Purpose of expansion tank and spider hose?

[NickL]

So how does it all work, specifically the expansion tank. Does the port on the spider hose draw coolant from it? Or does it flow coolant back to the expansion tank?

I'm just lost on how bmw's cooling systems work in these cars. I'm doing something completely different so I need to know what to do with the spider hose.

[nrubenstein]

The spider hose is a return hose. Water flows into the water pump at the front of the engine, sourced from the head via the heater core. The expansion tank doesn't flow per se. Excess coolant goes there because there's nowhere else to go, and needed coolant is sucked out.

[Nutzy]

It does flow continuously while the engine is running. Pop the cap off (when cold!) and you can see the coolant squirting in from the small hose coming from the radiator.

It serves a few purposes I think. 1. is expansion, 2. is a reservoir, 3. is a place to constantly 'bleed' the radiator of any caught up air.

I got a slightly altered take on it. The swirl tank burps the system of any gas and expansion then goes on to a non pressurized catch bottle.