corbin5754
Well-Known Member
ok if I have a 4" 260cfm fan exhaust setup would a 8" passive intake hole be good
Passive and exhaust question
- Thread starter corbin5754
- Start date
NinjaShamen
Well-Known Member
Ideally you'd want your exhaust hole to be bigger than your intake. Exhaust up high at one end of your area, intake down low on opposite side. And want your fan to pull the walls in a little, negative pressure.
|3E@Losetgreenz
Member
Common misunderstanding-->
[Wiki]
Standard cubic feet per minute (SCFM). However, this usually is not the case as the most important change between these two definitions is the pressure. To move air, either a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of air or other gas, it gets smaller. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume.
The term cubic feet per minute (CFM) is ambiguous when it comes to the mass of gas that passes through a certain point because gas is compressible. If the pressure is doubled, then, for an ideal gas, the mass of the gas that passes by will also be double for the same rate of flow in cubic feet per minute. For instance, a centrifugal fan is a constant CFM device or a constant volume device, meaning that, at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though mass flow rate through the fan is not.
[Wiki]
Standard cubic feet per minute (SCFM). However, this usually is not the case as the most important change between these two definitions is the pressure. To move air, either a positive pressure or a vacuum must be created. When positive pressure is applied to a standard cubic foot of air or other gas, it gets smaller. When a vacuum is applied to a standard cubic foot of gas, it expands. The volume of gas after it is pressurized or rarefied is referred to as its "actual" volume.
The term cubic feet per minute (CFM) is ambiguous when it comes to the mass of gas that passes through a certain point because gas is compressible. If the pressure is doubled, then, for an ideal gas, the mass of the gas that passes by will also be double for the same rate of flow in cubic feet per minute. For instance, a centrifugal fan is a constant CFM device or a constant volume device, meaning that, at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though mass flow rate through the fan is not.
AKgrower29
Well-Known Member
Hmm, I can appreciate the above information but am not sure what relevance it has to the OPs question. As far as Passive intakes your 8" will be plenty for your 4" exhaust. I in fact run a 6" exhaust at 435cfm and use a passive intake equivalent to a 8" and have no issues.
coreywebster
Well-Known Member
Yeah 8" passive is fine for intake with 4" active extraction. Just fine.
|3E@Losetgreenz
Member
Yes all thay is sound and will work well. But the principal of air exchange(s) for ventilation is to move air in volume or mass, which is measured in FPM. CFM is used to measure the rate of air exchanged which is dependent upon duct size and area as well as static pressure.
(Square ft) duct sizing formula= Cfm divided by FPM
(Square ft) duct sizing formula= Cfm divided by FPM
AKgrower29
Well-Known Member
Stating the breakdown of air movement and calculations of how it is scientifically directed means very little to this post if you can't relate it real word terms and answer the OP's questions. I'm not here to question your knowledge but just stating something with out applying it to help the OP is confusing and unproductive to the thread.
So please inform us how examining air movement from a different perspective will help the OP solve the question of how big of a passive intake he needs for his room to ventilate properly.
So please inform us how examining air movement from a different perspective will help the OP solve the question of how big of a passive intake he needs for his room to ventilate properly.
SouthCross
Well-Known Member
Go big on the passive. You can always block it partially off or open it up. Give yourself room for dialing in the space environment. Figure for an air filter too.
|3E@Losetgreenz
Member
If u cant comprehend a simple formula to calculate intake duct sizing, you may want to quite inquiring information on a forum that uses technical terms and principles.
Hire a professional technician that understands the scope of "using a calculator and 6th grade science" to cut a small opening the right size.
AKgrower29
Well-Known Member
Haha ok, I comprehend just fine all I was saying is that what you posted while informational didn't even remotely answer the OP's question and that if your goal is to inform us how to calculate air movement so we can actually size and active exhaust to a passive intake to better explain yourself instead of just going to Wikipedia and copy/paste information.
