High Pressuse AOF Fans

[syrupkid]

I am adding AUF and AOF with a new cast refractory grate and an airtight front for next season and I don't know what kind of fan to use for the AOF. I have done a lot of looking on the internet and I haven't really found anything. I want a fan that does at least 300 CFM at 3 inches of water. What do all of you guys have?

[BreezyHill]

I am using a fan out of an unused sawdust collector. Go to harbor freight on line and you can see it. This one has a 1 hp motor and puts out a boat load of air. I was looking at the bounce house fans as they are very available and rather cheap on clist and ebay. Just have to have a good way to slow them down. I have a router speed control that I am planning to use. A damper system will also work but the router controller was only $15 on ebay so I couldn't pass it up. I have a preheater designed and just have to find the time and build it. With the bounce house fan you may be able to use the one fan to feed both AUF & AOF. They put out a bunch of air.

[maple flats]

A boat load of air is not what you need. Look at this link:
http://www.uvm.edu/~pmrc/Combustion.pdf
In there is a list of blowers for various width evaporators. Get the specs on the one for your size rig, then shop around, but make sure the specs are right. HP air does not need a load of CFM, but it does need the right amount. The most important issue is the pressure. A squirrel cage blower will not do what is needed. It is the high pressure that mixes and burns the gasses for the highest efficiency.

[Springfield Acer]

For what it's worth, I set up my new 2 x 6 with OFA and UFA supplied by a Toro electric leaf blower that is mounted outside the shack in a small screened enclosure. The Toro is the only one that I could find that has a variable speed control built right in. I run mine at about half speed and it performs great. I probably ran it for around 100 hours last season without a hiccup. I set the speed where I want it on the blower and then have a switch inside to turn it on and off when adding wood. I run the speed up to full when I am cooling down at the end of a boil and have only coals left.

[tuckermtn]

Dayton 4C129 on my 2.5x8 that had air everywhere.

[nymapleguy607]

On my 2x6 I have a dayton 2C820 High pressure blower, I have been running a 3450 rpm motor but it pushes too much air, I want to put a 1725rpm motor on mine to cut back on the volume of air.

[RileySugarbush]

Jeff,

If you are using this for over fire air, a much better solution is to reduce the size of your nozzles. AOF works best with the highest possible velocity in the air jets, and that is a function of the pressure just upstream of the nozzles. Smaller nozzles means less air at high speed. A slower blower motor will give you less air and slower jets, which is not nearly as good.

[nymapleguy607]

The blower feeds both the over and under fire air, If I run with the dampers fully closed I still get good fast jets of air. I want the slower motor so the dampers will be able to control the air. Unfortunatley a smaller nozzle diameter will only make more air come from under the grates

[RileySugarbush]

If you are going to run both over and under air with one high pressure blower, you need to size the nozzles to give you the right amount of air at near full static pressure of the blower, at least 4 inches of h20, to get high velocity, which insures good mixing. Just getting lots of air isn't really the goal. You shouldn't throttle the high pressure air anywhere else unless you want to shut down for firing. Ideally, very small nozzles with very high pressure, sized for the perfect amount of oxygen is what you are after. If you get plenty of air at lower pressure, your nozzles are too large.


Bleeding off some of the air for medium to high flow under fire needs to be a throttle valve between the blower and the grate, to preserve the high pressure that is needed for over fire. Think of it like you are trying to fill the swimming pool while washing your sidewalk with a pressure washer by putting a tee in your high pressure hose. You can't do do either task well. Better to use a separate hose to fill the pool, and let the high pressure device do what it was designed for.

I run separate blowers to avoid this problem. A cheap squirrel cage for under, with an inlet damper to adjust, and a high pressure radial vane blower at 3450 for 4" of pressure above. It has an inlet damper, but is always open or closed fully. Over fire I am running about 100 CFM at 80 mph jets. That is good velocity, but I think too much air volume. I plan on trying a higher pressure blower and smaller nozzles next season, for higher jet velocity and less air.

[nymapleguy607]

I have talked with the people at CDL about what RPM motor is running on there intensofire arches, they are running the 1725rpm motors. Also one member on here who built a 2x10 intensofire clone said he is running his with a 3 phase motor and VFD, He has said he runs the blower around 2000rpm, he's basing this off the frequency the motor is running. As far as nozzle sizing goes, before building this arch I used a bunch if different nozzle combinations and I was never happy with the results. The nozzles in this arch are 1/2" and have worked about the best for me.

[lpakiz]

Yes, I'm the guy with the VFD. I highly recommend a 3 phase motor and the Variable Frequency Drive arrangement. You can dial in any amount of air. However, that will still leave you with the "balance" issue between the AOF and the AUF.
My blower runs into a "plenum" where there are three fairly tight-fitting dampers. Still, it is difficult to get the right balance, because of air leakage past the dampers. Take time to get a perfect fit, or use 2 blowers. Air Under Fire is a piece of cake for any blower. It doesn't take much air there to get a raging fire.


If I built another, I would seriously consider the 2-fan idea

[maple flats]

I very successfully run AOF and AUF from 1 blower. My blower is outside, under my feed tank and is plumbed to the evaporator with 4" PVC. It then splits into 2 at a y under the arch and both go to 3" there. Then they go thru a 3" ball valve. It took me a full season before everything seemed ideal, but I now don't ever change the valves. The AUF has it's valve open about 25% and the AOF valve is open about 65-70%. My nozzles are 3/8" black pipe spaced every 6" and angled down 15 degrees. My front is not air tight and thus I can see thru the slight space between the doors. Once the fire gets going there is a great turbulence in the firebox and my boil is fantastic. I originally shut down to fuel, but then my grandson forgot once and we got no smoke out the front. Since then I haven't shut the blower off to fuel. The unfortunate part is that I don't know what the CFM of the blower is, because it is about 100 yrs old and from a company no longer in business. It is about 15" in diameter, with 4 paddles about 3.5" wide and they throw the air at good pressure out the 3" outlet. I connected this to the 4" PVC to carry it into the sugarhouse. I'm not sure what my RPM's are but I could check. I run a 1725 motor then belt drive it to the blower. It is stepped down some but again, I forget how much. When setting it up we tested the pressure with a gauge and used the pulley that met the suggested pressure, 3" W.C.

[nymapleguy607]

Yes, I'm the guy with the VFD. I highly recommend a 3 phase motor and the Variable Frequency Drive arrangement. You can dial in any amount of air. However, that will still leave you with the "balance" issue between the AOF and the AUF.
My blower runs into a "plenum" where there are three fairly tight-fitting dampers. Still, it is difficult to get the right balance, because of air leakage past the dampers. Take time to get a perfect fit, or use 2 blowers. Air Under Fire is a piece of cake for any blower. It doesn't take much air there to get a raging fire.


If I built another, I would seriously consider the 2-fan idea

My blower does run into a plenum with 3 air dampers, my blower just has so much air it overwelms the dampers. I never had to fully open a damper, My blower is rated at 173cfm @ 5"h20. by reducing the motor speed to 1725rpm I can cut that in half and let my dampers work like they should.

[lpakiz]

Jeff,
That does sound logical. If you have WAY too much air, you could slow the blower, as long as you generate adequate PRESSURE for the AOF. The ball valve idea sounds good too, as you can achieve total control over both air flows.

[RileySugarbush]

But lower speed will reduce the pressure as well as flow. If I had 5" wc, I'd be very happy and wouldn't do anything to to reduce AOF flow except adjust nozzle size. Pressure = velocity, and more is better. For a given velocity, total nozzle area = flow rate, an more is not better, just right is best. Too much and it cools the flue gasses and increases the total flow rate, limiting heat transfer.

A throttle valve, like the ball valves mentioned, or a blast gate, can be used to adjust AUF as needed.