Trying to close out a project where the GC's HVAC subcontractor has meet the 0.1 inches of water between interior walls of (2) MCC rooms and (1) Control room. I've read threads on this site discussing ventilation but nothing regarding the outward velocity of 60 ft/min. Installed in each of these rooms is an air handling unit for heat and cool with a fresh air inlet to each unit. I may be knocking at the wrong house but I thought I would pose the following question anyway. If anyone of these units is at it's peak performance, will it be able to supply the required outward velocity of 60 ft/min without a stand alone pressurization system and all the doors are open? This is neglecting leakage through wall penetrations, gaps in conduit sealant, etc..
NFPA 496 Pressurization of Electrical rooms for Class II, Div II, Grp G
- Thread starter Krub
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petersonra
Senior Member
- Location
- Northern illinois
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- engineer
I think your questions are best directed to the engineer that designed the system and put his seal on the drawings.
- Location
- Mission Viejo, CA
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- Professional Electrical Engineer
Petersona is right, this is fundamentally an HVAC issue and the airflow through the openings would typically be calculated rather than measured. Instrumentation, such as it is, would be applied simply for the purpose of determining whether or not the airflow system was active.
My curiosity would lead me to ask what you were attempting to have the pressurizing system to do. Are you simply trying to keep dust out or somehow evacuate it?
My curiosity would lead me to ask what you were attempting to have the pressurizing system to do. Are you simply trying to keep dust out or somehow evacuate it?
It’s a simple problem but you don’t have the information. What you have most likely is just a manometer reading on the room which is static pressure. Two ways to get it.
Add up the doorway sizes in square feet multiplied by your velocity gives you ACFM (cubic feet per minute). Then the fan curves for the HVAC give you the maximum ACFM the fans put out at various pressures. You could try to find the pressure at the fan with the doors open but that’s way too complicated. Doors open is close to zero pressure. We don’t need to know the pressure with the doors closed...that is not the condition we are interested in. So based on almost no restrictions on the fans we can estimate maximum output (flow). Whether this is true or not depends on restrictions in the HVAC. Usually engineers would size for 2-3 times bigger than needed.
Also it’s trivially simple to measure. Buy a pitot tube. You obviously have a manometer. Standing outside a door aim the pitot tube into the wind and measure the pressure across both ports. Then remove the hose from the center piper and hold it so it sees outside air (outside doorway) so you measure static pressure. There is a simple calculation that gives you velocity (feet per minute) from the measurements. Now measure across the white doorway every foot and average the readings because it will be higher flow in the center and lower at the edges. Static pressure is fixed. You just need dynamic so readings can go pretty quick.
Be careful on designing these. At around 1/8” of pressure there is so much pressure on a door that either you can’t close or can’t open it. Also above 1” is going to make everyone’s ears pop.
What you are doing by the way is not the way it is normally done. Purged and pressurized systems are normally done in small panels, not the entire room. You just purge the electrical equipment, not everything. That way all you need is a small blower or even a compressed air line. Also I haven’t ever heard of doing the calculations with the door open. It is usually assumed that the performance requirement is with the panel closed and that occasional access is nit the normal condition...but then again the occupied space isn’t purged. Also I don’t even think it’s going to work well. Every time the wind blows the building is going to pressurize on one side to the point where the doors will be stuck shut.
Add up the doorway sizes in square feet multiplied by your velocity gives you ACFM (cubic feet per minute). Then the fan curves for the HVAC give you the maximum ACFM the fans put out at various pressures. You could try to find the pressure at the fan with the doors open but that’s way too complicated. Doors open is close to zero pressure. We don’t need to know the pressure with the doors closed...that is not the condition we are interested in. So based on almost no restrictions on the fans we can estimate maximum output (flow). Whether this is true or not depends on restrictions in the HVAC. Usually engineers would size for 2-3 times bigger than needed.
Also it’s trivially simple to measure. Buy a pitot tube. You obviously have a manometer. Standing outside a door aim the pitot tube into the wind and measure the pressure across both ports. Then remove the hose from the center piper and hold it so it sees outside air (outside doorway) so you measure static pressure. There is a simple calculation that gives you velocity (feet per minute) from the measurements. Now measure across the white doorway every foot and average the readings because it will be higher flow in the center and lower at the edges. Static pressure is fixed. You just need dynamic so readings can go pretty quick.
Be careful on designing these. At around 1/8” of pressure there is so much pressure on a door that either you can’t close or can’t open it. Also above 1” is going to make everyone’s ears pop.
What you are doing by the way is not the way it is normally done. Purged and pressurized systems are normally done in small panels, not the entire room. You just purge the electrical equipment, not everything. That way all you need is a small blower or even a compressed air line. Also I haven’t ever heard of doing the calculations with the door open. It is usually assumed that the performance requirement is with the panel closed and that occasional access is nit the normal condition...but then again the occupied space isn’t purged. Also I don’t even think it’s going to work well. Every time the wind blows the building is going to pressurize on one side to the point where the doors will be stuck shut.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I have been in more than one control room that was pressurized. They all had an entry way with a door at both ends. The idea being I gather that one door would always be closed.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
My next question, would be what was the basis of classifying the outside location in the first place; i.e., what was the Standard used to determine the electrical area classification?
StarCat
Industrial Engineering Tech
- Location
- Moab, UT USA
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- Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
FPM can be read directly with the correct type of Anemometer.
Test and Balance is the branch of HVAC that does this. There are instruments that have a sort of square grid sensor area that can read what is called traverse and get CFM and FPM down into very low ranges. Generally if there is some type of minimum flow required, the system is designed as variable air volume and can modulate with some head room to the requirement under different operating conditions.
Anemometers for Air Flow Measurement from Cole-Parmer
Airflow measurement is a critical component when assessing an HVAC system. It helps ensure balance, energy efficiency and cost effectiveness of the system. Read more in this informative article from Cole-Parmer.
www.coleparmer.com
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