centrifugal pumps are most commonly used
turbomachinery devices which are used to
raise pressure or induce flow in the
control volume they are radial flow
devices various kinds of centrifugal
pumps are available in market with
different construction details but
working principle behind all of them
remain same in this video we will
analyze working principle of a
centrifugal pump with single suction
semi-open impeller one of such pump is
shown in Figure with one part of its
casing removed for ease of understanding
working of centrifugal pump is simple as
the impeller rotates it sucks fluid from
outside through I of impeller the fluid
is pushed radially along the impeller to
the casing casing collects the fluid and
it is pumped out through discharge
nozzle we will go through main
components of a centrifugal pump in a
detailed way
first impeller impeller is the device
which rotates and transfer energy to
fluid it has got collection of vanes
fitted to a hub plate
next component is casing which collects
fluid from impeller in an efficient way
the casing has got a special shape with
area of cross-section increases from
Inlet to outlet as the impeller rejects
fluid throughout casing along length of
casing mass flow rate increases but
increasing area of casing helps in
maintaining almost same velocity thus
volute casing helps in converting
dynamic part of fluid energy to static
part
it will be interesting
struction details of volute casing
casing is made on tuvalu curves which
are an offset a three-dimensional volute
is made from this curves
a portion is removed from volute shape
in order to accommodate the impeller in
it
a discharge nozzle is fit at exit
portion of the casing most of the time
discharge nozzle is diverging in shape
for centrifugal pumps of small capacity
as we discussed impeller and casing or
its main components but for larger
centrifugal pumps there will be
additional diffuser blades also present
in order to reduce velocity further or
they aid in dynamic to static energy
conversion
now we will see how to predict pressure
rise blade velocities at inlet and
outlet are shown here fluid velocities
at inlet and outlet are also marked here
you can see fluid velocity increases
from Inlet to outlet due to energy
addition to fluid the work required for
changing inlet velocity condition to
outlet is given by this equation we will
go through such turbo machinery analysis
in a separate video here Q is the flow
rate and V theta represents tangential
velocity component of flow
for a centrifugal pump inlet velocity
will be parallel to radius so tangential
component at Inlet is zero
from here we can find what's the head
rise in meters of fluid outlet blade
angle beta can be derived in terms of
velocities
also flow rate through impeller is given
as flow area times radial velocity
so head rise in a centrifugal pump can
be derived in terms of flow rate
using this we can predict what's the
head rise as we change the flow rate for
a particular pump geometry and a
particular impeller angular velocity
most important parameter in this
equation is blade outlet angle beta
there can be three different pump
characteristics depending upon value of
this angle first case if beta is less
than 90 degree since this term is
positive in this case pressure head
decreases with increase in flow
these kinds of impellers are called
backward curved
if beta is 90 degree with flow rate
there is no change in pressure rise
because this term is zero here
they are called radial type
if beta is more than 90 degree pressure
increases with increase in flow rate
such blades are called forward curved
blades
the big question out of these which one
is the most suited for industrial use
to get answer for this question let's
see how power consumption varies with
discharge for each of these cases
for backward curved blades as pressure
head decreases with discharge power
consumption stabilizes with flow
in radial blades since pressure does not
have any connection with flow rate power
consumption increases linearly
in forward curved blades sense pressure
head increases with flow power
consumption increases exponentially
making the operation unstable this will
lead to burnout of motor
so backward curved blades which has got
self-stabilizing characteristics in
power consumption is the most preferred
one in industry
to know more practical aspects of
centrifugal pumps check the second video
thank you