Compressing Helium for Cryogenic Applications
To utilize helium gas for refrigeration, impurities must be filtered out first. To filter helium gas, the same methods that are used to filter compressed air can be used to filter oil out of helium. Before using equipment to complete this process, modifications need to be made to make it effective for use with helium.
The first of two main differences in filtration of helium versus air deal with acceptable levels of oil. Refrigeration systems that use helium have a much more stringent purity requirement. Air filtration systems allow for up to 1 ppm of oil; 0.1 ppm is the standard for cryogenic applications involving helium. This amount is the maximum level permitted with preferred concentrations at a much lower level.
The second difference of note is that air from a compressor carries less oil content than helium. Helium that has been run through a compressor carries more than seven times the weight of oil per unit than compressed air. Helium is also compressed to a higher psi than air, contributing to the higher levels of oil found.
In order to purify helium, filters with coalescing capabilities must be used. Since virtually all oil found in helium are in a liquid state, these filters are able to remove nearly 100% of the oil. Filters must have a coalescing element, generally composed of borosilicate glass fibers. These components collect the oil by catching the liquid droplets, merging them until they are too heavy to remain in the helium gas. Commercial filters offer highly efficient means of purifying helium.
One problem that can arise stems from purified gas coming back into contact with the liquid oil. To prevent this from happening, many manufacturers further enhance their filters with polyurethane foam or coarse glass fiber in an outer layer. Since the demands for helium purification are much higher than regular air, multiple stages of filtration may be required.
Higher efficiency filters actually drain at a slower rate than lower efficiency filters, especially when dealing with liquid oil from helium gas. This is important to note because if the drain backs up, the excess oil can re-entrain into the gaseous helium. In this case, a lower efficiency filter may be the better option. To achieve the best purification, filtration needs to be done in three stages.