Which of the following will happen if the thickness of the refractory wall of the furnace is increased?
Right Answer is:
The temperature on the outer surface of the furnace wall will drop
In a furnace we want to have high heat transfer inside the furnace; however, we do not want any heat loss through the furnace wall. Thus to prevent the heat transfer from the furnace to the atmosphere a bad heat conductor or a very good heat insulator is required. In the case of the furnace, the wall is prepared by multiple layers of refractory materials to minimize heat losses. Therefore, wall insulation is required to minimize the heat loss from the system to the environment or heat gain from the environment to the system (like cryogenic systems).
The furnace chamber provides space to conduct the activity in the furnace. This chamber design has many issues like shape, size, the ratio of various dimensions, refractory lining thickness, nature, furnace casing material with proper ports, etc. These factors are briefly discussed below:
The furnace chamber shape could be closed rectangular (e.g. reheating furnace), narrow rectangular (e.g. coke oven), closed trough (e.g. open-hearth furnace), closed hemispherical shape (e.g. electric arc furnace), open cylindrical shape (e.g. cupola), one end closed cylindrical shape (e.g. induction melting furnace), etc. The shape depends on the processing activity and the mode of heat
The dimensions of rectangular shape furnace (length, width, and height), cylindrical-shaped (height and diameter) furnace or hemispherical shaped (section diameter and center height) furnace depend on the scale of operation and process constraints. In the case of reheating furnace, the length of the furnace will depend on the object size to be heated and burner design. The width of the furnace will be decided on the number of pieces to be heated (load) at a time, while the height will be decided by the aerodynamics of the furnace for good gas flow and heat transfer. The coke oven chamber dimensions are decided by operating factors. The coke oven length depends on the oven capacity and availability of pusher arm length, the height is decided by the selection of heating chamber flue design and the minimum desired width is decided by the ability of a man to enter for construction and repair. In each case, there are compelling parameters to fix the dimensions depending on the furnace capacity.
Refractory thickness and nature
The nature of refractory is decided by the maximum temperature requirements and other working conditions like the chemical nature of materials in contact, working load, erosion and corrosion parameters, etc. Once the nature of refractory is decided, then the thickness of the refractory layer would depend on the inner working temperature, the thermal conductivity of the refractory, and the maximum temperature sustainable by the outer metallic casing.
Thermal conductivity and heat capacity:
in practical applications, refractory materials processing high thermal capacity as well as low thermal conductivity are required, depending upon (of course) the functional requirements. In most situations, a refractory that serves as a furnace wall should have a low thermal conductivity in order to retain as much heat as possible. However, a refractory used in the construction of the walls of muffles or retorts or coke ovens should have a high thermal conductivity in order to transmit as much heat as possible to the interior. The charge remains separated from the flame in these specific examples of installations.
The heat transferred or conducted by a refractory at a rate Q is given by the relationship:
Q = K.(T1 − T2) A ⁄ ΔX
T1 = Temperature on the hotter surface
T2 = Temperature on the colder surface
A = Area of refractory
K = Thermal conductivity
ΔX = Thickness
From this relationship, it follows that if the refractory wall is made thicker, the heat loss will be decreased. Alternatively, if the refractory brick is backed by an insulating layer, the same result ensues. These provisions tend to cause the refractory material to be at a higher average temperature. The increased temperature tends to soften the refractory and enhance the chemical attack on it. In this scenario, a choice must be made between increased heat savings and a reduction in the life of the refractory material. This particular problem has drawn considerable attention to the design of open-hearth furnaces for steel making. The question of insulating the roofs of these furnaces has been the subject of a great deal of deliberation.
The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks, the entrapped air in the pores acts as a nonconducting material.
The heat capacity of a furnace unit at a given temperature depends on bulk density, specific heat, and thermal conductivity of the refractory. The heat capacity and thermal conductivity of a refractory increase with increasing bulk density, and decrease with increasing porosity. A refractory with a high bulk density and a high thermal conductivity is desirable from the point of view of certain applications. With refractories used for heat storage and transmission, such as checker bricks, no conflict between properties arises, since high density is a desirable criterion both for stability and for increased heat storage and conductivity.
Furnace casing material
The furnace is generally encased in a steel structure for stability, support, and handling. The steel selected is generally common structural steel unless some furnaces demand heat-resisting steel for special applications. The steel casing thickness depends on the furnace size and loads subjected. The steel casing is made sufficiently strong to sustain the rough working conditions of the furnace. This casing has ports for the burner, gas exit, material charging and discharging door, etc.
The burners are necessary to combust the fuel for generating thermal energy for its efficient utilization in the furnace. The suitable burners are selected based on the type of fuel (coal, oil or gas) best suited for the furnace.