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Heat Transfer

To heat up or to cool down is always needed in a chemical plant Part of energy problem Source of energy: coal, natural gas, shale gas, nuclear reactor, solar, wind, hydraulic, geothermal etc. waste heat recovery transfer media: usually steam efficiency: can you estimate it for every transfer?

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C: Heat Transfer ExperimentsTubular heat exchanger: heat transfer coefficient h = f(Re); purpose - heating or cooling; existing in each factory; steam-electricity co-generation system汽電共生 ; Drying: operation; sources of heating: conduction (less frequently), convection (hot air), radiation (lamp (wavelength), solar light, xenon lamp, etc)

Picture taken from Wikipedia

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Light spectrum for a xenon lamp (Google)

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C1: Tubular Heat Exchanger

Very traditional; steam is often used as the heating source; boiler to generate steam (may need licence to operate this equipment)Overall heat transfer efficient (Uo or Ui): outside heat transfer coefficient ho, outside scale resistance Ro, tube heat transfer resistance kw, inside scale resistance Ri, inside heat transfer coefficient hi 1/Uo = 1/ho + Ro + xw/kw (Do/Dm) + Ri (Do/Di) + 1/hi (Do/Di); Dm = log mean dia.

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liquid-side film heat transfer coefficient: ho, hi; usually assume other heat transfer resistance negligible, Q = m Cp (T out – T in) = hi Ai Tlm, with Tlm = (T2 - T1)/log (T2/T1) we may have co-current flow, counter-current flow, etc. In general: Nu = F(Re, Pr, L/D) Nu = Nusselt number = h D/k =convective heat transfer coefficient/conductive heat transfer coefficient Pr = Prandtl number = viscous diffusion rate/thermal diffusion rate = Cp/k

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Many correlations proposed: e.g. Colburn j -factor: Nu = 0.023 Re^0.8 Pr^1/3 (for Pr: 0.7 – 160) L/D < 60: entrance effect may not be neglected A simplified correlation: hi = a V^0.8 (1+ 0.0146 T)steam: saturated, superheating, super-cooling vent: safety purpose Source of heat: natural gas, diesel fuel (C8 – C21; BP: 200 – 320oC), coal, solar, waste heat, etc. heavy oil: very viscous, > C60, high percentage of aromatics, naphthalene, high amounts of NSO; bottom product from distillation

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Ps

B4B5 B3 B2

Tc

Tc

B1

steam inlet valvesteam pressure

by pass valve

steamtrap

steam outlet

safety valve

cold water

hot water

Steam trap: to discharge condensate, non-condensable gases, with minimum loss of steam; usually automatic valve;

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Different designs

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Let’s talk of steam traps

(YouTube: by Armstrong Internationalhttp://www.youtube.com/watch?v=liRyxcCBTa0)

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C2: Drying

Simultaneous heat and mass transfer Free moisture content, meaning some water may be chemically bond to solid material, may need “dehydration” moisture inside pores: may be slow to evaporate from air point view: percentage humidity (relative to saturation humidity) wet bulb temperature (adiabatic saturation temperature) vs dry bulb temperature vs dew point; humidity chart

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Wet bulb globe temperature: originally developed by US marine corps. To determine heat stress of work WBGT = 0.7 Tw (wet bulb temp., humidity effect)+ 0.2 Tg (solar radiation) + 0.1 Td (dry bulb temp)

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pre-heat period, constant rate period, falling rate period (first & second falling rate); constant rate period reach “critical point”, then mass transfer becomes important

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Dryer: with hot air drying + IR heater For industrial operation: usually in continuous mode (on a belt), tunnel kiln, etc.

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Pictures taken from: Vandenbroek International website; drum dryer Spray drying; (others: fluidized drying, etc)

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Double shell rotary drum dryer

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In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue. (Wikipedia)