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
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
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
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.
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.
configurations are possible, depending on how the
infrared and convection heating methods are introduced
to the process.
combination ovens apply infrared and convection
in successive zones. These ovens are well suited
for curing coatings on complex parts.
combination ovens apply both infrared and convection
in a single zone. They commonly are used to dry
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.
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.
of consecutive combination ovens include faster line
speeds, accurate and simple process control, consistent
heating and undisturbed products or coatings.
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.
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.
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.
Oven Design Is Key.
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.
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,
or horizontal zoning to provide flexible yet efficient
application of infrared heat.
element layout and distribution to counter edge
reflective panels to reradiate heat, even when the
panels are dirty due to use and/or lack of maintenance.
element wiring to extend the electric infrared elements'
temperature sensors to provide optimum temperature
nonvibrating structure to lengthen the elements'
controls to meet the specific process and operator
compliance with NFPA and OSHA standards and regulations
to ensure safety.
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.
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.
design in the convection section should incorporate:
blowers to allow easy adjustment of air velocities.
blower capacity to provide high air velocity for
accelerated heat transfer. Large blowers optimize
airflow and ensure uniform temperatures throughout
plenums to distribute air uniformly for precise
temperature control (±2°F).
seals and heavy insulation to economically contain
heat and allow efficient operation.
designed and balanced exhaust to meet the specific
zones to allow quick operator handling and packaging
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.
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.
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
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
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:
absorbed by the product and carried out.
absorbed by conveyor and tooling and carried out.
absorbed by phase changes or chemical reactions
(such as fusing of powders) within the oven.
carried out by airflow through the exhaust system.
lost through the oven walls, roof and openings.
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.
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