Surface Loading for Electric Resistance Heating Element Materials
Surface Loading for Heating Elements
The surface loading on a heating element is one of the most important
factors in the design review for a heating element. The element
temperature is dependent on how much energy has to be transmitted
from the heating element to the surrounding area, the temperature
of the surrounding area, and finally the ease of heat transfer from
the element to the surrounding area.
The amount of energy to be transmitted and the desired temperature
of the area are generally known. The hard part is calculating the
heat transfer from the element to the surrounding area. There are
three modes of heat transfer, Conduction, Convection, and Radiation.
Conduction of energy from an element used in industrial processes
is generally relatively low (but can be important in the case of
imbedded heaters) and is generally not accounted for. Convection
plays a role in ovens operating at relatively low temperatures,
up to about (1400°F / 760°C), but the effect is reduced
as the temperature increases (with increasing temperature, the air
thins and can carry less energy).
That leaves radiation as the primary form. Unfortunately in the
formula for radiant heat transfer, there are two variables that
are moving targets, emissivity and the shape factor, These variables
are difficult to estimate to a level of accuracy needed to properly
calculate element temperatures. The emissivity of a heating element
varies with the temperature, but also with the type and amount of
oxide on the surface. The shape factor can become very complex based
on the exact configuration of the heating element (radiating onto
itself or ceramic supports, or even the closeness to the furnace
For these reasons guidelines are generally used that give acceptable
life to the heating elements. Even by using these guidelines, one
must be careful to follow the assumptions made.
In addition to the tables that show element watt loading for different
furnace temperatures, there are also tables that show recommended
wall loading. These tables are useful in generating the basic furnace
design and verifying that the desire amount of energy can be applied.
They cannot be used in evaluating a specific element design.
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