KAUSAR AHMAD KULLIYYAH OF PHARMACY Heat Transfer By Conduction 1 PHM3133 Dosage Design 1 2010/11

KAUSAR AHMADKULLIYYAH OF PHARMACY

Heat Transfer By Conduction

1

PHM3133 Dosage Design 1 2010/11

http://staff.iiu.edu.my/akausar

http://staff.iiu.edu.my/akausar

Contents2

Practical heat transfer

Heat transfer medium

Heat transfer through multiple layers

Heat transfer at boundary


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.


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


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?


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


Thermal conductivity of air


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


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


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


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


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


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


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


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?


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


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


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


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





Relationship between Energy and Temperature21

Temperature (K)

Ene

rgy


References22

Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of

Dosage Form Design. Churchill Livingstone.


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KAUSAR AHMAD KULLIYYAH OF PHARMACY Heat Transfer By Conduction 1 PHM3133 Dosage Design 1 2010/11