Bitten hero
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| posted on 28/10/07 at 09:44 PM |
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blow or suck-opinions please
Hi all, looking to purchase my rad fan and not sure what type to go for?engine is zzr1100 with a polo rad(will be in the pod and at a angle to the
car).cheers gary
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RazMan
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| posted on 28/10/07 at 10:37 PM |
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Definitely a sucker imo. My theory is that there is unobstructed airflow getting to the rad - what it does going out of the other side is less
important 
If you have a fan preventing air getting to the rad (or deflecting it) you are not giving the best chance of keeping everything cool.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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worX
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| posted on 28/10/07 at 10:46 PM |
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As above...
A "sucker" will simply increase the surface area of cold air hitting your radiator. But, you can only fit this if you have room!
Oh, and as an ideal, you want the Rad leaning forward slightly...
Steve
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02GF74
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| posted on 29/10/07 at 09:23 AM |
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I beg to differ.
The obstruction to air flow by the fan shroud is the same regardless of whether it is in front or behind the radiator. This is when the fan is doing
the cooling not forced air cooling at speed.
Second reason being that with a pull type fan, the biggest pressure you can ever achieve is 14 psi (1 atmosphere), a push type can easily exceed that.
Same caveat as above.
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RazMan
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| posted on 29/10/07 at 09:47 AM |
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quote:
Originally posted by 02GF74
The obstruction to air flow by the fan shroud is the same regardless of whether it is in front or behind the radiator. This is when the fan is doing
the cooling not forced air cooling at speed.
Second reason being that with a pull type fan, the biggest pressure you can ever achieve is 14 psi (1 atmosphere), a push type can easily exceed that.
Same caveat as above.
But surely the fan's airflow is only required at slow or stationary speeds. Under normal conditions (when the car is moving) the air movement
will far exceed what a fan can produce and so the fan will only partially block or deflect this air - rather like covering part of the core to reduce
the cooling effect in winter conditions (showing my age here).
Air is free to exit the back of the rad core in a number of ways unless there is a sealed shroud (but most are fairly 'leaky' to say the
least) In other words you need to put more air in to get more out.
[Edited on 29-10-07 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Thinking about it
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 posted on 29/10/07 at 09:59 AM |
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This could take over from the IRS De-Dion debate.
[Edited on 29/10/07 by Thinking about it]
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nick205
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| posted on 29/10/07 at 10:14 AM |
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Production cars tend be fitted with suckers rather than blowers (although that may be more to do with packaging than cooling I guess) which means
suckers might be more readily available/cheaper.
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02GF74
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| posted on 29/10/07 at 10:57 AM |
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RazMan - yes - fitting the fan behind is better when driven; but if fan is doing the cooling when staionary then in fornt is better - a confict
unfortunately.
Nick - about production cars, going round scrap yards, I thought majority of fans are mounted in front, maybe my memory is telling me what I want it
to?
[Edited on 29/10/07 by 02GF74]
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tks
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| posted on 29/10/07 at 11:50 AM |
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quote:
Originally posted by 02GF74
Second reason being that with a pull type fan, the biggest pressure you can ever achieve is 14 psi (1 atmosphere), a push type can easily exceed that.
Same caveat as above.
This is not true!
the fan in front doesn't has suck resistance but has the same output resistance!!
the fan behind the rad has suck resistance but hasn't output resistance so both could develop the same pressure!
think about it this way: the amount(volume) of air being moved will need to enter the fan and exit it!!!!
sow it doesn't make any difference on wich side the rad is! on each side of the fan the same airflow is generated.
Tks
The above comments are always meant to be from the above persons perspective.
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matt_claydon
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| posted on 29/10/07 at 12:54 PM |
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Actually 02GF74 is right.
Air moves through the radiator because the pressure is higher in front of it than behind.
With a suck fan, the minimum pressure you can generate behind the rad is a complete vacuum (0 atmospheres). The pressure in front of the rad is 1 atm
so you have a pressure differential of 1 atm.
With the fan in front, you can generate as much positive pressure as you like and the pressure differential will be that minus the 1 atm present at
the rear of the rad.
As an aside, exactly the same principle applies when pumping liquids - if you have the pump at the top, the highest you can lift a fluid is something
like 10m as the only thing forcing the liquid up the tube is atmospheric pressure. With the pump at the bottom you can lift as high as you like since
you can exert as much positive pressure on the fluid as the pump can produce. This is why oil is pumped by injecting another fluid into the chamber,
thus forcing the oil the many hundreds of metres required up to the rig.
[Edited on 29/10/07 by matt_claydon]
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tks
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| posted on 29/10/07 at 01:23 PM |
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I knew of the pump story, but that one isn't comparable with the fan example because the distance between where the medium is to the pump is the
same in the fan example but not in the pump example! obviously its clear that if i need to suck i need to generate a unther pressure! and you
can't genereate more unther pressure that there is pressure sow thats clear.
but the speed of the air through the fan is the same as the pressure difference over it.
sow its the same in the fan example!
just get it man, if you turn around a blower will it make any difference?? if you turn around the fan blade will it make any difference?? the answer
is no! it wont.
Tks
The above comments are always meant to be from the above persons perspective.
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matt_claydon
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| posted on 29/10/07 at 02:29 PM |
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That would be true if a fan always simply produced a given air speed at a given rpm.
That's not what happens though; you create a pressure with the fan and the air speed generated is a function of the pressure and the resistance
to flow. With a suck arrangement the maximum pressure you can generate is 1 atm and there is a corresponding maximum velocity. With a blow arrangement
you can generate whatever pressure you like (within reason!).
Whether this is actually relevant in the radiator is largely dependent on the restance to flow through the radiator. If the radiator is relatively
free-flowing compared to the desired air speed through it then you will never get to the point whe blow and suck are different.
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RazMan
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| posted on 29/10/07 at 03:45 PM |
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Let's not forget the original question guys - we are discussing a fan which is mounted on a moving vehicle and not a stationary one. When
the car is moving at 70 mph the tiny air movement that the fan produces will make no difference at all, and only comes into the equation when the car
is stationary or moving very slowly.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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tks
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| posted on 29/10/07 at 03:49 PM |
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yeah, sow behind is far away better more surface of the rad is expoded to the air!
behind the rad, the air will rise..
Tks
The above comments are always meant to be from the above persons perspective.
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