And since your gonna be using a good fan anyways, you might as well run ducting through everything, and throw the filter in there too, using one fan to pull air through your filter, to cool the tube, and for exhaust. You can run one fan to do all of this.
Dude, you are seriously wrong in this instance. Your first assumption, that you need a a high volume fan to cool the light depending on the wattage is wrong. Now before you get your panties in a wad, let me explain. I have used floro's, t-5's, hid's and a $600.00 led light, I will not just be discussing theory. I recently installed a 1000watt hps light over 2 of my tables. I have a 200cfm axial fan that I bought at Menards for $28.00, and I ran the ducting so I can put another light in it, and made an extra 180 degree bend in it to take care of the light leak. I have more feet of ducting in this sytem than he could fit in his room. The 1000watt light replaced 2-4'-8bulb t-5's, 440 watts each, and the temperature of the room dropped 2 degrees. You do not need a high powered fan to air cool the light and they don't get much bigger than a 1000 watt light.
In order to use a carbon scrubber you will need a can fan. The reason is the airflow resistance of the carbon filter, Period. You can not add more airflow with out changing the size of the ducting, you would have to end with at least an 8" duct, should be 10" and every bend is a flow restriction. All of the joints would need to be 'Y' connections not 'T''s.
How much does a 8" or 10" can fan cost? Fan, ducting, fitting etc for my 6" line, including the two vents, about $125.00. He could do his with 4" aluminum for a lot less and he already has a bathroom exhaust fan for his 'fresh air' system. I just hope he remembers the oscillating fan too.
You are better off seperating the systems, that is science. If you start with 4" ducts and use the old pi-r-square rule you will need 3x4xpi to exhaust air from the room at the rated cfm with no flow restrictions. no obstructions ( like a carbon scrubber ). It will cost more and be less effective for him to do what you suggest. One of the things you want to keep in mind, you need to create a slight negative air pressure for a carbon scrubber to be effective if you are using passive air intake. If you don't the air in the room will replace itself, some will flow out the intake or any small hole you have without ever going thru the carbon filter.
If you look at cutaway views of the 3 types of inline fans, you will see that a "can fan" is just that, a fan in a can, the circumference of the fan is larger than the ends. As an example a 4" can fan might be about 6" around. You could move just as much air with a 6" duct and an axial fan. Creating a flow restriction to have positive air pull?? In addition to that this places your fan in the warm air stream from the light, not the right place for it, that is a whole-nother story. VV
