have heard a lot about infrared lately. Some of it
is fact. Some of it is fiction. We will reinforce
the facts, and debunk the fiction, to give you the
information necessary to solve your oven needs. And
we hope we can help.
a little background is in order. Every oven transfers
heat. The three heat transfer technologies available
are conduction, radiation, and convection. Conduction
heating is the direct transfer of heat energy from
a source in direct contact with a product, like a
hot plate to a teapot. But conduction heating has
only limited use in industrial oven applications.
Therefore, radiation and convection
are the most practical heat transfer technologies
available for industrial applications. (Note: Induction
and microwave are heat generation technologies and
not included in this discussion of heat transfer technologies.
A description follows in the Alternative Technologies
we will begin with a discussion of radiation, or radiant,
heat transfer. Radiant heating is the direct transfer
of thermal energy to a part or coating by electromagnetic
radiation. Radiant energy is emitted from infrared,
ultraviolet, and radio frequency sources. Infrared
ovens, the fastest growing method of drying and curing
coatings, is ideal for rapidly transferring a large
amount of energy to a part quality. Since no carrier,
such as air, is required to transfer energy from the
source to the part, infrared heating creates no real
air movement and therefore is particularly effective
for powder coating.
ovens are powerful ovens for many heating, drying,
and curing processes. Infrared ovens can be gas infrared
or electric infrared, thereby providing a wide range
of infrared sources described on the following pages.
But to understand how an infrared oven can assist
you and your process, it is important to understand
the concepts and to separate the fact from the fiction.
few concepts are necessary for an understanding of
the design and operation of infrared ovens.
and absorbtivity. Infrared heating results from the
absorption of radiant energy by an object. The ratio
of radiant energy absorbed to the amount of energy
striking an object is called its emissivity. An object
that absorbs all of the radiant energy striking it
has an emissivity of one and is called a blackbody.
A perfectly reflective object reflects all the energy
striking it and has an emissivity of zero. All other
substrates, therefore, have an emissivity somewhere
between one and zero.
heat transfer. Infrared is a line-of-sight heat source.
That is, infrared can only directly heat those surfaces
visible to the source. Hence, it is difficult to heat
complex parts evenly using infrared heating alone.
Infrared wavelengths are longer than visible light
but shorter than microwaves. The radiant energy, or
wavelength, of an infrared element depends on its
temperature: The higher the temperature, the shorter
the peak wavelength. This is always true.
The energy output of a radiant source depends upon
the absolute source temperature raised to the fourth
power. As the source temperature increases, heat intensity
rate versus constant rate drying. Falling rate drying
refers to a process where drying becomes incrementally
more difficult. Drying pottery is an example. If the
energy input rate is increased to offset the additional
time required, the material may not accept it: Pottery,
for example, will crack. Constant rate drying refers
to a drying process limited only by the rate of energy
input. Drying water off a metal surface is an example.
usage charges versus demand charges. Power usage charges
are simply the variable price paid for electric energy,
often stated in cents per kilowatt hour. A significant
but often overlooked charge is the demand charge,
the fixed price paid for access to the energy, based
on the peak load. This number is often mistakenly
neglected in payback analysis.
efficiency versus power efficiency. Oven efficiency
is the percentage of energy actually transferred to
the part, taking into account all oven variables.
For example, a well insulated oven will have a higher
oven efficiency than a poorly insulated oven. Energy
efficiency is the percentage of energy actually converted
into heat. Electric infrared always has a higher energy
efficiency than gas infrared. But the low cost of
gas may actually overwelm the efficiency difference.
infrared versus gas convection. Close review of effectiveness
comparisons between electric and gas are often comparisons
between electric infrared and gas convection. For
fair comparison, be sure to compare infrared with
infrared: And convection with convection.
sources, emitters, elements, and heaters. These are
all words used interchangeably to describe gas and
electric sources of infrared heat.
convection versus forced convection. Natural convection
is the incidental flow of air created by heat. An
antiquated style of oven, called a gravity oven, actually
uses the tendency of hot air to rise to induce convection
airflow. In contrast, forced convection is actually
the intentional flow of air provided by fans and blowers.
The higher the air velocity and the greater the turbulence,
the more effective the heat transfer.
ovens are only effective in heating flat surfaces.
Flat surfaces are ideally suited for heating by infrared.
However, more complex, three-dimensional shapes can
also be heated with infrared. Three dimensional parts
can be rotated so that all sides are evenly exposed
to radiation as they pass through the oven. The heating
rate can also be varied from zone to zone to allow
sufficient soak times to heat internal regions of
a part. And natural conduction ameliorates this effect:
As can convection.
radiation works better in a vacuum with little or
no air movement. Air is virtually transparent to infrared
radiation. However, water vapor, carbon dioxide, and
other greenhouse gases do absorb infrared, such as
our atmosphere. But for the distance between the source
and the part of a few feet or less, the energy absorbed
by the air or gases will be negligible.
infrared penetrates more than long-wavelength infrared.
As a generalization, this statement is not true. For
example, metals do not transmit infrared radiation
at any wavelength. All infrared radiation striking
metal is absorbed or reflected at the surface. On
the other hand, some non-metals do transmit radiation.
These materials include water, glass, quartz, and
some ceramic and polymer materials. Each of these
materials will have a unique and complex pattern of
transmission, not simply a function of wavelength.
one wavelength is best for a given application. This
statement is blatantly false. There are many factors
that need to be considered. All wavelengths will most
likely work for a given application. But you need
to consider not only the heating rate, but also the
available floor space, maintenance requirements, source
durability, response time, system efficiency, initial
oven cost, energy consumption cost, conveyor speed,
part size variation, controllability, and aggravation
cost. All of these items need to be considered in
order to pick the right solution in a practical way,
of infrared. High rates of heat transfer are possible,
without air movement, with infrared heating. Intensities
of 3,000 to 25,000 BTUs per hour square foot provide
several process advantages including fast start-up,
fast line speeds, high heating efficiency, relatively
small space requirement, no disturbance or contamination,
quiet operation, and easy and effective zoning of
heating sources. And although operating cost, capital
expenditure, and replacement cost of infrared heating
may be higher than that of convection heating, quality,
and productivity improvements typically offset the
higher cost of installing and operating infrared ovens.
of infrared. Precise control of part temperature is
more difficult with infrared than it is with convection,
requiring more precise sensors and controls. This
is because infrared typically heats a part to a transient
temperature, not the equilibrium or "soak" temperature
a part would reach if heated for a long period of
time. Since infrared source temperatures range from
600° F to as high as 4,000° F, the equilibrium
temperature of a part heated solely by infrared energy
would far exceed the acceptable curing zone of most
coatings. Due to the high rate of heat transfer with
infrared heating, part exposure time in an infrared
oven must be closely controlled. Variations in exposure
time from part to part will produce far greater variations
in final part temperature than will the same variations
in time in a convection oven. And because infrared
only directly heats the areas it "sees", infrared
heating of complex parts is less uniform. Hence the
layout and location of source are therefore more critical
to satisfactory heating.
also gas infrared ovens,
ovens, and combination