Primary mirror cooling fan question

strongmanmike (Michael) · #1 01-03-2007, 09:51 PM

The rear cooling fan on my new ASA N12 Newtonian astrograph is wired such that it is sucking the air out of the tube rather than blowing it onto the back of the mirror.

It is a closed tube and there is hardly any room around the mirror and mirror cell and the fan is mounted in a carbon fiber plate that fills the entire rear of the tube:

http://www.pbase.com/strongmanmike20...72520276/large

I am not sure if it would be better to turn the fan around and have air blowing on to the back of the mirror or leave it as it is with the air being sucked out?

Any opinions?

Cheers

Mike

Photon · #2 01-03-2007, 10:05 PM

Particulary in electronics, and other devices, its customary to force air onto the device or surface to be cooled. I'd have thought the reciprocity theorem would have applied and it wouldn't matter, but the experts believe different. I understand when sucking away from the mirror, a vacuum or void will occur particulary in the centre where little air movement takes place so cooling is not as efficient. Try reversing the fan and see if you notice the difference. Do you have temp sensors mounted?
Regards IanG

strongmanmike (Michael) · #3 01-03-2007, 10:13 PM

Hi Ian

I was of the same opinion as you so I have already reversed the fan

I was hoping some gurus out there could assure me of the correct direction

. Bsically the main mirror is well enclosed in this design so I thought perhaps a cooling fan should indeed suck instead of blow..?

I don't have a temp sensor on the mirror but I guess that might be an interesting exercise? I guess a sensor outside the tube to measure ambient air temp would be a useful comparison too? Any recomendations? Jaycar perhaps?

Mike

matt · #4 01-03-2007, 10:16 PM

Mike

There are 2 schools of thought on this question of whether a cooling fan should direct air toward or away from the mirror.

I would ask why the manufacturers of your scope have the mirror configured this way. They obviously feel this is the right way???

Write an e-mail to them, before you change anything around.

Dennis · #5 01-03-2007, 10:28 PM

Just one thought Mike. Would sucking air out minimise the possibility of dust or foreign bodies being propelled into the OTA? Actively blowing air inside the OTA would more likely also introduce dust, pollens, etc into the cavity unless there was an effective filter?

Cheers

Dennis

strongmanmike (Michael) · #6 01-03-2007, 11:03 PM

Hi Dennis

As far as I was aware and expecting when I turned my fan on, was that air would be blown against the back of the primary mirror becasue that was all I had ever heard of. Now becasue the ASA scope is virtually fully enclosed at the back, perhaps sucking air out is better?

The pollen and dust thing might be true but the fan should be focusing on cooling the primary not keeping it clean

Mike

matt · #7 01-03-2007, 11:38 PM

But remember, Mike, a fan extracting air from your OTA is a fan removing warm air.

strongmanmike (Michael) · #8 01-03-2007, 11:47 PM

Ah yes, I know that Matt this is why I am confused and I hope somone can put me straight and explain it

I had emailed ASA but the short reply was just that it should suck not blow but without an explanation so I am not convinced...yet

I emailed them again tonight and asked for some clarification and explanation, so we'll see what they say

Mike

davidpretorius · #9 02-03-2007, 12:51 AM

Hi Mike,
Bird, Paul and I are all blowing air onto a surface to cool it better. In our cases, we are inserting an aluminium plate, into the equation, connecting it to some peltiers (ie cooling the plate below ambient) and actually using the fan to blow internal air onto this colder plate and then circulating around the mirror. The mirror is then cooling ahead of ambient dropping.

There seems to be marked differences in cooling rates when blowing onto as opposed to sucking away.

Given most times, the air in your tube will always be warmer than ambient (unless doing solar during the day), then blowing colder ambient air into a warmer system is better than trying to suck it out.

I have started a blog http://southcelestialpole.org.au/blo...d=21&blogid=11 This unit is for John K. It will sit on top of his JMI torus. Icemans is next for his 12". He sent me piccies and dimensions of the base of his new 12" today.

iceman (Mike) · #10 02-03-2007, 05:52 AM

I blow air onto the mirror, for the reasons stated above.

You can get an indoor/outdoor temperature sensor from Dick Smith for $30. I just fitted one to my 12" mirror.

Ambermile · #11 02-03-2007, 07:46 AM

Hmm, I blow air onto the 22", but it's not enclosed... nor are - I think - anyone elses that has answered. Maybe the fact that it's a closed system leads the designers to this decision purely on the merits of keeping the local insect population safe? (not to mention off the primary)

Arthur

rmcpb (Rob) · #12 02-03-2007, 08:45 AM

Sucking or blowing ambient air would produce the same cooling for the volume of air moved across the mirror regardless of the direction. However, as Ian has said when sucking from an enclosed system there is a pressure drop inside which will cause a drop in efficiency of the fan. If the fan was blowing then this inefficiency would not be there BUT if the air cannot get past the mirror quick enough then you will have back pressure which will reduce the efficiency as well. Is the mirror that tight in the OTA ie. <20mm clearance? If not then blowing should be more efficient for cooling.

Cheers

Omaroo (Chris Malikoff) · #13 02-03-2007, 09:14 AM

I'd rather a fan that blows Mike. I guess that you could, if you wanted to, place a fine-ish filter on the back of the fan and it would blow filtered air inside your tube. If you suck, then unfiltered air (and grunge) has already passed through the tube and on to your mirror before the fan extracts it.

Cheers
Chris

Dennis · #14 02-03-2007, 09:16 AM

Hi Mike

Just for info, it might be worth having a look at this review of a commercial "poke it up an SCT" cooler which discusses introducing dust into the "sealed" OTA of an SCT. The fan they use has a filter and the CN report indicates it will filter out particles larger than 0.25 microns.

Maybe this is why your fan sucks out rather than blows in?

Cheers

Dennis

strongmanmike (Michael) · #15 02-03-2007, 03:46 PM

Hmm? as I expected might be the case, it is hard to make a diffinitive suggestion given that my OTA design is not the standard Newtonian arrangement?

I have however just had another close look at the fan in operation and it isn't actually a closed system after all. There is in fact a gap of 2-3mm all the way around the back plate that lets air out that has been blown in behind the mirror. There is no bank up of air and it seems to be flowing nicely across the back of the mirror and mirror mount. A quick check with a wet palm confirms there is air coming out from all the way around the back plate and a tissue is sucked onto the fan so there appears to be no negative back pressure?

The fan is attached in relief on the inside of the back plate and is about 20mm thick. The fan is located above a hole in the mirror support and there is a gap of around 50-60mm or more between the front of the fan and the back of the mirror and about 20 - 30mm between the front of the fan and the back of the mirror support. The gap between the inside of the back plate and the back of the mirror is therefore some 70mm or more so there is plenty of room inside there for air flow.

My gut tells me (and it is a gut getting bigger and bigger these days

) that both sucking and blowing in this case will cool the mirror, it's just which is fastest that is the real question? Given the identification of the air being able to exit behind the mirror I'm further leaning to the blowing side though...?

I am not so concerned about the effects of blowing dust in now as it clearly has an exit route that is behind the primary mirror rather than in front. Of course some air may still be blown around in front of the mirror but the gap is very small from what I can see and there is even an apperture mask of around 2mm infront of the mirror.

I am waiting for a hopefuly diffinitive explanation from ASA soon?

Thanks for all the input guys it is most appreciated!

Mike

bird (Anthony Wesley) · #16 02-03-2007, 04:18 PM

Mike, blowing air onto the mirror will be better. If you have it sucking instead then some air will be sucked in around the edge of the OTA, flow through the inside and be blown out without ever going near your mirror - this air is basically wasted, and it might turn out to be a significant percentage of the total air.

But if you blow air onto the mirror then all the air is used. Remember that air is not a good conductor of heat, so you really can't rely on the mirror radiating its heat into the air inside the OTA, that's very inefficient. It's much better to force the air across the glass to improve the heat exchange.

cheers, Bird

strongmanmike (Michael) · #17 02-03-2007, 05:44 PM

Yes I tend to agree with your Bird.

Unless ASA get back to me with some compelling explanation as to why I shouldn't, I think I'll leave it as blowing rather than sucking....

Mike

GrahamL · #18 03-03-2007, 09:16 AM

I found this strange when I first hooked up my fan .

Sucking seems to work ok as such , will be interesting to hear what they have to say mike.

edit found this
http://www.fpi-protostar.com/bgreer/.../fanselect.htm

strongmanmike (Michael) · #19 03-03-2007, 02:00 PM

Thanks Graham

I think I have seen that article before but had forgotten about it, very enlightening thanks. Seems to suggest that in my case sucking may in fact be a viable alternative to blowing after all but it is still not diffinitive..? For example it seems to suggest that if appropriate venting is available sucking is an option but doesn't explain what appropriate venting actually equates to..?

Is your fan on your Dob? Is the mirror end of your OTA closed? Are you still runnning it in suck mode?

Cheers

Mike

rmcpb (Rob) · #20 03-03-2007, 02:36 PM

Mine is baffled (closed) and it blows, very efficient.