The recent introduction of oil-injected screw compressors in cryogenic systems for increased mechanical reliability has increased the possibility of helium contamination at the input to the refrigerator above the permissible/eve/of a few parts in 10 9. A new laser particulate detection instrument is described which continuously measures the/eve/ of particulate matter and oil aerosols directly in a high pressure gas stream. It was developed to monitor, in situ, helium at 15 atmosheres (1.5 MPa) that is used in a cryogenic refrigerator.
The instrument uses forward scattering of laser light to detect particulate matter and accomplishes high sensitivity and stability by using unique automatic compensation of the entire electro-optical train. High sensitivity and stability permit measurement of mass concentration for particles between 0.3 and 3 pm diameter in quantities down to 10 ugm m 3, corresponding to 5 parts in 10 9. The direct measurement of particulate matter transported by the process flow is independent of gas pressure, velocity, temperature and composition. Calibration results are discussed.
The instrument has proven very valuable for both process monitoring and process optimization. An example of compressor shut down is given.