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?

1

C: Heat Transfer Experiments

 Tubular 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 2

3

Light spectrum for a xenon lamp (Google) 4

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)  Basic equation: q = h A  T  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 (scale resistance usually small)  1/Uo = 1/ho + Ro + xw/kw (Do/Dm) + Ri (Do/Di) + 1/hi (Do/Di); Dm = log mean dia.

5

 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 6

Logarithmic mean temperature difference  Tlm  7

 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 (chemical element); bottom product from distillation 8

B1 safety valve steam inlet valve B5 steam pressure Ps B4 B3 B2 hot water Tc steam trap by pass valve cold water steam outlet  Steam trap: to discharge condensate, non condensable gases, with minimum loss of steam; usually automatic valve; Tc 9

Different designs 10

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 11

 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) 12

Humidity chart 13

Adiabatic saturation temperature – wet bulb temperature

 Air at T, H; water sprayed into to RH = 100%, system T = adiabatic saturation temperature  cs (T-Ts) + H  s = Hs  s; (H-Hs)/(T-Ts) = -cs/  s adiabatic saturation line (on T-H diagram)  Air at T, H flow over wet bulb to read T = wet bulb temperature  hy (T-Tw) = Mb k  w (Hw-H); (H-Hw)/(T-Tw)= hy/(Mb k  w) psychrometric line  cs  hy/Mb k Lewis relation 14

 pre-heat period, constant rate period, falling rate period (first & second falling rate);  constant rate period reach “critical point”, then mass transfer becomes important 15

Taken from: manuals for best practice dryer 16

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

17

 Pictures taken from: Vandenbroek International website; drum dryer  Spray drying; (others: fluidized drying, etc) 18

Double shell rotary drum dryer 19

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) 20