Air Flow Missing
Air Flow Missing
(OP)
Dear All,
I have installed a fresh air system as shown in the attached picture. However I could notice that the air flow rate that is being taken at the suction side of the AHU is not being delivered at the end of a 23 meter insulated flexible hose pipe that is used to distribute the air.
Can you please help me out in tracing what is the problem.
Regards,
Siril K Thodeti
I have installed a fresh air system as shown in the attached picture. However I could notice that the air flow rate that is being taken at the suction side of the AHU is not being delivered at the end of a 23 meter insulated flexible hose pipe that is used to distribute the air.
Can you please help me out in tracing what is the problem.
Regards,
Siril K Thodeti





RE: Air Flow Missing
RE: Air Flow Missing
RE: Air Flow Missing
RE: Air Flow Missing
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RE: Air Flow Missing
Also is the really minus 20C coming from the AHU?? That's enough to make 10% difference
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Air Flow Missing
RE: Air Flow Missing
My thinking goes in this manner:
Mass Flow of Inlet = Mass Flow of Outlet
Inlet Side:
Mass Flow of Dry Air + Mass Flow of Water in the Air = Mass Flow of dry Air + Mass Flow of water in the Outside Air ( This Water flow rate would be mostly zero as the cooling coil is at -17 deg C, most of the water would have condensed)
Hence My assumption is that the outlet air flow rate would be less.
Please correct me if my understanding is wrong.
Regards,
Siril K Thodeti
RE: Air Flow Missing
However, you failed to mention a "cooling coil," which drastically changes the temperature.
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RE: Air Flow Missing
Apologies if In case if I was not able to mention that there is a cooling coil. As a described earlier there is an AHU in which this cooling coil is hosed.
The cooling coil is bringing down 1300 CFM of air at a temperature of 25 Deg C, 50% RH to -17 (Negative) Deg C , 95% RH
Even If I take the density effect
Weight of Air Flow Delivered by AHU is / Weight of Air Flow Sucked by AHU is
= 800 x 1.35 ( Density of Air at -17 Deg C)/ 1300 x 1.15 ( Density of AIr at 25 Deg C)
= 72%
So 28% of air is missing after cooling coil
Weight of Air Flow Finally Reached / Weight of Air Flow Discharged by AHU is
= 420 x 1.3 ( Density of Air at -6 Deg C)/ 800 x 1.35 ( Density of AIr at -17 deg C)
= 55%
So totally 55% of air is missing after the Discharge Side of the AHU
Also the air is being discharged through an Rigid SS Pipe, whose inner surface is rough.
Also, when an OEM Claims that there AHU discharges 10000 CFM? It is on which side of the AHU? SUction SIde or Discharge Side ? My understanding till date is that it should be same. Please correct me if my understanding is wrong
RE: Air Flow Missing
RE: Air Flow Missing
RE: Air Flow Missing
You talk about volumetric flow rates in CFM, then use temperature and weights in Celsius and Kg. Mixing systems of units is ALWAYS a bad idea and inevitably leads to errors in calculations.
How big is the coil and how many points are you measuring? If you are measuring flow on the AHU discharge, where is the fan in the AHU and how close to the AHU are your measuring velocity?
RE: Air Flow Missing
Suction of air from conditioned room at a temperature of 25 deg C ---> Passing of AIr through Blower--> Passing of Air through Cooling Coil 1 that would cool the air from 27 deg C to 5 Deg C--> Passing of air through Cooling coil 2 that would cool the air from 5 deg C to -17 deg C
The drawing of the AHU is as attached.
From the AHU air is being distributed as shown in the attached picture.
We are measuring the volumetric flow rate at the following points:
1) At the suction side of the filter where we are getting 1300 CFM @ 16 deg C, 50% RH
2) 200 mm after the second cooling coil where we are getting 800 CFM at -20 deg C
3) At the end discharge point where we are getting 180 CFM at -5 deg C at point 1 and 220 CFM at -5 deg C at Point 2
RE: Air Flow Missing
After thinking that through, I’m slightly disappointed that I spent time looking at your post.
RE: Air Flow Missing
So from your post you are suspecting it to be following 2 probable reasons:
1) Air is getting leaked from the AHU itself
2) Some amount of air is staying within the pipe itself without being discharged at the end destination?
RE: Air Flow Missing
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Air Flow Missing
and you need to measure where you have stable flow, not right after obstructions etc.
RE: Air Flow Missing
RE: Air Flow Missing
RE: Air Flow Missing