How do you clean air ducts?

We use our truck mount vacuum machine and high pressure air blow gun to clean up the whole air conditioning system. We will first open some holes on the air ducts to hook up an 8 -10 inch diameter hose to the air conditioning system and connect this hose to our powerful truck mount vacuum machine (air volume 20000 cubic feet per minute). When the vacuum machine is turned on, it generates huge negative pressure. Contaminants in your air conditioning system would be extracted through the hose to the vacuum machine outside. At the meantime, we use a high pressure Tornado blower to clean up the air vents and floor/ceiling registers one by one and help draw the contaminants to the vacuum machine.