The Dangers of Cure Ovens
By: Simon Fridlyand
Cure ovens can emit deadly carbon monoxide, so it's critical for industrial facilities to meet established criteria for providing make-up air.
Cure or dry-off ovens in industrial facilities require proper installation and careful monitoring to ensure their operation won't endanger workers. The ovens are commonly used in the following applications:
* Parts drying after pre-treatment
* Adhesive curing
* Solvent, powder and water-based paint curing
* Pre-heat for out-gassing and powder applications
* Metal, plastic and other substrate heating to a desired temperature.
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In order to accommodate various sizes of parts, the conveyor openings into cure ovens are usually large, allowing gases and chemicals to escape into the workplace. |
Industrial establishments that use such ovens must make sure that there is no reverse flue situation. Reverse flue, essentially a mechanically induced downdraft, can cause flue gases and other harmful chemicals to be drawn into a plant.
This situation may take place if there is negative pressure in the building caused by substantial exhaust, or when a process is used that requires combustion air, while insufficient make-up air is provided for the building.
In order for the exhaust system to work properly, make-up air is required to replace air, while an amount equal to that removed. Building codes in Canada mandate that industrial establishments must provide make-up air systems to compensate for exhausted air.
Canadian codes require that an amount of fresh outside air is included in any replacement air calculation to maintain indoor air quality requirements. Make-up air can be provided via an independent system or in combination with the building's HVAC system.
Buildings that do not have a proper make-up air system might display some of the following conditions:
* Difficulties in opening doors
* Cold drafts
* Air contaminates from other areas (such as curing ovens) entering the building.
The main danger is that carbon monoxide and other products of combustion (whatever is being cured in the oven) may be coming out of the large conveyor openings in the ovens. One can easily assess the direction of the flow by using some paper tissue attached to a stick, like a flag, and watching the direction of the flow. This should be done through the entire height of the conveyor opening.
According to the Journal of the American Medical Association, carbon monoxide is responsible for 800 to 1,000 accidental deaths a year. It is the most common cause of unintentional poisoning deaths in the United States and the same applies to Canada. It is estimated that 10,000 people in the U.S. annually seek medical attention as a result of carbon monoxide poisoning.
Carbon monoxide, a colourless, odourless and tasteless gas, is slightly lighter than air, with a specific gravity of 0.97, It has an explosive range of 12.5% to 74%. It is lethal within minutes at 1.28% in air and will cause asphyxiation long before it poses an explosion danger.
Measured in the air, in parts per million (ppm), carbon monoxide exposure of 800 ppm for 45 minutes can cause flu-like symptoms, while exposure to the same 800 ppm for three hours can cause death.
Inhaled carbon monoxide is absorbed by the oxygen-carrying portion of the blood, the hemoglobin. The carbon monoxide and the hemoglobin combination is known as carboxy hemoglobin (COHb). Carbon monoxide is 240 times more likely to be absorbed by the hemoglobin than oxygen (O2). As a result, small amounts of carbon monoxide in the air can rapidly increase your COHb level.
Carbon monoxide replaces oxygen in the blood, depriving organs and cells of needed oxygen and preventing the release of oxygen in the blood, further reducing the oxygen available to the body. It is a one-two punch that can be fatal once the COHb level reaches 50% to 70%.
The confusion resulting from exposure to carbon monoxide can prevent a victim from leaving a contaminated atmosphere, even if the danger is realized.
Provincial regulations and the American Conference of Governmental Industrial Hygienists (ACGIH) also lists maximum allowable concentrations for human exposure to air contaminants in workplaces. The limits are expressed in TWA-TLV (time-weighted 40-hour-week average threshold limit value) for long-term exposure and STEL (short-term exposure limit) for a 15-minute exposure.
There are established testing procedures to identify concentrations of dangerous substances. If a facility exceeds the recommended TWA-TLV or STEL limits, proper make-up ventilation is a way to correct the situation.
Simon Fridlyand, P.Eng., president of S.A.F.E. Engineering, Toronto, can be reached at simonf@safeengineering.ca.
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