HEAT EXCHANGERS in food process engineering Energy balance methodology used to design industrial equipments 1 FIP-DES Bertrand Broyart, Violaine Athès Cristian Trelea

Outlook Motivation: thermal treatment of liquid food (recall) Types of heat exchangers – illustrations Design of heat exchangers – Heat transfer – Heat balances Tutorials

3 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

5 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 m 2 /m 3 of installation  tubes : 30 m 2 /m 3 of installation 2- Heat exchangers without wall: direct transfer  vapour injection  direct electrical heating (ohmic)

7 Principle of tubular heat exchangers Single tube Several tubes

8 Principle of plate heat exchangers

9 Plate heat exchangers

10 Scraped surface heat exchanger Highly loaded media (e.g. custard, cream) Texturing (e.g. ice-cream) Low compactness (1 m 2 / m 3 ) Motor Product outlet Product inlet Product Insulation Thermal fluid Rotor Scraper Thermal fluid inlet Thermal fluid outlet Scrapers Rotor Heat transfer wall Insulation Thermal fluid

11 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 wall Fluid 1 e Fluid 2 T1T1 T2T2 T w1 T w2 Q Convection (fluid 1) : Q = h 1. A. (T 1 - T w1 ) Conduction (wall) : Q = ( w / e. A). (T w1 - T w2 ) Convection (fluid 2) Q = h 2. A. (T w2 - T 2 ) h G = 1 / (1/h 1 + e/ w + 1/h 2 ) h2h2 h1h1 w Q = h g. A. (T 1 - T 2 ) 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):

13 Thermal conductivities: some orders of magnitude (W.m -1.K -1 ) Air = Water = 0.6 Stainless steel = 14 Glass = 0.8 Copper = < food products < 0.6 gas liquid solid << Oil Fat Milk Fruit juice Milk = 0.56

14 Convection coefficients: some orders of magnitude h (W.m -2.K -1 ) Air h = 5 … 50 Water h = 200 … 2000 Water State change L-V h = 2000 … h air h water h water L-V << Still Highly ventilated Stationary Flowing Boiling Vapour condensation PoorMediumGood

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) h int h ext R int R ext1 R ext2 Cold fluid Hot fluid A (  T) A + d  T1T1 T2T2 (dQ) T h1 T c1.. T h2 T c2 ThTh T h – dT h T c + dT c TcTc 15

 Valid in co-and counter-current  16 Design of heat exchangers: heat balance (ctd) Local heat flux through dA Local heat balance for hot and cold fluids Global heat balance for hot and cold fluids Final result with Logarithmic mean temperature difference (Δ = hot – cold)

0L  T 1  T 2 Cold Hot  T 1  T 2 0L 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 17 Liquid – liquid

 T 1  T 2 0L Condensing hot fluid Cold Condensation 0L  T 1  T 2 Cold fluid boiling Hot Boiling Co- and counter-current configurations of heat exchangers Special cases Inlet cold Inlet hot Outlet hot Co-current Outlet cold Counter-current Inlet cold Inlet hot Outlet hot Outlet cold 18  T  T 0L Cold Hot (formula for logarithmic mean temperature breaks down) With state change 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