Heat Transfer

Heat transfer is the result of one basic principle: heat will always move from warmer to colder areas. When a hot surface is surrounded by an area that is colder, heat will be transferred. Heat transfer is continuous and will always happen until adjacent areas are the same temperature. The heat transfer takes place by three methods:

conduction
convection
radiation

Conduction

Conduction is the process in which heat is transported on a molecular level. It can occur along or through a material or from one material to another. For conduction to occur between materials the materials must be in direct contact. Conduction takes place in solids, liquids and gases and from one to another.

The rate at which conduction occurs varies considerably according to the substance and its state. In solids, metals are good conductors with gold, silver and copper being amongst the best. The rate of conduction decreases in materials such as concrete and masonry, even less for wood, and then to the lowest conductors such as thermal insulating materials.

Liquids and gases are generally bad conductors but this is sometimes obscured by more heat transfer taking place by convection.

Convection

Convection occurs in liquids and gases. For any solid to lose (or gain heat) by convection it must be in contact with the fluid. Convection cannot occur in a vacuum. Convection results from a change in density in parts of the fluid, the density change being brought about by a change in temperature. When convection takes place solely through density change it is known as ‘natural convection’. If the displacement fluid is accelerated by wind or artificial means, the process is called ‘forced convection’. With forced convection, the rate of heat transfer is increased - substantially so in many cases. Even a slight breeze is enough to accelerate convection.

Radiation

The process by which heat is emitted from a body and transmitted across space is called radiation. Heat radiation is a form of wave energy in space (similar to radio and light waves). Radiation does not require any intermediate medium such as air for its transfer; it can readily take place across a vacuum. All bodies emit radiant energy. The rate of emission is governed by:

the temperature difference between radiating and receiving surfaces;
the distance between the surfaces; and
the emissivities of the surfaces - dull matt surfaces are good emitters / receivers, bright reflective surfaces are poor.

Requirements of an Insulation

In order to perform as an effective insulation, a material must restrict heat flow by any, and preferably, all three methods of heat transfer. Most can dramatically reduce conduction and convection elements by the cellular structure of the material. The radiation component is reduced by absorption into the body of the insulation and is further reduced by the application of a bright foil or outer jacket to the product.

Surface Emissivity

The effects of surface emissivity are exaggerated in high temperature applications and particular attention should be paid to the selection of the type of surface of the insulation system. Low emissivity surfaces, such as bright polished aluminum, reduce heat loss by inhibiting the radiation of heat from the surface to the surrounding ambient space. However, by holding back the heat being transmitted through the insulation, a dam effect is created and the surface temperature rises. This temperature rise can be considerable and, if insulation is being used to achieve a specified temperature, the use of a low emissivity system could well necessitate an increased thickness of insulation. For example a hot surface at 500°F insulated with 2” of product with a mean thermal conductivity of .35 and ambient temperature of 70°F would give a surface temperature of approximately 150°F, 130°F and 110°F when the outer surface is of low (bright aluminum = .04), medium (stainless steel = .3) or high (ASJ = .9) emissivity respectively.

Energy

Mechanical systems, no matter how well insulated, will need a continual input of energy to maintain desired temperature levels. The energy needed will be much smaller in a well insulated system than in an uninsulated or poorly insulated one - but it will still be needed.

Heat will transfer between the surrounding areas and the pipes, tanks and vessels (as well as the fluids contained within). In a hot system, additional energy is needed to compensate for the loss or the temperature of the fluid will fall. A well insulated vessel will maintain the heat of the contents for a longer period of time but it will never keep the temperature stable on its own. Thermal insulation does not generate heat as that it is a common misconception that thermal insulation automatically warms the object on which it is installed. If no heat is supplied to that object it will begin to cool.

The need for a constant supply of energy translates to the constant supply of fuel. With the understanding that most company’s look to minimize their dependence on fuel, Hudson Bay Insulation is focused on helping companies achieve their goal. We have developed our Insulation Energy Appraisal Program to educate companies on a low cost solution with near term ROI.

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