Static Pressure Question - Ducts and Carbon scrubbers

Hello friends,

I am in the process of designing a 400 watt grow cabinet, and I've got a couple questions.

I am planning to use an Elicent inline fan to suck through a DIY carbon filter, through a 6" (probably Hydrofarm) cool tube, and blow out of the cabinet. I found the spec table for the Elicent fans. It gives flow rate (in m3/hr) versus pressure (in pa). This information would be priceless if I only knew how much static pressure would be generated by my DIY carbon scrubber... The only static pressure numbers I have been able to find are on the Can Filter webpage. They list static pressures at "max CFM" for a couple of their filters (not all of them...). The two they list are 0.75" water for the Can 33, and something like 0.375" water for the Can 2600. My DIY carbon filter will have a bed thickness of 1", which I think is closer to the Can 2600 (but the 2600 is a 4" model...).

So... Could you guys throw out some guesses on what the static pressure through my DIY carbon scrubber will be? I am going to use a 6" inside diameter, 8" outside diameter, and about 15" long.

Nothin, huh? I'm not the type to just buy everything, plug it all in, and see if it works, which seems like the usual approach...

Ive been succesful with the couple of indoor grows i have done without going so far into detail. I'm not even sure what your asking and I'm sure other people are saying the same. Keep it simple just make sure you have adaquate air in and out with plenty of light and keep it cool. I dought anyone can come up with the numbers yor requesting but there are alot of people more advanced than me. I'm one of those trial and error guys. Sorry I couldnt help. :rasta:

Thanks for the reply. I went with the trial and error method for my first grow (CFLs in some stacked rubbermaid tubs). It worked out fine. When there wasn't enough airflow, i just cut more holes in the tub. The thing is, I am going to be building this cabinet as an eyepleasing piece of furniture. I'd rather not have yo make modifications after I build it initially. From looking around on here, I see others have cooled, ventilated and deodorized 400 watt cabinets with even just one 4" inline fan. I'll go for the 6" with nearly twice the cfm, just to be safe. A speed controller should help, too.

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Originally Posted by SpicySativa

Thanks for the reply. I went with the trial and error method for my first grow (CFLs in some stacked rubbermaid tubs). It worked out fine. When there wasn't enough airflow, i just cut more holes in the tub. The thing is, I am going to be building this cabinet as an eyepleasing piece of furniture. I'd rather not have yo make modifications after I build it initially. From looking around on here, I see others have cooled, ventilated and deodorized 400 watt cabinets with even just one 4" inline fan. I'll go for the 6" with nearly twice the cfm, just to be safe. A speed controller should help, too.

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Sounds like a solid plan. I would go with 4" inch fans though. 6" is a bit overkill, not to mention they are much louder than the their 4" inch counterparts. Do you plan on keeping the fan/s inside? Noise could be an issue if you do.

Spicy the static pressure will be dictated by the size of your carbon and how tightly it packs in your can. I would estimate 1/2" of pressure loss when clean and probably up to 1" when dirty.

Sounds kool. I have always thought about making a large cabinet like an intertainment center into a grow cabinet. I use a makeshift 4ml black plastic grow tent. I have a 6" 440cfm inline fan pulling through my 1000w hps fixture and exhausted out roof. The fan is kinda loud if you were going to put it inside. I have a 4" inline as well, not using it now but its much quieter. I have negative air in my enclosure and with no air coming out it controls smell accept at vent termination which is 20ft in the air. Seems to work very well. Good luck man sounds like a very kool idea please post some pics when you get it done would love to see it when it's finished.:rasta:

I have actually heard from several sources (local hydro shop dude included) that a 6" fan will be quieter than a 4" fan when running at the same CFM. So if a 4" would cut it running at full power, a 6" speed controlled down to the same CFM would be quieter while accomishing the same job.

The fan will be inside the cab. The cab will be in an unused bedroom, and I'm not breaking any laws out here in CA, so noise is not ALL that important anyway. My plan is to build a diy duct muffler if the noise bugs me.

I've seen a few posts showing various DIY duct mufflers, but here's my idea.... It's quite similar to how DIY carbon scrubbers are made, but instead of a cap on the end I'll have an opening. Picture about a 15" long section of 10" diameter steel duct. Inside that 10" duct, there will be a 6" diameter "roll" of hardware cloth (held in place just like with the diy carbon scrubber. The space between the mesh and the 10" duct will be filled with foam, batting, or something of that nature. A 10"x6" reducer will be one end, and a 10" flange will be on the other, and will be sealed up to the exhaust hole. I think it'll greatly reduce that whooshing sound air likes to make.

You are on the right track with the fans and noise reduction. A higher CFM fan dialed down will be quieter than lower CFM fan running at top speed. Also, noise can be greatly reduced simply by up sizing the ducts. So if it's a 6" duct, taper it up to 10" south of the fan and you will see considerable reduction in noise. I would do that before doing any experiments. That is essentially what a DIY duct muffler does anyway. See link in my signature for good info on fan sizing.

Without the specification on your filter and fan, the answer to your question is conjecture.

As a rule of thumb, as the static pressure increases, the horse power required to overcome the resistance is equal to the square of the increase. I.E: A 10% increase in static pressure requires 100% more horsepower to overcome it.

The best way that I have found without information, is to just measure static pressure, which is easy to do with a length of clear plastic tubing. If you drill a hole in the duct and attach a length of clear hose to it, it is possible to loop it around so that it makes a vertical â??Uâ?�, add water to the hose, and see how much higher it lifts the water in one leg vis a vis the other.

1â?� of water column is equal to .0361 pounds per cubic inch.

My 4â?� Vortec fan puts out less static pressure than my 6â?�s, which put out about 2.32 inches of water column.

Sound is also a function of air movement and blade tip speed. A 6â?� running at slow speed is significantly quieter than a 4â?� fan running at full speed.

As just another tongue in cheek number, the cfm is directly proportional to the fan rpm, but an increase in rpm requires the cube of the increase in horsepower. Better to get a big fan and slow it down than a little one and run it close to blade stall speed to achieve the required output.

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Originally Posted by Graywolf

The best way that I have found without information, is to just measure static pressure, which is easy to do with a length of clear plastic tubing. If you drill a hole in the duct and attach a length of clear hose to it, it is possible to loop it around so that it makes a vertical â??Uâ?�, add water to the hose, and see how much higher it lifts the water in one leg vis a vis the other.

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Agreed, the details of the fan involved are important. Wouldn't the diameter of the clear hose be a factor? If the diameter were a quarter inch it would take less pressure to lift the water than if the diameter were say 3". Regardless, my comments still stands. In general terms, a large fan dialed down will produce less noise than a small fan running at top speed. And again, increasing the diameter of the downstream duct will also greatly reduce noise.

The funny thing about pressure is that diameter doesn't matter. You could poke a pin hole in the bottom of the hoover dam, and the pressure would be the same as if you punched out a 3 foot hole. Of course more water would come out of the bigger hole, but the pressure is just dependent on the height of the water column above the hole, not how much water is actually above it. Hope that makes some kind of sense...

You're right, it would be easy to measure the static pressure at different flow rates once I have my system built. It wouldn't do me nearly as much good as knowing beforehand, so that I could design my cabinet around the ventilation system.

Sizing the fan for the grow box is pretty straight forward. Have a look a the fan link in my signature thread.

[quote=Graywolf]
the cfm is directly proportional to the fan rpm, but an increase in rpm requires the cube of the increase in horsepower.QUOTE]

Isn't horsepower increase the cube root of RPM2 minus RPM1?