Room pressure calculation

[punk_HVAC]

Hi,

I'm working on a project of HVAC design for day care facility with approx. total area of 13,200 sq. ft. and average ceiling height of 11.5 ft.
As per heat loss/heat gain calculations, total air supply is 19,200 CFM which includes 5,140 CFM of outdoor air as per ASHRAE 62.1.
Total washroom and general exhaust is 2,725 CFM (to run continuously during operating hours).
I need help to determine the building pressure. What is the calculation to find out if the building is under positive or negative differential pressure with outside?

Thank you.

 

[goutam_freelance]

Best is to run a CFD analysis with all inlet flows and outlet flows through ducting, exhaust etc. The whole ducting and fan system should be input. This will give you pressure distribution within the room showing low/high pressure areas.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[johncooling]

Why trolling?

 

[johncooling]

Punk_hvac if I understood your question corectly: if, in a space, supply airflow [cfm] > return airflow [cfm] then you've got positive pressure and if supply airflow [cfm] < return airflow [cfm] then negative pressure. The calculation you can find on this forum.

 

[nuuvox000]

You can look in the ASHRAE fundamentals handbook for pressurization equations. It's the section on smoke control I believe. However, unless it's a critical space, it's best to keep the building at a slightly positive pressure so just make sure there is a "little" more air going into the space than being taken out. Almost no way to calculate it beforehand since building construction will vary in tightness.

 

[punk_HVAC]

johncooling,
Yes I understand your point of positive and negative pressure.
I need some formula to understand how much positive pressure building will be holding?

 

[johncooling]

I agree with nuuvox, you realistically can not accurately calculate that, you can just pump slightly more cfm into the building then you pull out. Here is a thread about the equation

https://www.eng-tips.com/viewthread.cfm?qid=31557

If you want to accurately balance differential pressure at a given point (in a space), you would have to use pressure sensors and automation.