Often used to cure powder coatings,
combination ovens—ovens
that employ both infrared and
convection— usually connect infrared
and convection heating zones together
in 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.
United 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.
To 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
Standards’ president, Robert Wesch.
“Our capability was limited by the infrared
oven,” he said.
The 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 lightgauge
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.
Finding a System
The square and round light poles
produced at United Lighting Standards
vary in length from 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.
United 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 tubular
hardware.
The 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 C.A. Litzler Co. Inc.,
Cleveland.
“We exhaustively evaluated all three
approaches and decided to go with the
combination oven from C.A. Litzler,”
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.
With 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 C.A.
Litzler. 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
for shipment.
Electric or Gas? Why It Mattered
For this application, using gas infrared
provided two significant advantages
over electric infrared: Increased productivity
and lower operating and maintenance
costs.
Productivity. In analyzing the needs
of United Lighting Standards, C.A. Litzler
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 max. output to provide an engineering
safety factor, allow for increased production
and ensure rapid heat up from a
cold start.
“The 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. “C.A. Litzler
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.”
Operating 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, C.A. Litzler 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.
These 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.
Maintenance Costs. Rated at 5,000
hr under the best of 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.
C.A. Litzler 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 $350.
The cast-iron burners are designed to
operate on a premixed volume of air and
gas rather than relying on atmospheric
air for combustion. With premix burners,
the oven temperature can be held constant
while product loads fluctuate. Heat
input is modulated to match the control.
“When 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 Curing
Incorporating convection heat transfer
in this oven also provided two process
advantages: improved quality and enhanced
efficiency.
Quality. 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.
Efficiency. 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.
C.A. Litzler 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 heating alone.
The Right Combination
Oven testing and troubleshooting
was performed before the assembled system
was shipped, so downtime at United
Lighting Standards was reduced to six
days.
“We’ve 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.”
For United Lighting, infrared and
convection together was the right
combination.
Reproduced with permission from
Process Heating magazine
(Vol. 6, No. 7). Copyright 1999.