Kausar Ahmad

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Heat Transfer By Conduction

• Kausar Ahmad
• Kulliyyah of Pharmacy
• http//staff.iiu.edu.my/akausar/PHM3133.htm

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Contents

• Practical heat transfer
• Heat transfer medium
• Heat transfer through multiple layers
• Heat transfer at boundary

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Quiz Practical Heat Transfer

• Which mechanism of heat transfer .?
• You stir some hot soup with a silver spoon and
  notice that the spoon warms up.
• You stand watching a bonfire, but cant get too
  close because of the heat.
• It is hard for central air-conditioning in an old
  house to cool the attic.

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Heat transfer medium

• Water . water-bath
• Steam.fluid energy mill
• Oil..oil-bath
• Thermal liquid.????
• Airoven, spray drier, fluidised bed
• Steam..BOILER
• Pebbles/Sand/Iron ballshigh temperature
  equipment

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Thermal conductivity, k

• k thermal conductivity
• good thermal conductorshigh k
• k J/s-m-?C (?C or K)
• good thermal insulators low k
• Exercise
• What is k for vacuum? Polystyrene cup?

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Values of k (J/s-m-K)
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Rate of heat transfer

• H rate of heat transfer J/s Q/t
• H k A (TH-TC)/L Ttemperature,
  Ldistance
• Q/t k A ?T/ ?x .what is k? (a constant)

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Calculation Example 1

• Find HQ/t in J/s
• Q/t k A ?T/ ?x
• k0.080 J/s-m-C
• A35 m2
• ?xx0.02 m
• ?T TH-TC 25-0 25?C
• Plug in.
• H3500 J/s3500 Watts

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Heat transfer through multiple layers

• Air is better than wool! And cheaper!! Therefore
  important for insulation.Hencelayered clothing!
  Low k
• For effective heat transfer, choose material with
  high thermal conductivity.

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Heat transfer through multiple layers

• Q/t k A ?T/ ?x
• H/A ?T k/?x ?T/R
• R ?x/k Joules/s-m2, R is the thermal
  resistance
• R adds for multiple layers
• Q/tA ?T/(R1R2R3...)
• R1 x1/k1 etc

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Calculation Example 2

• Find HQ/t in J/s
• Key Point Continuity (just like fluid flow)
• H1 H2
• k1A(T0-TC)/x1 k2A(TH-T0)/x2
• solve for T0 temp. at junction
• then solve for H1 or H2
• answers
• T02.27 C
• H318 Watts

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Examples heat transfer through multiple layers

• Heat transfer between fluids..air heater
• Heat transfer through a wall .pot on stove
• Heat transfer in pipes and tubes. heat exchanger
• Heat exchange between a fluid and a solid
  boundaryfluidised bed

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Heat exchange between a fluid and a solid boundary

• At the boundary, heat transfer is influenced by
  conduction and convection
• H hA(T1 T1, wall), h is the film coefficient

T1, wall
A
T1
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Film coefficient, h (J/m2-s-K)
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Rate of heat transfer

• Taking into account k and h,
• K, thermal conductivity and h,film coefficient
• Q UAdT
• U is the overall heat transfer coefficient

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Quiz Gas as heat transfer medium

• E.g. fluidised bed
• What is the property of GAS?
• HOT AIR
• What is the mechanism of heat transfer?
• Contactive heat transfer!

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U values Overall heat transfer coefficient
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Common heat transport fluids

• from Perrys Chemical Engineers Handbook 6th Ed.

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Relationship between Energy and Temperature
Energy
Temperature (K)
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Heat transfer equipment Fluidised bed dryer

• Hence, drying of solids using fluidised bed
  technique is very popular!
• http//www.pharmaceuticalonline.com/product.mvc/Fl
  uid-Bed-Dryers-0002?VNETCOOKIENO

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Insulation

• Insulation for piping is critical to ensure
  minimum heat loss
• Typical insulators are
• Glass wool/rock wool
• Aluminum sheets

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References

• http//webcast.berkeley.edu/course_details.php?ser
  iesid1906978353
• This video demonstrates the process of using heat
  transfer ...9 min - www.youtube.com/watch?v
  ub5c65vcP-o
• Sand dryer