Dredge Nozzle Physics

[Sierra Sam]

Reduced the angle of attack. Will try it in Sierras this weekend. If pressure still insufficient I'll attempt to slide a sleeve into the jet to reduce diameter for pressure increase. Either it works or it won't. Got lotsa pipe and welding rod so failure is not an option; A bit of a time delay perhaps, but unless I die trying it's all good. Heck, it's fun trying, just as digging all day for a few specks glittering on the black mat in the morning sun

[Hoser John]

NO WAY JOSE-you then have lousy suction,cavitate easily,frothes like a divorce lawyer(or environutz)and it MUST utilize the force from hitting the wall with the EXTENDED SWEET SPOT (for every action there is a direct reaction in direct proportional ration) to get good suction. Dahlke minijet flop was for this reason-you drill a hole through the hose fast too without the steel jacket. tons a au 2 u 2 -John

[finegold]

Thanks John for the kind words, except for the "old" part of the Treasure Emporium reference.

The real credit for most of the jet/eductor design foundation goes to D&B Custom Dredge.

Dick Arthurs and Bill Cohenour were the masters of tenacious "cut and try". Many hours in the test tanks and on Piru Creek (when you still could dredge there).

From their work I marched on and also created a formula for getting you close to want you want. I say close because there are many hidden variables that will drive you nuts. Ie: workmanship, alignment, etc.

I don't profess any special knowledge on single eductors as the performance, using the same gpm and pressure, is significantly different than using a dual eductor.

Anyway my experience (primarily duals and my favorite quads) is 11.5 to 12 degrees on the angle of the eductor to the jet tube. Perfect center of jet tube discharge form the eductor. Lots of tack welding and cooling to minimize "creep".

Formula (presuming the previous was boring):
The area of the jet in sq. inchs times 7% = eductor bore in sq. inches

Example: 2 inch jet [Pi x (Radius of jet - Squared) = jet in sq in.]
2 inch jet = 3.14 x (1" (radius of jet) Squared)
3.14 x (1 x 1) = 3.14 sq in for a 2" jet
3.14 sq inches x 7% (.07) = .219 sq. Inch for eductor

Now for the tricky part: convert backwards and finally come up with a bore diameter.
Eductor size is .219 sq. In which we will call .22 - because it is easier.
Divide .22 by 3.14(Pi) = .07 , now find square root of ..07 which by calculator is .264 IMPORTANT: this is the radius of the eductor size - NOT the bore size!
Multiply .264 x 2 = .528". Refer to a conversion chart to find conversion of decimal inched to "regular" inches which is fractionally larger than 1/2".

At 30 psi the 1/2' eductor will require 41 g.p.m. Without getting into an explanation of friction loss of your ten foot, one inch diameter pressure hose, you will require 33.5 psi at the pump to provide the 30 psi at the eductor.

Anyone still awake?

[Geo-George]

May I add,......

Optimal Flow Velocity

The optimal flow velocity for a gravity flow dredge depends upon the specific requirements of the sluice being used. Most sluice manufacturers publish specifications for flow in gallons per minute. In general, you will find that most modern dredging sluices will require a flow velocity of between 10 and 15 feet per second to operate at their designed flow volume specification. Sluice designs tend to vary however, and it is recommended that you check the flow requirements for your specific sluice and adjust the layout of your gravity dredge so as to operate within the manufacturer's published specifications. Converting velocity (in feet per second) from the graph above to gallons per minute is relatively easy given that there are 7.481 gallons per cubic foot of water, and the volume of water in a one foot length of pipe is equal to the crossectional area of the pipe. This should roughly be pi multiplied by the radius squared. Volume multiplied first by the velocity, and finally, by 60 seconds per minute will equal the predicted flow of the gravity dredge in gallons per minute. I've calculated out conversion factors for the common dredge sizes in the table below. Just multiply the conversion factor from the table below with the velocity deturmined from the graph above to yield the flow in gallons per minute

Table for Converting Flow Velocity to Gallons Per Minute

Converting Flow Velocity
to Gallons Per Minute
Size
(inches) Conversion
Factor
2 =9.791
2.5= 15.30
3 =22.03
4 =39.17
5= 61.20
6= 88.12
8 =156.7
Although this example applies to a "Gravity" dredge, the priciple still applies.

[jimmy]

Finegold, Thanks for posting your jet formula (Nov. 97 Ak. pros. forum). Made 2 3" dual jets (my tubing for jet tubes was above or below calculated diameter). Both worked great, but each had their sweet spot (pump/eng rpm). I used Tiny tach to measure rpm and medium quality pressure gague for pump pressure. Was able to lift material 7' above surface of water from 3' below during testing. Thanks again, jimmy (dont know how to post pics)

[Sierra Sam]

Holy COW! Diggin dirt seems soooo much simpler! Sheesh guys, I was an Art major for a reason. Obviously didn't do me much good since I'm now tryin to fab my own rig to save a buck not buying one off the shelf.

The math of it all seems a bit overwhelming, but then not so. Seems the math is just part of it, then there's the sweet spot that emphasizes the reactionary suction factor. Looks like I'll be doin some weldin and a cuttin a few times. In fact, now you guys got me thinking and I see some serious experimentation in my future. Big concern this time of year is extreme short time water is available as creek is dry most months. The other is the Poison Oak that's rearing its ugly head EVERYWHERE!

And... Done! With initial refinement at least.