used to cure powder coatings, combination ovens -
ovens that employ both infrared and convection - usually
connect infrared and convection heating zones together
in a series: That is, a zone of infrared followed
by a zone of convection. This is logical given that
when curing powder coatings, the objective is first
to fuse the coating, then cure it. But an innovative
combination oven, installed in Michigan one year ago,
actually utilizes infrared and convection simultaneously
in each zone, creating a fast, yet flexible powder
coating curing oven.
Lighting Standards manufactures steel and aluminum
light poles at its plant located just outside Detroit.
When the company began operations in 1971, the poles
were hand sprayed with a liquid primer, then painted
with a liquid acrylic enamel finish. The paint drying
process took two to four hours, limiting production
to only 30 poles per day.
take advantage of the superior appearance and abrasion
resistance provided by powder coating, the company
replaced its liquid primer and paint line with a powder
coating line in 1987. But the powder line never quite
lived up to its promise, according to United Lighting
Standard's president, Robert Wesch. "Our capability
was limited by the infrared oven," he said.
problems with the electric infrared oven were many.
Foremost, curing poles coated with light-colored powder
was extremely slow because the infrared intensity
necessary to accelerate curing discolored the powder.
To prevent this, lamp intensity was reduced, and the
entire line was slowed when light-colored poles were
being cured. Additionally, the high intensity heat,
combined with the necessary curing time for the poles,
would cause light-gauge housings assembled to the
poles to discolor. United Lighting switched to painting
the housings separately from the poles and attaching
them after curing, but this added another process
step. Finally, the electric infrared elements were
expensive to operate and required significant maintenance
to replace and repair burned-out or broken elements.
square and round light poles produced at United Lighting
Standards vary in length form 10 to 40' with wall
thicknesses from 0.125 to 0.5". Typically tapered,
the poles are welded to heavy, 0.5 to 2" thick solid
bases that range from 10 to 17" in diameter. Pole
weight ranges from a 30 lb., 10' aluminum pole to
a 1,100 lb., 40' steel pole. A continuous overhead
conveyor transports the poles, base plate at the rear,
through the powder application line and curing system.
Lighting coats the poles with polyester TGIC powder
purchased from several suppliers. More than 70% of
the poles are coated bronze while black accounts for
another 10%, and about 8% are coated white. The remainder
are coated assorted colors depending on the intended
use. In addition to the poles, United Lighting also
powder coats the poles' associated flat, formed and
search for an improved curing system, headed by then
general manager Bernie Jenkins, focused on three options:
A new electric infrared oven, a catalytic infrared
oven or a combination gas infrared and convection
curing oven recommended by Thermovation Engineering
exhaustively evaluated all three approaches and decided
to go with the combination oven from Thermovation
Engineering," Jenkins said. Using both gas infrared
and convection together in each zone appeared to provide
the flexibility and controllability needed to accommodate
the broad range of parts, substrates, powders and
line speeds effectively and efficiently.
the new combination oven, poles first are cleaned
by shot blasting, then conveyed through the powder-application
unit. Once coated, the poles enter the oven designed
and built by Thermovation Engineering. The combination
curing system is a free-standing 32' long, 8' high
and 5' wide structural steel assembly with 4" thick,
double-insulated sheet metal floor, roof and wall
panels. In the first zone, powder fusion is achieved
by applying high-intensity gas infrared and low-velocity
convection. This combination rapidly brings the substrate
and powder to fusion temperature without disturbing
the powder. Once the coating has fused, the cure is
completed with the continued application of infrared
and moderate-velocity convection, which hold the substrate
and coating at the curing temperature. After exiting
the oven, the overhead conveyor carries the poles
laterally to where the poles are stacked and packaged
or Gas? Why It Mattered.
this application, using gas infrared provided two
significant advantages over electric infrared: Increased
productivity and lower operating and maintenance costs.
In analyzing the needs of United Lighting Standards,
Thermovation engineers determined that the existing
electric infrared oven yielded the kW equivalent of
1.18 million BTU/hr. Therefore, the burner capacity
for the gas infrared section of the combination system
had to be at least 1.6 million BTU/hr. to provide
the desired 30% increase in productivity. In fact,
the curing system was designed with a 2 million BTU/hr.
maximum output to provide an engineering safety factor,
allow for increased production and ensure rapid heat
up from a cold start.
catalytic system supplier recommended a system delivering
1 million BTU/hr., and the electric infrared supplier
recommended a 1.4 million BTU/hr. system," Jenkins
noted. "Thermovation engineers recommended a combination
curing system with 2 million BTU/hr. capacity. We
realized that the additional capacity would allow
us to reach our increased production goals while providing
for future needs."
costs. Gas infrared is significantly less expensive
to operate than electric infrared. A significant portion
of electric energy costs for the previous oven derived
from the monthly demand charges imposed on energy
consumed during periods of high demand. For purposes
of comparison, Thermovation engineers analyzed the
energy costs of an electric infrared system with a
demand capacity of 392 kW and a 300 kW average usage
level operating eight hours a day, 22 days per month.
With these figures, estimated monthly electrical energy
cost was $7,168.24 - of which almost 60% was attributable
to demand charges.
operating costs were compared with those of the proposed
1.6 million BTU/hr. gas infrared system. With the
same usage per month, gas charges were estimated at
$1,047.55. The significant savings were possible because
there are no utility demand charges for gas usage.
Thus, energy-related operating costs for the proposed
larger system were estimated at about $6 per hour
vs. almost $41 per hour for the previous system.
Costs. Rated at 5,000 hr. under the best conditions,
the glass lamps used in the electric infrared were
no match for the light poles. Replacement costs were
estimated at approximately $12,000 per year.
Engineering recommended using heavy-duty cast-iron
burners designed for long life under tough operating
conditions. These burners far outlast any electric
element and have three times the expected life of
less rugged formed-sheet metal burners. Experience
has proved this decision right: After one year of
continuous operation, just two of the 90 burners in
the oven have been replaced at a combined cost of
cast-iron burners designed to operate on a premixed
volume of air and gas rather than relying on atmospheric
air for combustion. With the premix burners, the oven
temperature can be held constant while product loads
fluctuate. Heat input is modulated to match the control.
we looked at these design elements and cost estimates,
we realized that the additional capital investment
for the combination oven was insignificant in comparison
to the operating and replacement costs of the electric
infrared oven," Jenkins said.
convection heat transfer in this oven also provided
two process advantages: improved quality and enhanced
Used in combination with infrared, convection heating
significantly improved coating quality for United
Lighting. By precisely controlling airflow and velocity,
convection provides efficient, effective heat transfer
that ensures accurate and uniform temperatures along
and across the parts. Adding convection particularly
improved the cure for the poles coated in light colors,
and its addition allowed the less-visible undersides
of the poles, light-gauge housings and miscellaneous
hardware to be cured more effectively.
Oven efficiency is the ratio of the heat input into
the product vs. the energy consumed by the oven. Electric
radiant elements typically have a radiant efficiency
(the ratio of radiant energy emitted vs. energy consumed)
of 60 to 90%. Gas infrared burners typically have
radiant efficiencies of 40% to 60%. In each case,
the remainder of the energy input (that which is not
converted directly to radiation) becomes heated air
within the oven.
engineers designed the oven to use this heated air
to provide additional heat to the product and offset
losses that typically occur through the exhaust and
enclosure. The moving air improves overall oven efficiency,
ameliorating the inherent radiant inefficiency of
gas infrared (when compared to electric infrared).
The additional convection heating system supplements
the preheated air, helping to heat the poles more
rapidly and uniformly than is possible with radiant
testing and troubleshooting was performed before the
assembled system was shipped, so downtime at United
Lighting Standards was reduced to six days.
met and exceeded our productivity, quality and cost-reduction
goals. Number of poles per shift through the system
is up more than 30%, and we're confident we can easily
raise this to 50% to 60%," company president Wesch
noted. "Light-colored poles cure as fast as dark colored
poles. And, we have reduced operating costs by more
than 60% while maintenance and replacement have been
reduced to almost nothing."
United Lighting, infrared and convection together
was the right combination.