Is there any responsibility for this issue for anyone outside of the equipment vendor? In the US, this would be entirely his problem. I presume you will make sure that the heater sees 270 C as a part of the acceptance test. If there's a problem, won't it surface then?
As far as the problem goes, physics is what physics is. If there is not enough room to clear the expansion of the hot piping, something is going to give. Most likely, it will be the outer wall that the headers psss through. Are you sure that there is not enoguh flexibility in the air cooler housing to make the problem go away?
If the air cooler housing can't flex sufficiently, then one or more of the axial restraints must be removed selectively, or else physics will remove one randomly and, very likely, noisily. Without seeing drawings of the equipment, it's kind of hard to give concrete advice or a rational recommendation.
It's very hard for me to believe that the equipment vendor has made this sort of mistake and does not understand the concequences. Another possible thing you need to look at - if the air cooler gets hot, will the support steel expand enough to increase the axial clearance for the headers? If that's the case, then you have no problem.
I realize nobody wants to spend the money to fix this right now, but if there is a problem somebody is going to have to fix it eventually. Better for someone to bite the bullet up front and fix the problem with construction trades than to have to shut down an operating plant and fix it with maintenance personnel (who are usually not much good at construction even though they think they are).
I repeat, make sure your acceptance test gets this unit up to the max rated temperature, and keeps it there long enough that if anything is going to break, it breaks right then. That should (if the contract with the equipment vendor is any good at all) make sure that the vendor fixes the problem before he turns over the equipment.
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CraigB