HVAC Deisgn Extract airflow

[kodzak1981]

Hi,

I am beginner at Ventilation and air conditioning and I have some beginners question. I am interested in how is the amount of extraction air determined?

In the books I read, it says "amount of extract air is usually the same as supply air"... However, I am working on some projects and I can see that amount of extract air is lower than supply air. How I understand, this is due to keep room over pressured. However nobody can give me an answer how is the extract airflow determined. Is it lower for 10% of supply, or 20...or there are some other guidance.

I appreciate your answer in advance.

Regards.

 

[Drazen]

it is common practice that over- or under-pressurization is achieved by specifying supply air to have 5-10% larger or smaller airflow than exhaust air.

 

[EnergyProfessional]

Unless you have a well defined room with known infiltration, a rule of thumb as mentioned by Drazen probably is your best bet.

For each space the code (or ASHRAE or whatever your guideline is) will tell you either OA supply, or exhaust rates. This is the one you need to meet. Then you just play around with the other one to either pressurize, or depressurize.

Ultimately inflow and outflow will be the same. the difference is made up by infiltration from high pressure to low pressure room. through doors, cracks etc. air is like energy, you can't jsut create it from nothing, nor can you make it disappear. Keep in mind the flow readings are not that accurate. with different temperature, pressure, and device inaccuracy a 50 cfm flow may as well be 40 or 60 cfm. and using volume is even more misleading. Technically we should talk about mass flow rates.

 

[kodzak1981]

Hi,

thanks for your reply on this.

Regards

 

[NeoMeche]

Further to the answers given above, refer to ASHRAE 62.1 or your local code.

http://www.eng-tips.com/viewthread.cfm?qid=339769

At my company, we typically use Drazens approach and positively pressurize spaces to 5% or so to minimize infiltration. We only do this for comfort cooling typically though, Labs and medical work are a whole different ballgame. With these applications you will need to worry about min air changes, min outside air changes, pressurization, re-circulation of air, etc.

An air balance can become a bit tricky once your start thinking about supply, return, exhaust, relief, ventilation, and infiltration.