Spray Bar hole size?

[redracer77]

Just wondering about hole size for a spray bar I am making,

Flow will be 3500-4000lph, 25mm pipe.

I was thinking 17 x 6mm holes as that is similar surface area as 25mm pipe.

Does that sound about right?

1 row of holes or 2 rows staggered?

[Ira]

Just wondering about hole size for a spray bar I am making,

Flow will be 3500-4000lph, 25mm pipe.

I was thinking 17 x 6mm holes as that is similar surface area as 25mm pipe.

Does that sound about right?

1 row of holes or 2 rows staggered?

Sounds fine, there's no real rules for it other than more and larger=more volume lower velocity, fewer and smaller= more velocity lower volume.

I'd also suggest putting a few holes pointing straight down from the spraybar to give some flow in the dead area under the bar, but that's just personal preference.

[redracer77]

Thanks, I did think about a few pointing down, will drill them now.

[David R]

Best just to experiment IMO. I usually glue all the joins on my return except for where the spraybar goes into the last elbow, that way you can change it up and play around with it.

[kinnadian]

Smaller holes = better velocity = less deadspots on the opposite end of the tank. You may want to start ay smaller holes than 6mm, depending on what your velocity requirements are for this spray bar (eg do you have extra powerheads in the tank?)

Smaller holes shouldn't put less total volume out, unless you go so small that you are introducing flow restrictions. If your pump is putting out a constant flow rate and pressure, smaller hole (ie less cross sectional area) will just result in greater velocity. Flow = Velocity x Area. That is until the point where you excessively drop the discharge pressure of the spray bar, due to the venturi effect, because the holes are introducing choked flow due to your velocity being sonic. It will be hard to achieve this with the flow rates we experience from return pumps/canisters.

[Ira]

Smaller holes = better velocity = less deadspots on the opposite end of the tank. You may want to start ay smaller holes than 6mm, depending on what your velocity requirements are for this spray bar (eg do you have extra powerheads in the tank?)

Smaller holes shouldn't put less total volume out, unless you go so small that you are introducing flow restrictions. If your pump is putting out a constant flow rate and pressure, smaller hole (ie less cross sectional area) will just result in greater velocity. Flow = Velocity x Area. That is until the point where you excessively drop the discharge pressure of the spray bar, due to the venturi effect, because the holes are introducing choked flow due to your velocity being sonic. It will be hard to achieve this with the flow rates we experience from return pumps/canisters.

You don't get a higher velocity without higher pressure, the smaller holes give a higher velocity by restricting the flow increasing the pressure. Pumps will not provide the same volume at a higher pressure resulting in a decreased volume.

If you want some simplified equations:

Flow=Velocity X Area

Velocity= Sqrt(Pressure)

So, Flow=sqrt(pressure)*Area

Area has no influence on velocity at a given pressure. But in the example of a pumped water system decreasing the outlet area results in an increase in pressure but the increased backpressure results in a decrease in flow from the pump. You can look at the back the box for almost any pump and find the curve of flow vs pressure.

[kinnadian]

You're really overstating the effect of increased backpressure in the spray bar itself, and overstating how much that backpressure will affect the flow rate of the pump. It might be up to 5% increased pressure for 3-5% overall decreased flow rate. And that's all beside the fact that ANY spraybar will induce backpressure on the pump and reduce flow rate, a single return outlet pipe will create the most flow.

In a high pressure system I would tend to agree, but when the differential pressure over the outlet holes are so small and the pressure from the pump is relatively low the overall reduction is minor.

If he is that pedantic about 3-5% extra flow rate, he should try to reduce all unnecessary bends and corners, try to have as minimal head pressure as possible (eg mount canister just below spray bar) and use an appropriate sized pipe diameter.

Number and spacing of holes will also optimise flow, if you want to get pedantic, as well as changing hole size as it gets further away from the return pipe connection and also spray bar diameter.

The total area of all holes combined (eg [Number of Holes] x 3.14/4*[Hole Diameter]) should be equal to the area of the return pipe of your pump (3.14/4*[Pipe Diameter]).

If he is using standard 16mm ID return piping, and has say 30 holes, then he should have the holes about 3mm (2.92 to be exact). 50 holes would be 2.26mm. 6mm holes are very big.

Regardless, redracer I recommend starting small at 3mm and spacing them out, see what velocity and flow you get (flow it into a bucket at the same height up as your tank will be), then if you want to increase, then increase the hole size.

[redracer77]

I went with a mix of 5 and 6mm holes, increased size further from the source, 17 holes spraying across/angle, 4 hole spraying down, 25mm pipe.

Tank is 1500l x 700w.

Seems good, I appear to only have 1 dead spot and have pointed a powerhead at it.