Minimum Diffuser/Runout TAB? 50cfm?

[ChillMode_VB757]

Greetings,

The technical guidance through my chain of command indicates that no ventilation diffuser shall be scheduled less than 50 cfm for the sake of "system balancing". I'm a design engineer, not a TAB guy. I'm pretty sure cfm has nothing to do with a measurable rate - rather it has more to do with velocity. I believe this 50 cfm rule of thumb is based on a minimum size 6" round duct. Let's say the duct was 5" or 4" round. What's the minimum airflow one could balance in smaller round duct with accuracy +/- 10%, using standard TAB/Cx firm equipment? Duct transverse vs. face of diffuser? Any advice would be appreciated, thanks.

From UFC 3-410-01:
DOAS air inlets and outlets must deliver airflow quantities equal to or
greater than 50 cfm (24 lps) through each device to allow for proper
system balancing

[WAYNE3298]

The 50 CFM rule is usually because capture hoods are useless for flows below that and frankly 50 CFM is too low for accuracy and repeatability. I have balanced airflows as low as 20 CFM but did it by pitot traverse of the duct. It takes a lot of time but can be done if you have a reasonable velocity.

I almost forgot. WELCOME TO THIS SITE YOU WILL LIKE IT.

[Artrose]

The 50 CFM rule is usually because capture hoods are useless for flows below that and frankly 50 CFM is too low for accuracy and repeatability. I have balanced airflows as low as 20 CFM but did it by pitot traverse of the duct. It takes a lot of time but can be done if you have a reasonable velocity.

I almost forgot. WELCOME TO THIS SITE YOU WILL LIKE IT.

Ha Ha........ It can be done 20 cfm...HA!...... Till the wind blows outside.....or the fart fan takes a crap..... Take care of that magic pencil there Mr. Wayne.......

[WAYNE3298]

The wind pressure has to exceed the duct SP to alter the airflow. It's the pressures not the CFM. The 20 CFM I had to do was in a lab and the wind had nothing to do with it. With a flow hood you would be pissing in the wind but not with an inclined manometer.

[Artrose]

The wind pressure has to exceed the duct SP to alter the airflow. It's the pressures not the CFM. The 20 CFM I had to do was in a lab and the wind had nothing to do with it. With a flow hood you would be pissing in the wind but not with an inclined manometer.

Please don't b.s. a b.s.'er.

Years ago I was tasked to balance a drug laboratory. The building design required pressure gradients to keep the nasties from migrating. The system had fume hoods inside the cracking chamber room, a dressing room, hallway, and an office area. (and that's a simplified description) The only Phoenix valves were on the fume hoods.
Although VFD's were available, the pneumatically trained antique engineer specified the use of inlet vanes and standard trane pancakes to achieve his pressure control. We satisfied the requirements, but let them know that we couldn't guarantee accuracy after we left, and that regular maintenance was absolutely vital.

Another brilliant move in an attempt to force T&B perfection was handed down to us by the military. Rather than the standard + - 10%, they decided that the new requirement was going to be 0-5%+. Or they wouldn't accept the job.

So, ok, I'll accept your claim that you can accurately measure flow with the clunky old red tube wonder, but let's take a trip out to that job this afternoon and check that repeatability.

[WAYNE3298]

I didn't say the CFM would stay forever. With design flows you are not comfortable with you put your concerns in the balance summary. You put in the summary also how you accomplished the task if you did and who with the client verified and approved the results. Some of my balance summaries were several pages long but not one of them came back to bite me in the ass. They saved my ass from engineers that didn't know as much as they thought. That is the purpose of the summary so do them every time and cover all the bases. It is the balancers job to flag problems and potential problems and I did a lot of that.
On any airflow the stability depends on the continuity of mass which is dependent on building conditions and how and when the controls respond to changes. You force building condition changes and put the result in the summary. It's not your responsibility to decide if the result of the changes are acceptable but it is your responsibility to document the performance.
I did several jobs with +/- 5%. I didn't like it any more than you but I accepted the job with that specified and so did you if you wanted and got the job. It can be done but again no matter the accuracy specified the stability depends on the continuity of mass. I have seen systems that when I started TAB couldn't maintain flow within 50%. You don't balance under those conditions. You inform all parties that the stability is unacceptable to meet the specified performance. I have been instructed in writing to balance systems that I knew were not going to work. In each case I responded in writing with my concerns again but agreed to do the work with no liability of the end result.
The only time I refused to do the balance as instructed was if it was unsafe. In those cases I identified the safety concerns and in every case the problem was resolved. I got several extras to fix jobs that I documented repeatedly that were safe but not workable but was instructed to balance anyway.
Airflows are subject to building conditions. An example is if you are controlling building static pressure at say 0.03 inches with the doors closed and someone decides to prop the doors open. Something that happens a lot also is in restaurants it is common for the cooks to turn off the hood make-up fan with the exhaust fan running.
I said all that to say this velocity in the duct and continuity of mass is crucial to measuring and maintaining airflow. The duct size is important to the balancer because you need a reasonable velocity to traverse any duct. Velocity is velocity no matter the CFM and without a reasonable velocity you can't get a good traverse because the distribution of the airflow/velocity which translates to velocity pressure in the duct will not meet the minimum requirements for even 90% accuracy.
I could traverse with the old reliable inclined manometer or with the high dollar electronic meters. They both have limitations and you should test both out so you can decide when to use them. The vast majority of my traverses were with electronics. I did enough research to know when to use every piece of equipment I owned and I owned a lot of test equipment.

[Artrose]

I didn't say the CFM would stay forever. With design flows you are not comfortable with you put your concerns in the balance summary. You put in the summary also how you accomplished the task if you did and who with the client verified and approved the results. Some of my balance summaries were several pages long but not one of them came back to bite me in the ass. They saved my ass from engineers that didn't know as much as they thought. That is the purpose of the summary so do them every time and cover all the bases. It is the balancers job to flag problems and potential problems and I did a lot of that.
On any airflow the stability depends on the continuity of mass which is dependent on building conditions and how and when the controls respond to changes. You force building condition changes and put the result in the summary. It's not your responsibility to decide if the result of the changes are acceptable but it is your responsibility to document the performance.
I did several jobs with +/- 5%. I didn't like it any more than you but I accepted the job with that specified and so did you if you wanted and got the job. It can be done but again no matter the accuracy specified the stability depends on the continuity of mass. I have seen systems that when I started TAB couldn't maintain flow within 50%. You don't balance under those conditions. You inform all parties that the stability is unacceptable to meet the specified performance. I have been instructed in writing to balance systems that I knew were not going to work. In each case I responded in writing with my concerns again but agreed to do the work with no liability of the end result.
The only time I refused to do the balance as instructed was if it was unsafe. In those cases I identified the safety concerns and in every case the problem was resolved. I got several extras to fix jobs that I documented repeatedly that were safe but not workable but was instructed to balance anyway.
Airflows are subject to building conditions. An example is if you are controlling building static pressure at say 0.03 inches with the doors closed and someone decides to prop the doors open. Something that happens a lot also is in restaurants it is common for the cooks to turn off the hood make-up fan with the exhaust fan running.
I said all that to say this velocity in the duct and continuity of mass is crucial to measuring and maintaining airflow. The duct size is important to the balancer because you need a reasonable velocity to traverse any duct. Velocity is velocity no matter the CFM and without a reasonable velocity you can't get a good traverse because the distribution of the airflow/velocity which translates to velocity pressure in the duct will not meet the minimum requirements for even 90% accuracy.
I could traverse with the old reliable inclined manometer or with the high dollar electronic meters. They both have limitations and you should test both out so you can decide when to use them. The vast majority of my traverses were with electronics. I did enough research to know when to use every piece of equipment I owned and I owned a lot of test equipment.

Requiring a balancer to certify (zero to 5% plus) is making a liar out of the balancer, and even if those numbers were actually achieved during the balance, they are very difficult, and sometimes impossible, to reliably reproduce throughout all of the systems during job commissioning.

A contract with those 0 to 5%+ specifications invites unrealistic expectations and misunderstandings. That other guy did it. You both have the same certifications. You should be able to do it too.

If I certify that I read 20 cfm on a drop, then the engineer and the building owner assume, and expect similar results on the next job. It doesn't matter how many notes you've left in your report. Just sayin. And we both know how difficult it can be to reliably read and reproduce that 20 cfm result with the equipment and installations that we deal with.

I've read many of your posts here, and I think we're both basically on the same page. I appreciate your knowledge sharing. Your mileage might vary.

The thing with me is, if I see something that looks like a bean, I'm going to call it a bean. I can only hope others would do the same for me.

And, a balance job is supposed to last forever provided Mr Crankshaft didn't screw with it, and it has a little regular maintenance.

[WAYNE3298]

On all the jobs that required +/- 5% balance I insisted the readings be verified by the owners rep. On those systems a digital hood was useless because the response is too quick even if you take several readings and have the meter average them. NEBB actually warns about repeatability of the digital hood but the analog hood is already a thing of the past. I explained that in person and in the balance summary to the owner and engineer. I do understand where you are coming from and have no hard feelings. I also mean no disrespect to you because I do respect your opinion and anyone that has worked in the TAB field.
I hope you read my post on the railroad crossing controls building. That was a dangerous situation and was made that way by the TAB company putting total trust in the digital hood. The controls engineer designed the cooling system based on the digital hood results and the controls overheated during the performance tests. The owner's engineer wouldn't tell me why they wanted me to measure the airflows when I had to drive 80 miles to take the readings and my competitor was within 3 miles of them.
I was pretty sure what happened and showed the owner's engineer how both hoods responded. He did tell me why he wanted me to take the readings after he had redesigned the cooling system and there were no problems.

[Artrose]

On all the jobs that required +/- 5% balance I insisted the readings be verified by the owners rep. On those systems a digital hood was useless because the response is too quick even if you take several readings and have the meter average them. NEBB actually warns about repeatability of the digital hood but the analog hood is already a thing of the past. I explained that in person and in the balance summary to the owner and engineer. I do understand where you are coming from and have no hard feelings. I also mean no disrespect to you because I do respect your opinion and anyone that has worked in the TAB field.
I hope you read my post on the railroad crossing controls building. That was a dangerous situation and was made that way by the TAB company putting total trust in the digital hood. The controls engineer designed the cooling system based on the digital hood results and the controls overheated during the performance tests. The owner's engineer wouldn't tell me why they wanted me to measure the airflows when I had to drive 80 miles to take the readings and my competitor was within 3 miles of them.
I was pretty sure what happened and showed the owner's engineer how both hoods responded. He did tell me why he wanted me to take the readings after he had redesigned the cooling system and there were no problems.

No disrespect intended here either .....but we still have a small disconnect...... The jobs I was referring to were spec'd ......Zero to 5% plus..... No minus. There was no cushion allowed on the bottom end.

Looking back I am relatively sure that they used this ridiculous parameter to use as leverage to weed out, or reject, the half-azzed outfits looking to make a quick buck off Uncle. It sure made our lives miserable though.

[WAYNE3298]

I noticed what you wrote 0-5% but thought you meant +/-. We are actually on the same page there because +/-5% is ridiculous but 5% one way is absolutely stupid. For some of them they really thought the tight spec gave them leverage. After doing design for 25 years and then going into TAB believe me I had a lot of fights with design engineers. Some of them after reading my resume tried to get me to do part of their jobs. A few times when they wanted to make me look wrong and bad they brought in another TAB company. They would then have a meeting with the owner present but that didn't work out well for them either. I had experience on both ends but nobody else did.

One you might enjoy if you haven't already read my post on it was the cooling tower with slip on flanges on the piping. I told the engineer if someone closed a valve with a pump running the pipe would blow apart. He didn't believe me but the control guy accidently did it with one of his control valves and the pipe blew apart dumping 45,000 GPM into the parking lot.

[Artrose]

I noticed what you wrote 0-5% but thought you meant +/-. We are actually on the same page there because +/-5% is ridiculous but 5% one way is absolutely stupid. For some of them they really thought the tight spec gave them leverage. After doing design for 25 years and then going into TAB believe me I had a lot of fights with design engineers. Some of them after reading my resume tried to get me to do part of their jobs. A few times when they wanted to make me look wrong and bad they brought in another TAB company. They would then have a meeting with the owner present but that didn't work out well for them either. I had experience on both ends but nobody else did.

One you might enjoy if you haven't already read my post on it was the cooling tower with slip on flanges on the piping. I told the engineer if someone closed a valve with a pump running the pipe would blow apart. He didn't believe me but the control guy accidently did it with one of his control valves and the pipe blew apart dumping 45,000 GPM into the parking lot.

No, I hadn't read that one. That's absolutely hilarious. Sometimes you just need to stand back a watch from a distance. Not that I haven't made my own share of mistakes.

Years ago, I had a young Mr. Know-it-all tell me one time that he was the Engineer and what he says goes. He's the same guy that a few weeks later ended up in the hospital because he blew himself up with an acetylene rig.

[WAYNE3298]

When I did design some of the best help I got was from contractors even though some were a pain in the ass. I designed a small heating water system in a boiler room and the budget didn't allow a site visit. The contractor called me and ask why I routed the boiler feed pump inlet pipe where I did. I told him that wasn't my first choice but to know it would fit where he wanted to install it I would have had to go to the site. He said it will fit so can I put it there. I told him send me a fax confirming it will go there and you take responsibility for any interference. He said OK but how long will it take you to answer my fax? I said how long will it take you to send it and he said 15 minutes. I said then I will answer it 15 minutes after I get it. He laughed and said I don't believe that because engineers can't make decisions that quick. He then ask since it will take less pipe to do it my way how much money do you want back. I said are you serious and he said yes. I said you just made me look better than I am and the install will be a cleaner install for the owner so why would I want money back. He said that's the way engineers are.
He sent me a fax and I answered it right back and included this was a no cost change. My phone rang about 5 minutes after I sent the fax and it was him. He apologized for not believing me and said you are the first engineer whose job I have done that can make a decision that quick and everybody else would have wanted money back.

I posted this to assure you that not all engineers are a pain in the ass just most of them.

I had engineers tell me I had to do things their way. I would ask them if they were going to certify the work and provide the guaranty. That shuts them up every time. None of them of course had included such nonsense in the spec.

[Artrose]

When I did design some of the best help I got was from contractors even though some were a pain in the ass. I designed a small heating water system in a boiler room and the budget didn't allow a site visit. The contractor called me and ask why I routed the boiler feed pump inlet pipe where I did. I told him that wasn't my first choice but to know it would fit where he wanted to install it I would have had to go to the site. He said it will fit so can I put it there. I told him send me a fax confirming it will go there and you take responsibility for any interference. He said OK but how long will it take you to answer my fax? I said how long will it take you to send it and he said 15 minutes. I said then I will answer it 15 minutes after I get it. He laughed and said I don't believe that because engineers can't make decisions that quick. He then ask since it will take less pipe to do it my way how much money do you want back. I said are you serious and he said yes. I said you just made me look better than I am and the install will be a cleaner install for the owner so why would I want money back. He said that's the way engineers are.
He sent me a fax and I answered it right back and included this was a no cost change. My phone rang about 5 minutes after I sent the fax and it was him. He apologized for not believing me and said you are the first engineer whose job I have done that can make a decision that quick and everybody else would have wanted money back.

I posted this to assure you that not all engineers are a pain in the ass just most of them.

I had engineers tell me I had to do things their way. I would ask them if they were going to certify the work and provide the guaranty. That shuts them up every time. None of them of course had included such nonsense in the spec.

I know it sounds like I don't like Engineers. Actually, some of my best friends are Engineers.

The ones I'll take to task are the ones who display a high and mighty. I fully realize I'm not always the sharpest pencil in the pencil box. I try to be humble most of the time, and I can help you achieve your goal in the best and most efficient manner, or I can help sink you.

We have another thread here (took the chance of getting banned for a personal attack), where I called one of those beans out. He fired right back with the high and mighty. And, I'm done.

[WAYNE3298]

I understand where you are coming from Artrose probably more than you realize. When I did TAB I had more trouble with the design engineers than anyone else. Almost none of them could handle being wrong even after you proved it conclusively. I had one of them try to prove me wrong when I told him he undersized an exhaust fan by running a pitot traverse in the middle of the 180 return on the fan outlet. The ones that like you like you a lot but the others hate you.
Another quick story I think is poetic justice. I had the job of balancing some trickling filters for a sewerage plant. The engineering company I did design for for 25 years did the design and had a doofus field resident engineer. As soon as I started work he came running out of his trailer and came to the fan I was testing. He said you realize you have to test that fan supplying air and exhausting it. I told him it isn't specified that way so I am only going to test them blowing in. He said I'll reject the balance if you do that. I said you stand right here and I'll test the first fan both ways and if the flow is within 10% I'm only going to test them blowing in. The airflow to my surprise was within about 3%. The fun part was I told him where to stand so he could see the gage and that was right where the air blew on him when exhausting. That moron stood there with pissy air blowing on him while I took my time running the traverse.
There is more to the story but I thought you might like that part.