Oil sealed vacuum pumps such as vane and piston pumps can tolerate a certain amount of water and other condensable vapors such as alcohols, acetone, and solvents. For the integrity of the sealant oils, and to achieve a good vacuum (lower than the vapor pressure of the liquid), it is best to prevent the condensables from entering the oil sealed vacuum pumps or removing them after they do enter the vacuum pump. The following discusses various methods for this removal and mostly refers to the condensate as water. The discussion of water is also applicable to other vapors and liquids, especially ones having a vapor pressure close to or higher than water.
Trapping and filtering condensable vapors upstream of
the vacuum pump:
Sorption: traps water by means of a membrane or media which will, “filter” a vapor to that of a liquid employing either adsorption or absorption. The membrane type typically coalesces the wetted air and the media type (example: activated alumina) absorbs the condensate.
Condensing or knock-out pot: vertically-configured tank, pot, or drum that
has surface area for wetted vapors to condense on. In addition to the
surfaces, separation is by gravity wherein the liquid falls to and is entrained at the bottom. The vapor travels upward at a designed velocity and exits at the top of the vessel. These containers will often contain a baffle or designed such that the air has a restricted or obstructed path, thus providing more surface area and for gravity to work for liquid to fall down and the air to travel up towards the exit.
Cold Trap: a container in which the traveling air comes in contact with coils or vanes full of a chilled fluid. The vapor will condense on the colder surface, change from a gas to a liquid, and collect onto the cold surface and/or drop to the bottom of the container.
Liquid Nitrogen Trap: similar in design to a cold trap except that the
intention is for the vapors to freeze onto the coils.
Removing condensates from the vacuum pump:
Gas Ballast: Gas Ballast means admitting gas (usually air, nitrogen or another non-compressable) during the compression stroke of the pump to change the ratio of non-condensables to condensables and prevent condensing because of the heat of compression (admitted air adds to the gas mass). Using gas ballast reduces the performance of the vacuum pump at low pressures. Gas ballast flows are limited to approximately 5% of the pump displacement. Gas ballast is very effective for condensables with vapor pressure close to or higher than water.
Hot Pump: A hot pump handles condensables, such as water, by keeping them in the vapor phase. Elevating the oil temperature above the boiling point of the
condensable, up to a maximum of 230ºF, can prevent condensing by pumping it
through the pump and out the exhaust like any other gas. A thermostatically
controlled device maintains the oil temperature and a high temperature rated oil
is incorporated to protect the vacuum pump. Hot pump operation can result in a
higher rate of oil degradation. The use of synthetic oil is recommended or
aggressive oil changes.
Decanting: Immiscible condensables can be handled through phase separation (vapor to a liquid) in the oil reservoir and drained off at the oil sump. Precaution must be taken to prevent the condensed liquid from entering the oil feed line of the vacuum pump. In addition, the inlet pressure of the vacuum pump can be affected adversely.
Knox Air Stripping: Knox air stripping involves the admittance of a compressed permanent gas blowing over the discharge valves of the vacuum pump to entrain vapors at the discharge. It is similar to using gas ballast but less efficient. However, it does not affect the inlet pressure of the vacuum pump
Vacuum Distillation or Vapor Handling: The vapor handling, or vacuum distillation system, reduces the total pressure at the exhaust of the rotary piston pump by pumping in series with the use of a small liquid ring vacuum pump. The liquid ring vacuum pump reduces the exhaust pressures of the piston pump oil. This together with an elevated oil temperature maintained using a thermostatically controlled valve on the cooling water line keeps the vapor from condensing in the oil. It is important to note that the total pressure must be less than the saturated vapor pressure at the oil temperature within the pump. The vapor handling method handles much more vapor than gas ballast, and it does not increase the inlet pressure of the rotary piston vacuum pump. This secondary pump methodology is most applicable to large oil sealed piston pumps which have a large quantity of oil. A key advantage of the vapor handling system is that the condensable can be recovered in the liquid ring pump and used as the process sealant if compatible with the process.
