One place I worked had a scrubber about 6' in dia x 15' tall - 424ft3 (3180 gallons equivalent = big!). It was a typical setup with a NaOH solution reservoir at the bottom and this solution was pumped to spray nozzles at the top. Above the reservoir was a perforated shelf that contained a layer of packing (saddles) about a foot, or more, thick (in Hong Kong, we used reject plastic flowers from a manufacturer). The fumes entered between the NaOH reservoir and the packing and the scrubbed air exited through the blower mounted at the top. Very similar to what is pictured in the link given by Acid_Bath76, except, there was no water overflow. There was a drum of 50% caustic soda sitting on the ground next to the scrubber and a pH probe constantly measured the pH of the NaOH solution in the scrubber. When the pH dropped to a certain level, a small pump automatically added 50% caustic from the drum until the pH was raised to the proper level.
http://www.engineeringtoolbox.com/scrubbers-air-washers-d_139.html
The variable blower was 2600 cfm max. At full speed, at a 10 second retention time, this would require a 2600/6 (because 10 sec. is 1/6 of a minute) = 433 ft3 scrubber volume (pretty close to the actual scrubber volume given above). They had several open fume hoods and NOx was generated in each one. Theoretically, the scrubber should have handled the fumes when the blower was on max, but it didn't. They had to turn it down to about 1800 to prevent a plume of very visible red fumes from exiting from the top - that's the way they adjusted it - visually. So, in this case, it required about a 2600/1800 x 10 = 14.4 second retention time to scrub the NOx.
Another place I worked used 12, 4-liter beakers for dissolving karat gold. On top of each beaker was a loose fitting plastic lid with a 1" plastic flex hose coming off the top. There was a hole that could be opened and closed on the lids to add acids. All the hoses were connected to an exhaust manifold. They also had a 50 gal Pfaudler kettle, with a lid and about a 3" dia exhaust pipe connected to the manifold. They had no open fume hoods where NOx was generated. The scrubber had a 275 cfm blower and was about 4' dia x 12' high (150 ft3) and was built the same as the one above. The scrubber volume was about 3 times what was theoretically needed - for both overkill and to allow for a little expansion.
The NOx is a lot more difficult (and slower) to scrub than many other types of acid fumes. The reason is that the NO produced by the dissolving reaction must be first oxidized to NO2 before it will scrub. This requires time and an oxidizer (oxygen, e.g.). Any unoxidized NO will go through the system and, when it's exhausted and it hits the air, it will be oxidized to NO2 = red fumes. Some people use a 2 stage scrubber, with H2O2 in the first stage and NaOH in the second. The peroxide oxidizes the NO to NO2 and this is scrubbed fairly easily by the NaOH. In this 2 stage system, a smaller total scrubber volume is needed.
As 4metals intimated, the way around all this is to use closed systems rather than wide open fume hoods. That way, a much, much smaller blower can be used and this would result in a much, much smaller scrubber being required. Less air opening (in) = less cfm needed (out) = less scrubber volume needed.
It's all math, folks!
Please correct me if any of my assumptions are wrong. One thing I've learned is that these systems are anything but intuitive. I've seen a lot of money wasted by intelligent people building exhaust and scrubber systems, intuitively, without doing the proper homework, that flat-out didn't work. I must admit that I was involved in a couple of these in the old days.
Chris
