# Heating of Slabs

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```					     Heating of Slabs
1. Heat Up Time Based on Empirical Data
2. Heat Up Time Based F.E.A. Analysis .
Heating of Slabs I.
• A furnace hearth 2.4 m by 1.8 m is covered with 3”
steel billets is to be heated from 25 to 1200 degC
with a semi luminous flame. What is the heating
capacity in net tons per hour and what is the heat
up time?
• You may assume that a semi luminous flame has a
luminosity of 63% (as a rule most flames are semi
luminous).
Heating of Slabs I.
• 1. Select SI units (we will work
with SI units in this example).
• 2. Select batch process and heat
treatment.
• 3. Click on arrow to display
processes.
• 4. Select steel annealing.
• 5. Select next.
Heating of Slabs I.
•   Enter dimensions of slab
•   Enter furnace temperature
•   Enter luminosity
•   Select Heat Loss and click
on “Calc Heat Losses”
Heating of Slabs I.
• Enter furnace dimensions
• Click on “Calculate heat
losses” to calculate heat
areas
• Select “Next”
Heating of Slabs I.
• Now click on “Calculate”

• Empirical Heat Up time is
51 min/slab
• Production Rate 2853
kg/hr @ 811 kJ/kg
• Next we will re-calculate
this example using F.E.A.
analysis
Heating of Slabs II
better estimate of heat up time:
• Emissivity of walls
• Emissivity of slab
• Heat capacity of slab
• Wall temperatures
Heating of Slabs II

•    Enter dimensions
•    Temperatures
•    Emissivities
•    Heat capacity
Heating of Slabs non absorbing
media. III
• Select Conduction in Solids
• Select Show Chart
• Select 3 segments and 20
rows
• Emissivity of bottom and
walls equal to 0.5
• Mid point (“T2”) reaches
1200*C after 45 min
Heating of Slabs in Absorbing
Media.III

• Suppose we have a gas fired
furnace with absorbing
atmosphere (CO2 and H2O)
• Enter production and flow rates
• Enter fuel (No#2) and Air flow
• Calculate Stack Gas losses
• This will calculate composition
of furnace gas and hence gas
emissivity
Heating of Slabs absorbing
media. III
• Set stop time to 50 min
and thickness to 75 mm

• Click on Calc

• Calculated Heat Up time
is approx 42 min
• Calculated gas emissivity
0.07
What You Learned

• To estimate heat up time for a slab

• To calculate temperature profiles and heat
up time for a bar/slab
Heating of Slabs
• Which method is more accurate?
• The first method requires very little information
and is therefore by nature a less accurate method.
However, for iron and steel it may be preferred
especially if emissivity values are uncertain.
• With accurate emissivity data the second method is
be preferable.

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 views: 8 posted: 9/29/2012 language: English pages: 13
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