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:
The Insulation can be classified into three groups according to the temperature ranges for which they are used.
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
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.
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 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.
Thermal conductivities of typical hot and cold insulation materials are given in table5.1 and table 5.2.
The most basic model for insulation on a pipe is shown below. r1 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)) x 10 W/m2-K
For vertical pipes,
h = (B + 0.009 ( Th – 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
r1 = Actual outer radius of pipe, mm
r2 = (r1 + tk)
r2 = (r1 + tk)
The heat flow from the pipe surface and the ambient can be expressed as follows
From the above equation, and for a desired Ts, Rl can be calculated. From Rl and known value of thermal conductivity k, thickness of insulation can be calculated.
