Fuel gas compressors installed in cogeneration systems must be highly reliable and efficient machines. The screw compressor can usually be designed to meet most of the gas flow rates and pressure conditions generally required for such installations. To an ever-increasing degree, alternative sources are being found for the fuel gas supply, such as coke-oven gas, blast-furnace gas, flare gas, landfill gas, and synthesis gas from coal gasification or from pyrolysis. A feature of the oil-free screw compressor when such gases are being considered is the isolation of the gas compression space from the bearing and gear lubrication system by using positive shaft seals. This ensures that the process gas cannot be contaminated by the lubricating oil, and that there is no risk of loss of lubricant viscosity by gas solution in the oil. This feature enables the compressed gas to contain relatively high levels of particulate contamination without danger of “sludge” formation, and also permits the injection of water or liquid solvents into the compression space, to reduce the temperature rise due to the heat of compression, or to “wash” any particulate matter through the compressor.

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