Newb with 8/71 based bench needs some help!

[TK]

Long story short - I started building a flow bench abut 10 years ago, but never got around to finishing it as a mate about 200m down the road had a nice SF600 which he was happy for me to use.

But now he's shifted workshop & is too far away.

Soooo, it's about time I finished my bench, but I've got a few Q's.

First up - drive motor requirements: I have a 8V71 blower, I stripped it years ago & checked it over - it's in pretty reasonable condition. I need to know how large a motor I will need to drive it, I need the ability to flow heads up to around 400-450cfm at 28" depression, I have an old 3-phase 5HP motor that needs a rewind - will it be powerfull enough to do what I want it to do? or will I need something like a 7.5HP motor?

Next - reversing the flow, initial thoughts are to simply switch over a couple of phases on the drive motor so it spins backwards & have a filter on the outlet/ inlet at the back of the bench to stop it picking up crap when running in reverse.
Any reason why this won't work?

I do not intend to run a speed control on the motor - gets a bit expensive to do with a relatively big 3-phase motor, instead I am going to use a throttle body of a 5L EFI engine with a fine screw thread to adjust the opening to bleed off pressure. Good/bad idea?

These may seem like fairly simple questions, but I've never built a bench before & have fairly limited time (busy performance engine shop - time building flow bench = unpaid time) so I only want to do stuff once.


Thanks for any help, there will be more questions later, but that'll be enough to get me on my way.

Tony

[TK]

Ok, answered a couple of my own questions - sent the motor to a rewind place for a quote, turns out it's a 7.5hp anyway & plenty good enough, they also tell me that reversing the direction of it will not hurt it at all, so that side of things is sorted.

Next question - roughly how fast should I spin the blower to get 400cfm @ 28" capacity?, motor runs at 1450rpm.

Also - I had intended to just seal the entire cabinet, including the blower & drive motor & have the throttle body on the side bleed air, wasn't going to worry about any seperate compartments etc in there, just having the hole in the top of the bench & oriface etc going straight into the cabinet with nothing else below them - bad idea, or will that work ok?

[blaktopr]

Hey Tony, welcome aboard. If you are to go to an orifice design, you would have to separate the chambers for before and after the plate to get your differential reading. Look for posts on settleing boxes for under the test cylinder also. You would need just two chambers, plate between the two, and a baffle above and below the plate. Use some of this to help you search on the subjects, they are here. The plate under the cylinder/tube has been discussed here and will not give accurate readings due to speeds and the like through the plate.

From what I encountered with a bleed device for depression control. Mine is crude. A slightly radiused hole and a curved PVC cap. So not like a throttle you are thinking about. But, when the air in the intake mode is drawing also through the "bleed" (mine was 3") for depression, the air entering into the wide open insides of the cabinet produces turbulence that makes the manometers a little jumpy. The more it is closed, the calmer it gets. You may want to look into having the throttle body in the center of two horn shaped or funnel shaped exit areas. One in the bench for intake, outside for exhaust. That way the air can enter or exit in a more "controlled" manner. Funny thing, it's the same thing we work on doing valve jobs and chamber designs in heads for this not to happen, but before you get there you have to combat the problem on the bench...but all same "science".

I can't tell you bout the CFM requirements/motor speeds though. But I know there are a few guys here who can.

Chris.

[TK]

Thanks for that, some food for thought.
I was going to run a pod filter on the outside of the throttle body which has a nice bellmouth, it would be easy enough to do the same minus the filter on the inside.
My setup is not a simple oriface type - I've got different size venturi's (50,100,200 & 300cfm - will be needing a bigger one as well) with 4 holes for the manometer lines drilled through to the smallest part of the venturi & all hooked together to give the best average of all 4 holes.
The venturis are interchangeable & fit in the bottom of the 4" pipe coming down from the bench top - I figured the venturi shape with a nice bellmouth off the bottom would be ok, may have to do a dodgy MSpaint picture to explain what I mean - I'll do it tomorow.

For the blower drive ratio I can always just pick some pulleys at random, spin it up to get a baseline & either increase or reduce the ratio as required to get the flow I need.

[TK]

A couple of the venturis:



Venturi roughly fitted to bench:



Blower etc, I was originally just going to run the tube from the venturi straight to the blower entry - but a bit of a think says that'll pulse WAY too much, intention now was to seal the bench instead with 18mm MDF & leave the blower entry open & just fit a baffle between the bottom of the venturi & the blower to smooth things out a little, the large volume of the enclosed bench should be enough to damp out the pulses from the blower:



Overview of the bench so far:

[49-1183904562]

Tony

I do not think the supercharger is going to like being spun backwards especially at the speeds needed. I would reconsider using a good VFD since this is not a load sensitive application you do not need a sensor less vector type and therefore should be able to do for similar or less money than 10 vacuum cleaner motors would cost you.

Rick

[TK]

Dang, I did actually think about any consequences to spinning the blower backwards - I was choosing to believe it'd be ok, thanks for shattering that belief

Oh well, I guess I can figure out some way to plumb it to reverse the flow reasonably easy, I'll post up what I come up with.
I'll have a chat with an electician I know about speed control for the motor - that may be a simpler/neater/better way anyway, depending on the cost of the speed controller.

Seeing as it looks like there are going to be some big changes to the basic design anyway would I be better off going to a simple oriface instead of the venturi's? - or even mounting the venturis in a different spot so there is a settling chamber before them?

Thanks for the help - if I hadn't found this site I probably would have plugged away & finished the bench to the original design & wondered why the thing never worked very well & why the blower broke, looks like there is gonna be some more effort involved in finishing it but the final product should be worth it - especially considering what a bench capable of the flow & depresion this should be able to do is a quite expensive thing in Australia.

The guy who's bench I used to flow heads on recently got a performance trends black box & software - looks good, so I think I'll invest in that setup as well.

[bruce]

If you haven't checked out this bench design take a look:

[2seater]

I don't know if the roots blower can be used in either direction myself, but I had planned on doing exactly that with the 8V-92 I have. The timing gears don't appear to have any bias for rotation and the rotors don't really have any contact. I do not have a drive end for the one I have, so that may make some difference?

[Tony]

A roots blower should work perfectly well running backwards at the very low pressures we are interested in.

I would be saving up for a VFD to run this blower. It will cut down noise, heat, starting inrush current at startup, and allow simple reversing. And it will hugely simplify things.

How much horsepower?

Suppose 28" test pressure only, neglecting any other pressure drops. That is 450 CFM x 1.0 psi divided by 296 = 1.52 air horsepower. A 7.5 Hp motor will do that easy.

How fast?
My little black book tells me an 8V71, which is the old style big diameter rotor type, has a displacement of 436 cubic inches per revolution. Or roughly 0.252 cubic feet per revolution.

If you want 450 cubic feet, you need 1,785 revolutions (per minute), neglecting air leakage around the rotors. With a VFD, the drive ratio becomes less important, because you can run the motor at speeds faster or slower than the rated fixed motor rpm.

One thing to think about. The inlet side has almost constant flow, but the output side pulses greatly. These things drone very loudly as they run, which is the outlet pulsing. You are going to need a large settling chamber to even out this pulsing, but the constant droning noise may still drive you nuts. It will sound like a big angry blowfly or bumble bee.

At 1800 rpm, there will be six rotor lobes discharging into the outlet port, or 10,800 pulses per minute. That is 180 Hz, that sucker is really going to hum at full speed, and it is quite likely going to really vibrate the panels of your bench.

[49-1183904562]

Tk Tony,

Is your supercharger Helical rotors?

With or without Rotor seals?

Last are the drive gears Helical? cut Any Backlash?

Rick

[Tony]

My bench uses a large diameter centrifugal blower, not a roots blower.

A roots supercharger should work very well indeed in a flow bench, because the "boost" pressure is so very low. The efficiency will be high too, the pulsing output, and the characteristic drone it produces are the only real drawbacks. But it will still be a lot quieter than a whole bunch of vacuum cleaner motors.

There will always be some slight gear backlash, there needs to room between the teeth for the oil. These blowers always have helical gears. Shimming one of these helical gears, is how the rotors are timed.

If set up properly they can be run in either direction without any problem. This is only true at the very low pressures we are using.

There is a very big difference between running a blower like this stone cold at 1psi and 2,000 rpm with a "little" electric motor, and running one at 25 psi and 10,000 rpm, requiring several hundred drive horsepower.

[TK]

You're a legend mate - cheers for that info, you're in Aus too yeah? - whereabouts?

Motor speed without VFD is 1450rpm - I was going to belt drive it, but direct drive may be an option if I can speed it up a little with a VFD for big heads, mind you it's already set up with motor mounts & belt tensioner for belt drive - may as well leave it like that.
I didn't realise a VFD could also reverse the direction, that's something I'll have to look into.
Electronic speed & direction control opens up the possiblity of fully computer controlled testing, wind the valve open to the desired lift, hit the go button & the laptop & bench does the rest & spits out a figure - nice.

A guy I know has a little bench with a 6/71 & 2.5hp motor - it's a pretty crappy thing but it's whisper quiet.

Even if mine is horribly loud I don't really care - it'll still be quieter than a lot of other things in my machine shop, I got earmuffs, I'll put up with it for the sake of a big ass flow bench.

The bench is pretty big, 1.2mx.75mx.75m, enough room for a decent volume to damp the pulses, an internal lining of something like foam rubber should help.

[TK]

Rotors are slightly helical - less pulsing than a straight rotor.
No seals - it's straight of a diesel engine.
Backlash is minimal & within spec - the thing is in good nick.

Thanks Tony - sounds like I'm on a winner.

Any idea on the location of the venturi's - or should I just switch to oriface plates?

[Tony]

I am located in Melbourne.

Yes the three lobe helical blowers are a lot better than the old two lobe egg beaters, that is for sure.

Direct coupling the blower and motor with a rubber coupling is what I would do, but the whole thing then becomes very long.
If you already have a pulley drive system there, then go for it.

A VFD opens up a lot of possibilities, including running up to frequencies a fair bit higher than 50/60Hz. Reversing is always a feature of the more modern drives, as is computer control, via an optional serial data cable.

These VFDs are not cheap, but it will open up a whole new world of possibilities if you get one.

Flow benches are always pretty loud, just from air roar, if nothing else.
A big centrifugal blower will be almost silent, except for air roar, which you cannot really do much about. Yours will have a characteristic drone, but you can always install some sound absorbing material into a settling chamber to make a big effective muffler. At least it will not scream the way all very high rpm vacuum motors do.

A VFD also allows you to slow down the blower, and that will vastly reduce noise and power consumption most of the time. An air bypass system and running the blower at constant high speed means the noise will always be much higher. A VFD also allows automatic gradual acceleration when starting up (this can be programmed into the VFD) and reduces the massive power surge at startup you will always have if you just switch the motor on.