Root pumps

The booster (blower or Roots) pumps are high-throughput pumps with a low compression ratio. A booster pump is schematically shown in Fig. 1.8. The gap between the two counter-rotating rotors is of the order of 0.1 mm. The rotation (about 50rps) of the rotors,... 

Fig. 1.8. Scheme of a Roots pump. Main components 1, inlet 2, two-lobe piston 3, exhaust 4, pump casing ... 

As shown in Figure 2.25, it is more efficient to use a backing pump combined with one or two roots blowers (Fig. 2.27). The backing pump requires only 40 m3/h capacity, combined with a roots pump of 200 m3/h. This system evacuates the 1000 L also in 8 min down to 0.01 mbar, but below 0.1 mbar the system has a capacity of 200 m3/h or 2.2 10 3 g/s at 0.05 mbar. Such a pumping system is preferable for freeze drying compared with a large two-stage pump alone. 

In the case of dust filters which are used to protect gas ballast pumps and roots pumps, the percentage restriction value for the various pressure levels are listed in the catalog. Other components, namely the condensate separators and condensers, are designed so that they will not reduce pumping speed to any appreciable extent. 

The following may be used as a rule of thumb for dimensioning vacuum lines The lines should be as short and as wide as possible. They must exhibit at least the same aoss-section as the intake port at the pump. If particular circumstances prevent shortening the suction line, then it is advisable, whenever this is justifiable from the engineering and economic points of view, to include a roots pump in the suction line. This then acts as a gas entrainment pump vi/hich reduces line impedance. 

Pumps which transport quantities of gas from the low pressure side to the high pressure side vi/ithout changing the volume of the pumping chamber (Roots pumps, turbomolecular pumps)... 

The quantity of gas effectively pumped by a Roots pump is calculated from the theoretically pumped quantity of gas Qth and the internal leakage QiR (as the quantity of gas which is lost) as ... 

For the efficiency of the Roots pump, the generally valid equation applies k... 

Fig. 2.18 Maximum compression kg of the Roots pump RUVAC WA 2001 as a function of fore vacuum pressure p ...

Normally a Roots pump will be operated in connection with a dovmstream rough vacuum pump having a nominal pumping speed Sy. The continuity equation gives ... 

The ratio S( Sy (theoretical pumping speed of the Roots pump / pumping speed of the backing pump) is termed the gradation kj. From (2.15) one obtains... 

Equation (2.16) implies that the compression k attainable with a Roots pump must always be less than the grading k, between Roots pump and backing pump since volumetric efficiency is always < 1. When combining equations (2.13) and (2.16) one obtains for the efficiency the well known expression... 

The characteristic quantities to be found in equation 2.17 are only for the combination of the Roots pump and the backing pump, namely maximum compression kg of the Roots pump and gradation k, between Roots pump and backing pump. 

With the aid of the above equations the pumping speed curve of a given combination of Roots pump and backing pump may be calculated. For this the following must be known ... 

The way in which the calculation is carried out can be seen in Table 2.3 giving the data for the combination of a Roots pump RUVAC WA 2001 /... 

Compression in a Roots pump is performed by way of external compression and is termed as isochoric compression. Experience shows that the following equation holds approximately ... 

Types of motors used with roots pumps... 

Fig. 2.20 Cross section of a Roots pumps with bypass line...

Fig. 2.21 Vacuum diagram - Roots pump with integrated bypass line and backing pump...

Like Roots pumps, claw pumps belong to the group of dry compressing rotary piston vacuum pumps (or rotary vacuum pumps). These pumps may have several stages their rotors have the shape of claws. 

Fig. 2.22 Diagram of a Roots pump with pre-admission cooling...

The ultimate pressure attainable with the DRYVAC 251 S or 501 S is -compared to the versions without integrated Roots pump - by approximately one order of magnitude lower (from 2 10 mbar to 3 10 mbar) and the attainable throughput is also considerably increased. It is of course possible to directly flange mount LEYBOLD RUVAC pumps on to the DRYVAC models (in the case of semiconductor processes also mostly with a PFPE oil filling for the bearing chambers). 

Therefore, the condenser must be backed by a gas ballast pump (see Fig. 2.41) and hence aiways works - iike the Roots pump - in a combination. 

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