ReplyShould the new forums be in Pro or open areas guys?
Recommendations?
Professional Member
see attached.
Former Site Administrator- Now Retired
Should the new forums be in Pro or open areas guys?
Recommendations?
Professional Member
If you do a duct velocity profile and plot the results you will discard Gil's method. In the right situation any number of alternative methods of measuring airflow will look good but will not work more often than they will. NEBB approves two methods of pitot traverse, the equal area method and log Tchebycheff. AMCA rates the accuracy probability of air testing and there is no way to rate accuracy when the only readings are in any spot in the duct. Log Tchebycheff is considered the most accurate but is more work than the equal area method. If you need to rely on your tests and or prove the accuracy of a pitot traverse you have to used industry standard and approved methods.
Professional Member
Dad,
I think it should be open in part because there are those that will not find it otherwise.
Professional Member
Donut shop pencil balancers are out there and honest TAB people hate them more than anyone else. Try to compete with their price. I was ask to determine the problem with a office space that was added to the basement of a building. The balance report couldn't have looked any better. I couldn't find airflow any where. I traced the problem back to the main duct and discovered that the hole in the main duct at the tap in had never been cut. I urged the owner to take action but he didn't want to which explains in part why that kind of thing happens.
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It's not electricity, but there can be rotating equipment.
Maybe start in the open area, then if need be, a second one could be opened in Pro. Just as there is a General in the open and a General in the Pro.
Originally Posted by Dad
Professional Member*
I'm guessing if you had good straight air, then okay. After a fitting, maybe not so much.
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Pro.
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Wayne makes a good point though.
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Professional Member
Dad, I guess the question might be , who would profit most from the forum.
We are here on Earth to fart around ......Kurt Vonnegut
You can be anything you want......As long as you don't suck at it.
USAF 98 Bomb Wing 1960-66 SMW Lu49
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Ultimately, wouldn't it be our customers?
Professional Member
I can see how that measurment method might get close. Like you mentioned, a nice long straight run. With where you measure is important. Air in a duct rubs against the duct material and tumbles causing turbulence. Maybe this guy took the time to see where he measured and got the same numbers as a standard traverse. Did some more tests and determined how to mimic a standard treverse.
A problem is an engineer might want to look at the job traverse report and for sure doesn't want to see four holes. He expects all the holes. Much more if his system has too much system effect.
We are here on Earth to fart around ......Kurt Vonnegut
You can be anything you want......As long as you don't suck at it.
USAF 98 Bomb Wing 1960-66 SMW Lu49
Professional Member
technocratic, Was my coil write-up written well enough to get the message across without turning off someone with minimal knowledge of the subject? If not criticize it and I will try to improve. Don't be afraid to ask questions. Someone here will know the answer.
Professional Member*
If you read the entire doc, it accounts for turbulence.
Professional Member
One huge problem with the 4 hole traverse is that there are instances where the airstream is so turbulent with swirl that pitot traverse is not possible. Many times some areas of the duct will have negative pitot readings which are recorded as zero airflow. One reason for mandating the minimum number of readings is to determine if the traverse is valid. 75 % of the readings must be greater than 10 % of the maximum value measured or the traverse is not valid. Readings taken in the center of the duct are seldom negative even when many other readings are. The 4 hole method will get you burned and when it does the engineer will not give you the benefit of doubt. You will be branded a liar no matter how honorable your intentions are.
Professional Member
I hope my response to the 4 hole traverse didn't sound harsh because it was only meant to help. Over the years I have done an estimated 12,000 to 15,000 pitot traverses. The leading edge of the velocity profile varies with what ever causes the airflow turbulence in the duct. To make initial airflow adjustment in ducts that had high airflows I ran a pitot traverse and calculated the average velocity pressure. By knowing the average velocity pressure you can take velocity pressure readings across the duct until you find the location closest to the average (we call that the sweet spot). Airflow was then adjusted until you obtained the new desired velocity pressure at that location. That method almost always gets you pretty close to the airflow you want but when you re-traverse the duct the sweet spot is usually in a different location which indicates the velocity profile changes with the velocity of the air.
Disclaimers on a balance report are meant to alert everyone concerned that the readings are dependent on tested conditions and will not repeat if those conditions are not replicated. One example of that is systems with diversities. Some of the VAV's cannot be calibrated unless some of the boxes are closed enough to force air to the remote VAVs. The balancer should be able to re-establish balanced conditions in most cases and demonstrate airflows to be within about 5% of that balanced. Occasionally that can't be done.
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Professional Member
We didn't drill holes for the 80-point traverse - went inside the AHU & measured the air coming out of the cooling coil (we were positioned on the outside of the coil so not to disturb the air flow - still 'tight' conditions).
The attached PDF shows the actual cfm results for all 80 points (equal spacing method). You can definitely see some odd disturbance values. Maybe that's the argument for taking a lot of readings. I'm not entirely convinced that my capture of the 4 "center' points (or a few more) will always work - but Gil's prediction did work for this particular AHU coil.
Again, there's the NBEE way & a 'quick & reasonably' accurate way that's appropriate for general cfm discovery.
Professional Member
tridiumtech
Did you measure airflow at the coil with the vel-grid?
Professional Member
I did read it all and when I accounted for turbulence I could understand how his numbers were arrived at. He just averaged the probability of how much turbulence would affect he average.
We are here on Earth to fart around ......Kurt Vonnegut
You can be anything you want......As long as you don't suck at it.
USAF 98 Bomb Wing 1960-66 SMW Lu49
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