Rotary piston pumps

Huff [23] found that reciprocating and rotary piston pumps were the most economical mechanical systems for their range of application. Obviously, the economic discussions are dependent on the vacuum expected and the local utility costs, plus the cost of maintenance. 

D = Sealed diameter, inches (estimates of nominal diameter acceptable) 

E = Evacuation factor, at final evacuation suction pressure, Tables 6-9 and 6-10 

P = Partial pressure of air in mixture, lbs/sq in. abs Pc = Absolute intake pressure of pump Pd = Piston displacement, cu ft/min Pn = Partial pressure of non-condensable gas pounds per square inch absolute (or other absolute units) Ps = Static pressure, atm 

Pv = Vapor pressure of condensable vapor, pounds per square inch absolute (or other absolute units) 

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In 1840 a hydrauHc power network, which involved large reciprocating pumps that were driven by steam engines, suppHed fluid power to London. However, concurrent technology in steam (qv) turbines and the electric generators outmoded such networks until hydrauHc systems were improved with the use of rotary pumps and oil. The rotary piston pump marked the transition from use of water to oil as the hydrauHc fluid (4). The use of vacuum-distilled, refined mineral oils were instmmental in the success of rotary axial piston pumps and motors such as the Waterbury variable speed gear... 

Vacuum capacities and operating ranges, table, 344, 355 Ejectors, 344, 357 Integrated systems, 344 Liquid ring pumps, 344 Rotary lobe blowers, 344 Rotary piston pumps, 344 Rotary vane pumps, 344 Vacuum equipment, 343 Applications diagram, 352 ASME Code, 344 Pumps, 382 Steam jets, 357 Vacuum flow,... 

Condensation of vapours must be avoided since it causes a deterioration of the oil water vapour condense at 234 torr at 70°C, which is the typical working temperature of the pump. To avoid condensation, most pumps have a ballast valve in the high-pressure stage. After the gas has been closed off from the inlet, some gas is bled in through the ballast valve. This causes the discharge valve to open sooner, with a reduction of the compression ratio. Rotary piston pumps have similar characteristics. 

The dependence of the factor from the desired pressure is shown in Fig. 2.75. It should be noted that the pumping speed of single-stage rotary vane and rotary piston pumps decreases below 10 mbar with gas ballast and below 1 mbar without gas ballast. This fundamental behavior is different for pumps of various sizes and types but should not be ignored in the determination of the dependence of the pump-down time on pump size. It must be pointed out that the equations (2.32 to 2.36) as well Fig. 2.75 only apply when the ultimate pressure attained with the pump used is by several... 

The pump-down times of rotary vane and rotary piston pumps, insofar as the pumping speed of the pump concerned is constant down to the required pressure, can be determined by reference to example 1. 

Oil-sealed rotary vaeuum pumps (Rotary vane pumps and rotary piston pumps)... 

A clean and dry vacuum system (qpy = 0) with V = 2000 I (as in example 1) is to be pumped down to a pressure of Pend - 10 tAbar. Since this pressure is smaller than the ultimate pressure of the rotary piston pump (Sg max ... 

Rotary vane/rotary piston pumps 1 -stage, p -10" mbar 2-stage, />ult - 10"3 mbar Dry pumps... 

Evacuation of the chamber is achieved by means of a series of pumps initially a rotary piston pump takes the pressure down from atmospheric to around 30 Torr, then a blower of the Rootes-type reduces it further to about 0.1 Torr lastly, an oil-diffusion pump brings the chamber down to the condition required for the evaporation cycle—in the region of 0.0002 Torr. 

Rotary piston pump studies made by Sadler12 demonstrated that the above attributes of piston pump design helped them outperform vane design pumps both in speed and vacuum achieved (Sadler compared pumps of comparable size). On the other hand, piston pumps had greater vibration. 

In addition, mechanical pump oils can be specialized for use in specific environments such as those with high-oxygen contents. Some are blended for use in specific types of pumps such as direct-drive pumps, belt-driven pumps, and rotary-piston pumps. As with most things, no single product fits the bill for all circumstances. Thus, there are many varieties, grades, and types of mechanical pump oils. 

A rotary-piston pump is an oil-sealed, positive-displacement vacuum pump. The oil both lubricates the pump and seals the discharge from the suction side of the pump. As the piston rotates, gas enters a chamber, as shown in Figure 5.2. Then, the inlet port closes, and the gas is compressed in the chamber until the discharge valve opens, exhausting the gas to the atmosphere. Possible contamination of the oil with condensable vapors, usually water, is a problem. One way condensation can be avoided is by reducing the partial pressure of the condensable gases by allowing air to leak into the cylinder, which is called a gas ballast. 

The various components of a rotary piston pump are shown in Figure 2.12. 

A key part of a rotary piston pump is the slide valve, and especially the inlet sleeve within it. One end of the slide valve (a cylinder or tube) is continually completely open to the work chamber the other end continually contacts (tides upon) the moving rotary piston. 

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