Issues in Furnace design
1) Source of energy in processing of raw materials is fossil fuel in most cases. Even if electric energy is used, it is also derived from fossil fuels. Thus energy efficient design of thermal enclosure is important; particularly heat losses should be as minimal as possible.
2) In chemical processing, fluid flow is important. Liquid and gases are flowing at high temperature so erosion and corrosion of the refractory is important. In addition, fluid flow also influences the rates of heat and mass transfer. The dead zones (dead zones are those areas in which no movement of solid and liquid takes place) should be avoided while designing the furnace chamber
3) Atmosphere in the furnace is also important to avoid oxidation of the material being heated
4) Control of furnace temperature is also an important issue. Overheating and under-heating lead to inefficient utilization of fuel and also overheating or under-heating of material. Furnace should be equipped with the temperature measurement and control devices
5) Furnaces are both batch and continuous type. In the continuous type for example in heating of ferrous material for hot working, the furnace chamber consists of preheating, heating and soaking zones. The material enters through the preheating zone and exits the soaking zone for rolling. But the flow of products of combustion is in the reverse direction. Furnace design is recuperative type in that material exits at the desired temperature from the soaking zone and the products of combustion discharge the preheating zone at the lowest possible temperature. Different types of continuous furnaces are in use, like walking beam type, pusher type, roller hearth type, screw conveyor type etc.
6) In the batch furnaces, the load is heated for the fixed time and then discharged from the furnace. There are different types of batch furnaces like box type, integral quench type, pit type and car bottom type
7) In many cases the furnace is equipped with either external heat recovery system or internal heat recovery system. In the external heat recovery system a heat exchanger like recuperator is installed outside the furnace. Here heat exchanger must be integrated with the furnace operation. In the internal heat recovery the products of combustion are recirculated in the furnace itself so that flame temperature is somewhat lowered. The objective is to reduce the NOx formation.
8) The products of combustion are moving at high speeds in the furnace. The flow of products of combustion is important to obtain rapid heat transfer and minimum thermal gradient.
Source: George E.Totten and M.A.H.Howes: Steel heat treatment handbook
P.Mullinger and B. Jenkins: Industrial and process furnaces
W. Trinks: Industrial furnaces
W. Trinks and M.M. Mawhinney: Industrial furnaces
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