Electric ovens
 

Electric ovens use electric infrared elements, or sources. Electric infrared sources are heated by current flowing through a resistance heating element. The element and the material surrounding the element are heated to an incandescent temperature.
All well designed electric ovens exhibit the following characteristics:
• vertical and horizontal zoning: To provide an effective, flexible, and efficient application of electric infrared heating to a specific process.
• Precise layout and distribution of elements: To incorporate shape factors, overcome an edge effect, and provide greater flexibility.
• Insulated reflective panels to reradiate heat: To provide reradiation, even when panels may be dirty.
• Insulated element wiring to provide additional life: To extend significantly the life of the infrared elements.
• Non-contact temperature sensors for control: To provide the optimum in temperature control.
• Rigid, non-vibrating structure: To lengthen the life of the elements.
• Custom control: To meet the specific needs of the process and the operators.

Equipment for paint drying
The provision of an oven in a production shop ensures speeding up of the paint drying operation. Depending upon the rate of production, the oven can be either batch type of tunnel type.

Batch type electric oven
This type of oven is largely used where there is batch production and where the volume does not justify a conveyorised installation.
Examples: General engineering workshops and fabrication shops, where different types of equipment are manufactured and production volume is low.

Tunnel type electric oven
This type of oven is preferred where production volume is fairly large and continuous.
Examples: Continuous production plants manufacturing sheet metals components for
Tractors two - wheelers, power tillers, typewriters, fans, refrigerators, air conditioners, etc.

Stoving methods
Stoving involves heat transfer from the heat source to the coating or paint film and two methods in general use for this purpose are convection and radiation. The method of stoving is recommended after taking into consideration the size, weight, shape and material of the component to be dried.

Convection heating
Convection heating is carried out by heating the air surrounding the article to be stoved. This is generally done by having a heating chamber where air is heated and the heated air is circulated inside the oven chambers by means of fans. Normal sources used to heat the inlet air are electricity, gas, steam or oil.

Convection ovens are suitable for both batch and continuous operation, depending on the work load.

A limitation of the convection type oven is that it will usually require to be started from
twenty minutes to half an hour before stoving operations can begin. This is because the air inside the oven will have to be heated to the required temperature, and this takes time.

Normally, the temperature in a convection oven is thermostatically controlled, so that the heat and therefore the fuel consumption is regulated depending upon the amount of work entering the oven.

As the object is heated by circulation of air, any shape and size of object can be dried by convection heating.

This method is generally used for drying of large castings, machined components and objects having a non uniform weight distribution.
Radiation Heating
This is attained by heating the source so that the source starts emitting infra-red radiation and this radiation heats up the paint film. The infra-red emission can be directed towards the object to be heated by means of suitably shaped reflectors if necessary. The absorption of the radiation takes place at the surface of the charge, and infra-red heating is thus essentially a surface heating process.

Since infra-red radiation is emitted in straight lines from the source or reflector, plain surfaces are most readily treated. In infra-red paint stoving, the temperature attained by the paint film depends upon the intensity of radiation on the painted surfaces, the time of exposure, and the mass of the article. The colour of the paint also plays a part in the speed with which the surface is heated. Black paints tend to absorb more heat, whereas a glossy white paint requires a longer time of exposure.

For infra-red heating, infra-red gas burners working of LPG gas, infra-red bulbs or infra-red electrical heaters can be used.