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Thread: Volumetric flow per 100kbtu/hr.

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  1. #1

    Volumetric flow per 100kbtu/hr.

    I’ve been thinking about the burner for my larger furnace and was just looking for some feedback. I’ve been reading here and on the web and I plan to use natural gas as fuel but thought I may as well consider LP and oil in case future needs change. So I was looking at several candidates for the air source and after some back of the napkin calcs, it appears if I could reliably deliver 100cfm of air to the furnace, that would have the potential for approximately up to 500kbtu/hr. The maximum intended single melt capacity of this furnace would be ~50lbs of aluminum. The following chart seemed to have some good summary info for estimating energy potential, volumetric flow, and AFR for common fuels.



    So according to this, a cubic foot of NG can produce about 1000 btu. At stoichiometric burn the air fuel ratio (AFR) to do so is approximately 10:1. So for 100kbtu/hr you’d need about 100ft3 of NG per hour or1.67ft3/min. For 10:1 AFR, that means 16.7ft3 of air per 100kbtu. You may want 10-20% excess air but in any case at most you’d need <=20cfm of air per 100kbtu/hr…..so 5x yields the 100cfm of air for 500kbtu/hr I mentioned.

    For propane, the AFR is about ~2.5 times greater but so is the energy per unit of fuel so it’s in the same ball park as far as energy potential for 100cfm of air.

    Oil is a little trickier on the AFR because the mist isn’t really a gas but assuming one could sufficiently atomize and completely burn the oil, it looked like a good performing oil burner would also be in the same ball park for 100cfm of air. Using the chart above, approximating the Heat of Combustion as ~20kbtu for a pound of oil, that means you need about 5lbs/hr to produce 100kbtu/hr. If the AFR is approximated at ~200ft3/lb oil, that means 1000ft3/hr or 16.7 cfm…….so all these fuels need about the same amount of air to produce 100kbtu. As an aside, if oil is ~7.5lb/ gallon, that means an oil burner that consumes .67 gal/hr is in the range of 100kbtu/hr.

    Does this seem about right?

    Now, as for the combustion temperatures achieved for each fuel, that’s a discussion for another day.

    Best,
    Kelly
    Last edited by kcoffield; 01-08-2017 at 08:03 PM.

  2. #2
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    Did you calculate in the compressed air required to atomize the oil..? I'm assuming you're brainstorming a spray nozzle because a drip would need more than 100cfm to convey the oil into the furnace.
    FLAME ON...!!!!

  3. #3
    Quote Originally Posted by J. Vibert View Post
    Did you calculate in the compressed air required to atomize the oil..? I'm assuming you're brainstorming a spray nozzle because a drip would need more than 100cfm to convey the oil into the furnace.
    I'm going to try to get there with NG but for oil, I didn't consider the additional compressed air. I thought it would be small compared the blower fed air, but even for drip, more than 100cfm? This is what I'm trying to get a handle on.

    From the above, wouldn't that produce a very, very, lean, burn.....with 100cfm would be 20% more air than (theoretically?) necessary to burn almost 3 1/2 gal/hr of oil and produce 500kbtu/hr. I was thinking that would be a monster of a burner. Isn't 300-350kbtu/hr pretty respectable....?

    Best,
    Kelly

  4. #4
    Quote Originally Posted by kcoffield View Post
    I'm going to try to get there with NG but for oil, I didn't consider the additional compressed air. I thought it would be small compared the blower fed air, but even for drip, more than 100cfm? This is what I'm trying to get a handle on.

    From the above, wouldn't that produce a very, very, lean, burn.....with 100cfm would be 20% more air than (theoretically?) necessary to burn almost 3 1/2 gal/hr of oil and produce 500kbtu/hr. I was thinking that would be a monster of a burner. Isn't 300-350kbtu/hr pretty respectable....?

    Best,
    Kelly
    Kelly,
    I run oil (waste and vegetable) and propane in the several furnaces I have here in the shop. I am afraid the specific math on some of this is beyond me, but I will agree that 100cfm for a drip style burner probably won't be enough. This is generally because the velocity of the air needed to atomize the waste oil blowing by the drip tube is very high. Most inexpensive blowers are not designed to produce high volume, high velocity air. If you ebay high output centrifical blowers you will see how much these real deal blowers cost. From my experience you have two workarounds to this problem. In order to get the velocity needed to atomize the fuel, you reduce the opening of your tuyere in order to get the highest velocity of air passing your drip tube. Because most blowers are not designed for this, your 100 cfm blower output will drop considerably due to the back pressure created by the smaller opening. The solution is of course to use a larger blower. How large? Large enough that you can push the (in your case) required 100 cfm through your specific tuyere size without being affected by back pressure. My experience has been that this usually ends up being a blower approximately 2-3 times larger (cfm) than you need. The other workaround is compressed air. In this model the blower is simply supplying enough oxygen to burn the fuel, although it will help atomize the fuel. All of the "work" of atomizing the fuel is done by the compressed air. You can easily create the velocity needed to atomize the fuel by forcing it through a small opening. Backpressure is not an issue. If you have compressed air, this is a great option. I use this style burner on my primary furnace, and it is by far my favorite. It is easier to light than a straight drip style burner, its dead simple, and can be adjusted quickly to whatever fuel you are using with just a few tweaks to a needle valve. Hope this helps
    ...Dave...

  5. #5
    I would love to see you run a couple of big ass ng ribbon burners in that thing. Seen the small one i built for my forge?
    It works a treat, runs on household pressures. You could TRY to get the gas company reset your regulator on your house, but im not sure if they would.. I do know there is a few things that are important to get the lazy ng flowing, but it wasnt hard, just a length of pipe. Run a search for ribbon burner, sorry im on a tablet at the moment.
    Visit me: WWW.HandcraftedLanterns.com
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  6. #6
    Quote Originally Posted by jagboy69 View Post
    I would love to see you run a couple of big ass ng ribbon burners in that thing. Seen the small one i built for my forge?
    I did see your build and liked the lost wax (crayon) approach you used and also the result it achieved. However, I think the design approach I took has locked me into a tubular burner in a tangential Tuyere, at least on the current build and for the foreseeable future. I think I'll be able to get enough NG into the furnace but may take some work to get the control and stability I'm looking for with the very low residential supply pressures. I have a good supply of NG in my shop but the meter and regulator on the main is quite a ways away and asking/getting the utility company to tap and supply NG at higher pressure in a residential setting would be an exercise in futility.

    Best,
    Kelly

  7. #7
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    I'll be honest with you Kelly... I'm fairly solid with math, but can't be bothered applying it to my furnace/burner.

    I know I have enough umphff behind my oil feed with the pump I use. I know I have enough combustion air with the BC blower I use. If I ever find I'm struggling reaching a usable pour because of either factor, I'll deal with it then.

    I guess I'm rather old school with how I run my rig. Flames exiting = rich, flames barely make it midway up the furnace wall = lean. To control it I can either tweak air or fuel.

    I rather attempt to wrap my brain around ramming up workable molds.
    FLAME ON...!!!!

  8. #8
    Quote Originally Posted by J. Vibert View Post
    I'll be honest with you Kelly...I'm fairly solid with math, but can't be bothered applying it to myfurnace/burner.

    I know I have enough umphff behind my oil feed with the pump I use. I know Ihave enough combustion air with the BC blower I use. If I ever find I'mstruggling reaching a usable pour because of either factor, I'll deal with itthen.

    I guess I'm rather old school with how I run my rig. Flames exiting = rich,flames barely make it midway up the furnace wall = lean. To control it I caneither tweak air or fuel.

    I rather attempt to wrap my brain around ramming up workable molds.


    Appreciate that J and the advice. Growing up I spent alot of time in the shop with my father. We built all kinds of things. A good dose of common sense and trial and error can go a long way. I have always loved building things and that’s why I went to school….to get better at it. The educational institutions will teach that Engineering is the reduction ofscience to practice. I think it’s also understanding the differences between theory and reality, what’s practical and what isn’t. Sometimes I go the other direction to identify and apply the science to what I see being practiced. I think doing both can enable better undertsanding and from that better results without as much iteration. Besides, I like it. It sooths my brain when I cant be using my hands and keeps me out of the bars. I know with certainty some of projects I've taken on in my professional life wouldn’t have stood a chance at success without understanding the science let alone have been delivered on a schedule or budget. It’s just the way I’m wired now but I don’t feel the need when it comes to the mundane.

    When it comes to furnace heat, the only constraint I have at the moment, whatever the source of heat may be, it will need to be introduced to the furnace through a 2” tangential Tuyere. Otherwise it’s a clean sheet of paper. The build journey is just as enjoyable for me as reaching the destination, which will only enable me to build more and better when casting time comes……and it will come. Please bear with me on all the analytical dribble in between.

    Quote Originally Posted by Junkyard View Post
    ….… but I will agree that 100cfmfor a drip style burner probably won't be enough. This is generally because thevelocity of the air needed to atomize the waste oil blowing by the drip tube isvery high…….. The other workaround is compressed air.

    With you and J that’s +2. -Got it. I have plenty of compressed air so if I was ever to go the route of liquid fuel most likely to be pressure fed/atomized for all the reasons you cite….dead simple.

    Hope this helps


    It does and thanks for the advice.

    ……..Because most blowers are not designed for this, your 100 cfm blower output will drop considerably due to the back pressure created by the smaller opening. The solution is of course to use a larger blower. How large? Large enough that you can push the (in your case) required100 cfm through your specific tuyere size without being affected by backpressure. My experience has been that this usually ends up being a blower approximately 2-3 times larger (cfm) than you need.


    Depending upon the style of the blower, there can be large differences between free air flow and what can be delivered against even very modest pressures. I always wonder if it’s the blower that needs to deliver 2x or 3x or if it’s just the case the blower can only deliver 1/x of that flow at that delivery pressure.....I surmise that's what you are saying because it seems to me, that if you actually have to deliver 2-3x the air flow necessary to burn the fuel, it would be creating a highly oxidizing furnace environment.

    I may be underestimating some of the waste oil burner energy values. In another thread the Indiscriminate Scavenger indicated on average he is burning ~5gal/hr of oil and melting 90lbs of iron in just over an hour. I’m estimating his runs at about 750kbtu/hr and IMO that’s a monster by home/backyard standards. For the foreseeable future my biggest melts will be a little more than half that mass, but aluminum, with most melts far less. I was figuring 300-400kbtu/hr would be plenty…….. Given I have NG, it’s inexpensive (like <=50 cents/ccfor 100kbtu), I could run a 300kbtu NG burner for an hour for $1.50, not have tostore fuel, and have essentially instant on. It’s hard to pass it up.

    So I looked for blowers for the air source and decided I really liked the OEM 3-stage central vacuum motors. They approach 100cfm offree air flow, do so at <13 amps on 120vac, and because they are multiple stage, are less sensitive to reduction in flow due to changes in back pressure caused by flow in the furnace and can actually develop a fair amount of head for what they are. They also have a universal motor making for easy speed control, >=700hr average life, are well supported with rebuild parts, and are a nice compact package (6” diameter, 7” long) with a 2” discharge port that is easy to plumb to. Downside; 20krpm, ear plugs not included! It should be capable of velocities similar to some of the higher performing commercial hand riers. As a bonus for analytical nerds like me, flow and tech data was available. If anyone is interested I can post a data sheet. I think it’s a good place for me to start given my initial desires are aluminum, and like I said enough air for potentially 500kbtu/hr….-should be ample. I got a new one for $45. It will still take some work for stable performance with low pressure NG and some fail safe controls.

    It would likely be an adequate air source for any pressurefed liquid fuel systems in the same energy range.

    I would still welcome any/all additional comments should I eventually graduate to bigger burners and hotter alloys melts. As an aside,only a fraction of the heat of combustion makes its way into the melted metal…..our furnaces just aren’t very thermally efficient. Now my electric is a different story in this regard.

    Best,
    Kelly
    Last edited by kcoffield; 01-13-2017 at 04:08 PM.

  9. #9
    Here’s the central vac motor and specs. The Ametek units sell for ~$90 shipping included but can be bought for $40-45 new/used if you watch. There are several knock offs of the same unit that can be bought any day for <=$69 free shipping. Just search eBay for “3 stage central vacuum Motor”and lowest $ first. There are several versions. I preferred the cast metal discharge horn but a plastic version is available for slightly less.







    Vacuum motor/fan assemblies can be decent candidates for furnace blowers. I chose the unit above for all the aforementioned reasons but if you search eBay for “vacuum motor” and do an advanced search above $22 you’llfind a lot of viable candidates. Or if you can just nab one from and old vacuum cleaner unit for free. Only thing is many/most won’t have a discharge shoot that is easy to plumb to so you may need to fab some discharge duct and you may also need to deal with controlling other voltages and/or DC for those source from battery powered hand held units.

    Best,
    Kelly

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