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Introduction to Heat Exchangers The basics of heat transfer

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Title: Introduction to Heat Exchangers The basics of heat transfer


1
Introduction to Heat Exchangers
  • The basics of heat transfer
  • What are exchangers for?
  • Main heat exchanger types

2
Lecture series
  • Introduction to heat exchangers
  • Selection of the best type for a given
    application
  • Selection of right shell and tube
  • Design of shell and tube

Q U A ?T
3
Contents
  • Why we need heat exchangers
  • The basics of their design
  • Some general features of exchangers
  • Different types of exchanger

4
What are heat exchangers for?
  • To get fluid streams to the right temperature for
    the next process
  • reactions often require feeds at high temp.
  • To condense vapours
  • To evaporate liquids
  • To recover heat to use elsewhere
  • To reject low-grade heat
  • To drive a power cycle

5
Example of an exchanger
Bundle for shell-and-tube exchanger
6
Feed-effluent exchanger
Feed-effluent exchanger
Exothermic reaction
Heat recovery
7
Distillation
Bottom product
8
Power cycle
Steam turbine
Condenser
Boiler
Feedwater heater
9
Q U A DT
yw
Thot
Tcold
  • We have thermal resistances in series

10
Local and mean values
  • Overall means from the hot side to the cold
    side including all resistances
  • However it is still at a particular point in the
    exchanger i.e. it is local
  • Hence you can have a local, overall coefficient
  • LOCALLY
  • FOR WHOLE EXCHANGER

11
Integrating local over the exchanger area
  • Local equation
  • Rearranging
  • and integrating

dQ
dA
Total area AT
12
Definitions of mean values
  • From previous slides
  • Comparing the two sides

13
Special case where Ts are linear with Q
  • Eqn. integrates to give log. mean temperature
    difference - LMTD

?Ta
14
Multipass exchangers
T1
  • For single-phase duties, theoretical correction
    factors, FT, have been derived
  • FT values are less than 1
  • Do not design for FT less than 0.8

T2
Temp.
t2
t1
Q
15
Typical FT correction factor curves
  • T,t
    Shell / tube side
  • 1,
    2 inlet / outlet

16
Thermal effectiveness
  • Stream temperature rise divided by the
    theoretically maximum possible temperature rise

T1,in
T1,out
T2,in
T2,out
17
Compactness
  • Can be measured by the heat-transfer area per
    unit volume or by channel size
  • Conventional exchangers (shell and tube) have
    channel size of 10 to 30 mm giving about 100m2/m3
  • Plate-type exchangers have typically 5mm channel
    size with more than 200m2/m3
  • More compact types available

18
Main categories of exchanger
Heat exchangers
Recuperators
Regenerators
Wall separating streams
Direct contact
  • Most heat exchangers have two streams, hot and
    cold, but some have more than two

19
Recuperators/regenerators
  • Recuperative
  • Has separate flow paths for each fluid which flow
    simultaneously through the exchanger transferring
    heat between the streams
  • Regenerative
  • Has a single flow path which the hot and cold
    fluids alternately pass through.

20
Double Pipe
  • Simplest type has one tube inside another - inner
    tube may have longitudinal fins on the outside

However, most have a number of
tubes in the outer tube - can have very many
tubes thus becoming a shell-and-tube
21
Shell and Tube
  • Typical shell and tube exchanger as used in the
    process industry

22
Shell-side flow
23
Complete shell-and-tube
24
Plate and frame
  • Plates hung vertically and clamped in a press or
    frame.
  • Gaskets direct the streams between alternate
    plates and prevent external leakage
  • Plates made of stainless steel or higher quality
    material
  • Plates corrugated to give points of support and
    increase heat transfer

25
Plate types
Corrugations on plate improve heart transfer give
rigidity Many points of contact and a tortuous
flow path
Chevron
Washboard
26
Flow Arrangement within a PHE
Gaskets arranged for each stream to flow
between alternate plates
Alternate plates (often same plate types inverted)
27
Air-cooled exchanger
  • Air blown across finned tubes (forced draught
    type)
  • Can suck air across (induced draught)

Finned tubes
28
ACHE bundle
29
Plate-fin exchanger
  • Made up of flat plates (parting sheets) and
    corrugated sheets which form fins
  • Brazed by heating in vacuum furnace

30
Can have many streams
7 or more streams are typical
31
Cooling Towers
  • Large shell with packing at the bottom over which
    water is sprayed
  • Cooling by air flow and evaporation
  • Air flow driven by forced or natural convection
  • Need to continuously make up the cooling water
    lost by evaporation

32
Agitated Vessel
  • Used for batch heating or cooling of fluids
  • An agitator and baffles promote mixing
  • A range of agitators are used
  • Often used for batch chemical reaction

33
Proprietary types
  • Types described so far are generic types
  • These can be made by any company with necessary
    skills (no real patent protection)
  • There are now many special, proprietary
    exchangers made by one company or a small number
    of companies under licence
  • One example is the printed circuit exchanger by
    Heatric

34
Printed circuit heat exchanger
  • Plates are etched to give flow channels
  • Stacked to form exchanger block
  • Block diffusion welded under high pressure and
    temperature
  • Bond formed is as strong as the metal itself

35
Distribution of typesin terms of market value in
Europe
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