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