Modifications to feed milling operations which satisfy the following conditions of this section are permitted by rule.(1) In conjunction with the installation of additional grain or feed storage silos, including bins used for loading out finished feed, all materials shall be transported in a closed conveying system when handled mechanically or pneumatically. Exhaust air to the atmosphere shall be vented through a fabric filter having a maximum filtering velocity of 4.0 feet per minute (ft/min) with mechanical cleaning or 7.0 ft/min with automatic air cleaning.(2) In conjunction with the installation of a pellet mill/pellet cooler system, the air from the pellet cooler shall be vented through a high efficiency cyclone collector which has a cone length at least twice the diameter of the cyclone.