Lesson 1. Heat exchangers

LESSON 1.
HEAT EXCHANGERS
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Contents (I)
•
•
•
•
•
Definition.
Classification.
Regenerators.
Mixers or direct contact heat exchangers.
Packed bed heat exchangers (Intercambiadores
de lecho compacto).
• Direct flame heat exchangers (Intercambiadores
de llama directa).
• Recuperators:
– Classification.
– Double-pipe heat exchangers.
– Cross-flow heat exchangers.
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Contents (II)
• Recuperators:
– Shell-and-tube heat exchangers.
– Compact heat exchangers.
– Plate or plate and frame heat exchangers.
• Fouling factors (Factores de impureza o
suciedad).
• Heat transfer coefficients (Coeficientes de
convección).
• Overall heat transfer coefficient (Coeficiente
global de transferencia de calor).
• Selecting the circulation side.
• Bibliography.
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Definition
• Device used to implement the process of heat
exchange between two fluids that are at
different temperatures and separated by a
solid wall.
• Many engineering applications: Heating, Air
Conditioning and Ventilation (H.A.C.V.) of
buildings, power generation, heat
recuperation, chemical processes...
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Classification
• According to the operation mode:
– Recuperators: both fluids pass simultaneously
over the two surfaces of the separating wall.
– Regenerators: both fluids pass alternatively over
the same surface of the separating wall.
– Mixers or direct contact: there is no separating
wall.
– Packed bed: it is a regenerator with a porous
medium as fixed matrix.
– Direct flame: unique fluid in contact with a flame
through a wall.
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Regenerators (I)
• Both fluids pass alternatively over the
same surface of the separating wall.
• Heat exchange in two stages:
1. The hot fluid heats the matrix.
2. The matrix heats the cold fluid.
• Matrix = separating wall.
• The matrix can be fixed or rotary.
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Regenerators (II)
• Matrix materials characteristics:
– High cp.
– High k in ⊥ direction to the flow and low in //
direction.
– High thermal diffusivity (α).
– Low thermal dilatation coefficient.
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Regenerators (III)
• Fixed matrix regenerator:
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Regenerators (IV)
• Rotary matrix regenerator:
– Both heat transfer
process occurs
simultaneously.
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Regenerators (V)
• Rotary matrix regenerator:
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Regenerators (VI)
• Rotary matrix regenerator:
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Regenerators (VII)
• Rotary matrix regenerator:
– Applications:
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Mixers (I)
• The heat is exchanged between the two fluids
without separating wall between them (direct
contact).
• There is heat and mass transfer (water
evaporation).
• Two examples:
– Cooling or refrigeration towers: used to
dissipate heat from an industrial process to
the atmosphere instead of to a river or sea.
– Evaporative cooler (enfriador evaporativo):
used in air conditioning installations. Air is
cooled by means of water evaporation.
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Mixers (II)
• Cooling or
refrigeration towers:
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Mixers (III)
• Cooling or refrigeration
tower diagram:
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Mixers (IV)
• Cooling or
refrigeration tower
diagram:
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Mixers (V)
• Evaporative cooler diagram:
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Packed bed heat exchanger
• It is a regenerator
where the matrix
is a porous
medium.
• Figure:
regenerator of
double fixed
matrix for
continuous
operation.
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Direct flame heat exchanger
• Heat
transmission by
means of
radiation mode
(flame) and
convection
mode (exhaust
or flue gases).
• Diagram of an
industrial boiler
or steam
generator:
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Recuperators (I)
• Both fluids pass simultaneously over the two
surfaces of the separating wall.
• Classification: according to the flow arrangement
and according to the construction type.
• Double-pipe heat exchangers (Intercambiadores
de tubos concentricos o doble tubo):
– Parallel flow (flujo paralelo).
– Counter-flow (contraflujo).
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Recuperators (II)
• Double-pipe heat exchangers:
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Recuperators (III)
• Cross-flow heat exchangers
(Intercambiadores de flujo cruzado):
– Tubes finned (both fluids unmixed).
– Tubes unfinned (one fluid mixed and the other
unmixed).
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Recuperators (IV)
• Cross-flow heat exchangers:
– Used to cool a liquid with air instead of water
(shell-and-tube heat exchangers).
– Advantage: Elimination of water dependence
and high cost of its treatment.
– Disadvantage: Require more space and
produce more noise.
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Recuperators (V)
• Cross-flow heat exchangers:
– Two types:
• Forced flow fan:
– The fan is below the tubes.
– Advantage: high turbulence and heat transfer.
– Disadvantage: low outlet speeds ⇒ recirculation
problems.
• Induced flow fan:
– The fan is over the tubes.
– Advantages: needs less power and no
recirculation problems.
– Disadvantages: higher cost and more noise.
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Recuperators (VI)
• Shell-and-tube heat exchangers
(Intercambiadores de carcasa y tubos o de
pasos múltiples):
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Recuperators (VII)
• Shell-and-tube heat exchangers:
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Recuperators (VIII)
• Shell-and-tube heat exchangers:
– Advantage: Great number of tubes ⇒ increase
of heat transfer area.
– Disadvantage: Increase of heat transfer area ⇒
increase of cross-sectional area ⇒ decrease of
flow speed ⇒ decrease of heat transfer
coefficient.
– Less efficient than the counter-flow heat
exchnagers.
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Recuperators (IX)
• Shell-and-tube
heat exchangers:
– Components.
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Recuperators (X)
• Shell-and-tube
heat exchangers:
– Components.
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Recuperators (XI)
• Shell-and-tube heat exchangers:
– Subdivided according to the number of passes
of each fluid through the tubes and the shell.
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Recuperators (XII)
• Shell-and-tube heat
exchangers:
– Baffles to increase the
turbulence and to
create a component of
cross flow.
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Recuperators (XIII)
•
Types of shell-and-tube heat exchangers:
1. With fixed tubular plates:
•
•
•
•
Types: welded, riveted, with expansion joint,
double-plate.
Longitudinal movement due to dilatation not
permitted.
Operation limited to shell and tube
temperature difference under 50 ºC.
Welded type: no possibility of dismantling ⇒
chemical cleaning and fluid not corrosive.
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Recuperators (XIV)
•
Types of shell-and-tube heat exchangers:
2. With “U” tubes:
•
•
•
•
•
Inlet and outlet of the tubes at the same side
of the exchanger.
Tubes curved in “U” shape at the other side.
Bigger shell than the previous.
Lower manufacturing cost than the previous
(saving in connections).
Chemical cleaning of the tubes due to the
curves shape.
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Recuperators (XV)
•
Types of shell-and-tube heat exchangers:
3. With floating head:
•
•
Longitudinal movement due to dilatation
permitted.
Higher manufacturing cost and construction
difficulty.
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Recuperators (XVI)
•
Types of shell-and-tube heat exchangers:
– Comparison:
≈ 0.75
Fixed
plates
≈ 0.85
Yes
No
Yes
Yes
No
Yes
Yes
Yes
Yes
Chemical
Chemical
Chemical or
mechanical
Chemical or
mechanical
Chemical
Chemical or
mechanical
“U” tubes
Relative cost
Dilatations
absorbed
Shell change
allowed
Tubes
change
allowed
Tubes
cleaning
method
Shell
cleaning
method
Floating head
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Recuperators (XVII)
• Compact heat exchangers (Intercambiadores
de calor compactos):
– The objective is to obtain a high heat transfer
area per volume unit (up to 700 m2/m3).
– Formed by complex arrangements of tubes
(plane or circular) with fins (rectangular or
annular):
• Plate fin, tube fin or parallel plate heat
exchangers.
– Used when one of the fluids is a gas.
– Small cross-sections ⇒ laminar flow.
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Recuperators (XVIII)
• Compact heat exchangers:
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Recuperators (XIX)
• Compact heat exchangers:
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Recuperators (XX)
• Plate or plate and frame heat exchangers
(Intercambiadores de placas):
– Formed by several thin, corrugated plates
joined together by means of gaskets (juntas)
or welded, depending of the liquids inside.
– The whole is compressed by a rigid frame,
formed by a fixed plate in one side, a mobile
plate on the other, a guide bar and a carrying
bar.
– An arrangement of parallel flow channels is
created. One fluid travels in the odd numbered
channels and the other in the even.
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Recuperators (XXI)
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Recuperators (XXII)
• Plate heat exchangers:
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Recuperators (XXIII)
• Plate heat exchangers:
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Recuperators (XXIV)
• Plate heat exchangers:
– Corrugated plates
provide:
• Greatest surface area
for heat transfer.
• Maximum turbulence
of the flow.
• Optimum fluid
distribution.
• Minimal fouling.
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Recuperators (XXV)
• Plate heat exchangers:
– Operation limitations:
• Welded plates: 350 ºC
and 40 bar.
• With gaskets: 260 ºC
and 20 bar.
– Disadvantage: higher
pressure drops than
shell-and-tubes.
– Advantage: easy
cleaning (dismantle)
and counter-flow
arrangement.
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Recuperators (XXVI)
• Design and calculation of plate heat
exchangers:
4 Ac
– Re = f (Dh) with Dh = P
mojado
=
4·a ·e
≈ 2·e
2·(a + e )
if a >> e.
– a = width of the plate; b = height of the plate; e
= separation between plates.
– There are convection correlations specific for
plate heat exchangers ⇒ consult bibliography.
– Same calculation methods used for
recuperators but with specific formulas ⇒
consult bibliography.
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Recuperators (XXVII)
• Design and calculation of plate heat exchangers:
U ·Su
2·U · S u
≈
n + 1 ( m& u ·c p )min ( m& u ·c p ) min
– Definition of Plate Number, PN: PN = 2·n ·
– n = number of plates; Su = unit surface of one plate
(projected for corrugated plates); m& u = unit mass flow per
channel.
n+1
& = m& u
– ST = n·Su and m
so PN and NTU are related:
2
• If NTU = PN, the heat exchanger is well designed.
• If NTU < PN, the heat exchanger is oversized, the
power exchanged is higher than the prescribed one.
• If NTU > PN, the heat exchanger is undersized and it
transfers less power then the required.
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Recuperators (XXVIII)
•
Types of plate heat exchangers:
a) One pass per channel of each fluid, pure counterflow.
b) Multi-pass U arrangement.
c) Multi-pass Z arrangement.
d) Complex arrangements.
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Fouling factors
Fluid
Sea water (T < 50 ºC)
Sea water (T > 50 ºC)
River water
Treated boiler feed water
Liquid gasoline and oil
Refrigerant liquids
Methanol, ethanol
Natural gas
Steam (non-oil bearing)
Steam (oil bearing)
Compressed air
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R′f′
(m2·K/W)
0.0001
0.0002
0.0005
0.0002
0.0004
0.0002
0.0004
0.0004
0.0001
0.0004
0.0002
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Heat transfer coefficients
Process
Natural or free
convection
Natural or free
convection
Forced convection
Forced convection
Phase change
convection
Phase change
convection
Fluid
h (W/m2·K)
Gas
2 - 25
Liquid
50 – 1,000
Gas
Liquid
25 - 250
50 – 20,000
Boiling
2,500 – 25,000
Condensation 5,000 – 100,000
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Overall heat transfer coefficient
Fluid combination (hot – cold)
U (W/m2·K)
Water - Water
1,000 – 2,500
Water - Oil
110 - 350
Ammonia - Water
1,000 – 2,500
Gases - Water
10 - 250
Steam condensing- Water (in tubes)
1,000 – 6,000
Steam condensing - Gases
25 - 250
Ammonia condensing – Water (in tubes) 800 – 1,400
Alcohol condensing – Water (in tubes)
250 - 700
Finned-tube heat exchanger (Water in
25 – 50
tubes, Air in cross flow)
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Selecting the circulation side
• Thermal point of view: hot fluid in the shell side if
the objective is to cool it.
• Corrosion: more corrosive fluid inside the tubes.
• Pressure: Highest pressure fluid inside the tubes.
• Dilatations: Hot fluid inside the tubes.
• Fouling: dirtiest fluid through the side with
easiest cleaning.
• Viscosity: highest viscosity fluid through the
more turbulent side (shell).
• Pressure drop: fluid with lower flow resistance
through the tubes.
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Bibliography (I)
• F. P. Incropera and D. P. De Witt,
Fundamentals of Heat and Mass Transfer, 5th
ed., John Wiley & Sons, Inc., New York, 2002.
• F. Kreith y M. S. Bohn, Principios de
Transferencia de Calor, 6ª ed., Thomson,
Madrid, 2002.
• A. Bejan, Heat Transfer, 1st ed., John Wiley &
Sons, Inc., New York, 1993.
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Bibliography (II)
• E. Torrella, J. M. Pinazo, R. Cabello,
Transmisión de Calor, 1ª ed., Servicio de
Publicaciones, Universidad Politécnica de
Valencia, Valencia, 1999.
• J. Irigaray, Tecnología Energética II, Unicopia,
Servicio de Publicaciones de TECNUN, San
Sebastián, 2002.
• http://www.alfalaval.com.
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