Donaldson Coolant Filters: Cooling systems are typically partial flow (by-pass) filters, with less than 10% of coolant flowing through the filter. We offer a range of Donaldson filters designed to work in a variety of operating environments and maintenance schedules. Use of the correct filter is important to maintain the proper balance to prevent over-concentration (silicate dropout) or under-concentration, which leads to corrosion, liner pitting and other system problems.