Energy efficiency in Thermal utilities
Chapter 5: Insulation & Refractories
5.1 Purpose of Insulation
A thermal insulator is a poor conductor of heat and has a low thermal conductivity. Insulation is used in buildings and in manufacturing processes to prevent heat loss or heat gain. Although its primary purpose is an economic one, it also provides more accurate control of process temperatures and protection of personnel. It prevents condensation on cold surfaces and the resulting corrosion. Such materials are porous, containing large number of dormant air cells. Thermal insulation delivers the following benefits:
1.Reduces over-all energy consumption
2. Offers better process control by maintaining process temperature.
3. Prevents corrosion by keeping the exposed surface of a refrigerated system above dew point
4. Provides fire protection to equipment
5. Absorbs vibration
5.2. Types and Application
The Insulation can be classified into three groups according to the temperature ranges for which they are used.
Low Temperature Insulations (up to 90°C)
This range covers insulating materials for refrigerators, cold and hot water systems, storage tanks, etc. The commonly used materials are Cork, Wood, 85% magnesia, Mineral Fibers, Polyurethane and expanded Polystyrene, etc.
Medium Temperature Insulations (90 — 325°C)
Insulators in this range are used in low temperature, heating and steam raising equipment, steam lines, flue ducts etc. The types of materials used in this temperatures range include 85% Magnesia, Asbestos, Calcium Silicate and Mineral Fibers etc.
High Temperature Insulations (325° C — above)
Typical uses of such materials are super heated steam system, oven dryer and furnaces etc. The most extensively used materials in this range are Asbestos, Calcium Silicate, Mineral Fibre, Mica and Vermiculite based insulation, Fireclay or Silica based insulation and Ceramic Fibre.
Insulation material
Insulation materials can also be classified into organic and inorganic types. Organic insulations are based on hydrocarbon polymers, which can be expanded to obtain high void structures
Example: Thermocol (Expanded Polystyrene) and Poly Urethane Form (PUF).
Inorganic insulation is based on Siliceous/Aluminous/Calcium materials in fibrous, granular or powder forms.
Example: Mineral wool, Calcium silicate etc. Properties of common insulating materials are as under:
Calcium Silicate: Used in industrial process plant (Figure 5.1) piping where high service temperature and compressive strength are needed. Temperature ranges varies from 40 °C to 950 °C .
Glass mineral wool: These are available (Figure 5.1) in flexible forms, rigid slabs and preformed pipe work sections. Good for thermal and acoustic insulation for heating and chilling system pipelines. Temperature range of application is —10 to 500 °C
Thermocol: These are mainly used as cold insulation for piping and cold storage construction.
Expanded nitrite rubber:
This is a flexible material that forms a closed cell integral vapour barrier.
Originally developed for condensation control in refrigeration pipe work and chilled water lines; nowa-days also used for ducting insulation for air conditioning.
This is available in a range of forms from light weight rolled products to heavy
rigid slabs including preformed pipe sections. In addition to good thermal insulation properties, it can
also provide acoustic insulation and is fire retardant.
Use of Moulded Insulation
Lagging materials can be obtained in bulk, in the form of moulded sections; semi - cylindrical for
pipes, slabs for vessels, flanges, valves etc. The main advantage of the moulded sections is the ease
of application and replacement when undertaking repairs for damaged lagging.
The thermal conductivity of a material is the heat loss per unit area per unit insulation thickness per
unit temperature difference. The unit of measurement is W/m?/m°C or W/m°C. The thermal conductivity
of materials increases with temperature. So, thermal conductivity is always specified at the mean
temperature (mean of hot and cold face temperatures) of the insulation material.
Thermal conductivities of typical hot insulation materials is given in Table 5.1
5.3. Calculation of Insulation Thickness
The most basic model for insulation on a pipe is shown in Figure 5.2.rl shows the outside radius of the pipe
r2 shows the radius of the pipe + insulation.
Heat loss from a surface is expressed as
H=hXAx(T,-Ta)
Where, h = Heat transfer coefficient, W/m2-K
H = Heat loss, Watts
Ta = Average ambient temperature, °C
Ts = Desired/actual insulation surface temperature, °C
Th = Hot surface temperature (for hot fluid piping), °C & Cold surface temperature for cold
fluids piping)
For horizontal pipes, heat transfer coefficient can be calculated by:
h = (A+ 0.005 (T, — Ta)) x 10 W/m2-K
For vertical pipes,
h=(B + 0.009 ( T, — Ta)) x 10 W/m2-K
Using the coefficients A, B as given below.
k = Thermal conductivity of insulation at mean temperature of Tm, W/m-C
tk = Thickness of insulation, mm
rl = Actual outer radius of pipe, mm
r2 = (rl + tk)
The heat flow from the pipe surface and the ambient can be expressed as follows
Equivalent thickness of insulation for pipe, Etk.
Insulation of any system means capital
expenditure. Hence the most important factor in
any insulation system is to analyse the thermal
insulation with respect to cost. 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. An illustrative case is
given in Figure 5.3. Each industry has different fuel cost and boiler efficiency. These values can be
used for calculating economic thickness of insulation. This shows that thickness for a given set of
circumstances results in the lowest overall cost of insulation and heat loss combined over a given
period of time. The following Figure 5.4 illustrates the principle of economic thickness of insulation.
The simplest method of analysing whether you should use 1” or 2” or 3” insulation is by comparing
the cost of energy losses with the cost of insulating the pipe. The insulation thickness for which the
total cost is minimum is termed as economic thickness. Refer fig 5.4 The curve representing the total
cost reduces initially and after reaching the economic thickness corresponding to the minimum cost,
it increases.
The determination of economic thickness requires the attention to the following factors.
1. 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
Procedure for calculating Economic thickness of insulation
To explain the concept of economic thickness of insulation, we will use an example. (Refer Table 5.2)
Consider an 8 bar steam pipeline of 6” dia having 50-meter length. We will evaluate the cost of energy
losses when we use 1”, 2” and 3” insulation to find out the most economic thickness.
A step-by-step procedure is given below.
1. Establish the bare pipe surface temperature, by measurement.
2. Note the dimensions such as diameter, length & surface area of the pipe section under consideration.
3. Assume an average ambient temperature. Here, we have taken 30°C.
4. Since we are doing the calculations for commercially available insulation thickness, some
trial and error calculations will be required for deciding the surface temperature after putting
insulation. To begin with assume a value between 55° & 65°C, which is a safe, touch temperature.
5. Select an insulation material, with known thermal conductivity values in the mean insulation
temperature range. Here the mean temperature is 111°C. and the value of k = 0.044 W/m*C for
mineral wool.
6.Calculate surface heat transfer coefficients of bare and insulated surfaces, using equations
discussed previously. Calculate the thermal resistance and thickness of insulation.
7. Select r2 such that the equivalent thickness of insulation of pipe equals to the insulation thickness
estimated in step 6. From this value, calculate the radial thickness of pipe insulation = r2-rl
8. Adjust the desired surface temperature values so that the thickness of insulation is close to the
standard value of 1” (25.4 mm).
9. Estimate the surface area of the pipe with different insulation thickness and calculate the total
heat loss from the surfaces using heat transfer coefficient, temperature difference between pipe
surface and ambient.
10. Estimate the cost of energy losses in the 3 scenarios. Calculate the Net Present Value of the
future energy costs during an insulation life of typically 5 years.
11. Find out the total cost of putting insulation on the pipe ( material + labor cost)
12. Calculate the total cost of energy costs and insulation for 3 situations.
13 Insulation thickness corresponding to the lowest total cost will be the economic thickness of
insulation.
Note that the total cost is lower when using 2” insulation, hence 2”size is the economic insulation
thickness.
5.5 Simplified Formula for Heat Loss Calculation
Various charts, graphs and references are available for heat loss computation. The surface heat loss
can be computed with the help of a simple relation as given below. This equation can be used up to
200 °C surface temperature. Factors like wind velocities, conductivity of insulating material etc has
not been considered in the equation.
S = [10+(Ts-Ta)/20] x (Ts-Ta)
Where,
S = Surface heat loss in kcal/hr m2
Ts= Hot surface temperature in °C
Ta= Ambient temperature in °C
Total heat loss/hr (Hs) = SxA
Where,
A is the surface area in m2
Based on the cost of heat energy, the quantification of heat loss in Rupees can be worked out as under:
Case Example
Steam pipeline 100 mm diameter is not insulated for 100 metre length supplying steam at 10 kg/cm?
to the equipment. Find out the fuel savings if it is properly insulated with 65 mm insulating material.
Assumptions:
Boiler efficiency — 80%
Fuel Oil cost - Rs.15000/tonne
Surface temperature without insulation — 170°C
Surface temperature after insulation — 65°C
Ambient temperature — 25°C
Existing Heat Loss
S =[10+ (Ts-Ta)/20] x (Ts-Ta)
Td = 170°C
Ta = 25°C
S =[10+(170-25)/20] x (170-25) =2500 keal/hr-m2
S1 = S =Existing heat loss (2500 kcal/hr-m2 )
Modified System
After insulating with 65 mm glass wool with aluminum cladding the hot face temperature will be
65 °C
Ts — 65 °C
Ta — 25 °C
Substituting these values
S = [10+(65-25)/20] x (65-25)
= 480 kcal/hr m2
S2 = S =Existing heat loss (480 kcal/hr-m2 )
Cold Insulation Features
Cold Insulation should be considered and where operating temperature are below ambient where
protection is required against heat gain, condensation or freezing. Condensation will occur whenever
moist air comes into contact with the surface that is at a temperature lower than the due point of the
vapour. In addition, heat gained by uninsulated chilled water lines can adversely affect the efficiency
of the cooling system. The most important characteristics of a suitable Cold insulation material have
following features:
1.Low thermal conductivity
2.High water resistance, and
3.Durability at low temperature
Other properties like easy workability, negligible capillary absorption should also be taken into
consideration while making a selection. The insulation system is only as good as its vapour barrier
and the care with which it is installed.
Material Selection for Cold Insulation
Selection of insulation materials should be carefully considered where the possibility of steam purging
of the equipment is required or for other reasons which may cause the temperature to be increased to
a level that exceeds the maximum limiting temperature of the insulation materials, 1.e., material then
deteriorate. Examples of cold insulation include Urethane Foam, Expanded Polystyrene, Resin bonded
glass wool, Resin Bonded Glass wool, and Phenolic Foam.
Economics of Cold Insulation:
Unlike hot insulation system, the concern area in Cold Insulation is the heat gain into the refrigerated
space, which leads to increase in the refrigeration load (TR) & energy consumption as a consequence.
The cost of heat gain can thus be assessed & evaluated against cost of additional cold insulation
thickness, to optimize overall energy consumption & cost in refrigeration system.
5.7 Refractories
Any material can be described as ‘refractory,’ if it can with stand the action of abrasive or corrosive
solids, liquids or gases at high temperatures. Examples are: fireclay, alumina, magnesite, chrome
magnesite, dolomite etc. Refractory materials are made in varying combinations and shapes and for
different requirements of high temperature processes carried out in metal extraction, cement,
glassmaking, manufacturing, ceramic etc.
The general requirements of a refractory material can be summed up as :
1.Ability to withstand high temperatures.
2.Ability to withstand sudden changes of temperatures.
3.Ability to withstand action of molten metal slag, glass, hot gases, etc.
4.Ability to withstand load at service conditions.
5.Ability to withstand load and abrasive forces.
6.Low coefficient of thermal expansion.
Should be able to conserve heat.
7.Should not contaminate the material with which it comes into contact.
5.8 Properties of Refractories
Some of the important properties of refractories are:
Size :
The size and shape of the refractories is a part of the design feature. It is an important feature
in design since it affects the stability of any structure. Accuracy and size is extremely important to
enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.
Bulk density:
A useful property of refractories is bulk density, which defines the material present in
a given volume. An increase in bulk density of a given refractory increases its volume stability, its
heat capacity, as well as resistance to slag penetration.
Porosity: The apparent porosity is a measure of the volume of the open pores, into which a liquid can
penetrate, as a percentage of the total volume. This is an important property in cases where the refractory
is in contact with molten charge and slags. A low apparent porosity is desirable since it would prevent
easy penetration of the refractory size and continuity of pores will have important influences on
refractory behaviour. A large number of small pores is generally preferable to an equivalent number
of large pores.
Cold crushing strength:
The cold crushing strength, which is considered by some to be of doubtful
relevance as a useful property, other than that it reveals little more than the ability to withstand the
rigors of transport, can be used as a useful indicator to the adequacy of firing and abrasion resistance
in consonance with other properties such as bulk density and porosity.
Pyrometric cone equivalent (PCE):
Temperature at
which a refractory will deform under its own weight is
known as its softening temperature which is indicated by PCE. Refractories, due to their chemical complexity, melt progressively over a range of temperature. Hence refractoriness or fusion point is ideally assessed by the cone fusion method. The equivalent standard cone which melts to the same extent as the test cone is known as the pyrometric cone equivalent.
Thus in the Figure 5.5 refractoriness of Sample A is much higher than B and C. The pyrometric cone
equivalent indicates only the softening temperature. But, in service the refractory is subjected to loads
which would deform the refrectory at a much lower temperature than that indicated by PCE. With
change in the environmental conditions, such as reducing atmosphere, the P.C.E. value changes
drastically.
Refractoriness under load (RUL):
The refractoriness under load test (RUL test) gives an indication
of the temperature at which the bricks will collapse, in service conditions with similar load.
Creep at high temperature:
Creep is a time dependent property which determines the deformation
in a given time and at a given temperature by a material under stress.
Volume stability, expansion, and shrinkage at high temperatures:
The contraction or expansion
of the refractories can take place during service. Such permanent changes in dimensions may be due
to several operational factors.
Any material when heated, expands, and contracts on cooling. The
reversible thermal expansion is a reflection on the phase transformations that occur during heating and
cooling.
Thermal conductivity:
Thermal conductivity depends upon the chemical and mineralogical
compositions as well as the glassy phase contained in the refractory and the application temperature.
The conductivity usually changes with rise in temperature. In cases where heat transfer is required
though the brick work, for example in recuperators, regenerators, muffles, etc. the refractory should
have high conductivity. Low thermal conductivity is desirable for conservation of heat by providing
adequate insulation.
The provisions for back-up insulation, conserves heat but at the same time it increases the hot face
temperature and hence the demand on the refractory quality increases.
Light weight refractories of low thermal conductivity find wider applications in the moderately low
temperature heat treatment furnaces, where its primary function is usually conservation of energy. It
is more so in case of batch type furnaces where the low heat capacity of the refractory structure would
minimize the heat storage during the intermittent heating and cooling cycles.
5.9 Classification of Refractories
Refractories can be classified on the basis of chemical composition and use and methods of manufacture
is given in Table 5.3.
5.10 Typical Refractories in Industrial Use
Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and
atmospheres encountered different types of refractories are used. Typical installations of refractories
are shown in Figure 5.6
Fireclay Refractories
Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of
aluminium silicates with various amounts of silica ranging from SiO2 content of less than 78% and
containing less than 44% of AL2O3
Table 5.4 shows that as the quantity of impurities increases and the amount of Al2O3 decreases, the
melting point of fireclay brick decreases. Owing to its relative cheapness and widespread location of
the raw materials used to manufacture firebricks, this material finds use in most furnaces, kilns, stoves,
etc.
High Alumina Refractories
Alumino silicate refractories containing more than 45% alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100%. The refractoriness of high alumina
refractories increases with increase in alumina percentage. The applications of high alumina refractories includes the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
Silica Brick
Silica brick (or Dinas) is a refractory material containing at least 93% SiO, The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces. In addition to high fusion point multi-type refractories, the other important properties are their high resistance to thermal shock (spalling) and their high refractoriness.
It finds typical use in glass making and steel industry. The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached.
This behaviour contrasts with that of many other refractories, for example alumino silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. Other advantages are flux and stag resistance, volume stability and high spalling resistance.
Magnesite
Magnesite refractories are chemically basic materials, containing at least 85% magnesium oxide. They are made from naturally occurring magnesite (MgCO₂). The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO, ratio of less than 2 with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags.
Chromite Refractories
Chrome-magnesite refractories are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. The magnesite-chromite refractories on the other hand are suitable for service at the highest temperatures and in contact with the most basic slags used in steel melting. Magnesite-chromite usually ahs a better spalling resistance than chrome-magnesite.
Zirconia Refractories
Zirconium dioxide (ZrO,) is a polymorphic, material. There are certain difficulties in its usage and fabrication as a refractory material. It is essential to stabilize it before application as a refractory. This is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization and quantity of stabilizer as well as the quality of the original raw material. Zirconia refractories have a very high strength at room temperature which is maintained upto temperatures as high as 1500°C. They are, therefore, useful as high temperature constructional materials for furnaces and kilns. The thermal conductivity, of zirconium dioxide is found to be much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Since Zirconia exhibits very low thermal losses and does not react readily with liquid metals, it is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Zirconia is a useful refractory material for glass furnaces primarily since it is not easily wetted by molten glasses and because of its low reaction with them.
Oxide Refractories (Alumina)
Alumina refractory materials which consist of aluminium oxide with little traces of impurities are often known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water and super heated steam, and in most inorganic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850°C.
Monolithics
Monolithic refractories (single piece cast in the shape of equipment such as one for a ladle shown in Figure 5.7) are replacing the conventional type fired refractories at a much faster rate in many applications including those of industrial furnaces. The main advantages being:
1.It eliminates joints which is an inherent weakness
2.Method of application is faster and skilled measures in large number are not required
3.Transportation and handling are simple
4.Offers better scope to reduce downtime for repairs
5.Offers considerable scope to reduce inventory and eliminate special shapes
6.It is a heat saver
7.Has better spalling resistance
8.Has greater volume stability
Various means are employed in the placement of monolithics like ramming, casting, gunniting, spraying, sand slinging, etc. Ramming masses are used mostly in cold applications where proper consolidation of the material is important. The same practice can be adopted with both air setting and heat setting materials. Proper ramming tools need to be selected.
Castables by name implies a material of hydraulic setting in nature. Calcium aluminate cement being the binder, it will have to be stored properly to prevent moisture absorption. Further its strength starts deteriorating after a period of 6 to 12 months.
Insulating materials
Insulating materials greatly reduce the heat losses through walls. Insulation is effected by providing a layer of material having a low heat conductivity between the internal hot surface of a furnace and the external surface, thus causing the temperature of the external surface reduced.
The insulating materials may be classified into the following groups
1.Insulating bricks
2.Insulating Castables
3.Ceramic fibre
4.Calcium silicate
5.Ceramic coating
Insulating materials owe their low conductivity to their pores while their heat capacity depends on the bulk density and specific heat. Structure of air insulating material consists of minute pores filled with air which have in themselves very low thermal conductivity, excessive heat affects all insulation material adversely, but the temperatures to which the various materials can be heated before this adverse effect occurs differ widely. Clearly, therefore, the choice of an insulating material must depend upon its effectiveness to resist heat conductivity and upon the temperature that it will withstand.
One of the most widely used insulating materials is diatomite, also known as kiesel guhr which is made up of a mass of skeletons of minute aquatic plants deposited thousands of years ago on the beds of seas and lakes. Chemically this consists of silica contaminated with clay and organic matter. Wide ranges of insulating refractories with wide combinations of properties are now available.
The important physical properties of some insulating refractories are shown in the Table 5.5.
Castables and Concretes
Monolithic linings and furnace sections can be built up by casting refractory insulating concretes, and by stamping into place certain light weight aggregates suitably bonded. Other applications include the formation of the bases of tunnel kiln cars used in the ceramic industry. The ingredients are similar to those used for making piece refractories, except that concretes contain some kind of cement, either Portland or high-alumina cement.
Ceramic Fibre
Ceramic fibre (Figure 5.8) is a low thermal mass insulation material, which has revolutionalised the furnace design lining systems. Ceramic fibre is an alumino silicate material manufactured by blending and melting alumina and silica at temperature of 1800-2000°C and breaking the molten stream by blowing compressed air or dropping the melt on spinning disc to form loose or bulk ceramic fibre. The bulk fibre is converted to various products including blanket, strips, veneering and anchored modules, paper, vacuum formed boards and shapes, rope, wet felt, mastic cement etc. for insulation applications. The properties of ceramic fibre in comparison with with conventional refractories is given in Table 5.6
Fibres are usually produced in two temperature grades based on Al,O, content. A recent addition is
Zr O, added alumino silicate fibre, which helps to reduce shrinkage levels thereby rating the fibre for
higher temperatures. Continuous recommended operating temperature for fibres are given in the
following Table 5.7.
These fibres are generally produced in bulk wool form and needled into blanket mass of various densities ranging from 64 to 190 kg/m’. Converted products and over 40 different forms are made from blankets to suit various requirements.
The characteristics of ceramic fibres are a remarkable combination of the properties of refractories
and traditional insulation material.
Lower Thermal Conductivity
The low thermal conductivity — 0.1 kcal/m hour deg C at 600°C for 128 kg/m? density blanket
—allows construction of thinner linings with the same thermal efficiency as that of conventional
refractories. Hence, for the same outer envelope dimension the furnace volume is much higher.
It is 40 % more effective than good quality insulation brick and 2.5 times better than asbestos
product. Insulating property of ceramic fibre is better than calcium silicate product.
Light Weight
Average density of ceramic fibre is 96 kg/m. It is one tenth of the weight of insulating brick
and one third that of asbestos / calcium silicate boards. For new furnaces structural supports
can be reduced by 40%.
Lower Heat Storage
Ceramic fibre linings absorb less heat because of lower density. Furnace can be heated and
cooled at faster rates. Typically the heat stored in a ceramic fibre lining system is in the range
of 2700 - 4050 kcal/m? (1000 — 1500 Btu/Ft’) as compared to 54200-493900 kcal/m? ( 20000
— 250000 Btu/Ft’ ) for conventionally lined system.
Thermal Shock Resistant
Ceramic fibre lining resist thermal shock due to their resilient matrix. Also faster heat up and
cool down cycles are possible thereby improving furnace availability and productivity.
Chemical Resistance
Ceramic fibre resist most of the chemical attack and is unaffected by hydrocarbons, water and
steam present in flue gases.
Mechanical Resilience
This property permits fibre lined furnaces to be shop fabricated and shipped to site in assembled
form without fear of damage.
Low Installation Cost
No special skills are required as application practices are standardised. Fibre linings require
no dry out or curing times and can be heated to the capacity of the burners after installation is
completed without concern for cracking or spalling.
Simple Maintenance
In case of physical damage the defective section can be quickly removed and a replacement
piece added. Whole panel sections can be prefabricated for fast installation with minimal
down time.
Ease of Handling
All product forms are easily handled and most can be quickly cut with a knife or scissors.
Vacuum formed products may require cutting with a band saw.
Thermal Efficiency
The low thermal conductivity of ceramic fibre can be advantageously made use of by the lesser
lining thickness and reduced furnace volume. The fast response of ceramic fibre lined furnace
also allows for more accurate control and uniform temperature distribution within the furnace.
The other advantages offered by ceramic fibre are summarized below:
1.Light weight furnace
2.Simple steel fabrication work
3.Low down time
4.Increased productivity
5.Additional capacity
6.Low maintenance cost
7.Longer service life
8.Higher thermal efficiency
9.Faster response
High Emissivity Coatings
Emissivity, the measure of a material’s ability to both absorb and radiate heat, has been considered by
engineers as being an inherent physical property which like density, specific heat and thermal
conductivity, is not readily amenable to change. However, the development of high emissivity coatings
now allows the surface emissivity of materials to be increased, with resultant benefits in heat transfer
efficiency and in the service life of heat transfer components. High emissivity coatings are applied in
the interior surface of furnaces. The Figure 5.9 shows emissivity of various insulating materials
including high emissivity coatings. High emissivity coating shows a constant value over varying
process temperatures.
The application of high-emissivity coatings in furnace chambers promotes rapid and efficient transfer
of heat, uniform heating, and extended life of refractories and metallic components such as radiant
tubes and heating elements. For intermittent furnaces or where rapid heating is required, use of such
coatings was found to reduce fuel or power to tune of 25-45%. Other benefits are temperature uniformity
and increased refractory life.
Furnaces, which operate at high temperature, have emissivities of 0.3. By using high emissivity coatings
this can go upto 0.8 thus effectively increasing the radiative heat transfer.
Selection of Refractories
The selection of refractories for any particular application is made with a view to achieve the best
performance of the equipment furnace, kiln or boiler and depends on their properties. Further, the
choice of a refractory material for a given application will be determined by the type of furnace or
heating unit and the prevailing conditions e.g. the gaseous atmosphere, the presence of slags, the type
of metal charge etc. It is, therefore, clear that temperature is by no means the only criterion for selection
of refractories.
Any furnace designer or industry should have a clear idea about the service conditions which the
refractory is required to face. The furnace manufacturers or users have to consider the following points,
before selecting a refractory.
1.Area of application.
2.Working temperatures.
3.Extent of abrasion and impact.
4.Structural load of the furnace.
5.Stress due to temperature gradient in the structures and temperature fluctuations.
6.Chemical compatibility to the furnace environment.
7.Heat transfer and fuel conservation
8.Cost considerations.
It is therefore, essential to have an objective evaluation of the above conditions. A proper assessment of the desired properties would provide guidelines for selection of the proper refractory materials.
It would be important to mention here that the furnace manufacturer or a user is also concerned with
the conservation of energy. Fuel can be saved in two ways: either by insulation or by faster working.
Both these methods give low energy cost per tonne of product.
Heat Losses from Furnace Walls
In furnaces and kilns, heat losses from furnace walls, affect the fuel economy substantially. The extent
of wall losses depends on:
1.Emissivity of walls;
2. Conductivity of refractories;
3. Wall thickness;
4. Whether furnace or kiln is operated continuously or intermittently.
Heat losses can be reduced by increasing the wall thickness, or through the application of insulating
bricks. Outside wall temperature and heat losses for a composite wall of a certain thickness of firebrick
and insulation brick are much lower due to lesser conductivity of insulating brick as compared to a
refractory brick.
In the case of batch furnace operation, operating periods (“‘on”) alternate with idle periods (“off”).
During the off period, the heat stored in the refractories in the on-period is gradually dissipated, mainly
through radiation and convection from the cold face. In addition, some heat is obstructed by air flowing
through the furnace. Dissipation of stored heat is a loss, because the lost heat is at least in part again
imparted to the refractories during the next “on” period, thus expending fuel to generate the heat. If a
furnace is operated 24 hr. every third day, practically all of the heat stored in the refractories is lost.
Solved Example:
A steam pipeline of 250 mm outer diameter & 100 meters long is insulated with 150 mm Mineral wool
insulation. As an energy conservation measure, the management has upgraded the existing Mineral
wool insulation with efficient calcium silicate insulation. Calculate the economics in terms of payback
if the insulation is upgraded at a cost of 20 lakhs.
Given:
1.Operating hours : 8000
2.Boiler efficiency : 87%
3.Fuel Oil Cost : Rs. 45,000 per ton
4.GCV of the fuel : 10,200 kcal/kg
5.Thickness of Mineral wool insulation : 150 mm
6.Thickness of Calcium Silicate insulation : 100 mm
7.Surface temperature with Mineral wool insulation : 70 °C
8.Surface temperature with Calcium silicate insulation : 55°C
9.Ambient temperature : 30 °C
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Chapter 6
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