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10:23 Jul 31, 2023 

I would understand "sparing" as a clumsy way of saying that these fans are "spares" - redundant back-ups ready to take over if the "main" fans fail. Or they may be "surplus to basic requirement" so that the whole thing can keep going if one fan fails.

There MUST be some "main fans" to insure input and output of fresh and used air, and they ought to be some redundancy built into the system - it's important enough for that.

So "Number of Diesel + 1" could mean that if one fan is what is needed to supply one diesel machine with fresh air, there has to be one "spare fan" available.

"3 x 50%" for exhaust fans would means you need 3 fans each having 50% of the total capacity needed, so if one fails you still have 100% of what's needed to get used air out.

IOW suitable fans sparing = suitable level of redundancy.

That's what would make sense.

05:10 Jul 31, 2023 

en suppléance

22:44 Jul 28, 2023 

This is a decidedly curious use of this term in EN, and it might be worth checking that there is no error / typo / scanno in your source text. In fact, was it maybe written by a non-native speaker of EN? If so, it might represent a mistranslation from their mother tongue?
The only thing I can think of that might just about make technical sense here would be if it was intended to have the sense of 'with redundancy' — i.e. at least 2 fans capable of relaying each other if necessary; it is common these days to run both fans alternately, so they wear out equally, and are also being tested.
It could also conceivably mean a 'fail-over' situation, where if one fan should fail, the other ones takes over its job.
If it did have the sense of 'sparingly', as interpreted by Florian, I suppose it might be taken as meaning 'with a bare minimum specification' — i.e. the smallest fans they can get away with. Although technically that would seem like a bad idea in the context given. But I don't think it would mean 'as few fans as possible'.