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Convection
ovens use convective heat transfer, whether the energy
source is gas, electric, or steam. Convection is the
indirect transfer of thermal energy by the circulation
of a heated carrier, such as air. The greater the
air velocity and the turbulence in a convection oven,
the faster the transfer of heat.
Convection
ovens supply air at temperatures not far above the
desired curing temperature of the part or coating.
That is, the part or coating is typically heated to
an equilibrium temperature, usually between 150°F
to 1000°F, depending on the application: 400°F
is common for many coating applications. The rate
of heat transfer is often 500 to 2,000 BTUs per hour
per square foot, far less intense than most infrared
ovens, but far more uniform.
High
velocity convection.
High
velocity convection is a unique form of forced convection.
In contrast to natural convection ovens, where air
flows incidentally as the result of the heat, forced
convection is the intentional flow of air provided
by fans and blowers. One step further, high velocity
convection uses large amounts of air to create velocities
at the upper limit that the process will allow. Where
forced convection is like a breeze at the beach, high
velocity convection is like a tornado. High velocity
convection is ideal for many preheating and drying
applications.
Advantages
of convection. Convection ovens are ideal for uniformly
transferring heat to a part. Convection
is the most common type of oven. The advantages include
accurate control of temperature, simplified burner
control, uniform, consistent heating regardless of
part size or shape, low capital expenditure, and low
maintenance cost.
Disadvantages
of convection. The primary limitations of convection
ovens result from its
low rate of heat transfer and its inherent requirement
for rapid air movement, which is unacceptable for
powder applications. These characteristics can result
in slow start up, slow line speeds, greater need for
oven space, and possibility of disturbing or contaminating
unfused powders.
Well
designed convection ovens exhibit many of the following
characteristics:
•
Large blower capacity to provide high velocity air
and uniform heating. Heat transfer is accelerated
by air velocity. Large blowers provide the flexibility
to optimize airflow with little effort. A high rate
of air circulation insures uniform temperatures
throughout the oven.
•
Air plenums for uniform air distribution. Perfectly
and flexibly designed plenums to distribute air
accurately and uniformly at ± 2°F.
•
Air seals and heavy insulation to contain heat.
To operate efficiently and economically.
•
Inverter controlled blower for air velocity control.
To provide adjustment of air velocities simply and
easily by the push of a button, manually or automatically.
•
Proper exhaust. Designed and balanced to meet the
specific exhaust needs of
the powder or finish being cured.
See
also infrared
and combination ovens.
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Convection
Ovens.
