09012006

1
Heat Exchanger Design

• Shell-and-Tube Exchangers
• Ref McCabe, Smith, Harriott, 7th ed., Chapter
  15

2
Learning Objectives

• What is a shell-and-tube exchanger?
• How to select the number of tube passes
• How to select the baffle pitch
• What are the temperature patterns?
• How to correct the LMTD for multi-pass
• How to rate an existing exchanger
• How to select a standard exchanger

3
Shell-and-tube Exchangers

• Provide large amounts of heat transfer area--up
  to 5000 ft2--in relatively small space
• Available in standard sizes--specified by
  TEMA--of shell diameters and tube counts
• Number of tubes per shell depends on shell
  diameter, tube diameter, and tube pitch
• Tube lengths available in multiples of 4 ft,
  e.g., 8, 12, 16, 20 ft

4
Shell-and-Tube Exchanger
P baffle pitch Ds shell inside diameter

• Cross sectional area of tubes
• Outside heat transfer area

Di tube inside diameter Do tube outside
diameter
5
Selection of number of tube passes

• Inside film coefficient changes with the 0.8
  power of the number of tube passes
• Pressure drop changes with the 2.8 power of the
  number of tube passes
• Example

Npasses hi ?p, psi
6
Selection of baffle pitch

• Shell fluid velocity is inversely proportional to
  the baffle pitch (distance between baffles)
• ho increases with velocity to the 0.6 power
• ?p increases with velocity to the 2.8 power

P, inches ho ?p, psi
7
Temperature Patterns
8
Correction to LMTD
For one shell pass, 2, 4, 6, tube passes
McCabe, Smith, Harriott, 7th ed., Figure
15.6(a), p. 447
9
Correction to LMTD
For two shell passes, 4, 8, 12, tube passes
McCabe, Smith, Harriott, 7th ed., Figure
15.6(b), p. 448
10
LMTD Correction Factor FG

• When either fluid temperature is constant, FG 1
• It does not matter which fluid is in the shell
  and which is in the tubes
• It does not matter in which direction the shell
  fluid flows
• For cross flow, see McCabe, Smith, Harriott,
  7th edition, Figure 15.7, page 452.

11
Rating an existing exchanger

• Exchanger exists and we know
• Number of tubes and tube passes, Nt, Npasses
• Tube diameter, length, and rating, Do, Di, L, xw
• Shell diameter, baffle pitch, tube pitch, Ds, P,
  p
• Compute area of heat transfer
• Compute design coefficient

12
Exchanger Rating (continued)

• Compute film coefficients and clean over-all
  coefficient
• Compute available fouling resistance
• If fouling resistance is large enough, the
  exchanger is suitable for the service

hi from Sieder-Tate ho from Donohue
13
Selecting a Standard Exchanger

• Estimate the heat transfer coefficient U (see
  McCabe, Smith, and Harriott, 7th ed., Table 11.2,
  page 343)
• Calculate an estimate of the area and number of
  tubes
• Select an appropriate shell diameter from TEMA
  tube count tables
• Rate the selected exchanger.

14
Summary

• Shell-and-tube heat exchangers provide large heat
  transfer area in a compact space
• Fluid velocities are controlled by the number of
  tube passes and baffle spacing
• LMTD must be corrected for the parallel/countercur
  rent patterns
• Select and rate exchangers by determining the
  available fouling resistance