Melt time?

Discussion in 'Burners and their construction' started by 0maha, Oct 24, 2021.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Positive Displacement is a generalism thrown around when speaking about compressors. In my view it only applies to machines like recip pistons, rotary vane, flooded screw compressors, etc, that displace positive sealed volumes. I have only partial recall of your blower, but I can say with fair certainty it's a centrifugal (or radial) compressor and whether they are the the lower performing so-called squirrel cage or more refined turbine varieties, they are not positive displacement and all suffer the same leakage you refer to (it's just a matter of degree), and the volumetric flow produced will be a strong function (decreased flow) of (increasing) upstream pressure.

    To illustrate the point, completely close your gate. The flow and velocity will both go to zero but the blower will continue to run.

    The gate creates pressure drop which increases the delivery pressure required from the blower, which in turn reduces the volumetric flow it is delivering. Although the velocity may be high in the vicinity of the gate, when the flow returns to the full diameter of the pipe, the velocity will be reduced by the reduced volumetric flow. The velocity is just the volumetric flow (ft3/min) divided by the cross sectional area of the delivery tube (ft2)........ft/min.

    -Closing your gate is delivers lower velocity and volumetric flow of air.

    Best,
    Kelly
     
  2. Melterskelter

    Melterskelter Gold Banner Member

    Roger that.

    Denis
     
  3. Melterskelter

    Melterskelter Gold Banner Member

    And on any compressor of whatever design closing a gate reduces air (compressible fluid) flow. Whether screw compressor, piston, squirrel cage, whatever, a gate cuts flow. It’s a simple matter of increasing resistance to flow with a gate decreases flow. Always. If the pressure generated by whatever compressor exceeds design limits of a gate, the gate will break. A stronger properly designed for the conditions gate will cut flow.

    If one wants to maintain a volume of flow through a compressor of whatever design for the sake of the compressor, then a “Y” fitting with a gate on each of the two legs will allow adjustable, predictable, consistent flow through each leg.

    Denis
     
  4. HT1

    HT1 Gold Banner Member

    I did your test, with the gates closed the blower BLEW off the inlet pipe , so I did it again standing on the blower, the blower made a sound roughly like a turbine with a bearing going out, so I turned it off, no rush as soon I stepped off the blower it blew an inch off the pipe releasing the air ,

    and I can assure you all my furnace is not operating they way you are all saying, but this I will point out, my gate, is at the inlet of the burner, so perhaps moving it back farther would cause it to act differently (shrug)
    V/r HT1
     
  5. 0maha

    0maha Silver

    I've been noodling on that myself. At the moment, I'm thinking something like this:

    [​IMG]

    One gate going to the burner/tuyere, the other as something of an adjustable waste gate so the fan isn't eating all the backpressure when you've got the primary gate throttled down.
     
    Melterskelter likes this.
  6. Petee716

    Petee716 Gold Banner Member

    My current setup is similar to this in principle. I was previously using a router speed controller which I used to control my vintage Kirby vacuum cleaner. That vacuum finally bit the dust (um, yeah) and I switched to a leaf blower. The speed controller got all melty inside and quit working shortly afterwards. So I went to an airflow like you show here. I was assured that the extra dump wasn’t necessary but I did it anyway. It works fine and adjusts for the amount of needed air for the various stages of propane startup, midtemp oil burn, and full throttle.

    Pete
     
  7. 0maha

    0maha Silver

    In the spirit of "anything worth doing is worth over doing", I put this valve together:

    [​IMG]

    Mostly, it's two halves 3D printed, with an aluminum valve thing in between.

    Here's the input side, after a bit of paint. My blower (a shopvac for now) attaches to the end, then the valve allows you to proportion the air flow between the furnace and the waste gate.

    [​IMG]

    Here it is in action. Seems to be working.

    [​IMG]

    Between the pressurized fuel delivery and the additional combustion air, this setup seems to be working really well. I just finished a melt from cold to pour in right at an hour. That's a huge improvement from where I started.

    The 3D model is here: https://a360.co/3r7XNa4

    Fair warning. The parametric modeling is pretty much a hot mess, but you're welcome to play around with it if you like.
     
    Clay, Tobho Mott and Mark's castings like this.

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