Depressure of a room adjacent to other rooms in depressure 1

[Isaarri]

Hello to all,

I am a mechanical engineer entering the HVAC world and have stroke upon a case that neither my colleagues nor I can give a clear answer to.

We are designing a building that is compounded by zones with different depressure levels, the building is surrounded by air at atmospheric pressure. We are trying to achieve the depressures in the rooms by maintaining the supply airflow constant and varying the extraction airflow. The depression is achieved by extracting more air than is supplied.

The extraction airflow to achieve the depression is calculated as the sum of the supply air + infiltrations + leakages (Qext = Qsupply + Qinf + Qleak).

The leakages and infiltrations considered are the ones which take place through the doors.

The image attached shows a representation of the distribution of the rooms.

The problem arrives when the extraction airflow is calculated (formula in red) after de calculation of the infiltrations/leakages. It results that in some of the areas, especially the room shaded in grey (room 1), the leakages are greater than the infiltrations and, therefore, the extraction airflow is lower than the supply airflow, creating a contradiction as it would not create a depression.


This can not be like this, but after checking all the calculations it seems that they are right and that the problem is in the concept or the configuration.


What do you think? is there another method to calculate extraction airflow in depressurised areas adjacent to rooms more depressurised?


Thank you in advance!

 

[MintJulep]

Flow is not pressure.

Rather, flow is driven by pressure.

 

[IRstuff]

the leakages are greater than the infiltrations and, therefore, the extraction airflow is lower than the supply airflow, creating a contradiction as it would not create a depression.

Your statement does not seem correct. If the total inflow is higher than the total outflow, you must have a negative differential, since flow goes from higher pressure to lower pressure.

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[Isaarri]

My apologies, I am not a native english speaker. Yes, the final intention for controlling pressure values is to control airflow.

 

[Isaarri]

Could you elaborate on the answer, please?

Thank you in advance.

 

[MintJulep]

See this thread.

thread403-43928:

 

[IRstuff]

Could you elaborate on the answer, please?

Was this not part of your education? Everything that can flow, flows from higher pressure/concentration to lower pressure/concentration. If there's inflow into Rm 1, then its internal pressure must be lower than the outside pressure.

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[nuuvox000]

Read through chapter 8 of the ASHRAE cleanroom design guide and that should help you (attached).

 

[Isaarri]

Yes, I have studied a lot of fluid dynamics. My question was because I don't understand your answer. If the inflow is higher than the outflow, you have more air in the room and therefore there is a positive differential. At least as I understand it.

 

[IRstuff]

If the inflow is higher than the outflow, you have more air in the room and therefore there is a positive differential. At least as I understand it.

No, consider pumping air into a tire that's already at 35 psi; you need to have the pump have a higher pressure to get more air in. Conversely, the higher pressure within the tire causes the air inside to want to mingle with the lower pressure air outside.


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[FMJalink]

Hi Isaari,

If I do understand you correctly, the design balance calculations show that with room 1 there is a lot of leakage to rooms 2 and 3. In those rooms 2 and 3 that large leakage flow should be removed to stay on -25 Pa. If the balance equations for rooms 2 and 3 have extraction flows taking this into account, the system should designed correctly.

Relatively large leakage flows will make it difficult to control the pressure in the rooms, specially if these flows are of dynamic origin as opening doors.

Good luck with solving your problem.

 

[MedicineEng]

Does Room 1 have infiltration (air coming in from outsite) from other places that you didn't account for?
For instance a cable tray penetration or a misplaced ceiling tile or even a lighting box.
All of the above can be a source of incoming air into the room.

Here in our property we had an issue with one of our tehcnical rooms that was at negative pressure without an reason as there was only a FCU in full recirculation mode (no exhaust, no fresh air).
After some head scratching we found that the cable trays that were going to this room pass through an extraction tunnel next to this technical room and because these were not air tight,basically the extraction tunnel was pulling air from the room through the cable tray. so our solution was to stuff the cable tray with expanding foam to reduce the negative pressure. Not entirely successful, but much better than what we had first place.

Please let us know if you manage to find what was the cause for the lack of negative pressure on your room.