Gattling-Style Pitot Tube Bench - My new design.. thoughts?

[larrycavan]

Yes, exhaust sealing is where the problems show up in a hurry. Doesn't mean it's not solveable though, just more complicated...

[Ken73]

Well, after a LOT of thought - I think I'm going to ditch the rotating disc idea in favor of three knife-gate valves. However, to put a twist on it, rather than have them manually actuated individually, I'm going to have a crank-type mechanism that will open and close all of them so that only one of them can be open at a time. I'll have two slider controls on the control panel; one for intake/exhaust flow (with automatic manometer switching also) and one for the flow range (1.5", 2", or 3" tubes.)

I'll also be able to do automatic manometer switching with this setup so I don't have to plug/unplug any tubing.

[Tony]

I rather like the idea of fixed tubes and stoppers too, it solves a whole lot of potential problems, and it is simple.

Or instead of just plain push in rubber stoppers, how about a hinged self aligning flap mounted onto a shaft, rather like an internal turbo wastegate flapper? It could be arranged with an over centre spring with a shaft and lever extending outside the cabinet.

The spring would then hold the flap either hard one way (shut) or hard the other way (open), rather like a toggle light switch. Changing flow ranges could then be done in about half a second. And from the position of the levers you will know what range you are in at a glance.

[Ken73]

I'm not so sure I like the stopper or flapper concept; it seems like they'd seal great in one direction, but pop right out in the other direction. The waste gate valves eliminate this entirely. I *could* do the flappers or stoppers with the same bell-crank type mechanism I'm looking at doing but it wouldn't be quite as efficient.

I'm trying to draw up the design so I can scan it in for everyone to see.

[larrycavan]

[trader]

Larry, I think you already understand this, so I'll give my take on the issue.

The commercial avering pitot's have a minimum and maximum (CFM) operating range for the different pipe sizes. (like orifices) They also have a operational range in differential pressure (D/P in. W.C.). With the orifice, you are working in a D/P % range of 100%, for multiple orfices.

It is basically the same with the pitot (in my mind).

The min and max cfm flow ranges for the pipes(commercial averaging pitot) are 1.5" 10 to 300 cfm. The 2" pipe is 20 to 500 cfm. The 3' pipe is 40 to 800 cfm. In a 1.5" pipe, the operating range is 0.38 to 115 (D/P in. W.C.) and in a 2" pipe it is 0.75 to 75 (D/P in. W.C.) and in a 3" pipe it is 0.39 to 35 (D/P in. W.C.) to match the cfm numbers listed. These numbers are rounded out and air @ 60f & 14.7 psia.

The obvious concern, is, that if you are using a water (incline) manometer. (115"wc, 75"wc, 35"wc) Even with some commercial electronic (D/P) manometers that have that high a range, they will have less resolution to the right of the decimal point. Look at the differential pressure you have to generate to cfm range.

I feel the 3" pipe has a more realistic D/P and cfm range for a single pipe system, with electronics. What did he say? KISS.....keep it smart & simple.

This may not be entirely right, but just how I see it...

trader

[Ken73]

And to think my original flowbench was a single 3" tube. LOL.

What I did experience with that flowbench though, was a lack of resolution with lower valve lifts. Of course, I was only flowing about 2" of water at that point. The blowers I had were not suitable for a flowbench by any means, but at higher valve lifts I could actually get a little flow and a decent reading. It was somewhat accurate in the range I needed for intakes - 150-300 cfm. (I had to interpolate up to 28" H2O obviously.)

There are some items I want to be able to flow that are small - i.e. TBI IAC passages. Some that are big; Quadrajet carburetors, air cleaners. And of course, some that are right in the middle; cylinder heads, intakes, etc.

I do definitely agree Larry - doing this with gates will allow me to build it faster. That was the prime concern; after thinking about it, I figured it would take me a while to get the rotating disc working. Granted, the bell-crank assembly I want to do isn't exactly simple either, but I can manually operate the valves too, if I can't get it built.

[larrycavan]

[Ken73]

Okay, here's the latest drawing. I didn't realize I could use Visio so easily to draw this up, so I whipped this up in a few hours this morning;



You can see the bell-crank assembly in the top, and the three tubes. Mind you, this isn't 100% perfect but it's as close to scale as I could get it in Visio. The gray dashed line on the right side in the middle is a passage that allows me to pass the control wires to/from the control panel. It's on the exposed motor side (i.e. not the side that the motors feed into the pitot tubes.)

The view on the bottom left is a front view; the intake/exhaust selector disc can be seen. The motor box is pretty obvious; I didn't draw the flapper valve assembly as I didn't think it was necessary to illustrate it (but there will be one installed, on the intake side.) The intake/exhaust selector disc will be moved via a simple cable system; it only needs to travel about a foot, or 90 degrees to change flow direction.

The motors will feed the 6"x6" square tube on the left side, which goes up to a 6"x6" plenum that feeds all three tubes. Just below the 90 degree bends at the top are the gate valves. The other 6"x6" tube on the right is exposed to open air at the top. I'm hoping that despite the amount of bends, the larger 6" square tube will help with flow loss from the motor compartment.

As for the size, it's a 24"x28" footprint for a reason - that's the space I have in my shop for it. LOL. I have a 9'+ ceiling in the shop so 87" of height is no big deal.

I also realize the two smaller tubes need to be necked down at a shorter length; I'm opening that one up for discussion. Bruce had original guided me with my first bench, and had me run it 6xD up and downstream. Now I'm hearing 10xD up, 5xD down? The problem with that is, this is a bi-directional bench. I'm going to make some bi-directional averaging pitot tubes for this one, unlike what I previously had; two averaging pitot tubes facing away from each other, spread about 1/2" apart.

Oh, and for now it will run with manometers - there will be some gadgetry (which I've already figured out but it's a pain to draw) to switch manometers automatically whether it be intake vs. exhaust or any of the three tubes. There will only be two slide controls on the front panel; one for intake/exhaust, and one for the range selection. There will also be 7 switches for motors and a single router control to run the primary motor. Eventually I'm going to build my own flow computer that will run the motors as well as show CFM and depression (and connect to a computer.)

[bruce]

[Tony]

Ken, one idea that has just occured to me instead of a crank could you use some windscreen wiper motors to operate the gate valves ? The park wiper contact will automaticaly give you a "home" position, and all it would need would be an additional external microswitch to stop the motor in the "valve open" position.

If you plan to build your own flow bench computer and write software for it, an electricaly driven range changing system would not be too difficult to incorporate.

The standard crank on a wiper motor would have just about the right stroke for a gate valve, and mechanicaly it would be dead simple.

[Ken73]

Actually the crank will be operated either by a knob on the control panel (with three positions) or a slider along the front of the control panel. The intake/exhaust will most likely be actuated with a slider. I didn't see any need to automate the range control; I figured I'd know what range to select.

The computer solution unfortunately is a ways off. I'm going to make one from scratch from a BasicX24 microcontroller (I have a couple of them laying around) to run the motors and display the depression and CFM. I realize there are units out there that do this already, but I'd like to have a little fun and do it myself.