1. Heat Transfer By Conduction
Kausar Ahmad
Kulliyyah of Pharmacy
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2. Contents Practical heat transfer Heat transfer medium
Heat transfer through multiple layers
Heat transfer at boundary
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3. Practical Heat Transfer
You stir some hot soup with a silver spoon and notice that the spoon warms up.
You stand watching a bonfire, but can’t get too close because of the heat.
It is hard for central air-conditioning in an old house to cool the attic.
Mechanism of heat transfer
Hot molecules have more KE than cold molecules
High-speed molecules collide with low-speed molecules
net result:
energy transferred to lower-speed molecules
heat transfers from hot to cold
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4. Heat transfer medium Water ………………….…. Steam……………………. Oil………………..………
Thermal liquid………..….
Air…………………….
Pebbles/Sand/Iron balls…
……. water-bath …….fluid energy mill …..oil-bath ……….???? …oven, spray drier …high temperature equipment
Exercise
How does a greenhouse work?
Why does fibreglass wall insulation reduce heat loss?
How does a thermos bottle work?
Why do objects at same temperature feel thermally different?
Do ice cubes freeze faster if trays are filled with warm water?
Why do you feel cool standing in front of an open refrigerator?
Does a microwave oven cook from the inside out?
Do beverages remain cold longer if left in warm air or set in cooler water?
If you leave a refrigerator door open in an insulated room, will the room get colder?
When it rains, mist forms on the inside of the windscreen if the air-conditioner is switched off. On the other hand, with the air-conditioner on, mist forms on the outside. Why?
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5. Thermal conductivity, k
k = “thermal conductivity”
good thermal conductors ----high k
[k] = J/s-m-C (C or K)
good thermal insulators … low k
Exercise
What is k for vacuum? Polystyrene cup?
Vacuum, k = 0
Polystyrene, k=0.08
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6. Values of k (J/s-m-K) Material Temperature/K k Uses Copper 373 (100C)
379 ?
Graphite
323 (50C)
138
Glass wool
373
0.062
Piping insulation
Water
0.67
circulation
Air
473 (200C)
0.0311
fluid
Steam
0.0235
energy mill
At 50 deg. C, k for copper is 399. Explain.
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7. Thermal conductivity of air
Temperature
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8. Rate of heat transfer
H = Q/t = rate of heat transfer, Unit: J/s H = k A (TH-TC)/L Q/t = k A T/ x TH
Hot TC
Cold L
Area A
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9. Find the rate of heat transfer
Q/t = k A T/ x
T = TH-TC = 25C
Plug in….
Q/t = x 35 x 25/0.02
H=3500 J/s
H=3500 Watts
Inside:
TH = 25C
Outside:
TC = 0C
Wood:
thickness x = 0.02 m
area A = 35 m2
k = J/s●m●C
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10. Heat transfer through multiple layers
Air is better than wool! And cheaper!! Therefore important for insulation.Hence…layered clothing! Low k
For effective heat transfer, choose material with high thermal conductivity.
Δx = x1 + x2 + x3 T
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11. 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 boundary……fluidised bed
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12. Find the rate of heat transfer in multiple layers
Assume H1 = H2
k1A(T0-TC)/x1 = k2A(TH-T0)/x2
solve for T0 = temp at junction
T0=2.27 C
then solve for H1 or H2
H=318 Watts
x1 = 0.02 m A1 = 35 m2 k1 = J/s-m-C
x2 = m A1 = 35 m2 k1 = J/s-m-C
Inside:
TH = 25C
Outside:
TC = 0C
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13. Thermal Resistance R1 = x1/k1 etc 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/ kx = T/(R1+R2+R3+...)
R1 = x1/k1 etc
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14. Insulation
Insulation for piping is critical to ensure minimum heat loss
Typical insulators are
Glass wool/rock wool
Aluminum sheets
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15. 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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16. Film coefficient, h (J/m2-s-K)
Fluid h
Water (heat-exchanger)
Gases
17-285
Organic solvents
Oils
…….why?
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17. Overall heat transfer coefficient
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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18. U values Overall heat transfer coefficient
Convection
Sea-breeze…. 1
Radiation
Else, heat from sun produces roasted human…… 2
Indirect i.e. through wall conduction
House is our shelter… 20
Contactive mechanism i.e. gaseous phase heat carrier passes directly through the solids bed
200
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19. Common heat transport fluids
from Perry’s Chemical Engineers’ Handbook 6th Ed.
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20. Heat transfer equipment: Fluidised bed dryer
Hence, drying of solids using fluidised bed technique is very popular!
What is the mechanism of heat transfer?
Contactive heat transfer
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21. Relationship between Energy and Temperature
Temperature (K)
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22. References
Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of Dosage Form Design. Churchill Livingstone.
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