6 replies to this topic

[jimmason]

in the ongoing discussion for how to do a high vac, heat tolerant, non-fouling, low cost, 12vdc, quiet and efficient vac/blower for gasifier starting, i wanted to make sure everyone saw terry's very interesting contribution.  this solution came across the yahoo wood gas list a month or so ago. it has most of the desired qualities, minus the quiet and efficient ones, which are less important anyways.

the idea is to use the common pass throw multistage vac motors to run a relatively low pressure ejector.  as usual, the jet is the air, and it produces the vac which pulls the gas.  same way we do on the standard gek ejector venturi, though with high pressure compressed air for the jet.

the central vac motors will do enough jet to get an eturi to work.  you use a jet of about half the diameter of the entrainment tube.  i think terry was using a 3/4" jet and 1.5" entrainment.  i'd go a little smaller on the jet.  

vac motors are usually mutistage and produce between 80" to 120" or so h2o vac.  they are much higher vac than the common centrifugal blower options, as they are multistage.  they stack up 2 or 3 centrifugal rotors, the output of one routed back to the center of the next stage for another acceleration.

at one point we did some tests on low pressure eturis and found you could produce about half the vac on the suck side as you had pressure on the jet side.  we were doing these tests for internal gasifier tar recycling schemes, but same motive problem.  i'm somewhat vague in the memory of the "half" relationship.  maybe bear can correct the details.

anyways, here's terry's solution on youtube.

Here it is on youtube.



http://www.youtube.c.../11/A1ERzMGPKBg

the central vac motors are available in both 120/240ac and 24/36vdc.  unfortunately i can't find any 12vdc off the shelf.  the 24 and 36 dc ones are used for sweepers, carpet cleaners and other mobile cleaning equipment.   at least the 24vdc ones at half power will still produce impressive vac.  or, run a dc booster.  or, get them rewound, as we've been doing with the fans in the other thread.

the fans in the other thread produce 11" of h2o.  we thought this was adequate.  but once we add a filter, air mix, and actual flow, you end up with under 5" at the reactor.  this isn't enough.  so we now see we really want a 20" or so blower for the best of all possible worlds.  this is at the top end of what the single stage boiler fans are intended to do.  the vac fans are directed more at these higher vac but lower flow situations-- like what we have in a small scale gasifier.

here's some sources to get your started.  many more are findable via google.  and used ones are likely already around the homestead in an unused vac for free . . .

http://www.electricm...ametek_menu.htm

http://www.commercia...omel_c_435.html

http://www.centralvacuumonline.com/motors.html


the most interesting dc one i have found so far is the Lighthouse 116515-13 24dc.  this is an all metal one in a bypass configuration.  see here and scroll down. http://centralvacuum.../lighthouse.htm

here's the picture and performance graph.

[jimmason]

trying to keep the record straight, i also see stephen abbadess did one of these low pressure versions back in the day when all of us were figuring out the jet pumps.  i forget he was driving his on a vacuum cleaner motor.  we were driving ours on an air compressor, thus we each ended up with quite different sizing.

here's a link to a video of stephen's early version.  he described it as follows:

"Try a 3/4 inch mixing tube with a 1/4 inch air supply blown by a vacuum cleaner on a dimmer. Here is an early version in operation:"



the other thing to remember about the sizing of these is the potential for flame backup.  if you are mixing air into the gas before the eturi, you now have a combustible mixture and you need to keep the velocity through the entrainment tube about the flame speed, lest the flame back up into your jet area.

we found we needed to go down to 3/4" plumbing to solve the flame back up problem.  the details of this and other dimensions in the basic gek eturi are here:

http://wiki.gekgasif...ix swirl burner

also, stephen's name "jet ejector pump" is really much better than our "ejector venturi".  these devices actually do not operate on venturi principles.  they are momentum transfer devices between two different velocity streams.  "jet pump" is probably what we should really call them.  but for some reason we started calling them "ejector venturis".  

j

[Jonathan_Spreadborough]

I was always opposed to pulling my start up gas through my filtration, That way I never needed 20" of vacuum. I would think that you could make some modification to that 12v fan you found, change the gasket material to handle a little heat and pull your gas off just after your cyclone filter.

[bauereri]

I'd like to put one of these ejector pump units together to play around with.  Is there anywhere a person can go to learn more about how to size the indvidual components?  I'm pretty green at this, so I would prefer to read first and act later rather than do a bunch or trial and error.

Thanks,

Eric

[jimmason]

eric,

here are the plans for the basic gek ejector:  http://wiki.gekgasif...ix swirl burner



for the low pressure versions it seems about 1/4" for the jet, into a 3/4" entrainment tube.  for the high pressure versions, you use a smaller jet diameter.

it is much easier with an air compressor supplying the jet.  the low pressure jet ones are challenged getting to the high vac numbers we ideally want.

jim

[bauereri]

Hi Jim,

I've been studying the design of the eturi.  I have a GEK kit on the way and I will definitly start it up using the compressed air jet.  However, in the end I would like to move to a low pressure jet so I don't need to put so much work on my air compressor.  I'd rather tear up old, cheap vacuum cleaner motors! :)

Are there any main principals I need to know like ratio of entrainiment tube diameter to mixing tube?  Or distance of jet to entrainment tube?  Ratio of entrainment tube to gas inlet diameter? Anything else like that?

I also have a question about the air/fuel mixture when using the low pressure jet.  I imagine that the air through the jet would be signigficant enough to affect the air/fuel mixture.  Is it possible to have adequate mixture control when using the low pressure jet?

Thanks,

Eric

[jimmason]

bauereri said:

Hi Jim,

I've been studying the design of the eturi.  I have a GEK kit on the way and I will definitly start it up using the compressed air jet.  However, in the end I would like to move to a low pressure jet so I don't need to put so much work on my air compressor.  I'd rather tear up old, cheap vacuum cleaner motors! :)

yes, we all agree with this, thus all the recent testing to see how to save the pleasures of an eturi, and not need the full air compressor.

bauereri said:

Are there any main principals I need to know like ratio of entrainiment tube diameter to mixing tube?  Or distance of jet to entrainment tube?  Ratio of entrainment tube to gas inlet diameter? Anything else like that?

I also have a question about the air/fuel mixture when using the low pressure jet.  I imagine that the air through the jet would be signigficant enough to affect the air/fuel mixture.  Is it possible to have adequate mixture control when using the low pressure jet?

it seems we all end up in the realm of 1:3 ratio (or 1:2-1:4) of the diameter of the jet to entrainment tube.  or at least for the lower pressure versions.  higher pressure jets can have bigger ratios.  with using an air compressor, we use an 1/8" jet on a 3/4" entrainment tube.  

the length of the entrainment tube should be 5-7x the diameter of the entrainment tube.  ideally you have a nice converging section transition into the entrainment tube, and a nice diverging section transfer out of it.  we approximate this with plumbing bell reducers for the gek ones, as we are trying to DIY this.

if you want the best practice dims, see the jacoby tarbox site here: http://www.jacobytar....com/index.html

click the data library and download their design specs.

and yes, the problem you get into with the low pressure ejectors is you can get too much air to still have the proper mixture.  we have not yet fully characterized these ratios with the lower pressure ones.  but this is why we were using the smallest jet possible on the high pressure ones.

still, the 1/4 jet and 3/4 entrainment with a vac seems to stay below the 1:1 ratio you want.  it will impact a bit when you can light the flare, as in the beginning you are pulling lots of air through the system, and don't need to mix in as much.  this is a minor issue on a diy rig, but something we consider as we step towards making this a fully automatic, no smoke, no smell, everything like a diesel genset type solution.  


the bigger issue on the low pressure vac drive is the power consumption it takes to drive the jet meaningfully.  to get anywhere close to what is needed, we've sourced a 3 stage 24vdc vac blower.  it pulls 600watts.  the power consumption of this on a car battery, run for 15+ minutes on start up, allowing for failure, entropy and the inevitable crap batteries, then wanting to start the engine, gets challenging.  the current in house theory is that to get the needed vac on an ejector with a dc pump drive, we'll need to consume an amount of watts that is not realistic for a regular one car battery rig.

so right now we're back at blowers and overdriving them.  we have a 10" stall vac blower at 12vdc posted about elsewhere.  we were rewinding it to get 15-20" still at 12vdc, but couldn't keep the fets intact.  higher end ones and lots of cooling fins kinda worked, but it was tempermental.  so now we're staying with the 12vdc winding, but using a voltage boost to about 17vdc.  this keeps the amps about the same through the fets, and their switching is much more efficient at higher voltages, thus they tend to not generate as much heat.

this we hope will get us 10" of usable vac at flow, but to help this out we're now separating the gas and air drive.  the premix solution doubles the volume you need to pull, and you take on the flame back up problem into the blower.  to solve this you end up wth some sort of constriction to keep the velocity up enough that flame can't back up.  we posted this solution elsewhere around here.  it works, but its touchy and you can get failures.  and more importantly, the constriction now ups the blower performance you need to push the gas/air through it.

putting all this together, we've decided to try running two blowers- one for gas and one for air.  this both lowers the blower performance needed, and eliminates the back up problem.  we still very much want to stay with a premixed flare as it burns much cleaner, and we want to add the chp exchanger right on top of it.  we want all this enclosed and not passive draw mixing so there is no smoke leakage downlow.  this means we need active mixing into a closed chamber, and thus the two blowers.

here's the double blower test we're working on inhouse at the moment. no idea if this is a good idea yet.  we'll hopefully see early this week what it will do.





j