At an intake to a duct, Static pressure inside the duct is converted to Velocity pressure which makes the air move.

Air Flow (cfm) = K sqrt(SP) * Duct Area



Now all the air around your intake is sitting still and it has to be pulled into the intake and accelerated up to the velocity of the air travelling in the duct.



All the air being pulled into the inlet causes an effect called the vena contracta. Some of the air around the inlet isn't moving at all while some of the air is compressed and expands as the velocity pressure is converted to static pressure. The duct static pressure and entry losses are related to the size of the vena contracta.



The shape of the intake is very important in reducing the hood entry losses.



Sp = (1 + Fd)Vp

Vp= Sp/(1 +Fd)

Assuming a Fd of 0.96, Vp = Sp/1.96 = 0.5 *Sp

so the Velocity pressure is only 1/2 the Static pressure with the so effectively we are losing 1/2 our air flow at the entrance to the duct.

The way to combat Static Pressure losses at the intakes is to reduce the Velocity of the air at the intake by increasing the area of the intakes.

INTAKE SIZING

Getting the intakes right is the biggest challenge in setting up a growbox. The intake area needs to be at least equal to the exhaust area and up to 2x to reduce losses.

A = pi * (radius)^2 = (pi *diameter)^2/4

Areas for common duct and pipe sizes



Thanks to Freezerboy for the nice picture. This is a common mistake when using a number of small holes to equal one big one. The only solution is to calculate the area for the big hole and divide by the area of the smaller holes to get the number of small holes needed.



In this case one 9" hole needs 80 x 1" holes to be equivalent.

I'd like to point out that making one big hole is better than a bunch of little holes as each little hole has it's own Vena Contracta losses which do add up.

Darkroom Vents

I see lot's of guys using darkroom vents which are an excellent way to light trap the intake. From my observations, it appears that they are an obstruction to the airflow and seem to cut the air flow down by 1/2.

[TheShroomDude]

Great Thread Red!!!

[RudeLand]

Very useful info, thanks.

Great read red!

So much info I have to re-read it a few more times. Just my style! K+ when it allows me again. Don't let me forget! lol

[K.J]

Originally Posted by FreezerBoy

What stops the smell is Negative Pressure.

You can use intake fans but you'd better know what you're doing. You can't pump out more than you bring in but you CAN pump in more than you pump out. That's called positive pressure. Where pressure inside is greater than outside the cab. Positive pressure forces air out of the cab through any crack it can find thus eliminating the usefullness of filters.

Traditional passive intake uses 3 equal sized holes. One at the top for a powered exhaust and two passive intakes at the bottom. Now air rushes out as fast as it comes in. No buildup of pressure. Air rushing through the cab creates a pressure drop or, negative pressure. Pressure inside is less than outside the cab (it's why wings lift an airplane or a curveball curves) Because of negative pressure, all air in the vicinity will try to enter the cab. The only way out is through the filter.

That's a great summary Freezerboy. Thanks for writing it.

[nag-champa]

i cant afford for any smell to leak through any opening, so is it recommended that i run the exhaust 24/7? i have a room right next to the grow room, if i leave a window open in the next room, will the cracks in the door be enough for air exchange?

i dont know if its possible yet, but would it be safer if the room was sealed?

[nag-champa]

excuse my manors.
thanks for the help

Here's a good comparison between the different fan types based on Noise, Air Volume and Static Pressure.








Let's make some noise about noise. Our ears are some pretty special instruments that can pick up very faint sounds to all the way to a rock concert at over 100 db.

Noise level is computed using the formula 10Log(Sound Pressure Level). What's important to know is that every 3db difference in sound is double the noise. Every 10db difference is 10 times the noise.

A 100 db rock concert is 10000000000 times louder than a whisper.

Another measurement to know about is SONNES

1 Sonne = 40 db and 4 Sonnes = 60 db where each sonne is 5 db starting at 40 db.

Bathroom fans are measured in Sonnes to make it easy for the average shopper to compare noise levels. A high quality bathroom fan runs 1 sonne and most of the cheaper ones are 3 sonnes or about 55 db.

Computer fans operate a 20-30db and are 300x quieter than a bathroom fans at 55 db and 10,000x quieter than a centrifical fan at 60-70 db.

AIR VELOCITY

While we are talking about noise, a big source of noise in ventilation comes from the rushing sound of the air itself.

Air Flow is the velocity of the air in ft/min * the Area of the duct flow in square feet.

Cubic Feet per Minute = (ft * ft * ft) / min

I just helped out a friend that has a 440 cfm fan hooked up to a 4" duct and he can't stand the noise of it. My suggestion is to increase the duct size or connect the duct to a large cardboard box to drop the air speed down while maintaining the air flow rate.



This is the basic technology behind a muffler or silencer. Increase the area of the ducting to drop the airspeed. Add a grill or grid to even out the flow of air to reduce turbulence and the noise will be significantly reduced.

[messn'n'gommin']

Had to bump this! One of the most asked about problems here. You made it short and to the point and easy to understand and FreezerBoy's summation is as good as it gets! From an old LD, "high" school dropout, I vote sticky!!!!

Namaste, mess

MATCHING FANS AND FILTERS


Matching a fan and a filter can be pretty tricky. You'll see lot's of lively discussions on this topic on this website. Most people rely on a salesman at a growshop or buy a matched fan and filter combo from a reputable manufacturer. Let me tell you that the curves can be easily created and you can verify easily if a fan and filter combination will be suitable for your grow setup.

Each Fan Manufacturer will provide a Flow vs Pressure curve. You'll need to get this to create the Fan Curve.

CANFAN Fan Specifications





The other curve that you need is the Filter Performance Curve.

The filter performace curve follows the formula

CFM = K * SQRT(Pressure)

The Manufacturer will give you a CFM rating at a certain air pressure in the units inches of water. That's all you need to determine the K constant value for that filter.

K = CFM / SQRT(Pressure)

CANFAN Filter Performance Constants


CANFAN was nice to give us a few operating points for thier fan and filter combo's. That's all we need to get the K value for that filter.

Here's the CANFAN Filter Performance Curves



Now that we have our Fan Curve and our Filter Curves, let's put them on the same graph so we can see the operating points where the two curves cross.






Here's the operating points from where the fan curve crosses the filter curve.