Liquid ring vacuum pumps of the series LEM/LEL are single-stage displacement pumps of uncomplicated and robust construction with the following particular features like non-polluting due to nearly isothermal compression, oil-free (no lubrication in the working chamber), handling of nearly all gases and vapours, small quantities of entrained liquid can be handled, easy maintenance and reliable operation, low noise and nearly free from vibration, protection against cavitation as standard, incorporated dirt drain, incorporated central drain and no metallic contact of the rotating parts.