Heat Treatment Furnace Design

Heat Treatment Furnace Design - Heat treatment furnace is a type of furnace that is used to provide a heat treatment at a work piece such as normalizing, tempering, annealing, hardening, hot forming, stress relieving, etc. The heat required for the heat treatment process varies depending on the type of material and heat treatment processes that need. In general, heat treatment furnace has a temperature between 400 to 1200‘C.

Generally, the design of the heat treatment furnace includes the calculation of power requirement, wall furnace design, heater design, determined of thermocouple and thermocontrol, design of boogie system and door system. But in this post is only described about estimated energy and power requirement.

If you want to design a heat treatment furnace, you must be know about user requirement are:

• Maximum Temperature, the temperature is very influential in the design of furnace because it will be based on calculation of power requirement, heater design, wall furnace design, and determined of thermocouple and thermo control.
• Work piece material and heat treatment process, if the maximum temperature unknown, you can see the charts or the tables of material temperature. An example of a table for the material temperature in VCN-150, VCL-140 and EMS-45 (standard Bohler) is displayed below.

• Furnace capacity includes the volume of the furnace chamber and the weight of material to be processed. As well as the temperature, furnace capacity will also be based on calculation of power requirement and wall furnace design.
• Heating time target and the available power. Time and electric power have a relationship of mutual influence. The goal faster heating time, it takes more power, and otherwise.

Estimated energy requirement for the heat treatment furnace

If the data is known, you can calculate the energy needed to heat the furnace. At the time of furnace work, the heater element will heat the air in the furnace chamber. The hot air will be absorbed by the furnace walls and work piece in the furnace chamber. Therefore, the total energy required to heat the furnace is:

Note:
Q_R   = Energy Requirement (Joule)

Q_m   = Energy for Work piece Heating (Joule)

Q_a   = Energy for Air Heating (Joule)

Q_w   = Energy for Heat Storage (Joule)

The energy to heat the workpiece is calculated on the basis of the heat capacity that can be received by the workpiece on a specific temperature change. Each material has a different specific heat capacity. Here is a table of the specific heat capacity of certain specified materials.

If the initial temperature of workpiece is T1 and heated to a temperature T2, the energy required for heating the workpiece can be calculated by the following equation:

Note:
Q_m   = Energy for work piece heating (Joule)

m     = Mass of work piece (kg)

c_m    = Specific heat capacity of work piece (Joule/kg ^oC)

T₁    = Initial temperature (^oC)

T₂    = Final temperature (^oC)

Energy for air heating is the energy used to heat the air in the furnace chamber. If the initial air temperature is T1 and heated to a temperature T2, the energy required for heating air can be calculated by the following equation:

Note:
Q_a   = Energy for air heating (Joule)

V_a    = Air volume (m³)

C_a   = Air heat capacity is 1200 Joule/m^{3 o}C

T₁    = Initial temperature (^oC)

T₂    = Final temperature (^oC)

Heat storage is heated air that is absorbed by the furnace wall. The furnace wall function is resist the heat in the chamber. Therefore, the furnace wall is made of a material having a thermal conductivity which is very low such as firebrick, insulating bricks, ceramic fiber, calcium silicate and rockwool. Although able to withstanding the heat, the furnace wall can still absorb heat depending on the material heat capacity used for the furnace wall. Then, the value of the heat capacity of the furnace wall for some materials indicated.

Energy required to replace the heat absorbed by the furnace wall is by the following equation:

Note:
Q_w   = Energy for heat storage (Joule)

V_w   = Volume of wall furnace (m³)

C_w   = Heat capacity of wall material (Joule/m^{3 o}C)

T₁    = Initial temperature (^oC)

T₂    = Final temperature (^oC)

If ‘L’ is the furnace length, ‘W’ is the furnace width and ‘H’ is the furnace height, so volume of the furnace wall is:

 

Estimated power requirement for the heat treatment furnace

The power required to heating process in the furnace chamber for on the time is by following equation:

Note:
P_R     = Power requirement (Watt)

Q_R     = Energy requirement (joule)

t         = Time heating (sec)

Determining the electric power to be greater than the power requirement or divided by the efficiency of the furnace so that the heat generated on time. Furnace efficiency is regarding the heat loss that occurs in the furnace chamber.

That is a description about Heat Treatment Furnace Design on the topic of energy and power requirement estimated. Other topics related to Heat Treatment Furnace Design  will be described in other post like the wall furnace design, heater design, determined of thermocouple and thermocontrol, design of boogie system and door system.

Furnace calculation formula and table are reference to the handbook of Heat Transfer by Holman and Technical Bulletin by Applied research services.

If you find misconceptions in the furnace calculation, please provide the correction in the comment box.