5/20/2013
Convection Baking
Kevin G. Knott
Technical Sales Manager
Franz Haas Machinery of America, Inc.
Convection Baking
What are we baking today
The products we bake often decide the type of oven heat we use to bake it 1
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Convection Baking
Total Heat on the Product =
Conductive + Radiant + Convective
Convection Baking
Heat transfer in Baking Ovens
Convection
(Newtons equation) k (Newtons equation) k = h x A x = h x A x (T
(To – Ts) •
q is the heat transfer rate in W
•
A is the area normal to direction of heat flow in m2
h is the convective heat‐transfer coefficient in W/m2 C
•
To is the temperature of the air immediately before impinging on the product
•
Ts is the surface temperature of the product
Radiation
(Stefan Boltzman equation) q (Stefan Boltzman equation) q = = ee x A x s x A x s (T
(Te – Tr)4
•
•
•
•
•
q is the radiant emission in W
A is the area of the emitting surface in m²
e is the Stefan‐Bolzman constant, numerically equal to 5.676 x 10E‐08 W/m2 x K4
s is a compounded constant covering the emissivity of the two surfaces and their geometrical relationship
is a compounded constant covering the emissivity of the two surfaces and their geometrical relationship
Te and Tr are the temperatures of the warm and colder bodies respectively
Conduction
(Fourier’s Law) q = k x (Fourier’s Law) q = k x D
DT
•
•
•
q is the conduction in W
k is the thermal conductivity W/m x K4
DT is the differential temperature
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Convection Baking
Air Plenum Oven Roof/ Walls
Radiation
Convection
Conduction Conduction: Radiation:
Convection:
Heat transferred to the product from the belt.
Heat transferred to the product from all surfaces at higher temperatures.
Heat transferred to the product from the air currents. The faster the air hits the product the higher the heat transfer.
Convection Baking
Cyclo therm oven ‐ Radiant
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Convection Baking
Direct Gas Fired (DGF) – Free Convection
Convection Baking
Indirect Fired Convection
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Convection Baking
Why Convection?
Air Plenum Products on Baking Band
Boundary Layer
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Convection Baking
Characteristics of Ovens with high convection •Lower air temperature
Air Plenum •Efficient heat transfer
•Fast change over & reaction time
•No flash heat
•Lower fuel consumption
•Excellent drying abilities
E ll
d i
bili i
•Even coloring of products
•Simple operation of oven/full repeatability
Convection Baking
Direct Fired Convection
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Convection Baking
Indirect vs. Direct heating
•Indirect heating is cleaner
•Explosion relief is much simpler
•Energy consumption is identical or less
•Drying properties are better
•Humidity control is independent of h ti
heating capacity (temperature)
it (t
t )
Convection Baking
Hybrid Ovens
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Convection Baking
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Questions?
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