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HEATING
PRINCIPLES
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The
transfer of heat from a warm object to a cooler one takes place
by one of three methods or a combination thereof. These methods
are: conduction, convection or radiation.
CONDUCTION
Conduction of heat occurs when faster moving molecules pass on some
of their energy to adjacent molecules which are slower-moving, i.e.
at a lower temperature. This may occur within a solid or between
a solid and an adjacent fluid such as air. In any heated building
or enclosure, heat is conducted or transmitted from the warm inside
air to the inside surfaces, then through the wall or roof, to the
cooler outside surface and on to the outside air.
CONVECTION
Convection transfer of heat involves the mixing of warm and cool
particles of fluid. The mixing may come as a result of density differences
due to temperature differences which is natural convection, or,
if the mixing is produced by mechanical means, forced convection.
In a heated building, convection losses occur when cold outside
air enters a building, mixes with the warmer inside air, and then
exits through an exhauster or through doors, cracks, etc.
 RADIATION
Heat transfer by radiation differs from the transfer of heat by
conduction or convection in that it does not need matter to accomplish
the transfer. Radiated heat is usually termed infra-red. This is
just one of the several forms of radiation (see the electromagnetic
spectrum, Figure #1). Infra-red is transmitted at the speed of light,
186,000 miles per second, in a straight line with minimal loss to
the air. It can be aimed, reflected or focused by materials that
have a highly reflective surface. Bright aluminum for example. When
infra-red strikes an absorptive object such as concrete, wood, water,
paint, skin or clothing it is converted into heat at the surface.
Surrounding air is then warmed by conduction and convection. The
best example of this transfer of heat is from the sun to the earth
without loss of heat to outer space.
Radiation or infra-red energy is emitted by all matter that is above
absolute zero (-460°F). The net transfer of heat is from one
object to a cooler object.
Warm objects, including people inside a heated building lose or
radiate heat to the cooler inside surfaces of the walls. The walls
conduct heat to the outside surface and then lose heat by radiation,
conduction and convection to the outside.
HOW INFRA-RED WORKS
The amount of radiation produced by a perfect radiator is expressed
by the Stephan-Boltzman Law, where:
For ordinary objects, non-perfect radiators, Q is reduced by multiplication
of the object's emissive power (always less than one). Thus, at
normal temperatures the amount of infra-red radiation produced by
an object is relatively low, but as the temperature is increased,
radiation increases significantly.
For example: An object at 80°F (540° absolute temperature)
with an emissive power of 0.85 will produce 124 BTUH/ft2.
When its absolute temperature is doubled to 1080° its output
is increased sixteen fold to 1,984 BTUH/ft2. If its absolute
temperature is quadrupled to 2160° its output increases two
hundred and fifty six fold to 31,744 BTUH/ft2. |
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