HEAT EXCHANGERS. OUTLINE Definition & classification of heat exchangers Heat exchangers...

HEAT EXCHANGERS

OUTLINEDefinition & classification of heat exchangers

Heat exchangers according to construction•Plate heat exchangers•Extended surface heat exchangers•Tubular heat exchangers

Heat exchangers according to phase change•Condensers•Evaporators

WHAT IS A HEAT EXCHANGER?

They are devices specifically designed for the efficient transfer of heat from one fluid to another fluid over a solid surface.

WHAT ARE HEAT EXCHANGERS USED FOR?

They have the function to transfer heat as efficiently as possible. Heat exchangers are widely used in :

I. refrigeration

II. air conditioning

III. space heating

IV. electricity generation

V. chemical processing

CLASSIFICATION OF HEAT EXCHANGERS

Heat exchangers may be classified according to the following main criteria:

• Recuperators and regenerators• Transfer processes: direct contact and indirect

contact• Geometry of constructions: tubes,plates and

extended surfaces• Phase change mechanisms: condensers and

evaporators• Flow arrangements: parallel, counter and cross flow

RECUPERATORS

• The conventional heat exchangers with heat transfer between two fluids.• Hot steam A recovers some of the heat from stream B.

REGENERATORS

• Storage type heat exchangers. The same flow passage (matrix) is alternately occupied by one of the two fluids.

• Thermal energy is not transfered through the wall.

TRANSFER PROCESSES

• Heat transfer between the cold and hot fluids through a direct contact between these fluids.

• Examples: Spray and tray condensers,cooling towers

1. Direct contact type heat exchangers:

•Heat energy is exchanged between hot and cold fluids through a heat transfer surface.

•The fluids are not mixed

2. Indirect contact type heat exchangers:

FLOW ARRANGEMENTS

1. Paralel Flow Heat Exchangers:

• Two fluid streams enter together at one end, flow through in the same direction, and leave through at the other end

2. Counter Flow Heat Exchangers:

• Two fluid streams flow in opposite directions.

3. Cross Flow Heat Exchangers:

• The direction of fluids are perpendicular to each other.

BASIC CRITERIAS FOR THE SELECTION OF HEAT EXCHANGERS

Process specifications

Service conditions of the plant environment,

resistance to corrosion by the process

Maintenance, permission to cleaning and

replacement of components

Cost- Effectiveness

Site requirements, lifting, servicing,capabilities

PLATE HEAT EXCHANCERS

– GASKETED PLATE

– SPIRAL PLATE

– LAMELLA

• Limited to below 25 bar and 250ºC

• Plate heat exchangers have three main types : gasketed ,spiral heat exchangers and lamella

• The most common of the plate-type heat exchangers is the gasketed plate heat exchanger

GASKETED PLATE HEAT EXCHANGER

• The most common of the plate-type heat exchangers is the gasketed plate heat exchanger

SPIRAL PLATE HEAT EXCHANGER

• Ideal flow conditions and the smallest possible heating surface

LAMELLA

• Consisting of cylindrical shell surrounding a number heat transfering lamellas.

• Similar to tubular heat exchanger

ADVANTAGES

• Plate heat exchangers yield heat transfer rates three to five times greater than other types of heat exchangers.

• The design of the plate heat exchanger allows to add or remove plates to optimize performance, or to allow for cleaning, service, or maintenance with a minimum of downtime.

• Plate exchangers offer the highest efficiency mechanism for heat transfer available in industry.

DISADVANTAGES

• Plate exchangers are limited when high pressures, high temperatures, or aggressive fluids are present.

• Because of this problem these type of heat exchangers have only been used in small, low pressure applications such as on oil coolers for engines.

2. EXTENDED SURFACE HEAT EXCHANGERS

- PLATE FIN HEAT EXCHANGER

- TUBE FIN HEAT EXCHANGER

PLATE FIN HEAT EXCHANGER

• For gas to gas applications.

• Widely used in cryogenic, energy recovery, process industry, refrigeration and air coditioning systems.

TUBE FIN HEAT EXCAHNGER

• For gas to liquid heat exchangers.

• Used as condersers in electric power plant, as oil coolers in propulsive power plants, as ir cooled exchangers in process and power industires.

TUBULAR HEAT EXCHANGERS

• are so widely used because the technology is well established for making precision metal tubes capable of containing high pressures in a variety

of materials.

There is no limit to the range of pressures and temperatures that can be accommodated.

TUBULAR HEAT EXCHANGERS

SHELL AND TUBE

DOUBLE-PIPE

SHELL AND TUBE HEAT EXCHANGERS

SHELL AND TUBE HEAT EXCHANGERS

•are the most commonly used heat exchangers in oil refineries and other large chemical processes. •are used when a process requires large amounts of fluid to be heated or cooled. • provide transfer of heat efficiently.•use baffles on the shell-side fluid to accomplished mixing or turbulence.

SHELL AND TUBE HEAT EXCHANGERS

• tube : strong, thermally

conductive, corrosion

resistant, high quality

• outer shell : durable, highly

strong

• inner tube : having effective

combination of durability,

corrosion resistant and

thermally conductive

APPLICATIONS:

• Oil refining,

• Vapor recovery systems,

• Permanent engines,

• Industrial paint systems.

SHELL AND TUBE HEAT EXCHANGERS

U - TUBE HEAT EXCHANGERS

FIXED TUBE HEAT EXCHANGERS

FLOATING HEAD HEAT EXCHANGERS

U - TUBE HEAT EXCHANGERS heat exchanger systems consisting of straight

length tubes bent into a U-shape surrounded by a

shell.

U - TUBE HEAT EXCHANGERS

• Both initial and maintenance costs are reduced by

reducing the number of joints.

• They have drawbacks like inability to replace

individual tubes except in the outer row and inability to

clean around the bend.

U - TUBE HEAT EXCHANGERS

• Examples : reboilers, evaporators and Kettle

type. • They have enlarged shell sections for

vapor-liquid separation.

FIXED TUBE HEAT EXCHANGERS have straight tubes that are secured at both

ends to tube sheets welded to the shell.

FIXED TUBE HEAT EXCHANGERS

• They are the most economical type design.• They have very popular version as the heads

can be removed to clean the inside of the

tubes.• Cleaning the outside surface of the tubes is

impossible as these are inside the fixed part.• Chemical cleaning can be used.

FLOATING HEAD HEAT EXCHANGER

one tube is free to float within the shell and the other is fixed relative to the shell.

FLOATING HEAD HEAT EXCHANGERS

• A floating head is excellent for applications

where the difference in temperature between the

hot and cold fluid causes unacceptable stresses

in the axial direction of the shell and tubes.• The floating head can move, so it provides the

possibility to expand in the axial direction.• Design allows for bundle to be removed for inspection,

cleaning or maintenance.

FLOATING HEAD HEAT EXCHANGERS

• Examples : kettle boilers which have dirty

heating medium.• They have the most highest construction cost of

all exchanger types.

DOUBLE-PIPE HEAT EXCHANGERS

• They consist of one pipe concentrically located inside a

second, larger one.

• Cold and hot liquid respectively

flows in the gap of inner pipe

and sleeve pipe.

• Structure is simple and heat

transmission is large.

DOUBLE-PIPE HEAT EXCHANGERS

• utilize true counter-current

flow which maximizes the

temperature differences

between the shell side and

tube side fluids.

DOUBLE-PIPE HEAT EXCHANGERS

• When the process calls

for a temperature cross,

it is the most efficient

design and will result in

fewer sections and less

surface area.

DOUBLE-PIPE HEAT EXCHANGERS

ADVANTAGES:

• Operates in true counter current flow permitting

extreme temperature cross.

• Economically adaptable to service differentials.

• Ideal for wide temperature ranges and differentials.

• Provides shorter deliveries than shell and tube due to

standardization of design and construction.

PHASE CHANGE HEAT EXCHANGERS

1.Reboilers

(Evaporaters)

2.Condensers

1)REBOILER to generate vapor to drive fractional distillation separation

Most Common Reboiler’s TypesKettle Reboilers Forced Recirculation ReboilersThermosiphon Reboiler

Kettle Reboilers

Major factors influence reboiler type selection:

Plot space available

Total duty required

Fraction of tower liquid traffic vaporized

Fouling tendency

Temperature approach available

Temperature approach required

Kettle ReboilersAdvantages Insensitive to

hydrodynamicsHigh heat fluxes are

possibleCan handle high

vaporizationSimple pipingUnlimited area

DisadvantagesAll the dirt collects and

non volatiles accumulate

Shell side is difficult to clean

Difficult to determine the degree of mixing

Oversize shell is expensive

Thermosiphon Reboiler

Thermosiphon Reboiler

operate using natural circulation with process flow on the shell side

process flow on the tube or shell side in vertical units.

not require a pump for recirculation have sensible heat transfer followed by

nucleate boiling.

Forced Recirculation Reboilers

Forced Recirculation Reboilers

• These reboiler types have two mechanisms of heat transfer: sensible heat transfer followed by nucleate boil-ing.

• Process flow is typically on the tube side of a standard exchanger in the vertical position.

2)CONDENSERS

a) Water-Cooled Condensensers

Horizontal shell and tube

Vertical shell and tube

Shell and coil Double pipe

b) Air- Cooled Condensers

Phases:1) de-super-heating2) Condensing3) Subcooling

Single-Pass Condenser

SELECT AN WATER-COOLED CONDENSER

…IF:

1. Adequate water supplies are available from tower, city or well sources.2. Water supply is of good quality.3. Heat recovery is not practical or unimportant.4. Plant ambient temperatures consistently exceed 95°F.5. Ambient air is polluted with large dust and dirt particles.

ADVANTAGE & DISADVANTAGES

1. Offer lower capital investment.2. Operates more efficiently on hot summer days.3. Easier to operate.4. Does not offer summer ventilation.

SELECT AN AIR-COOLED CONDENSER

...WHEN:

1. Adequate water supply not available from tower or well sources.2. Water supply is not of good quality.3. Heat recovery is practical and important.4. Plant ambient temperature will not consistently exceed 95°F.5. Ambient air is not polluted with large dust and dirt particles.

ADVANTAGE & DISADVANTAGES

1. Somewhat more costly to purchase and operate.2. Gives less cooling on hot summer days.3. Consumes more electricity.4. Offers summer ventilation and winter supplement heating.

OUTLINEDefinition & classification of heat exchangers

Heat exchangers according to construction•Plate heat exchangers•Extended surface heat exchangers•Tubular heat exchangers

Heat exchangers according to phase change•Condensers•Evaporators

REFERENCES Andreone, C.F., Tubular heat exchanger inspection, maintenance, and repair,

McGraw-Hill, NY, 1998 Couper JR,Penry W.R., Fair J.R., Walas S.M., Chemical Process Equipment,

Elsevier Inc, 2005 Incropera,F.P.,Dewitt D.P., Fundamentals of Heat and Mass Transfer, 5th ed.,John

Wiley & Sons Inc., NY,2000 Kakaç, S. Heat exchangers, CRC Press, Fla, 1998 Shah, R.K.,Psekulis D., Fundamental of Heat Exchanger Design, John Wiley

&Sons Inc., NY,1999 http://chentserver.uwaterloo.ca/courses/Che025Lab/perry/Chap11.pdf http://en.wikipedia.org/wiki/Heat_exchanger#Flow_arrangement http://www.advantageengineering.com/fyi/110/advantageFYI110.php http://www.buildingdesign.co.uk/mech/guntner/dry-air-coolers.htm http://www.engineeringpage.com/heat_exchangers/tema.html http://www.martechsystems.com/downloads/tech_managingreboilerops.pdf http://www.me.wustl.edu/ME/labs/thermal/me372b5.htm http://www.pacificconsultant.net/compact_heat_exchanger.htm http://www.rwholland.com/hairpin.htm http://www.taftan.com/thermodynamics/EXCHANGE.HTM http://www.thomasnet.com/about/exchangers-heat-shell-tube-26641001.html