Coolant compatibility

Definition and importance 

Evaluating the compatibility of coolants is looking into the performance of coolant mixtures and how the coolant properties are influenced when being mixed. The performance of the separate coolants is compared with the performance of the coolant mixtures. The evaluations are mainly part of an OEM coolant approval process where new coolants require being compatible with the approved in service coolants. The studies allow the OEM to provide recommendations to the field and reduces potential issues with the cooling system. During compatibility studies the effects on the main functions of a coolant (freezing, boiling protection, heat transfer properties, materials protection) are evaluated. The base fluid determines the first three; the protection of the materials is however determined by the additive package. When mixing coolants both an evaluation of the influence on performance and environmental/toxicological aspect are important.

Influence of the base fluid 

For coolants with different base fluids, the effects on freezing, boiling protection and heat transfer properties are evaluated. e.g. when mixing a monoethylene glycol (MEG) and monopropylene glycol (MPG)-based coolant, the difference in viscosity between both coolants and influence on heat transfer properties should be taken into consideration. The MPG coolant with higher viscosity reduces the fluidity and lowers the pump ability at lower temperatures, therefore having a negative effect on the heat transfer efficiency during start up. 

Influence of the additives 

The additives present in a coolant are a well-balanced mixture of different components (such as corrosion inhibitors, pH buffers, stabilizers, anti-oxidants, antifoams, dyes). Mixing of coolants can disrupt this balance. By changes in the additive concentrations sometimes resulting in instability, pH changes and deposit formation. As a result the coolant mixture can show a loss in corrosion protection performance and a reduction in heat transfer performance. Knowing that the corrosion protection properties of different corrosion inhibitor technologies are very much concentration dependent one can derive that an unbalanced additive package can be formed either by mixing coolants of different additive technologies (e.g. OAT, lobrid or hybrid Si-OAT, P-OAT, traditional) however also by mixing similar additive technologies. The difference in protection mechanism and depletion rates of certain corrosion inhibitors explains why mixing of coolants can influence the performance and life of the separate coolants. Although present at low concentrations the dyes present in the coolants need to be compatible in respect to obtain an acceptable color and fluid stability. 

How reduce issues caused by coolant mixing in the field? 

One of the methods advised to avoid mixing of coolants in the field is to fully drain a coolant from the cooling system and to rinse the system several times with inhibited water prior introduction of the new coolant. However often coolants get contaminated anyhow. Therefore, OEM’s include coolant compatibility tests to the coolant approval program to provide adequate recommendations. As mixing of coolants can result in an increased risk of additive instability and a reduction in corrosion protection performance, the advanced test program most often involves corrosion tests such as glassware corrosion tests (ASTM D1384 type), hot surface corrosion tests (ASTM D4340 type), dynamic heat transfer testing and fleet tests. Earlier studies have indicated that evaluations under static and dynamic conditions can result in opposite conclusions (SAE940769/SAE 2000-01-1977). A combination of test conditions is therefore preferred and can provide broader insights. Because the recommendations from the OEMs are based on thoroughly testing it is preferred to follow these guidelines rather than depending on a simple miscibility test at room temperature.