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In each footage from ISS I see this thick tube going through the hatches into Soyuz spacecraft, ending in the descent module.

What is it for?

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Ventilation.

In microgravity, hot/cold air does not move the same way, so you do not get natural mixing or movement of air.

Sleeping astronauts get a air stream pointed at them so that they do not get enveloped in a cloud of carbon dioxide and die.

When docked at the ISS the Soyuz is mostly powered down, or runs of the ISS power and life support systems. After all they are designed to stay docked for 200= days.

Mir was full of these things between the modules. The ISS design was a bit better and seems to require fewer. But the Soyuz vehicles still need the ventilation.

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    $\begingroup$ I still wonder why it must be so thick... it's not like Soyuz is exposed to some conditions vastly different than the rest of the ISS, and maintaining the balance shouldn't require some huge air flows... $\endgroup$ – SF. Jan 24 '16 at 2:19
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    $\begingroup$ Weight. It doesn't need some huge air flow, pressure within the flex duct is barely above 1 atm as is evident by its paper thin walls not bulging outwards between spring reinforcement at all. If they'd go with smaller diameter duct, pressure would have to be increased to support same volumetric flow rate, which would increase pressure differential, require thicker walls and make the duct heavier. $\endgroup$ – TildalWave Jan 24 '16 at 17:32
  • $\begingroup$ if you have ever worked in home HVAC systems, old houses, being retrofitted, often use a narrower system to transport air, called high pressure. They are about 2 inches wide, and much thicker/stiffer/stronger hoses than the more flexible 5-6 inch hoses for all the reasons Tildal offered. $\endgroup$ – geoffc Jan 24 '16 at 17:52

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