forming of molded paper-pulp products (egg cartons and other packaging). Other applications include soil remediation, where contaminated ground water is drawn from wells by vacuum. In petroleum refining, vacuum distillation also makes use of liquid-ring vacuum pumps to provide the process vacuum. Liquid-ring compressors are often used in vapor recovery systems. Design Single- and multi-stage Liquid-ring systems can be single- or multistage. Typically a multistage pump will have up to two cascaded compression stages on a common shaft. In vacuum service, the attainable pressure reduction is limited by the vapor pressure of the ring-liquid. As the generated vacuum approaches the vapor pressure of the ring-liquid, the increasing volume of vapor released from the ring-liquid diminishes the remaining vacuum capacity. The efficiency of the system declines as the limit is approached. Single-stage vacuum pumps typically produce vacuum to 35 Torr (mm Hg) or , and two-stage pumps can produce vacuum to 25 Torr, assuming air is being pumped and the ring-liquid is water at or less. Dry air and 15 °C sealant-water temperature is the standard performance basis, which most manufacturers use for their performance curves. Recirculation of ring-liquid Some ring-liquid is also entrained with the gaseous discharge stream. This liquid is separated from the gas stream by other equipment external to the pump. In some systems, the discharged ring-liquid is cooled by a heat exchanger or cooling tower, and then returned to the pump casing. In some recirculating systems, contaminants from the gas become trapped in the ring-liquid, depending on system configuration. These contaminants become concentrated as the liquid continues to recirculate, and eventually could cause damage and reduced life of the pump. In this case, filtration systems are required to ensure that contamination is kept to acceptable levels. In non-recirculating systems, the discharged hot liquid (usually water) is treated as a waste stream. In this case, fresh cool water is used to make up the loss. Environmental considerations are making such "once-through" systems increasingly rare. Liquid selection Liquid-ring vacuum pumps can use any liquid compatible with the process as the sealant liquid, provided it has the appropriate vapor pressure properties. Although the most common sealant is water, almost any liquid can be used. The second most common sealant liquid is oil. Since oil has a very low vapor pressure, oil-sealed liquid-ring vacuum pumps are typically air-cooled. For dry chlorine gas applications, concentrated sulfuric acid is used as the sealant.