048

208 gpm at 60 lb/in2 gage 47.3 m3/h at 4.1 bar gage

The size is obtained from Figure 4. Enter Figure 4b at Q2 = 308 gpm (70 m3/h) and discharge head (H2 — H3) = 23.1 — ( — 20) = 43.1 ft [7 — ( — 6.1) = 13.1 m]; read eductor size of 4 in (102 mm) based on the discharge connection.

NOTE: If there were any appreciable length of run on the discharge line, it would be necessary to calculate the pressure drop in this line and recalculate the eductor size after adding the line loss to the discharge head required. Frictional losses on the suction side must also be included. In the example chosen, however, 100 gpm (22.7 m3/h) in a 4-in (102-mm) suction line 20 ft (6.1 m) long will have negligible frictional loss, less than 0.25 ft (0.08 m) H2O.

P SUCTION FLOW »" MOTIVE FLOW

FIGURE 5 Characteristic performance of an eductor

P SUCTION FLOW »" MOTIVE FLOW

FIGURE 5 Characteristic performance of an eductor

Performance Characteristics Figure 5 illustrates the performance characteristics of eductors. Note the sharp break in flow rate below the design point. For this reason, all eductors are not designed for a peak efficiency. It is often advantageous to have a wide span of performance with lower efficiencies rather than a peak performance with very limited range. (See Section 4.1; where, also, the validity of assuming a straight-line head-vs.-suction flow characteristic is discussed.)

Applications Beside the obvious advantages of being self-priming, having no moving parts, and requiring no lubrication, eductors can be made from any machinable material in addition to special materials, such as stoneware, Teflon,®* heat-resistant glass, and fiberglass. The applications throughout industry are too numerous to mention, but some of the more common will be discussed here. The type of eductor is determined by the service intended.

general purpose eductobs Table 1 is a capacity (flow rate) table for a general purpose eductor used for pumping and blending. This type of eductor, illustrated in Figure 6, has a broad performance span rather than a high peak efficiency point. Standard construction materials for this type of eductor are cast iron, bronze, stainless steel, and PVC. Typical uses include cesspool pumping, deep-well pumping, bilge pumping aboard ship, and condensate removal.

The following problem illustrates the use of Table 1.

example 2 Pump 30 gpm (6.81 m3/h) of water from a sump 5 ft (0.61 m) below ground. Discharge to drain at atmospheric pressure. Motive water available is 40 lb/in2 (2.8 bar) gage.

Solution Enter left side of Table 1 at 5 ft (1.5 m) suction lift and 0 lb/in2 (bar) gage discharge pressure. Read horizontally across to 40 lb/in2 (2.8 bar) gage operating water pressure. Read 9.6 gpm (2.18 m3/h) suction and 7.3 gpm (1.66 m3/h) operating fluid. These values are obtained in a 1-in (25.4-mm) eductor with a capacity ratio of 1.0.

To determine the capacity ratio of the required unit, divide the required suction by the quantity handled in 1-in (25.4-mm) eductor:

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