A method of isolating or separating one conducting body from another by means of nonconductors to prevent or reduce the transfer of heat; also called heat insulation.
Heat, a form of energy, normally is associated with visual action such as fire, steam, or boiling water. These more obvious conceptions of heat actually represent a small com¬ponent. All matter contains relative amounts of heat except at absolute zero ( — 459.6°F. or — 273.ı°C), at which tem¬perature molecular activity ceases. The heat capacity of objects is a relative measure based on the weight, specific heat, and temperature above a specified temperature level, such as abso¬lute zero, freezing, or boiling.
Forms and Compositions of Insulation. Heat insula¬tions are available in a number of shapes or structures, many of which are designed to serve a specific use. Compositions are animal, vegetable, and mineral, and combinations, of these types. Because of the nature of the service for which insulating materials are designed, most are relatively light in weight and hence somewhat fragile. Except for specific forms such as bricks, blocks, and boards that are known to possess a definite physical strength, most insulations should not be expected to provide a great deal of structural value.
Loose Fill. Loose-fill insulation usually is poured or packed in bulk between confining structural members. Loose-fill compositions used for specific temperature requirements are asbestos powder, cork granules, diatomaceous earth powder, powdered gypsum, mineral-wool (rock, slag, or glass) pellets, shredded paper, magnesia powder, silica-gel powder, shred¬ded wood fibers, vermiculite (expanded mica) granules, pumice, pearlite, and lightweight slags. Certain loose-fill insulations are used as aggregates in lightweight concrete.
Flexible. Flexible forms of insulation (blankets, sheets, batts and felts) facilitate installation by wrapping, nailing, or the use of adhesives. They are well adapted for use on curved surfaces. Flexible products are available in many compositions, some of which incorporate surface-reinforcing mediums of various types; materials include asbestos felt, cane, cattle hair, cotton, hemp, jute, kapok, mineral wool, paper, seaweed, rubber foam, and wood fibers.
Rigid. Rigid insulation forms sometimes are classified as blocks, boards, bricks, sheets, or slabs. Many rigid insulation products consist of combinations of different raw materials, with or without internal binders, air spaces, or surface treatments. Rigid insulations are nailed or wired in place or installed with adhesives. Their compositions include asbestos, calcium-silicate, cellular glass, cellulose acetate, cork, diato-maceous earth, fire clay, gypsum, magnesia, cane, mineral wool, paper, rubber foam, straw, vermiculite, and wood. In the building field, insulation boards are used as sheathing, as a plaster-base material, and for interior finish. Such uses may depend on their structural strength, appearance, or acoustical performance rather than on their thermal value.
For Pipe. Pipe insulations are preformed into rigid half cylinders to fit specified pipe and tubing diameters, or, for large pipe sizes, are furnished in segments and in flexible form. Some prefabricated pipe lines employ loose-fill insula¬tion packed in between the pipe surface and the exterior protective jacket. Rigid pipe insulations are banded or wired in place; flexible forms are wrapped around the pipe and secured with twine or wire. Rigid pipe-insulation composi¬tions include asbestos, calcium-silicate, cellular glass, cork, diatomaceous earth, magnesia, mineral wool, and vermiculite. Flexible compositions are of hair felt and mineral wool. Insulated prefabricated piping systems are packed with asbestos fiber or mineral wool.
Cements. Insulating cements come in dry powder or pellet form and require only the addition of water to mix to troweling consistency. They are used on equipment having irregular contours, such as valves, pipe fittings, and turbines, and may be applied as a finish coat over other insulation forms. Compositions include asbestos, diatomaceous earth, magnesia, mineral wool, vermiculite and pearlite.
Reflective. Reflective insulations effectively reduce the transfer of radiant heat when their low-emissivity surfaces adjoin an air space. Aluminum foil is a widely used reflective type. It is furnished in single- and multi-layer forms with or without a paper backing and is used in combination with insulation boards, flexible insulations, and pipe insulation. Steel sheets about 0.006 inches (0.015 cm) thick also are used as reflective insulation, principally in refrigeration.
Factors in Selection. The selection of different insula¬tion compositions for specific uses is determined broadly by the ability of the material to withstand continued exposure to the design operating temperature without disintegration. Animal, vegetable, and mineral compositions may be used where temperatures do not exceed 175°F. to 200°F. (79°C. to 93°C.) as in refrigerated installations and in the building field. Most mineral insulation compositions are satisfactory for heated applications up to 6oo°F. (315°C), and some are applied in different forms throughout the temperature range from below zero to 18oo°F. (— 18°C. to o.82°C). Above 18oo°F., diatomaceous earth products and lightweight insu¬lating bricks are used.
The choice of one form of insulation over another is gov¬erned largely by their respective physical properties. For example, where heavy floor loads are encountered, as in ice-storage plants and open-hearth furnaces, the compressive strength of the insulation may be of more practical signifi¬cance than the thermal conductivity. For aircraft and in the transportation field generally, lightweight insulation forms having high thermal resistance are ordinarily preferred.
The ease of installing different materials has a decided bearing on their selection since in many cases the cost of in¬stallation exceeds the value of the insulation itself. The ma¬jor markets for insulation are appliances, buildings, power and process equipment, and transportation.