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  1. #1
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    Feb 2006
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    Default New Blower for gas powered forge

    My current mains powered gas forge uses an all plastic 1800W variable speed Chinese/domestic vacuum cleaner I found during hard rubbish kerbside collection.
    This vacuum cleaner works OK but it has some limitations;
    - one is the screeching noise it makes is a bit irritating
    - the next thing is the air flow rates are low and this limits the control of the gas flames.
    - the next thing is that being all plastic it's a bit of a concern being used so close to the forge.

    Recently I picked up a small (9") naked impeller with a sweet German built in 1/3 HP motor from Eskimo and have been exploring ways of turning it into an efficient blower.

    The plan I used is the Bill Pentz budget impeller design (see piece of paper on bench) which uses an efficient spiral design.
    Bills plan if for a 14" blower so I rescaled all the measurements to suit the 9" impeller.
    Bills' plan also uses MDF for the top and bottom (or front and back) and galv sheet for the impeller housing spiral but as this thing is going to be used near a forge I decided to use sheet metal instead of the MDF.

    The 9" impeller and motor from Eskimo is the black thingo shown in this picture
    New Blower for gas powered forge-bits-jpg
    The front and back of the impeller housing are cut out of panels scavenged from a clothes dryer
    The inner spiral (at back of bench) is made from some galv sheet metal and some ally angle cut and then bent to the spiral shape.
    The spiral ally and galv are then pop riveted together to provide a spiral shape

    Next the spiral is pop riveted to one of the sides and some space/clamp down rods added
    New Blower for gas powered forge-aplusb-jpg

    Then the front panel is screwed on top. I was going to use rivets but I decided to use 4 mm machine screws in case I needed to take it apart!
    This will also make it a lot easier to seal properly.
    New Blower for gas powered forge-aplusbplusc-jpg

    The impeller/motor was removed from its original base and attached to a 5 mm thick circular ally sheet
    New Blower for gas powered forge-impeller-jpg
    The impeller is held onto the top panel with 5/32" countersunk screws that screw into nutserts attached to the impeller housing


    The low restriction (150 mm diameter) entry was given some rounded intake edges along the lines the bell mouth hoods I use on my dust collectors
    New Blower for gas powered forge-bellmouthentry-jpg

    I just assembled the bits, made some temporary electrical connections and fired it up and it generated 3.5" of water column pressure which should in theory produce a flow rate of 400 CFM and when I measured it I got 411 CFM which is 6 times more than the Vaccum cleaner is able to generate.
    This is without any sealant or gaskets so the performance should improve a bit once the joints are sealed.
    The noise levels are about the same as the vacuum cleaner (~80db, maybe the new blower is a touch louder) but the frequencies output by the blower is a lot less irritating than the vacuum cleaner.

    Todo list
    - add a mesh cover to the air intake,
    - make up some more permanent electrical connections
    - adapted output connections to the forge.
    Attached Images Attached Images

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  3. #2
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    Default

    Very nice Bob. I will be watching to see the end result. I need to do the same thing one day.

    Dean

  4. #3
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    Default

    bloody hell Bob...have'nt you got anything better to do...

    now if that was me I would have just stuck it on the top of a fabricated box with a tube connected to the opposite side of the fan inlet ring... but I take short cuts when i can
    as I said, backward curve fans are able to deliver their capacity at high static pressures so as long as there is not too much.
    Most manufacturers do this these days to allow easier duct configuration and to save space.

    ps fantastic sheetmetal work...my Dad would luv to see that ...he was tinsmith/sheet metalworker

  5. #4
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    Cheers Guys.

    ...have'nt you got anything better to do...
    This flu I have is still hanging around and I'm still coughing up great lumps of yellow so I can only do about 90 minutes before I need a lay down.
    This lightweight sort of work is also about all I have the strength for at the moment.

    I did a bit more on this yesterday, sealed it all up with "no more gaps" and added a fan speed controller and tidied up the outlet so I can connect to the forge air ducting. After sealing it all up I retested the pressure but sealing it made no difference and I then noticed that when the outlet was closed off, air was coming OUT of the edges of the inlet!!

    This means the gap between the impeller housing and top of the impeller is too great and I will have to instal a barrier to reduce this effect. Luckily I didn't pop rivet that panel to the impeller spiral so I can easily take it apart.

  6. #5
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    Default trying

    Trying to remember the impeller clearances to blower housing that BP suggested for the 15 and 16 inch impellers in his CV design - wasn't it between 1/2 inch and 1/4inch (6mm)... (no less)?

    If your using a smaller impeller presumably the clearances are a little tighter than that?

    You are a damn clever fella Bob, no 2 ways about that!

    As a suggestion only,



    Could you not maybe fit the VFD controller in this empty space - with its window display and controls showing thru the outside cover, to "integrate" the unit?

    When tested and your happy with it - could you maybe fill the void areas on the outsides of the spiral, with say ex-panda foam or something to reduce the sound levels a little?

  7. #6
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    Default

    Some good ideas there TT.

    The speed controller is inside a diecast box fixed by screws to the back of the impeller. The reason for this is so that when I remove the impeller/motor from the housing (as I have had to do about a half dozen times already) no AC wire disconnection is required, the impeller/motor/controller/AC cord&plug all comes away as one unit.

    Most of the noise appears to comes from the inlet and outlet and I can't really block those off. The outlet noise is suppressed once the unit is connected to the forge so that is not really the problem. If necessary I would duct the intake to the outside of the shed but let me see where the forge is going to live before I go cutting holes in the shed.

    I measured the gap between the top if the impeller and the fan housing and it was about 16 mm. I made up a spacer to reduce the gap, I haven't tested it yet - can't do more than ~1.5 hours in the shed before I get too knackered.

  8. #7
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    Default If

    If your to decrease the clearance of the impeller to the housing (at the top and bottom - not the outside arc of the blades) i.e. between the flat sides made from flat sheet off the dryer machine...

    Is it possible that the pressure differential (suction) created when operating, will pull that round flat metal support ring inwards 1 mm or 2mm say, under outside air pressure, and further reduce the clearance of the impeller to the other flat side ...possibly?

    I was just thinking - if the round aluminium plate that the impeller motor etc is attached to, was a little larger in diameter than the circumference of the impeller housing spiral.. it would get more support against movement in and out under air pressure differential in operation (supported by the walls of the spiral) - than the section of thin flat metal plate will afford the current smaller dia aluminium disc now mounting the impeller.

    This way if you have to bring the clearances down further...(with thicker spacers) you might remove any potential movement of that section of semi flexible flat metal sheet, because the walls of the spiral will support under the edge of the larger dia aluminium disc.

    Obviously requires a new larger dia aluminium disc to be cut tho, maybe the old one will come in useful for some other project perhaps?.

    On the opposite side..you could just use alloy angle bracing fixed across the flat metal sheet, to stiffen the metal sheet, if required.

    Understand completely about the flu thing - could take a month or so before you come good possibly.

    Slowly, slowly, catchee monkey...

    Maybe it should have a backup compressed air pressure cylinder as well, for event of a power failure?... to give you enough air supply to buy the time needed, to shut off the gas supply, if the air stops unexpectedly - so the forge flame doesn't flame out into the shop, looking for oxygen to sustain the flame (as you suggested)?
    That or a emergency halon dump system out of a commercial boat, fitted in the shed, to deny it oxygen (but you'd have to then wear scuba to operate it safely)!

    Just suggestions.

  9. #8
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    Quote Originally Posted by Timless Timber View Post
    If your to decrease the clearance of the impeller to the housing (at the top and bottom - not the outside arc of the blades) i.e. between the flat sides made from flat sheet off the dryer machine... Is it possible that the pressure differential (suction) created when operating, will pull that round flat metal support ring inwards 1 mm or 2mm say, under outside air pressure, and further reduce the clearance of the impeller to the other flat side ...possibly?
    The static pressure at zero flow is low, 3.5" of WC, which translates to about 0.125 psi and I can see the outer casing twitch or flex inwards (less than 1mm) when I quickly cover the inlet, but of course when there is flow the pressure drop is much less so the casing does not visibly pull inwards.

    Maybe it should have a backup compressed air pressure cylinder as well, for event of a power failure?... to give you enough air supply to buy the time needed, to shut off the gas supply, if the air stops unexpectedly - so the forge flame doesn't flame out into the shop, looking for oxygen to sustain the flame (as you suggested)?
    Just suggestions.
    Yeah I have thought of this, in fact some gas forges use compressors as air supplies but these are nearly all LPG gas forges which I understand do not need as much forced air volume, as the higher gas pressures can be used to produce a gas jet that drags in extra air from the atmosphere. I might have to look at it again.

    I'm currently working on a valve to auto shut off for the gas if the power fails or the power to the blower is accidentally turned off.
    Better still would be an air flow sensor so that I can wire that into the control circuit so that it cuts the gas when the air flow stops.

  10. #9
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    Default

    No biggie, just showing the diecast speed control box and the front grill.
    I'm not going to do any more on it for the moment as it puts out more than enough air
    grill.jpg Spcontr.jpg

  11. #10
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    Default Have

    Have you dispensed with the bell mouth on the inlet, in favor of the grill?

    Looks good...

    Do you buy those VFD's Bob or are they scavenged off other scrapped / skip binned equipment?

  12. #11
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    Quote Originally Posted by Timless Timber View Post
    Have you dispensed with the bell mouth on the inlet, in favor of the grill?
    The bell mouth is still there - it's underneath the grill.
    In practice once connected to the forge the bell mouth (BM) is redundant because the 2" outlet that connects to the forge is so restrictive that it does not matter whether I use a BM or not.. The reason I whipped up a BM was because I can make them very quickly and I wanted to see what max flow this little impeller could output

    Do you buy those VFD's Bob or are they scavenged off other scrapped / skip binned equipment?
    The motor is single phase so it cannot use a VFD.
    I'm just using a light dimmer switch. The control is poor (ie min to max is over a short range) but it will do for what I need.

  13. #12
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    how much is 'more than enough"

    might have smaller one coming up.

    ps great job Bob

  14. #13
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    alternative...is to greatly restrict airflow (preferably supply) rather than speed control. The fans are non-overloading

  15. #14
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    Quote Originally Posted by eskimo View Post
    alternative...is to greatly restrict airflow (preferably supply) rather than speed control. The fans are non-overloading
    Hi Eskimo,

    The Air flow is ultimately greatly restricted because it has to pass through the restriction of 2 x 20 mm (3/4") diam torches inside the forge.
    I'm actually getting a max air speed/flow about 1/2 that of the vacuum cleaner (VC) at the torch which on reflection is to be expected because the VC is a low flow but high pressure device.

    We're about to build a charcoal powered forge for my nephew. We'll be trying the new blower out on this forge to see how it goes.

    Cheers

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