Balancing suction air flow at different areas on pipe

I have a T-shaped pipe that is connected to a vacuum pump. Along one side, there are 4 areas of 9 holes where air will get sucked in from the surroundings. It's used to pick up some small objects.

The problem is the 2 areas of holes that are closer to the center have a much higher intake flow/pressure compared to the 2 further areas, that it renders those 2 less-/ineffective.

Hole sizes and the general positions of the hole areas are fixed, so I'm thinking of balancing it by adjusting the number of holes at each area instead. (edit: Currently can't use a flow regulator in this application, any other suggestions instead of changing number of holes are welcome as well)

I am not trying to achieve the exact same suction pressure for each area, but rather to have sufficient suction pressure at each area. Will changing the number of holes in each area be able to balance the suction pressure? My guess was reducing the number of holes for the 2 middle areas or increase for the 2 end areas, but I'm not sure by how many. If you could provide a general idea/example on how to calculate this, or an alternative method to accomplish this, that would be great.

• Does this answer your question? How to divide the water flow into outlets with equal flow each?. The question is about liquid, but its as applicable to gases.
– NMech
Commented Jul 14, 2021 at 9:36
• @NMech The answers given don't seem to apply or are already applied. Diameter of all holes are equal; area of main pipe seems large enough; and I can't use a flow regulator in this application. Would the fact that air is being sucked into the holes from the surroundings instead of air flowing out make any difference, or theoretically is it the same? Commented Jul 14, 2021 at 10:01
• Your question doesn't tell us if the object being picked leaks air or not. If it doesn't then there will be no flow when the vacuum cups contact and the pressure will quickly equalise across the whole system. If the object is leaky then you will have a pressure drop along each branch in proportion to the flow. Can you edit your question to clarify? Commented Jul 14, 2021 at 12:59
• I would use tape to cover three (or more) of the holes in the central two groups of nine. That would help balance flow. Commented Jul 14, 2021 at 13:21
• @JimClark Thanks! Was thinking of trying that. Just wondered if there was a more analytical way to determine the optimal number of holes rather than trial and error. Commented Jul 15, 2021 at 0:30

The two critical variables for flow rate will be resistance and diameter/area of flow.

The holes in the outer zones could be over drilled to increase their diameters.

If that is not an option then reducing the number of holes in the central zone and increasing the number of holes in the out zone would help.

Alternatively, as others have stated, you could cover some holes in the central zone with tape to block them. As an alternative to this, the resistance could be increased and hole diameters reduced by initially covering all the holes in the central zone with very sturdy tape which has good adhesive properties. After this, using a smaller diameter pin or spike, smaller diameter holes can be placed in the tape where the existing hole are.

If this is successful an adapter could be made for fit over the existing holes which has the holes with the diameters and number of holes required. The adapter could be screwed or clamped to the suction manifold.

I would suggest separate channels to each set of holes with orifice plates to limit the flow to / from each.

That way you can control each set individually.

The other solution will be to over-specify the flow rate so that the poorest one is sufficient...

It will be a next to impossible task to try and analytically predict the number/position/size of the holes in order to have exactly the same flow. Even if you do for a temperature and a set humidity, at different temperatures you might have unacceptable results.

If you are really interested in equalising the flows, a better alternative is to use a flow regulator, which will ensure a standard flow rate. You can set each one, before each of the four openings. Its a robust and cost effective solution compared to the alternatives.

If you don't care too much about equality of the flow, and drilling new holes/different size is not an option, Jim Clark's comment is imho your best bet. I.e. you could try covering with tape some of the holes, and see by trial and error which combination provides you with adequate flow.

• Currently I can't use a flow regulator in this application. I don't need it to be exactly the same flow, but the 2 hole areas closer to the center have a significantly higher suction pressure than the outer 2 that the outer 2 are less effective. Commented Jul 14, 2021 at 10:04
• Sorry if I wasn't clear, the main problem I have isn't to achieve the exact same pressure, but rather for all 4 areas to have sufficient suction flow/pressure. Commented Jul 14, 2021 at 10:15
• Α pressure regulator and a flow regulator are two different things.
– NMech
Commented Jul 14, 2021 at 11:13
• Just read your edit, thanks for answering! Was just wondering if there was a more analytical way to determine the optimal number of holes rather than through trial and error. Commented Jul 15, 2021 at 0:33
• There are semi-empirical ways where you can calculate the pressure drop along the channel and through the opening (however they are for much simpler geometries) and then use an equation to estimate the flow. An expert could also use CFD (Computational fluid dynamics) to predict the flow in the system (however it requires a fair amount of computational power and a lot of experience to get something more than pretty pictures).
– NMech
Commented Jul 15, 2021 at 3:41

you could add a diverter fin like a n inverse V inside to give favore to the side openins and deny direct suction from center openings. as per my sketch.

• Thanks for the suggestion! Will try to experiment with this design as well. Commented Jul 15, 2021 at 0:25