Thermal Insulation

Thermos Flask

In cold places, we want hot coffee or hot tea, hot food.

We design for reducing heat flow, e.g. heat exchanger, building insulation, thermos flask and so on. Thermal insulation materials must have low thermal conductivity. This is achieved by trapping air or some other gas inside small cavities in a solid. Low conductivity of gas to inhibit heat flow. Heat can ,however, be transferred by natural convection inside the gas pockets and by radiation between the solid enclosure walls.

Apparent thermal conductivity of an insulating material accounting for conduction through solid material, and convection through the air space as well as radiation.

There are essentially three types of insulation materials :

1. Fibrous

Fibrous materials consist of small diameter particles or filaments of low density that can be poured into a gap as “loose-fill” or formed into blankets. Fibrous materials have high porosity.

  • Mineral wool for temperature below 700°C
  • Fiberglass for temperature below 200°C
  • Refractory fibers such as Alumina or Silica for temperature between (700°C to 1700°C)

2. Cellular

Cellular insulation are closed or open cell materials that are usually in the form of flexibile or rigid boards. Low density or low heat capacity and good compressive strength are their advantages. E. g.

  • Polyurethane
  • Expanded polystyrene foam.

3. Granular

Granular insulation consist of small particles of inorganic materials bonded into desired shapes or used as powder. E. g.

  • Perlite powder
  • Diatomaceous Silica
  • Vermiculite

Sometimes reflective sheets are used for provide insulation. Two or more thin sheets of metal with low emittance are placed parallel to each other to reflect the radiation back to its source, e. g. Thermos bottle.

Apart from low thermal conductivity, Insulating material should have

  • Structural rigidity
  • Low density
  • Less degradation
  • Good chemical stability
  • Low cost

Polyurethane and polystyrene generally lose between 20 to 50% their insulation quality during first year of use.

For high temperature applications, refractory materials are used. They can be form of bricks which can withstand temperature upto 1700°C.

Insulation pays for itself from energy it saves. Insulating a surface properly requires a one time capital investment, but it’s effects are dramatic and long term. The payback period of insulation is usually under 2 years ☕😊

Hot coffee from thermos bottle

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