How to Choose the Correct Tube Wall Thickness for a PTFE Exchanger in a High Vacuum Service?

A PTFE shell and tube condenser is used to recover a valuable solvent under deep vacuum to reduce its boiling point. The shell side is virtually in a vacuum. The thin and flexible PTFE tubes not only carry the cooling water but also resist the crushing force of the external atmosphere pushing inwards. A normal thin-walled tube, totally appropriate for positive pressure, can abruptly collapse flat like a drinking straw, stopping the flow and wrecking the thermal performance. The tube wall thickness for vacuum service is not designed to contain pressure but to prevent a catastrophic collapse. Choosing the proper tube wall thickness for PTFE exchanger vacuum service depends on an understanding of buckling physics, thermal trade-offs, and support solutions.

The Physics of Collapse Under External Pressure
How a Vacuum Destroys a Tube
When one side of a heat exchanger is evacuated (e.g. to 10 mbar abs or less), the external air pressure (about 1 bar or 14.7 psi) acts uniformly on the outside of each tube. The interior of the tube may be at atmospheric pressure or perhaps slightly higher. This external load causes a hoop compressive stress in the wall of the tube. A thin-walled flexible PTFE tube (with a comparatively low elastic modulus of roughly 0.5 GPa compared to 200 GPa for steel) would bend and collapse into an oval or flattened shape with a pressure much lower than the yield stress of the material. The hoover is an invisible giant hand squeezing on the tube. The only thing that prevents it from crushing is the thickness of the wall.

Supporting the Tubes: The Baffle Spacing's role
The unsupported length of the tube between baffles or tubesheets has a great influence on collapse resistance. A tube supported at 200 mm intervals will stand much better to external pressure than a tube supported at 500 mm intervals. For high vacuum service, baffle spacing is frequently lowered to 100–150 mm. Closer baffles result in higher shell-side turbulence and better heat transmission, to partly compensate the thermal cost of the thicker wall. However, increasing the number of baffles increases the pressure drop on the shell side, and may demand a bigger shell diameter to keep the flow area.

In addition, the design should allow for a vacuum on the tube side and a positive pressure on the shell side. In floating head or U-tube construction, either side can be evacuated, and the tube wall must be strong enough to prevent collapse from the side under vacuum. In these circumstances the thicker wall is selected and the exchanger is generally designed as a 'full vacuum' rating on both sides.

Practical Guidelines for Selecting
Step by Step Process
Define the level of hoover: Give the absolute pressure on the low side (i.e. 10 mbar, 50 mbar). The worst situation is full vacuum (0 millibar absolute) and requires the strongest design.

• Choose tube sizes : Choose an outer diameter (OD) that is a compromise between heat transfer, fouling, and mechanical strength. The normal sizes are 6 mm or 8 mm OD for hoover service. Larger diameters (10 mm and up) are not practicable.

Use the manufacturer's collapse pressure data or a FEA to estimate the needed wall thickness 6 mm OD is a cautious starting point, 0.8 mm thickness at room temperature. 1.0-1.2 mm may be necessary for greater temperature or larger OD.

Set baffle spacing: Span unsupported not more than 150 mm. For long tubes (more than 2 m) extra intermediate supports or tie-rods with spacers are utilised.

Evaluate thermal performance: Calculate U value with selected wall thickness If the U-value is too low (e.g. less than 200 W/m2K) think about raising the tube count or length, or accept a bigger exchanger.

Validate with manufacturer: Proven PTFE exchanger vendors have empirical collapse data for their tube dimensions. A written guarantee is required that the equipment will function without collapse under the specified vacuum and temperature conditions.

Example specification PTFE tube outer diameter 6 mm. Wall thickness: 1.0 mm + / -0.05 mm Tube material: virgin PTFE. Maximum unsupported span 120mm Shell side design pressure: complete vacuum (0 mbar abs). Tube side design pressure: 3 bar g. Maximum working temperature: 90'C. The vacuum in the exchanger shell side should be 10 mbar absolute with tubes at atmospheric pressure. Tubes should not be visibly collapsed or ovalled.'

Technical Accuracy: Calculation of Collapse Pressure
The failure of a tube under external pressure is a buckling failure, not a yield failure. First approximation is given by standard formulae for thin-walled tubes (e.g. Timoshenko formula for elastic buckling), however non-linear stress-strain behaviour and creep of PTFE need to be validated empirically. Most makers of PTFE exchangers test sample tubes for hydrostatic external pressure. The measured collapse pressure for a 6mm OD tube with 1.0mm wall thickness at 20°C is often 1.2-1.5 bar, giving a safety factor of 1.2-1.5 above complete vacuum. At 90 °C the collapse pressure might be as low as 0.8-1.0 bar and therefore a bigger safety margin is needed. Hence, supports are spaced more closely or the walls are made thicker for elevated temperature vacuum service.

Conclusion: Living in the Invisible Squeeze
The thickness of the wall of a vacuum PTFE exchanger has to be a fight against the crushing force of the atmosphere. And a thicker, stiffer and wellsupported tube wins the fight. The tubes are capable of withstanding full vacuum without collapse because of a minimum wall thickness of 0.8-1.0 mm and closely spaced baffles (unsupported span 150 mm). The U-value is improved by the thicker wall but the alternative-a collapsed, clogged tube-is unacceptable. The silent, invisible pressure of the air outside is the most strong force in a vacuum system and a properly selected tube wall thickness is the only defence against it.