Page 144 - Fluid mechanics, heat transfer, and mass transfer

122 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

. What is the effect of increased impeller size in the pump . Up to what percent decrease in impeller diameter for a
casing? centrifugal pump, is tolerable to keep the efficiency
& Increases maximum discharge pressure for which levels high?
downstream equipment might not be designed. & 20%.
& A 10% increase in impeller size might result 30% . How is impeller size selected for a given pump casing?
increase in amperage on the motor. & A general rule of thumb is to select an impeller that is
& Amperage, A, required by the motor is given by one size smaller than the maximum size so that it can
be replaced with a larger one without replacing the
A /ðHeadÞðsp:gr:ÞðLiquid flow rateÞ= casing.
& A pump that requires maximum size impeller
motor efficiency: ð5:15Þ
should not be selected. When put into service, it
may be found that the selected impeller is too small.
. Illustrate graphically how impeller size determines
In such a case, the pump has to be replaced, which
capacity and head developed by a centrifugal pump.
is expensive when compared with choosing initially
& Figure 5.17 is illustrative of impeller size, capacity,
a larger pump and changing impeller to a larger
and head developed relationships for a centrifugal
size. Cost of impeller is insignificant compared to
pump.
pump.
& For the same flow rate, head developed increases with
& Start-up conditions must be considered while select-
increased diameter of the impeller.
ing the impeller. May be the pump has to put up
50% more head to establish initial circulation than
2
h 2 ¼ h 1 ðd 1 =d 2 Þ : ð5:16Þ normal.
& While increasing impeller size, the consequences of
& For the same head developed, flow rate (capacity)
increase in shut-in pressure must be examined and
increases with impeller size. taken care of.
& Motor selection should be based on the largest size
Q 2 ¼ Q 1 ðd 2 =d 1 Þ: ð5:17Þ impeller that can fit into the pump and not on size of
actual impeller used.
& Amperage for the motor increases with increased
. “Speed control for a centrifugal pump will not affect its
impeller diameter. efficiency.” Comment.
& Changes in speed up to 20% will not adversely affect
3
A 2 ¼ A 1 ðd 2 =d 1 Þ : ð5:18Þ efficiency.
. How are pulsations reduced through design of a cen-
trifugal pump?
& Number of vanes: Increasing number of vanes.
& Radial clearance between tip of the impeller and the
stationary collector.
& Type of impeller: Pulsations tend to increase with
increase in specific speed of the impeller. Thus, the
radial impeller has the lowest pulsations, followed by
Francis and axial-type impellers.
& Impeller tip speed: Higher tip speeds generally mean
larger pulsations.
& Staggering impellers in multistage machines: Stag-
gering reduces pulsations.
. What is the difference in operating centrifugal pumps in
series and parallel?
& Options for handling high head or high flow applica-
tions include using pumps in series or parallel.
& When running in series, the heads are added and the
FIGURE 5.17 Effect of impeller size on capacity versus head total capacity is equal to that of the pump with the
developed for a centrifugal pump. smallest capacity.