Sagging of Tubes in a Heat Exchanger Tube Bundle

With the 14 mm tubes and the 1.1 m between supports (assuming that some tubes are supported by every other baffle), this is right at the maximum unsupported span per TEMA (Tubular Exchanger Manufacturer's Association) limits for Cu tubes at this diameter.

I don't think that the sag in and of itself is an issue. What can be an issue with long spans between supports, is vibration. If tubes are vibrating due to the shellside flow and contact each other, that will cause premature tube failures.

Monitor it for damage and progression of damage. If there is some contact damage and you have access, you can insert flat-bars between the tubes half-way between the baffles. Secure the outer ends of these to some kind of rim to keep them from moving.

If you have to buy another, pay attention to the baffle spacing and the vibration evaluation.

GA Lawrence, you have correctly pointed out the real problem now- "vibration". All the tubes are supported at 8 places. The finned tubes are supported by a split hexagonal shape made of 1mm thick sheet metal resting on each other. Tubes at the middle are more affected. Sagged tubes could be be moved easily, even by finger. Hot air is entering from one side. Exchanger manufacturer says air velocity in shell is only 0.9 m/s and they have designed for much more velocity & they have made hundreds of exchangers with similar design.

It appears to me that supports are delicate, may be good enough for a steady air velocity but not for the sudden change in air velocity during start-up, tripping and surging of compressor. Exchangers are bought-out item for compressor manufacturer. Expecting more comments from the learned members.

GA, for adding more clarity to question, seldom done by most OP's."finned tubes are supported by a split hexagonal shape made of 1mm thick sheet metal resting on each other"It appears to be the root cause of the problem. Acceleration and deceleration during surging and unplanned tripping of the machine is extremely high. Tube supports should be designed to take care of it. The split of the hexagonal shape sheet metal support may be opening up or getting flattened due to low thickness, allowing tubes to move up & down or side wise causing sagging. Putting any plate across to support, as suggested earlier by some one may affect heat transfer. A rigid support with allowance for thermal expansion may be the answer, which shall be done by OEM of exchanger.