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Topic : Insulation MaterialName : Sathis Kumar .SEmail : sathis_svg@rediffmail.com

Selection of insulation material

Name : Sathis Kumar .S

Designation : Energy Engineer

Company Name : Dr.Ambedkar Institute of ProductivityNational Productivity Council, Chennai.

Postal Address : P-248, MMDA Colony,Arumbakkam, Chennai-600 106.

Fax No : 044-26254904

Email Id : sathis_svg@rediffmail.com

User Id : sathis_svg

mailto:ajchoudhary@dtps.bses.commailto:sathis_svg@rediffmail.commailto:sathis_svg@rediffmail.commailto:ajchoudhary@dtps.bses.com

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Mineral Fibre, Mica and Vermiculite based insulation. Fireclay or Silica based insulationand Ceramic Fibre etc.

The following table describes the characteristics of various insulating materials and howthey should be used.

TYPE OF INSULATION APPLICATION RECOMMENDATIONS

PolystyreneAn organic form made by

polymerizing styrene

Suitable for low temperatures(-167 oC to 82 oC). Mainly used incool rooms, refrigeration pipingand concrete retaining structures

Advantages Rigid andlightweightDisadvantages Combustible,has a low melting point, is UVdegradable, and susceptible toattacks by solvents

PolyurethaneMade by reacting isocyanates andalcohols. Made in continuousslab or foamed in situ.

Suitable for low temperatures (-178 oC to 4 oC). Mainly used incool rooms, refrigeratedtransports, deep freezing cabinets,

refrigeration piping and floor andfoundation insulation

Advantages Closed cellstructure, low density and highmechanical strengthDisadvantages Combustible,

produces toxic vapours and has atendency to smoulder

Rockwool (mineral fibre)Manufactured by melting basaltand coke in a cupola at about1500 oC. Phenolic binders used.

Suitable for temperatures up to820 oC. Mainly used to insulateindustrial ovens, heat exchangers,driers, boilers and hightemperature pipes

Has a wide density range and isavailable in matts, blankets, looseform or preformed for pipeinsulation. It is chemically inert,non-corrosive and maintainsmechanical strength duringhandling

FibreglassFormed by bonding long glassfibres with a thermo setting resinto form blankets and batts, semi-

rigid boards, high density rigid boards and preformed pipesections

Suitable for temperatures up to540 oC. Mainly used to insulateindustrial ovens, heat exchangers,driers, boilers and pipework

Will not settle or disintegratewith ageing.Fibreglass products are slightlyalkaline PH9 (neutral is PH7).

It should not promote or accelerate the corrosion of steel, provide it is protected fromexternal contamination

Calcium SilicateMade from anhydrous calciumsilicate material reinforced with anon-asbestos binder. Available inslab form of various sizes.

Suitable for temperatures up to1050 oC. Mainly used to insulatefurnace walls, fire boxes, back-uprefractory, flue lining and boilers

Has a minute air cell structure,has a low thermal conductivityand will retain its size and shapein its useable temperature range.It is lightweight, but has goodstructural strength so it canwithstand mechanical abrasion.It will not burn or rot, is moistureresistant and non-corrosive

Ceramic fibreMade from high purity aluminaand silica grains, melted in anelectric furnace and blasted byhigh velocity gases into lightfluffy fibres. Made in a variety of forms

Suitable for temperatures up to1430 oC. Mainly used to insulatefurnace and kiln back-uprefractory, fire boxes, glass feeder

bowls, furnace repair, inductioncoil insulation, high temperaturegaskets and wrapping material

Suitable for many applications because of the variety of forms.These include cloth, felt, tape,coating cements and variformcastable (fire brick)

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SELECTION OF INSULATING MATERIALS

A few important factors, which should be considered while choosing proper insulatingmaterials are:

a. The operating temperature of the system

b. Type of fuel being firedc. Resistance of the materials to heat, weather and adverse conditionsd. Thermal conductivity of the materiale. Thermal diffusivity of the materialf. Ability of the material to withstand the various conditions viz. thermal shock,

Vibration, Chemical attack etc.g. Resistance of the material to retard the spread of flameh. Permeability of the materiali. Total cost, including the cost of material, installing and thereafter its maintenance.

HEAT LOSS CALCULATION

Consider a pipe having R 1, R 2 as inside and outside radius over which an insulation is provided, whose radius is R 3. We need to find out the linear heat loss or heat loss per unitlength. For that we need to calculate the over all heat transfer coefficient (U) of the pipeincluding the insulation.

Where,U-the overall heat transfer coefficienthi & ho are the convective heat transfer coefficient.

Generally, the heat transfer coefficient of ambient air is 40 W/m 2 K. This coefficient canof course increase with wind velocity if the pipe is outside. A good estimate for anoutdoor air coefficient in warm climates with wind speeds under 15 mph is around 50W/m 2 K.

The total heat loss per unit length(Q/L) can then be calculated by:

(2)

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Since heat loss through insulation is a conductive heat transfer, there are instances whenadding insulation actually increases heat loss. The thickness at which insulation begins todecrease heat loss is described as the critical thickness. Since the critical thickness isalmost always a few millimeters, it is seldom (if ever) an issue for piping. Criticalthickness is a concern however in insulating wires. Figure shows the heat loss vs.

insulation thickness for a typical insulation. It's easy to see why wire insulation is kept toa minimum as adding insulation would increase the heat transfer.

INSULATION THICKNESS

Your economics depends on the cost of insulation, which in turn depends upon thethickness of insulation, that you are going to provide. So it is very important that youdetermine the exact insulation thickness that is required.

Method-1

This formula can be used for flat surfaces:

Thickness = k hkCal Loss Heat

C eTemperatur SurfaceC eTemperatur Operating *

)/()(deg)(deg

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Note:

The important factor, which affects the thickness is the emissivity(k). So higher the k,higher the insulation thickness required and hence higher the cost. By using the meanvalue of the insulating material, we get lower k value, and therefore a lower insulation

thickness.

To find the k value, find the mean temperature:

2eTemperatur SurfaceeTemperatur Operating +

Based on this mean temperature, k values can be obtained from Heat Transfer Handbooks.

Method-2

Readymade tables are available to calculate the Economic Thickness of Insulation.

Example of Economic Thickness Determination:

Using the table (Adapted from Perrys Chemical Engineers Handbook) below, assuminga 8.0 inch pipe at 345 0C in an indoor setting with an energy cost of $3.792/million kJ,what is the economic thickness?

Answer: Find the corresponding block to 203.2 mm (8.0 inch) pipe and $3.792/millionkJ energy costs, we see temperatures of 343.3 0C, 371.1 0C and 565.6 0C. Since our temperature does not meet 565 0C or 371 0C, we use the thickness before it. In this case,

for 343.30

C, 50.8 mm (2 inch) thickness of insulation is sufficient. At 3710

C, it can beincreased to 63.5 mm (2.5 inch) thickness of insulation.

Economic thickness charts from other sources will work in much the same way as thisexample.

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Conclusion

Insulation definitely saves money, but above that it very vital in safety point of view.Pipes that are readily accessible by workers are subject to safety constraints. Therecommended safe "touch" temperature range is from 54 0C to 65 0C. Insulation

calculations should aim to keep the outside temperature of the insulation around 600

C.