HEAT EXCHANGERSin food process engineering

Energy balance methodology used to design industrial equipments

1

FIP-DES

Bertrand Broyart, Violaine AthèsCristian Trelea

Outlook

• Motivation: thermal treatment of liquid food (recall)

• Types of heat exchangers – illustrations• Design of heat exchangers– Heat transfer– Heat balances

• Tutorials

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Operation

Objective

Thermal treatment of liquid food (recall)

Stabilisation Texturing

- Concentration- Pasteurisation- Sterilisation- Chilling

- Cooking- Crystallisation- Emulsification

- Enzyme inactivation - Micro-organism destruction - Water activity reduction

- Viscosity increase- Phase change (e.g. gel formation, freezing …)

Types of equipment for thermal treatment

1- Heat treatment AFTER conditioning

Autoclaves

2- Heat treatment BEFORE conditioning

Heat exchangers

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Sterilisation cycle in an autoclave

Retort temperature

Product temperature

Time (min)

Heating

Cooling

Main types of heat exchangers• Sales : € 600 M per year in Europe for food industries• price of a sterilisation line: several M€

1- Heat exchangers with a wall: indirect transfer tubular geometry : concentric tubes plane geometry : plates, blades extended geometry : fins on plates or tubes

Configuration :• co-, counter-, cross- current

Compactness : plates: de 150 à 300 m2/m3 of installation tubes : 30 m2/m3 of installation

2- Heat exchangers without wall: direct transfer vapour injection direct electrical heating (ohmic)

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Principle of tubular heat exchangers

Single tube

Several tubes

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Principle of plate heat exchangers

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Plate heat exchangers

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Scraped surface heat exchanger

Highly loaded media(e.g. custard, cream)

Texturing(e.g. ice-cream)

Low compactness(1 m2 / m3)

MotorProduct

outlet

Product inlet

Product

InsulationThermal fluidRotor

Scraper

Thermal fluid inlet

Thermal fluid outlet

Scrapers

Rotor

Heat transfer wallInsulationThermal fluid

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An industrial production chain

Homogenisation

Storage Conditioning

Heat treatments

Raw milk

UHT milk

Design of heat exchangers: heat transfer

Stationary heat transfer through a plane wallFl

uid

1

e

Flui

d 2

T1 T2Tw1 Tw2

Q

Convection (fluid 1) :

Q = h1 . A . (T1 - Tw1)

Conduction (wall) :

Q = (w / e . A) . (Tw1 - Tw2)

Convection (fluid 2)

Q = h2. A . (Tw2 - T2)

hG = 1 / (1/h1 + e/w + 1/h2)

h2h1w

Q = hg . A . (T1 - T2 )After eliminating wall temperatures, one can write the heat flux as a function of fluid temperature difference only:

The global heat transfer coefficient corresponds to 3 thermal resistances in series (fluid 1 + wall + fluid 2):

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Thermal conductivities: some orders of magnitude (W.m-1.K-1)

Air = 0.025

Water = 0.6

Stainless steel = 14

Glass = 0.8

Copper = 380

0.1 < food products < 0.6

gas liquid solid< <

OilFat

MilkFruit juice

Milk = 0.56

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Convection coefficients: some orders of magnitude h (W.m-2.K-1)

Air

h = 5 … 50

Water

h = 200 … 2000

WaterState change L-V

h = 2000 … 10000

h air h water h water L-V< <

Still Highly ventilated

Stationary Flowing

Boiling Vapour condensation

Poor Medium Good

Consider à local heat flux dQ in a  « slice of fluid » between A and A + d :

Design of heat exchangers: heat balance

A

O (inlet) A (outlet)

hint

hext

Rint

Rext1

Rext2

Cold fluid

Hot fluid

A

(T)

A + d

T1T2

(dQ)

(dQ)

Th1

Tc1

.

.

Th2

Tc2

ThTh – dTh

Tc + dTcTc

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2121 hhphhccpcclmg TTcmTTcmTAhQ

Valid in co-and counter-current 16

Design of heat exchangers: heat balance (ctd)

Local heat flux through dA chg TTdAhQd

Local heat balance for hot and cold fluids

cpcchphh dTcmdTcmQd

Global heat balance for hot and cold fluids

2121 hhphhccpcc TTcmTTcmQ

Final result

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21

Ln TT

TTTlm

with

Logarithmic mean temperature difference

(Δ = hot – cold)

0 L

T1

T2

Cold

HotT1 T2

0 L

Cold

Hot

Co- and counter-current configurations of heat exchangers

Inlet cold

Inlet hot

Outlet hot

Co-current

Outlet cold

Counter-current

Inlet cold

Inlet hot

Outlet hot

Outlet cold

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Liquid – liquidLiquid – liquid

T1

T2

0 L

Condensing hot fluid

Cold

Condensation

0 L

T1T2

Cold fluid boiling

Hot

Boiling

Co- and counter-current configurations of heat exchangersSpecial cases

Inlet cold

Inlet hot

Outlet hot

Co-current

Outlet cold

Counter-current

Inlet cold

Inlet hot

Outlet hot

Outlet cold

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T

T

0 L

Cold

Hot

constant Tcmcm phhpcc

(formula for logarithmic mean temperature breaks down)

With state change

With state change

Same fluid on both sides

Same fluid on both sides

Tutorials

• Design a heat exchanger– calculate heat transfer coefficients– calculate the necessary area

• Compare co- and counter-current configurations

• Compare water and steam heating• Consider the effect of fouling