subject: m.e. 108, 143 fundamentals of heat exchanger design wednesday, july 27, 2011 lecturer:...

Subject: M.E. 108 , 143 Fundamentals of Heat Exchanger Design

Wednesday , July 27, 2011

Lecturer: Prof. Gennaro J. Maffia

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ME-108 Principles of Heat Exchanger Design, Part IThis seminar provides an overview of the equipment used for the transfer of heat in the process industries, including that used in chemical plants, refineries, and cogeneration power plants. The transformation of the basic heat balance into a T-Q curve and the resulting zone analysis to produce an effective design is covered. Selection of the appropriate TEMA designation, as well as economic and environmental considerations will be addressed. The seminar will conclude with a discussion of the techniques, such as Pinch Technology, in developing a heat exchanger network.

ME-143 Principles of Heat Exchanger Design, Part IIThis seminar provides a continuation of the discussion of the equipment used for the transfer of heat. Specific examples and case studies will highlight analysis techniques and step-wise procedures. Emphasis will be placed on the calculation of the film coefficient for a variety of different geometries and how to compose an overall heat transfer coefficient. Transfer line exchangers, plate-fin exchangers and Joule-Thompson exchangers are among the examples covered.

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ME-108 Principles of Heat Exchanger Design, Part I

overview of the equipment used for the transfer of heat in the process industries,

including that used in chemical plants, refineries, and cogeneration power plants.

basic heat balance into a T-Q curve and the resulting zone analysis to produce an effective design

TEMA designation, as well as economic and environmental considerations will be addressed.

Pinch Technology

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Fundamentals of Heat Exchanger Design

This seminar provides an overview of the used for the transfer of heat in the process industries, including that used in chemical plants, refineries, and cogeneration power plants.

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The transformation of the basic heat balance into a T-Q curve and the resulting zone analysis to produce an effective design is covered. Selection of the appropriate TEMA designation, as well as economic and environmental considerations will be addressed.

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The seminar will conclude with a discussion of the techniques, such as Pinch Technology, in developing a heat exchanger network.

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Dr. Gennaro Maffia, D.E., MBA After twenty years of industrial experience with large multinational companies, Dr. Maffia served as the Chairman of the Chemical Engineering Department at Widener University until 2007. He is currently Prof. of Chem. Eng at Manhattan College, Prof Emeritus at Widener University and Adjunct Prof. of Chem. and Biol. Engr. at Drexel University. Prof. Maffia is a member of many professional and honor societies as well as being an associate of several consulting companies. His research interests include the development of environmental remediation and biomedical technologies bases on water retention properties of unraveled bovine hide collagen. He holds a doctorate from Dartmouth College and a MBA from NYU.

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Gennaro J. (Jerry) Maffia 610-499-4089 (-4059 fax) Widener University KH-269C [email protected] One University Place, Chester, PA 19013 PROFESSIONAL EXPERIENCE: current-1992 Professor of Chemical Engineering & Department Chair Department of Chemical Engineering, Widener University, Chester, PA current-1996 Founder and Director, Collagen Research Group, Widener University, Chester, PA current-2002 Chairman, ACS Continuing Education Committee (Philadelphia Section) current-1991 Associate, Becht Engineering, Lincoln Park, NJ current-1992 Visiting Professor of Chemical Engineering, Drexel University, Philadelphia, PA

1992-1988 Manager, Technology Development, ARCO Chemical Co., Newtown Square, PA 1988-1979 Principal Engineer, ARCO Chemical Co., Newtown Square, PA 1979-1978 Senior Engineer, Air Products & Chemicals Co., Allentown, PA 1978-1973 Senior Engineer, ABB Lummus Co., Bloomfield, NJ EDUCATION: 1988 Doctor of Engineering, Dartmouth College, Hanover, NH 1977 MBA (Economics), New York University, NY, NY 1973 Master of Chemical Engineering, Manhattan College, Riverdale, NY 1972 Bachelor of Chemical Engineering, Manhattan College, Riverdale, NY

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PROFESSIONAL ACTIVITIES: Adjunct/Visiting Professorships (since 1983): Widener U. (University College), Drexel University,

Temple U., Pennsylvania State U., DCCC Short Courses and Industrial Courses (since 1993): PECO, Inc., ARCO Chemical Co., Sun Co., ACS Continuing Education Courses, Harvard Technology Inc. Training Courses Consulting: 30+ clients since 1992 including government agencies, chemical processing

industry, refineries, pharmaceutical companies, biotechnology start-ups (notable: US-DOJ, US-DOE, Verax, Inc.), currently 4 clients (Fall, 2005 – Summer, 2006)

Professional Societies/Other Organizations: American Chemical Society (Board of Directors), AICHE (former president - DVS), ISPE, ASEE, AAUP, ALCA, CCN (co-founder)

Honor Societies: Tau Beta Pi (chapter advisor), Phi Kappa Phi, Phi Beta Delta Recent Honors: 2002/3 Lindback Award for Teaching, 2003 Zandi Award, Who’s Who in Am. Ed. Service: Several university committees (notable: Budget Committee, GO Team, Middle States,

Assessment Committee (SOE)), professional society committees (notable: ACS Continuing Education, AICHE-DVS Executive Committee, AICHE Admissions), community service organizations (notable: Meals on Wheels, Childreach), community outreach activities (CHEER program, Engineering Summer Camp, Graduate Teaching Fellows Program)

RESEARCH/GRANTS:

Major Research Topic: Collagen Based Technologies – manufacture of high surface area proteins and applications in environmental remediation, biotechnology and biomedical engineering

Publications/Presentations: 150+ publications and presentations; 20+ invited lectures/seminars, 4 US Patents, 2 World Patents

Thesis Students Supervised: 15 completed (1996 – present), 6 on-going – all Masters level Current Research Grants: NSF-CRDF, FPRF, USDA, Lindbergh Foundation, Nanotechnology Institute,

Ben Franklin Partnerships, Widener University Internal Grants 2004-6 International Trips (research, consulting, collaborations): Plymouth, UK; Prague, CZ; Dubai,

UAE; Abu Dhabi, UAE; Kishinev, MD – current research colleagues in Kishinev, MD and Lyons, FR; Zlin, CZ

Brief Biography (Text): http://quantum.soe.widener.edu:281/gjm.htm

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• Founded in 1821, Widener University is a multi-campus, independent, metropolitan university. It offers a student-centered learning environment where course work connects to societal issues. Dynamic teaching, active scholarship, personal attention, and experiential learning are key components of the Widener experience.

• The university provides a unique combination of liberal arts and professional education through its eight schools and colleges.

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Collagen Research Lineage

Started at Dartmouth ‘85cell cultureVerax - FBBR

Widener Environmental Angle

flocculationfiltrationfractionation

Tailor Made Substrates

Other Applications

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Collagen Research Group Department of Chemical EngineeringWidener University

Time Capsule

1985 – 1988 Dartmouth College - Initial Research1985 – 1995 Verax Corp., Lebanon, NH1994 – 2006 Collagen Research Group, Widener U

150+ students, professors, and colleagues have worked on protein research at Widener since 1994

HS summer interns and summer campers –50-75 per year since 1996

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Collagen Research Group Department of Chemical Engineering

Major Collagen Projects

manufacture and applications of the main productscollagen dispersions

water treatmentenvironmental remediationintermediate

lyophilized collagen matricescontrolled releaselost protein technology

sterile, crosslinked collagen matricesbiotech/biomedicaltissue engineering

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Donna Stauffer and Prof. Maffia

Brian Coll, Megan Winkelman, and Prof. Maffia

Jennifer Hubbell

Freshman and Prof. Maffia Prof. McNeil, Charlene Emlet, Eric Shaw and Donna Stauffer

Audra Rizzetto

Widener UniversitySome Chemical Engineering Students and Professorscontact: [email protected]

Monica Beistline

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CRG

C

B

T

M

A

P

EnvironmentalExtractionFiltration AidFlocculation and Settling

milling and dispersion (smart material)

BiotechnologyCell Immobilization reactor design biosensing tissue engineeringBiomolecule DeliveryControlled Release

MiscellaneousDispersing AgentInfrastructureInsulationMicro-encapsulationCoating of Medical DevicesAdditive for Photocatalyst CoatingLost Protein

& Catalyst Manufacture16

AFTER

MIL

LIN

G A

ND

W

AS

HIN

G

50 - 100 nm diameter

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LRSM Determination of the Morphology of (U of PA)Freeze Dried Collagen Substrates (taken byAnnie Venable, ’00 and current grad student)

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Heat Transfer

Directstream mixingcheap

not viable in most situations

Indirectstreams do not mix

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Heat Transfer Mechanisms

Conduction

Convection

Radiation

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Conduction is the transfer of thermal energy from a region of higher temperature to a region of lower temperature through direct molecular communication within a medium or between mediums in direct physical contact without a flow of the material medium.

Metals (eg. copper) are usually the best conductors of thermal energy. Fluids (liquids (except liquid metals) and gasses) are not typically good conductors.

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Convection is a combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium.

Unlike the case of pure conduction, now currents in fluids are additionally involved in convection. This movement occurs into a fluid or within a fluid, and cannot happen in solids. In solids, molecules keep their relative position to such an extent that bulk movement or flow is prohibited, and therefore convection does not occur.

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Radiation is transfer of heat through electromagnetic radiation. Hot or cold, all objects radiate energy at a rate equal to their emissivity times the rate at which energy would radiate from them if they were a black body.

No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum.

Both reflectivity and emissivity of all bodies is wavelength dependent. The temperature determines the wavelength distribution of the electromagnetic radiation as limited in intensity by Plank’s law of black-body radiation.

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Conduction

Convection

Radiation

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Shell and tube heat exchanger

indirect heat transfer

most common type of heat exchanger in oil refineries and other large chemical

processes

suited for higher-pressure applications

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consists of a shell (a large pressure vessel) with a bundle of tubes inside it.

One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.

The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.

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Basics

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side).

Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa.

This process is best represented by a T-Q diagram

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Bottom Line

At every point in the HX, the hot stream must be hotter than the cold stream by a distance called the driving force

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Tem

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Heat Transfer Rate or Duty

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The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes.

In this way, waste heat can be put to use. This is a great way to recuperate energy.

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Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side.

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In large power plants with steam-driven turbines, shell-and-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which can be recycled back to be turned into steam, possibly into a shell-and-tube type boiler.

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Shell and tube heat exchanger design

There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.

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http://en.wikipedia.org/wiki/Image:U-tube_heat_exchanger.PNG

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http://en.wikipedia.org/wiki/Image:Straight-tube_heat_exchanger_1-pass.PNG

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http://en.wikipedia.org/wiki/Image:Straight-tube_heat_exchanger_2-pass.PNG

In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which typically have U-tubes.

They are used to boil water recycled from a surface condenser into steam to drive the turbine to produce power.

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Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell.

In a single pass heat exchanger, the fluid goes in one end of each tube and out the other.

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Surface condensers in power plants are often 1-pass straight-tube heat exchangers.

In other facilities such as refineries, two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler.

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There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes.

Countercurrent heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams.

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Many companies however do not use single pass heat exchangers because they can break easily in addition to being more expensive to build.

Often multiple heat exchangers can be used to simulate the countercurrent flow of a single large exchanger.

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Selection of tube material

To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes.

Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation.

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Tube material

The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion.

All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials, typically metals. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross-contamination and possibly loss of pressure.

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The Tubular Exchanger Manufacturers Association, Inc.

(TEMA) is trade association of leading manufacturers of shell and tube heat exchangers, who have pioneered the research and development of heat exchangers for over sixty years.

The TEMA Standards and software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.

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TEMA members are market-aware and actively involved, meeting several times a year to discuss current trends in design and manufacturing.

The internal organization includes various subdivisions committed to solving technical problems and improving equipment performance.

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TEMA

Choosing the right supplier is an important and difficult decision. Users need to be absolutely sure that their manufacturer is reliable, producing heat transfer equipment that is safe, effective, and economical. Considerable time and money are spent having a heat exchanger designed and built to exact specifications, design codes and requirements.

TEMA, was founded in 1939 and has grown to include a select group of member companies. Members adhere to strict specifications.

TEMA members develop and update today's standards. TEMA Standards and Software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.

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TEMA

TEMA has also developed engineering software that complements the TEMA Standards in the areas of flexible shell elements (expansion joints) analysis, flow induced vibration analysis and fixed tubesheet design and analysis.

This state-of-the-art software features a materials data-bank of 38 materials. The programs handle many complex calculations, so users can focus on the final results.

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TEMA

Before a company can even become a member of TEMA, it must have a minimum of 5 years of continuous service in the manufacture, design and marketing of shell and tube heat exchangers. All TEMA companies must have in-house thermal and mechanical design capabilities, and thoroughly understand current code requirements and initiate strict quality control procedures.

Additionally, all welding must be done by the company's own personnel, and the company must have its own quality control inspectors.

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Tubular Exchanger Manufacturers Association, Inc.

8th Edition of the Standards of the Tubular Exchanger Manufacturers Association

sections have been reviewed to incorporate new data which were not available at the time of the 1988 printing, including suggestions which resulted from the

extensive use of the Standards by both manufacturers and users of shell and tube heat exchangers.

cooperation of the American Petroleum Institute (API) and the American Society of Mechanical Engineers (ASME)

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TEMA’s Eighth Edition:

Metrification has been included where feasible and appropriate.

Methods for calculating several types of floating head backing rings have been added.

A method for incorporating pass partition rib area into flange design has been incorporated.

The vibration section has been expanded and vibration amplitude for vortex shedding and acoustic resonance have been added.

Nozzle flange pressure/temperature rating tables from ASME Standard B16.5-1996 w/ 1998 addenda are included.

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TEMA’s Eighth Edition:

New materials have been included in coefficient of thermal expansion, modulus of elasticity, and thermal conductivity tables.

Design equations for double tubesheets have been added.

A method for calculating the mean metal temperature for tubesheets has been added.

Stress multipliers have been added to account for the stiffness of knuckles on flanged and flued expansion joints.

Suggested calculation methods have been incorporated for both vertical and horizontal supports.

Design methods have been added for lifting lugs.

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More on T-Q

• Ticket to HX design and rating

• Basis for Heat Integration

• Used for process design

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QU

ICK

RE

VIE

W

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Industrial Furnaces

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Overview and Content

• Background• Definitions• Key Areas

Steam reformer

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Fired Heaters are used widely in chemical processing and petroleum refining plants

supply heat to various process fluids by burning fuel in a combustion chamber

fuel is usually oil or gas or a combination

burners are placed on the floor of the heater or on the wall

fuel + air oxidation products

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Radiation (T4)

Conduction (DTsolid)

Convection (DTconvective film)

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Radiation

tubes are installed along the walls and roof

combustion chamber called radiant section

heat is transferred to the tube wall primarily by radiation

flue gases are then passed through the convection section of the heater

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Characterized by extended surfacesfins or studs improve heat transferincrease effective area required area governed by LMTD and Uo

Economical heat recoverysteam generationboiler feed water preheatcombustion air preheat

Flue gas passes through a stack to the atmosphere

Convection Section

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Convection Section

Typical Indirect heat transfer mechanism

T bulk outsideoutside filmoutside foulingtube wallinside film inside fouling

T bulk inside

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In large heatersmost of the heat is transferred in the radiant section

Convection section increases the overall efficiency of the heater

Some fired heaters, for very low heat duty services, have no convection section

thermal efficiency lowest capital investment.

Most fired heaters have both a radiant and convection section

New-fired heaters air preheating system NOx reduction system

Radiant Efficiency => Radiant Duty/Fired DutyOverall Efficiency => Absorbed Duty/Fired Duty

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Process Fluid:

Fluid characteristics of the process fluids should be considered before designing a heater.

for example,

Very high viscosity fluids have tendency to attain very high film temperature, as the fluid in the film does not readily mix with the bulk fluid.

This results in uneven distribution of heat in the fluid and develops hot spots, where vaporization and degradation occurs.

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Heat Duty:

Total furnace heat duty

sum of heat transferred to all process streams, including auxiliary services such as steam superheaters.

Amount of heat duty affects the selection of type and configuration of heater.

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What Flue Gas Temperature Tells Us

Flue gas temperature is an indication of how effectively combustion heat is being transferred to the boiler water. In general, lower flue gas temperature indicates better heat transfer and higher overall efficiency. It means that less energy is going up the stack and, everything else being equal, more is going into the water. If you find flue gas temperature gradually increasing over time, this indicates gradually deteriorating heat transfer within the boiler. This could be caused by soot buildup on the fire side of the heat transfer surfaces, or scale buildup on the water side. A flue gas temperature increase of 100°C indicates a boiler efficiency decrease of 4 to 5 per cent.

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Average Radiant Heat Flux:

Average radiant heat flux rate is very important parameter for design of a fired heater.

Higher the design radiant flux, less the heat transfer surface, smaller the heater and lower the cost.

Unduly high radiant rates, however, result in higher maintenance cost due to shortened life of components and coke deposition.

Allowable average radiant heat flux rate is a function of various factors such as heater type, feedstock, service, coil outlet temperature etc. and, therefore, established by experience.

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Mass Flow Velocity:

Cracking and polymerization occur in the film of the fluid inside the tube wall surface.

To minimize coking and fouling in coils, fired heaters should be designed with high enough mass velocities.

However, too high a mass velocity will cause a high coil pressure drop, resulting in high pumping or compressor costs, increased design pressure of the coils and upstream equipment and possible erosion at the heater return bends.

Design mass velocity is usually kept in the range of 250 to 350 lb/sec-ft2. Under turndown conditions, mass velocity should be kept above 150 lb/sec-ft2 in order to prevent excessive coking and fouling of the coils.

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Vaporization in Process Fired Heaters:

It is desirable to avoid a situation when the liquid or partially vaporized stream reaches a point within the heater in which it is 100% vaporized.

Foreign material or polymer formed in tankage, which does not vaporize, may deposit on the tube and cause coking.

Therefore, limit the maximum vaporization to about 80%.

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Tube size, Number of Passes and Fluid Pressure Drop:

A combination of the tube size and number of passes is selected to satisfy the mass flow velocity, throughput and fluid pressure drop requirements.

Tube diameters are normally selected from standard tube sizes, in the range of 3 to 8 inches.

Non-standard sizes can also be used when design parameters cannot be met with standard sizes.

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Turndown:

Turndown requirements are set by process considerations.

In general, turndown rates of 60% can be used without falling below mass velocity rates needed to prevent excessive coking rates.

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Burner Turndown:

Burner turndown is a function of burner design and the type of fuel.

However, burner turndown does not normally affect furnace turndown, but burners can be turned off or excess air increased when furnace is operated at greatly reduced firing rates.

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Stack Temperature and Optimum Heater Efficiency:

The economic stack temperature or the optimum efficiency of the heater is a function of fuel value, inlet oil temperature, investment cost of the incremental convection section and the required rate of return from incremental investment.

Stack temperature usually ranges from 350°F to 700°F, however, a temperature of 250°F can be achieved for low sulfur fuel using air preheater.

Stack temperature must be high enough to prevent acid condensation on the convection section inlet tubes and air preheater.

Most of the new fired heaters have convection sections and air preheater. As a result, heater efficiencies have increased to more than 90%.

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Tube/Coil Materials:

Heater tubes are usually made from carbon steel, alloy steel or stainless steel pipes.

Tubing material is selected based on service life, corrosion resistance and cost.

Allowable stresses in the tube material decrease with increasing temperatures, therefore, higher tube temperatures require thicker tubewalls or higher alloy-content.

Carbon steel is the most widely used material for heater tubing where corrosion resistance is relatively mild.

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Applications

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In addition to the books by Reed and Zink, Inc.

A nice reference: http://heaterdesign.com/design0.htm 102

Fired Heaters or Furnaces, are used for various purposes in the Refining & Petrochemical Industry, Power Generation Facilities and in Homes and Apartments, etc.

In refineries applications include:

Air Heater, Boilers, Reboilers, Vacuum Charge Heater, Crude Furnace, Hydrocracker, Pyrolysis Furnace, Reforming Furnace, Visbreaker Furnace, etc.

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Industrial burners handbook By Charles E. Baukal 107

The first step in the refining process is the separation of crude oil into various fractions or straight-run cuts by distillation in atmospheric and vacuum towers.

The main fractions or "cuts" obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residuum

At the refinery, the desalted crude feedstock is preheated using recovered process heat. The feedstock then flows to a direct-fired crude charge heater where it is fed into the vertical distillation column just above the bottom, at pressures slightly above atmospheric and at temperatures ranging from 650° to 700° F

All but the heaviest fractions flash into vapor. As the hot vapor rises in the tower, its temperature is reduced.

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In order to lower the temperatures in further distilation of residual crude from the atmospheric tower, reduced pressure.

Vacuum towers may produce gas oils, lubricating-oil base stocks, and heavy residual.

Vacuum towers are typically used to separate catalytic cracking feedstock from surplus residuum.

VACUUM DISTILLATION PROCESS

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Hydrocracking Process.In the first stage, preheated feedstock is mixed with recycled hydrogen and sent to the first-stage reactor, where catalysts convert sulfur and nitrogen compounds to hydrogen sulfide and ammonia.

Limited hydrocracking also occurs.After the hydrocarbon leaves the first stage, it is cooled and liquefied and run through a hydrocarbon separator. The hydrogen is recycled to the feedstock. The liquid is charged to a fractionator. Depending on the products desired (gasoline components, jet fuel, and gas oil), the fractionator is run to cut out some portion of the first stage reactor out-turn. Kerosene-range material can be taken as a separate side-draw product or included in the fractionator bottoms with the gas oil.

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VISBREAKING PROCESS

Visbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy crude-oil residue without affecting the boiling point range. Residual from the atmospheric distillation tower is heated (800-950 degrees F) at atmospheric pressure and mildly cracked in a heater.

It is then quenched with cool gas oil to control overcracking, and flashed in a distillation tower. Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils. Middle distillates may also be produced, depending on product demand. The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum flashed in a stripper and the distillate recycled.

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Fired Heater Components

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http://upload.wikimedia.org/wikipedia/commons/1/1a/Furnace2-en.svg

Burner

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Fire

d H

eate

r W

ith A

ir P

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at

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Fired heaters are major consumers of energy in the chemical process industries (CPI), especially at petroleum refineries and petrochemical plants--accounting for as much as 70% of total plant consumption in some instances. While most plant engineers and operators are aware of the importance of controlling excess oxygen in fired heaters, they often overlook a key determinant of efficient heater operation: the control of their draft, namely, the negative pressure inside the vessel with respect to the atmosphere.

Importance of Draft

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In most fired heaters, the draft is maintained at almost four times the value recommended.

At the other end of the spectrum, some heaters run with no draft--in fact, with positive pressure at the radiant arch (the transition zone between the radiant and convection sections).

Neither situation is desirable; they can cause considerable loss of energy, and can even be hazardous.

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ZONE ANALYSIS

defined as regimes of phase changes where the overall heat transfer coefficient (Uo) will vary

Using T-Q (Temperature-Heat) diagrams are the best way to pinpoint zones.

Chemical #1 enters the shell at 200 0C as a superheated vapor. In Zone 1, it releases heat to the tubeside chemical (Chemical #2).

Zone 1 ends just a Chemical #1 begins to condense.

The tubeside (Chemical #2) enters as a liquid or gas and does not change phase throughout the exchanger.

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ZONE ANALYSIS

Chemical #1 leaves Zone 1 and enters Zone 2 at its boiling temperature, Tb1. T* marks the temperature of Chemical #2 when Chemical #1 begins to condense.

In Zone 2, Chemical #1 condenses to completion while Chemical #2 continues to increase in temperature. The temperature of Chemical #2 when Chemical #1 is fully condensed is denoted at T**.

Finally, in Zone 3, both chemicals are liquids. Chemical #1 is simply liberating heat to Chemical #2 as it becomes a subcooled liquid and exits the shell at 100 0C.

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ZONE ANALYSIS

Defining zones is one of the most important aspects of heat exchanger design. It is also important to remember that if a process simulator does not support zoned analysis you should model each zone with a separate heat exchanger.

Thus, the previous illustration would require 3 heat exchangers in the simulation and also may require three separate bundles in the real plant.

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GJM Procedure, part 1

• Assemble Physical Properties• Assemble Service Requirements• Construct T-Q Diagram• Establish Zones and Calculate LMTD• Guess a Uo• Calculate Required Surface Area

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• Pick Tube Dimensions – size, gauge, length

• Calculate Number of Tubes• Calculate Flow Characteristics on

Shell and Tube Sides– Velocity, Re, Pr, Nu

• Calculate hi, ho

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• Calculate Uo• Check versus Assumed Uo• Iterate or Goal Seek (Excel) Until the Uo

Matches• Calculate Pressure Drop

http://quantum.soe.widener.edu:281/HX1.xls

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If using a process simulator, obtaining the physical properties of the streams are easy.

Get the physical properties for each zone separately to ensure accuracy, but in some cases it is acceptable to use an average value.

This would be true for sensible heat transfer since the material is not changing phase or undergoing a truly significant temperature change (over 1000C).

Physical properties will include: heat capacity, viscosity, thermal conductivity, density, and latent heat (for phase changes). These are in addition to the boiling points of the streams at their respective pressures.

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Guessing Uo

Estimates for the heat transfer coefficients can be found in most textbooks

Main equation Q=UoADTlm

The above equation must be used to get an area for each zone, then add them together.

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Tubular, heating or cooling W/m2-KGases at atmospheric pressure inside and outside tubes 5-35Gases at high pressure inside and outside tubes 150 - 500 Liquid outside (inside) and gas at atmospheric pressure inside (outside) tubes 15 - 70 Gas at high pressure inside and liquid outside tubes 200 - 400 Liquids inside and outside tubes 150 - 1200 Steam outside and liquid inside tubes 300 - 1200 Tubular, condensation Steam outside and cooling water inside tubes 1500 - 4000 Organic vapors or ammonia outside and cooling water inside tubes 300 - 1200 Tubular, evaporation steam outside and high-viscous liquid inside tubes, natural circulation 300 - 900 steam outside and low-viscous liquid inside tubes, natural circulation 600 - 1700 steam outside and liquid inside tubes, forced circulation 900 - 3000 Air-cooled heat exchangers2) Cooling of water 600 - 750 Cooling of liquid light hydrocarbons 400 - 550 Cooling of tar 30 - 60 Cooling of air or flue gas 60 - 180 Cooling of hydrocarbon gas 200 - 450 Condensation of low pressure steam 700 - 850 Condensation of organic vapors 350 - 500 Plate heat exchanger liquid to liquid 1000 - 4000 Spiral heat exchanger liquid to liquid 700 - 2500 condensing vapor to liquid 900 - 3500 131

The determination of U is often tedious and needs data not yet available in preliminary stages of the design. Therefore, typical values of U are useful for quickly estimating the required surface area. The literature has many tabulations of such typical coefficients for commercial heat transfer services.

Lower values are for unfavorable conditions such as lower flow velocities, higher viscosities, and additional fouling resistances. Higher values are for more favorable conditions.

Coefficients of actual equipment may be smaller or larger than the values listed. Note that the values should not be used as a replacement of rigorous methods for the final design of heat exchangers, although they may serve as a useful check on the results obtained by these methods.

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What geometric configuration is right for the exchanger? Once you've selected a shell diameter, tubesheet layout, baffle and tube spacing, etc., it's time to check your velocity and pressure drop requirements to see if they're being met.

Experienced designers will usually combine these steps and actually obtain a tube size that meets the velocity and pressure drop requirements and then proceed.

Some guidelines may be as follows: 3/4 in. and 1.0 in. diameter tubes are the most popular and smaller sizes should only be used for exchangers needing less than 30 m2 of area.

If your pressure drop requirements are low, avoid using four or more tube passes as this will drastically increase your pressure drop.

133

This is where you'll spend much of your time in designing a heat exchanger. Although many textbooks show

Nu=0.027(NRE)0.8(NPR)0.33

as the "fundamental equation for turbulent flow heat transfer", what they sometimes fail to tell you is that the exponents can vary widely for different situations.

For example, condensation in the shell has different exponents than condensation in the tubes. Use this fundamental equation if you must, but you should consult a good resource for accurate equations.

134

L. Spiral heat exchangers are often used to slurry interchangers and other services containing solidsM. Plate heat exchanger with gaskets can be used up to 320 °F (160 °C) and are often used for interchangingduties due to their high efficiencies and ability to "cross" temperatures. More about compact heat exchangerscan be found at:

or an online list can be found at one of the two following addresses:http://www.cheresources.com/uexchangers.shtmlhttp://www.processassociates.com/process/heat/uvalues1.htm

K. Double pipe heat exchangers may be a good choice for areas from 100 to 200 ft2 (9.3-18.6 m2)

10 °F (5 °C) for refrigerants.I. Cooling tower water is typically available at a maximum temperature of 90 °F (30 °C) and should be returned to the tower no higher than 115 °F (45 °C)J. Shell and Tube heat transfer coefficient for estimation purposes can be found in many reference books

E. Flows that are corrosive, fouling, scaling, or under high pressure are usually placed in the tubesF. Viscous and condensing fluids are typically placed on the shell side.G. Pressure drops are about 1.5 psi (0.1 bar) for vaporization and 3-10 psi (0.2-0.68 bar) for other servicesH. The minimum approach temperature for shell and tube exchangers is about 20 °F (10 °C) for fluids and

C. A 1 ft (30 cm) shell will contains about 100 ft2 (9.3 m2)A 2 ft (60 cm) shell will contain about 400 ft2 (37.2 m2)A 3 ft (90 cm) shell will contain about 1100 ft2 (102 m2)D. Typical velocities in the tubes should be 3-10 ft/s (1-3 m/s) for liquids and30-100 ft/s (9-30 m/s) for gases

Heat Exchangers

A. For the heat exchanger equation, Q = UAF (LMTD), use F = 0.9 when charts for the LMTD correctionfactor are not availableB. Most commonly used tubes are 3/4 in. (1.9 cm) in outer diameter on a 1 in triangular spacing at 16 ft (4.9 m) long.

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Performance comparisonThis chart compares the performance of different heat exchanger technologies, including plate-fin, oil cooler, and tube-fin heat exchangers. Performance is shown as Q/ITD, the heat load divided by the difference in incoming temperature of the liquid and air. Units are not shown so that technologies can be compared regardless of size.As many heat exchangers are customized, a range of typical values is shown for each technology. All performances are compared using water as the cooling fluid.

Fluid compatibilityCoolant compatibility with wetted surfaces must be considered when selecting a heat exchanger technology. A copper fluid path is compatible with water and most common coolants. A stainless steel fluid path is necessary when using deionized water and other corrosive fluids. Aluminum offers excellent performance with ethylene glycol/water mixture (EGW), oils and other fluids, but is not compatible with untreated water. The table below shows fluid/heat exchanger compatibility.

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http://www.lytron.com/standard/heat-exchangers-6000.htm

http://www.lytron.com/standard/he_aluminum.asp

http://www.lytron.com/standard/heat-exchangers-plate-fin.htm

http://www.lytron.com/standard/heat-exchangers-brazed-plate.htm

ME-143 Principles of Heat Exchanger Design, Part II

This seminar provides a continuation of the discussion of the equipment used for the transfer of heat. Specific examples and case studies will highlight analysis techniques and step-wise procedures. Emphasis will be placed on the calculation of the film coefficient for a variety of different geometries and how to compose an overall heat transfer coefficient. Transfer line exchangers, plate-fin exchangers and Joule-Thompson exchangers are among the examples covered.

137

Energy Balance

Outline

• Heating and cooling requirements • Indirect or direct heat transfer• Various types of heat exchangers

– Mechanical design and process arrangement

– Key vendors and producers • Tubular Exchanger Manufacturers Association

138

What is Heat ?

• Universe: matter and energy• Energy causes the atoms and

molecules to always be in motion

• Motion of atoms and molecules creates heat or thermal energy

• In space, matter still has a very small amount of heat energy

139

What is Heat ?

Cal Tech Image Using IR

140

Force, Pressure, Work, Energy, Power

• Force: Newton, (kg)(m)(s-2)• Pressure: Pascal, (N)(m-2)• Work: Joule, (N)(m)• Energy: Joule, (N)(m)• Power: Watt, (J)(s-1)

• Example: a 60 W light bulb expends 216 kJ every hour of use and costs the

consumer about 1 cent (US)

141

• Energy– 1 BTU = 1055 J– 1 BTU = 1.055 kJ– 1 kWh = 3600 kJ

• Energy in Colloquial Terms– kWh = 1 kJ/s of power expended for

1 h– Cost (based on PECO):

$0.165/kWh

142

Current Energy Production Picture – US and World-Wide

• Language of Energy Production and Usage

• Magnitude – Energy Needs– Environmental Impact

• as a energy and environmental primer: • in combustion: 1 kg C yields ~ 3.7 kg CO2 and ~

33 MJ

143

Unit Levels – SI Prefixes• 1 EW exa• 103 PW peta• 106 TW tera• 109 GW giga• 1012 MW mega• 1015 kW kilo• 1018 W-----• 1021 mW milli• 1024 mW micro

144

145

World Energy Consumption

Fuel typePower in TW

Energy in EJ

Oil 5.6 180

Gas 3.5 110

Coal 3.8 120

Hydroelectric 0.9 30

Nuclear 0.9 30

Geothermal, wind,solar, wood

0.13 4

Total 15 471

as a reference: 174,000 TW (174 PW) incoming solar power

146

Value of Energy- Energy Information Administration

Price Summary

Year

2007 2008 2009 2010

WTI Crude ($/barrel) 72.32 99.57 60.12 72.42

Gasoline ($/gal) 2.81 3.26 2.34 2.7

Diesel ($/gal) 2.88 3.8 2.47 2.88

Heating Oil ($/gal) 2.72 3.38 2.51 2.78

Natural Gas ($/mcf) 13.03 13.67 11.92 11.56

Electricity (cents/kwh) 10.65 11.36 11.64 11.4

Basic Heat Balance The First Law of Thermodynamics:

expression of the principle of conservation of energy

states that energy can be transformed but is not created or destroyed

147

Heat Transfer

Directstream mixingcheap

not viable in most situations

Indirectstreams do not mix

148


Conduction

Convection

Radiation

149

Conduction

Transfer of thermal energy (heat) from a region of higher temperature to a region of lower temperature by direct contact

(Second Law of Thermodynamics)

150

Conduction

Direct molecular communication without a flow of material

Metals (eg. copper) are usually the best conductors of thermal energy.

Fluids are not typically good conductors.

HX tubes are usually made of copper, CS, or SS

151

Convection

combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium.

T bulk

T surface

convective film coefficient

152

This movement occurs into a fluid or within a fluid, and cannot happen in solids

In solids, molecules keep their relative position to such an extent that bulk movement or flow is impossible

153

Radiation

Transfer of heat through electromagnetic radiation

Objects radiate energy at a rate equal to their emissivity times the rate at which energy would radiate from them if they were perfectradiators

154

Radiation

No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum

155

Radiation

Reflectivity and emissivity of all bodies is wavelength dependent

Temperature determines the wavelength distribution of the electromagnetic radiation

156

HOT Stream – loses heat

COLD Stream – gains heat

Direct Mixing

157



Indirect Heat Exchangecountercurrent

158



Indirect Heat Exchangecocurrent

159

Heat Transfer EquationsBeginnings of a T-Q Curve

mQ

KKkg

kJ

h

kgTTmcQ p

21

Q, kJ/h heat transferred

, l latent heat

cp, specific heat160


Indirect heat transfer

Most common type of heat exchanger in oil refineries and other large

chemical processes

Suited for higher-pressure applications

161


Consists of a shell (a large pressure vessel) with a bundle of tubes inside it.

One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.

The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.

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Shell and tube heat exchanger design

There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets.

The tubes may be straight or bent in the shape of a U, called U-tubes.

163

The Tubular Exchanger Manufacturers Association, Inc.

(TEMA) is trade association of leading manufacturers of shell and tube heat exchangers, who have pioneered the research and development of heat exchangers for over sixty years.

The TEMA Standards and software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.

TEMA

166

TEMA Mission

Users need to be absolutely sure that their manufacturer is reliable, producing heat transfer equipment that is safe, effective, and economical.

Considerable time and money are spent having a heat exchanger designed and built to exact specifications, design codes and requirements.

167

TEMA, was founded in 1939 and has grown to include a select group of member companies. Members adhere to strict specifications

TEMA members develop and update today's standards. TEMA Standards and Software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design

TEMA History

168

TEMA members are market-aware and actively involved, meeting several times a year to discuss current trends in design and manufacturing

The internal organization includes various subdivisions committed to solving technical problems and improving equipment performance

TEMA Outreach and Standards

169

ThomasNet for HX Vendors

170

ThomasNet for HXsSide-by-Side Comparisons

171

Heat Exchangers: Shell & Tube Icon

Tube Side

She

ll S

ide

172

Heat Exchangers: Shell & Tube Icon

173

Overview of HX Design – Part 1

• Equipment used for the transfer of heat energy

• Basic heat balance as a T-Q curve • Appropriate TEMA designation• Economic and environmental considerations • Pinch Technology for heat exchanger

networks

174

The fluids can be either liquids or gases on either the shell or the tube side.

In order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes.

In this way, waste heat can be put to use. This is a great way to recuperate energy.

Fluid Flow Arrangements

175

Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangers

Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side.

Sensible Heat Transfer vs Phase Change

176

In large power plants with steam-driven turbines, shell-and-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which can be recycled back to be turned into steam, possibly into a shell-and-tube type boiler

Amongst the Largest HXs are…..

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Heat Exchangers: Shell & Tube Arrangement

179

TEMA

Front, Shell, Rear

Designation

182

Sample

CFU (top) AKT (bot)

183

CFU Designation

(C) Channel Integral with

Fixed Tube Sheet

(F) Two Pass Shell with Longitudinal Baffle

(U) U-tube (two pass tubes)

184

AKT Designation

(A) Channel and Removable

Cover

(K) Kettle

(T) Pull Through Floating Head

185

Heat Transfer Equations- sign convention

• Exothermic – gives off heat

– Usually written as negative• Endothermic – absorbs heat

– Usually written as positive• GJM – “think positively”

– Using words like heat removed and heat absorbed and plot on the same axis

186

Direct Heat Transfer Example

Name HOT Air COLD Air 1

Vapour Fraction 1.0000 1.0000 1.0000

Temperature [C] 300.0 25.00 164.2

Pressure [kPa] 400.0 400.0 400.0

Molar Flow [kgm... 100.0 100.0 200.0

Mass Flow [kg/h] 2895 2895 5790

Liquid Volume Fl... 3.291 3.291 6.583

Heat Flow [kJ/h] 8.099e+005 -2564 8.074e+005187

Indirect Heat Transfer Example

Name HOT Air COLD Air 1 2

Vapour Fraction 1.0000 1.0000 1.0000 1.0000

Temperature [C] 300.0 25.00 100.0 227.7

Pressure [kPa] 400.0 400.0 400.0 400.0

Molar Flow [kgm... 100.0 100.0 100.0 100.0

Mass Flow [kg/h] 2895 2895 2895 2895

Liquid Volume Fl... 3.291 3.291 3.291 3.291

Heat Flow [kJ/h] 8.099e+005 -2564 2.151e+005 5.923e+005188


Name HOT Air COLD Air 1 2

Vapour Fraction 1.0000 1.0000 1.0000 1.0000

Temperature [C] 300.0 25.00 164.2 164.2

Pressure [kPa] 400.0 400.0 400.0 400.0

Molar Flow [kgm... 100.0 100.0 100.0 100.0

Mass Flow [kg/h] 2895 2895 2895 2895

Liquid Volume Fl... 3.291 3.291 3.291 3.291

Heat Flow [kJ/h] 8.099e+005 -2564 4.037e+005 4.037e+005189

Basics

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side).

Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa.

This process is best represented by a T-Q diagram

190


Pla

te a

nd

Fra

me

Hea

t E

xch

ang

er

191

2nd Law of Thermodynamics - Bottom Line

At every point in the HX, the hot stream must be hotter than the cold stream by a distance called the driving force

192

J-T Heat Exchanger System With Brazed Aluminum Type plate Exchanger

193

Composite Curves in the Plate Exchanger

composite cold curve

194


Step 1: Gather duty requirements and physical properties

Step 2: Construct a T-Q Curve

Step 3: Guess an Overall Heat Transfer Coefficient

Step 4: Calculate the LMTD

Step 5: Calculate the F (correction factor)

Step 6: Calculate the Heat Transfer Area

195


Step 7: Assign the tube specifications – diameter, BWG and length

Step 8: Calculate the outside surface area per tube

Step 9: Calculate the number of tubes to provide the required area

Step 10: Calculate the velocity - shell side and tube side

Step 11: Calculate the tube and shell side coefficients

Step 12: Calculate the Overall Heat Transfer Coefficient

Step 13: Check with Step 3, adjust and recalculate

196

http://quantum.soe.widener.edu:281/HX.xls197

http://quantum.soe.widener.edu:281/HX1.xls 198

http://quantum.soe.widener.edu:281/HX1.xls

Tubular, heating or cooling W/m2-KGases at atmospheric pressure inside and outside tubes 5-35Gases at high pressure inside and outside tubes 150 - 500 Liquid outside (inside) and gas at atmospheric pressure inside (outside) tubes 15 - 70 Gas at high pressure inside and liquid outside tubes 200 - 400 Liquids inside and outside tubes 150 - 1200 Steam outside and liquid inside tubes 300 - 1200 Tubular, condensation Steam outside and cooling water inside tubes 1500 - 4000 Organic vapors or ammonia outside and cooling water inside tubes 300 - 1200 Tubular, evaporation steam outside and high-viscous liquid inside tubes, natural circulation 300 - 900 steam outside and low-viscous liquid inside tubes, natural circulation 600 - 1700 steam outside and liquid inside tubes, forced circulation 900 - 3000 Air-cooled heat exchangers2) Cooling of water 600 - 750 Cooling of liquid light hydrocarbons 400 - 550 Cooling of tar 30 - 60 Cooling of air or flue gas 60 - 180 Cooling of hydrocarbon gas 200 - 450 Condensation of low pressure steam 700 - 850 Condensation of organic vapors 350 - 500 Plate heat exchanger liquid to liquid 1000 - 4000 Spiral heat exchanger liquid to liquid 700 - 2500 condensing vapor to liquid 900 - 3500

199

OutlineAssessing the performance of a heat exchangerValue of energy transferredCapital cost of the heat exchangerEconomic performanceEconomic sensitivity

200

Assessing the Performance of a Heat Exchanger

OutlineLeveraging issues, such as the level of energy

transferredFouling, cleaning and sparing philosophyFuture of heat exchanger designNew technologyBreakthrough design options

201

Assessing the performance of a heat exchanger

Calculate the required heat transfer area and the associated cost

Calculate the cost of any utility or utility savings

Perform an economic analysis based on delta costs and delta returns – that is will additional surface area of a heat exchanger provide a high enough reduction in utility costs

202

Assessing the performance of a heat exchanger

Ene

rgy

Cos

t

Heat Exchanger Area

203

Value of Level of Heat/Cooling

204


205


206

Basics of Pinch Technology for Heat Integration

Pinch technology presents a simple methodologyfor systematically analyzing chemical processes and the surrounding utility systems with the help of the First and Second Laws of Thermodynamics.

The temperature approach is the minimum allowable temperature difference (DTmin) in the stream temperature profiles, for the heat exchanger unit.

207

Basics of Pinch Technology for Heat Integration, continued….

The temperature level at which DTmin is observed in the process is referred to as "pinch point" or "pinch condition"

The pinch defines the minimum driving force allowed in the exchanger unit.

208

Objectives of Pinch Technology for Heat Integration

Pinch Analysis is used to identify energy cost and heat exchanger network (HEN) capital cost targets for a process and recognizing the pinch point

The procedure first predicts, ahead of design, the minimum requirements of external energy, network area, and the number of units for a given process at the pinch point

209

Composite Curves for Hot and Cold Streams

211

T-Q Curves for Composite Streams

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30 35 40

Q, mm BTU/h

Tem

per

atu

re,

F

212

Sliding the Cold Curve

213

Slide a Bit Further

214

Schematic of shell and tube geometry

215

Fouling Buildup on Shell and Tube Side 217

Sam

ple

HT

Cs,

BT

U/h

-ft2 -

F

218

Novel Heat Exchangers

220

Novel Exchangers

222

Kinex Heat Exchangers

223

True Countercurrent Exchangers

224

Plate Exchangers

225

Questions

1. Give an example of direct heat transfer

2. Give an example of indirect heat transfer

3. Energy is transferred from hot to cold (T/F ?)

4. What is the main S&T HX design equation?

5. What is a T-Q diagram?

6. Does the T-Q for the cold material intersect the T-Q curve for the hot material?

7. Discontinuities on the T-Q diagram represent phase change (T/F?)

8. What is the main trade organization for S&T HXs?

9. What does LMTD stand for?

10. Provide a reasonable Uo for a gas-gas HX?

226