Vacuum pump/blower conversion needed

[Al_The_Man]

I have used these regenerative blowers in the past, they make exellent sources for vacuum tables. http://cgi.ebay.com/ws/eBayISAPI.dll...866984481&rd=1
Until I noticed it was rated at 400hz!
The question I have, I usually see them rated as inches of mercury for vacuum use, this one only shows pressure in h2o. I have searched for some kind of correllation between these ratings with no luck, anybody have an idea of the relationship?
It is too bad this one is 400hz, who uses this apart from the aircraft industry?
Al

[sol]

http://xtronics.com/reference/convert.htm

If I recall correctly appx. 27 3/4 inches water column = 1 psi

Don't ya just love this non metric measure?

[unterhaus]

The vacuum conversion would be by the ratio of the densities. If you have inches of water and want inches of mercury, divide by the density of mercury and multiply by the density of water.

The motor may just be rated to 400hz for use with a vfd.

[sol]

And that ratio is 13.61 for Water - Mercury. (Just mercury's specific gravity.)
Anyway 80 inches of water is less than a decent shop vac draws...they run around 100 inches.

[Al_The_Man]

Originally Posted by sol Anyway 80 inches of water is less than a decent shop vac draws...they run around 100 inches.

I would assume the difference is in the capacity, as at 5hp regenerative, this should be more efficient and much higher capacity?
Al

[fyffe555]

1 atmosphere = 406.8 in water = 29.92 in mercury = 14.7 psi = 1.013 Bar = 1.033 kgf/cm2...

[sol]

Originally Posted by Al_The_Man I would assume the difference is in the capacity, as at 5hp regenerative, this should be more efficient and much higher capacity? Al

I'd think so ....
Hmm... I do not see anything in the Ebay ad that tells the CFM or other volume/time value. The thing sure looks like it ought to move lots of air, but how much?

For comparison and wild guesses...
A good shop vac moves over 100 Cubic Feet a Minute of air at a pressure/vacuum of 100 inches of water.
Shop dust collectors, those blower with a bag jobs (1 to 2 hp) move from 600 to 1800 CFM at a vac of only 5 to 12 in. of water.
It would seem that 5 hp at 80 inches of water should be moving a major volume of air doesn't it?
Sorry I can't be more helpful.

[Al_The_Man]

I looked at the specs on the 5hp Gast units I had been using and they specify max vacuum at 88 h2o with a free air flow of 215cfm, I checked their web site and they now have a really good handbook on compressors and vacuum selection etc http://www.gastmfg.com/pdf/vacpresshdbk.pdf
The other thing I have never been able to get a definitive answer for ( although I have a theory) is to why a impeller style pump, air or liquid run at maximum load with both inlet and outlet totally unrestricted, if you throttle either the intake or discharge the load decreases proportionately, as when you block the end of your house vacuum the load goes right off the motor and because its a universal motor the rpm goes way up, with and induction motor the rpm goes up to rated rpm and the current goes to minimum.
Al

[sbrpollock]

Al:

The phenomenon you're talking about is called "Cavitation". When you throttle the inlet of a Centrifugal pump, the impeller slings "out" more material (Water or Air) than it can pull in. This creates low pressure areas inside the pump. If it is severe enough, (like if the inlet is blocked off completely) the majority of the impeller will be roating in a vacume or close to it. This is what unloads the motor and causes the increase in RPM. Cavitation is very bad for both processes and equipment. I've seen it cause severe erosion and pitting inside of cast iron pumps. It also causes hammering in outlet piping and leads to failures of pipe, valves, and other equipment.

VERY IMPORTANT:
Throttling the outlet side of pumps by partly closing the outlet can be dangerous and should NOT be done!

In many situations it is common to have a check valve somewhere near the inlet of a pump to keep it from loosing its prime when it is shut down. For example, when a pump is pulling water out of a pit, a check valve in the pit keeps the water from running backwards into the pit when the pump is shut off. This is called a foot falve. Now, Picture this: If you have a pump running in this situation and you close the outlet of the pump completely, you create a closed vessel. The water can't get out through the outlet because it is closed, and it can't get out through the inlet because the check valve blocks the reverse flow. In this situation the impeller just churns the water and it has nowhere to go. This causes the water to heat up, and if the pump is powerfull enough, it will boil the water and turn it to steam! This steam pressure, since it has nowhere to escape can build up to high enough levels to cause the pump to explode! I had some 60 HorsePower water pumps that were equipped with pressure reliefs on them just like boilers have. I accidentally found out once that these particular pumps were capable of heating the water up to the boiling point and making enough steam pressure to pop those relief valves in about two or three minutes of operation with the outlet blocked.

[Al_The_Man]

I found this phenomenon out when I installed a large pump as a washer and it kept nuisance blowing the overloads, the manufacturer of the pump suggested throttling the output until the current came down. The problem they said was that the outlet was virtually wide open. I had no problem after that.
Al

[trubleshtr]

I found this free download handy, has almost every conversion in a nice little lay out, might help you out....

http://www.joshmadison.com/software/convert/