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 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
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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)
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.
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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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Logarithmic mean temperature difference
Tlm
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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
(chemical element); bottom product from distillation
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B1
safety valve
steam pressure
steam inlet valve
Ps
B5
B4
B3
B2
hot water
Tc
steam
trap
Tc
by pass valve
cold water
steam outlet
 Steam trap: to discharge condensate, noncondensable gases, with minimum loss of steam;
usually automatic valve;
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Different designs
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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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Humidity chart
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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
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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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Taken from: manuals for best practice dryer
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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)
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