Process Heating 1997.

   
 

Combination Ovens for Paint and Powder Finishing. Consistent drying and curing of solvent-borne, waterborne and high solids paints and powder coatings may be best achieved in ovens using both infrared and convection heating. But, careful attention to the idiosyncrasies of each technology is needed to realize their synergistic benefits.

The two most common technologies for paint and powder finishing are infrared and convection heating. However, the heat transfer characteristics and production requirements of each are distinct. Because today's coatings, particularly those for automotive finishing, have diverse curing and drying requirements, rarely is one heat transfer method, by itself, the optimum solution for a finishing application.

Often, the most efficient, cost-effective oven design is one that melds infrared and convection heating, taking advantage of each method's unique strengths while minimizing the limitations. To be most effective, such ovens, called combination ovens, are designed with specific curing and production requirements in mind.

The Combination Concept. Combination infrared/convection systems are those in which the primary heat source is radiant elements but which also includes a forced convection system in the same enclosure. Combination systems often are used in finishing to augment heat transfer (using radiation) or improve temperature accuracy and uniformity (using convection). The latter is particularly true for complex shapes like wheels and engine blocks.

The two heating methods work in concert to provide the optimum temperature profile. The radiant heat source transfers the bulk of the energy required to raise the parts and coating to curing temperature. Convection heating holds the parts at temperature, allowing all coated surfaces to dry or cure uniformly and completely. Combination ovens can provide the speed of infrared heating and the accuracy and uniformity of convection in a productive, precise oven.

Three configurations are possible, depending on how the infrared and convection heating methods are introduced to the process.

  • Consecutive combination ovens apply infrared and convection in successive zones. These ovens are well suited for curing coatings on complex parts.
  • Concurrent combination ovens apply both infrared and convection in a single zone. They commonly are used to dry waterborne coatings.
  • Mixed combination ovens utilize concurrent and consecutive combination zones in a single enclosure. They are designed to provide flexibility, allowing processors to dry and cure solvent- borne, waterborne, high solids and powder coatings in one unit rather than several specialized ovens.

Consecutive Combination Ovens. This design utilizes a two-stage heating profile. First, it rapidly brings a product fusion or curing temperature. Second, it holds the product and/or coating at a precise equilibrium curing temperature for a specified time to provide the desired result. The high initial heat transfer rate is achieved with an infrared section, then a convective section holds the product at the equilibrium point for the requisite dwell time.

Advantages of consecutive combination ovens include faster line speeds, accurate and simple process control, consistent heating and undisturbed products or coatings.

Concurrent Combination Ovens. These ovens rapidly transfer heat to a product and/or coating while simultaneously circulating air over it to provide the desired result. This profile is achieved by using infrared and convection heating in the same zone. Concurrent combination ovens are suited for applications where temperature-sensitive products or coatings require high rates of mass transfer for example, water removal. Because mass transfer requires effective air movement to allow water or solvents to continue evaporating, this design often is used in drying applications.

Like consecutive combination ovens, the concurrent combination design provides fast line speeds, accurate control and consistent heating. This design also promotes effective moisture removal or solvents dilution.

Mixed Combination Ovens. This oven's primary objective is to provide flexibility. Its design incorporates infrared and convection in concurrent and/or consecutive zones, providing a hybrid oven capable of drying and curing a wide spectrum of finishes.

Careful Oven Design Is Key.

The first step in designing any combination oven is establishing the heat balance. In addition to satisfying a basic heat-balance calculation to determine gross energy input for the convection section, more complex calculations must be performed to determine the component losses that make up the radiant section's heat balance. Residence times and heat intensities are more critical in radiant sections, demanding accurate calculations. With iterative computer-based and spreadsheet analyses, it is possible to accurately analyze parameters - changes in a product's heat transfer coefficient with rising temperature, for example - with a minimum of simplifying assumptions that compromise calculation accuracy. Process testing provides real-world results that can be scaled up or cross checked with the mathematical calculations.

Infrared Section. Infrared heating's high heat transfer rate ensures fast line speeds and high productivity without compromising the finish. Good design for any infrared section, whether gas-fired or electrically powered, should include:

  • Vertical or horizontal zoning to provide flexible yet efficient application of infrared heat.
  • Precise element layout and distribution to counter edge effects.
  • Insulated, reflective panels to reradiate heat, even when the panels are dirty due to use and/or lack of maintenance.
  • Insulated element wiring to extend the electric infrared elements' life.
  • Noncontact temperature sensors to provide optimum temperature control.
  • Rigid, nonvibrating structure to lengthen the elements' life.
  • Custom controls to meet the specific process and operator requirements.
  • Strict compliance with NFPA and OSHA standards and regulations to ensure safety.

Although electric heat sources usually provide greater radiant efficiency than gas-fired sources, radiant efficiency alone does not dictate overall oven efficiency. 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. In general, electric infrared is efficient and has powerful zoning capabilities. By contrast, gas-fired infrared sources burn a fuel/air mixture to heat a metal or ceramic to an incandescent temperature. In general, gas infrared provides great intensity and low operating costs.

Convection Section. In this section, the part being cured is held at a precise temperature for the amount of time necessary to provide the desired finish. Uniform, precise temperature control during finishing ensures consistent physical and visual properties.

Good design in the convection section should incorporate:

  • Inverter-controlled blowers to allow easy adjustment of air velocities.
  • Large blower capacity to provide high air velocity for accelerated heat transfer. Large blowers optimize airflow and ensure uniform temperatures throughout the oven.
  • Air plenums to distribute air uniformly for precise temperature control (2°F).
  • Air seals and heavy insulation to economically contain heat and allow efficient operation.
  • Properly designed and balanced exhaust to meet the specific application's needs.
  • Cooling zones to allow quick operator handling and packaging after finishing.

Heat Transfer Rates. Generally, a convection oven is designed to transfer 500 to 2,000 BTU/hr-ft2. By contrast, radiant ovens can transfer from 3,000 to 25,000 Btu/hr-ft2. Radiant's high heat transfer rate can substantially reduce the time required to cure a finish, but radiant heating is suitable only for those processes that can withstand the high heat transfer rate.

Time/Temperature Relationship. Convection ovens are designed to generate an ambient air temperature not far above the desired part temperature. No matter how long a part is left in the oven, it never can exceed the equilibrium temperature of the convection zone.

Due to the high temperature of its sources, radiant heating does not heat parts to an equilibrium temperature. Rather, parts are heated to a transient temperature, which depends upon the part's thermal capacity, its exposure time, and the net heat transfer between the part, its environment and the oven's radiant sources. Thus, exposure time to radiant heating must be accurately controlled to achieve uniform results. Exposure time variations in a radiant zone will produce far greater variations in final product temperature and therefore, the final quality of the finish than would a convection zone.

If accurate temperature control is desired, the best design may be a convection zone operated with a long residence time and a supply air temperature equal to, or only slightly above, desired part temperature. More sophisticated control strategies are required to achieve precise temperatures in parts heated by radiant sources.

Oven Efficiency. Efficiency is a term frequently used and misused in connection with finishing systems. In a typical finishing application, energy input is distributed among five factors:

  • Heat absorbed by the product and carried out.
  • Heat absorbed by conveyor and tooling and carried out.
  • Heat absorbed by phase changes or chemical reactions (such as fusing of powders) within the oven.
  • Heat carried out by airflow through the exhaust system.
  • Heat lost through the oven walls, roof and openings.

When evaluating claims of efficiency, be sure to understand the definition being used. For example, one vendor may consider the first three items useful, while another may consider the last three. Without a common definition of efficiency, both are right.

Obviously, neither infrared nor convection heating alone can meet all finishing heating requirements. But, when joined together in a synergistic oven design capitalizing on the strengths of each method, an optimum oven design can be achieved that will ensure high productivity yet provide consistent, quality finishing at a low cost.