LESSON 1. HEAT EXCHANGERS

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Energy Technology A. Y. 2006-07 1

LESSON 1.

HEAT EXCHANGERS

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.

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Energy Technology A. Y. 2006-07

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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.

Definition

• Device used to implement the process of heat

exchange between two fluids that are at different temperatures and separated by a

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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.

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.

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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.

Regenerators (III)

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Regenerators (IV)

• Rotary matrix regenerator:

– Both heat transfer

process occurs simultaneously.

Regenerators (V)

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Regenerators (VI)

• Rotary matrix regenerator:

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.

Mixers (II)

• Cooling or

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Mixers (III)

• Cooling or refrigeration tower diagram:

Mixers (IV)

• Cooling or refrigeration tower diagram:

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Mixers (V)

• Evaporative cooler diagram:

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:

Recuperators (I)

• Both fluids pass simultaneously over the two

surfaces of the separating wall.

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Recuperators (II)

• Double-pipe heat exchangers:

Recuperators (III)

• Cross-flow heat exchangers

(Intercambiadores de flujo cruzado):

– Tubes finned (both fluids unmixed).

– Tubes unfinned (one fluid mixed and the other

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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.

Recuperators (V)

• Cross-flow heat exchangers: – Two types:

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Recuperators (VI)

• Shell-and-tube heat exchangers

(Intercambiadores de carcasa y tubos o de

pasos múltiples):

Recuperators (VII)

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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.

Recuperators (IX)

• Shell-and-tube

heat exchangers:

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Recuperators (X)

• Shell-and-tube heat exchangers: – Components.

Recuperators (XI)

• Shell-and-tube heat exchangers:

– Subdivided according to the number of passes

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Recuperators (XII)

• Shell-and-tube heat

exchangers:

– Baffles to increase the

turbulence and to create a component of cross flow.

Recuperators (XIII)

• Types of shell-and-tube heat exchangers:

1. With fixed tubular plates:

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Recuperators (XIV)

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.

Recuperators (XV)

3. With floating head:

• Longitudinal movement due to dilatation permitted.

• Higher manufacturing cost and construction difficulty.

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Recuperators (XVI)

– Comparison:

“U” tubes Fixed

plates Floating head

Relative cost ≈ 0.75 ≈ 0.85 1

Dilatations

absorbed Yes No Yes

Shell change

allowed Yes No Yes

Tubes change allowed

Yes Yes Yes

Tubes cleaning

method

Chemical Chemical Chemical or mechanical Shell cleaning method Chemical or mechanical Chemical Chemical or mechanical

Recuperators (XVII)

• Compact heat exchangers (Intercambiadores

de calor compactos):

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Recuperators (XVIII)

• Compact heat exchangers:

Recuperators (XIX)

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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 (XXII)

• Plate heat exchangers:

Recuperators (XXIII)

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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.

Recuperators (XXV)

• Plate heat exchangers: – Operation limitations:

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Recuperators (XXVI)

• Design and calculation of plate heat

exchangers:

– Re = f (D_h) with 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. e e a e a P A D mojado c h 2· ) ·( 2 · · 4 4 _≈ + = =

Recuperators (XXVII)

• Design and calculation of plate heat exchangers: – Definition of Plate Number, PN:

– n = number of plates; S_u= unit surface of one plate (projected for corrugated plates); = unit mass flow per channel.

– S_T= n·S_uand so PN and NTU are related:

• 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 min p u u min p u u c m S U c m S U n n ) · ( · · 2 ) · ( · · 1 · 2 PN & & ≈ + = u m& 2 1 + =m n m& &_u

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Recuperators (XXVIII)

• Types of plate heat exchangers:

a) One pass per channel of each fluid, pure counter-flow. b) Multi-pass U arrangement. c) Multi-pass Z arrangement. d) Complex arrangements.

Fouling factors

Fluid

R

_f

′′

(m2·K/W) Sea water (T < 50 ºC) 0.0001 Sea water (T > 50 ºC) 0.0002

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Heat transfer coefficients

Process Fluid h (W/m2·K) Natural or free

convection Gas 2 - 25

Natural or free

convection Liquid 50 – 1,000 Forced convection Gas 25 - 250 Forced convection Liquid 50 – 20,000

Phase change

convection Boiling 2,500 – 25,000 Phase change

convection Condensation 5,000 – 100,000

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

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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.

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.

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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.