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Heat Transfer By Conduction

Heat Transfer By Conduction. Kausar Ahmad Kulliyyah of Pharmacy. http://staff.iiu.edu.my/akausar. Contents. Practical heat transfer Heat transfer medium Heat transfer through multiple layers Heat transfer at boundary. Practical Heat Transfer.

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Heat Transfer By Conduction

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  1. Heat Transfer By Conduction Kausar Ahmad Kulliyyah of Pharmacy http://staff.iiu.edu.my/akausar PHM3133 Dosage Design 1 2010/11

  2. Contents • Practical heat transfer • Heat transfer medium • Heat transfer through multiple layers • Heat transfer at boundary PHM3133 Dosage Design 1 2010/11

  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. PHM3133 Dosage Design 1 2010/11

  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 PHM3133 Dosage Design 1 2010/11

  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? PHM3133 Dosage Design 1 2010/11

  6. Values of k (J/s-m-K) PHM3133 Dosage Design 1 2010/11

  7. Thermal conductivity of air Thermal conductivity Temperature PHM3133 Dosage Design 1 2010/11

  8. TH Hot L TC Cold Area A 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 PHM3133 Dosage Design 1 2010/11

  9. Outside: TC = 0C Inside: TH = 25C Wood: thickness x = 0.02 m area A = 35 m2 k = 0.080 J/s●m●C Find the rate of heat transfer Q/t = k A T/ x • T = TH-TC = 25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

  10. T 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 PHM3133 Dosage Design 1 2010/11

  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 PHM3133 Dosage Design 1 2010/11

  12. Outside: TC = 0C Inside: TH = 25C x2= 0.075 m A1 = 35 m2 k1 = 0.030 J/s-m-C 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 = 0.080 J/s-m-C PHM3133 Dosage Design 1 2010/11

  13. Thermal Resistance 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 PHM3133 Dosage Design 1 2010/11

  14. Insulation • 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

  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 PHM3133 Dosage Design 1 2010/11

  16. Film coefficient, h (J/m2-s-K) PHM3133 Dosage Design 1 2010/11

  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 PHM3133 Dosage Design 1 2010/11

  18. U valuesOverall heat transfer coefficient PHM3133 Dosage Design 1 2010/11

  19. Common heat transport fluids • from Perry’s Chemical Engineers’ Handbook 6th Ed. PHM3133 Dosage Design 1 2010/11

  20. Heat transfer equipment: Fluidised bed dryer • 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

  21. Relationship between Energy and Temperature Energy Temperature (K) PHM3133 Dosage Design 1 2010/11

  22. References Aulton, M. E. (Ed.) (1988). Pharmaceutics – The Science of Dosage Form Design. Churchill Livingstone. PHM3133 Dosage Design 1 2010/11

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