Compression of helium by an oil-sealed rotary screw compressor releases as much as 4,000 parts per million by weight (ppm) of liquid and vapor oil impurities into the helium gas. Any helium gas used for cryogenic applications must contain less than 0.1 ppm of impurities, which means the gas must be filtered first. In general, the methods used for filtering oil from compressed air can also be used for removing oil from helium. However, the oil-separation equipment for compressed air must be modified significantly to be effective in removing oil from helium.
The two main differences between air filtration and helium filtration are:
The purity requirements for helium refrigeration systems are far more stringent than those for normal air filtration. Filtration of oil in air to 1 ppm is typically satisfactory; for cryogenic helium applications, 0.1 ppm is often considered the maximum tolerable level, with even lower concentrations preferred. The output oil content in helium from a rotary screw compressor is much higher than the output oil content in air from the same compressor. This is in part due to the fact that the compressor is a volume-handling device—the weight of oil per unit weight of helium is 7.25 times more than the weight of oil per unit weight of air. In addition, helium is commonly compressed to 200 psig or higher, while compressed air is typically about 125 psig or less. The higher output pressure results in higher volumetric oil loading.
Removing liquid oil from helium
At least 99.99 percent of the oil in helium occurs as liquid droplets. Coalescers can remove virtually all of the liquid oil contamination. All commercial filters for high-efficiency separation of liquid oil from gas utilize continuous coalescing filters. The typical coalescing filter element is composed of a thin mat of borosilicate glass fibers, formed into a cylinder and structurally bonded by resin or by internal and external perforated supports. Separating compressor oil from helium – Parker HannifinWith proper design, this type of filter element can readily capture 99.99 percent or higher for 0.3 to 0.6 micron oil particles and droplets. If the filtered contaminant is a solid particle, it remains attached to the surface of the filter fiber, held there by Vander Waals forces. However, liquid droplets captured on the fibers can migrate down the length of a fiber to crossover points with other fibers. Here the liquid droplets grow into large drops, which eventually are forced through the depth of the filter to the downstream surface by the flow of gas through the filter. By the time they reach the downstream surface, the droplets have become so large that they are too heavy to be re-suspended in the gas stream, and instead drain down the vertical filter tube. Clean gas exits the filter housing. If gas flow direction through the filter tube is inside-to-outside (as is common with commercial coalescing filters), the coalesced liquid drains from the outer surface of the filter tube into the sump of the filter, where it is removed by an automatic drain.
Discussion and recommendations
One of the challenges with any continuous coalescing filter results from the fact that the filtered gas and coalesced liquid both exit from the downstream side of the filter element. Any re-entrainment of the coalesced liquid will contaminate the clean gas. For air filtration, most manufacturers fabricate their coalescing filter elements with an outer layer of coarse glass fiber or polyurethane foam to act as an entrainment separator. These entrainment separator layers are, however, not 100 percent efficient. In a helium system (or any especially wet system), there is significant liquid carryover from the first coalescing filter. Therefore, more than one stage of coalescing filtration is required for helium filtration. A second challenge is that a coalescing filter that coalesces oil drains more slowly from a higher-efficiency filter than from a lower-efficiency filter. If oil enters the filter more rapidly than it can drain from the filter, the excess re-entrains into the exit gas, regardless of the efficiency rating of the filter. For this reason, a very high-efficiency filter used as a first-stage coalesce in a helium system will likely result in much poorer coalescing, compared to a lower-efficiency filter. Based on these and other observations regarding helium systems, it is recommended that a three-stage filtration system be used for removing liquid oil from helium. A Parker Balston multiple stage filtration system (see Table 1), for example, is designed to consistently reduce liquid oil content in helium from several thousand ppm to less than 0.1 ppm.