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.
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.
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.
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)|
For titanium, thickness is as low as 1 mm.
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 Pass Geometry
There are three main types of tube passes: ribbon, quadrant, and H-band.
Tube pitch is the shortest center-to-center distance between tube.
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
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.
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