Factors affecting the efficency of water aspirators

[Hunterc]

Is the maximum attainable vacuum from a water aspirator directly proportional to the pressure being forced through the tube of the aspirator?

Are there any factors other than water temperature that will affect the vacuum performance of an aspirator?

My idea goes like this, I've got an aspiratior like the one in the picture I posted, it's got a 5/8" inlet that I plan on connecting to a quick disconnect fitting so I am able to connect the aspirator directly to my electric pressure washer to increase the pressure of the water being used to create a vacuum.

 

[Dullard]

I believe that your 'aspirator' is of the 'eductor' type. They work this way:
Motive water is fed to a converging nozzle. This increases velocity and reduces pressure (See Bernoulli). There is a short 'open' section at the exit of the converging nozzle where the 'suction port' connects - this is the 'vacuum.' The motive water (and anything 'vacuumed up') then enter a diverging nozzle - pressure increases / velocity decreases (Bernoulli, again).

The answer to your question is 'No.' The vacuum is not directly proportional to the motive flow. However:
Increased motive flow will (to a point) increase the volume capacity of the eductor. The pressure at some fixed flow will be lower with higher motive flow. The exact behavior of the eductor depends on the design - the starting diameter of the diverging nozzle has a significant influence on the capacity of the device. Beyond a certain motive flow, 'clean' entry into the diverging nozzle becomes the constraint.

With no suction flow, the 'vacuum' will (theoretically, depending on how optimized) be very close to the vapor pressure of the water for any motive flow in the design range of the device.

Don't connect this to a pressure washer. OR: If you must, use eye protection and remove pets/children from the area.

 

[berkeman]

I plan on connecting to a quick disconnect fitting so I am able to connect the aspirator directly to my electric pressure washer to increase the pressure of the water being used to create a vacuum.

As mentioned by @Dullard it is probably not a good idea to try to get a stronger vacuum by using a pressure washer. The Wikipedia articles says that you can increase the vacuum by connecting several steam stages in series -- have you thought of trying that? Have you looked at using steam instead of water?

https://en.wikipedia.org/wiki/Aspirator_(pump)

In order to avoid using too much steam, a single steam-ejector stage is generally not used to generate vacuum below approximately 10 kPa (75 mmHg).[1] To generate higher vacuum, multiple stages are used; in a two-stage steam ejector, for example, the second stage provides vacuum for the waste steam output by the first stage. Condensers may be used between stages to reduce the load on the later stages. Steam ejectors with two, three, four, five and six stages may be used to produce vacuums down to 2.5 kPa, 300 Pa, 40 Pa, 4 Pa, and 0.4 Pa, respectively.[1]

 

[berkeman]

Also, have you looked at just getting a traditional vacuum pump? What is the application?

 

[Hunterc]

As mentioned by @Dullard it is probably not a good idea to try to get a stronger vacuum by using a pressure washer. The Wikipedia articles says that you can increase the vacuum by connecting several steam stages in series -- have you thought of trying that? Have you looked at using steam instead of water?

https://en.wikipedia.org/wiki/Aspirator_(pump)

Yes Dullard sufficiently scared me when he said keep all children and pets out of the area
I saw the steam aspirator designs but the application is going to be used mostly for evaporating and reclaiming solvents some of which will be flammable so I'm scared that the heat from the steam could be a source of ignition.
I can't use my rotary vane pump as a source of vacuum when I'm evaporating solvents because the flow rate would be too high and would suck out the solvent vapor before it could condense which would be bad for the pump to say the least. So I opted to hook up my jet pump a 300 gallon water tank and hooked up my aspirator and it works good for the application but I wish the vacuum achieved was as low as my rotary vane.
I'm going to start reading about the multiple stage steam aspirators.

 

[berkeman]

the application is going to be used mostly for evaporating and reclaiming solvents some of which will be flammable

The Wikipedia link also mentions that you have to be careful about using aspirators to generate a vacuum used for some chemicals -- disposal of the water can raise EPA issues...

In the past, water aspirators were common for low-strength vacuums in chemistry benchwork. However, they are water-intensive, and depending on what the vacuum is being used for (i.e. solvent removal), they can violate environmental protection laws such as the RCRA by mixing potentially hazardous chemicals into the water stream, then flushing them down a drain that often leads directly to the municipal sewer.

 

[Hunterc]

The Wikipedia link also mentions that you have to be careful about using aspirators to generate a vacuum used for some chemicals -- disposal of the water can raise EPA issues...

Yes and to minimize pollution I recirculate the same water in the tank and use a cold trap.
I also run the water through a carbon filter after I've used it to evaporate anything other than ethanol.

 

[Dullard]

FYI:

Your eductor is about as small as they get. There are several companies (Fox, Mazzei, others) who have larger units on the shelf - I don't know what your vapor Flow/Pressure requirement is, but it's probably in the range of commercial availability.

If your vane pump is 'too big,' it can be throttled to reduce the flow rate. Maybe I don't understand?

 

[Hunterc]

The motor

FYI:

Your eductor is about as small as they get. There are several companies (Fox, Mazzei, others) who have larger units on the shelf - I don't know what your vapor Flow/Pressure requirement is, but it's probably in the range of commercial availability.

If your vane pump is 'too big,' it can be throttled to reduce the flow rate. Maybe I don't understand?

The motor that drives my vane pump is single phase and I was told that variable frequency drives shouldn't be used on single phase motors like mine.

 

[Dullard]

It's true that you can't run the single-phase motor at low speed. I'm talking about a ball/butterfly valve at the pump inlet. Run the pump at full speed - just restrict the inlet cross-section.

 

[Hunterc]

It's true that you can't run the single-phase motor at low speed. I'm talking about a ball/butterfly valve at the pump inlet. Run the pump at full speed - just restrict the inlet cross-section.