I suspect the answer is “no” - or at least “not for useful purposes” but given fluid dynamics contains some amazing and counter-intuitive results, where better to ask than here?
The use case is in dust control for woodworking. I am the proud owner of a shiny new SawStop PCS cabinet saw. The next step is serious dust control in my workshop. Dust control is a fraught issue, and I have discovered there is a lot of bad information out there (surprise surprise), and sadly getting things right isn’t trivial. For a safe workshop you need both chip clearance - which is easy, and fine dust control - which is not. Most home gamer tools have small (2" or smaller) dust ports designed for use with a shop vacuum cleaner. These have a laser focussed point of dust collection and require relatively high pressure differences and relatively low flow rates. Dust control outside these ports is hard, and you need high volume, relatively low pressure difference flows, typical of the 4" and preferably bigger ports on larger machines. Dust collection systems are typically based upon a materials handling blower which is low pressure high volume. So there will be such a blower - probably of the order of 1000cfm plus.
But this is useless for many ports on tools, including the blade guard dust port on my cabinet saw, not to mention the drop saw and so on. One answer is to have two dust collection blowers, and two sets of pipe runs. That seems a trifle wasteful. So, the question is:
Is there a viable venturi (or similar) design that can take say a 500cfm flow of say 7" pressure and provide a 100cfm flow of maybe 20"? Going the other way seems to work; at the extreme we have steam powered ejectors, and at the domestic level things like the Dyson flow multiplier fan. But going the other way seems hard, and I can’t find anything. Any ideas?
Powering a high speed blower from airflow through a turbine run by the low pressure side is one option, but very complex, and I suspect the efficiency would be poor at best. What would be great is device with no moving parts that can be used as a form of adaptor.
Like you, I recognize eductors that create a large low pressure flow from a small high pressure one. No, I don’t know of any eductor or other device that does the opposite without any moving parts. Obviously it would be possible to create any pressure and flow combination from any other pressure and flow combination with turbines driving fans, as long as the resulting power is less than the original power by enough to handle the inefficiency of the mechanism.
If I wanted to do what you want to, I’d just buy a blower suited to these applications.
Thanks guys, I have continued to mull over this, and look around. I’m still pretty despondant that it is intrinsically impossible to do what I want, but it is valuable to see some of the wider ranges of workable devices. Maybe more later.
In my head I was working out a dual system using a shop vac set-up for the tools and the air multiplier using compressed air to drive it as a catch all for the finer particulate in the air of the shop as long as it’s not too large.
Although on further reflection if you are narrowing the pipe diameter (say 7" to 2 ") you’re basically accomplishing the task, No? Low pressure, high flow rate necked down becomes higher pressure, less flow rate. HVAC ducting in reverse…
I think the problem the OP wants to solve is one of impedance, in the specific form of pressure per flow ratio. Is it intrinsically impossible to convert flow energy at low impedance into flow energy at high impedance? Certainly not, because for example you could build a combination of turbines that would do it. It is obvious that eductors can convert the impedance downward. So far nobody has thought of something as simple that does the conversion in the other direction, and by “simple” I mean with only one physical part that doesn’t move. But the turbine combination could have two parts, one moving. That’s still pretty simple.
Yup, you have my (as OP) notion exactly right.
The issue seems to be that at low pressures the air is in its incompressible flow regime, and it just doesn’t want to play in the same way as it does at high flow rates. But my working knowledge of fluid dynamics stops with memories of being bored rigid studying Navier Stokes equations a few zillion years ago.
Some of the links above provide for some quite interesting reading, but so far that is nothing that is going to beat adding a second vacuum system.
I don’t think that’s it. If your question had been about water, we would have had the same answers, but it’s more incompressible than air in this discussion.
You can convert high flow/low pressure to high pressure by using a nozzle.
You can get low-flow vacumn out of a narrow high-speed (high speed) air jet with a venturi.
That’s in principle. The most pressure you can get that way is air pressure.
Valves like that do exist, (Venturi Vacuum Pumps and A/C vents) and are used to create low flows from high-pressure air, but if they were any good at power transfer, I don’t think turbines would exist.
Since the Venturi Vacuum Pumps are very low flow and high pressure, and A/C system are relatively high flow and low pressure, those two cases straddle the case you’ve described. I take that to mean that your in-between case would be technically possible if anbody really wanted to do that, and had enough power to start with.
(Perhaps such a vacumn transformer does exist, and I just don’t know about it. I’m no expert)
Nozzles don’t increase the pressure. At least, not the total pressure. You can divide pressure into static pressure and velocity pressure, and a nozzle does convert static pressure to velocity pressure, but it doesn’t increase the total.