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Oven Types For Coil Coating Lines

Author: Chris Baldwin – Thermal Engineer, Bronx International

The following type of paint curing ovens have been successfully applied to the paint curing process on a continuous coil coating line:-

  • Induction Ovens
  • Flotation Convection Ovens
  • Catenary Convection Ovens
  • Catenary Infrared Ovens

Induction ovens have been applied successfully to the curing of paint coatings but are not commonly used; and unless there is a unique reason as to why an induction oven would be preferred such as, to save space, electricity is cheaper than gas or gas is not available then the use of induction ovens on a continuous coil coating line would not be recommended.

Flotation convection ovens are ovens that use high velocity hot air jets flowing through specially designed nozzles to both lift the strip and provide the heating medium.

Flotation ovens avoid the need to tension the strip and prevent the creation of a catenary or sag in the strip which thus reduces the height requirement of the oven particularly on faster lines requiring long ovens.

Also because of the high jet velocities required to support the strip the ovens tend to be shorter than catenary type convection ovens.

However the distance between the nozzles and the strip is small and thus access for re-threading the strip is difficult and although the high velocity jet stream provides accelerated heating this is often at the sacrifice of the quality of the finished painted surface.

Catenary convection ovens rely on external bridles to tension the strip at both ends as it passes through the oven. The level of tension is controlled to maintain a pre-determined strip catenary position through the oven.

Because the strip is supported externally the jet nozzles within a catenary convection oven only have a single function which is to provide the hot air jet that cures the coated surface of the strip. This allows the jet velocities to be selected to better suit paint surface finish quality.

Although either flotation convection ovens or catenary convection ovens could be heated using electricity, oil or gas it is conventional for convection ovens used on continuous coil coating lines to be heated with natural gas or some form of bulk gas such as LPG or LNG. Further the gas firing can be either direct or indirect.

There is often concern expressed about using direct gas firing to heat convection ovens especially when producing high quality requirement products such as appliance casings but modern gas supplies and oven designs have proven that direct gas firing can be used even when a high quality painted surface is required.

It is also worth mentioning that if necessary convection paint curing ovens can be installed in the vertical plane however this is not common due to the additional cost of the supporting structure and increasing the building roof height.

The development of Infrared ovens using short wave frequencies or frequencies below the infrared frequency range (often referred to as near infrared) have been successfully applied to the curing of paint coatings on coil painting lines.

Infrared oven curing times as low as 3 seconds have been achieved under laboratory conditions but for successful application in a production environment the addition of break zones between the banks of infrared emitters has been found to be necessary and thus overall infrared oven dwell times are typically 9 to 11 seconds (for polyester coatings).

As with convection paint curing ovens, infrared paint curing ovens can be installed in either the vertical or horizontal plane, however because infrared ovens are generally half the length of an equivalent convection oven and do not require bulky combustion boxes, infrared ovens can be more readily applied in the vertical plane and can be applied well into combination galvanising and painting lines that are often restricted for space.

Another advantage of infrared ovens particular those in the near infrared range is the ability of the oven to switch the heating process on and off almost immediately.

The infrared ovens have thus been applied on non-continuous lines coil coating lines with great success.

However, the electric power requirements of infrared ovens can become excessive as the line speed increases and thus the running cost can also be excessive.

Further, as there is no requirement for the pre-heating of make-up air with infrared ovens as there is with convective ovens, then when used with a VOC oxidation process there is no possibility to recover energy from the exhaust stream to reduce the oven running costs; hence it is essential to use the most thermally efficient oxidation process available which means using a regenerative thermal oxidiser (unless there is some external heat requirement not associated with the continuous coil coating line such as supply of heat to a direct heating network, factory space heating, steam generation plant, etc., that would require the use of a recuperative thermal oxidiser).

Also, before opting to use an infrared paint curing oven, it is recommended that the paint suppliers are contacted to ensure that infrared curing is suitable for the paint coatings to be used. Typically modified paint formulations are required when using infrared ovens and some coatings may specify curing via convective heat transfer only.

In general infrared painting systems find suitable application when space is limited, gas is not available, and line speed is low or when the line is a low speed stop-start line. Even if gas is not available but, line speeds start to move above 40 m/min then it would be recommend that the installation of a bulk gas storage system is investigated such that gas convection ovens can be used.

It is worth mentioning that there are gas infrared ovens available on the market but as yet these ovens tend to be used for drying of water based coatings and have not been developed for use with the curing of solvent based coatings on a continuous coil coating line.

With the type of thermal oxidiser and paint curing oven selected the next item on the continuous coil coating line thermal system to be considered is the paint coater room.

The size of the coater room has a significant effect on the size and thus cost of the overall thermal system.

There are both  health & safety and emissions regulations to be satisfied when designing a coater room ventilation system.

To ensure a safe environment for the coater operators to work in the solvent concentration within a coater room, the solvent must be kept below certain limits and thus the coater room requires adequate ventilation.

Further, the air exhausted from the coater room comes under standard emissions control and thus VOC oxidation of coater room air is required.

The general rule of thumb is to ventilate the coater room at 60 volume changes per hour and therefore every additional cubic metre of coater room volume could potentially increase the size of the VOC oxidation system.

Experienced suppliers of continuous coil coating line thermal systems use specialist techniques that limit the amount of air that needs to be processed through the VOC oxidation system and thus limit the size and cost of the equipment.

Once the three main components of the coil coating line thermal system, i.e. curing oven, oxidiser and coater room have been selected then the basic system flow diagram can be created and linked together with interconnecting ductwork and the main exhaust stack.

Air must be exhausted from the paint curing oven at a rate that guarantees that solvent concentration is always below 25% of the upper explosion limit (LEL) of the solvent being processed (i.e. Factor of Safety of 4:1) or if the appropriate continuous solvent monitoring is installed then the solvent concentration limit within the oven can be allowed to rise to 50% of the LEL (i.e. Factor of Safety of 2:1).

Knowing the size of the paint coater room, the coater room exhaust rate can also be calculated and combined with the calculated curing oven exhaust rate the system flows can be specified and balanced both on a volume and energy basis to complete the basic system design.

Additional heat recovery systems, such as heat exchangers to provide hot water for heating the coil coating line cleaning section or for general factory heating can then be added as required thus completing the concept design procedure.

Although the above selection procedure is relevant today we must not fall under the misconception that this is a final selection procedure. As emissions regulations become more stringent, fuel sources change and coatings advance then the selection procedure that is relevant today may not be relevant tomorrow.

The development of thermal systems for coil coating is not at its final destination it will continue to move forward and thus each time an application is presented to ensure that best available technology is used then the selection process must be reappraised by thermal engineers who are expert in the field of continuous coil coating.

Disclaimer: This information is for general information purposes only and should be viewed as such. For detailed, precise information for your upcoming coil coating line, its best to speak to a Bronx Technologist.

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