I'm trying to build a kind of tea leaf blower (the machine will blow cut tea leaves.) I bought a car air conditioner blower fan (link below) and I connected my blower system (technical drawing link below.)

The air flow isn't enough to make the tea leaves fly, so I have to increase the air velocity. What can I do to increase the air velocity? I have 100 watt power limit.

There are 13 nozzles on the tube. Each nozzle is 20 mm in diameter. The tube is 1 meter long, the tube diameter is 70 mm tapering to 40 mm beginning to end.

What I tried and the results:

  1. Motor powered with 12 V:
  • Fan only:
    • The motor draws 5 A
  • Fan connected to tube:
    • Current drops to 4 A (48 W)
    • Average air flow velocity in nozzle: 42 km/h (highest value average)
  1. Motor powered with 18 V:
  • Fan only:
    • The motor draws 9 A
  • Fan connected to tube:
    • Current drops to 5 A (90 W)
    • Average air flow velocity in nozzle: 57 km/h (highest value average)

Motor specifications:

  • Brushed DC motor
  • Factory usage fan RPM is 2700
  • 12V
  • 487 m^3/h airflow (average air flow velocity 49 km/h)

I tried with 18V, RPM is 3700, average air flow velocity 64 km/h.

My project image:

enter image description here

Radial fan image:

enter image description here

Radial fan datasheet: https://1drv.ms/u/s!Ao1QAv8AtzQgo0uh3r90yp1b5lZv?e=ulLm7v

  • 1
    \$\begingroup\$ If factory usage is 12V then I would stick with that and try not to push the motor to its limits (unless you are using it for short amounts of time). I'm not really sure what a tea leaf blower does or requires so if you can give some more information? Please also note that no one here has any idea of your project and therefore the airflow values and other information about the build itself are rather meaningless unless you add some context. Please edit your question to have relevant information and a clear question (I'm not really sure what I'm being asked???) \$\endgroup\$ Nov 19 '20 at 8:17
  • \$\begingroup\$ Maybe this video can give some information how tea leaf harvester working. youtube.com/watch?v=dOJfZMFLV7A thish machine is working with petrolium, Gasoline agriculture is forbidden in my country and I working on similar machine with battery model. \$\endgroup\$ Nov 19 '20 at 10:49

It's normal for a centrifugal fan like you have to speed up as the exit is restricted. This means you've reduced the mass of air passing through the fan, the motor is doing less work on the airflow, so the load on the motor is less, and so the speed increases.

If the motor is intended for car use, then 14 V is a reasonable voltage to run it on long term, this is the voltage at which cars tend to charge their batteries. While it will run for a while at 18 V, that's not recommended if you want the motor to last.

The fan you have bought is a high-speed low pressure fan. You'll notice from the fan datasheet that the blades are canted forward. Any stall in the output airflow will radically reduce the output pressure. You can turn speed into pressure with a flare from the fan to the plenum.

The nozzles you are using, with their small cyclindrical section and their right-angle connection to the plenum will generate a high back-pressure. You need to design the airflow into the nozzles from the plenum more carefully. Amongst the many things that may help you increase velocity after the nozzles are

  • increase the bore of the nozzles
  • reduce the number of nozzles
  • add a diffuser flare to the fan outlet
  • flare the diameter from the plenum to each nozzle
  • use a larger radius of bend from the plenum to the nozzles
  • use a higher pressure fan, perhaps cascade two fans of the type you have
  • maybe replace the individual nozzles with a single slot in the plenum, the flare may be easier to achieve if you build the plenum with a single strip folded into a tear-drop cross-section
  • \$\begingroup\$ How about starting with a battery-powered leaf blower motor/fan assembly insteadi? \$\endgroup\$ Nov 19 '20 at 15:32
  • \$\begingroup\$ @Mark Leavitt Yes, Yes I’m working on it but the result was disappointing. I built the device on my own, I just couldn't get enough air flow. I have done many analyzes and tests in SolidWork, but I could not get close values. Often the airspeed was too low. I could not buy the leaf blowers sold in the market because they are expensive. I've examined the drawings of these machines, but somehow it doesn't work like them. if you have any advice I will listen \$\endgroup\$ Nov 20 '20 at 6:44
  • \$\begingroup\$ @Neil_UK thanks for your information. Before I use motor with 18V I ask the factory can I use this motor 18V, they said yes but in short period, Also I checked temperature motor while using in 18V( I applied only for if I increase rpm could I get enough velocity) . There similar product in japan which working with gasoline( youtube.com/watch?v=dOJfZMFLV7A ). This machine using similar blower system but it has more power because internal-combustion engine. Also, I measured each nozzle air velocity and about 130 – 120 km/h. But its really so much. \$\endgroup\$ Nov 21 '20 at 5:45
  • \$\begingroup\$ In my opinion if I can obtain about 90 km/h velocity the machine could easily throw it in bag. Also, this gasoline machine nozzle is 15mm diameter. My nozzle size was 20mm diameter. I will share some result of different size of nozzle diameter analysis which I take in SolidWorks. Also, when I add some radius nozzle junction point the nozzle lost some velocity. Now I working on individual nozzle If you interest, I can share result. Lastly, which type fan can give more pressure, axial or radial fan ? Different size nozzle comparison: 1drv.ms/u/s!Ao1QAv8AtzQgo0yJ72smXFXiod5Q?e=SSX48Z \$\endgroup\$ Nov 21 '20 at 5:46

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