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120vac fan setup.

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120vac fan setup.

Unread postAuthor: Ryan » Fri Nov 04, 2011 4:34 pm

I have been working on my propane potato cannon for a while and I purchased a metal 120vac powered fan to stir the fuel air mixture. My only problem is being able to power it. I could probably use a motorcycle battery and a power inverter but Im trying to make it spacious, easy to carry and as efficient as I can. Any help would be greatly appreciated because as of now it's really the only thing giving me trouble.
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Unread postAuthor: jrrdw » Fri Nov 04, 2011 5:33 pm

Don't use a fan with brushes in the motor. The spark that they give off will cause the cannon to misfire. Use a brush-less computer fan, they will run off of a 9v battery.

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Unread postAuthor: jimmy101 » Fri Nov 04, 2011 6:07 pm

Most DC fans are brushed and they probably won't fire a spudgun. I would be concerned about a 120VAC motor, particularly if it is brushed (some AC motors have brushes, some don't).

Best bet is to just get a small DC motor, brushless is nice but not required. Just about any small DC motor can be adequately run from a single 9V battery.
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Unread postAuthor: Mr.Tallahassee » Fri Nov 04, 2011 6:59 pm

Jimmy, jrrdw is right. A brushed DC motor will most likely ignite the mix if metered correctly. The best bet is to get a PC fan as they are brushless and have 0 chance of igniting the mix (unless a winding breaks and arcs). PC fans come in all different shapes and sizes. As to using an AC fan, I have seen the brushes while the motor is running. It will ignite prematurely.
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Unread postAuthor: Ryan » Fri Nov 04, 2011 7:13 pm

The one I have is a brushless pc fan and works better than other dc powered fans but I need a simple attachable way to power it.
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Unread postAuthor: jimmy101 » Fri Nov 04, 2011 8:32 pm

Mr.Tallahassee wrote:Jimmy, jrrdw is right. A brushed DC motor will most likely ignite the mix if metered correctly. The best bet is to get a PC fan as they are brushless and have 0 chance of igniting the mix (unless a winding breaks and arcs). PC fans come in all different shapes and sizes. As to using an AC fan, I have seen the brushes while the motor is running. It will ignite prematurely.

Wrong again.

Many people have tried to ignite a properly fueled propane+air gun with a DC motor. Even though you can see sparks at the brushes it won't ignite the mix. Not sure why but best guess is that there is too little energy in the spark, which would probably put the energy at less than about 0.5mJ.

Brusless fans are nice but many many many guns have used standard DC fans without problem. DR was a big fan of them and used cheapo "personal fans" for Kmart with good results.

A 120 VAC brushed fan may well be a different matter but a typical 3V to 12V DC fan has never been shown to be capable of ingiting a perfect propane+air mix.
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Unread postAuthor: Mr.Tallahassee » Fri Nov 04, 2011 10:29 pm

The minimum ignition energy of a propane air mix has been calculated around 0.25mJ in a 5.2% fuel/air ratio. Whether or not it will ignite the mix depends on how accurate the mix is, how well it is mixed, and how much power is being applied to the motor. I personally would not risk it. PC fans are easy to come by. I believe it's better safe than sorry.
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Unread postAuthor: jimmy101 » Sat Nov 05, 2011 12:22 pm

Mr.Tallahassee wrote:The minimum ignition energy of a propane air mix has been calculated around 0.25mJ in a 5.2% fuel/air ratio. Whether or not it will ignite the mix depends on how accurate the mix is, how well it is mixed, and how much power is being applied to the motor. I personally would not risk it. PC fans are easy to come by. I believe it's better safe than sorry.
Last sentence is certainly true. Even if a fan only works one in a thousand times that would be once too many.

I'm aware of the various "minimum energies" that have been estimated. I generally use the 0.5mJ value.

However, that value is pretty misleading. You need the minimum energy to be expressed in a minimum volume. Just smacking the chamber with your hand probably puts a Joule or two of energy (thousands of times the "minimum energy" for ignition) into the chamber but that won't ignite the mix. The correct spec is that there is a minimum energy density required. That energy density doesn't have to exist throughout the chamber, it only needs to be present at a small spot in the chamber. As far as I know there are no firm numbers on what the density might be. A BBQ piezo generates something along the lines of 5mJ. To get that energy to ignite the fuel mix it needs to be in a sufficient small volume so that the energy density is high enough. For a piezo we could estimate the energy density assuming a 3mm (1/8") long spark that is a cylinder of radius 0.1mm (WAG). The spark's volume is 0.024 mm^3 and the energy density is 200GJ/m^3 (GJ=gigajoules). So that is a rough estimate of what is required to ignite a stoichiometric mix of propane in air 100% of the time. The minimum energy would be perhaps one or two orders of magnitude less than that.

My guns are always properly mixed and fueled and they still won't ignite with a small brushed DC fan running at 9V. As I mentioned before DR has made many guns mixed with small brushed cheapo DC fans and never had an ignition caused by the fan. A small DC motor running at 9V and assuming a coil resistance of 10 Ohms is pulling about 1 Amp and 9 Watts (9J/sec). Figure the spark duration is about 1mS so that's 9mJ, 20 to 40 times the "minimum ignition energy". So you might think it should work but doesn't.

And finally, "whether or not it will ignite the mix depends on ..." is a bit misleading. An ignition event is a fairly haphazard event, there is a range of energies (more precisely E densities) over which the perfect mix will sometime ignite and sometime not. Large rapid changes in chemical processes and properties tend to be inconsistently triggered. This happens in ignition events (including spontaneous ignition of flammable material caused by high temperature and "open cup" flammability measurements), boiling and freezing of liquids, etc. Heck this happens in much simpler things like the transition from laminar to turbulent flow of air through a pipe. Big changes in state often have inconsistent onset conditions. To counter this spud gunners generally just use an ignition source that is orders of magnitude more than the "minimum energy" required to ensure that the gun always ignites.
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Unread postAuthor: SpudBlaster15 » Sat Nov 05, 2011 2:57 pm

I can confirm that small brushed DC fans pose no ignition hazard for most fuel/air mixtures at standard pressure.

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I used this personal camping fan (Actually several of them, since they would tend to burn out at higher voltage inputs) in my first advanced combustion launcher for several months with no issues. The fuel was precisely metered MAPP gas, the mixing was nearly ideal, and the fan/motor housing was far from perfectly sealed. Still, even at double the rated operating voltage, no inadvertent ignition event ever occurred.

As for why this was the case, there are a number of factors that may have contributed, most of them relating to the energy of the arcs and the movement of gases inside the motor housing. Not only are the sparks generated in a brushed motor very small and lacking in energy, they occur within the space between closely mating parts, which the fuel likely has much difficulty diffusing into. Additionally, the airflow in a running electric motor is quite turbulent, and this turbulent flow may produce something of a pressure gradient around the armature which prevents air/fuel mixture from reaching it.

In the end, I would consider the risk of accidental firing to be so small that these fans are perfectly safe to use in combustion launchers.
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Unread postAuthor: jrrdw » Sat Nov 05, 2011 3:42 pm

There still a chance of misfire even if it never happened to you or anyone you know. It's happened to somebody or the question/statement of risk would not be there. We build and use these cannons at our own risk...

Got a idea for a power source like the OP ask? Lets hear it.
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Unread postAuthor: SpudBlaster15 » Sat Nov 05, 2011 4:29 pm

jrrdw wrote: It's happened to somebody or the question/statement of risk would not be there.


The question of risk comes from the fact that a brushed electric motor generates electrical arcing, and similar mechanisms (Albeit orders of magnitude more energetic) are known to reliably ignite air/fuel mixtures.

While it's certainly appropriate to have initial concern in light of this information, more detailed analysis shows that it's not really a valid comparison, and the hazards associated with using a brushed fan in a combustion launcher are mostly hearsay.

Many people have used personal camping fans in cannons, and some have even attempted to build a functional ignition system based on a brushed electric motor, yet at far as I can determine from my readings, not a single incident of ignition has ever been reported. While it's not entirely conclusive, this is pretty good evidence of a safe practice.

jrrdw wrote:Got a idea for a power source like the OP ask? Lets hear it.


The OP hasn't specified the running current of the fan in question, or provided the name of the manufacturer and model number for reference, so it's difficult to recommend a power supply with the information given.
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Unread postAuthor: jrrdw » Sat Nov 05, 2011 4:39 pm

To be that one, would really really suck! Wouldn't it?
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Unread postAuthor: Ryan » Sat Nov 05, 2011 6:09 pm

I know and assume all risks involved. All I am really wanting to know is a way to power a 120 volt ac computer fan that is small enough to be mounted on the cannon wether its an inverter and battery setup or what.
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Unread postAuthor: dewey-1 » Sat Nov 05, 2011 6:33 pm

Ryan wrote:I know and assume all risks involved. All I am really wanting to know is a way to power a 120 volt ac computer fan that is small enough to be mounted on the cannon wether its an inverter and battery setup or what.


Why?
It will be cheaper and a whole lot easier to buy the correct DC fan than to buy an inverter!

What are the specifications on this "120 VAC Metal Fan"?
Supply a link or model number.
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Unread postAuthor: jrrdw » Sun Nov 06, 2011 7:07 am

There should be a spec. sticker on your fan. Give us those numbers and we can better help you.

Hey....If you have the money to spend Black & Decker makes a portable power pack that will supply 110 ACv for as long as the internal battery holds out. DeWalt makes them as well. Those are the two off the top of my head anyway...
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