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
(No Transcript)
5
(No Transcript)
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
(No Transcript)
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