Ultimate pressure Roots pumps

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). 

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 ... 

The ultimate pressure of a Roots blower/backing pump combination is approximately that of the backing pump divided by the compression ratio (k0) for the Roots pump at this pressure. In practice, however, the calculated value should be multiplied by a factor of 4 to 5 to take into account performance deficiencies of the backing pump. 

For this reason, in large production systems, three-stage vacuum pumps are used. These consist of two-stage rotary vane pumps and a Roots pump. The greatest advantage of such a pump combination lies in the large volume flow rate at low pressures and in the low ultimate total pressure attained (Figure 14.13). 

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