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ME421 Heat Exchanger and Steam Generator Design

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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


1
ME421Heat Exchanger andSteam Generator Design
  • Lecture Notes 6
  • Double-Pipe Heat Exchangers

2
Introduction
3
Introduction
  • DP HEX one pipe placed concentrically inside
    another
  • 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
    necessary

4
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5
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6
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
    fluids)
  • 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

7
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8
Thermal / Hydraulic Design
Inner Tube
  • Use correlations to find HT coefficient and
    friction factor
  • Total pressure drop

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

Study Example 6.1 (detailed analysis)
9
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
    areas

10
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

11
Thermal / Hydraulic Design (continued)
  • The heat transfer equation is (heat duty
    equation)
  • 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)

12
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.

13
  • 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
    arrangement.
  • 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.

14
  • For one-series hot fluid and n1-parallel cold
    streams,
  • For one-series cold fluid and n2-parallel hot
    streams,
  • Then, the total heat transfer rate is

15
  • 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.

16
Total Pressure Drop
  • Total pressure drop includes frictional pressure
    drop, entrance and exit pressure drops,
    static-head, and the momentum-change pressure
    drop.
  • 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.

17
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
    nozzles.
  • 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
    details.

18
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
    side
  • 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
    gaskets.
  • For low heat duty applications, simple
    constructions, easy assembly, lightweight
    elements and minimum number of parts contribute
    to minimizing costs.

19
IPS inch per second (unit system) NFA net flow
area
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