INTRODUCTION TO. Chapter 15

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Rosamund Mathews
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1 INTRODUCTION TO HEAT EXCHANGERS Chapter 15

2 What is a Heat Exchanger? A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact.

3 Classification of heat exchangers

4 Heat exchangers are classified according to Transfer process Number of fluids Degree of surface contact Design features Flow arrangements Heat transfer mechanisms

5 Fig. 1 Fluidized-bed heat exchanger.

6 Fig. 2 Heat transfer surface area density spectrum of exchanger surfaces ( Shah, 1981).

7 Fig. 3 (a) Shell-and- tube exchanger with one shell pass and one tube pass; (b) shell-and- tube exchanger with one shell pass and two tube passes.

8 Fig. 4 Standard shell types and front- and rear-end head types (From TEMA, 1999).

9 Fig. 5 Gasketed plate-and-frame heat exchanger.

10 Fig. 6 Plates showing gaskets around the ports (Shah and Focke, 1988).

11 Fig. 7 Section of a welded plate heat exchanger.

12 Fig. 8 Bavex welded- d plate heat exchanger.

13 Fig. 9 Spiral plate heat exchanger with both fluids in spiral counter flow.

14 Fig. 10 (a) Lamella heat exchanger; (b) cross section of a lamella heat exchanger, (c) lamellas

15 Fig. 11 Printed-circuit cross flow exchanger

16 Fig. 12 Corrugated fin geometries for plate-fin heat exchangers: (a) plain triangular fin; (b) plain rectangular fin; (c) wavy fin; (d) offset strip fin; (e) multilouver fin; (f) perforated fin.

17 Fig 13 (a) Individually finned tubes; Fig. 13 (a) Individually finned tubes; (b) flat (continuous) fins on an array of tubes.

18 Fig. 14 Individually id fin tubes.

19 Fi 15 H t h l t t d Fig. 15 Heat wheel or a rotary regenerator made from a polyester film.

20 Classification according to transfer process Indirect contact type Direct contact type Direct transfer Storage Fluidized bed Immiscible fluids Gas-liquid Liquid-vapour Single-phase Multiphase

21 Classification according to number of fluids Two-fluid Three-fluid N-fluid (N > 3) Classification according to surface compactness Gas-to-liquid Liquid-to-liquid and phase-change Compact β 700 m 2 /m 3 Non-compact β < 700 m 2 /m 3 Compact β 400 m 2 /m 3 Non-compact β < 400 m 2 /m 3

22 Classification according to design or type Tubular Plate-type t Extended surface Regenerative PHE Spiral Plate coil Printed circuit Plate-fin Tube-fin Gasketed Welded Brazed Ordinary Separating wall Heat-pipe wall Double-pipe Shell-and-tube Spiral tube Pipe coils Cross-flow Parallel flow Rotary Fixed-matrix Rotating to tubes to tubes hoods

23 Classification according to flow arrangements Single-pass Multipass Counter flow Parallel flow Cross flow Split flow Divided flow Extended surface Shell-and-tube Plate Cross- Cross- Counter flow parallel flow Compound flow Fluid 1 m passes Fluid 2 n passes Parallel counter flow m-shell passes n-tube passes split-flow Divided-flow

24 Classification according to heat transfer mechanisms Single-phase convection on both sides Single-phase convection on one side, Two-phase convection on other side Two-phase convection on both sides Combined convection and radiative heat transfer

25 Classification according to process function Condensers Liquid-to-vapor phase-change exchangers Heaters Coolers Chillers

26 Convective heat transfer d Fluid1 vägg Fluid2

27 Overall heat transfer coefficient Q& = UA Δt m 1 = Δt TR m

28 Expression for overall thermal resistance 1 1 b 1 1 TR = + + w + + α A α A λ A α A α A i i i w vl Fo o ó Fi o

29 Values of the heat transfer coefficient W/m2K Air atmospheric pressure 5-75 Air pressurized Water, liquid Organic liquids Boiling Condensation

30 Correlations for the heat transfer coefficient Nu = hl/k = function (flow velocity, physical properties, geometry) = function (Re, Pr, geometry)

31 General research needs How to achieve more compact heat exchangers High thermal efficiency Balance between enhanced heat transfer and accompanied pressure drop Material issues especially for high h temperature t applications Manufacturing methodology Fouling Non-steady operation

1 Classification of Heat Exchangers

1 Classification of Heat Exchangers A variety of heat exchangers are used in industry and in their products. The objective of this chapter is to describe most of these heat exchangers in some detail using