Roots pumps applications

Fig. 2.1 i Areas of application for Roots pumps and condensers pumping water vapor (o.G. = without gas ballast)... 

Flence the Roots pump RUVAC WA 1001 would be the suitable pump. The permissible remaining moisture in the product determines the attainable ultimate pressure. The relationship between the ultimate pressure and the remaining moisture is fixed for every product but different from product to product. LEYBOLD has many years of experience to its record regarding applications in this area. Assume that a 0.1 % residual moisture content is required, for which the necessary ultimate pressure is 6 10 mbar. During the fast 5 h the remaining 6 % of the moisture content, or 5 kg of water, is removed. At a mean pressure of about 0.65 mbar, 2000 m /h of vapor is evolved. Two possibilities are offered ... 

Because of the problems that can potentially arise with oil-sealed pumps, there is an increasing trend to use dry pumps in many applications. Various types are available including Roots pumps backed with suitable pumps (see Section 3.2.3), multi-stage Roots pumps, screw-, hook-and-claw- and scroll pumps. Dry piston pumps and diaphragm pumps are also available but their pumping speed range is restricted. The... 

Roots pumps must be combined with a backing pump/pump-set for normal vacuum applications. Used in combination with any of a range of backing pumps (oil-sealed rotary, dry, liquid-ring) Roots pumps allow higher pumping speeds and lower pressures to be achieved than can be obtained by the backing pump on its own. 

Chapter 3 summarised initially the various types of vacuum pump available and the pressure ranges in which they normally operate. Subsequent sections dealt specifically with types of pump and, in some cases, to support calculations, reviewed the operating principles and characteristics. For example, aspects of oil-sealed rotary pump operation were discussed (Examples 3.1-3.5) and Roots pumps, widely used in applications where large gas loads at pressures in the rough-medium range have to be handled, were examined (Examples 3.7-3.9). 

However, in chemistry applications spring-loaded valves have been found to be at least failure prone. When pumping substances which cause sticking of such a bypass valve, its function is always endangered. There exists the considerable risk that in the event of a valve function failure, the Roots pump is damaged since under these conditions the permissible pressure difference can be exceeded. 

As mentioned above, the rate of mass flux into the monitor is independent of pressure and dependent on the square root of absolute temperature. However, further corrections do exist when mass, by G.C. analysis, is converted to mg/m3 or PPM. As with rotometers and a pump, the corrections are not the simple application of the gas laws. 

A more suitable and modern approach in coimection with chemistry applications is that of smoothly cutting-in a Roots vacuum pump in to a process without relying on a bypass valve but instead by using a frequency and thus speed controlled motor. Now the Roots vacuum pump wUl no longer start to operate at a certain operating pressure at full speed. Under certain circumstances it may be started even at atmospheric pressure, but under speed, respectively frequency control, and at a lower rotational speed and thus a low theoretical volumetric speed. The characteristic is the same as for a Roots vacuum pump equipped with a bypass valve, except that now the valve can no longer fail because it is missing. 

Also, for approximate cavitation similarity, the velocities of the fluid relative to the pump should be about the same. This means that the head generated by the model and that by a larger unit will then be the same, as shown. Equating Ng on this basis then gives the applicable speeds of a different size unit. This is inversely as the square root of relative capacities. Also, the sizes of the model and prototype for cavitation similarity will be inversely as their rpm, and directly as the square root of capacities when geometrically similar. 

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