kausar ahmad kulliyyah of pharmacy heat transfer by conduction 1 phm3133 dosage design 1 2010/11
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KAUSAR AHMADKULLIYYAH OF PHARMACY
Heat Transfer By Conduction
1
PHM3133 Dosage Design 1 2010/11
http://staff.iiu.edu.my/akausar
Contents2
Practical heat transfer
Heat transfer medium
Heat transfer through multiple layers
Heat transfer at boundary
PHM3133 Dosage Design 1 2010/11
Practical Heat Transfer3
1. You stir some hot soup with a silver spoon and notice that the spoon warms up.
2. You stand watching a bonfire, but can’t get too close because of the heat.
3. 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…
……. water-bath
…….fluid energy mill
…..oil-bath
……….????
…oven, spray drier
…high temperature equipment
PHM3133 Dosage Design 1 2010/11
4
Thermal conductivity, k5
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
Values of k (J/s-m-K)6
Material Temperature/K
k Uses
Copper 373 (100C) 379 ?
Graphite 323 (50C) 138 ?
Glass wool 373 0.062 Piping insulation
Water 373 0.67 circulation
Air 473 (200C) 0.0311 fluid
Steam 373 0.0235 energy mill
PHM3133 Dosage Design 1 2010/11
Thermal conductivity of air
PHM3133 Dosage Design 1 2010/11
7
Temperature
The
rmal
con
duct
ivity
Rate of heat transfer8
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
PHM3133 Dosage Design 1 2010/11
Find the rate of heat transfer9
Q/t = k A T/ x
T = TH-TC = 25C
Plug in….
Q/t = 0.080 x 35 x 25/0.02
H=3500 J/s
H=3500 Watts
Inside:
TH = 25C
Outside:
TC = 0C
Wood: thickness x = 0.02 marea A = 35 m2
k = 0.080 J/s●m●C
PHM3133 Dosage Design 1 2010/11
Heat transfer through multiple layers
10
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.T
PHM3133 Dosage Design 1 2010/11
Δx = x1 + x2 + x3
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
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
x2 = 0.075 m A1 = 35 m2 k1 = 0.030 J/s-m-C
Inside:
TH = 25COutside:
TC = 0C
PHM3133 Dosage Design 1 2010/11
x1 = 0.02 m A1 = 35 m2 k1 = 0.080 J/s-m-C
Thermal Resistance13
Q/t = k A T/ xH/A = T k/x = T/R
R = x/k [Joules/s●m2], R is the thermal resistance
R “adds” for multiple layersQ/tA = T/ kx = T/(R1+R2+R3+...)
R1 = x1/k1 etcPHM3133 Dosage Design 1 2010/11
Insulation14
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 = hA(T1 – T1, wall), h is the film coefficient
AT1
T1, wall
PHM3133 Dosage Design 1 2010/11
Film coefficient, h (J/m2-s-K)16
Fluid h
Water (heat-exchanger)
1700-11350
Gases 17-285
Organic solvents 340-2840
Oils 57-680 …….why?
PHM3133 Dosage Design 1 2010/11
Overall heat transfer coefficient17
Taking into account k and h,
k, thermal conductivity and h,film coefficient
Q = UAdT
U is the overall heat transfer coefficient
PHM3133 Dosage Design 1 2010/11
U values Overall heat transfer coefficient
18
ConvectionSea-breeze….
1
RadiationElse, heat from sun produces roasted human……
2
Indirect i.e. through wall conductionHouse is our shelter…
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 fluids19
from Perry’s Chemical Engineers’ Handbook 6th Ed.
Fluid Temperature (oF) Pressure (psig)
Steam 200-1100 0-4500 Water 300-400 90-230 Oil 30-600 0 Molten salts 290-1100 0 Silicon compounds 100-700 0
Flue gas or air 30-2000 0-100
PHM3133 Dosage Design 1 2010/11
Heat transfer equipment: Fluidised bed dryer20
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 Temperature21
Temperature (K)
Ene
rgy
PHM3133 Dosage Design 1 2010/11
References22
Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of
Dosage Form Design. Churchill Livingstone.
PHM3133 Dosage Design 1 2010/11
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