ME421 Heat Exchanger and Steam Generator Design

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ME421Heat Exchanger andSteam Generator Design

• Lecture Notes 7 Part 2
• Shell-and-Tube Heat Exchangers

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Basic Design Procedure

• Flow rates compositions, temperatures,
  pressures.
• Process Eng ? Design Eng
• Shell and head types, baffles, tube passes, etc.
• Preliminary design/analysis
• Use heat transfer and pressure drop correlations

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Preliminary Design

• Estimate heat transfer coefficients and fouling
  resistances.
• Tables 8.4 and 8.5 give h and U values for
  various cases
• Estimating h is preferred (Table 8.4)
• With h, Rfs, Rw, and overall surface
  efficiencies (in case of fins on either side)
  estimated, evaluate the overall heat transfer
  coefficient
• This is the most general expression, also
  estimate Uc.
• Take F 1.0 for counterflow HEX (single tube
  pass), or F 0.9 for any even number of tube
  passes.

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Preliminary Design (continued)

• Estimate heat load
• Calculate ?Tlm,cf
• Estimate the size of the HEX
• This area is also related to tube diameter do and
  number of tubes Nt
• The objective is to find the number of tubes with
  diameter do, and shell diameter Ds to accommodate
  the number of tubes, with the given tube length.

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Preliminary Design (continued)

• Shell diameter, Ds is
• CL is the tube layout constant
• CL 1.0 for 90o and 45o, CL 0.87 for 30o and
  60o
• CTP is the tube count calculation constant
• CTP 0.93 for one tube pass
• CTP 0.90 for two tube passes
• CTP 0.85 for three tube passes
• PR is the tube pitch ratio, PT/do
• Number of tubes, Nt is

See Example 8.1
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Rating of the Preliminary Design

• If HEX is available, skip preliminary design and
  proceed with rating only. If rating shows that Q
  and/or pressure drop requirements are not
  satisfied, select a different HEX and iterate.
• If not, preliminary design output is the rating
  input. Calculate the heat transfer coefficients
  and pressure drops.
• If length is fixed, rating output is outlet
  temperatures if heat load is fixed, rating
  output is HEX length.

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Rating of the Preliminary Design (continued)

• Tube side Chapters 3 4 for heat transfer
  coefficient and pressure drop calculations
  (two-phase flow later)
• Shell side more complicated
• If rating output is not acceptable, modify
• HEX cannot deliver the heat required increase h
  or area
• To increase hi, increase um in tubes, thus number
  of passes
• To increase ho, decrease baffle spacing or
  decrease baffle cut
• To increase area, increase length or shell
  diameter, or use shells in series
• ?ptube gt ?pall decrease number of tube passes or
  increase tube diameter (thus decrease tube
  length, increase shell diameter and number of
  tubes)
• ?pshell gt ?pall increase baffle spacing, tube
  pitch and baffle cut, or change type of baffles

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Shell Side Analysis
Kern Method (simple method) Shell Side Heat
Transfer Coefficient

• Baffles increase heat transfer coefficient due to
  increased turbulence, tube correlations are not
  applicable
• Without baffles, h can be based on De, similar to
  double-pipe HEX, and Chapter 3 correlations can
  be used
• On the shell side, McAdams correlation for Nu

square
triangular
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Kern Method (simple method) Shell Side Heat
Transfer Coefficient (continued)

• Gs (shell side mass velocity) can be evaluated
  from
• where is the bundle crossflow area at the
  center of the shell
• Ds shell diameter
• C clearance between adjacent tubes
• B baffle spacing
• PT pitch size
• Gs evaluated here is a fictional value because
  there is actually no free-flow area on the shell
  side. This value is based on the bundle crossflow
  area at the hypothetical tube row possessing the
  maximum flow area corresponding to the center of
  the shell

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Kern Method (simple method) Shell Side Pressure
Drop

• Depends on the number of tubes the fluid passes
  through in the bundle between baffles and the
  length of each crossing.
• The following correlation uses the product of
  distance across the bundle, taken as Ds, and the
  number of times the bundle is crossed.
• ?s (?b/?w)0.14
• Nb L/B 1 is the number of baffles
• (Nb 1) is the number of times the shell fluid
  passes the tube bundle
• f takes into account entrance and exit losses
• where

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Kern Method (simple method) Tube Side Pressure
Drop

• Total pressure drop including sudden expansions
  and contractions during a return (for multiple
  tube passes)
• Ignore second term if single tube pass
• See Example 8.2 for the application of Kern
  method on Example 8.1

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Bell-Delaware Method (complex method)

• Shell side flow is complex, combines crossflow
  and baffle window flow, as well as baffle-shell
  and bundle-shell bypass streams and other complex
  flow patterns
• Five different streams are identified A, B, C,
  E, and F
• Bell-Delaware method takes into account the
  leakage and bypass streams, most reliable method
  for shell side
• B-stream is the main stream, others reduce it and
  change shell side temperature profile, thus
  decrease h
• A leakage through tube/baffle clearance, C
  bundle bypass stream, E baffle bypass stream, F
  multi tube pass

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Bell-Delaware Method Shell Side Heat Transfer
Coefficient

• hideal is the ideal heat transfer coefficient
  for pure crossflow in an ideal tube bank
• Js are correction factors
• ji is the Colburn j-factor for an ideal tube
  bank (Figures 8.15-8.17, depend on shell side Re,
  , tube layout, and
  pitch size or correlation 8.25)
• As is the crossflow area at the centerline of
  the shell for one crossflow between baffles, As
  Ds CB/PT
• Note that Res is different for this method
  (based on do)

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Bell-Delaware Method Shell Side Heat Transfer
Coefficient (continued)

• Correlation for the Colburn j-factor for an ideal
  tube bank
• a1 a4 from Table 8.6 in book
• Correlation for ideal friction factor
• b1 b4 from Table 8.6 in book as well

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Bell-Delaware Method Shell Side Heat Transfer
Coefficient (continued)

• Correction factors (Js)
• Jc is the correction for baffle cut and spacing.
  For a large baffle cut, 0.53 for no tubes in
  window, 1.0 and for small windows with a high
  window velocity, 1.15.
• Jl is the correction factor for baffle leakage
  effects (A- and E-streams). Putting baffles too
  close increases leakage. Typical value 0.7 - 0.8.
• Jb is the correction factor for bundle bypassing
  effects and shell and pass dividers (C- and F-
  streams). For small clearance between outermost
  tubes and shell for fixed tube sheet
  construction, 0.9. For a pull-through rotating
  head, 0.7.
• Js is the correction factor for variable baffle
  spacing at the inlet and outlet. Usually between
  0.85 and 1.0.
• Jr applies if Res lt 100. If Res gt 100, Jr 1.0.
• The combined effects of all Js is 0.6.

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Example 8.3

• Given specifications for a HEX, first perform
  preliminary design, then detailed thermal
  analysis
• Compares the heat transfer coefficient on the
  shell side, evaluated using three methods
• Kern Method (note the different equation for As,
  but gives the same result as As DsCB/PT)
• Taborek Method (just a different Nu correlation
  than McAdams, other procedures same as Kern
  Method, but Res is based on do, not De)
• Bell-Delaware Method (Res is again based on do
  not De)
• All three methods give comparable ho as a result
• Then, hi, Uc, Uf (Rft given in the problem), Af,
  Ac are calculated
• OS is evaluated as 43, but it should not exceed
  30 in design specifications. Therefore, a new OS
  is assumed (20) and Rft is recalculated, which
  will help determine a suitable cleaning schedule.
  With this OS, the new Af and Ds are found.
• With these new constructional parameters, the
  design must be re-rated (you can do this as an
  exercise)

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Bell-Delaware Method Shell Side Heat Pressure Drop

• The total nozzle-to-nozzle pressure
• drop has 3 components
• Entrance and exit
• Internal
• Window

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• Entrance and Exit
• Affected by bypass but not by leakage
• Effect due to variable baffle spacing
• where ?pbi is the pressure drop in an equivalent
  ideal tube bank in one baffle compartment of
  central baffle spacing
• Rb is the correction factor for bypass flow (C-
  and F-streams), 0.5-0.8 depending on the
  construction type
• Nc is the number of tubes crossed during flow
  through one crossflow in HEX
• Ncw is the number of tube rows crossed in each
  baffle window
• Rs is the correction factor for the entrance and
  exit section having different baffle spacing (see
  literature for tabulated correction factors)

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• Internal
• Interior crossflow section (baffle tip to baffle
  tip)
• where Rl is the correction factor for baffle
  leakage effects (A- and E-streams), 0.4-0.5
• Nb is the number of baffles

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• Window
• Affected by leakage but not by bypass
• Combined pressure drop in all windows
• where ?pwi is the pressure drop in an equivalent
  ideal tube bank in the window section

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• The total pressure drop over the shell side is
  then
• The pressure drop in nozzles must be calculated
  separately
• ?pbi is calculated from
• fi from Figs. 8.15 8.17 or correlation 8.26
• For an ideal baffle window section, ?pwi is
  calculated from

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• See literature for Dw, Aw, and correction
  factors.
• Number of tube rows crossed in one crossflow
  section, Nc
• Lc is the baffle cut distance from
• baffle tip to inside of shell

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Bell-Delaware Method Shell Side Heat Pressure
Drop (continued)

• Number of tube rows crossed in each window, Ncw
• Number of baffles, Nb
• If Bi B Bo, then Nb L/B 1
• The total shell side pressure drop of a typical
  shell-and-tube HEX is about 20-30 of the
  pressure drop that would be calculated without
  taking into account baffle leakages and tube
  bundle bypass effects.
• Read the Chapter on Shell-and-Tube HEX from D.
  Biniciogullaris M.S. Thesis, PDF document on web.

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Example 8.4

• Given the HEX designed in Example 8.3, and other
  specifications, calculate the shell-side pressure
  drop using Bell-Delaware method to see if HEX is
  suitable.
• Takes into consideration all factors mentioned in
  the previous 7 slides.
• Compares the result with that obtained through
  Kern method.
• ?pBD lt ?pK, about 48.

Example 8.5

• Complete design of a HEX for given process
  specifications with the Kern method.
• The example can be repeated with the
  Bell-Delaware method as an execise.