In furnaces and kilns, heat losses from furnace walls, affect the fuel economy substantially. The
extent of wall losses depends on:
i) emissivity of walls;
ii) conductivity of refractories;
iii) wall thickness;
iv) whether furnace or kiln is operated continuously or intermittently.
Different materials have different radiation power (emissivity). The emissivity of walls coated with aluminium paint is lower than that of bricks. Fig. 5.10(A) shows the coefficient of heat dissipation for the following conditions:
a) rough vertical plane surface.
b) Vertical aluminium painted walls The variations of thermal conductivity for typical refractory materials (silica brick, fireclay brick and insulation brick) with temperature is depicted in Figure(B). Thus at a mean temperature of 600 °C, conductivity of the insulation brick is only 20 per cent of that for fireclay brick.
Heat losses can be reduced by increasing the wall thickness, or through the application of insulating bricks. Outside wall temperature and heat losses for a composite wall of a certain thickness of firebrick and insulation brick are much lower due to lesser conductivity of insulating brick as compared to a refractory brick.
In the case of batch furnace operation, operating periods (‘on’) alternate with idle periods (‘off’). During the off period, the heat stored in the refractories in the on-period is gradually dissipated, mainly through radiation and convection from the cold face. In addition, some heat is obstructed by air flowing through the furnace. Dissipation of stored heat is a loss, because the lost heat is at least in part again imparted to the refractories during the next ‘on’ period, thus expending fuel to generate the heat. If a furnace is operated 24 hr. every third day, practically all of the heat stored in the refractories is lost.
But if the furnace is operated 8 hrs. per day, not all the heat stored in the refractories is dissipated. For a furnace with firebrick wall (350 mm) it is estimated that 55 per cent of the heat stored in the refractories is dissipated from the cold surface during 166 hours idle period. Furnace walls build of insulating refractories and encased in a shell reduce flow of heat to the surroundings. Inserting a fibre block between the insulating refractory and the steel casing can further reduce the loss. The general question one asks is how much heat loss can be reduced by application of insulation. The answer is that it depends on the thickness of firebricks and of the insulation and on continuity of furnace operation.
To sum up, the heat losses from the walls depend on:
· Inside temperature.
· Outside air temperature.
· Outside air velocity.
· Configuration of walls.
· Emissivity of walls.
· Thickness of walls.
· Conductivity of walls.
The following conclusions can be drawn:
· Thickness of walls and Conductivity of walls can be easily controlled by the furnace fabricator.
· As the wall thickness increases, the heat losses reduce.
· As thickness of insulation is increased, heat losses reduce.
· The effect of insulation in reducing heat losses is more pronounced than the increase of wall thickness. Roughly 1 cm of insulation brick is equivalent to 5 to 8 cm of refractory (firebrick).
· In intermittent furnaces, thin walls of insulating refractories are preferable to thick walls of a normal refractory for intermittent operation since less heat is stored in them.
· One approach to achieve less heat storage capacity would be to utilise insulating material itself to form the inner refractory lining. Robust refractories with fairly good strength and spalling resistance can be used for temperatures in the range of 1300 °C. They are termed as hot face insulation.
· Hot face insulating bricks are lighter than normal refractories, weighing only one-third to one-half as much. Therefore, heat storage in the hot face insulation is very much reduced.