (Ts-Ta)
Ts = 170oC
Ta = 25oC
S = [10+(170-25)/20] x (170-25) = 2500 Kcal/hr-m^2
S1 = S =Existing heat loss (2500 Kcal/hr-m^2 )
Modified System
After insulating with 65 mm glass wool with aluminium cladding the hot face temperature will be 65oC
Ts – 65oC
Ta – 25oC
Substituting these values
S = [10+(65-25)/20] x (65-20)
= 480 K.cal/hrM^2
S2 = S =Existing heat loss (480 Kcal/hr-m^2 )
Fuel Savings Calculation
Pipe dimension = (^) 100 mm φ & 100 m length
Surface area existing (A1) = 3.14 x 0.1 x 100 = 31.4 m^2
Surface area after insulation (A2) = 3.14 x 0.23 x 100 = 72.2 m^2
Total heat loss in existing system (S1 x A1)
= 2500 x 31.4 = 78500 Kcal/hr
Total heat loss in Modified system (S2 x A2)
= 480 x 72.2 = 34656 Kcal/hr
Reduction in heat loss = 78500 – 34656 = 43844 Kcal/hr
No. of hours operation in a year = 8400 hours
Total heat loss (Kcal/yr) = 43844 x 8400 = 368289600
Calorific value of fuel oil = 10300 Kcal/kg
Boiler efficiency = 80%
Price of fuel oil = Rs.15000/Tonne
Yearly fuel oil savings = 368289600/10300 x 0.
= 44695 kg//year
