Rotary pumps speed

TABLE 6.28 Rotary-Pump Speed Reduction for Various Liquid Viscosities... 

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... 

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

Since changes in pump speed affect volumetric output, some pumps are rated by their displacement. Pump displacement is the amount of fluid the pump can deliver per cycle or complete rotation. Since most pumps use a rotary drive, displacement is usually expressed in terms of cubic inches per revolution. 

The nature of the power supply. If the pump is to be driven by an electric motor or internal combustion engine, a high-speed centrifugal or rotary pump will be... 

Fig. 1.7. Typical pumping speed vs. inlet pressure of a two-stage rotary vane pump.

Example 1.2. A diffusion pump with constant speed Sx = 301/s below 10-3 torr has a critical forepressure pK = 0.12 torr. It is connected to a rotary pump by a tube of length L = 150cm (see Fig. 1.15). 

We wish to choose conservative values for the tube diameter D and the pumping speed S3 of the rotary pump. 

Of course, the pressure p3 at the inlet of the rotary pump must be less than 0.08 torr, say 0.04 torr. Hence the speed S3 at the inlet of the rotary pump must be at least ... 

Rotary pumps forcibly transfer liquid through the action of rotating gears, lobes, vanes, screws etc, which operate inside a rigid container. Normally, pumping rates are varied by changing the rotational speed of the rotor. Rotary pumps do not require valves in order to operate. 

Rotary pumps can normally be divided into two classes small liquid cavity high speed pumps and large liquid cavity low speed pumps. 

Fig. 2.11 Motor power of a rotary piunger pump (pumping speed 60 mVh) as a function of...

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... 

Example A vacuum chamber of 500 I may have a total surface area (including all systems) of about 5 m. A steady gas evolution of 2 10 mbar l/s is assumed per m of surface area. This is a level which is to be expected when valves or rotary feedthroughs, for example are connected to the vacuum chamber. In order to maintain in the system a pressure of 1 10 mbar, the pump must have a pumping speed of... 

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. 

Sgj = effective pumping speed at the rotary vane pump at the... 

While the transfer and process chambers are constantly evacuated, the entrance and exit chambers must be periodically vented and then evacuated again. Due to the large volumes of these chambers and the short cycle times, a high pumping speed is required. It is provided by combinations of rotary vane pumps and Roots pumps. For particularly short cycle times gas cooled Roots pumps are also used. 

Positive-displacement pumps and those that approach positive displacement will ideally produce whatever head is impressed upon them by the system restrictions to flow. The maximum head attainable is determined by the power available in the drive (slippage neglected) and the strength of the pump parts. A pressure relief valve on the discharge side should be set to open at a safe pressure for the casing and the internal components of the pump such as piston rods, cylinders, crankshafts, and other components which would be pressurized. In the case of a rotary pump, the total dynamic head developed is uniquely determined for any given flow by the speed at which it rotates. 

---