02GF74
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 posted on 11/12/09 at 11:18 AM |
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kinda fundamental question - talk to me about air locks
how engine do airlocks cause overheating?
surely the water being forced by the pump can squeeze past the air????
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mr henderson
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| posted on 11/12/09 at 11:32 AM |
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That's a very good question, I will be interested to read the answers.
I love that avatar, BTW
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Bluemoon
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| posted on 11/12/09 at 11:32 AM |
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Yes, but the water ways split inside the engine, so there is more than one way for the water to go complicating matters... It will take the easy
path.
Also think upside-down U bend water can pass with an air pocket at the top but the flow is restricted, and the air-pocket can't be removed..
i.e. to get it out you would need an air bleed at the top of the bend..
It's probably even worse than that if you think about the air expanding as it gets hot the air in the trapped in an upside down U bend gets
bigger (as the heater tank vents) causing more restriction..
Dan
[Edited on 11/12/09 by Bluemoon]
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blakep82
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| posted on 11/12/09 at 11:35 AM |
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if you have an air lock, there will be bits inside the water way which will not be immersed in water and so the heat won't be taken away.
water will pass through it, but only a much smaller amount
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02GF74
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| posted on 11/12/09 at 12:06 PM |
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have to admit I am fining it hard to visualise.
let's keep i simple to upside down U e.g. as per thermostat housing or top of radiator.
one of two things happen when water is being pumped:
a) the water pushes the air through the system
b) the water flows "under the air" - so as ^^^ say, a lot less will flow - not sure I can picture that.
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mr henderson
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| posted on 11/12/09 at 12:10 PM |
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Perhaps it depends where the air lock is. Most of us have topped up the water system in a car, then run the engine for a bit, then had to top it up
again, because the pump has pushed the air around with the water.
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blakep82
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| posted on 11/12/09 at 12:17 PM |
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like this
 
Description
you're right, i didn't mean less will flow, but it won't flow as easy.
normally the pump will be pumping water up as fast as its coming down, gravity will help keep water moving up. and no real effort on the pump
with too much air the pump willbe pumping against gravity also
[Edited on 11/12/09 by blakep82]
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blakep82
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| posted on 11/12/09 at 12:19 PM |
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quote:
Originally posted by mr henderson
Perhaps it depends where the air lock is. Most of us have topped up the water system in a car, then run the engine for a bit, then had to top it up
again, because the pump has pushed the air around with the water.
that wouldn't strictly be an air lock though would it? an air lock would be permanantly trapped air i think
________________________
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craig1410
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| posted on 11/12/09 at 12:52 PM |
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If it helps to visualise then pretend it isn't air, pretend it is a piece of cork or polystyrene instead. If we are talking about a U bend then
the air/cork/polystyrene will stay more or less in position and will restrict the flow capacity of the hose.
I don't think it will tend to expand much when hot, as one of the posters above said, because the system is pressurised and this will counteract
the expansion due to temperature to a large extent. If anything I would expect the air volume to decrease slightly because the expanding water volume
(due to heat) will be more 'powerful' than the expanding air. This is after all why we run pressurised systems, to prevent water turning
to steam.
I hope this helps,
Craig.
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stevebubs
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| posted on 11/12/09 at 12:53 PM |
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Correct and whilst the pressure of the water will compress the air, it will only compress until the pressure of the 2 liquids (gas is a liquid) is
equal.
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Bluemoon
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| posted on 11/12/09 at 01:03 PM |
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quote:
Originally posted by craig1410
I don't think it will tend to expand much when hot, as one of the posters above said, because the system is pressurized and this will counteract
the expansion due to temperature to a large extent. If anything I would expect the air volume to decrease slightly because the expanding water volume
(due to heat) will be more 'powerful' than the expanding air. This is after all why we run pressurised systems, to prevent water turning
to steam.
I hope this helps,
Craig.
Yes and no. If there is not to much air, yes. Air will expand to a larger extent than water as it's a gas pressure is proportional to
temperature, if the system was vented.
Thus if the pressure becomes high enough (because of an airlock of sufficient cold volume) venting via the pressure cap occurs allowing the air to
expand.
Having worked this out the temperature increase will be too small, to allow this i.e. temperature increase by ~100C would cause a 1/3rd pressure
increase thinking about it for a gas, that's not enough to open the pressure cap (would require 100% pressure increase i.e. 15psi or so).
Water expansion is probably the dominate effect as the temperature increases..
Thanks Craig that made me think a bit harder about the physics!
So this effect will have little to do with airlocks, if anything the water expansion will be the dominate one then, if anything will help as the
airlock volume will be squashed as the water expands, probably decreasing the effect of an airlock.
Dan
[Edited on 11/12/09 by Bluemoon]
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Dusty
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| posted on 11/12/09 at 01:13 PM |
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Other effects? Suppose the air is under the thermostat as in a crossflow. Thermostat will not open. Air also replaces some of the water in the system
reducing efficiency. Suppose the air is trapped in the head. Hot spots, poor cooling, localised boiling?
That it does reduce flow is clear from all the old cars you drove that had poor heaters that warmed up when you bled them.
Probably because the pumped water will flow via the line of least resistance. Offered an option of pushing the water column a few inches uphill past
an empty length of hose or downhill through full hose it goes down. In the pic below if you keep uprating the pump you will get to a stage where the
lower loop offers the same resistance to flow as the hydrostatic pressure difference of the columns in the upper loop and flow will then start over
the top. Sort of. Many factors to consider.
Also depends on the power of the pump. I suspect most are only just up to the job.
[Edited on 11/12/09 by Dusty]
 
Rescued attachment cool.JPG
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Steve G
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| posted on 11/12/09 at 01:17 PM |
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I'd say fairly simply put, if you have an air lock which will end up at the highest point this will either restrict or block the flow of
coolant.
The energy from combustion (ie heat) needs to be dissipated - this is normally done by being transferred to the coolant, and to then be transferred to
heat the air flowing across the radiator.
If the flow is restricted around the cooling system, you lose or restrict the capacity for the heat energy to be removed from the engine and so the
metal will get hotter and hotter (and probably warp). The coolant in contact with this metal getting hotter and hotter will soon turn to steam and so
pressurise the system with fairly obvious consequences. Something will give eventually - be it the engine warping, a pressure cap releasing, or a
coolant hose going pop.
Does that sound logical??
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Findlay234
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| posted on 11/12/09 at 01:27 PM |
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With an expansion tank set up the more locked in air you have the less fluid there is in the system to remove heat. Water has a much higher specific
capacity and conductivity than high pressure air.
Any air in the system, whether it stays in situ, partially blocks the fluid flow or even just flows around the system with the water will reduce the
over cooling effect due to the reduced volume of working fluid.
As far as i can work out (in my head) there should be no situation where a bubble of air actually stops water moving round the system. It should act
just like a fluid albiet a less dense fluid so more likely to rise to the top of any U bends especially in low pressure cases such as the engine being
off or where you have very large diameter tubing. Large diameter tubing, where the cross sectional area is far greater than needed for your water
pumps flow rate will allow air to form bubbles in a U bend. This bubble shouldnt stop any water from still flowing around but it will reduce the
capacity to remove heat from the bend itself and by reducing the overall level of coolant fluid (the water) from the system the overall cooling
efficiency will go down. There shouldnt be an issue with this happening within the engine beacuse the internal water ways "should" be
designed to match the pumps flow rate so should push any air out.
If there was a case where the air formed a blockage then as the pump continues to spin the water in front of the air block should gain pressure and
the water behind the air block should loose pressure. This pressure differential will move the blocking fluid 'Air'
Therefore an air bubble should not stop your fluid moving around the system.
I will now say that it depends on the size of the bubble. if you have a bubble so large that it takes up a large percentage of the coolant volume then
the pump may get to a point where it starts to cavitate or even just spin in air. therefor no fluid will be moving around at all. I think this is a
very rare case and nothing to do with bubbles, this is a major loss of coolant fluid.
Oh ive also thought that you if you had a badly designed system that could maintain a large air bubble around the thermostat then this would
'lock' out the rest of the system and no coolant would pass through the radiator. but this shouldnt happen unless your thermostat is at
the top of your system and uses large diameter tubing...
[Edited on 11/12/09 by Findlay234]
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02GF74
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| posted on 11/12/09 at 01:51 PM |
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quote:
Originally posted by craig1410
If it helps to visualise then pretend it isn't air, pretend it is a piece of cork or polystyrene instead. If we are talking about a U bend then
the air/cork/polystyrene will stay more or less in position and will restrict the flow capacity of the hose.
I hope this helps,
hmmmm, I think I may have cracked it - I wasn't thinkg of the air being under pressure.
refering to the U-bend above.
With engine from cold, water will be pumped under the "lump" of air without any problems but as the system is sealed, the water will
expand thus increasing the pressure within the system.
As the air becomes under higher pressure, it stays at the highest point being less dense but the pump will find it harder to pump the water past
it.
My analogy.
Take a gulp of water, stick a toilet inner tube over yer gob with your hand over the other end. Now try to spit the water out. I reckon your hand
will get wet.
Now do the same but this time put the air in the tube under pressure. Unless you have the lungs of Geoff Capes, you are unlikley to squirt much, if
any, water out yer gob.
one more thing.
but you could say what if the air was water, then how can the pump ever work?
I think the difference here is that the water would be same density so is not resisting flow as the mnuch less dense air. If the pump was powerful
enough (or the tube small enough) , it would push the air round the system. maybe?
It would be interesting to see this happen with a transparent tube.
[Edited on 11/12/09 by 02GF74]
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Findlay234
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| posted on 11/12/09 at 02:27 PM |
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quote:
Originally posted by 02GF74
quote:
Originally posted by craig1410
If it helps to visualise then pretend it isn't air, pretend it is a piece of cork or polystyrene instead. If we are talking about a U bend then
the air/cork/polystyrene will stay more or less in position and will restrict the flow capacity of the hose.
I hope this helps,
hmmmm, I think I may have cracked it - I wasn't thinkg of the air being under pressure.
refering to the U-bend above.
With engine from cold, water will be pumped under the "lump" of air without any problems but as the system is sealed, the water will expand
thus increasing the pressure within the system.
As the air becomes under higher pressure, it stays at the highest point being less dense but the pump will find it harder to pump the water past
it.
My analogy.
Take a gulp of water, stick a toilet inner tube over yer gob with your hand over the other end. Now try to spit the water out. I reckon your hand
will get wet.
Now do the same but this time put the air in the tube under pressure. Unless you have the lungs of Geoff Capes, you are unlikley to squirt much, if
any, water out yer gob.
one more thing.
but you could say what if the air was water, then how can the pump ever work?
I think the difference here is that the water would be same density so is not resisting flow as the mnuch less dense air. If the pump was powerful
enough (or the tube small enough) , it would push the air round the system. maybe?
It would be interesting to see this happen with a transparent tube.
[Edited on 11/12/09 by 02GF74]
No this is not a good analogy, your hand is the blockage in your system not the air. the cooling system is also a closed loop system so better to
think in these terms... attached image...
 
Rescued attachment untitled1.JPG
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Findlay234
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| posted on 11/12/09 at 02:30 PM |
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The fluid will continue to move around regardless of the air in the system (to a point where the air takes over most of the system and the pump starts
to cavitate)
The cork analogy is also wrong because we are talking about a fluid here not a solid. ok it is a fluid under pressure but even a very viscous fluid
under high pressure (maybe use oil in a shock absorber) doesnt act like a cork.
In a closed loop system and as long as there are no 'solid' blockages to the loop then at any pressure the pump stength is not important.
you could have a computer fan in there (as long as the casing and components can handle the pressure) then it will be able to pump the fluid
around.
If the pressure in the system is at 5Bar then that doesnt mean the pump is working against 5bar because at rest the pressure behind the pump is also
at 5bar. the pump creates the pressure differential for the the fluid to flow. for a given fluid flow rate the pump only needs to be strong enough to
overcome the resistance to flow which will only come from the friction (including blockages). The viscosity of the fluid changes the level of friction
and although its dependant on pressure the difference is very small.
[Edited on 11/12/09 by Findlay234]
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02GF74
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| posted on 11/12/09 at 02:45 PM |
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well I am back to square one - as ^^^^ , the water will be pumped around.
so if air lock is in thermostat hoiusing (e.g. crossflow, so what, water is still getting round.
or in top of radiatr - so the top 1 or 2 cm see air but water is flowing thoruugh the vast majority of the radiator.
hence how is the air lock bad?
getting more confused ... 
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Findlay234
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| posted on 11/12/09 at 02:46 PM |
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This is all from memory by the way people so I stand to be corrected.. 
Cheers
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Steve G
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| posted on 11/12/09 at 02:53 PM |
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Water may well be pumped around - but the flow will be restricted to a greater or lesser degree by the air bubble. Think of a pipe in the shape of an
upturned "U" containing an air bubble and try blowing the water through. It'll take more effort than if there were no bubble. The
water pump can only give a certain amount of effort.
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02GF74
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| posted on 11/12/09 at 03:00 PM |
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quote:
Originally posted by Steve G
Think of a pipe in the shape of an upturned "U" containing an air bubble and try blowing the water through. It'll take more effort
than if there were no bubble.
I'm trying but nothing's happening.
Why?
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Findlay234
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| posted on 11/12/09 at 03:02 PM |
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An air bubble that occurs in your thermostat (and doesnt move) and stops (or locks) it is bad because then the cooling fluid will only move around the
internal waterways and not through the radiator. thereby limiting the volume of cooling fluid and limiting the surface area for heat exchange to the
atmosphere (something the radiator does very well)
If you have an air bubble in the radiator then you again limit the volume of fluid in the system and it can be hard to remove because the total cross
section of the radiator can be larger than the waterways to and from it and larger than the pumps capacity therefore creating a nice low pressure area
for the bubble to congregate. The bubble also has the effect of blanking off a portion of the radiator... not good. I think off the top of my head its
best to either have an air bleed at the top of the rad or to have the inlet to the rad at the bottom and the outlet at the top so the air flows with
the fluid as best it can...
A non moving air bubble in the pump is bad because you pump will cavitate and as its not an air pump will become useless. this would have to be a very
badly designed system with the pump at the top or a system that had run very dry.
Air bubbles in the rest of the system will lower the volume of working fluid so will make the system less efficient but shouldnt stop it working..
The only time i think i could ever see an air bubble stopping fluid flow (apart from the cavitation mentioned) would be a very tall system where the
pumps strength was unable to overcome the pressure created by the weight of the water above it. This would have to assume that there was more fluid on
the high side of the system and would factor in gravity.. I doubt any of our systems are too weak to overcome the changes in potential energy seen.
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Findlay234
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| posted on 11/12/09 at 03:06 PM |
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Where are you having a problem??
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02GF74
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| posted on 11/12/09 at 03:10 PM |
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the upside down u-bend.
if I take my garden hose, and hold it so that there is an upside down u-bend, even at lower pressure, I reckon water is gonna flow.
I am not getting how the air at the top of the U is stopping the water from flowing.
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Findlay234
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| posted on 11/12/09 at 03:24 PM |
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where are you talking about? in a toilet? in your car cooling system?
Do you actually have an issue where an 'air lock' is actually restricting flow or are you trying to hypothetically understand it?
Cheers
Fin
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