## Sunday, April 22, 2012

### Thermal Insulation Of Piping

Thermal insulation plays a major role in piping engineering. It reduces the heat loss and hence improves the economics of the process. There are also various other advantages for thermal insulation which will be described below.

What will the piping engineer learn from this section ?
The piping engineer will gain the following skills after studying this section:
1. What is the purpose of insulation?
2. How to optimise the thickness vs cost of insulation?
3. What are the various types of insulating materials?
4. How to select an insulating material?

### 1. Purpose of Insulation

• Energy saver • To maintain process condition • For personnel protection • To retard heat transfer
Note: A recent article in Chemical Engineering ( March 1997) mentions the US practice of deciding insulation thickness for personnel protection on the basis of surface after insulation not more than 52 deg C.

### 2.  Fundamentals of Heat Transfer

Conduction : Transfer of heat energy  within a body or between two bodies in physical contact.
Convection : The transfer of heat by the movement of parts of a fluid within the fluid , because of difference in the density, due to the difference in the temperature of the parts of the fluid.
Radiation: The transfer of heat energy from a high temperature body, through space, to another lower temperature body.

### 3.  Thermal Conductivity

The rate of steady state heat flow through a unit area of a homogeneous material, induced by a unit temperature gradient in a direction perpendicular to that unit area.

### 4.  First Law of Thermodynamics

Energy  can neither be created nor destroyed . It can only change form. The first law of thermodynamics cannot be proved mathematically  but no process in nature is known to have violated the first law.

### 5.  Second Law of Thermodynamics

It is possible to construct a device that operates in a cycle and converts all amount of heat into work, while exchanging heat with a single source.

### 6.  Heat

‘Heat’ is defined as the form of energy that is transferred between two systems, or a system and its surrounding, by virtue of temperature difference. Heat transfer to a system is positive, and heat transfer from a system is considered –ve.

### 7.  Conductive Heat Transfer

(Fourier’ s law of heat transfer) When temperature gradient exists in a body, energy transfer from a high temperature region to a low temperature region takes place. We say energy is transferred by conduction and heat transfer rate per unit area is proportional to the normal temperature gradient.
Or   Q/A  α  dt/dx
Or Q  = - kA. dt /dx
The - ve sign is used to satisfy the second law of thermodynamics

### 8. Convective Heat Transfer

It is seen that velocity of a fluid layer at the wall is zero. So, the heat must be transferred only by conduction.
The temperature gradient varies at the rate at which the fluid carries the heat, which in turn depends upon the velocity and other thermal properties of the fluid.

Equation 2 relates h with thermal conductivity and temperature gradient at the wall, and is used for the determination of ‘h' experimentally.

Thermal Conductivity: It is the physical property of a substance and characterizes the ability to transfer heat.

If Q = Rate of heat transfer

A = Area of heat flow, normal to direction of flow

k = Coefficient of thermal conductivity

dt/dx = Temperature gradient in the direction of heat flow

Q = -kA dt/dx   or   k = - (Q/A). dx/dt

#### Properties of Thermal Conductivity

1. It differs with each substance.
2. It depends on structure and other physical properties.
3. It depends upon temperature.
K = K0 ( 1+ α t)
The above equation shows that dependence of thermal conductivity on temperature is linear.

### 9. Conductivity through Thick Plane Homogeneous Wall

Assumptions
1. Steady state flow of heat takes place.
2. Thermal conductivity remains constant.
3. No heat flow takes place along y or z axes.

Conductance through a thick plane homogeneous wall with conductance
varying with temperature :
Consider
α = +ve for insulating material
= - ve for conductive material

Assume

### 11. Thermal Conductivity of Various Materials at 0 deg C

Conductivity
• For most pure metals it decreases with increasing temperature.
• For gases and insulating material, it increases with rise in temperature.

Non–metallic Solids

Liquids

Gases

### 12. Heat Conduction through Hollow Cylinder

By analogy with the equation of heat transfer through wall, this equation can
be written as

### 15. Various forms of Insulation Material

1. Loose fill
2. Blankets
3. Moulded blocks
4. Sheets
5. Bricks/Blocks

### 16. Physical Properties of Insulating Material

Depending upon application / uses the following physical properties of insulating material should be considered:
1. Density
2. Compressive strength
3. Flexibility
4. Mouldability
5. Moisture absorption
6. Aging
7. Effect of temperature
8. Flammability
9. Ease of application

### 17. Properties of Some Common Insulating Material

1. EPS/PUF : Good for cold insulation, mouldable, but highly inflammable.
2. Mineral wool: Good for ambient temperature /high temperature application , non inflammable.
3. Refractor y / Ceramic : Good for ver y high temperature, high compressive strength, no moisture absorption.

### 18. Method of Application of Insulating Material

Figure 1.

Typical insulation system on a vertical cold room wall:

1. Wall
2. Bitumen layer
3. Vapour barrier
4. Wooden batten
5. Insulation slab - EPS
6. Bitumen seal coat
7. Wire netting
8. Sand/ cement plaster

Figure 1.
Typical insulation system on a hot pipe

### 19. Design Parameters for Insulation System

1. Controlling heat loss from hot piping
2. Providing personal protection
3. Providing personal comfort in commercial building
4. Reducing heat gain by cold piping
5. Reducing surface condensation
6. Economic optimisation of energy conservation
Insulation thickness in mm data:

Note :
3. Density of glass wool : 80 – 100 Kg/m3
4. Surface temperature after insulation = 55 – 60 deg C
5. Insulation upto 65 mm- single layer, otherwise two layers.

### 20. Specific Weight and Conductivity of Materials

Selection of insulating material:

The following are the steps in the selection of insulation material/system:
1. Select design criteria
2. Specify inside/outside temperature
3. Specify insulation material
4. Find optimum insulation thickness
5. Specify application techniques
6. Finalise complete insulation system

### 21. Insulation Specification Sheet

1. Material: Calcium silica or Perlite (Thermal conductivity: 0.046 Kcal/mh deg C)

2. Material: Styrofoam ( for cooling water / chilled water)

### 22. Numericals

Solution

2. In the given condition find :
a) Thermal resistance
b) Conductance
c) Hourly heat loss

3. Consider a cold storage wall of area 5m x 2.5 m insulated with three layers each of brick (120 mm thk), cork ( 80 mm thk) & wood panel (25 mm thk). The inside face temp. is 00C and outside face temp. is 200C. Find a) Heat infiltration through wall in 24 hours, b) Also find interface temp t2 and t3.
Given thermal conductivity of each material as

To calculate interface temp:
Since heat transfer through each layer is same

#### Summing Up

We have learnt in this lesson about the importance of insulation and the role of the piping engineer in it. It is concluded that with proper insulation, a lot of revenue can be saved. Also not only insulation increases the profit of the company, it also contributes in improving the safety of the process plant.

#### Self-assessment

Please indicate either true or false:

1. The purpose of insulation is to maintain process conditions.
2. The US practice of deciding insulation thickness for personnel protection on the basis of surface after insulation is not more than 520C.
3. Conduction is transfer of heat energy due to difference in density.
4. Second law of thermodynamics: It is possible to construct a device that operates in a cycle and converts all amount of heat into work, while exchanging heat with a single source.
5. Velocity of fluid layer at the wall is maximum.
6. Thermal Conductivity is the physical property of a substance and characterises the ability to transfer heat.
7. Dependence of thermal conductivity on temperature is non-linear.
8. Conductivity for most pure metals decreases with increasing temperature.
9. For gases and insulating material, conductivity increases with rise in temperature.
10. Hot insulation material examples : Expanded polystyrene (EPS), Polyurethane Foam (PUF).

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