INSULATION

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:

Reduces over-all energy consumption

 Offers better process control by maintaining process temperature.
 Prevents corrosion by keeping the exposed surface of a refrigerated system above dew
point
 Provides fire protection to equipment
 Absorbs vibration

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
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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 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 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 nitrile 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; now-a-days also used for ducting insulation for air conditioning.

Rock mineral wool: 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-m2/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.
Calcium silicate sections Glass Mineral wool.

Thermal conductivities of typical hot and cold insulation materials are given in Tables below.

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Calculation of Insulation Thickness
The most basic model for insulation on a pipe is shown In the Figure. show the outside radius of
the pipe r2 shows the radius of the Pipe+ insulation.

Heat loss from a surface is expressed as

H = h X A x (Th–Ta)

Where

h = Heat transfer coefficient, W/m2–K

H = Heat loss, Watts
Ta = Average ambient temperature, ºC
Ts = Desired/actual insulation surface temperature, ºC
Th = Hot surface temperature (for hot fluid piping), ºC & Cold surface temperature for cold
fluids piping) For horizontal pipes, heat transfer coefficient can be calculated by:
h = (A + 0.005 (Th – Ta)) × 10 W/m2-K
For vertical pipes,
h = (B + 0.009 ( Th – Ta)) × 10 W/m2-K

Using the coefficients A, B as given below.

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Economic Thickness of Insulation (ETI)
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. 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

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the cost of insulating the pipe. The insulation thickness for which the total cost is minimum is
termed as economic thickness.

The determination of economic thickness requires the attention to the following factors.

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

Procedure for Calculating Economic Thickness of Insulation

To explain the concept of economic thickness of insulation, we will use an example.

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:

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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/m2 °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-r1

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.

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