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# Unit 1 3 Centrifugal pump (1)

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```HYDRODYNAMIC
MACHINES III
Fluid Mechanics: Fundamentals and Applications by
Yunus A. &Ccedil;engel and John M. Cimbala, 4th Edition,
2018.
Mr. L MASHEANE
OFFICE:
BHP 152
EMAIL:
lmasheane@cut.ac.za
TELEPHONE NUMBER: 051 507 3683
DYNAMIC PUMPS
There are three main types of dynamic pumps
that involve rotating blades called impeller
blades or rotor blades, which impart
momentum to the fluid.
They are sometimes called rotodynamic
pumps or simply rotary pumps.
Rotary pumps are classified by the way flow
exits the pump: centrifugal flow, axial flow,
and mixed flow
The impeller (rotating portion) of the three main
categories of dynamic pumps: (a) centrifugal
flow, (b) mixed flow, and (c) axial flow.
DYNAMIC PUMPS
Centrifugal-flow Pump: Fluid enters axially (in the same direction as the axis
of the rotating shaft) in the center of the pump, but is discharged radially (or
tangentially) along the outer radius of the pump casing.
For this reason centrifugal pumps are also called radial-flow pumps.
DYNAMIC PUMPS
Axial-flow Pump: Fluid enters and leaves axially, typically along the outer
portion of the pump because of blockage by the shaft, motor, hub, etc.
DYNAMIC PUMPS
Mixed-flow Pump: Intermediate between centrifugal and axial, with the flow
entering axially, not necessarily in the center, but leaving at some angle
between radially and axially.
CENTRIFUGAL PUMPS
CENTRIFUGAL PUMPS
Centrifugal pumps and blowers can be
easily identified by their snail-shaped
casing, called the scroll.
They are found all around your home; in
dishwashers, hot tubs, clothes washers
and dryers, hairdryers, vacuum cleaners,
kitchen
exhaust
hoods,
bathroom
exhaust fans, leaf blowers, furnaces, etc.
They are used in cars; the water pump in
the engine, the air blower in the
heater/air conditioner unit, etc.
Centrifugal pumps are ubiquitous in
industry as well; they are used in building
ventilation systems, washing operations,
cooling ponds and cooling towers.
A typical centrifugal blower with its
characteristic snail-shaped scroll.
MAIN PART OF
CENTRIFUGAL PUMP
CENTRIFUGAL PUMP
There are three types of centrifugal pump based on impeller blade geometry: (a)
Backward-inclined blades, (b) radial blades, and (c) forward-inclined blades.
Centrifugal pumps with backward-inclined blades are the most common. These
yield the highest efficiency of the three because fluid flows into and out of the
blade passages with the least amount of turning.
Centrifugal pumps with radial blades (also called straight blades) have the
simplest geometry and produce the largest pressure rise of the three.
Centrifugal pumps with forward-inclined blades produce a pressure rise that is
nearly constant.
CENTRIFUGAL PUMP
The three main types of
centrifugal pumps are those with
(a) backward-inclined blades, (b)
radial blades, and (c) forwardinclined blades; (d) comparison
of
net
head
and
brake
horsepower performance curves
for the three types of centrifugal
pumps.
VELOCITY TRIANGLE OF
THE CENTRIFUGAL PUMP
Tangential velocity (U)
Absolute velocity (V)
Relative velocity (Vrelative)
Water enters
radially, hence, the
angle between
tangential velocity,
U1, and absolute
velocity, V1 is 90
degree.
WORK DONE BY THE
CENTRIFUGAL PUMP
HEADS OF A PUMP
1. Static Head
It is the vertical distance between water levels in the sump and
the reservoir.
Let Hs = Static head on the pump
hs = Height of the centre line of pump above the
sump level (Suction head)
hd = Height of the liquid level in the tank above the
centre line of pump (Delivery head)
Then Hs = hs + hd
HEADS OF A PUMP
2. Total Head
It is the total head which has to be developed by a pump to
deliver the water form the sump into the tank. Apart from
producing the static head, a pump has also to overcome the
losses in pipes and fittings and loss due to kinetic energy at
the delivery outlet.
Let H = Total head
hfs = Losses in suction pip
hfd = Losses in delivery pipe
hf = Total friction loss in pipe = hfs + hfd
Vd = Velocity of liquid in delivery pipe.
Then H = hs+ hd + hfs + hfd + Vd2/(2g)
HEADS OF A PUMP
3. Manometric Head
It is usually not possible to measure exactly the losses in the pump
casing. So, a term known as manometric head is introduced. It is the
rise in pressure energy of the liquid in the impeller of the pump.
If two pressure gauges are installed on the suction and the delivery
sides as near to the pump as possible, the difference in their reading
will give the change in the pressure energy in the pump or the
manometric head.
Hm = Manometric head of the pump
Hms = Reading of the pressure gauge on the suction side
Hmd = Reading of the pressure gauge on the delivery side.
Then Hm = Hmd - Hms
LOSSES AND EFFICIENCIES
The following are the important efficiencies of centrifugal pump
 Manometric efficiency
 Mechanical efficiency
 Overall efficiency
LOSSES AND EFFICIENCIES
LOSSES AND EFFICIENCIES
LOSSES AND EFFICIENCIES
LEARNING ACTIVITY 1.3.1
A centrifugal pumps delivers 50 liters of water per second to a
height of 15 m through a 20 m long pipe. Diameter of the pipe is
14cm. Overall efficiency is 72%, and the coefficient of friction
0.015. Determine the power needed to drive the pump. [13,726kW]
LEARNING ACTIVITY 1.3.2
The internal and external diameters of the impeller of a centrifugal
pump are 200mm and 400mm, respectively. The pump is running at
1200r.p.m. The vane angles of the impeller at inlet and outlet are
20&ordm; and 30&ordm;, respectively. The water enters the impeller radially and
velocity of flow is constant. Determine the work done by the
impeller per unit weight of water per second. [44,1 Nm/N]
LEARNING ACTIVITY 1.3.3
A centrifugal pump has an eye diameter of 200mm and outer
diameter of 350mm. At the impeller inlet, the width (passage depth)
of the impeller is 90mm while at the outlet it is 45mm. If the vane
outlet angle is 30&ordm;, the discharge is constant at 0,18m3/s and the
speed is constant at 1450rev/min. Determine:
(a) The angle at inlet for zero whirl and smooth flow on to the
blade [11,83&ordm;]
(b) The theoretical impeller torque [638,5Nm]
(c) The head increase across the impeller, and [54,9m]
(d) The theoretical power transferred to the fluid [96,94kW]
Neglect vane thickness.
THANK YOU
Contact details
L. MASHEANE
MECHANICAL ENGINEERING
DEPARTMENT
Tel.: 051 507 3683
E-mail: lmasheane@cut.ac.za
www.cut.ac.za | Bloemfontein (051) 507 3911 | Welkom (057) 910 3500
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