4.1. Open-hearth furnace

Figure 3 shows the simplest form of furnace. This is little different to heating a component on a brazing hearth except that the furnace arch reflects heat back onto the component and provides rather more uniform heating. In this furnace a gas or oil burner plays directly onto the charge. Heat is also reflected onto the work from the furnace lining as previously mentioned. The advantages and limitations of this type of furnace are as follows:

Advantages

• Low initial cost.

• Simplicity in use and maintenance.

• Fuel economy since it heats up quickly.

Limitations

• Uneven heating.

• Poor temperature control.

Figure 3 Gas-heated open hearth furnace

4.2. Semi-muffle furnace

Figure 4 shows a semi-muffle furnace. This is an improvement upon the open-hearth furnace previously described. The flame from the burner does not play directly onto the charge, but passes under the hearth to provide ‘bottom heat’. This results in more uniform heating.

Figure 4 Gas-heated semi-muffle furnace

The advantages and limitations of this type of furnace are as follows:

Advantages

• Comparatively low initial cost.

• Simplicity in use and maintenance.

• Fuel economy.

• Fairly rapid heating.

• Heating is more uniform than for the open-hearth type of furnace.

• Limited atmosphere control can be achieved by varying the gas–air mixture through a system of dampers. The flue outlets are situated just inside the furnace door so that any atmospheric oxygen that may leak past the door is swept up the flue before it can add to the scaling of the work.

• Reasonable temperature control.

• Reasonable atmosphere stability due to the greater mass of the furnace lining compared with the open-hearth type furnace.

Limitations

• Heating is still comparatively uneven compared with more sophisticated furnace types.

• Atmosphere control is somewhat limited. Although oxidation can be reduced by careful control of the gas–air mixture, some scaling will still take place and there will still be flue gas contamination of the work.

4.3. Muffle furnace (gas heated)

Figure 5 shows a full muffle furnace. You can see from the figure that the work is heated in a separate compartment called a muffle. The work is completely isolated from the flame and the products of combustion. The advantages and limitations for this type of furnace are as follows:

Advantages

• Uniform heating.

• Reasonable temperature control.

• Good temperature stability due to the mass of refractory material forming the muffle and the furnace lining compared with the mass of the work.

• Full atmosphere control is possible. Any sort of atmosphere can be maintained within the muffle since no combustion air is required in the muffle chamber.

Limitations

• Higher initial cost.

• Maintenance more complex and costly.

• Greater heat losses and slow initial heating result in lower fuel economy unless the furnace can be operated continuously.

Figure 5 Gas-heated muffle furnace