Swirl Pot and Expansion Bottle Query
Discussion
Can anyone explain how the header "expansion" tank/ Swirl pot system works on the V6 2.9 set up, I have just noticed that on mine the neck of the expansion bottle is below the neck of the swirl pot. I understand the basics of the operation but I would have thought that water level in the expansion vessel would have to be higher than that of swirl pot in order for air to be expelled as the swirl pot extracts it from the coolant, if the amount of air in the swirl pot is insufficient to clear the pipe to the header tank then will this not just act as another air lock?, I am wondering if this is why most Swirl pots are always only 2/3 full, if I raise the header tank the air would presumably exit to it and water would replace it into the swirl tank thus keeping it full.
Or have I got this completely wrong.Any thoughts?
Or have I got this completely wrong.Any thoughts?
There are some cars that are connected up differently, but the vast majority of cars I've seen are connected up like this:
Top hose into the swirl tank at the top, out at the bottom.
Degas hose comes out of the swirl tank at the top, connects to the bottom of the expansion tank.
Overflow from the top of the expansion tank to fresh air.
The swirl tank and expansion tank both have fittings to take a cap and they will both accept a pressure cap or a blanking cap.
If the system is all in good order, it will work best with the blanking cap on the swirl tank and the pressure cap on the expansion tank.
As long as you have one blanking cap and one pressure cap, the design will work quite well with the caps either way round.
There are some fairly common faults which can develop which mean the system will only work properly *on some individual cars* with the caps a particular way round.
How the expansion tank works depends which way round you have the caps.
If you have a blanking cap on the swirl tank then water/air can move freely between the two tanks. As the cooling system pressure varies with temperature and revs this causes a continual tidal flow back and forth between them. Because of the way the hoses are connected, the flow out of the swirl tank will tend to include air / bubbles from the top of the tank; the flow back from the expansion tank will tend to take the degassed water from the bottom of the tank. Hence this arrangement uses the expansion tank as an air separator and continually transfers air into the expansion tank (where it will naturally seep out past the pressure cap).
If you have the pressure cap on the swirl tank this tidal flow does not occur. The expansion tank is essentially stagnant and unpressurised, and just acts as an overflow catch tank for the swirl tank (which now has to act as a swirl tank and expansion tank). This doesn't work quite as well but it does work. Any water/air that escapes past the pressure seal on the top of the swirl tank will dribble into the catch tank. Eventually when the engine is switched off and has cooled down the pressure will drop. If it drops so far that there is a slight suction, the cooling system will pull water back from the catch tank, past a one-way valve in the pressure cap, and back into the swirl tank. And that works too, as long as all the necessary seals and valves are in good order.
Top hose into the swirl tank at the top, out at the bottom.
Degas hose comes out of the swirl tank at the top, connects to the bottom of the expansion tank.
Overflow from the top of the expansion tank to fresh air.
The swirl tank and expansion tank both have fittings to take a cap and they will both accept a pressure cap or a blanking cap.
If the system is all in good order, it will work best with the blanking cap on the swirl tank and the pressure cap on the expansion tank.
As long as you have one blanking cap and one pressure cap, the design will work quite well with the caps either way round.
There are some fairly common faults which can develop which mean the system will only work properly *on some individual cars* with the caps a particular way round.
How the expansion tank works depends which way round you have the caps.
If you have a blanking cap on the swirl tank then water/air can move freely between the two tanks. As the cooling system pressure varies with temperature and revs this causes a continual tidal flow back and forth between them. Because of the way the hoses are connected, the flow out of the swirl tank will tend to include air / bubbles from the top of the tank; the flow back from the expansion tank will tend to take the degassed water from the bottom of the tank. Hence this arrangement uses the expansion tank as an air separator and continually transfers air into the expansion tank (where it will naturally seep out past the pressure cap).
If you have the pressure cap on the swirl tank this tidal flow does not occur. The expansion tank is essentially stagnant and unpressurised, and just acts as an overflow catch tank for the swirl tank (which now has to act as a swirl tank and expansion tank). This doesn't work quite as well but it does work. Any water/air that escapes past the pressure seal on the top of the swirl tank will dribble into the catch tank. Eventually when the engine is switched off and has cooled down the pressure will drop. If it drops so far that there is a slight suction, the cooling system will pull water back from the catch tank, past a one-way valve in the pressure cap, and back into the swirl tank. And that works too, as long as all the necessary seals and valves are in good order.
MnS said:
tozerman said:
My swirl pot is always full to the brim and my expansion tank is always a third full!
Cheers.....Tony..
to GreenV8S explanation which is very much like mine last year, I think we all need to accept that both ways work if there are no faults in the system and both have advantages and disadvantages.Mark
Edited by mep12345 on Saturday 11th April 16:41
GreenV8S said:
If you have a blanking cap on the swirl tank then water/air can move freely between the two tanks. As the cooling system pressure varies with temperature and revs this causes a continual tidal flow back and forth between them. Because of the way the hoses are connected, the flow out of the swirl tank will tend to include air / bubbles from the top of the tank; the flow back from the expansion tank will tend to take the degassed water from the bottom of the tank. Hence this arrangement uses the expansion tank as an air separator and continually transfers air into the expansion tank (where it will naturally seep out past the pressure cap).
Thanks Peter,Thats pretty much what I visualised, I just struggled with air gathering at a lower point rather than highest.I'm sure I have read a number of posts on here whereby people say their swirl pot is about 1/3 empty most of the time though.
magpies said:
what happens if you dont have the swirl pot as per most other production cars? - I have an S1 which I purchased without engine (and swirl pot) I have a 24v engine to fit.
The swirl pot is just a reservoir doubling as a crude air separator. It's not common to have a full flow swirl pot and you can use a more conventional system if you prefer. For example I've ditched the whole thing and replaced it with a swirl tank that connects between the top hose and the pump inlet. Because I have an entry stat, this is energised all the time even when the engine is cold, which makes it more effective. If you make this sort of change you do need to make sure the layout is thought through so filling/venting works OK and the degas system is effective. Mine was all custom made but there's no reason you couldn't achieve a similar setup using standard production components.
Sorry to all but got drawn in again 
Roy,
There was a thread about a year ago titled "stirring a hornets nest", but I can't find it through the search facility where I gave an explanation of the system and how it should work (I am an Engineer who works with fluid systems) and both ways round should work if the system is fault free, each with there own advantages and disadvantages. If you read the TVR bulletin carefully you'll notice it corrects an earlier one, so who says they didn't recorrect this one? In particular the first paragraph doesn't make sense (i.e. the donor car system would have run at the same (or similar) system pressure to TVR as most vehicles run at around 1.4 psi cooling system pressure (check the numbers on the caps at Halfords and see how little variation there is) so "blowing off the level sensor or cracking the bottle" etc is just complete rubbish).
In sum from a system design perspective it doesn't matter - do whichever you feel happy with and your car works best with as the system can cope with either and both have benefits and disadvanages when compared to each other. I prefer the pressure cap on the expansion bottle as I prefer to avoid the pressure pulsing disadvantage of having them the other way round, but each to they're own.
I will now and not get drawn in again - I promise
Mark
Roy,
There was a thread about a year ago titled "stirring a hornets nest", but I can't find it through the search facility where I gave an explanation of the system and how it should work (I am an Engineer who works with fluid systems) and both ways round should work if the system is fault free, each with there own advantages and disadvantages. If you read the TVR bulletin carefully you'll notice it corrects an earlier one, so who says they didn't recorrect this one? In particular the first paragraph doesn't make sense (i.e. the donor car system would have run at the same (or similar) system pressure to TVR as most vehicles run at around 1.4 psi cooling system pressure (check the numbers on the caps at Halfords and see how little variation there is) so "blowing off the level sensor or cracking the bottle" etc is just complete rubbish).
In sum from a system design perspective it doesn't matter - do whichever you feel happy with and your car works best with as the system can cope with either and both have benefits and disadvanages when compared to each other. I prefer the pressure cap on the expansion bottle as I prefer to avoid the pressure pulsing disadvantage of having them the other way round, but each to they're own.
I will now
and not get drawn in again - I promiseMark
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