Transcript Heat Transfer By Conduction
Heat Transfer By Conduction
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K A U S A R A H M A D K U L L I Y Y A H O F P H A R M A C Y
http://staff.iiu.edu.my/akausar PHM3133 Dosage Design 1 2010/11
Contents
2 Practical heat transfer Heat transfer medium Heat transfer through multiple layers Heat transfer at boundary PHM3133 Dosage Design 1 2010/11
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Practical Heat Transfer
3 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. PHM3133 Dosage Design 1 2010/11
Heat transfer medium
Water ………………….…. Steam…………………….
Oil………………..……… Thermal liquid………..….
Air…………………….
Pebbles/Sand/Iron balls… 4 ……. water-bath …….
fluid energy mill …..oil-bath ……….????
…oven, spray drier …high temperature equipment PHM3133 Dosage Design 1 2010/11
Thermal conductivity, k
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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?
PHM3133 Dosage Design 1 2010/11
Material Copper Graphite Glass wool
Values of k (J/s-m-K)
6 Uses Temperature/ K 373 (100 C) 323 (50 C) 373 k 379 138 0.062
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Piping insulation Water Air Steam 373 473 (200 C) 373 PHM3133 Dosage Design 1 2010/11 0.67
0.0311
0.0235
circulation fluid energy mill
Thermal conductivity of air
7 PHM3133 Dosage Design 1 2010/11 Temperature
Rate of heat transfer
8 H = Q/t = rate of heat transfer, Unit: J/s
T H Hot
H = k A ( T H T C )/ L Q/t = k A T/ x L
T C Cold
Area A PHM3133 Dosage Design 1 2010/11
Find the rate of heat transfer
9 Outside: T C = 0 C Inside: T H = 25 C Q/t = k A T/ x T = T H -T C = 25 C Wood: thickness x = 0.02 m area A = 35 m 2 k = 0.080 J/s ● m ● C Plug in….
Q/t = 0.080 x 35 x 25/0.02
H=3500 J/s H=3500 Watts PHM3133 Dosage Design 1 2010/11
Heat transfer through multiple layers
10 Δx = x1 + x2 + x3 T 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.
PHM3133 Dosage Design 1 2010/11
Examples: heat transfer through multiple layers 11 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 PHM3133 Dosage Design 1 2010/11
Find the rate of heat transfer in multiple layers 12 x 1 = 0.02 m A 1 = 35 m 2 k 1 = 0.080 J/s-m-C Assume H 1 = H 2 k 1 A(T 0 -T C )/x 1 = k 2 A(T H -T 0 )/x 2 solve for T 0 = temp at junction T 0 =2.27 C
Outside: T C = 0
C Inside: T H = 25
C
then solve for H 1 or H 2 H=318 Watts x 2 = 0.075 m A 1 = 35 m 2 k 1 = 0.030 J/s-m-C PHM3133 Dosage Design 1 2010/11
Thermal Resistance
13 Q/t = k A T/ x H/A = T k/ x = T/ R R = x/k [Joules/s ● m 2 ], R is the thermal resistance R “adds” for multiple layers Q/tA = T/ k x
R
1 = T/(R 1 +R 2 +R 3 +...)
= x
1
/k
1 etc PHM3133 Dosage Design 1 2010/11
Insulation
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Insulation for piping
is critical to ensure minimum heat loss Typical insulators are Glass wool/rock wool Aluminum sheets PHM3133 Dosage Design 1 2010/11
Heat exchange between a fluid and a solid boundary 15 At the boundary, heat transfer is influenced by conduction and convection : H =
h
A(T 1 – T 1, wall ),
h
is the film coefficient T1, wall A PHM3133 Dosage Design 1 2010/11 T 1
Fluid
Film coefficient, h (J/m
2
-s-K)
16 h Water (heat-exchanger) 1700-11350 Gases 17-285 Organic solvents Oils 340-2840 57-680 …….why?
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Overall heat transfer coefficient
17 Taking into account k and h, k, thermal conductivity and h,film coefficient Q =
U
AdT U is the overall heat transfer coefficient PHM3133 Dosage Design 1 2010/11
U values
Overall heat transfer coefficient 18 Convection Sea breeze….
1 Radiation Else, heat from sun produces roasted human…… Indirect i.e. through wall conduction House is our shelter… 2 20 Contactive mechanism i.e. gaseous phase heat carrier passes directly through the solids bed 200 PHM3133 Dosage Design 1 2010/11
Common heat transport fluids
19 from Perry’s Chemical Engineers’ Handbook 6th Ed.
Fluid Steam Water Oil Molten salts Silicon compounds Flue gas or air Temperature ( o F) Pressure (psig)
200-1100 300-400 30-600 290-1100 100-700
0-4500 90-230 0 0 0 30-2000 0-100
PHM3133 Dosage Design 1 2010/11
Heat transfer equipment: Fluidised bed dryer 20 Hence, drying of solids using fluidised bed technique is very popular!
http://www.pharmaceuticalonline.com/product.mvc/Fluid-Bed-Dryers 0002?VNETCOOKIE=NO PHM3133 Dosage Design 1 2010/11
Relationship between Energy and Temperature 21 PHM3133 Dosage Design 1 2010/11 Temperature (K)
References
22 Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of Dosage Form Design. Churchill Livingstone.
PHM3133 Dosage Design 1 2010/11