Venturi injector suction not adequate

[kunalvanjare]

Hey guys, we've been tasked with providing solutions for reducing coolant consumption by our client using automation. One of those is a system to reclaim the coolant dripped off of the chips from a Chip Trolley in a CNC Machine. Around 15-20 Litres of coolant+water emulsion is wasted everyday, even more on certain machines. And this data is for one machine. Considering the total number of machines in our client's Machine Shop, we believe this would go some way in solving coolant consumption issues for them.

So basically we have designed a Venturi Injector. This was designed by taking some inspiration from existing Venturi designs from the internet. I am in no way an expert on Fluid Mechanics or CFD. The motive liquid in the injector is the Coolant pumped by the Coolant Pump (The unit is connected to the discharge of the pump by a Tee). In the suction port, we have connected a 6mm OD PU Tube and at the other end of the tube is a Suction Block with filters. This block is to be placed on the Chip Trolley. Please see attached file for the drawings.

So in this lockdown, since it is difficult to visit clients, we took trials with our own small pump used for transferring water in our office. We connected the flexible hose from the discharge of the pump into the inlet of the Venturi with a Hose Nipple & Clamp. Our small pump is a 0.5 HP domestic pump that discharges approx. 35 LPM. We placed the Suction Cup into a box with some amount of water.

What we saw was the venturi worked to some extent, as in it sucked the water through the cup, but it would stop sucking liquid after a point. And this happened in multiple trials. We even changed the 6mm OD tube into a 8mm OD tube, but the result was the same. We need the system to reclaim all the water (or coolant). Also we noticed that the outlet of water from the venturi would fluctuate i.e the flow-rate of water kept varying as the suction was taking place. Maybe the flow-rate from the pump is not constant, I need to verify that.

Yes I know I would probably need to take more trials with a bigger pump, but I was wondering if you guys could let me know if I am going wrong anywhere and if there were some immediate changes I can make to achieve this goal. I have filmed one of the trials which can be found on this link - https://photos.app.goo.gl/LLWaS6m3QR4aEqPp6
Maybe that might help you'll get a better idea of what I am doing.

Any advice would be great. Thanks.

 

[Lnewqban]

The lower you can locate that Ventury respect to the level of coolant to be removed, the better it will work.

 

[jrmichler]

The best solution is to buy an eductor designed for your particular application. Then you will know exactly what the performance will be. I have had good experience with Fox Valve eductors in a similar application: https://www.foxvalve.com/liquid-eductors/introduction/.

If you really want to build your own, take a close look at the cross section in the link above.

 

[Joe591]

I've had a look at your video and your venturi pump seems to work pretty well. The water that's left at the bottom of the bucket is miniscule compared to what has been sucked up. At that point your venturi starts to suck up air instead of the remaining water, which is what you can expect at that point. I don't think your goal of sucking up that last little bit of fluid is realistic.

Also, how much pressure is at your pumps output? if the pump output pressure it too high then you won't really achieve a proper vacuum at the venturi throat. For your dimensions of 20mm at inlet and 9mm at throat you only exchange about 40 kPa of pressure for increased velocity at the throat and that's from bare basic bernouilli calculations (calculations that are so basic they are probably wrong). If the pressure at the venturi inlet is too high you simply won't achieve vacuum at the throat. Whatever pump you select will have to match the venturi otherwise you will not have good results.

It would be interesting to see how you came up with your dimensions. Care to post your method?

 

[kunalvanjare]

I've had a look at your video and your venturi pump seems to work pretty well. The water that's left at the bottom of the bucket is miniscule compared to what has been sucked up. At that point your venturi starts to suck up air instead of the remaining water, which is what you can expect at that point. I don't think your goal of sucking up that last little bit of fluid is realistic.

Also, how much pressure is at your pumps output? if the pump output pressure it too high then you won't really achieve a proper vacuum at the venturi throat. For your dimensions of 20mm at inlet and 9mm at throat you only exchange about 40 kPa of pressure for increased velocity at the throat and that's from bare basic bernouilli calculations (calculations that are so basic they are probably wrong). If the pressure at the venturi inlet is too high you simply won't achieve vacuum at the throat. Whatever pump you select will have to match the venturi otherwise you will not have good results.

It would be interesting to see how you came up with your dimensions. Care to post your method?

The trials which I have taken here are not quite representative of the actual application of the product. The 'vacuum cup' will be placed on a Bin with a lot of swarf/chips out of which the stuck coolant keeps dripping onto the base of the bin. The cup will not be immersed like I had done during my trials. There usually is just a thin layer of coolant. So I need the suction to be much stronger than what I had in the trial.

I don't know the pressure at the pumps output. I have attached the performance chart as per the catalog. It is a small 0.5 HP domestic pump so I am assuming it is not meant for high pressure applications. I will be trying out my unit on actual Coolant pumps in a CNC, but i need to be satisfied with the Venturi during my own trials before I take it to a machine shop.

Finally, I used the cross-sectional view of a venturi I found online during my research as guide. I know it was a shot in the dark, but I never expected this to work even this much. So I guess there's only room for improvement.

 

[kunalvanjare]

Also, I have now realized the diameter of the right side of the nozzle must be higher than the end point of the left nozzle (part where the liquid exits). Attached image FYR. Also, I guess the suction diameter needs to be higher? Any advice on this and anything else will be appreciated. Thanks.

 

[Dullard]

I think that your basic problem is your definition of 'strong suction.' At very low flow (suction port), your eductor will operate at a low pressure. Once you start sucking air, the pressure increases. The only way to accomplish what (I think) you want to do is to severely restrict the suction cross-section at the interface with the water (where air also enters). This will allow you to suck air with sufficiently high velocity and sufficiently low volume to also carry water to the eductor. You are essentially trading volume capacity for pressure - a 'reverse nozzle' effect.

 

[Tom.G]

How about making your coolant collection/drip pan funnel shaped, or at least sloped, and put the suction tube at the low spot?

That approach works well for the millions of cars on the road where the oil pan has a well/cistern/sump that the oil pump draws from.

Cheers,
Tom

 

[kunalvanjare]

I think that your basic problem is your definition of 'strong suction.' At very low flow (suction port), your eductor will operate at a low pressure. Once you start sucking air, the pressure increases. The only way to accomplish what (I think) you want to do is to severely restrict the suction cross-section at the interface with the water (where air also enters). This will allow you to suck air with sufficiently high velocity and sufficiently low volume to also carry water to the eductor. You are essentially trading volume capacity for pressure - a 'reverse nozzle' effect.

If I have understood you correctly, you are talking about the suction cup which I plan to leave on the Chips trolley. I have only made 25mm(width)*2mm (depth) grooves on the cup for the suction (see the attachment). This had occurred to us while designing that if the groove size would be too big, the unit would suck in a lot of air. The 2mm depth(or height) of the groove was decided based upon inputs of the actual liquid level of coolant in the bin, received from the client.

 

[kunalvanjare]

How about making your coolant collection/drip pan funnel shaped, or at least sloped, and put the suction tube at the low spot?

That approach works well for the millions of cars on the road where the oil pan has a well/cistern/sump that the oil pump draws from.

Cheers,
Tom

Hi Tom, this would have been ideal but the problem is that the Coolant (or chip) collection bins are supplied by the CNC Machine Tool manufacturer and aren't in our scope.

 

[Tom.G]

Hi Tom, this would have been ideal but the problem is that the Coolant (or chip) collection bins are supplied by the CNC Machine Tool manufacturer and aren't in our scope.

OK. To be a bit OCD about this, affix a sloped false bottom in the bins (maybe sloped to one corner).

 

[Joe591]

I think your pump is operating of off its intended curve and to the right of it, which might explain some of the erratic behaviour. That means the current pump is oversized and producing more flow than it was intended to.

 

[Dullard]

It is convenient to think of an eductor's motive flow as 'divorced' from the 'suction' flow, but it's also wrong; for low motive pressures, it's extremely wrong. For a liquid-motive eductor, the relationship between motive pressure and motive flow may be very accurately predicted by treating the converging section of the eductor as an orifice, where the downstream pressure is the 'suction' pressure. As you increase the suction pressure (also orifice downstream pressure), the motive flow is reduced (and the motive pressure increases [pump-dependent]). If you're seeing eductor outlet flow variation, this is probably the reason.

 

[kunalvanjare]

Hi guys, my second design (based on a Fox Valve venturi) is almost ready for trials. I will take my trials and comment here soon.

Meanwhile I was wondering if you'll could shed some light on the Vacuum Cup. This cup shall stick to the bottom of the Swarf Bin. It has slots on its bottom for allowing the liquid to enter. I am attaching the design of the Cup in this post.

Would be great if you'll could comment on this. I'm spending so much time on developing a venturi. I'd hate it if the problem actually was in the Vacuum Cup.

As described, the function of the Cup is to attach itself to the bottom of the Bin. Due to the vacuum created by the venturi, the liquid from the Bin is sucked through this cup. Let me know if you'll need any more info. Thanks :)

 

[Lnewqban]

...
Would be great if you'll could comment on this. I'm spending so much time on developing a venturi. I'd hate it if the problem actually was in the Vacuum Cup.

As described, the function of the Cup is to attach itself to the bottom of the Bin. Due to the vacuum created by the venturi, the liquid from the Bin is sucked through this cup. Let me know if you'll need any more info. Thanks :)

Why do you need that 16° cone?
It seems a complication to machining.
What prevents the part from lifting one side some, either by effect of the hose or the metal debris or both?
I would make it as wide and low as possible.
Any chance to add some magnetic capability, like additional magnet-clamp?

 

[kunalvanjare]

Why do you need that 16° cone?
It seems a complication to machining.
What prevents the part from lifting one side some, either by effect of the hose or the metal debris or both?
I would make it as wide and low as possible.
Any chance to add some magnetic capability, like additional magnet-clamp?

Yes making it wide and low like any vacuum suction cups is something I was thinking about as well. I don't know how I can add a magnetic clamp on this. Ideally it should stick to the surface due to the low-pressure (partial vacuum) created inside due to the venturi. But with those 25x2mm slots, will a proper vacuum ever be created?

The 16 degree cone was simply to provide a gradual taper or more surface area for the liquid.

 

[Dullard]

The cup should work fine until the (reduced) depth of the liquid allows air into the slots. At that point (unless your eductor is very large), gas will 'swamp' your eductor. If you're relying on vacuum to keep the cup in place, you may have a problem at this point.

 

[jrmichler]

Here's a suggestion. Take a piece of steel heavy enough to stay in place under its own weight, say about 1" or 2" thick by 4" or 5" square or round. Drill and tap for 3 or 4 screws to be used for setting the gap. Drill and tap a center hole for the hose. Add a chamfer to the bottom of the center hole.

Adjust the screws to get the gap that works best.

This is a case where it's quicker and easier to build something and try it than to try to calculate the best possible solution. I mention steel. You could just as well use aluminum, plastic, or even a block of wood. It would just need to be larger in order to be heavy enough to stay in place.

 

[kunalvanjare]

Here's a suggestion. Take a piece of steel heavy enough to stay in place under its own weight, say about 1" or 2" thick by 4" or 5" square or round. Drill and tap for 3 or 4 screws to be used for setting the gap. Drill and tap a center hole for the hose. Add a chamfer to the bottom of the center hole.

Adjust the screws to get the gap that works best.

This is a case where it's quicker and easier to build something and try it than to try to calculate the best possible solution. I mention steel. You could just as well use aluminum, plastic, or even a block of wood. It would just need to be larger in order to be heavy enough to stay in place.View attachment 267280

Thanks for this. One query... It appears as if the block would be resting on the screws extending from the bottom. Assuming the height of the block is approx. 28mm & I use 1.25" long screws (31.75mm), the screws would be extending past the block by 3.75mm. Will the venturi produce enough suction to suck the liquid through that gap? And here, the entire block would be resting on something similar to dowel(locating) pins right?

 

[jrmichler]

Those screws are adjusted to get the gap that works the best. They should be at least 1.5" or 2" long for a 1" thick block. The block rests on the tips of the screws. You can set the gap to less than 0.1 mm or more than 1 mm. Experiment to find what works best.