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Thermal insulation is the reduction ofheat transfer (i.e., the transfer ofthermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence.[1] Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.
Heat flow is an inevitable consequence of contact between objects of differenttemperature. Thermal insulation provides a region of insulation in whichthermal conduction is reduced, creating athermal break orthermal barrier,[2] orthermal radiation is reflected rather than absorbed by the lower-temperature body.
The insulating capability of a material is measured as the inverse ofthermal conductivity (k). Low thermal conductivity is equivalent to high insulating capability (resistance value).[3] Inthermal engineering, other important properties ofinsulating materials are productdensity (ρ) andspecific heat capacity (c).
Thermal conductivityk is measured inwatts-per-meter perkelvin (W·m−1·K−1 or W/mK). This is becauseheat transfer, measured aspower, has been found to be (approximately) proportional to
From this, it follows that the power of heat loss is given by
Thermal conductivity depends on the material and for fluids, its temperature and pressure. For comparison purposes, conductivity under standard conditions (20 °C at 1 atm) is commonly used. For some materials, thermal conductivity may also depend upon the direction of heat transfer.
The act of insulation is accomplished by encasing an object in material with low thermal conductivity in high thickness. Decreasing the exposed surface area could also lower heat transfer, but this quantity is usually fixed by the geometry of the object to be insulated.
Multi-layer insulation is used where radiative loss dominates, or when the user is restricted in volume and weight of the insulation (e.g.emergency blanket,radiant barrier)
For insulated cylinders, acritical radius blanket must be reached. Before the critical radius is reached, any added insulation increases heat transfer.[4] The convective thermal resistance is inversely proportional to the surface area and therefore the radius of the cylinder, while thethermal resistance of a cylindrical shell (the insulation layer) depends on the ratio between outside and inside radius, not on the radius itself. If the outside radius of a cylinder is increased by applying insulation, a fixed amount of conductive resistance (equal to 2×π×k×L(Tin-Tout)/ln(Rout/Rin)) is added. However, at the same time, the convective resistance is reduced. This implies that adding insulation below a certaincritical radius actually increases the heat transfer. For insulated cylinders, the critical radius is given by the equation[5]
This equation shows that the critical radius depends only on theheat transfer coefficient and thethermal conductivity of the insulation. If the radius of the insulated cylinder is smaller than the critical radius for insulation, the addition of any amount of insulation will increase heat transfer.
Gases possess poorthermal conduction properties compared to liquids and solids and thus make good insulation material if they can be trapped. In order to further augment the effectiveness of a gas (such as air), it may be disrupted into small cells, which cannot effectively transfer heat bynatural convection. Convection involves a larger bulk flow of gas driven by buoyancy and temperature differences, and it does not work well in small cells where there is little density difference to drive it, and the high surface-to-volume ratios of the small cells retards gas flow in them by means of viscousdrag.
In order to accomplish small gas cell formation in man-made thermal insulation, glass and polymer materials can be used to trap air in a foam-like structure. This principle is used industrially in building and piping insulation such as (glass wool),cellulose,rock wool,polystyrene foam (styrofoam),urethane foam,vermiculite,perlite, andcork. Trapping air is also the principle in all highly insulating clothing materials such as wool, down feathers and fleece.
The air-trapping property is also the insulation principle employed byhomeothermic animals to stay warm, for exampledown feathers, and insulating hair such as natural sheep'swool. In both cases the primary insulating material is air, and the polymer used for trapping the air is naturalkeratin protein.
Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion ofglobal energy consumption. Building insulations also commonly use the principle of small trapped air-cells as explained above, e.g. fiberglass (specificallyglass wool),cellulose,rock wool,polystyrene foam,urethane foam,vermiculite,perlite,cork, etc. For a period of time,asbestos was also used, however, it caused health problems.
Window insulation film can be applied inweatherization applications to reduce incoming thermal radiation in summer and loss in winter.
When well insulated, abuilding is:
In industry, energy has to be expended to raise, lower, or maintain the temperature of objects or process fluids. If these are not insulated, this increases the energy requirements of a process, and therefore the cost and environmental impact.
Space heating and cooling systems distribute heat throughout buildings by means of pipes or ductwork. Insulating these pipes using pipe insulation reduces energy into unoccupied rooms and preventscondensation from occurring on cold and chilled pipework.
Pipe insulation is also used on water supply pipework to help delay pipe freezing for an acceptable length of time.
Mechanical insulation is commonly installed in industrial and commercial facilities.
Thermal insulation has been found to improve thethermal emittance ofpassive radiative cooling surfaces by increasing the surface's ability to lower temperatures below ambient under direct solar intensity.[6] Different materials may be used for thermal insulation, includingpolyethyleneaerogels that reduce solar absorption and parasitic heat gain which may improve the emitter's performance by over 20%.[6] Other aerogels also exhibited strong thermal insulation performance for radiative cooling surfaces, including asilica-aluminananofibrous aerogel.[7]
Arefrigerator consists of a heat pump and a thermally insulated compartment.[8]
Launch and re-entry place severe mechanical stresses on spacecraft, so the strength of an insulator is critically important; the failure of insulating tiles on theSpace ShuttleColumbia caused the shuttle airframe to overheat and break apart during reentry, killing the astronauts on board. Re-entry through the atmosphere generates very high temperatures due to compression of the air at high speeds. Insulators must meet demanding physical properties beyond their thermal transfer retardant properties. Examples of insulation used on spacecraft include reinforcedcarbon-carbon composite nose cone andsilica fiber tiles of theSpace Shuttle. See alsoInsulative paint.
Internal combustion engines produce a lot of heat during their combustion cycle. This can have a negative effect when it reaches various heat-sensitive components such as sensors, batteries, and starter motors. As a result, thermal insulation is necessary to prevent the heat from the exhaust from reaching these components.
High performance cars often use thermal insulation as a means to increase engine performance.
Aircraft engines such as turbojets use thermal barriers on components such as the combuster sections/afterburner for heat protection.[9][10][11]
Thermal insulation is integral to greenhouse design, enabling controlled environments for plant growth and energy efficiency. Common insulation techniques include the use of double-layer films and multi-wall polycarbonate panels, which trap air between layers to reduce heat transfer while maintaining sufficient light transmission for photosynthesis. These measures improve energy efficiency and support year-round crop production, significantly reducing heating costs.
Insulation performance is influenced by many factors, the most prominent of which include:
It is important to note that the factors influencing performance may vary over time as material ages or environmental conditions change.
Industry standards are often rules of thumb, developed over many years, that offset many conflicting goals: what people will pay for, manufacturing cost, local climate, traditional building practices, and varying standards of comfort. Both heat transfer and layer analysis may be performed in large industrial applications, but in household situations (appliances and building insulation), airtightness is the key in reducing heat transfer due to air leakage (forced or natural convection). Once airtightness is achieved, it has often been sufficient to choose the thickness of the insulating layer based on rules of thumb. Diminishing returns are achieved with each successive doubling of the insulating layer.It can be shown that for some systems, there is a minimum insulation thickness required for an improvement to be realized.[12]
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