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Choosing the best exchanger for a given process application


Choosing the best exchanger for a given process application Lecture series Introduction to heat exchangers Selection of the best type for a given application ... – PowerPoint PPT presentation

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Title: Choosing the best exchanger for a given process application

Heat Exchanger Selection
  • Choosing the best exchanger for a given process

Lecture series
  • Introduction to heat exchangers
  • Selection of the best type for a given
  • Selection of right shell and tube
  • Design of shell and tube

The steps
  • Coarse filter
  • Rejecting those exchangers which will not be
    suitable on the grounds of operating pressure and
    temperature, fluid-material compatibility,
    handling extreme thermal conditions
  • Fine filter
  • Estimating the cost of those which may be suitable

Coarse filter
  • Use information next few slides to reject those
    exchangers which are clearly out of range or are
    otherwise unsuitable
  • The information is summarised in the table
  • At this stage, if in doubt, include the exchanger
    (poor choices are likely to turn out expensive at
    the fine filter stage)

General points
  • Tubes and cylinders can withstand higher
    pressures than plates
  • If exchangers can be built with a variety of
    materials, then it is more likely that you can
    find a metal which will cope with extreme
    temperatures or corrosive fluids
  • More specialist exchangers have less suppliers,
    longer delivery times and must be repaired by

Thermal effectiveness
  • Stream temperature rise divided by the
    theoretically maximum possible temperature rise

Double pipe
  • Normal size
  • 0.25 to 200m2 (2.5 to 2000 ft2) per unit
  • Note multiple units are often used
  • Built of carbon steel where possible

Advantages/disadvantages of double-pipe
  • Advantages
  • Easy to obtain counter-current flow
  • Can handle high pressure
  • Modular construction
  • Easy to maintain and repair
  • Many suppliers
  • Disadvantage
  • Become expensive for large duties (above 1MW)


Scope of double pipe
  • Maximum pressure
  • 300 bar(abs) (4500 psia) on shell side
  • 1400 bar(abs) (21000 psia) on tubeside
  • Temperature range
  • -100 to 600oC (-150 to 1100oF)
  • possibly wider with special materials
  • Fluid limitations
  • Few since can be built of many metals
  • Maximum e 0.9
  • Minimum DT 5 K

Shell and tube
  • Size per unit 100 - 10000 ft2 (10 - 1000 m2)
  • Easy to build multiple units
  • Made of carbon steel where possible

Advantages/disadvantages of ST
  • Advantages
  • Extremely flexible and robust design
  • Easy to maintain and repair
  • Can be designed to be dismantled for cleaning
  • Very many suppliers world-wide
  • Disadvantages
  • Require large plot (footprint) area - often need
    extra space to remove the bundle
  • Plate may be cheaper for pressure below 16 bar
    (240 psia) and temps. below 200oC (400oF)

Scope of shell and tube Essentially the same as a
double pipe
  • Maximum pressure
  • 300 bar(abs) (4500 psia) on shell side
  • 1400 bar(abs) (21000 psia) on tubeside
  • Temperature range
  • -100 to 600oC (-150 to 1100oF)
  • possibly wider with special materials
  • Fluid limitations
  • Few since can be built of many metals
  • Maximum e 0.9 (less with multipass)
  • Minimum DT 5 K

Plate and frame
  • Plates pressed from stainless steel or higher
    grade material
  • titanium
  • incoloy
  • hastalloy
  • Gaskets are the weak point. Made of
  • nitrile rubber
  • hypalon
  • viton
  • neoprene

Advantages of plate and frame
  • High heat transfer - turbulence on both sides
  • High thermal effectiveness - 0.9 - 0.95 possible
  • Low ?T - down to 1K
  • Compact - compared with a ST
  • Cost - low because plates are thin
  • Accessibility - can easily be opened up for
    inspection and cleaning
  • Flexibility - Extra plates can be added
  • Short retention time with low liquid inventory
    hence good for heat sensitive or expensive
  • Less fouling - low r values often possible

Disadvantages of plate frame
  • Pressure - maximum value limited by the sealing
    of the gaskets and the construction of the frame.
  • Temperature - limited by the gasket material.
  • Capacity - limited by the size of the ports
  • Block easily when solids in suspension unless
    special wide gap plates are used
  • Corrosion - Plates good but the gaskets may not
    be suitable for organic solvents
  • Leakage - Gaskets always increase the risk
  • Fire resistance - Cannot withstand prolonged fire
    (usually not considered for refinery duties)

Scope of plate-frame
  • Maximum pressure
  • 25 bar (abs) normal (375 psia)
  • 40 bar (abs) with special designs (600 psia)
  • Temperature range
  • -25 to 1750C normal (-13 to 3500F)
  • -40 t0 2000C special (-40 to 3900F)
  • Fluid limitations
  • Mainly limited by gasket
  • Maximum e 0.95
  • Minimum DT 1 K

Welded plates
  • Wide variety of proprietary types each with one
    or two manufactures
  • Overcomes the gasket problem but then cannot be
    opened up
  • Pairs of plates can be welded and stacked in
    conventional frame
  • Conventional plate and frame types with
    all-welded (using lasers) construction have been
  • Many other proprietary types have been developed
  • Tend to be used in niche markets as replacement
    to shell-and-tube

Air-cooled exchangers
Advantages of ACHEs
  • Air is always available
  • Maintenance costs normally less than for water
    cooled systems
  • In the event of power failure they can still
    transfer some heat due to natural convection
  • The mechanical design is normally simpler due to
    the pressure on the air side always being closer
    to atmospheric.
  • The fouling of the air side of can normally be

Disadvantages of ACHEs
  • Noise - low noise fans are reducing this problem
    but at the cost of fan efficiency and hence
    higher energy costs
  • May need special features for cold weather
  • Cannot cool to the same low temperature as
    cooling tower

Scope of Air Cooled Exchangers
  • Maximum pressure - tube(process) side 500 bar
  • Maximum temperature 600oC (1100o F)
  • Fluids subject to tube materials
  • Size per unit 5 - 350m2 (50 - 3500ft2 ) per
    bundle (based on bare tube)

Plate Fin Exchangers
  • Formed by vacuum brazing aluminium plates
    separated by sheets of finning
  • Noted for small size and weight. Typically, 500
    m2/m3 of volume but can be 1800 m2/m3
  • Main use in cryogenic applications (air
  • Also in stainless steel

Scope of plate-fin exchanger
  • Max. Pressure 90 bar (size dependent)
  • Temperatures -200 to 150oC in Al
  • Up to 600 with stainless
  • Fluids Limited by material
  • Duties Single and two phase
  • Flow configuration Cross flow, Counter flow
  • Multistream Up to 12 streams (7 normal)
  • Low DT Down to 0.1oC
  • Maximum DT 50oC typical
  • High e Up to 0.98
  • Important to use only with clean fluids

Printed Circuit Exchanger
  • Very compact
  • Very strong construction from diffusion welding
  • Small channels (typically 1 - 2 mm mean hydraulic
  • Can be made in stainless steel, nickel (and
    alloys), copper (and alloys) and titanium

Scope of PCHE
  • Maximum Pressure 1000bar (difference 200bar)
  • Temperature -200 to 800oC for stainless
    steel but depends on metal
  • Fluids Wide range
  • but must be low fouling
  • Normal Size 1 to 1000m2
  • Flow configuration Crossflow or counterflow
  • Effectiveness ? up to 0.98
  • Low ?T Yes
  • Thermal cycling Has caused problems

  • Which exchanger types can be used for condensing
    organic vapour at -60oC and 60 bar by boiling
    organic at -100oC and 70 bar?
  • Would you modify your choice if the boiling
    stream were subject to fouling requiring
    mechanical cleaning?

Heat exchanger costing - fine filter
  • Full cost made up of
  • Capital cost
  • Installation cost
  • Operating cost
  • The cost estimation method given here is based
    only on capital cost - which is the way it is
    often done
  • Note installation costs can be as high as
    capital cost except for compact exchangers
  • Installation cost considerations can predominate
    on offshore plant

  • The cost estimate method given here is for the
    preliminary plant design stage - scoping
  • Note that we are trying to estimate the cost of
    an exchanger before we have designed it
  • Full design and cost would be done later

Quick sizing of heat exchangers
  • We estimate the area from

FT correction factor
  • This correction accounts for the two streams not
    following pure counter-current flow
  • At the estimation stage, we do not know the
    detailed flow/pass arrangement so we use
  • FT 1.0 for counter flow which includes most
    compact ant double-pipe
  • FT 0.7 for pure cross flow which includes
    air-cooled and other types when operated in pure
    cross flow (e.g. shell-and-tube)
  • FT 0.9 for multi-pass
  • FT 1.0 if one stream is isothermal (typically
    boiling and condensation)

Estimating U
  • This may be estimated for a given exchanger type
    using the tables from ESDU (given below)
  • These tables give U values as a function of Q/?T
    (the significance of this group will become clear
  • Example high pressure gas cooled by treated
    cooling water in a shell-and-tube, where
  • Q/?T 30 000 W/K
  • gives U 600 W/m2K
  • This includes typical fouling resistances

Estimating cost
  • This has often been done by multiplying the
    calculated area, A, by a cost per unit area
  • But, when comparing exchangers, U and A vary
    widely from type to type. It is also difficult
    to define A if there is a complicated extended
  • Hence, ESDU give tables of C values where C is
    the cost per UA - using 1992 prices
  • Note, from our basic heat transfer equation
  • UA Q / DT

  • ESDU gives tables for a range of heat exchanger
    types but we can only include here those for
    shell-and-tube and plate-and-frame
  • Full data Item 92013 is available from
  • ESDU International plc
  • 27 Corsham Street
  • London N1 6UA
  • Tel 0171 490 5151 Fax 0171 490 2701

Steps in calculation
  • Calculate ?Tln and hence estimate ?T
  • Determine Q/?T
  • Look up C value from table
  • To determine C at intermediate Q/?T, use
    logarithmic interpolation - see next slide
  • Calculate exchanger cost from - Cost C(Q/?T)
  • Taking the last shell-and-tube example, C 0.4.
    Hence, Cost 0.4 X 30 000 12 000
  • Make sure that you take account of footnotes in

Logarithmic interpolation

Where the Vs are the values of Q/?T. V1 and V2
are the values either side of the required value
  • Heat Exchanger Advisor
  • Helps guide you through the selection process
  • Does the coarse and fine filter steps in one and
    provides extensive help text