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Design of Shell and Tube Heat Exchanger – Tube Side Construction

In my previous post, design of shell and tube heat exchanger – layout design, I shared how to select fluid allocation, which one should be in tube side and which one should be shell side, general rule for allowable fluid velocity, pressure drop, and temperature approach. This post will be the continuation of design of shell and tube heat exchanger series, which specifically explain about construction detail in tube side.

Want to learn from the beginning?
Read my first post about step-by-step design of shell and tube heat exchanger. Still curious? Read my second about about layout design of shell and tube heat exchanger.

 

This is mind map of this topic.

Design of shell and tube heat exchanger - tube design
Design of shell and tube heat exchanger – tube design

Diameter

Available tube inside diameter is ranging from 16 mm (5/8 in) to 50 mm (2 in). To get economical and compact design, tube with diameter of 3/4 in – 1 in are usually used. But, when fouling is expected and low pressure drop is required, use tube with inside diameter 1 in. As a general guide, use inside diameter 3/4 in as a good trial to start design.

Thickness

Tube thickness is selected to withstand pressure and give adequate corrosion allowance. Table below shows standard wall thickness for steel tubes.

Outside diameter (mm)Wall thickness (mm)
161.21.62.0--
20-1.62.02.6-
25-1.62.02.63.2
30-1.62.02.63.2
38--2.02.63.2
50--2.02.63.2

For titanium, thickness is as low as 1 mm.

Length

This table shows preferred length of heat exchanger.

Preferred length of tube
6 ft (1.83 m)
8 ft (2.44 m)
12 ft (3.66 m)
16 ft (4.88 m)
20 ft (6.10 m)
24 ft (7.32 m)

Longer tube will reduce number of tubes. In the end, it will reduce shell diameter. For general rule, ratio of tube length to shell diameter is usually fall within 5 to 10.

Tube Side Passes

Tube side passes is selected to give required tube-side design velocity and taking maximum advantage of available pressure drop. Higher velocity will give higher heat transfer coefficient. Number of tube passes is ranging from 1 to 16 passes. However the standard design has one, two, or four tube passes. Figure below shows tube passes for two, four, and six passes.

Tube passes
Tube passes

Tube Pass Geometry

There are three main types of tube passes: ribbon, quadrant, and H-band.

 

Tube pass geometry
Tube pass geometry

Tube Pitch

Tube pitch is the shortest center-to-center distance between tube.

Tube pitch, (a) 30 deg triangular; (b) 90 deg rectangular
Tube pitch, (a) 30 deg triangular; (b) 90 deg rectangular

For triangular tube design (30 deg or 60 deg), tube pitch is 1.25 multiply by OD (outside diameter). If OD is 20 mm, then tube pitch is 25 mm. If OD is 25 mm, then tube pitch is 31.25 mm.

For square pattern, tube pitch is:

  • 1.25 multiply by OD, or
  • Minimum cleaning lane between adjacent tube is 6 mm (OD + 6 mm), whichever greater

For example we have 25 mm OD square pattern, which tube pitch to be used?

  • 1.25 x OD = 1.25 x 25 mm = 31.25 mm
  • based on minimum cleaning lane = 25 mm + 6 mm= 31 mm. Then, used 31.25 mm tube pitch

Tube Arrangement

There are four tube arrangement designs:

  • 30o triangular
  • 60o rotated triangular
  • 90o square
  • 45o rotated square

Table belows show the differences between those tube arrangements.

Criteria30 deg triangular60o Rotated triangular
Arrangement
Heat transferHigh
Pressure dropHigh
ApplicationLess fouling fluid
Fixed tube sheet design
Tube accommodationAccommodate more tubes
Effect on shell sizeSmall shell size
LimitationLimited to clean shell side service
Ease on shell side mechanical cleaning-
PopularityPopularLess popular compared to 30 deg triangular
Criteria90o square45o rotated square
Arrangement
Heat transferLow
Pressure dropLow
ApplicationHeavily fouling fluid
Floating head design
Pressure drop at shell side is constraint and flow is turbulentFor laminar or turbulent flow of a single phase fluid or condensing fluid on shell side
Tube accommodationAccommodate less tubes
Effect on shell sizeLarge shell size
Limitation-
Ease on shell side mechanical cleaningEasily
Popularity-

That’s all I want to share. However this is not the last! There will be the last series of design of shell and tube heat exchanger post which is shell construction design. After that, I plan to give you a bonus, so please continue reading my new post, comment, and share if you like!

Referece: Chemical Engineering Design – Coulson

Feature image from here.

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