This is a fork of a different, but related thread about drying feedstock air to assist with filament drying:
This fork was created so as not to dilute that thread, which is extremely worthwhile on its own, even more so without this tangent distracting from it.
Vision Miner recommends that the make-up air feeding into a blast oven have a dewpoint of -40C or better:
Maybe that’s an overly conservative interpretation of their footnote, but for exactly that reason that’s how I’m parsing it.
Ya, I don’t think so… $499. 
I was providing a link to the -40C dewpoint reference (for those who want to check the original source), but, unfortunately, it came off looking like a link/advertisement for their drying kit. 
Outside my control. Honest. Ironically, their own drying kit doesn’t provide pre-dried make-up air, so just ignore that part of the reference link. It’s the footnote that’s of relevance for answering @user_3026326371 's question (above). Thus far it’s the only reference I’ve found which provides a hard number prescriptively.
NOW I see where you are going and where the minus 40 come from 
I think you got the temperature confused with the DEW POINT.
Assuming we don’t want to dry soaked filament and considering the max the plastic can accumulate:
If the airstream drying the filament comes OUT (assuming dry air going in) with a moisture content of 105 taken from the filament then to reach a dew point of minus 40 degrees Celsius we only need about minus 16 on the cold side/cold trap.
That however is at AMBIENT pressures.
If we would use a vacuum assisted dryer the temps would go up big time, even above the zero mark with ease.
I can boil water with my crappy vacuum pump and even better with the proper one.
The beauty of freeze drying, as that is what we are talking about here, is this:
Even with a moderate vacuum the boiling temp of the water goes down while the dew point goes up.
The LOWER the dew point is set the more water can be made to condense in a fixed period of time.
I’ve made a quick a dirty fruit dehydrator for a friend a few years back.
He already had an ice maker so I did not bother with a compressor unit for refrigerant.
Instead we used a 30 litre stock pot and added a 15mm perspex lid to with - with a silicone seal ring.
A little two way valve to hook up a little fridge compressor acting as a vacuum pump.
Fruit went onto trays and the pot into an ice bath.
The water literally condensed everywhere and there was no need to keep the compressor running, worked much faster than those hot air dryers and the fruit looks much nicer as well.
So are we now talking about freeze drying one or two rolls at once or are we talking about using this concept in a big box and at ambient pressures ?
the latter case. More specifically, to dry the makeup air to a dewpoint of -40C or lower. Baby steps.
I take it back. It’s time for me to throw in the towel on this narrow minded approach. Even if it’s technically viable, it now looks as thought it’s wasteful economically. You’re right: it should be far cheaper to apply some kind of a vacuum while heat drying than to cryogenically generate bone dry make-up air to be used while heating under normal atmospheric pressure. Strangely, however, almost nobody seems to be doing it that way, or even talk about how one day they might be doing it that way. Why is that? Is there a yet-to-be-revealed gotcha in that approach? Unfortunately, CNCKitchen never tested this option. He only tried applying a vacuum, but not while simultaneously also heating. He should have, but he didn’t.
