Backing pumps

When the water jacket is in place, the tubing below the jacket is bulged and the backing pump side arm D is added—far enough from the ring seal to cause no strain in the latter. The return drain tube E for mercury can now be attached. 

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. 

Torikai et al. [5.4] sprayed aqueous solutions of Mn2+, Co2+, and Ni2+ sulfate into LN, and produced uniform, spherical particles, which were freeze dried in a bottle at -80 °C. This was connected to two cold traps (LN2), a diffusion pump, and a backing pump. Freeze drying of approx. 10 g took 2-3 days. The dry product in the form of Mn3C02Ni (S04)6-15-16 H2 O could be transformed into a fine spinelle powder at 900-1000 °C during 1 hour. 

Example The vacuum system of non-benchtop mass spectrometers consists of one to three rotary vane pumps and two or three turbo pumps. Rotary vane pumps are used for the inlet system(s) and as backing pumps for the turbo pumps. One turbo pump is mounted to the ion source housing, another one or two are operated at the analyzer. Thereby, a differentially pumped system is provided where local changes in pressure, e.g., from reagent gas in Cl or collision gas in CID, do not have a noteworthy effect on the whole vacuum chamber. 

The throughput value is important in determining the size of the backing pump in relationship to the size of a high vacuum pump with which it is connected in series in order to ensure that the backing pump will be able to take off the gas moved by the high vacuum pump (see Section 2.32). 

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 ... 

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

Sixties, mercury as the medium was almost completely replaced by oil. To obtain as high a vapor stream velocity as possible, he allowed the vapor stream to emanate from a nozzle with supersonic speed. The pump fluid vapor, which constitutes the vapor jet, is condensed at the cooled wall of the pump housing, whereas the fransported gas is further compressed, usually in one or more succeeding stages, before it is removed by the backing pump. The compression ratios, which can be obtained with fluid entrainment pumps, are very high if there is a pressure of 10 mbar at the inlet port of the fluid-entrainment pump and a backing pressure of 10 2 mbar, the pumped gas is compressed by a factor of 10 ... 

LEYBOLD manufactures pumps such as the TURBOVAC 55 with an integrated Holweck stage (screw-type compressor) and, for example, the HY.CONE 60 or HYCONE 200 with an integrated Siegbahn stage (spiral compressor). The required backing pressure then amounts to a few mbar so that the backing pump is only required to compress from about 5 to 10 mbar to atmospheric pressure. A sectional view of a HY.CONE is shown in Fig. 2.52a. 

Comparatively, then, a pump set consisting of a Roots pump, condenser, and backing pump, which can transport 100 kg/h of vapor and 18 kg/h of air at an inlet pressure of 50 mbar, has a power requirement of 4 -10 kW (depending on the quantity of air involved). A steam ejector pump of the same performance requires about 60 kW without altering the quantity of air involved. 

---