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**Heat Transfer By Conduction**

Kausar Ahmad Kulliyyah of Pharmacy PHM3133 Dosage Design /11

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**Contents Practical heat transfer Heat transfer medium**

Heat transfer through multiple layers Heat transfer at boundary PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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**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? PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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**Values of k (J/s-m-K) Material Temperature/K k Uses Copper 373 (100C)**

379 ? Graphite 323 (50C) 138 Glass wool 373 0.062 Piping insulation Water 0.67 circulation Air 473 (200C) 0.0311 fluid Steam 0.0235 energy mill At 50 deg. C, k for copper is 399. Explain. PHM3133 Dosage Design /11

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**Thermal conductivity of air**

Temperature PHM3133 Dosage Design /11

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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 PHM3133 Dosage Design /11

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**Find the rate of heat transfer**

Q/t = k A T/ x T = TH-TC = 25C Plug in…. Q/t = x 35 x 25/0.02 H=3500 J/s H=3500 Watts Inside: TH = 25C Outside: TC = 0C Wood: thickness x = 0.02 m area A = 35 m2 k = J/s●m●C PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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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 boundary……fluidised bed PHM3133 Dosage Design /11

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**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 = 25C Outside: TC = 0C PHM3133 Dosage Design /11

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**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/ kx = T/(R1+R2+R3+...) R1 = x1/k1 etc PHM3133 Dosage Design /11

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Insulation Insulation for piping is critical to ensure minimum heat loss Typical insulators are Glass wool/rock wool Aluminum sheets PHM3133 Dosage Design /11

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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 PHM3133 Dosage Design /11

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**Film coefficient, h (J/m2-s-K)**

Fluid h Water (heat-exchanger) Gases 17-285 Organic solvents Oils …….why? PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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**Common heat transport fluids**

from Perry’s Chemical Engineers’ Handbook 6th Ed. PHM3133 Dosage Design /11

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**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 PHM3133 Dosage Design /11

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**Relationship between Energy and Temperature**

Temperature (K) PHM3133 Dosage Design /11

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References Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of Dosage Form Design. Churchill Livingstone. PHM3133 Dosage Design /11

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