2. Contents
Project Description
Contents
Introduction
Type of Heat Exchangers
Reuperator
Regenerators
Plate Type Heat Exchangers
Plate Fin Heat Exchangers
Plate Shell Heat Exchangers
Spiral Heat Exchangers
Multi Pass Tube Exchangers
Shell and Tube Heat Exchangers
Fin Tube Heat Exchangers
Other Heat Exchanger
Heat Exchangers Problems
Fouling
Precautions
Heat Exchangers Cost
3. What is Heat Exchanger ?
A heat exchanger is a device that is used to transfer thermal energy 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.
In heat exchangers, there are usually no external heat and work interactions.
Typical applications involve heating or cooling of a fluid stream of concern and
evaporation or condensation of single or multicomponent fluid streams.
In other applications, the objective may be to recover or reject heat, or sterilize,
pasteurize, fractionate, distill, concentrate, crystallize, or control a process fluid.
In a few heat exchangers, the fluids exchanging heat are in direct contact. In
most heat exchangers, heat transfer between fluids takes place through a
separating wall or into and out of a wall in a transient manner.
In many heat exchangers, the fluids are separated by a heat transfer surface,
and ideally they do not mix or leak.
5. Recuperative Heat Exchangers….?
A recuperator is a special purpose counter-flow energy recovery heat
exchanger positioned within the supply and exhaust air streams of an air handling
system, or in the exhaust gases of an industrial process, in order to recover the waste
heat.
In many types of processes, combustion is used to generate heat, and the
recuperator serves to recuperate, or reclaim this heat, in order to reuse or recycle it.
The term recuperator refers as well to liquid-liquid counter flow heat exchangers used
for heat recovery in the chemical and refinery industries and in closed processes such
as ammonia-water or LiBr-water absorption refrigeration cycle.
Recuperators are often used in association with the burner portion of a heat engine,
to increase the overall efficiency.
For example, in a gas turbine engine, air is compressed, mixed with fuel, which is then
burned and used to drive a turbine. The recuperator transfers some of the waste heat
in the exhaust to the compressed air, thus preheating it before entering the fuel
burner stage.
Since the gases have been pre-heated, less fuel is needed to heat the gases up to
the turbine inlet temperature. By recovering some of the energy usually lost as waste
heat, the recuperator can make a heat engine or gas turbine significantly more
efficient.
6. Regenerative type Heat Exchangers
The regenerator is a storage-type heat exchanger.
The heat transfer surface or elements are usually referred to as a
matrix in the regenerator.
To have continuous operation, either the matrix must be moved
periodically into and out of the fixed streams of gases, as in a rotary
regenerator.
Gas flows must be diverted through valves to and from the fixed
matrices as in a fixed matrix regenerator.
7. Plate type Heat Exchangers ?
These are composed of multiple, thin, slightly separated plates that have
very large surface areas and small fluid flow passages for heat transfer.
This stacked plate arrangement typically has lower volume and cost than the
shell and tube heat exchanger.
Advances in gasket and brazing technology have made the plate type heat
exchanger increasingly practical.
When used in open loops, these heat exchangers are normally of the gasket
type to allow periodic disassembly, cleaning, and inspection.
There are many types of permanently bonded plate heat exchangers, such
as dip brazed, vacuum brazed, and welded plate varieties.
They are often specified for closed loop applications such as refrigeration.
Plate heat exchangers also differ in the types of plates that are used.
Some plates may be stamped with "chevron", dimpled, or other patterns,
where others may have machined fins or grooves.
8. Plate Heat Exchangers
• Turbulence helps prevent particles
from settling or plating out. Particle
build up creates
“Resistance/Fouling” inside
equipment
• Industrial plates are usually designed
with a 8mm pressing depth
• PHE are designed to pass particles
up to half of the plate pressing
depth
o Example- A 8mm plate can pass a
particle 4mm (or 4000 microns)
o Note: Lakos JPX Separators remove
particles down to 74um microns. eJPX
down to 44um.
9. Why Plate Heat Exchangers
•Very effective way to transfer heat
•Compact in size
•High sheer stress on plates
-This helps to reduce fouling
U-Values range from 500-1000
10.
11.
12. Spiral heat exchangers
A spiral heat exchanger (SHE), may refer to a helical (coiled) tube configuration, more
generally, the term refers to a pair of flat surfaces that are coiled to form the two
channels in a counter flow arrangement.
Each of the two channels has one long curved path. A pair of fluid ports are
connected tangentially to the outer arms of the spiral, and axial ports are common,
but optional.
The main advantage of the SHE is its highly efficient use of space. This attribute is often
leveraged and partially reallocated to gain other improvements in performance,
according to well known tradeoffs in heat exchanger design.
A compact SHE may be used to have a smaller footprint and thus lower all-around
capital costs, or an oversized SHE may be used to have less pressure drop, less
pumping energy, higher thermal efficiency, and lower energy costs.
Self-cleaning Water filters are used to keep the system clean and running without the
need to shut down or replace cartridges and bags.
The SHE is good for applications such as pasteurization, digester heating, heat
recovery, preheating and effluent cooling.
13.
14. Shell and Tube Heat Exchanger
Shell and tube heat exchangers consist of series of tubes.
A set of tubes is called the tube bundle and can be made up of
several types of tubes: plain, longitudinally finned, etc.
Shell and tube heat exchangers are typically used for high pressure
applications with pressures greater than 30 bar and temperatures
greater than 260 °C.
Shell and tube heat exchangers are robust due to their shape.
Mostly the ends of each tube are connected to plenums or water
boxes through holes in tube sheets.
The tubes may be straight or bent in the shape of a U, called U
tubes.
16. Why shell-and-tube?
CEC survey: S&T accounted for 85% of new exchangers supplied to oil-
refining, chemical, petrochemical and power companies in
leading European countries. Why?
Can be designed for almost any duty with a very
wide range of temperatures and pressures
Can be built in many materials
Many suppliers
Repair can be by non-specialists
Design methods and mechanical codes have
been established from many years of experience
17. Scope of shell-and-tube
Maximum pressure
Shell 300 bar (4500 psia)
Tube 1400 bar (20000 psia)
Temperature range
Maximum 600oC (1100oF) or even 650oC
Minimum -100oC (-150oF)
Fluids
Subject to materials
Available in a wide range of materials
Size per unit 100 - 10000 ft2 (10 - 1000 m2)
Can be extended with special designs/materials
18.
19. There are many types of shell-tube
heat exchanger but the most
common types in use are :-
U-Tube Heat Exchanger
Straight-Tube ( 1-Pass )
Straight-Tube ( 2-Pass )
21. When to use U-Tube type Heat Exchangers?
It is used where the temperature difference
between the shell side and tube side fluids is
quite great….WHY?
Because the tubes are free to expand since
the tube bundle is fastened to only one tube
sheet.
22. Straight Tube Shell-and-Tube Heat Exchangers
One Shell Pass and One Tube Pass
Baffles are used to establish a cross-flow and to induce turbulent mixing of the
shell-side fluid, both of which enhance convection.
The number of tube and shell passes may be varied, e.g.:
One Shell Pass,
Two Tube Passes
Two Shell Passes,
Four Tube Passes
23. Factors affecting the choice of the shell arrangements
The amount of cooling and heating required
The pressure drop
The type of service [for instance the shell arrangement
that provides space for vapors to accumulate is the kettle
type re-boiler
24. Use of Baffles In Shell And Tube Heat Exchangers
Why are baffles used?
To support the tube
To direct the fluid stream across the tube
To improve the rate of heat transfer
Shell
Tubes
Baffle
25. Problems of Conventional Shell & Tube
Zigzag path on shell side leads to
Poor use of shell-side pressure drop
Possible vibration from cross flow
Dead spots
Poor heat transfer
Allows fouling
Recirculation zones
Poor thermal effectiveness,
26. Comparison of shell side geometries
Twisted
tube
Segmental
baffles
Helical
baffles
ROD
baffles
Good p Y N Y Y
High shell N Y Y N
Low fouling Y N Y Y
Easy
cleaning
Y With square
pitch
With square
pitch
Y
Tube-side
enhance.
Included With inserts With inserts With inserts
Can give
high
Y N N Y
Low
vibration
Y With special
designs
With double
helix
Y
27. Air cooled heat exchanger
Used for cooling and condensation and used when cooling water is in
short supply or expensive
They can also be competitive with water cooled units even when water
is plentiful
Most common used in petroleum and gas processing industries
Main components
Air cooled exchangers consist of banks of finned tubes over which air is
blown or drawn by fans mounted below or above the tube
If the fan is mounted below the tubes the unit is termed forced draft unit
and if the fan is mounted above the tubes the unit is termed induced daft
28. Air cooled heat exchanger
Forced draft air cooled heat exchanger
[cross flow]
29. LMTD………?
The LMTD is a logarithmic average of the temperature difference between the hot and
cold feeds at each end of the double pipe exchanger.
The logarithmic mean temperature difference is used to determine the temperature
driving force for heat transfer in flow systems, most notably in heat exchangers.
The larger the LMTD, the more heat is transferred. The use of the LMTD arises
straightforwardly from the analysis of a heat exchanger with constant flow rate and fluid
thermal properties.
We assume that a generic heat exchanger has two ends "A" and "B“ at which the hot
and cold streams enter or exit on either side; then, the LMTD is defined by the logarithmic
mean as follows:
LMTD = (ΔTa - ΔTb) / ln(ΔTa/ΔTb)
ΔTA is the temperature difference between the two streams at end A, and ΔTB is the
temperature difference between the two streams at end B.
Q = U.A.(LMTD)
Where Q is the exchanged heat duty, U is the heat transfer coefficient and A is the
exchange area. Note that estimating the heat transfer coefficient may be quite
complicated.
30. Controlling A in Simulator
A = Ntubes π Dtubes Ltubes
Shell
Shell Diameter and pitch determines Ntubes
Tubes
Dtubes
Ltubes
Tube pitch-The transverse pitch is the shortest distance
from the center lines of two adjacent tubes.
Tube pitch ratio 1.25 to 1.5 typically
31. Controlling U in a Simulator
For a given heat duty and geometry - U determines the HX
area
Steps
Identify the controlling heat transfer resistance
ho-Manipulate the shell side Reynolds number
Shell diameter
Tube pitch
Number of baffles
hi-Manipulate the tube side Reynolds number
Tube diameter
Number of tubes (shell diameter and tube pitch)
Number of passes
If odd things happen check to see that you have the same controlling heat
transfer resistance
33. Temperature Levels
In Heat Exchangers
Near-optimal minimum temperature approaches in heat exchangers
depend on the temperature level as follows:
10°F or less for temperatures below ambient,
20°F for temperatures at or above ambient up to 300°F,
50°F for high temperatures,
250 to 350°F in a furnace for flue gas temperature above inlet process fluid
temperature.
36. Other Problems
Temperatures Cross Each Other
Non-functioning Exchanger
To solve increase approach ΔT
Condensation/Evaporation
Heat transfer with multiple heat
transfer coefficients in a single
apparatus
Various regimes of boiling
Various regimes of condensation
37. Fouling
• Fouling occurs when any type of particles
both organic or inorganic plug or plate out
on heat transfer surfaces creating a
resistance to transfer energy
• There are two types of fouling
– Macro-fouling
– Micro-fouling
38. Macro Vs Micro Fouling
• Macro-fouling
– Sand
– Silt
– Scale
– Rust
– Mineral deposits
Example- CaCO3
• Micro-fouling
– Biological growth
– Algae
– Bacteria
– Mussels
• Micro-fouling is
controlled by water
treatment
39. Problems of Fouling
• Many
contaminants mix
together to form
larger deposits
– Example- CaCO3
mixed with sand
makes concrete
• It is these large
particles that
create problems
40. Fouling
• Are dissolved solids and
particles under 40
micron a problem?
• Typically no, as they do
not precipitate out of
solution until they reach
120F, or if the ph is out of
balance
• The Bigger the
Particle….The Bigger the
Problem
41. Factors affecting the kind and degree of fouling
1. The materials used in the heat exchanger
► Some materials corrode faster than others providing corrosion
products which decrease heat transfer
► Rough surface provides cavities for the build up of deposits
2. Fluid velocity
Affect the fouling rate [as the velocity increase the fouling rate decrease]
42. Prevention from Fouling of
Plate Heat Exchanger
• Using a separator
prior to a PHE
reduces the
Particulate Fouling
Factor (PFF) and
provides a huge
energy savings
– Example U-value of
500 x PFF.0001 = 5%
energy savings
– Example U-value of
1000 x PFF.0001= 10%
energy savings
43. Fouling In Plate Heat
Exchangers
• It is sometimes cheaper to
buy a separator than it is
to buy replacement
gaskets for a PHE
• Full flow separators reduce
PHE maintenance by a
factor of 7
• Every PHE should have a
energy saving separator to
maintain the designed
temperature approach
44. Fouling in Shell and Tube Heat
Exchangers
• Prone to fouling
especially during low
flow or downturn
• Particles tend to settle
with laminar flow
46. How to handle the problem of fouling
►Antifoulants prevent the formation of deposits
►Inhibitors [as corrosion inhibitors] prevent chemical reactions
which might cause deposits to build up
►Frequent cleaning of the H.X [maintenance]
47. Corrosion of heat exchangers
Another series problem in heat exchangers is corrosion
Severe corrosion can and does occur in tubing and very often with common
fluids such as water.
To avoid corrosion
►Proper material selection based on full analysis of the operating fluids,
velocities and temperatures is a must.
►Heavier gauge tubing is specified to offset the effect of corrosion followed by
proper start up operating and shut down procedure.
►Protection of the heat exchanger from corrosion [e.g. cathodic protection]
►Treatment of the cooling water used and using inhibitors.
48. Heat exchangers vibration
Vibration of the tubes as a result of the flow of the shell side past them
is important phenomena specially when the H.X size and flow quantities
of flow are increased
Vibration effects
►Vibration has a bad effect on both tubes and shell
►The joints between the tubes and tube sheet can fail due to vibration
causing leakage
►It causes leakage in the joints between shell and tubes
►Increase the shut down time to repair the H.X
49. Factors affecting tube vibration
Tubes geometry [layout]
Material of construction
Means of support
Heat exchanger size
Flow quantities
50. How to avoid vibration
Using inlet support baffles
Using double segmental baffles [improve tube support]
Using j shell type [ divided flow type to reduce the shell velocity]
Inlet support
baffles
Double-segmental baffles
51. How to Protect HT Equipment
• Basin Sweeping
Filtration
• Full Flow Filtration
• Side Stream
Filtration
• Closed Loop
Filtration
52. How to Protect HT Equipment
• Note- Basin Sweeping is
the preferred method
for protecting most
industrial equipment as
it is the ONLY form of
filtration that protects
heat exchangers,
nozzles, and the
cooling tower.
• 95% of all particulate
problems start in the
cooling tower.
Notes de l'éditeur
85 per cent is a higher figure than in the pie chart in lecture 1. The difference is that the above figure is for the limited range of industries shown while the pie chart is for all industrial applications.
