I hope people can answer various parts of this question:
a) Can you help me define “ventilation” concerning commercial buildings? My reference book will say it is amounts of Outside Air (OA) needed to dilute odors in buildings and keep the air fresh…as set by Code and ASHRAE stds. OK, but then they also say ventilation combats infiltration by maintaining air pressure just high enough to offset infiltration. BUT THEN! The examples only show how cfm is used to combat infiltration…isn’t there a pressure associated with this? (Let’s assume a one-room building) So, I’m asking about the air pressure in that room. How many inches of water will it be above atmospheric?
b) Also, a simple example they give seems illogical. A room with an exhaust fan has an infiltration rate of 500 cfm and the fan exhausts at 200 cfm. They claim you MUST provide 700 cfm ventilation air to exfiltrate 500 cfm and exhaust 200 cfm. But, cant’ we say the 500 cfm infiltrated will just be 500 cfm exfiltrated? To me, the example is saying 500 cfm infiltrated + 700 cfm ventilation air from a blower WOULD BE 1200 cfm total INTO the room! Is there some unspoken assumption that 500 cfm is being returned, or something?
c) When supplying air to a room, do you add the cfm req’d for cooling that room PLUS the ventilation requirement to find the total Supply Air (SA) req’d?
Seeing as this is your second HVAC question in as many days, I can’t help but wonder why exactly you need the answers to these questions? Are you taking a class where you are unable to get good answers from you instructor? I am in the HVAC design industry and can answer a few of the questions and I know there are others on the boards who can give better answers than me, but I am reluctant to answer homework assignments for you.
How can I prove to you this is NOT a homework assignment? I’m working on something that needs some quick and dirty answers to give me some food for thought. I found one HVAC board, but the activity is too low to get answers in real time. I’m just trying to read up on the subject. If it eases your conscience, tell me of some other HVAC board with activity…
What else can I tell you? I have my ME degree, and over 14 yrs, I’ve worked in a smorgasborg of ME applications making me a Jinx of all trades, and a master of none. I’d ask my reference book, but it won’t answer me.
If you could please just explain what sense it makes to require 700 cfm in ventilation when you already got 500 cfm infiltrating into the area? I know the right answer is that ventilation = infiltration + exhaust (in this example), by why? Why should the blower have to provide an additional 500 cfm, when mother nature’s already giving me the 500 cfm for free? If anything, the blower should provide the 200 cfm that will be exhausted. I WAG the simple answer is of a practical nature - to maintain so much conditioned air? Otherwise, 500 cfm of unconditioned air might make the room uncomfortable? Is the reasoning just as simple as that? - Jinx
Have you tried Eng-Tips Forums? All sorts of engineering forums, not just HVAC.
No. Generally the supply air cfm includes the OA cfm. For instance, if the room required 2 tons of cooling and your coil was specified for 500 cfm/ton, the total amount of air into the room would be 1000 cfm. Now, if the OA requirements of the space is 10% outside air, then you would need to provide 100 cfm of outside air. The return air duct would provide 900 cfm of return air from the room and the outside air duct would provide 100 cfm from outside. These would be mixed in the mixing box secion of the air handler and the entire 1000 cfm would be blown across the coil and back into the room. Either an exhaust fan or a relief vent would be needed to exhaust 100 cfm out of the space if you didn’t want to pressurize the room.
It seems to me that 500 cfm infiltration and 200 cfm conditioned air into a building space is way out of reason. Such a space has by far too much leakage and if it comes in one side it will also go (leak) out the other.
I worked at a large facility where there were two large HVAC systems supplied conditioned air to laboratories and offices.
One system supplied a ‘front’ office building with four office/lab wings. Sufficient air was filtered, heated/cooled, and delivered by FD fans to the inhabited parts of the building. Attic equipment and storage areas were supplied separately. Exhaust was accomplished by exhausting office air to the hallway and constant ID fans pulling air into the labs and exhausting thru hi-eff. filters.
Second system supplied both hot and cold air streams to mixer boxes to supply offices. Room air exhausted to hallways, and returned to HVAC center by ID fans where it was filtered and mixed with fresh air to be heated/cooled for the FD supply fans.
Systems were kept in balance as opening a outside door did not result in a signifigant flow of air. Air infiltartion was insigifigant, probably a fraction of a percent of the volumes handled.
QUESTIONS: Whence the source of the 500 CFM infiltration and 299 CFM HVAC?
What size space under consideration?
What type construction?
