2 fans sucking air out, or 1 fan sucking air in, 1 sucking out?

[DavidSWP]

Question as in subject title..

[HDTVman]

How about 2 fans blowing in? Blowing in cold air is more effective than trying to suck out hot air with the same fan(s).

Later BZ

[jewilson]

Whichever one you do you must have an airflow path. The ability to circulate air from a source that's not as hot as what you’re trying to cool and a method to exhaust the heated air.

You might also want to look for low noise fans, these are normally lower in power and don't produce CFM above 85. Noise should be around 35dB, the lower the better.

[DavidSWP]

I was going to use 2 noise blocker fans that are rated at about 35CFM and 21db each, just because I have them lying around. As for airflow path I was just going to have the fans (both extracting air) at one side of the box and vents on the other side to let air in. If I had one fan extracting and one fan sucking air in I was going to have the fans facing each other on either side of the box. This is to cool a 250Watt MH lamp in a fairly large enclosure.

[HDTVman]

It's better to blow in cold air. Cold air is more dense that hot air. The mass of the air flow for a given size fan is greater if you blow in cold air as opposed to trying to suck out hot air when using an open vent to complete the air flow circuit.

Later BZ

[jackinnj]

Originally posted by jewilson
Whichever one you do you must have an airflow path. The ability to circulate air from a source that's not as hot as what you’re trying to cool and a method to exhaust the heated air.

You might also want to look for low noise fans, these are normally lower in power and don't produce CFM above 85. Noise should be around 35dB, the lower the better.

It's like a transmission line problem -- you don't want a lot of impedance mismatches with vortexes etc.,

There's a really neat circuit in the March 4 issue of EDN for a simple thermal fan controller -- using a thermistor in the error amplifier loop of a Maxim PWM chip. The little trick with the unit is that it has a way of jump-starting the fan:

http://www.reed-electronics.com/ednm...s/382779f1.pdf

so you only run the fan full blast when you need to.

[jewilson]

HDTVman,

That what I would have said.

[DavidSWP]

How come most fan setups suck out hot air rather than blowing in cold air then?

If you blow in cold air aren't you just going to pressurise the box with more air that will get hot? Unless you have really big vents in which case I don't think the air flow will be very good. Isn't it better to have extractor fans making the inside of the box lower pressure and so that new cold air gets sucked through the vents?
I might be talking utter crap here, but most systems do use a system that sucks hot air out.

[bobharry]

Whatever you do with the fans, make sure they are positioned in your enclosure so the air gets some heat before it hits the bulb, because blowing cold air on a metal halide increases the risk of it exploding. I agree that if you are chosing between sucking air out or pushing air in, sucking it out is more effective (too long to explain if your don't see my view, and Davids).

[HDTVman]

Originally posted by DavidSWP
How come most fan setups suck out hot air rather than blowing in cold air then?

If your talking about computers, maybe so, but for high power tube equipment and the like a presurerized system is the most common. Pressurized systems allow the air flow to be directed where it is needed and filteing out the dust is much easier.

[i]If you blow in cold air aren't you just going to pressurise the box with more air that will get hot? Unless you have really big vents in which case I don't think the air flow will be very good. Isn't it better to have extractor fans making the inside of the box lower pressure and so that new cold air gets sucked through the vents?
I might be talking utter crap here, but most systems do use a system that sucks hot air out. [/B]

It depends on how much heat your talking about and that high wattage bulb will need a lot of cooling. It's all about mass flow and a X size fan will move a greater mass of air if the air is cold compaired to hot.