227

The motive line size is now chosen by selecting a reasonable velocity and frictional loss. Choosing a 2-in (51-mm) pipe size, velocity is 5.57 ft/s (1.70 m/s) and frictional loss is 6.1 ftH2O (1.86 mH2O). The revised operating head becomes 188.6 - 6.1 = 182.5 ft (57.4 - 1.86 = 55.6 m) and in USCS units Rq = 0.92182.5/40.83 - 1 = 0.9

then Q1 = 100/1.9 = 52.6 gpm (22.7/1.9 = 11.9 m3/h). (This value is close enough so that a third trial is not necessary.) The suction flow that can be delivered is then in USCS units 100 - 52.6 = 47.4 gpm in SI units 22.7 - 11.9 = 10.8 m3/h priming eductobs—water-jet exhausters Eductors are often used as priming devices for mechanical pumps. In this application, the eductor is used to remove air rather than water. Liquid jets are not well suited for pumping noncondensables; therefore, the capacities are low. However, the volume being primed is usually small, and so the low capacity is not a factor. When larger volumes are involved, such as condenser water boxes, it is more feasible to use an exhauster. The water-jet eductor of Figure 6 is converted to a water-jet exhauster by replacing the jet nozzle with a solid-cone spray nozzle. Evacuating rates and capacity tables for such a unit are shown in Figure 18 and Table 8. Eductors have approximately one-fifth the air-handling capacities of water-jet exhausters when supplied with similar motive quantities and pressures.

example 5 From Figure 18 and Table 8, determine size and water consumption to exhaust 15 standard ft3/min (0.42 m3/min) of air at 20 inHg (508 mmHg) abs discharging to atmosphere using 60 lb/in2 (4.14 bar) gage motive water at 80°F (27°C).

Solution Enter Figure 18 at 80°F (27°C) (1); read horizontally to the suction pressure 20 inHg (508 mmHg) abs (2); project vertical line to 60 lb/in2 (4.14 bar) gage motive pressure (3); project a horizontal line for the capacity of a 1-in (25-mm) exhauster (4); divide desired flow by the capacity of a 1-in (25-mm) unit, which is 1.9 standard ft3/min, (0.054 m3/min), to find capacity ratio: 15/1.9 = 7.9 (0.42/0.054 = 7.9).

The capacity ratio table shows that a 3-in (76-mm) exhauster with a capacity ratio of 9.0 is required. The motive water quantity from Table 8 is 86 gpm (19.5 m3/h). Note: Table 8 gives water consumption at 15 inHg (381 mmHg) abs; because flow varies as the square of pressure differential across the nozzle, the exact flow is obtained as follows:

Nozzle upstream pressure:

in USGS units 60 + 14.7 = 74.7 lb/in2 abs in SI units 4.14 + 1.01 = 5.15 bar abs

Nozzle downstream pressure:

14.7 lb/in2

30 inHg

FIGURE 18 Capacity curve of water-jet exhausters (Schutte and Koerting)

in SI units

508 mmHg abs

V 762 mmHg)

0.67 bar abs

Operating differential:

in USCS units in SI units

TABLE 8 Approximate water consumption of water-jet exhausters, gpma-

Water pressure, lb/in2 (bar) gage

Size, in (mm)

30 (2.1)

40 (2.8)

60 (4.1)

80 (5.5)

100 (6.9)

3 (76.2)

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Survival Treasure

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