Coolant filters are used in diesel engines of trucks and machinery. When it comes to construction they resemble can spin-on filters. Those filters function in the engine cooling system as by-pass filters which only a small part of the engine cooling liquid goes through. The main function of these filters is liquid coolant filtration and efficient trapping of solid contaminants. The filter medium is made out of special filtration materials which stop dust, corrosion products and sludge, formed in the degradation process of substances protecting the cooling system from scale, corrosion and premature erosion caused by cavitation. Special cubes of active substances – corrosion inhibitors – are used in most of coolant filters. They are slowly released into the coolant and they replenish the level of components being degradated in the exploitation, maintaining a constant, high level of liquid thermal conductivity and its anticorrosion and anticavitation properties.