Roots pumps calculations

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 ... 

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 /... 

Permissible cut-in pressure will depend on the reduction ratios of the roots pump as against the roughing pump and is calculated by dividing the permissible pressure differential Ap, g by the compression ratio, reduced by the value of 1 ... 

If the pumping speed vs pressure characteristic of a Roots pump + backing pump is required, the calculation starts by assuming values for p0]1t and obtaining values of Sback corresponding to pout from Sback vs p curves (usually provided by the manufacturer of the backing pump) where in this case, p = pout. 

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. 

A Roots pump (Sth = 150 m3 h1) is backed by a directly connected dry piston pump. Calculate the pumping speed of the combination given the data in Tables 3.5 and 3.6, provided by the manufacturer. 

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). 

Calculation Process There exists a theoretical grading between the Roots pump and the backing pump of 1 10. 

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). 

When running the calculation for the described combination of a Roots pump offering a theoretical pumping speed of 1000 m h and a backing pump with a pumping speed of 250 m h and a maximum permissible pressure difference... 

The chamber in Example 4.11 has a volume of 5 m3. The volumes of the diffusion pump and the connecting line to the chamber are 100L and 660 L, respectively. The volume of the connection between the Roots and diffusion pumps is 10 L. Calculate the response time for 95% of the final signal on the leak detector when it is connected in positions 1 and 2. 

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

In Figure 8-29, the pump-out vane thickness at the root is (gf+ whereas at the tip it is tf . In the back of the impeller, the pump-out vanes start at a diameter <4, whereas on the front side they start at These values are plugged into Equation 8-34 to obtain hi each case and to calculate axial thrust. 

Referring to Figure 8-29, when pump-out vanes are added in the back shroud, liquation 8 34 is then used to calculate the value of P at the root of the pump out vanes P ... 

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