Great forum, I am looking at building a flow bench based on the MSD design. I have read most of the articles that have been posted here and I am requiring clarification on the following. I would appeciate help regarding the ideal position to locate the brass tubes for measuring the plenum chambers. Also the ideal location for the test pressure probe, I have read articles where this probe should be located in the test stand. Or can it be located in the plenum and where? Also should the test orifice be inline with the measuring orifice?
Glenn
Orifice Probe Location - Orifice Probe Location
4 posts
• Page 1 of 1
by 84-1074663779 » Tue Mar 01, 2005 7:39 pm
Glenn, the trick is to measure static air pressure in what might be a fast moving and possibly turbulent air-stream. There are several effective ways to do this.
One way is to slow down the air in some sort of large plenum space, and measure the static pressure in a quiet corner, or measure it in several different locations and combine the pressures to get a reliable average.
Sometimes this is just not possible. A very effective way is to build just the static port part of a pitot tube. Google "pitot tube" for a lot of useful information and some pictures. This is basically a small pipe with a blanked off smooth spherical nose pointing directly into the moving airflow. About ten pipe diameters back from the nose you drill four pin holes at ninety degrees around the circumference. The outside of the pipe must be very smooth and aerodynamic. This works wonderfully well and can measure test pressure just below the head, or anywhere else in a fast moving air-stream. A bit of small diameter copper capillary tube salvaged from an old refrigerator, with a neat dome of solder on the end, and polished up on the outside with fine emery is very easy to make.
Ideally an orifice plate needs undisturbed air upstream if it is going to be consistent. Having a high velocity jet of turbulent air blasting straight at it, or across it, is just not going to work. The flow coefficient for that orifice is going to change very dramatically with every slight change in upstream flow condition, and it will be unusable. The MSD bench design is particularly bad in that respect.
Others that have copied this design have had to move either the orifice turret disc or test hole so they are no longer in direct line. You will know because there is no consistency between the quoted orifice diameters and rated flow numbers for each size of orifice in the article. An orifice bench that is working really well should have exactly four times the flow rating for a doubling of orifice diameter. It is easy to test, and when you get the numbers to come out right like that, you have really achieved something.
It is always the largest orifice and highest flow that is the problem. Get that working right, and the smaller sizes will all fall into line.
One way is to slow down the air in some sort of large plenum space, and measure the static pressure in a quiet corner, or measure it in several different locations and combine the pressures to get a reliable average.
Sometimes this is just not possible. A very effective way is to build just the static port part of a pitot tube. Google "pitot tube" for a lot of useful information and some pictures. This is basically a small pipe with a blanked off smooth spherical nose pointing directly into the moving airflow. About ten pipe diameters back from the nose you drill four pin holes at ninety degrees around the circumference. The outside of the pipe must be very smooth and aerodynamic. This works wonderfully well and can measure test pressure just below the head, or anywhere else in a fast moving air-stream. A bit of small diameter copper capillary tube salvaged from an old refrigerator, with a neat dome of solder on the end, and polished up on the outside with fine emery is very easy to make.
Ideally an orifice plate needs undisturbed air upstream if it is going to be consistent. Having a high velocity jet of turbulent air blasting straight at it, or across it, is just not going to work. The flow coefficient for that orifice is going to change very dramatically with every slight change in upstream flow condition, and it will be unusable. The MSD bench design is particularly bad in that respect.
Others that have copied this design have had to move either the orifice turret disc or test hole so they are no longer in direct line. You will know because there is no consistency between the quoted orifice diameters and rated flow numbers for each size of orifice in the article. An orifice bench that is working really well should have exactly four times the flow rating for a doubling of orifice diameter. It is easy to test, and when you get the numbers to come out right like that, you have really achieved something.
It is always the largest orifice and highest flow that is the problem. Get that working right, and the smaller sizes will all fall into line.
- 84-1074663779
by reptar28 » Mon Mar 07, 2005 1:00 am
Thanks Tony.
I am very much a novice on this subject and your advise is appreciated. I have taken your advise onboard and I have altered my flowbench design. Just on the sizing of the plentum chambers I was considering designing my flowbench with both plentums having similar sizes of approximately 40"(Length)x20"(Height)x24"(Deep) and having the test hole and measuring Orifce as far apart as I can. What could be the effect of having too big a plentum chamber and would the sizes I have indicated above be to big.
Thanks
Glenn
I am very much a novice on this subject and your advise is appreciated. I have taken your advise onboard and I have altered my flowbench design. Just on the sizing of the plentum chambers I was considering designing my flowbench with both plentums having similar sizes of approximately 40"(Length)x20"(Height)x24"(Deep) and having the test hole and measuring Orifce as far apart as I can. What could be the effect of having too big a plentum chamber and would the sizes I have indicated above be to big.
Thanks
Glenn
- reptar28
- Posts: 2
- Joined: Tue Feb 22, 2005 9:49 pm
- Location: Australia
by 84-1074663779 » Thu Mar 10, 2005 6:26 pm
A plenum just cannot be too big, the bigger the better. But even a small one is going to be better than none if it is carefully thought out.
One point to watch is that the mechanical loads on large areas of panel can rise to be pretty extreme. twenty eight inches is almost exactly 1psi, or 144 pounds of air pressure per square foot. Three feet square is 1,296 pounds if force trying to burst/crush your flow bench.
Heavy steel angle and bolts might be more appropriate than glue and wood-screws for a large bench.
Some vacuum motors can easily generate over a hundred inches of pressure if the flow is accidentally blocked.
It is easier to seal your bench if it operates under vacuum, all the joints will compress. High positive pressures can be a worry and are going to leak more readily.
Build the largest plenums and flow areas you possibly can into your bench, physical space permitting, you will never regret doing it.
One point to watch is that the mechanical loads on large areas of panel can rise to be pretty extreme. twenty eight inches is almost exactly 1psi, or 144 pounds of air pressure per square foot. Three feet square is 1,296 pounds if force trying to burst/crush your flow bench.
Heavy steel angle and bolts might be more appropriate than glue and wood-screws for a large bench.
Some vacuum motors can easily generate over a hundred inches of pressure if the flow is accidentally blocked.
It is easier to seal your bench if it operates under vacuum, all the joints will compress. High positive pressures can be a worry and are going to leak more readily.
Build the largest plenums and flow areas you possibly can into your bench, physical space permitting, you will never regret doing it.
- 84-1074663779
4 posts
• Page 1 of 1
Who is online
Users browsing this forum: No registered users and 24 guests