Test pressure control

[1960flh]

Still in my design phase but came to find these Solid State controllers at a reasonable price in comparison to variacs.
I spoke to the owner Henry Payne a very nice guy and racer himself. his products the TBP-18 (25 amp) and 18D-SW (10 amp will handle 38 peek) are both nice, the -SW will accommodate options for automated control but more costly.






Merry Christmas to you all

Rick

[Tony]

Once an automatic test pressure control system becomes this sophisticated, there are some pretty powerful advantages of using a large single three phase blower run from a commercial variable frequency drive.

* The pressure/flow characteristic of a vacuum cleaner motor changes very steeply, particularly the two stage higher pressure vacuum motors that most people use. As you make a series of increasing flow measurements, the power to the motors needs to be constantly adjusted. The pressure/flow characteristic of a large diameter single stage centrifugal blower will be almost flat. These things develop almost constant pressure over a very wide flow range, so any automatic test pressure control system has much less to do.

* Ac/dc brush type motors are very sensitive to the applied voltage, in fact that is how the speed is varied. But they are also very susceptible to sudden mains supply voltage variations, and the speed will also change substantially with mechanical load. Motor Rpm will change with airflow, and so will pressure. Completely block the flow to any vacuum cleaner, and hear the motor speed up. Motor performance also changes with motor winding temperature, which will vary all over the place, because it is cooled by the airflow, which is changing.... A three phase induction motor driven from a variable frequency drive is by design completely immune to mains voltage variation. But more importantly the SPEED is controlled by the supply FREQUENCY, the motor voltage within reason is far less critical. A three phase motor hardy changes in running speed from zero load to flat out maximum horsepower, it sure will draw a lot more current, but the running speed remains virtually constant. So a three phase motor and VFD system will have a rock solid stable operating Rpm, whereas a vacuum motor will be wandering up and down in Rpm all over the place with changes in internal temperature, flow, and mains voltage.

* Closed loop test pressure control is especially easy with a VFD, because the better ones already have this feature inbuilt. It only requires a manifold absolute pressure sensor from an EFI vehicle, (or even better, a commercial digital manometer), and a control knob to set the test pressure. With anything else, you get to develop your own closed loop feedback system.

For a basic flow bench, vacuum motors are very simple and cost effective, with some sort of manual flow control.

Once you start thinking in terms of high resolution data acquisition, and automatic control of test pressure, and other automated measurement techniques, the old vacuum cleaner motors are less than an ideal source of bench airflow. They are just not going to be stable enough to get the sort of repeatable numbers that the rest of the system may be capable of.

While a high quality variable frequency drive will not come cheap, the three phase motors cost almost nothing. The VFD will have its own internal closed loop speed control system, and quite likely a serial computer communications interface, where you can program the required test pressure into it. It all comes in one attractive box ready to go, and the final results will be far better.

The vast reduction in noise level of a large diameter three phase blower is an added bonus, even flat out it will be as quiet as a window airconditioner, but at more normal operating flows and pressures it will be almost silent. No possibility of burning out the three phase motor or wearing out the bearings either. Vacuum cleaner motors are built to have a rather limited life. This becomes especially true at high test pressures, where the motors will often be running at full flat out power, but with reduced (or even zero) internal cooling airflow. A three phase motor will always have its own independent enclosed fan and cooling system, so it can run flat out continuously with the bench flow completely blocked.

[1960flh]

On this topic, in my design I am planning on a two stage pump configuration of two banks of 6 to 8 motors per bank with back flow valves on the intake side of each secondary motor (primary being the speed controlled motors). I am considering using solid state motor speed control to vary depression. I see no issue when running in a mode where both stages are running only on the primary motors A Supporting B, but as needed when I have to turn on the secondary motors (one or more in each stage running full speed and one in each stage speed) how this will effect the effectiveness and sensitivity of my depression control especially at the transition points of each additional motor need. I hope this makes sense, ??? Any thoughts?

[1960flh]

Oh and A HAPPY Horspower FILLED NEW YEAR TO YOU ALL,

RICK

[Tony]

Try it and see.

Before constructing the final flow bench, just stuff some motors into a rough box of some sort, then build and test your motor control system. It will be a lot easier to work on, and make any required design improvements.

It is amazing how an initial concept can morph into something quite different once you begin doing some testing. Once it is working to perfection, the final flow bench will then be a work of art, not something that has been chopped around and modified several times.

[1960flh]

Thanks Tony, That was where I was headed as I wanted to prove that there, was even a measurable benefit to the second stage. At that my thought was to measure the depression created with a one inch orifice at full speed single stage then measure the depression with the second stage to see what benefit was gained. Is my thinking correct?

Rick

[Tony]

You are definitely on the right track Rick, testing your blower against various sized orifice loads.

Your best bet is to make a series of fairly crude plastic or sheet metal orifice plates, with holes from perhaps half an inch up to two inches in several steps. Nothing fancy or super accurate needed here.

Then with just one blower motor, see how much depression the blower can pull across each different sized orifice hole.

Using the orifice formula, flow = 13.55 times the square root of pressure, multiplied by orifice diameter squared. That will give you values of flow and pressure for each different sized hole. From that, a curve of flow versus pressure can be plotted on a sheet of graph paper for one blower.

The shape of the curve will vary hugely with different style, size and type of blower. Some can maintain almost constant pressure over a very wide flow range. Others develop massive blocked flow pressure, but the pressure falls off extremely rapidly with increasing flow.

At least then you will know what your blowers can do, and then it is possible to figure out how many will be required, and if any significant advantage can be gained by flowing them in series instead of parallel.

This can all be very instructive, and will give you a very good basis for deciding if your intended design can reach the anticipated flow and pressure with the available mains power available. It is ultimately the combined current draw of all those motors that will set the limit on maximum airflow.

[1960flh]

[color=#000000]Thanks Tony,

I did not think of multiple orifice sizes, but I learn quickly. I am using the Surplus Center motors so I should be able to test these motors in both singe and two stage configurations in the next week to ten days. I will post the results when I'm done.

I need to make a simple monometer, I understand the principle, but am confused on the tube diameter and fluid SG needed. If I am doing simple testing and will create a 96

[bruce]

Isn't there some pretty indepth spreadsheets posted here on the forum somewhere with the flow curves of the Surplus Center blowers? I seem to recall something being posted a year or two ago? Guess I need to use my own search function . . . I could be all wrong on this also

Here is the post I was thinking of:



Edited By bruce on 1167691010

[1960flh]

Bruce,

Thanks

I did not think the Surplus Center 16-1234 motor was an Ametek motor, and could not find any data in the sheet relating to them. Any help would be appreciated.

Rick

[Tony]

If an official manufacturers specification and pressure versus flow curve is available, that would be ideal. But I would still quickly confirm that the motors you have are exactly the same ones with a practical test.

The basics of a manometer are very simple. Air pressure causes a difference in fluid level between the two sides. Nothing else really matters, there is water at one level at one end of the tube, and water at a higher level at the other end.

Wells, sloping tubes, or changes in tube diameter do not effect the basic difference in two water levels. If there is six inches of pressure,it will shift as much water as it must to raise the water on one side six inches above the other.

The easiest way to test your blower is with a bucket full of water, and any transparent tube reaching up to the ceiling. The blower suction pulls the water up the tube, and you just measure the difference in height to the water in the bucket. Costs nothing, and is dead accurate. The tube does not even have to be perfectly straight.

[1960flh]

[color=#000000]Will do; I am curios to see the effects of a

[Tony]

Ordinary standard vacuum motors come in single stage and two stage configuration. That is, there can be either one or two rotors connected to the same drive motor internally.

The two stage motors can obviously develop far higher peak blocked flow pressure. That is definitely a desirable feature for a vacuum cleaner. But they also have a far steeper fall off in developed pressure with increasing flow. That is not a very helpful characteristic for a flow bench.

Which would be better ? A single stage blower that develops fifty inches with blocked flow, and full maximum rated CFM at forty inches of pressure.

Or two double stage blowers in series that can develop over two hundred and fifty inches with blocked flow, falling to the same forty inches at full rated CFM?

With multi rotor or series connected blowers, the test pressure is going to change fairly significantly with small changes in flow through what is being tested. With a single stage blower, the test pressure will still change, but only very slightly.

It has to do with the pressure ratio developed across each rotor stage. The pressure ratios don't add, they MULTIPLY.

That may or may not be an issue, but a fairly stable test pressure is always more of a joy to use than something that has to be chased all over the place over a range of progressively stepped flow measurements.

So I cannot get terribly excited about multi rotor blowers, or placing centrifugal blowers in series. If your blower cannot reach the desired pressure, more rotor Rpm, or a larger diameter rotor is a much more useful solution than cascading blowers in series.

Another thing to keep at the back of your mind is the danger of bursting or imploding your bench. Twenty eight inches of pressure is almost exactly 1 psi. It doesn't sound like much, until you realise that every square foot of bench sees 144 Lbs of force. At 112 inches of pressure, the force rises to 576 Lbs per square foot.

I will leave it to you to decide how a three foot by four foot glued and screwed particle board panel might react to several tons of suddenly applied air pressure.

It is remarkably easy to accidentally block the airflow. It happens in the blink of an eye, and can blow the fluid out of your manometers, or do some severe damage. So if your series connected blowers can develop obscenely high zero flow peak pressures, BEWARE...

A relatively flat pressure versus flow blower characteristic curve makes for a much nicer to use and safer flow bench.

Some sort of safety pressure relief system may be worth a thought if you have a very peaky blower pressure versus flow characteristic.

[SWR]

[larrycavan]