Handbook of Civil Engineering Calculations

FIGURE 13. System-head curves with recirculation flow.

(Chemical Engineering.)

the head-capacity characteristic curves of the pump and system. (2) Choose the next larg-
er diameter pump impeller to maintain a discharge flow of QA to tank A 9 Fig. 11, and a re-
circulation flow QB to tank B, Fig. 11. (3) Compute the recirculation flow Qs at the pump
shutoff point from Qs = QB(Hs/H 0 p)Q-^5. (4) Calculate the minimum safe flow Qmin for the
pump with the larger impeller diameter. (5) Compare the recirculation flow Qs at the
pump shutoff point with the minimum safe flow Qmin. IfQ 5 ^ gmin, the selection process
has been completed. If Q 5 < gmin, choose the next larger size impeller and repeat steps 3,
4, and 5 above until the impeller size that will provide the minimum safe recirculation
flow is determined.
This procedure is the work of Mileta Mikasinovic and Patrick C. Tung, design engi-
neers, Ontario Hydro, as reported in Chemical Engineering magazine.

PLfAfP CHOICE TO REDUCE ENERGY

CONSUMPTION AND LOSS

Choose an energy-efficient pump to handle 1000 gal/min (3800 L/min) of water at 6O^0 F
(15.6^0 C) at a total head of 150 ft (45.5 m). A readily commercially available pump is pre-
ferred for this application.

Calculation Procedure:

1. Compute the pump horsepower required
   For any pump, bhpi = (gpm)(H^(s)l?>96Qe, where bhpt = input brake (motor) horsepower
   to the pump; Ht = total head on the pump, ft; s = specific gravity of the liquid handled; e =
   hydraulic efficiency of the pump. For this application where 5=1.0 and a hydraulic effi-
   ciency of 70 percent can be safely assumed, bhpt = (1000)(150)(1)/(3960)(0.70) = 54.1
   bhp (40.3 kW).

Flow rote,gal/min(L/s)

Pump

and

system heads,

ft(m)