The useful range of a liquid ring vacuum pump may be extended using an atmospheric air ejector on the suction flange. It acts as an extra pumping stage. As the name implies, the motive for this ejector is air at atmospheric pressure. While the suction pressure of the liquid ring vacuum pump is in the 50-75 torr range, the ejector can pull the process down to the 5-10 torr range.
A common application would be on a two-stage cold water degasifier. Two parallel atmospheric air ejectors, one operating at 20-25 torr and the other operating at 10-15 torr, maintain different vacuum in the two different compartments of the degasifier. They both discharge to the same pressure, usually 60-75 torr. The two discharges are manifolded together and sent to the same liquid ring vacuum pump or bank of parallel pumps.
It should be noted that the liquid ring pump does have to compress both the suction air and the motive air back to atmospheric pressure. This results in the necessity of a larger liquid ring pump than would be required if a steam ejector and condenser were used ahead of the pump. (The motive steam would condensable, so the pump would only have to handle the suction air.)
Although we do have standard units, the great majority of our CR series atmospheric air ejectors are custom designed for maximum efficiency. This allows the backing liquid ring vacuum pump to be as small as possible. We can retrofit almost any existing liquid ring vacuum with one of these units.