SVT Cooling Fan: Push air in, or pulls air out?

[ryanrex]

my svt seems to be drawing air out from the inside. it's a magnavox era. maybe some one modded it through the years

 

[GC4]

I would think that it would be better to blow the air into the chassis. The jet of air that the fan would make blowing into the chassis is much more efficient (and powerful) then the air that a fan could suck out of the chassis ex. the force of air being sucked into your mouth vs. blown out. Now I'm not an electrical engineer but my fluids mechanics/heat transfer courses would lead me to believe that. Could be different when working with electronics though. Just my two cents.

 

[metron]

Sounds like ignorance prevails yet again.

Exhausting air out would be the more efficient way of cooling tubes, end of story...but hey, not all of us have the benefit of being electronic technicians and engineers that actually know what we're talking about. Because something is a certain way, doesn't make it the right way.

Being an electronic technician or engineer you should be knowledgable enough to understand that ignorance always prevails.

I asked the same thing some time ago. Seems like its better to have fan exhaust the hot air from the tubes. More efficient right? Right. Except I was assured by a few knowledgable folks that the fan is for blowing air on the tranny not for exhausting hot air from the tubes.

The correct answer is IN.

 

[Toptube]

You don't work around electronics much do you? This may be true in an open space, but that's not what we're dealing with.

I've been building computers for 12 years. Computer processors have probably the highest TDP of any consumer accessible man made thing.

If you have one fan, you blow it straight at the hottest thing or the main heatsink/radiator.



*and regarding dust: air pushing in or pushing out, you are going to get dust buildup inside the amp. That's what happens with active cooling.

 

[Jerrold Tiers]

Fan blows IN. Actually cools everything a bit better that way. Lots of air turbulence, and a fairly good circulation over the hot parts.

Tubes are not too fussy about cooling, but transformers are.

Pulling air OUT of a very open chassis area like that means that cool air just "wafts in" wherever it wants, and there is actually much less even cooling.

I have designed a lot of air-cooled stuff, and there are cases where pulling air OUT is better. In general, the SVT ain't one of them.

 

[JimmyM]

i'm glad you piped in, jerrold. this might have started a run on blown transformers with everyone reversing their fans.

my tech told me that the fan keeps the whole amp cool, not just the transformers. he said never run it without a grill in front because the air bounces off of it and into the rest of the amp. don't know how true that is but it seems to make sense.

btw, i'm afraid my fan has about had it. i have to crank it with my finger to get it going. any of you fan experts know what's the best modern fan to replace the fan in a 69?

 

[Gopher Bob]

if you have 1 fan on the computer it should always blow the hot air out...

Even on my giant heatsink theres a fan on its side and it blows the air through the heatsink with the ultimate goal of moving the hot out!

air is not a good thermal conductor... that is why blowing the air into the unit is not going to cool things down as much as getting the already hot air away from the components.


I'm not expert but this is my opinion based on what I've read over the years and it makes more sense than any argument for blowing air in.

edit: reading the post above the one above makes sense though with the SVT... if a particular component needs the fan then its another story.

 

[THB_bassman]

btw, i'm afraid my fan has about had it. i have to crank it with my finger to get it going. any of you fan experts know what's the best modern fan to replace the fan in a 69?

Jimmy...I just got a replacement fan - from Fliptops - for my '73-74 SVT. It costs $25.00 and fits with no modifications. Well, except I didn't have a new connector to use; had to cut off and re-use the old one; soldered and shrink-tubed the connection.
But...I can't say if it's the same as on your '69. I guess it may not work, but they don't offer another type/model fan for earlier SVTs.
(Hope this is helpful to you, after your generous help with my cab questions. Thanks again.)

Also, regarding the whining, turbo-like noise - I noticed the same thing. When I reversed the fan airflow direction (so it was pulling air from the front & exhausting out the back) it became almost totally silent.
But after reading the entire thread here, I'll change it back to blowing in from the back, and use spacers as described for reducing the noise.

Great thread, thanks to everyone.

 

[Surly]

I like the idea of a radiator on my amp. A fan pulling air in with coolant lines running through the chassis.

 

[SurferJoe46]

Kinda funny that no-one thinks about the effect of air cooling from a purely physical angle: The law of thermodynamics.

Air that is "drawn" out by a fan, is doing so by creating a vacuum or negative pressure area and therefor the air molecules are spaced further apart and since heat (a POSITIVE factor) requires the air on which it can "attach" (a bad word but fits for this description) to and be removed from the heat source.

BUT if the air is positively forced into an area or over a heated device, then the air molecules are tighter packed and under a slight pressure, resulting in more efficient heat transfer from the heat source.

The fact that an engine cooling fan is behind the radiator is just because it's easier to place it there for aesthetics and less problematic to designers who have bigger fish to fry by producing cleaner body lines. So this is NOT an arguable point.

The fact is: POSITIVE pressure produces more cooling effect, whilst vacuum and the resulting lowered pressure produces a less-packed molecular vehicle and cannot carry as much heat per comparative like volume of a denser-packed molecular airflow.

Then there's always the zoned attention to actual flow directly at/over the heat source, which is much easier to accomplish with directed flow unlike a generally ambient, vague and un-directed exhaust flow.

This is why ice is used to chill your drinks and not just cold water. The ice has more energy absorptive capacity than the equivalent amount of cold water. Besides, more water just destroys the alcohol content - but that's a totally different discussion.

 

[Groover]

This turned out to be a very interesting thread. I'm glad Jerrold jumped-in on this one too.

The main reason I asked was because my amp has obviously been tinkered with it before me owning it, and I was skeptical on how things were put back together (considering the fan was held to the grill with only two screws, instead of four).

I'll put it back on with airflow pointing inwards, and use the spacers to get rid of the whirling. I may even put a plastic cowl from a computer processor fan Dell PC's have some that may work.

I'm thinking the cowl woul go in front of the transformer, so the air would circulate towards the other tranformer instead of losing alot of pressure (and loss) by relying on the front grill to act as duct.

Then again, these things have worked just fine since 1969, so I'm sure the way it is now is just fine too.

Jimmy, sounds like your old fan motor needs brushes. If you have the Rotron fan, those look to me very well built. Maybe if you can find an old-school electric motor repair shop they may be able to pop-it apart and put some fresh brushes in? May be just easier to use the mentioned above from fliptops though...

Thanks for replies.

 

[Billy P]

Fan blows IN. Actually cools everything a bit better that way. Lots of air turbulence, and a fairly good circulation over the hot parts.

Tubes are not too fussy about cooling, but transformers are.

Pulling air OUT of a very open chassis area like that means that cool air just "wafts in" wherever it wants, and there is actually much less even cooling.

I have designed a lot of air-cooled stuff, and there are cases where pulling air OUT is better. In general, the SVT ain't one of them.

My 2005 SLM SVT CL blows hot air out the front. It was never modified.

 

[Jerrold Tiers]

My 2005 SLM SVT CL blows hot air out the front. It was never modified.

never said we were consistent.......

It's not a huge deal either way, really, but IMO blowing air in circulates it better IN THIS CASE.

And it cooks the fan less.

 

[eastcoasteddie]

The fact that an engine cooling fan is behind the radiator is just because it's easier to place it there for aesthetics and less problematic to designers who have bigger fish to fry by producing cleaner body lines. So this is NOT an arguable point.

There have been MANY vehicles that have had the fans in front of the radiator, probably because they didn't fit into the engine bays of these tiny things. Sometimes it ISN'T easier to put it there....

anyway, back on topic...

 

[THB_bassman]

This turned out to be a very interesting thread. I'm glad Jerrold jumped-in on this one too.

The main reason I asked was because my amp has obviously been tinkered with it before me owning it, and I was skeptical on how things were put back together
...

I'm glad you asked, because if you hadn't I probably would have. I was in the same situation as you, and couldn't seem to locate a definitive answer searching google or the forum. I'd also thank Jerrold as a valuable resource.

It is really amazing though, how much quieter the fan runs when I reversed it. I was going to leave it that way until now.
This isn't to dismiss the knowledgeable and thoughtful folks with opposite opinions...your input made very good sense to me and seemed viable. And while it may not make a tremendous difference in the life of this SVT, I just prefer to leave things as the designers intended (fan blowing into the head) but I still want to try the spacer idea to quiet down the noise that sucking the air through those little holes in the screen makes.
Talkbass saves the day again, and reminds me it's time to re-up my contributing membership.

 

[yeeharacing]

My recommendation would be to get information directly from the manufacturer.

There seems to be way too many variables in this thread to get a good idea of what is the proper direction.

With many variations over the years, the internal designs most likely have changed between each revision. Your airflow is critical and depending on the position of tubes and other equipment requiring cooling depends on which direction the fan should blow.

Cooling may be effective and may allow the amp to work but when pushed the cooling may not be adequate.

With years of working with computers behind me changing the direction of a fan will allow a computer to run at normal speeds. Start playing a highly graphical game and pushing your processor it will build heat quickly and not be able to maintain temperature.

 

[Pilgrim]

That would also lead to a much lessened life span for the fan as it will be sucking hot air through itself and heating up excessively.

Paul

Fans are cheap and easy to replace. Tube amps are not. Higher CFM fans are also easy to find.

I'd probably keep it blowing whichever direction the manufacturer set it up. If I were concerned, I'd find a higher CFM fan that would fit.

 

[yeeharacing]

Fans are cheap and easy to replace. Tube amps are not. Higher CFM fans are also easy to find.

And has anyone stopped to think about how PCs move air? They all exhaust air out the back, drawing it in through the front. Perhaps there's a clue there....for those of an observant turn of mind.

Not exactly, the cooling depends on the case design... granted that most cases are designed to exhaust but not all.

That is why I'm saying to contact the manufacturer or the most reliable resource to answer this question. It depends on the internal design and airflow through the case.

 

[Toptube]

if you have 1 fan on the computer it should always blow the hot air out...

Even on my giant heatsink theres a fan on its side and it blows the air through the heatsink with the ultimate goal of moving the hot out!

right, the point is its pushing the air into the heatsink, which is where all of the heat from the processor is localized. Yes, some of the new heatsinks are "towers" that allow you to push air into the heatsink, but also have it oriented so that the air is generally pushed/exhausting semi-directly from the case. (a second fan can be placed directly in line as an exhaust fan, then you have a nice push/pull setup.) Traditional heatsinks though have the fan facing down, in which case the fan blows down, at the heatsink. You can turn the fan around to pull away, but in all tests I've seen with just one fan, pulling away is much less efficient.

getting a little off topic, though.

air is not a good thermal conductor... that is why blowing the air into the unit is not going to cool things down as much as getting the already hot air away from the components.

Talking about bass heads now:

As has already been stated, most Fan's CFM rating is based on blowing(pushing), not pulling. Fans generally create more pressure and turbulence when blowing. An at all smart design will have the hottest components in the fan's direct path, if not right in front of it. Blowing into the case will bring cool air directly in and over whatever needs it most. Pushing in still naturally creates pressure to push air out the other vents. This will maximize the amount of cool air and allow it to move more quickly so it has less time to heat up before hitting the components.

If the fan were setup to pull out with something large like a head, the air inside would be moving much more slowly which means less turbulence and less focus. your cooling would be based only on the ambient temp of the head's interior since the cool air would be passively pulled in from vents on the opposite side. By the time the air made its way across to the components that need it, it would be warmer, slower, and less dense, thus giving less cooling benefit.

pulling and/or having exhaust fans only makes sense in multiple fan setups or with really small enclosures in which pulling pressure is greater and all of the components are very efficient so only require passive cooling---like a powersupply.

Air is actually a pretty good conductor of heat, but by natural design air doesn't have a lot of density. So we make up for this by moving more air more quickly with larger and/or faster fans. The general problem being the faster you spin a fan, the more noise it makes.

 

[Hoss]

any of you fan experts know what's the best modern fan to replace the fan in a 69?

Not being an expert by all means, but I replaced the fan in my 74 with this: [Invalid or Expired Link Removed]

Seemed to have the best CFM/noise relation thats why I ordered it.

After replacing it:
A lot less noise than original fan but about the same CFM.