Fan Questions

Started by david roberts, 10/23/2005 01:46AM
Posted 10/23/2005 01:46AM Opening Post
I plan to install a fan on my old Coulter 13.1 inch, F4.5 reflector. It has a thin mirror. The red tube is 16 inch diameter.

I bought a 3-inch brushless 12VDC cooling fan from Radio Shack ($20) and a battery attachment. My goal is to remove the air layer hugging the primary.

My questions are:

1. Is this fan to big or to small? I tried a 2-inch fan which I didn't think would blow air much to the other side of the tube. Does that matter? Is a 3-inch to much fan?

2. Should I install 2 fans on the tube or is one enough?

3. If one is all I need, I want to install it on top of the tube. Does it make a difference in performance where I install it, i.e. on the side of the tube?

4. Should I install it to blow air over the mirror or reverse it to pull air across the mirror? Would the reverse help with air currents in the tube? By the way, there are three holes in the original Coulter mirror cell.

5. The fan seems to run smooth but do I need a damper between the fan and the tube, like rubber or form?

6. Should I add a fan or two to the back of the mirror cell? Should they pull or push air.

7. How far forward should I place the fan from the face of the mirror.

8. I there an optimum fan arrangement?

All suggestions appreciated
Posted 10/23/2005 07:24AM #1
Check the Yahoo group Dob-fans.
Posted 10/23/2005 05:04PM #2
Hi David,
With a solid tube the one fan would be enough if it is about 30-35 cfm flow rated. If you mount it on a plate on the rear of the tube, then it will work well if you pull air out of the bottom. Solid tubes might be better cooled in this way. The jury is still out. Technically blowing on the back of the mirror cools the mirror the fastest, but with a solid tube you have to cool the upper tube too.
You should mount it with some foam or soft material to absorb the vibration, Velcro used to hold it in place on the tube is good.
Here is a Hardin with one of my fan modules installed.

Attached Image:

fblue's attachment for post 97585

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 10/23/2005 05:23PM #3
I mounted one 4 on the side of my 13". It vented out the top and bottom of the tube as I had an open cell. Vibration -is- an issue, and counter measures must be taken. Or high power observing is nixed.

Pretty much any air flow helps. I don't think 3" is too little. But you could certainly go more.

There is no optimum fan arrangement. There are just too many variables for that. The mass of the mirror and how much heat it can store, an open vs closed tube, the size of tube and volume of air to be displaced, the type of closed tube itself and how much heat it stores, pyrex vs plate glass, the type of mirror cell and how air flows around it, baffling for intrussive light,... etc.

I found that what worked best was to move a lot of air, first. Get rid of as much of the latent heat as possible, from the mirror, the cell and the tube. Then run just enough air across the mirror to optimize the image at the ep. At the eyepiece is where it counts. I concluded that each night was an experiment on how fast to run the blower after initial cooling.

Posted 10/23/2005 06:30PM #4
The best advice is simply to start moving air through the tube as Floyd suggests (choose your direction) and experiment. I have found that using more fans, or a bigger fan than one would think, is a good thing. This way you can drop the voltage to reduce the fan speed and still get enough air movement to provide the necessary cooling. This helps to reduce the vibratin potential from operating the fans while viewing at high power. It's at high power where the most noticable improvments are noticed.

Gary Weber
Posted 10/24/2005 02:36AM #5
Goggle is great. The web is better. Checkout this site, if you haven't already:

http://www.fpi-protostar.com/bgreer/miscpages/fanselect.htm