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Heat exchangersTypes of heat exchangersUses in chemical processes

Heat transfer fluidsLow temperature heat transfer fluidsMechanism for heat transfer

Heat transfer by conductionConduction steady stateThermal conductivityConduction through cylindrical vesselsLog mean wall area

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Heaters (sensible heat changes)Coolers (sensible heat changes)Condensers (also change of state, V to L)

Evaporators (also change of state, L to V)

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Shell and tubeDouble pipePlate

Finned tubes/gas heatersspiralVessel jacketsReboilers and vapourisers/evaporators

EtcDirect/indirect

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Chemical reactors (jackets, internal heatexchangers/calandria)Preheating feedsDistillation column reboilersDistillation column condensersAir heaters for driersDouble cone driersevaporatorsCrystallisersDissolving solids/solutionProduction support services HVAC, etcHeat transfer fluidsetc

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Steam (available at various temperatures andpressures)Cooling water (15 oC)Chilled water (5 oC)

Brines (calcium chloride/water fp. -18 deg cent.at 20% by mass; sodium chloride/water fp. -16.5deg cent. at 20 % by mass)Methanol/water mixturesEthylene glycol/water mixturesPropylene glycol/water mixtures (fp. -22 degcent at a concentration of 40% by mass)Silicone oils (syltherm)

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Considerations:Temperature(s) requiredFreezing point

ViscositySpecific heatDensity

Hazardous properties

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ConductionConvectionRadiation

Driving force for heat transfer temperaturedifference. Heat will only flow from a hotterto a colder part of a system.

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Fouriers lawdQ/dt = -kA(dT/dx)

dQ/dt rate of heat transfer

k thermal conductivityA area perpendicular to direction of heat

transfer, xdT/dx temperature gradient in direction x

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Q = k mA ( T/x )or q = k m ( T/x )

q heat flux, J/s m-2

Q - rate of heat transfer, J/skm mean thermal conductivity,A area perpendicular to the direction of heat

transfer, m2

T temperature change (T 1 T2), Kx length, m

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Material k (300K), Wm -1 K -1

Copper 400

Water 0.6

air 0.03

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Q = k2 LT/ln(r 2/r 1)L length of tube/cylinderr2 external radius

r1 internal radius

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Q = k mAL (T1 T2)r2 r1

where A L= 2 L (r 2 r1)ln(r 2/r 1)

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Assumptions:Thin wall and therefore x w is small and sincek is large, term x w/kA Lis negligible comparedto other termsAlso A terms cancel as they areapproximately equal

1/U = 1/h1

+ 1/h2

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Enthalpy lost by hot fluid = enthalpy gained bycold fluid

(Assume negligible heat losses to surroundings)

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Co-current operationCounter-current operationTemperature profiles

LMTD

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T1

T2

t1

t2

T1 = 455K; T2 = 388K; t1 = 283K; t2 = 372K

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P = (372 283)/(455 283) = 0.52R = (455 388)/(372 283) = 0.75

Therefore F= 0.87 (from graph)To obtain maximum heat recovery from the hot

fluid, t2 must be as high as possible.T2 t2 is known as the approach temperature.

Paul Ashall, 2008

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Used for calculating h values in circular tubesunder turbulent flow conditions.

Nu = 0.023 Re 0.8 Pr 0.4

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q = e (T 14 T24)

q heat fluxe emissivity (0 to 1) typically 0.9 Stefan-Boltzmann const.(5.67 x 10 -8 W

m -2 K-4)

T1 temp. of body (K)T2 temp. of surroundings

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PassesshellMultiple tubes

BafflesCo- and counter current operation

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Individual separated thin corrugatedparallel s.s. platesGaskets separate platesGap approx. 1.4mmPlate and frame arrangementHigh h values at relatively low Re valuesand low flow ratesCan operate with small TReduced foulingEasy cleaning

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Radial finsLongitudinal finsFor air/gas heaters where film heattransfer coefficients on gas side will bevery low compared with condensing steamon the other side of tube. Rate of heattransfer is increased by increasing surfacearea on side of tube with the limiting(low) heat transfer coefficient (gas side).