into the hammer mill 30 through recycled particulate matter inlet 62 the carrier gas 64 generally has no significant particulate matter within it however the presence of particulate matter within the carrier gas 64 is not troublesome unless it is larger than the holes present in the screen 42 the carrier gas 64 enters the hammer mill 30 through the holes in the screen forcing product inward against the normal centrifugal force of the hammer mill 30 and out through outlet 40 a and through screen 42 and through outlet 40 b the velocity of the carrier gas 64 can be regulated by the number and size of the holes in screen 42 and the volume of carrier gas vacuumed through outlet 40 a the vacuum at outlet 40 a is regulated by the revolutions per minute rpm of the fan motor 78 the greater the flow of carrier gas 64 the greater the velocity of the carrier gas 64 through the screen 42 in hammer mill 30 if the volume of carrier gas 64 remains constant the larger the holes and or the increase in number of holes in screen 42 will result in a lower velocity of carrier gas 64 through the hammer mill 30 the more volume.