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remwel
23-02-2011, 08:11 AM
I have a slight problem with this thing. A Building we have just completed a Fit-Out job has on each floor four AHU units distributing air to the floor. Two units have 20,000 cfm capacity, one has 18,000 and the other a 16,000 cfm. After all duct installations has been completed we did the balancing work. Upon achieving a balanced air flow adjustment i found something amiss in our data gathering.

On one of the AHU with a 20,000 cfm design capacity, our total read of the balanced air flow is 22,373 cfm. I have my team recheck the data and did another reading and the total remained almost the same (+ or - 100 cfm).

AHU C Rm1 capacity – 20,000cfm : Final Balancing Record - 22,373cfm

Am i missing something here? or the AHU cfm design (or most AHU for that matter) does constitute a percent factor of ±10 ~11% at full load (60 Hz)?

I just happen to start on AC systems technicals so if anyone can please be nice enough to enlighten me.

Tesla
23-02-2011, 11:11 AM
Hi remwel and welcome to our forum.
Firstly I am coming from the other side I have lots of experience on AHUs and VAVs from the BMS prospective and have carried out measurements in big systems. Generally we allow 10% leakage from the AHU to the outlets (supply air). And from design figures with regard to measurement get + or - 10% depending on design and modification over time. So you need to take into account where the measurement is taken from AHU or SA outelt. Also take into account accuracy of the measurement - did you get an independent company for the second measurement? or were the original techs sitting in the pub duplicating documents - it does happen. If your measurements are true and full (the whole system measured like fan speed, static pressures, air volumes ahu and outlets) you should be able to pin point the area of problem. Another consideration is the system designed on 60 Hz with 50 Hz gear?
Let us know a few more details for a better answer like the actual figures of all parameters taken.
There should be two system measurements one with full fresh air (all VAVs open) and one with full recirculation (all VAVs open).
Not a big problem in your case as you have more than enough which could be fixed with reduced static pressure control via VSD or pulley diameter.

Koopy
23-02-2011, 11:28 AM
Haha usually the issue is lack of air flow not too much. Can I have your excess?

remwel
23-02-2011, 11:44 AM
Thank you sir for that bit of a clarification. I'm particularly new with ACMV/HVAC, i was more on Piping works prior to my shift in the field. Basically i have that notion (from an engineering basics perspective) of 10% leeway particular to the fan/blower designs, i just have to get that out of my chest with a professional opinion.

To touch up some of the areas tho, our system is run on BMS with VAV controled air flow on each 1 or 2 or 3 or 4 supply diffusers. We adopted the methods of an independent testing commission, as for myself, i have done simple balancing prior to this on a smaller scale (non-BMS non-VAV plant office Duct / AC system running on closed loop chilled water system).
We conducted the test on all VAV @F/O - fresh air full. All instruments (Flow hood, Balometer et,al) calibrated a month before the test proper so no worries there. Balancing record as stated above taken as a sum of all SA diffusers. I have also taken data on the straight run at the AHU supply side with a reading of 20,251 cfm - taken a week prior to the balancing proper. so that is basically the case.

Goin' back a bit, i just checked with the copy of the AHU fan curve and i was also relieved to find my presumption true...i guess no need to go graphical on it here anyways.

Thank you

remwel
23-02-2011, 12:18 PM
Haha usually the issue is lack of air flow not too much. Can I have your excess?


:Dno problem sir, i'll send the excesses to you via Bluetooth LOLz

Tesla
23-02-2011, 01:07 PM
Thanks for getting back to us remwel. To save energy on your project you could specify variable static pressure dependent on load via VSD through the BMS (unless already specified by another consultant/contract). Looks like you got it sorted though and cleared any doubt about the results. It would be good to get some feed back once the project is complete and up and running for the rest of us to learn from. Feel free to air any queries in the future for opinions, or your own opinions regarding any threads.

remwel
23-02-2011, 02:50 PM
Will do Sir Tesla, Thanks for a your warm welcome.

Actually I'm also trying to get a good grip now with the BMS logic as it is now operating and running. Its a steep learning curve the way i see it.

Thanks again for the help.

Brian_UK
23-02-2011, 11:26 PM
The in duct airflow reading is the most accurate. the sum of the terminal readings will provide a factor to correct them to the true airflow reading.

Magoo
24-02-2011, 04:51 AM
If you can replicate the total design conditions then you would be seriously the luckiest commissioning engineer on this planet.
You have to average everything and come up with +/- average performance figure. Generally if you are with-in 10% +/-, you are on the button and that should accepted by any half sensible consultant.
Good luck. If not acceptable ask for design conditions to be provided. Been there done that. Confussion is the best defence.

MilosBog
24-02-2011, 08:30 PM
10% Wow, somebody can dream about it..In the DIN tolerance in certain cases is up to 25%.....Are you maybe from India? They are always precise.