A furnace is a device used for heating. The name derives from Latin fornax, oven. In the context of industrial applications it can be defined as an equipment to melt metals for casting or heat materials for change of shape ( rolling, forging etc) or change of properties (heat treatment).

Depending on the type of furnace different forms of fuel or energy used are electricity, coal, diesel oil, furnace oil etc.

Design & Operation

Based on the method of generating heat, furnaces are broadly classified into two types namely combustion type (using fuels) and electric type. In case of combustion type furnace, depending upon the kind of combustion, it can be broadly classified as oil fired, coal fired or gas fired.

  1. Based on the mode of charging of material furnaces can be classified as (i) Intermittent or Batch type furnace or Periodical furnace and (ii) Continuous furnace.
  2. Based on mode of waste heat recovery as recuperative and regenerative furnaces.
  3. Another type of furnace classification is made based on mode of heat transfer, mode of charging and mode of heat recovery. Schematic of general classification of furnaces is as shown in the figure below.

Figure 1: Classification and type of the Furnace

Characteristics of an Efficient Furnace

Furnace should be designed so that in a given time, as much of material as possible can be heated to a uniform temperature as possible with the least possible fuel and labor. To achieve this end, the following parameters can be considered.

  • Determination of the quantity of heat to be imparted to the material or charge.
  • Liberation of sufficient heat within the furnace to heat the stock and overcome all heat losses.
  • Transfer of available part of that heat from the furnace gases to the surface of the heating stock.
  • Equalization of the temperature within the stock.
  • Reduction of heat losses from the furnace to the minimum possible extent.

Performance Evaluation of Furnaces

The fuel required for combustion is cleaned, preheated and burnt in the combustion zone of the furnace. Even if the hot combustion gases impart most of the heat to the stock, it is seen in practice that much heat is lost.

Some of the heat passes into furnace walls and hearth. Another portion of heat is lost to the surroundings by radiation and convection from the outer surface of the walls. Heat is also lost through cracks and openings with gases escaping through doors. Every time the door is opened, considerable amount of heat is lost. In some cases, the heat is lost from stock, which protrudes out. Finally much of the heat is lost through the free gases either as sensible heat or as in complete combustion. Economy in fuel can be achieved if the total heat that can be passed on to the stock is as large as possible.

Furnace efficiency

Thermal efficiency of furnace is defined as the percentage of heat input that is effectively utilized to heat the stock. There are two methods of assessing furnace efficiency.

Direct method

The efficiency of furnace can be judged by measuring the amount of fuel needed per unit weight of material.

The quantity of heat to be imparted (Q) to the stock can be found from

Indirect method

In indirect method, furnace efficiency is calculated after subtracting sensible heat loss in flue gas, loss due to moisture in flue gas, heat loss due to openings in furnace, heat loss through furnace skin and other unaccounted losses:

Figure 2: Typical energy losses in Furnaces

Payback of energy saving options

There are many economically viable option to improve furnace efficiency. Past experiences based on energy audits conducted in industries in Nepal has shown that many energy saving options have a payback period for investments of less than 2 years.

Table 1: Payback period of energy saving options (ESPS, 2005)


  • ESPS, 2005. Environmental Sector Support Program conducted by DANIDA, Component 2 – Promotion of Cleaner Production in Industries.
  • Bureau of Energy Efficiency India (BEE), 2005: Guide book for National Certification Examination for Energy Managers and Energy Auditor - BOOK 2- Energy Efficiency in Thermal Utilities. Chaper 4 – Furnace.

Improving Furnace Efficiency


  • Establish a management information system on loading, efficiency, and specific fuel consumption.
  • Prevent infiltration of air, using doors or air curtains.
  • Monitor O2/CO2/CO ratios and control excess air level.
  • Improve burner design, combustion control, and instrumentation.
  • Ensure that the furnace combustion chamber is under slight positive pressure.
  • Use ceramic fiber linings in the case of batch operations.
  • Match the load to the furnace capacity.
  • Retrofit with heat recovery devices.
  • Investigate cycle times and avoid extended hours of runtime and excess heating.
  • Provide temperature controllers.
  • Ensure that the flame does not touch the stock.

Figure 3: Improvement of heat distribution in furnace


  • Repair damaged insulation.
  • Use an infrared gun to check for hot wall areas during hot weather.
  • Ensure that all insulated surfaces are clad with aluminum lining.
  • Insulate all flanges, valves, and couplings.

Waste Heat Recovery

  • Recover maximum heat from flue gases.
  • Ensure upkeep of heat transfer surfaces by regular cleaning.
  • Using waste heat for other process

Figure 4: Waste heat recovery for furnaces