Measuring Back Pressure Without an Engine 1

[ChrisPoyaa]

There are many techniques on how to measure backpressure in an exhaust system that is already connected to an engine. How can you measure the backpressure in an exhaust system if it is not going to be connected to one though? What could you use to represent the airflow and backpressure to match what the engine would create (or something proportional) to see if the backpressure is within the specifications? The engine is a 3.0L V6 VM Turbo Diesel. Thanks for the help!

 

[MikeHalloran]

I built a show display for exhaust risers that used a 'high pressure blower' from McMaster-Carr, with a 1HP 115/230/60 motor. According to some very rough calcs, the airflow approximated the idle exhaust flow of a Cummins 6BT-ish marine engine.

The blower would pretty regularly trip a 15A/115V breaker because of its instantaneous starting current of ~49A. I added a timed relay so it started at reduced current (~35A) for 6 seconds, which was enough time to get it mostly spun up.
If I had it to do over, I'd have bought a 3phase motor and a VFD that could run it from a single phase input and ramp it up to speed while limiting the current a bit more.

To use a blower like that, you'll have to measure your pressure with some precision (think Dwyer Instruments) and extrapolate along the assumed system curve from the one point you can measure with the blower.

Oh. A round car air filter glued to the blower inlet will keep birds out of it and reduce the noise a little. An acoustic labyrinth on inlet and outlet will help more.




Mike Halloran
Pembroke Pines, FL, USA

 

[ChrisPoyaa]

Thanks Mike! Yeah I'm going to get some precise pressure guages. This testing is going to be inside a facility and is going to be happening many times a week, so a blower that regulary trips a breaker would not be sufficient since we'll be working off of 110 V outlet. You said you did some rough calculations which is where I am stuck at now. We measure backpressure at maximum exhaust gas flow rate which of 900 kg/hr. So I'm thinking we could find a blower that can blow an amount proportional to the 900 kg/hr and then calcuated what the back pressure would have been at maximum exhaust flow.

Chris

 

[MikeHalloran]

Okay, that can work.
You need to have a way to check your calculations on an actual engine, at least once. Regularly would be even better.

If you're limited to a 115V supply, you're limited to a ~1HP motor, unless you can deal with the noise and maintenance issues of a brushed universal motor, in which case you can probably get to ~2HP. ... Okay, with a 3ph motor and a VFD, you might be able to get to 2HP. At least get a 20A circuit.

Back up there. Do check the facility; the tin shop I worked in had only one 115V outlet on the factory floor; everything else was higher voltage and 3 phase.



Mike Halloran
Pembroke Pines, FL, USA

 

[dgallup]

Why would you measure backpressure many times a week? Is it going to change?

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[ChrisPoyaa]

Yeah true, I will give the facility I belive there might be higher voltage outlets. I wonder how much mass flow a 1 or 2 HP motor can create?

We have to measure parts from certain batches before they leave the plant to make sure they are within the backpressure specifications. There is the DOC/DPF (DOC – Diesel Oxidation Catalyst ; DPF – Diesel Particulate Filter) and the SCR (Selective Catalyst Reduction) components that together make the exhaust system.

The full assembly backpressure can have no more than 850 mbar. The question is how to create the pressure in which to be measured.

 

[Tmoose]

http://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.html

 

[MikeHalloran]

Thanks, Tmoose.
I'm getting at least 4HP, neglecting all losses and uncertainties,
to feed 900 kg/hr of standard-ish air
against a backpressure of 850mbar,
and omitting any attempt to account for the engine's actual
turbo exhaust temperature, not revealed.

That's a huge backpressure limit.
Much larger turboDiesels have limits as low as 50mbar.

I hope some number somewhere includes a little budget for the exhaust system outside of the DPF,etc.




Mike Halloran
Pembroke Pines, FL, USA

 

[ChrisPoyaa]

Yeah Mike, 3-4 HP seems about right for 900 kg/hr of air flow thanks.
850 mbar of backpressure does seem quite high but hey I'm not going to question Chrysler's specs.

The engine's actual exhaust temperature is 450 degrees Celsius.

Does there need to be some sort of conversion of numbers since the 900 kg/hr involves the different gasses in the exhaust while the 4 HP blower will only be using air (different density gasses)?

 

[MikeHalloran]

I'm getting 9.8 HP to push 900 kg/hr of 450C air through an arbitrary resistance to develop a backpressure of 850 mbar.

Leaving the resistance unchanged, it takes 1.98HP to push 900 kg/hr of 70F air through it and the backpressure is 349 mbar.


This would be a good time for you to talk to Chrysler to reach agreement on the specifics of a test that will be agreeable to everyone.





Mike Halloran
Pembroke Pines, FL, USA

 

[Lou Scannon]

One thing to be mindful of when making assumptions about scaling factors to convert from specified conditions to test conditions, is whether the flow through the actual restriction is turbulent, or laminar. This will require knowledge of the nominal size of the individual flow paths, and perhaps some estimate of the surface roughness, if this is a significant fraction of the nominal size.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[GregLocock]

Why not use an accumulator?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

An accumulator is certainly a possibility.
Use a modest compressor to charge a big air tank.
Dump the tank contents through a large fast acting valve connected to the UUT, and measure the pressure at the connection point.

The pressure will not be static of course, but will be a decay curve starting near the initial tank pressure and decaying to zero. It will probably decay very quickly, because we have been working here with a very high backpressure limit that is probably not even close to representing a new, clean system.

The tricky part, IMHO, is that Chris will have to work with Chrysler's Engineering and Quality organizations to develop a test procedure and a limit curve that all will find acceptable.



Mike Halloran
Pembroke Pines, FL, USA

 

[ChrisPoyaa]

The back-pressure testing is not yet required by Chrysler, this is just planning ahead for when it is required so Chrysler is going to be left out of the discussion for the moment.

850 mbar represents a new, clean system and 1100 mbar represents an aged system with ash and soot accumulated. These seem like accurate representations of the backpressure for an entire system at maximum exhaust gas flow rate.

I really do like the idea of a compressor and an accumulator actually. I would need a compressor that could build up over 1100 mbar of pressure, so lets say 1500 mbar to be safe, and that also could keep up with the air flow rate of 900 kg/hr. Those seem like very high pressure and flow rate numbers to achieve though.

 

[1gibson]

Only you know the actual constraints, but seems like a lot to invest in a huge compressor/accumulator setup when you could do this with an engine on a stand, in a ventilated area. You could probably even buy the complete vehicle and still come out with money in your pocket.

 

[dgallup]

You could pump the reservoir up far higher than your test pressure, I would probably go for 150 PSI or more. Then put a precision pressure regulator in to drop pressure to about 3 bar. Downstream of that put a properly designed sonic nozzle that will flow your 900 kg/hr. Since the flow in the nozzle is sonic, it will flow exactly the same as long as the pressure ratio is low enough. All you have to do then is measure the back pressure. You should be able to hit steady state conditions in a couple of seconds or less. The flow rate will be independent of the tank pressure and the back pressure as long as the ratio across the nozzle is enough to achieve sonic flow. I think you could do this test with the average home shop compressor and a little plumbing.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Lou Scannon]

I wonder if you could tailor a microturbine to produce the approximately correct exhaust gas parameters, for a hot gas stand type test bench? It could be oversized, with a controlled wastegate to maintain controllable constant conditions.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[GregLocock]

An engine would be cheaper, for a backyard operation, but in an industrial setting the facility cost will soon turn it into a 50000 dollar job. Your customer may object to getting dirty exhausts as well.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Lou Scannon]

Hmmn, my assumption was sampling, not 100% testing, so if the test rendered the UUT unsaleable, that would be a manageable expense.
For 100% testing, filtered pipeline natural gas as a fuel for the microturbine should be acceptable.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[MikeHalloran]

The microturbines with which I am slightly familiar _really_ don't like backpressure.



Mike Halloran
Pembroke Pines, FL, USA