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Air Flow Missing

Air Flow Missing

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.

Siril K Thodeti

RE: Air Flow Missing

Sum all the air outlets and if the total is less than the air intake there is a leak somewhere.A smoke test might confirm the location/locations.

RE: Air Flow Missing

The entire flexible hose is insulated and cladded with the 24 Gauge Aluminium. So I am not able to do those things. Otherwise is there anything like Back pressure of the fan or something like that which is coming into the place?

RE: Air Flow Missing

If you are confident that there is no leak,it could be that you are reading air flow incorrectly!Airflow measurement is not like water flow measurement,lot of things can influence its accuracy such as straight runs of duct upstream/downstream of measuring point.For eg the accuracy of pitot tube increases with air velocity.So air flow measured around 5m/s is likely to be more accurate than airflow measured at 2m/s.

RE: Air Flow Missing

Inlet is 1300. Sum of your numbers is 1200, but no number on the 6" hose.

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

Mass of air flow in= mass of air flow out; CFM in can be either <, = or > CFM out due to temperature change; energy in + internal energy change = energy out. I'll skip the entropy aspect of the system.[

RE: Air Flow Missing

Hello Everyone,

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.

Siril K Thodeti

RE: Air Flow Missing

My bad, I have checked the calculations. For my conditions of 27 Deg C, 50% RH the humidity ratio is 0.01KG/KGda. So the water is hardly 1.1% of the total weight.

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

How do you measure airflow? any measurement is a bit inaccurate. Especially all methods relying on pressure (i.e. pitot tubes). Between some leaks (you always have a few %), changes in density, and all the airflow station inaccuracy you see how it never will match.

RE: Air Flow Missing

We are measuring this using an anemometer.

RE: Air Flow Missing

First, what are the units in your equations?
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

The flow of the air path is as follows:

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

Brain numbing. You show 1300 cfm of outdoor air in. A mystery plenum relieving 800 cfm (500 cfm left), some unknown quantity out a “bypass damper” (500-? Is left). Then some unknown quantity going out a 150 mm flex hose, then 400 cfm serving your loads. So I guess I’d have to guess the 150 mm flex hose has 100 cfm.

After thinking that through, I’m slightly disappointed that I spent time looking at your post.

RE: Air Flow Missing

@ Chas,
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

Given that the unit says 700 cfm, I'm surprised you're getting 1300. The air flow through that filter will not be uniform I suspect. Measure air velocity at multiple locations to get an average speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air Flow Missing

Are you sure you are measuring correctly? You need to traverse properly. I always see balancers that barely take 3 measurements due to laziness and ignorance.
and you need to measure where you have stable flow, not right after obstructions etc.

RE: Air Flow Missing

If you are wanting to measure air flow inside the unit, you need to measure at a point where the air flow is the most uniform. That is NOT 200mm from the back face of the coils. Using an anemoter inside the unit, the most accurate place to measure is going to be right on the back face of the coils. hold the anemoter directly against the coil. Also take at minimum nine (9) velocity readings, more = better.

RE: Air Flow Missing

dbill73: minimum 25, see my document above

RE: Air Flow Missing

Let me think... ok I thought...

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