Filter drier
NOTICE
Danger of blocked and clogged control valves due to dry ice!
Danger of blocked and clogged control valves due to dry ice!
The solubility of water in gaseous R744 is much lower than in other refrigerants. Therefore, especially in low temperature applications, even a relatively small amount of moisture can freeze out of the refrigerant.
Install a generously sized filter drier and a sight glass with a moisture indicator for R744!
- Use pure molecular sieve inserts. An additional filter with mesh size ≤ 25 µm is advantageous.
- Use R744 with low water content (< 5 ppm) and sight glasses with a moisture indicator (Refrigerant requirements and filling process).
- It is essential to follow the evacuation instructions (Checklist for commissioning, point 4.)!
- Also: Observe the maximum allowable pressures of the filter drier and ask the manufacturer about suitability for use with added oils for wear protection!
Hydrate formation
- The solubility of water in gaseous R744 is lower than in other refrigerants and depends on temperature and pressure. For:
- Gaseous R744: Water solubility decreases with increasing pressure and constant temperature.
- Liquid R744: Water solubility increases with increasing pressure and constant temperature.
- Gaseous and liquid R744: Water solubility decreases with decreasing temperature.
- The phase characteristic of the mixture of water and R744 shows in the p-t-diagram a large area in which hydrate can form (see figure below, blue area). Hydrate is corrosive, can dissolve metals and cannot be adsorbed by the molecular sieves on the filter driers. Therefore, the filter driers should not be installed in plant areas where hydrate could be present.
- To avoid not only the formation of hydrate, but also the formation of acid (hydrolysis), ice and to prevent corrosion, only generously sized and suitable filter driers and filter drier inserts should be used (see also safety note above).
Conclusion:
The use and effectiveness of filter driers in the system are limited to the suction side of the medium and low temperature compressor stages.
Blue: Hydrate
Yellow: R744 boiling point curve