Effects of CO2 in air on pH of ethylene glycol based Coolant
The chemistry of the coolant mixture circulating through Rockwell liquid cooled VFDs undergoes constant change during production times. The DI Cartridge maintains the coolant’s conductivity while other chemical parameters are in a constant state of flux.
Evaporation is common particularly in dry climates. When evaporation has occurred, this means the coolant’s freezing point has changed. In addition, the coolant mixture scrubs carbon dioxide from air lowering the coolant’s pH level. Evaporation is easily dealt with;changing pH because of CO2 adsorption is not.
When evaporation takes place, the reservoir can be topped up using Coolant -10C, -45C, or -55C (the three available mixtures) Rockwell Part #:80025-784-81, 80025-784-51, 80025-784-9. The product choice will depend on which freezing point is required and determined by the local climate.
The change in pH because of CO2 adsorption is slow but decreases steadily over time and cannot be adjusted. When the coolant mixture becomes more acidic, it also becomes very corrosive. This is why the VFDs must be flushed and refilled with new Coolant at least once every 1-2 years (min-max) to prevent possible drive down time.
To minimize the deleterious influence of the lowering of pH of the coolant solution, a unique inhibitor was formulated into all coolant mixtures. The corrosion inhibitor is important and recommended that only Rockwell approved Coolant -10C, -45C, or -55C be used to maintain integrity and prevent nullification of warranty. Not only does the inhibitor protect metal surfaces within the fluid loop, it also helps protect the metal housing the VFD is contained in should a leak occur.
Normal operating pH of Coolant -10C, -45C, and -55C is between 6 and 8. After six months of operation, the pH can drop as low as 5 and after one-two years, the pH of the Coolant can be as low as 3.5 – 4.5. This is very acidic and very corrosive to metal in particular. There is no way to buffer the pH back to 6 – 8 without drastically increasing the conductivity of the coolant. Buffering the pH would compromise the drives conductivity control and likely shut down the drive.
Dumping and refilling VFDs with new Coolant brings the pH back in the 6-8 range helping to protect pump seals, control valves, copper piping, rubber tubing, PVC plumbing and other materials the coolant comes in contact with.
The pH of the coolant is a difficult parameter to measure. The low conductivity of the coolant mixture means there are few ions to measure and all digital pH measurements are based on the free flow of ions through a pH probe. The pH tends to jump around and is not accurate. Additionally, it is not known with precision what pH value should be used to make a recommended dump and refill call. This is why the best marker is time. As an example, everyone is familiar with automobiles change-oil light when it blinks on after 5000 kms. This is the recommended path moving forward to indicate when the drive should be flushed and refilled with new Coolant -10C, -45C or -55C.