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Failures in curing furnaces (II)

Identify the most frequent failures and causes in infrared, electrical,  gas and conventional furnaces.

by Eng.  Gregorio Garza* Gas Furnaces

They use the same principle of convection for the curing of paint, i.e. they heat air and by means of it raise the temperature of the piece in order to carry out the curing process.    As you can see, the principle is the same as that of an electric oven, only here heat is generated by the combustion of gas instead of using an electrical resistance.  Other differences are that the gas furnace has more safety systems since the gas is very flammable. Some security systems are:

a).  Sweeping  System They are timers that perform a sweep with pure air before injecting gas and generating the ignition spark to avoid an explosion.   

b).  Low and high pressure  solenoid These detect that the gas pressure is adequate.

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c).  Flame detector    Detects that the gas is indeed in combustion.    

Process Failures

Excessive gas consumption
Generally, it is due to poor carburetion of the burner (wrong air-fuel ratio)  for such a process temperature.

Hot oven walls
This is due to poor quality insulation or poor oven design, regardless of what temperature the oven is worked at, the outside temperature of the oven must be such that it is supported to the touch, no more than 40°C.

    3.  Explosion or conato of fire by internal generation of VOC
    In some cases there are coatings that when curing emit a high volume of VOC and flammable substances, definitely, a gas furnace can not be used in this type of process.

Mechanical or Control Failures

a)The burner does not turn on

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It may be due to:

a) Lack of gas (check supply).
    b)  Erroneous air-fuel ratio (carburete correctly).
    c)  Ignition electrode in poor condition or out of place  (adjust or replace electrode).
    d)  Air in gas lines (purge gas line).
    e)  Pressure out of range (check supply).
    f)  Damaged flame detector (replace).

    2.  Combustion chamber smoking or at very high temperature
    This failure is usually due to poor airflow within the combustion chamber, so all the heat generated by the combustion of the gas stays there, causing an overheat, it can also be due to a poorly adjusted burner.

Temperature out of control
It is very difficult to control the temperature, it rises well above the set point and goes back down and thus the variation is maintained, this may be because very sensitive thermocouples are used that immediately see the change in only a portion of the air that is being recirculated and not the total system. This is corrected by changing the type of thermocouple or using a thermo well.

Infrared Furnaces

Infrared furnaces operate on a different principle than conventional furnaces, which use radiation which refers to the transmission of energy by means of waves or light such as infrared rays.  so it begins to emit infrared rays thus releasing energy, which is then absorbed by the painted parts. 

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There are two types of infrared furnaces:  shortwave  (halogen and quartz lamps)  and long  wave (resistive element).  Both work on the same principle, only shortwave furnaces are more efficient since they do not require preheating and can cure the parts as soon as they enter the furnace.

Process Failures

    1.  The parts do not cure properly or do not reach the desired temperature

    This is a very characteristic failure of infrared furnaces and most people think that there is a problem in the furnace, because not knowing the technology, they adjust it incorrectly.  

Infrared furnaces can normally be controlled in two ways:  one by percentage and one by controlling the temperature of the resistive element. This is where most people get confused and believe that this temperature is what will reach the piece they are going to cure and it is not so, for example:  "If I need to cure a piece at 150° F, it would be a mistake to adjust my oven to 150° F, since this is only a reference temperature that controls the temperature of the resistive element and therefore the amount of infrared rays that it releases.  The correct thing is to make a correlation table where by means of tests the temperature to which the furnace must be adjusted so that the piece reaches 150 ° F is determined. "

    The adjustment temperature will depend on many factors, such as type of thermocouple, position of this with respect to the resistive element, color of the part, etc.  It may be that the oven or element has to be adjusted to 450 ° F for the piece to reach a temperature of 150 ° F and this in no way means that the oven is failing.

    2.  Parts burned or deformed by heat
    This is another very common failure in IR furnaces and is usually linked to two things:

    a)  Time of residence. Because here the heating is not carried out by convection, the part may be gaining temperature  regardless of the ambient temperature and that it is still exposed to IR rays so if the piece is left inside the oven longer it can run the risk of burning or deteriorating.

    b)  Position of thermocouples:  If for some reason, a part hits a thermos and changes its position with respect to the emitter or resistive element, it can run amok emitting its maximum power in IR generation, so the piece is now exposed to a greater amount of energy and its temperature will rise without control.

    3.   Incorrect thermography: Most ovens are zoned, this is in order to make the pieces gain more heat at first so that their temperature rises quickly and then only gives energy to maintain a certain temperature.  Most people adjust all areas to the same temperature and this is a very common mistake, the right thing to do is to adjust the first zones to a higher temperature than the last ones.

Mechanical or Control Failures

1.Explosion of contactors

This is a very common failure in those furnaces that use mercury contactors, this is a liquid metal that is used in these contactors as a switch, when excited with voltage, it expands and closes the circuit, acting as a conductor. The problem is that if this contactor presents an internal leak, the mercury of one phase, makes contact with that of another phase, thus causing a short circuit that results in an explosion inside the control board that is not of alarming proportions, but definitely, can cause a conato of fire.

    2.  Short circuits inside the oven
    This regularly occurs in shortwave furnaces where quartz bulbs are used, sometimes the pieces hit the quartz tubes, breaking them and leaving the tungsten filament exposed as if it were a voltage line and when this touches the ground, it causes a short circuit.  This is why it is recommended that the parts are well fastened when entering the oven.

    3.  Too high temperature values
    This is another common problem in long-wave furnaces and is usually due to the thermocouple making contact with the resistive element.  In this case it is necessary to evaluate whether the ceramic oxide filler is in good condition and if so, only the thermocouple is recommended, otherwise the panel must be changed.

    These are some of the most common failures that in my twelve years of experience in furnace manufacturing I have identified. I remind you that an oven in poor condition can make a fire or worse, such as an explosion, so it is advisable not to operate a furnace in poor condition just for not stopping production: remember that the consequences can be fatal.

*Thierica. [email protected]

Author: Vanesa Restrepo

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