Suction capacity

Calculate the suction capacity, horsepower, discharge temperature, and piston speed for the following single-stage double acting compressor. 

Two of the machines will operate full-time the third will serve as a standby. For such a situation, the suction header would be sized for the suction capacity of two machines. In practice, it may be better to run all three machines at reduced speed. Then when one comes down for some reason, the other two can speed up to carry the full load. Of course, this type of operation cannot be accomplished with a fixed speed drive. For the situation described, the design load is still the full capacity of two machines or four cylinders. 

The slip S is the fraction of suction capacity loss. It consists of the volumetric efficiency loss fraction 1, the stuffing box loss fraction 1, and the valve loss fraction 1. The slip S is... 

Low-volume air sampler A low-volume air sampler is an apparatus having a suction capacity of up to 201/min. It is used for one continuous sampling lasting from several days to 1 week. Filter papers having a diameter of 5 cm and an active-carbon cartridge can be attached as a collecting device. 

The suction capacity of the vacuum pump depends on the size of the receiver and the method of charging. The higher the suction capacity, the faster the necessary ultimate pressure in closed position, measured by vacuum sensors, will be reached. 

In Figure 3.5, a double-sided pressurised machine is described. It possesses two inter casings with suction and pressure ports, so that an exchange of the medium to be compressed can take place via both front faces of the compressor stage. This type of arrangement is necessary if large suction capacities are to be realised with comparatively long impellers. 

However, the power requirement of the compression to be made available by the ring fiquid is also reduced by the reduction of the restrictor losses. The fiquid ring is decelerated less, so that the cross section passed through is reduced. Thus, the area available for the gas is enlarged, so that an increase in the suction capacity is to be denoted. 

The arrangement of the valves such that the surface is not covered leads to a pulsating gas discharge. A constant movement of the valves results from this. In comparison to the flaps, the valve bodies are subject to greater wear by the constant impact load. In the case of sufficient decrease in the diameter of the balls, the balls fall through the drillings. The result is a reduction in the suction capacity combined with an increase in the power consumption. 

One can infer from both of these equations that with increasing power consumption, the liquid ring decelerates more strongly (Eq. (3.4)) and thus the suction chamber is reduced (Eq. (3.5)). It is to be recognised, for example that highly viscous liquids not only raise the power consumption of the vacuum pump but also impair the suction capacity. 

If the condensable components are contained in the pumped flow, then these, depending on the material properties, condense during contact with the operating liquid. The chamber becoming open due to the condensation is filled up by gas flowing in, so that an increased suction capacity is to be denoted as... 

Because of the functional principle of LRVPs, an intensive phase contact exists bettveen the operating liquid and the medium to be compressed. Therefore, the available working chamber Vis partially filled by the vapour of the ring liquid V p in addition to the medium to be pumped. Thus, the suction capacity is reduced by the vapour of the ring liquid ... 

However, one finds no hydraulic suction capacity in the catalogue. The characteristics described here apply mostly to the compression of dry air and of an operating water temperature of approximately 18 C(15"C- -3K heating). Thus, in these values, the modification of the suction capacity by the vapour of the ring liquid and by the temperatures of gas and ring liquid are already considered. 

However, if the operating conditions diverge from those in the catalogue, then first the hydraulic and hence the actual suction capacity can be determined from the characteristics with aid of Eq. (3.11). Thus, taking into consideration, the conditions specified in the catalogue yields... 

LRVPs should be operated with suction pressures below the double vapour pressure of the ring liquid delivered on the pressure side with a gas ejector. With the aid of the ejector, the gas to be sucked off is compressed and the LRVP is fed with higher pressure. The result is an increase in the suction capacity of the working point. Furthermore, the noise developments are prevented by flow separations. 

Figure 3.13 shows the different behaviours of a vacuum pump, which was examined, on the one hand, with a conventional pump casing with inter casing and valve flaps and, on the other hand, with a fluidic optimised control cover. Equivalent suction capacities are to be established over wide ranges. However, the power requirement has decreased noticeably by the employment of the casing cover. 

Furthermore, the comparatively large cross section and the missing dead space make the formation of deposits more difficult. Since it is missing, the suction capacity can no longer be affected by defective valves (Figures 3.14 and 3.15). 

It is to be inferred from the performance data described in Figure 3.18 that in the case of LRVPs with port cylinder, the suction capacity and the power consumption are affected only marginally by the water quantity conveyed. The port cylinder permits the conveyance of three to four times the liquid quantity compared to the... 

Direct reduction of the pumped process gas prior to entering the vacuum pump is possible but seldom used in practice in relation with LRVPs. The gas is relaxed following the reduction. This results in a larger pressure difference on the vacuum pump, which reduces the suction capacity (Figure 3.29). 

Increasing the speed over the rated speed can be used to increase the suction capacity almost proportionately to the speed. The power requirements increase disproportionately during this process. 

With further divergence of the boiling pressure of the ring liquid, the gas entering via the propellant nozzle as well as the restrictor losses in the narrow mixing tube produces a decrease in the suction capacity of the combination, so that an operation without gas ejector makes sense. 

In each case, the maximum suction capacities can be obtained by the installation of a bypass parallel to the gas ejector according to Figure 3.34. If, for example with the evacuation of a tank up to a pressure of 42 mbar (cf. Figure 3.36) the valve VI remains closed and V2 opened (cf. Figure 3.34), then the greater suction capacity of the vacuum pump is first available. If valve V2 is closed now and valve VI opens, in the further course of the evacuation the greater suction capacity of the combination of gas ejector and vacuum pump can be utilised. 

The pressures and suction capacity obtainable with the aid of gas ejectors are limited by the suction capacity of the vacuum pump, since the propellant gas used is always to be compressed once again. 

If greater suction capacities with low pressures are to be realised, then with the availability of water vapour, steam jets are often used. The vapour used as propellant gas can still be thermally compressed in a condenser prior to entering the... 

It is to be taken into account that with the employment of rotary piston vacuum pumps, the temperature of the fluid fed to the liquid ring compressor is higher than with the other combinations. In addition, this heat is to be dissipated by the ring liquid, so that the temperature of the liquid ring will be higher and with it the suction capacity will be lower. 

Calcifications in LRVPs lead to a rise in the power consumption and sometimes interfere substantially with the suction capacity by constriction of the throughput cross section. 

Figure 4.7 Influence of the discharge pressure on the suction capacity, for constant suction pressure.

A simple method to influence the operational behaviour of a jet compressor is to throttle the motive pressure by a control valve. With a control as shown in Figure 4.9, the motive steam pressure can be reduced and thereby the motive mass flow rate is similarly reduced. This means that for lower motive flows the critical discharge pressure is also reduced. But as previously explained, this has no direct influence on either the suction capacity or the suction pressure, as long as the actual discharge pressure is lower than the critical one. 

The adaptation of the suction capacity of an ejector can not only be achieved by influencing the motive fluid supply, but also by measures on the suction side. Here, the suction capacity can be influenced by a control valve in the suction line or by an additional ballast load (Figure 4.12). 

---