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Blower moter requirement

  1. Jan 23, 2009 #1
    I have a custom need for a blower motor that I need some advice on. I have built a fireplace grate out of 1.5" schedule 80 pipe. It will sit in the coals of the fire, and I would like to blow air through the pipe to circulate that collected heat back into the house making the fireplace more efficient. I need a fan that will do the job.

    I have a 2" black pipe I need the fan to feed into (which will be reduced down to 1.5" pipe after about 6'), it will go through about 30' of pipe total, that has about twenty-five 90 degree elbows in it. I've been told that relates to a static pressure of between 7-8 inches.

    The size limitations for the fan is 12"x12"x24", and I have only 120V AC to the area.

    Any recommendations on where I can get a fan to provide around 75-100cfm output and not sound like a jet engine?

    Thanks!
     
    Last edited: Jan 23, 2009
  2. jcsd
  3. Jan 23, 2009 #2

    Q_Goest

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    Hi fireplace. Welcome to the board. I stuck your numbers into a calculator as follows:
    1.5" ID pipe, ~30 feet long, 25 elbows, 75 CFM air.

    Resulting pressure drop is about 1.3 psi. No good. Typical squirl cage blowers can't create anything like that much dP and if they could, it would be very loud.

    If you want that much flow, you need to put the 1.5" pipes in parallel. Putting 10 such pipes in parallel and making them shorter (say 6' with 2 elbows) results in about 0.1 inches of water dP which is much more realistic.
     
  4. Jan 23, 2009 #3
    If I put them in parallel, I wouldn't be able to have a common exhaust, would I? If I fed 10 lines from one pipe using T's, would air flow equally through all lines if I tied the exhaust pipe into all 10 lines by using more T's?

    Wouldn't the air flow through the pipes decrease dramatically as you progress from pipe A through pipe G? See the (very crude) "drawing" below. You have an in line T'ing off at 7 pipes (A-G), and then T'ing back into the outlet.

    Pipes (A-G)
    G F E D C B A
    ---------------------- blower in
    | | | | | | |
    .......fire...........
    | | | | | | |
    ---------------------- heat out


    Would this work??
     
    Last edited: Jan 23, 2009
  5. Jan 23, 2009 #4

    Q_Goest

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    Your drawing is pretty good. You should have a common inlet (aprox. 5") and a common outlet (5"). Actually, 5" pipe is very uncommon, so go with 6". . .

    Between those two you can put your 1.5" pipes which T into the 6" headers. The air flow won't change much from pipe to pipe because the headers have such low dP through them and should be relatively short (just long enough to T all 10 pipes in).

    Oh, and you don't need sched. 80. I'd suggest going with a much thinner wall at least on the 6" headers.
     
  6. Jan 23, 2009 #5
    What would i lose if the headers stayed 1.5"? Or even just went up to 3" pipe?

    The expense of this would balloon if I had to get 6" pipe and the T's (reducing down to 1.5" (if they even make them)). and somehow I would have to get from a 2" pipe to a 6" pipe. I guess I could do a bunch of cutting into the pipe and welding, but that isn't very cheap either.

    Is there basically no way to do this without going this route?

    Thanks for this advice. I really do appreciate it!
     
  7. Jan 23, 2009 #6

    Q_Goest

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    Well... if your headers aren't actually in the fire box, and are not exposed directly to the heat of the fire, nor to the potential for smoke (ie: carbon monoxide) then there's no reason you can't make the headers out of sheet metal. They don't even have to be round because there's no significant pressure. You can make some square/rectangular boxed sheet metal headers and connect them to your 1.5" pipes as long as the headers are isolated from the fire.

    I haven't checked a smaller pipe size, but I suspect going down to 3" wouldn't work.
     
  8. Jan 23, 2009 #7
    Therein lies the rub... the headers will be in the firebox and in direct contact with the logs and coals. The logs will lay across the 1.5" pipes and rest against the headers as well.

    Thanks for the advice on this. I guess I will just have to try and figure something out.
     
  9. Jan 23, 2009 #8

    russ_watters

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    I suspect that it would be cheaper to build a heat exchanger with parallel tubs (it really is the correct way to do it) than to buy a blower - if you can even find one - that has that performace. You should google "fireplace heater" and "fireplace blower" and see what is out there. You may get some design tips, such as with this: http://www.woodlanddirect.com/Firep...ace-Heaters-Blowers/Spitfire-Fireplace-Heater

    Note that the tubes are in parallel and there is only one header and a draw-through fan. I suspect these designs also use induction to add more air before it is discharged, to cool it.
     
  10. Jan 25, 2009 #9
    Q, I need to make sure I understand this before I continue the design. You say in the previous situation that the area of the manifold needed to be 10x the area of the outgoing individual pipes. I assume that is because I was running out with 10 lines through the coals? If I were to only run out with 8 lines, then it would only need to be 8x the area of the coals size line, correct?

    For example, if I use 1" pipe (area .785" sq) as the lines through the coals, and I use 3" (area 7.065" sq) manifold, then I could accommodate 9 lines (7.065/.785=9) across the coals, correct?

    Also, the distance from manifold to manifold would need to be at least 16" to give this thing enough space to build a decent fire in. How would this effect my operation?

    Thanks,
    Jacob

    (Russ, the unit you linked only works with open doors... ie your heated house air still goes up the chimney. I need to be able to close doors, and have the fire burn air coming in through my open ash doors, saving the heated air in my house, while still adding even more heated air from this unit.)
     
  11. Jan 25, 2009 #10

    russ_watters

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    The area of the manifold doesn't really need to be any larger than the total area of the branch pipes. If you use six 1.5" pipes, a 3.5" pipe would be plenty. Since area is a square function of diameter while perimeter is a linear function, friction goes down as diameter goes up (at a constant velocity).
     
  12. Jan 25, 2009 #11

    Q_Goest

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    Hi Jacob,
    Just as a rule of thumb, I generally try to make flow passages 'constant area'. That isn't to say they have to be, and in this case, the manifolds could be slightly smaller in area.

    To do this right, you should have a blower selected and a blower curve available. A blower curve shows the pressure generated by the blower on the Y axis and the flow rate on the X axis. You can then create a curve for your system which intersects this blower curve, and where they intersect, you have the 'operating point'.

    If you can provide a blower curve for the unit you want to use, and if you can say exactly what one of these 1.500" sched 80 pipes will look like (ie: length and number of bends) and if you can sketch where the blower might be on this system and where the outlet is, I could run some quick calcs on the design and tell you what you need. But without a bit more info, it gets difficult to nail down anything specific.
     
  13. Jan 26, 2009 #12
    Okay, this is the drawing I made of the grate. Should have all dimensions and info. I don't have a blower curve, because that's what I'm really trying to determine is what kind of fan I need for this purpose. I don't know what will work until I know the static pressure, it seems. Can you help?

    Thanks!

    [​IMG][/IMG]
     
    Last edited: Jan 26, 2009
  14. Jan 27, 2009 #13

    Q_Goest

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    Hi Jacob,
    Thanks. That helps a bit. The long lengths of 2" diameter pipe are a serious problem. There's no need to have a 4" header and 6 parallel 1.5" pipes if you have all that 2" pipe. The 2" pipe is what's going to drive the pressure loss. You could have 2, 1.5" pipes instead of 6 and you could have them coming off the 2" pipe, which would give you the same flow area as the 2" pipe, but then you'd need to find a 2" diameter blower. Maybe something off a car. Something this small is going to provide on the order of 5000 to 10000 Btu/hr with a flow of 20 to 40 CFM. These are very rough numbers. To be accurate, we need to locate a specific blower and have the blower curve.
     
  15. Jan 27, 2009 #14
    I see. This is bad. Is this design any better at all than my first idea of snaking through the coals in one long continuous 1.5" snake?

    I am considering changing the location of the input to outside, allowing me to hook up a powerful (and therefore loud) fan that may be able to do the job. I will look into it for specs on the motor.

    I can't tell me how much effort you have saved me.
     
  16. Jan 27, 2009 #15

    Q_Goest

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    Can you explain what you're trying to accomplish? What kind of heat output?

    Why do you need all the piping?
     
  17. Jan 27, 2009 #16
    I am trying to accomplish getting as much heat from the fire as possible. The more pipe I have in contact with the coals, the more heat will transfer into the pipe, and therefore into the house, providing I can find a motor that will blow it! I guess the point is finding the right balance. The fireplace was designed with two 2" hidden pipe going from inside the hearth to inside the firebox so I could blow air form a hidden location and back out into the house without having to keep the fireplace doors open. The grate also needs to perform as a normal grate would that holds the fire off the ground and allows air underneath so the fire burns well.

    That's about it.

    Instead of a blower curve, could you work with a CFM and a Static Water Lift? Vacuum cleaners tend to only provide this information. I think with these whole-house vacuum systems, there would have to be a lot of pipe and 90's they would have to pull through.
     
  18. Jan 27, 2009 #17

    Q_Goest

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    Yes, that's what you're looking for.
     
  19. Jan 27, 2009 #18
    Ok, so I learned vacuum motors would last a much shorter time than I thought. I need a regenerative motor. I have attached a curve for one below. Please take a look at the curve for the 400P model. It has a 1.5" output, by the way.

    If I used this in either configuration I proposed, how would it perform, and which way would it perform better?

    Thank you again.

    [​IMG][/IMG]
     
  20. Jan 27, 2009 #19

    Q_Goest

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    ok, one of these would work. They develop a lot more pressure than a conventional blower. I'll look at it and get back to you.
     
  21. Jan 27, 2009 #20
    Q,
    By the way, I'm thinking that blower may be overkill, but I could be totally wrong since I have no idea of the static pressure created by either of my designs. If you follow this link it will show the curves for many different model numbers of another manufacturer's blowers all on the same graph. Maybe I could use a smaller unit than the one I presented?
    http://www.aquacave.com/NPDPopImage.aspx?pi=http://www.aquacave.com/images/AE-SW-S41_1.jpg

    It's a 1HP motor on the regenerative blower that curve details, and maybe a 1/2 would work, or even less. I honestly have no idea how many CFM I should have, but I know that most of the blowers that come with most of these fireplace heater units are 100CFM, but that's at 0 pressure. I am sure it's down significantly when it runs through its chambers (even though there wouldn't be as many twists and turns as mine). If I could go with a smaller, quieter motor (and less expensive), I would much prefer that, but it is more important to do it right.

    Basically, what I need help with is:
    -What is the static pressure introduced by the 2 systems I designed, or to cut the work in half, maybe just of the latter, manifold design?
    -What kind of CFM do I want (I realize it can't be too much or I would blow cold air, too little and I wouldn't be maximizing my efficiency and the pipes wouldn't be cooled enough and would not last as long).
    -How many BTU would that ideal CFM provide?

    And if you don't have time for any of this, which I could easily imagine being the case, can you point me somewhere I could pay for this service? I don't even know where to start for something like this. It's incredibly valuable information, and I can't tell you how much I thank you already for the money you have saved me from me doing this the way I would have done it without your help!
     
    Last edited: Jan 27, 2009
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