Roots Vacuum Pumps

A Roots vacuum pump (see Fig. 2.17) Is a rotary positive-displacement type of pump where two symmetrically-shaped Impellers rotate Inside the pump casing past each other In close proximity. The two rotors have a aoss... 

If the pump is protected using a diaphragm-type pressure switch, then the pump will be switched on automatically. If a combination of roots pump and roughing pump is to convey highly volatile substances such as liquids with a low boiling point, then it is advisable to use a roots pump which is equipped with an integral bypass line and a valve which will respond to a pre-set pressure. Example Roots vacuum pumps RUVAC WAU / WSU. 

N cuum technology acceptance specifications for rotary piston vacuum pumps Part 1 - Rotary piston and vane type rotary vacuum pumps in rough and medium vacuum range Part 2 - Roots vacuum pumps in medium vacuum range ... 

A Roots vacuum pump has two symmetrical impellers, contra-rotating (at 50 or 60 Hz) inside a housing (see Figure 3.5). 

A Roots vacuum pump with a theoretical pumping speed (Slh) of 500 m3 h l is backed by a dry pump designed for the chemical industry. Under working conditions, the dry pump has a speed of 50m3h 1. 

Figure 23. Vacuum plant with nonlubricated sliding vane vacuum pumps to reduce pollution of wastewater a) Reactor b) Precondenser c) Roots vacuum pump d) Intermediate condenser e) Sliding vane rotary pump (two stage) f) Final condenser g) Condensate collection vessel...

Fig. 8 Combination with rotary lobe and Roots vacuum pump, Dr.-Ing. K. Bnsch GmbH...

In this case, several vacuum pumps of different sizes are connected in series. A constant flow of volatile vapour components is extracted from the recipient. If the vapour undergoes no phase change and isothermic compression occurs, then the product of working pressure and intake capacity remains constant (perfect gas law). If the pressure increases, the volume must become less than before the compression. A Roots vacuum pump (Fig. 9) is a machine which, under normal operating conditions, can gener-... 

Positive displacement pumps diaphragm vacuum pumps, liquid ring vacuum pumps, rotary vane and rotary piston vacuum pumps as well as Roots vacuum pumps (dry compressing claw and screw pumps are not yet mentioned here but also belong to this group of pumps). 

Figure 5.7 Lobe type (Roots) vacuum pump.

Initially Roots vacuum pumps served the purpose of assembling pump combinations, so-called pump systems. The main reason for this is the relatively high pumping speed they offer in connection with their small size, however, usually with a rather small pressure difference which can be overcome. 

The pressure difference which can be overcome by Roots vacuum pumps in the pump chambers in which the pumped gas is not compressed, is limited. Their great advantage is based on the extremely favourable specific construction volume (construction volume per cubic metre of pumping speed). Roots vacuum pumps offering pumping speeds of over 10 000 m h are being manufactured. 

Roots Vacuum Pumps with Bypass Valve, Respectively with Frequency Controlled Motor... 

Figure 5.11 Roots vacuum pump with internal bypass valve.

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. 

The lower the intake pressure drops in the course of the process, the faster the Roots vacuum pump may revolve. This operating approach makes sense since at lower pressures the same gas quantities take up an ever increasing volume. Thus at low pressures generally also a higher pumping speed is needed. 

Figure 5.12 depicts the smooth increase in the total pumping speed of a pump combination with a Roots vacuum pump with bypass valve, respectively without this valve but with frequency control. 

The volumetric efficiency / of a Roots vacuum pump is calculated as... 

When using the designation k for the amount of compression present between the intake port and the exhaust port of a Roots vacuum pump, then the following... 

The maximum compression of a Roots vacuum pump, a characteristic quantity for any Roots vacuum pump, is obtained in accordance with DIN 28 426 Part 2 at zero throughput and is designated as k, where / = 0. 

S.3.4.4 Dimensioning Combinations of Roots Vacuum Pumps with Backing Pumps... 

The intended combination of Roots vacuum pump (5(, ) and backing pump (5y) defines the theoretical grading... 

When combining the same Roots vacuum pump with a backing pump providing a pumping speed of 250 m h there is no problem. The theoretical grading amounts to 1 4, that is the maximum compression across the Roots pump is taken to be from 20 to 80 mbar. The therefrom resulting pressure difference of 60 mbar is permissible. 

When considering the aforementioned combination (grading 1 10) now at an intake pressure upstream of the Roots vacuum pump of 5 mbar, the requirements regarding the maximum pressure difference are fulfilled. 

At a grading of 1 10 and at an intake pressure of 5 mbar, a maximum pressure of 50 mbar will have to be assumed on the delivery side of the Roots vacuum pump. The pressure difference will now even in the most extreme case amount to only 45 mbar and is thus within the permissible range. 

In order to be able to completely describe a pump combination as to its operating data, all that is now lacking is the calculation of the maximum cut-in pressure for the Roots vacuum pump. Meant here is the conventional mode of operating the Roots pump at a full rotational speed, without bypass valve and a programmed fixed cut-in pressure (through a pressure switch or through a pressure measurement arrangement from the side of the plant, for example). 

As already discussed, compression within a Roots vacuum pump being an outer compression is isochoric. 

Thus the rating for the drive motor for a given size of Roots vacuum pump depends on the permissible maximum pressure difference and the amount of mechanical power loss depending on the design in which also the amount of power required for starting up the pump must be taken into account. 

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