Liquid Ring Designs
Today two main types of liquid ring vacuum pumps can be found, the radial flow or conical pump and the axial flow or flat port plate pump. Inside the two main designs three main types of liquid ring vacuum pump sub-designs can be found, the true single stage, the single stage variable port plate and the two-stage pump design.
The true single stage axial flow design is pictured above with its large inlet port on the right and smaller discharge port pictured on the left side. It features a suction port, impeller and discharge port. The final vacuum the pump can achieve is designed through the distance between the inlet and discharge port. True single stage pumps can achieve roughly 26"Hgv or 4"HgA and if the discharge is roughly atmospheric, say 30"HgA then the compression ratio is 30:4. As the gas enters the pumps inlet its conveyed and compressed until it reaches the beginning of the discharge port where almost all of the gas is then exhausted. Of course not all of the gas escapes and some finds its way back to the pumps inlet where it re-expands to the operating pressure.These rough vacuum pumps are one of the most durable pumps ever made and are used on a multitude of rough and wet vacuum applications.
The single stage variable port plate design is only available with the axial flow pump. It's construction includes the suction port, impeller and a valved discharge port shown above. If the final vacuum is determined by the total distance between the inlet and discharge port, then increasing the distance between them is the solution to increasing the compression ratio. The variable port plate only allows as much air to discharge as required while maintaining good efficiency through out the low pressure curve. In general a single stage flat port plate liquid ring vacuum pump can achieve 28.9"HgV or roughly 1"HgA and with an atmospheric discharge our compression ratio is roughly 30:1. As the gas is conveyed and compressed it enters the discharge area but it's not allowed to escape all at one time (as with the true single stage) as the port plate has multiple small slots (as shown below) that are covered by individual teflon valves. As the gas enters the compression cycle, only the valves that need to react will operate creating a more efficient Hp/cfm ratio, and lower service liquid usage than traditional designs. The final horizontal port is not valved as this is the final fail safe for all the gas to be exhausted before intake is reached in the compression cycle.
The third and final liquid ring is the two-stage design pump wich uses two impellers in series to achieve a final pressure of 28.9"Hgv. The first stage is 3 times as large as the second stage and if we assume the first stage has a displacement of 100 and the second stage has a displacement of 33 then our staging ratio is 100/33 or 3:1. For example, if our pump is "blanked off" generating the best vacuum it possibly can produce, then our compression ratio can be said to be roughly 30:1. The theoretical interstage pressure is then found by examining the 1st stage inlet pressure and the final stage discharge pressure, or in our example 5.5:1. Some two-stage liquid ring pumps can be favored over single stage valve and conical pumps for increased capacity per Hp at approximatly 27"Hgv to 28.9"Hgv.
Two-stage liquid ring machines should not be operated below 27"Hgv due to the designed staging ratio of the pump. For example,assume the pump is now operating at 20"HgA at the inlet and maintain our 30"HgA discharge condition as before, then our compression ratio becomes 30/20 or 1.5r. The first stage displacement of 100cfm/1.5r = 67cfm discharge into the inlet of the second stage, which carries a displacement of 33cfm, so our capacity can be choked by the 33cfm second stage limitation and over compression results. The interstage pressure may be found by examining the inlet pressure and staging ratio, or 60"HgA at the interstage. Interstage bypass valves are available to compensate for operating pressures above 27"Hgv to relieve over compression and reduce the operating horsepower.