PUMP,FANS & COMPRESSORS

Pumps
• Transportation of liquids is facilitated through
pipes either by gravity or by pumps.
• Pump is a device which increases mechanical
energy of the liquid increasing either its velocity,
pressure or elevation.
Pumps

Positive displacement Pumps Centrifugal pumps

`` Positive displacement pumps Centrifugal pumps
1 They apply pressure directly on to They generate high rotational
liquid by a piston rotating in an velocities converting resulting kinetic
cylinder which alternatively filled and energy of liquid into pressure energy.
emptied by liquid.
2 Closing the delivery valve results in Closing the delivery valve results in no
serious after effects. suction and discharge. The
mechanical energy is wasted in the
form of heat energy
3 Reciprocating pump, Gear Pump, Lobe Centrifugal pump is an example of
pump, vane pump, screw pump are non-positive displacement pump.
examples
4 Used for high pressure and low Used for low pressure and high
discharge discharge
 Centrifugal Pump
Main part of centrifugal pump
The following are the main part
of centrifugal pump
• 1. Impeller
• 2. Casing
• 3. Suction pipe with a foot
valve and strainer
• 4. Delivery pipe
 Components of centrifugal Pump
1. Impeller
The rotating part of a centrifugal pump is called impeller. It is
consists of a series of backward curved vanes. The impeller is
mounted on the a shaft which is connected to the shaft of an
electric motor
2. Casing:
The casing of a centrifugal pump is similar to reaction turbine. It
is an air tight passage surrounding the impeller and is designed
in such a way that the kinetic energy of water discharged at the
outlet of the impeller is converted in to pressure energy before
water leaves the casing and enters the delivery pipe.
 Components of centrifugal Pump
3. Suction pipe with foot valve and a strainer
A pipe whose one end is connected to the inlet of the pump and
other end dips into water sump is known as suction pipe. A foot
valve which is a non return valve or one way type valve is fitted
at the lower end of the suction pipe. The foot valve is opened
only in upward direction. A strainer is also fitted at the lower end
of the suction pipe.
4. Delivery pipe
A pipe whose one end is connected to the outlet of the pump
and other end delivers the water at a required height is known
as delivery pipe.
 Heads in a pump
1. Suction head
2. Delivery head
3. Static head
4. Manometric Head
Suction head: It is the vertical
distance between the liquid level
in the reservoir and eye line of
the pump. It is denote by hs.

Delivery head: It is the vertical

height of liquid surface in
overhead tank to which liquid is
to be delivered above the
centerline of the pump.

Static head: It is the vertical distance

between liquid levels in the reservoir
and overhead tank. Or it is the sum
of suction head and delivery head.
It is denote by H, then,
H =hs+hd
Manometric head: The head against which the centrifugal pump has to
work is called as Manometric head. It is denoted by Hm
 Cavitation
• Cavitation is defined as the
phenomena of formation of vapour
bubbles of a flowing liquid in a region
where the pressure of the liquid falls
below its vapour pressure and the
sudden collapsing of these vapour
bubbles in a region of higher pressure.
• When the vapour bubbles collapse , a
very high pressure is created.
• The metallic surfaces, above which
these bubbles collapse, is subjected to
these high pressure, which cause
pitting action on the surface. Thus
cavities are formed on the metallic
surface and also considerable noise
and vibrations are produced.
 Effect of cavitation
The following are the effects of cavitation:
1. The metallic surfaces are damaged and cavities are formed
on the surfaces.
2. Due to sudden collapse of vapour bubble, considerable noise
and vibrations are produced.
3. The efficiency of a turbine decreases. Due to pitting action,
the surface of the turbine blades becomes rough and the
force exerted by water on the turbine blades decreases.
Hence, the work done by water or output horse power
becomes less and thus efficiency decreases
 Precaution against cavitation
The following precautions should be taken against cavitation:

1. The pressure of the flowing liquid in any part of the hydraulic

system should not be allowed to fall below its vapour
pressure. If the flowing liquid is water, then the absolute
pressure head should not be below 2.5 m of water.
2. The special materials or coatings such as aluminum –bronze
and stainless steel, which are cavitation resistant materials ,
should be used.
Priming
 Net Positive Suction head-NPSH

To avoid cavitation, pressure at the suction side must be higher than Vapour
pressure of the liquid which is being pumped through the centrifugal pump. This
difference of suction pressure head and vapour pressure head is known as Net
Positive Suction head (NPSH)
Net Positive Suction Head(NPSH) = Net head at Pump inlet – vapor pressure head
of liquid to be pumped

Applying Bernoulli’s equation between at pump inlet and reservoir

 NPSH…. Continue

The right hand side of above

equation is the total suction
head. Hence NPSH is equal to
total suction head. Thus NPSH
may also be defined as the
total head required to make
the liquid flow through the
suction pipe to the pump
impeller.
 NPSHA and NPSHR
• For pump installation, a distinction is made between the required
NPSH and the available NPSH.
• The value of required NPSH is given by the pump manufacturer.
This value can also be determine by experimentally.
• For determining its value, the pump is tested and minimum value of
hs is obtained at which pump gives maximum efficiency without any
objectionable noise (i.e. Cavitation free). The required NPSH varies
with the pump design, speed of the pump and capacity of the pump.
• When the pump is installed , the available NPSH is calculated from
equation

• In order to have cavitation free operation of centrifugal pump , the

available NPSH should be greater than the required NPSH.
 Efficiency Of Centrifugal Pump

In case of centrifugal pump, the power is transmitted from the

shaft of the electric motor to the shaft of the pump and then to the
impeller. From the impeller the and then, the power is given to
water.
Thus power is decreasing from the shaft of the pump to the
impeller and then to the water.
the following are the important efficiency of the pump:
1. Manometric efficiency(man )
2. Mechanical efficiency (mech )
3. Overall efficiency (o )
 Efficiency Of Centrifugal Pump
MANOMETRIC EFFICIENCY
Manometric head
 man =
Head Imported By Impeller To Water

MECHANICAL EFFICIENCY
Rotor Or Impeller Power
mech =
Shaft power
OVERALL EFFICIENCY
Power Output Of Pump
o =
Power Input Of Pump
 Characteristic curves of centrifugal
pump
• Characteristics curves of centrifugal pumps are defined as
those curves which are plotted from the results of a number of
tests on the centrifugal pump.
• These curves are necessary to predict the behavior and the
performance of the pump when the pump is working under
different flow rate , head and speed. The following are the
important characteristic curves for pumps.
1. Main characteristic curves.
2. Operating characteristic curves and
3. Constant efficiency or Muschel curves
 Main characteristic curves
The main characteristic curves of
a centrifugal pump consists of
variation of head (manometric
head, Hm), power and discharge
with respect to speed.

For plotting curves of manometric

head versus speed, discharge is kept
constant,
For plotting curves of discharge
versus speed , manometric head is
kept constant, and

For plotting curves of power versus speed manometric head and

discharge are kept constant
 Operating characteristic curves
If the speed is kept constant , the
variation of manometric head , power
and efficiency with respect to
discharge gives the operating
characteristic curves of the pump.

The input power curve shall not pass

through the origin. It will be slightly
away from the origin on the y-axis, as
even at zero discharge some power is
needed to overcome the mechanical
losses.
 Constant efficiency curve

For obtaining constant

efficiency curves for a
pump, the head versus
discharge curves and
efficiency versus discharge
curves for different speed
are used.

For plotting constant efficiency curves (Also known as Iso-efficiency curves),

horizontal lines representing constant efficiency are drawn on the  - Q curves. The
points at which these lines cut the efficiency curves at various speeds are transferred
to the corresponding H~Q curves. The points having the same efficiency are then
joined by smooth curves represents the iso efficiency curves.
Fans, Blowers and Compressors
 Mechanical Energy Addition

Pumps – liquid Fan, Blower and Compressor - Gas

• Fans Blowers and Compressors are used to deliver gas at higher

pressure as compared to initial state.
Sl No Name of the Equipment Pressure rise and
Compression Ratio
1 Fan 1.5 atm, 1.11
2 Blowers 1.5 – 2.7 atm, 1.11-
1.20
3 Compressors 2 – 1000 atm, greater
than 1.2

• Pumps & Fans do not have an appreciable changes in density (Flow

of fluid is incompressible)
• Blowers and Compressors - density change of fluid is appreciable.
 Fans
Large fans are usually centrifugal, operating on exactly the same
principle as centrifugal pumps. Their impeller blades, however, may be
curved forward, this would lead to insatiability in a pump but not in a
fan
– Centrifugal flow Fan ( Air flow changes twice)
– Axial Flow Fan ( No change in direction of air)
 Centrifugal fans

• Radial fans can generate high static pressures to range of 1.8 atm.
• Design is simple, used for high temperature applications.
• Forward curved fans are used for moving large air volume against relatively low air
pressures
• Designed for low temperature applications
• Backward inclined fans power consumption drops within airflow range
• Non-overloading as the changes in static pressure does not overload the motor.
 System characteristics and Fan curve
Characteristic curves for Fan are called as “Fan Curves”
Developed for a given set of operating conditions.
• Fan volume
• System static pressure
• Fan speed & Brake Horse Power
 System resistance in Fans
• System resistance refers to Static
Pressure losses in the system (losses
encountered in short radius elbows
and narrow ducts)
• It varies as the square of the volume
of air flowing the system
• Effect – less volume of air which is
being pumped through the system.
• Resistance decreases as the flow
decreases.
• To determine the volume of air fan
will produce it is important to know
the system resistance characteristics.
 Fan efficiency

• Fan efficiency is expressed in terms of Total efficiency and Static Efficiency

• Total Pressure in inches of water gauge

• Air flow in CFM
• BHP is Brake horse power
• 6362 is unit consistency factor
Compressors
 Compressors
• Primary objective of compressors is to
compress the fluid into small volume while
increasing the pressure and temperature of
the fluid.
 Compressors

Positive displacement Centrifugal

Compressors Compressors
• Used for discharge applications up to 6 atm. • Series of impellers rotating at high
• Suited with high discharge and medium speeds inside a casing
pressure applications • Handle 340,000m3/hr of air or
• Operate in a similar manner as that of process gas at pressure of 20 atm.
reciprocating pumps • Interstage cooling is required in
• Prevention of leak and temperature rise is high pressure units
more important
• Cylinder head and walls are jacketed to
facilitate refrigerant cooling
• Multistage compressors are used in order to
achieve higher compression ratios
Compressor efficiency
 Vacuum pumps
• A compressor that takes suction at a pressure below atmospheric and
discharges against atmospheric pressure is called a vacuum pump.
• Any type of blower or compressor, reciprocating, rotary or centrifugal can be
adapted to vacuum practice by modifying the design to accept very low
density gas at the suction and attain the large compression ratios necessary
• As the absolute pressure at suction decreases, the volumetric efficiency drops
and approaches zero at the lowest absolute pressure attainable by pump.
• Usually the mechanical efficiency is also lower than for compressors. The
required displacement increases rapidly as the suction pressure falls, so large
machine is needed to move much gas.
• The compression ratio used in vacuum pumps is much higher than in
compressors, ranging up to 100 or more, with a correspondingly high
adiabatic discharge temperature. Actually however , the compression is nearly
isothermal because of the low mass flow rate and the effective heat transfer
from the relatively large area of exposed metal.