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ME421 Heat Exchanger and Steam Generator Design


ME421 Heat Exchanger and Steam Generator Design Lecture Notes 6 Double-Pipe Heat Exchangers Introduction Introduction DP HEX: one pipe placed concentrically inside ... – PowerPoint PPT presentation

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Title: ME421 Heat Exchanger and Steam Generator Design

ME421Heat Exchanger andSteam Generator Design
  • Lecture Notes 6
  • Double-Pipe Heat Exchangers

  • DP HEX one pipe placed concentrically inside
  • One fluid flows through inner pipe, the other
    through the annulus
  • Outer pipe is sometimes called the shell
  • Inner pipe connected by U-shaped return bends
    enclosed in a return-bend housing to make up a
    hairpin, so DP HEX hairpin HEX
  • Hairpins are based on modular principles they
    can be arranged in series, parallel, or
    series-parallel combinations to meet pressure
    drop and MTD requirements add-remove as

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Usage Areas / Advantages
  • Sensible heating / cooling, small HT areas (up to
    50 m2)
  • High pressure fluids, due to small tube diameters
  • Suitable for gas / viscous liquid (small volume
  • Suitable for severe fouling conditions (easy to
    clean and maintain)
  • Finned tubes can be used to increase HT surface
    per unit length, thus reduce length and Nhp
  • Outside-finned inner tubes most efficient when
    low h fluid (oil or gas) flows through annulus
  • Multiple tubes can be used inside the shell
  • Used as counterflow HEX, so they can be used as
    an alternative to shell-and-tube HEX

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Thermal / Hydraulic Design
Inner Tube
  • Use correlations to find HT coefficient and
    friction factor
  • Total pressure drop

  • Same procedure as above, but use
  • Hydraulic diameter, Dh 4Ac/Pw for Re
  • Equivalent diameter, De 4Ac/Ph for Nu
  • For a hairpin HEX with Bare Inner Tube,
  • Dh Di - do
  • De (Di2 - do2)/do

Study Example 6.1 (detailed analysis)
Thermal / Hydraulic Design (continued)
  • For a hairpin HEX with Multitube Longitudinal
    Finned Inner Tubes
  • Get Dh and De using
  • Unfinned, finned, and total outside HT surface

Thermal / Hydraulic Design (continued)
  • Overall HT coefficient based on outer area of
    inner tubes
  • where
  • is the overall surface efficiency
  • Area ratios At/Ai and Af/At are needed
  • Rw is for bare tube wall
  • hf is the efficiency of a rectangular
    continuous longitudinal fin (for other types of
    fins, use references)
  • h affects fin efficiency have the fluid with
    the poorest HT properties on the finned side

Thermal / Hydraulic Design (continued)
  • The heat transfer equation is (heat duty
  • The design problem, in general, includes
    determining the total outer surface area of the
    inner tubes from the above equation.
  • If the length of hairpins is fixed, then Nhp can
    be calculated.
  • U can also be based on the inner area of the
    inner tubes, Ai
  • For counterflow and parallel flow arrangements,
    no correction is necessary for ?Tm. However, if
    hairpins are arranged in series/parallel, a
    correction must be made (later).
  • Study Example 6.2 (detailed analysis)

Parallel / Series Arrangement of Hairpins
  • If the design indicates large Nhp, it may not be
    practical to connect them all in series for pure
    counterflow. A large quantity of fluid through
    pipes may result in ?p gt ?pallowable
  • Solution Separate mass flow into parallel
    streams, then connect smaller mass flow rate side
    in series. This is a parallel-series arrangement.
  • If such a combination is used, the temperature
    difference of the inner pipe fluid will be
    different for each hairpin.
  • Thus, in each hairpin section, different amounts
    of heat will be transferred and true mean
    temperature difference, ?Tm will be different
    from the LMTD.

  • The true mean temperature difference in
  • becomes
  • dimensionless quantity S is
  • For n hairpins, S depends on the number of
    hot-cold streams and their series-parallel
  • Simplest case is to either divide the cold fluid
    equally between n hairpins in parallel or to
    divide the hot fluid equally between n hairpins
    in parallel.

  • For one-series hot fluid and n1-parallel cold
  • For one-series cold fluid and n2-parallel hot
  • Then, the total heat transfer rate is

  • In the previous equations, it is assumed that U
    and cp of the fluids are constant, and the heat
    transfer rates of the two units are equal.
  • Graphs are available in literature for LMTD
    correction factor F as well.
  • If number of tube-side parallel paths is equal to
    the number of shell-side parallel paths, regular
    LMTD should be used.

Total Pressure Drop
  • Total pressure drop includes frictional pressure
    drop, entrance and exit pressure drops,
    static-head, and the momentum-change pressure
  • Frictional pressure drop is
  • For frictional pressure drop, use correlations
    from Chapter 4 or Moody diagram. Add equivalent
    length of the U-bend to the L in tube-side (Dh
    di) pressure drop.
  • You may need to account for the effect of
    property variations on friction factor.

Total Pressure Drop (continued)
  • Entrance and exit pressure drops through inlet
    and outlet nozzles is evaluated from
  • where Kc 1.0 at the inlet and 0.5 at the
    outlet nozzle.
  • Static head is ?pf ??H, where ?H is the
    elevation difference between inlet and outlet
  • For fully developed conditions, momentum-change
    pressure drop is
  • In all pressure drop calculations for design,
    allowable ?p must be considered.
  • Cut-and-twist technique increases h in
    longitudinal finned-tube HEX. See book for ?p

Design and Operational Features
  • In hairpin HEX, two double pipes are joined at
    one end by a U-tube bend welded to the inner
    pipes, and a return bend housing on the
    shell-side. The housing has a removable cover to
    allow removal of inner tubes.
  • Double-pipe HEX have four key design components
  • shell nozzles
  • tube nozzles
  • return-bend housing and cover plate on U-bend
  • shell-to-tube closure on other side of hairpin(s)
  • The longitudinal fins made from steel are welded
    onto the inner pipe. Other materials can be
    joined by soldering.
  • Multiple units can be joined by bolts and
  • For low heat duty applications, simple
    constructions, easy assembly, lightweight
    elements and minimum number of parts contribute
    to minimizing costs.

IPS inch per second (unit system) NFA net flow