Pumps and Fans - Chapter Notes, Thermal Engineering, Mechanical Engineering Notes | EduRev

: Pumps and Fans - Chapter Notes, Thermal Engineering, Mechanical Engineering Notes | EduRev

 Page 1


4-245
© 2000 by CRC Press LLC
4.8 Pumps and Fans
Robert F. Boehm
Introduction
Pumps are devices that impart a pressure increase to a liquid. Fans are used to increase the velocity of
a gas, but this is also accomplished through an increase in pressure. The pressure rise found in pumps
can vary tremendously, and this is a very important design parameter along with the liquid ?ow rate.
This pressure rise can range from simply increasing the elevation of the liquid to increasing the pressure
hundreds of atmospheres. Fan applications, on the other hand, generally deal with small pressure
increases. In spite of this seemingly signi?cant distinction between pumps and fans, there are many
similarities in the fundamentals of certain types of these machines as well as with their application and
theory of operation.
The appropriate use of pumps and fans depends upon the proper choice of device and the proper
design and installation for the application. A check of sources of commercial equipment shows that
many varieties of pumps and fans exist. Each of these had special characteristics that must be appreciated
for achieving proper function. Preliminary design criteria for choosing between different types is given
by Boehm (1987).
As is to be expected, the wise applications of pumps and fans requires knowledge of ?uid ?ow
fundamentals. Unless the ?uid mechanics of a particular application are understood, the design could
be less than desirable.
In this section, pump and fan types are brie?y de?ned. In addition, typical application information is
given. Also, some ideas from ?uid mechanics that are especially relevant to pump and fan operation are
reviewed.
Pumps
Raising of water from wells and cisterns is the earliest form of pumping (a very detailed history of early
applications is given by Ewbank, 1842). Modern applications are much broader, and these ?nd a wide
variety of machines in use. Modern pumps function on one of two principles. By far the majority of
pump installations are of the velocity head type. In these devices, the pressure rise is achieved by giving
the ?uid a movement. At the exit of the machine, this movement is translated into a pressure increase.
The other major type of pump is called positive displacement. These devices are designed to increase
the pressure of the liquid while essentially trying to compress the volume. A categorization of pump
types has been given by Krutzsch (1986), and an adaptation of this is shown below.
I. Velocity head
A. Centrifugal
1. Axial ?ow (single or multistage)
2. Radial ?ow (single or double suction)
3. Mixed ?ow (single or double suction)
4. Peripheral (single or multistage)
B. Special Effect
1. Gas lift
2. Jet
3. Hydraulic ram
4. Electromagnetic
II. Positive displacement
A. Reciprocating
1. Piston, plunger
a. Direct acting (simplex or duplex)
Page 2


4-245
© 2000 by CRC Press LLC
4.8 Pumps and Fans
Robert F. Boehm
Introduction
Pumps are devices that impart a pressure increase to a liquid. Fans are used to increase the velocity of
a gas, but this is also accomplished through an increase in pressure. The pressure rise found in pumps
can vary tremendously, and this is a very important design parameter along with the liquid ?ow rate.
This pressure rise can range from simply increasing the elevation of the liquid to increasing the pressure
hundreds of atmospheres. Fan applications, on the other hand, generally deal with small pressure
increases. In spite of this seemingly signi?cant distinction between pumps and fans, there are many
similarities in the fundamentals of certain types of these machines as well as with their application and
theory of operation.
The appropriate use of pumps and fans depends upon the proper choice of device and the proper
design and installation for the application. A check of sources of commercial equipment shows that
many varieties of pumps and fans exist. Each of these had special characteristics that must be appreciated
for achieving proper function. Preliminary design criteria for choosing between different types is given
by Boehm (1987).
As is to be expected, the wise applications of pumps and fans requires knowledge of ?uid ?ow
fundamentals. Unless the ?uid mechanics of a particular application are understood, the design could
be less than desirable.
In this section, pump and fan types are brie?y de?ned. In addition, typical application information is
given. Also, some ideas from ?uid mechanics that are especially relevant to pump and fan operation are
reviewed.
Pumps
Raising of water from wells and cisterns is the earliest form of pumping (a very detailed history of early
applications is given by Ewbank, 1842). Modern applications are much broader, and these ?nd a wide
variety of machines in use. Modern pumps function on one of two principles. By far the majority of
pump installations are of the velocity head type. In these devices, the pressure rise is achieved by giving
the ?uid a movement. At the exit of the machine, this movement is translated into a pressure increase.
The other major type of pump is called positive displacement. These devices are designed to increase
the pressure of the liquid while essentially trying to compress the volume. A categorization of pump
types has been given by Krutzsch (1986), and an adaptation of this is shown below.
I. Velocity head
A. Centrifugal
1. Axial ?ow (single or multistage)
2. Radial ?ow (single or double suction)
3. Mixed ?ow (single or double suction)
4. Peripheral (single or multistage)
B. Special Effect
1. Gas lift
2. Jet
3. Hydraulic ram
4. Electromagnetic
II. Positive displacement
A. Reciprocating
1. Piston, plunger
a. Direct acting (simplex or duplex)
4-246
© 2000 by CRC Press LLC
b. Power (single or double acting, simplex, duplex, triplex, multiplex)
2. Diaphragm (mechanically or ?uid driven, simplex or multiplex)
B. Rotary
1. Single rotor (vane, piston, screw, ?exible member, peristaltic)
2. Multiple rotor (gear, lobe, screw, circumferential piston)
In the next subsection, some of the more common pumps are described.
Centrifugal and Other Velocity Head Pumps
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily
because these pumps offer low initial and upkeep costs. Traditionally, pumps of this type have been
limited to low-pressure-head applications, but modern pump designs have overcome this problem unless
very high pressures are required. Some of the other good characteristics of these types of devices include
smooth (nonpulsating) ?ow and the ability to tolerate non?ow conditions.
The most important parts of the centrifugal pump are the impeller and volute. An impeller can take
on many forms, ranging from essentially a spinning disk to designs with elaborate vanes. The latter is
usual. Impeller design tends to be somewhat unique to each manufacturer, as well as ?nding a variety
of designs for a variety of applications. An example of an impeller is shown in Figure 4.8.1. This device
imparts a radial velocity to the ?uid that has entered the pump perpendicular to the impeller. The volute
(there may be one or more) performs the function of slowing the ?uid and increasing the pressure. A
good discussion of centrifugal pumps is given by Lobanoff and Ross (1992).
A very important factor in the speci?cation of a centrifugal pump is the casing orientation and type.
For example, the pump can be oriented vertically or horizontally. Horizontal mounting is most common.
Vertical pumps usually offer bene?ts related to ease of priming and reduction in required net positive
suction head (see discussion below). This type also requires less ?oor space. Submersible and immersible
pumps are always of the vertical type. Another factor in the design is the way the casing is split, and
this has implications about ease of manufacture and repair. Casings that are split perpendicular to the
shaft are called radially split, while those split parallel to the shaft axis are denoted as axially split. The
latter can be horizontally split or vertically split. The number of stages in the pump greatly affects the
pump-output characteristics. Several stages can be incorporated into the same casing, with an associated
increase in pump output. Multistage pumps are often used for applications with total developed head
over 50 atm.
Whether or not a pump is self-priming can be important. If a centrifugal pump is ?lled with air when
it is turned on, the initiation of pumping action may not be suf?cient to bring the ?uid into the pump.
Pumps can be speci?ed with features that can minimize priming problems.
There are other types of velocity head pumps. Jet pumps increase pressure by imparting momentum
from a high-velocity liquid stream to a low-velocity or stagnant body of liquid. The resulting ?ow then
FIGURE 4.8.1. A schematic of a centrifugal pump is shown. The liquid enters perpendicular to the ?gure, and a
radial velocity is imparted by clockwise spin of the impeller.
Page 3


4-245
© 2000 by CRC Press LLC
4.8 Pumps and Fans
Robert F. Boehm
Introduction
Pumps are devices that impart a pressure increase to a liquid. Fans are used to increase the velocity of
a gas, but this is also accomplished through an increase in pressure. The pressure rise found in pumps
can vary tremendously, and this is a very important design parameter along with the liquid ?ow rate.
This pressure rise can range from simply increasing the elevation of the liquid to increasing the pressure
hundreds of atmospheres. Fan applications, on the other hand, generally deal with small pressure
increases. In spite of this seemingly signi?cant distinction between pumps and fans, there are many
similarities in the fundamentals of certain types of these machines as well as with their application and
theory of operation.
The appropriate use of pumps and fans depends upon the proper choice of device and the proper
design and installation for the application. A check of sources of commercial equipment shows that
many varieties of pumps and fans exist. Each of these had special characteristics that must be appreciated
for achieving proper function. Preliminary design criteria for choosing between different types is given
by Boehm (1987).
As is to be expected, the wise applications of pumps and fans requires knowledge of ?uid ?ow
fundamentals. Unless the ?uid mechanics of a particular application are understood, the design could
be less than desirable.
In this section, pump and fan types are brie?y de?ned. In addition, typical application information is
given. Also, some ideas from ?uid mechanics that are especially relevant to pump and fan operation are
reviewed.
Pumps
Raising of water from wells and cisterns is the earliest form of pumping (a very detailed history of early
applications is given by Ewbank, 1842). Modern applications are much broader, and these ?nd a wide
variety of machines in use. Modern pumps function on one of two principles. By far the majority of
pump installations are of the velocity head type. In these devices, the pressure rise is achieved by giving
the ?uid a movement. At the exit of the machine, this movement is translated into a pressure increase.
The other major type of pump is called positive displacement. These devices are designed to increase
the pressure of the liquid while essentially trying to compress the volume. A categorization of pump
types has been given by Krutzsch (1986), and an adaptation of this is shown below.
I. Velocity head
A. Centrifugal
1. Axial ?ow (single or multistage)
2. Radial ?ow (single or double suction)
3. Mixed ?ow (single or double suction)
4. Peripheral (single or multistage)
B. Special Effect
1. Gas lift
2. Jet
3. Hydraulic ram
4. Electromagnetic
II. Positive displacement
A. Reciprocating
1. Piston, plunger
a. Direct acting (simplex or duplex)
4-246
© 2000 by CRC Press LLC
b. Power (single or double acting, simplex, duplex, triplex, multiplex)
2. Diaphragm (mechanically or ?uid driven, simplex or multiplex)
B. Rotary
1. Single rotor (vane, piston, screw, ?exible member, peristaltic)
2. Multiple rotor (gear, lobe, screw, circumferential piston)
In the next subsection, some of the more common pumps are described.
Centrifugal and Other Velocity Head Pumps
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily
because these pumps offer low initial and upkeep costs. Traditionally, pumps of this type have been
limited to low-pressure-head applications, but modern pump designs have overcome this problem unless
very high pressures are required. Some of the other good characteristics of these types of devices include
smooth (nonpulsating) ?ow and the ability to tolerate non?ow conditions.
The most important parts of the centrifugal pump are the impeller and volute. An impeller can take
on many forms, ranging from essentially a spinning disk to designs with elaborate vanes. The latter is
usual. Impeller design tends to be somewhat unique to each manufacturer, as well as ?nding a variety
of designs for a variety of applications. An example of an impeller is shown in Figure 4.8.1. This device
imparts a radial velocity to the ?uid that has entered the pump perpendicular to the impeller. The volute
(there may be one or more) performs the function of slowing the ?uid and increasing the pressure. A
good discussion of centrifugal pumps is given by Lobanoff and Ross (1992).
A very important factor in the speci?cation of a centrifugal pump is the casing orientation and type.
For example, the pump can be oriented vertically or horizontally. Horizontal mounting is most common.
Vertical pumps usually offer bene?ts related to ease of priming and reduction in required net positive
suction head (see discussion below). This type also requires less ?oor space. Submersible and immersible
pumps are always of the vertical type. Another factor in the design is the way the casing is split, and
this has implications about ease of manufacture and repair. Casings that are split perpendicular to the
shaft are called radially split, while those split parallel to the shaft axis are denoted as axially split. The
latter can be horizontally split or vertically split. The number of stages in the pump greatly affects the
pump-output characteristics. Several stages can be incorporated into the same casing, with an associated
increase in pump output. Multistage pumps are often used for applications with total developed head
over 50 atm.
Whether or not a pump is self-priming can be important. If a centrifugal pump is ?lled with air when
it is turned on, the initiation of pumping action may not be suf?cient to bring the ?uid into the pump.
Pumps can be speci?ed with features that can minimize priming problems.
There are other types of velocity head pumps. Jet pumps increase pressure by imparting momentum
from a high-velocity liquid stream to a low-velocity or stagnant body of liquid. The resulting ?ow then
FIGURE 4.8.1. A schematic of a centrifugal pump is shown. The liquid enters perpendicular to the ?gure, and a
radial velocity is imparted by clockwise spin of the impeller.
4-247
© 2000 by CRC Press LLC
goes through a diffuser to achieve an overall pressure increase. Gas lifts accomplish a pumping action
by a drag on gas bubbles that rise through a liquid.
Positive-Displacement Pumps
Positive-displacement pumps demonstrate high discharge pressures and low ?ow rates. Usually, this is
accomplished by some type of pulsating device. A piston pump is a classic example of positive-
displacement machines. Rotary pumps are one type of positive-displacement device that do not impart
pulsations to the existing ?ow (a full description of these types of pumps is given by Turton, 1994).
Several techniques are available for dealing with pulsating ?ows, including use of double-acting pumps
(usually of the reciprocating type) and installation of pulsation dampeners.
Positive-displacement pumps usually require special seals to contain the ?uid. Costs are higher both
initially and for maintenance compared with most pumps that operate on the velocity head basis. Positive-
displacement pumps demonstrate an ef?ciency that is nearly independent of ?ow rate, in contrast to the
velocity head type (see Figure 4.8.2 and the discussion related to it below).
Reciprocating pumps offer very high ef?ciencies, reaching 90% in larger sizes. These types of pumps
are more appropriate for pumping abrasive liquids (e.g., slurries) than are centrifugal pumps.
A characteristic of positive displacement pumps which may be valuable is that the output ?ow is
proportional to pump speed. This allows this type of pump to be used for metering applications. Also a
positive aspect of these pumps is that they are self-priming, except at initial start-up.
Very high head pressures (often damaging to the pump) can be developed in positive-displacement
pumps if the downstream ?ow is blocked. For this reason, a pressure-relief-valve bypass must always
be used with positive-displacement pumps.
Pump/Flow Considerations
Performance characteristics of the pump must be considered in system design. Simple diagrams of pump
applications are shown in Figure 4.8.2. First, consider the left-hand ?gure. This represents a ?ow circuit,
and the pressure drops related to the piping, ?ttings, valves, and any other ?ow devices found in the
circuit must be estimated using the laws of ?uid mechanics. Usually, these resistances (pressure drops)
are found to vary approximately with the square of the liquid ?ow rate. Typical characteristics are shown
in Figure 4.8.3. Most pumps demonstrate a ?ow vs. pressure rise variation that is a positive value at
zero ?ow and decreases to zero at some larger ?ow. Positive-displacement pumps, as shown on the right-
hand side of Figure 4.8.3, are an exception to this in that these devices usually cannot tolerate a zero
?ow. An important aspect to note is that a closed system can presumably be pressurized. A contrasting
situation and its implications are discussed below.
The piping diagram show on the right-hand side of Figure 4.8.2 is a once-through system, another
frequently encountered installation. However, the leg of piping through “pressure drop 1” shown there
can have some very important implications related to net positive suction head, often denoted as NPSH.
In simple terms, NPSH indicates the difference between the local pressure and the thermodynamic
saturation pressure at the ?uid temperature. If NPSH = 0, the liquid can vaporize, and this can result in
a variety of outcomes from noisy pump operation to outright failure of components. This condition is
called cavitation. Cavitation, if it occurs, will ?rst take place at the lowest pressure point within the
piping arrangement. Often this point is located at, or inside, the inlet to the pump. Most manufacturers
specify how much NPSH is required for satisfactory operation of their pumps. Hence, the actual NPSH
(denoted as NPSHA) experienced by the pump must be larger than the manufacturer’s required NPSH
(called NPSHR). If a design indicates insuf?cient NPSH, changes should be made in the system, possibly
including alternative piping layout, including elevation and/or size, or use of a pump with smaller NPSH
requirements.
The manufacturer should be consulted for a map of operational information for a given pump. A
typical form is shown in Figure 4.8.4. This information will allow the designer to select a pump that
satis?ed the circuit operational requirements while meeting the necessary NPSH and most-ef?cient-
operation criteria.
Page 4


4-245
© 2000 by CRC Press LLC
4.8 Pumps and Fans
Robert F. Boehm
Introduction
Pumps are devices that impart a pressure increase to a liquid. Fans are used to increase the velocity of
a gas, but this is also accomplished through an increase in pressure. The pressure rise found in pumps
can vary tremendously, and this is a very important design parameter along with the liquid ?ow rate.
This pressure rise can range from simply increasing the elevation of the liquid to increasing the pressure
hundreds of atmospheres. Fan applications, on the other hand, generally deal with small pressure
increases. In spite of this seemingly signi?cant distinction between pumps and fans, there are many
similarities in the fundamentals of certain types of these machines as well as with their application and
theory of operation.
The appropriate use of pumps and fans depends upon the proper choice of device and the proper
design and installation for the application. A check of sources of commercial equipment shows that
many varieties of pumps and fans exist. Each of these had special characteristics that must be appreciated
for achieving proper function. Preliminary design criteria for choosing between different types is given
by Boehm (1987).
As is to be expected, the wise applications of pumps and fans requires knowledge of ?uid ?ow
fundamentals. Unless the ?uid mechanics of a particular application are understood, the design could
be less than desirable.
In this section, pump and fan types are brie?y de?ned. In addition, typical application information is
given. Also, some ideas from ?uid mechanics that are especially relevant to pump and fan operation are
reviewed.
Pumps
Raising of water from wells and cisterns is the earliest form of pumping (a very detailed history of early
applications is given by Ewbank, 1842). Modern applications are much broader, and these ?nd a wide
variety of machines in use. Modern pumps function on one of two principles. By far the majority of
pump installations are of the velocity head type. In these devices, the pressure rise is achieved by giving
the ?uid a movement. At the exit of the machine, this movement is translated into a pressure increase.
The other major type of pump is called positive displacement. These devices are designed to increase
the pressure of the liquid while essentially trying to compress the volume. A categorization of pump
types has been given by Krutzsch (1986), and an adaptation of this is shown below.
I. Velocity head
A. Centrifugal
1. Axial ?ow (single or multistage)
2. Radial ?ow (single or double suction)
3. Mixed ?ow (single or double suction)
4. Peripheral (single or multistage)
B. Special Effect
1. Gas lift
2. Jet
3. Hydraulic ram
4. Electromagnetic
II. Positive displacement
A. Reciprocating
1. Piston, plunger
a. Direct acting (simplex or duplex)
4-246
© 2000 by CRC Press LLC
b. Power (single or double acting, simplex, duplex, triplex, multiplex)
2. Diaphragm (mechanically or ?uid driven, simplex or multiplex)
B. Rotary
1. Single rotor (vane, piston, screw, ?exible member, peristaltic)
2. Multiple rotor (gear, lobe, screw, circumferential piston)
In the next subsection, some of the more common pumps are described.
Centrifugal and Other Velocity Head Pumps
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily
because these pumps offer low initial and upkeep costs. Traditionally, pumps of this type have been
limited to low-pressure-head applications, but modern pump designs have overcome this problem unless
very high pressures are required. Some of the other good characteristics of these types of devices include
smooth (nonpulsating) ?ow and the ability to tolerate non?ow conditions.
The most important parts of the centrifugal pump are the impeller and volute. An impeller can take
on many forms, ranging from essentially a spinning disk to designs with elaborate vanes. The latter is
usual. Impeller design tends to be somewhat unique to each manufacturer, as well as ?nding a variety
of designs for a variety of applications. An example of an impeller is shown in Figure 4.8.1. This device
imparts a radial velocity to the ?uid that has entered the pump perpendicular to the impeller. The volute
(there may be one or more) performs the function of slowing the ?uid and increasing the pressure. A
good discussion of centrifugal pumps is given by Lobanoff and Ross (1992).
A very important factor in the speci?cation of a centrifugal pump is the casing orientation and type.
For example, the pump can be oriented vertically or horizontally. Horizontal mounting is most common.
Vertical pumps usually offer bene?ts related to ease of priming and reduction in required net positive
suction head (see discussion below). This type also requires less ?oor space. Submersible and immersible
pumps are always of the vertical type. Another factor in the design is the way the casing is split, and
this has implications about ease of manufacture and repair. Casings that are split perpendicular to the
shaft are called radially split, while those split parallel to the shaft axis are denoted as axially split. The
latter can be horizontally split or vertically split. The number of stages in the pump greatly affects the
pump-output characteristics. Several stages can be incorporated into the same casing, with an associated
increase in pump output. Multistage pumps are often used for applications with total developed head
over 50 atm.
Whether or not a pump is self-priming can be important. If a centrifugal pump is ?lled with air when
it is turned on, the initiation of pumping action may not be suf?cient to bring the ?uid into the pump.
Pumps can be speci?ed with features that can minimize priming problems.
There are other types of velocity head pumps. Jet pumps increase pressure by imparting momentum
from a high-velocity liquid stream to a low-velocity or stagnant body of liquid. The resulting ?ow then
FIGURE 4.8.1. A schematic of a centrifugal pump is shown. The liquid enters perpendicular to the ?gure, and a
radial velocity is imparted by clockwise spin of the impeller.
4-247
© 2000 by CRC Press LLC
goes through a diffuser to achieve an overall pressure increase. Gas lifts accomplish a pumping action
by a drag on gas bubbles that rise through a liquid.
Positive-Displacement Pumps
Positive-displacement pumps demonstrate high discharge pressures and low ?ow rates. Usually, this is
accomplished by some type of pulsating device. A piston pump is a classic example of positive-
displacement machines. Rotary pumps are one type of positive-displacement device that do not impart
pulsations to the existing ?ow (a full description of these types of pumps is given by Turton, 1994).
Several techniques are available for dealing with pulsating ?ows, including use of double-acting pumps
(usually of the reciprocating type) and installation of pulsation dampeners.
Positive-displacement pumps usually require special seals to contain the ?uid. Costs are higher both
initially and for maintenance compared with most pumps that operate on the velocity head basis. Positive-
displacement pumps demonstrate an ef?ciency that is nearly independent of ?ow rate, in contrast to the
velocity head type (see Figure 4.8.2 and the discussion related to it below).
Reciprocating pumps offer very high ef?ciencies, reaching 90% in larger sizes. These types of pumps
are more appropriate for pumping abrasive liquids (e.g., slurries) than are centrifugal pumps.
A characteristic of positive displacement pumps which may be valuable is that the output ?ow is
proportional to pump speed. This allows this type of pump to be used for metering applications. Also a
positive aspect of these pumps is that they are self-priming, except at initial start-up.
Very high head pressures (often damaging to the pump) can be developed in positive-displacement
pumps if the downstream ?ow is blocked. For this reason, a pressure-relief-valve bypass must always
be used with positive-displacement pumps.
Pump/Flow Considerations
Performance characteristics of the pump must be considered in system design. Simple diagrams of pump
applications are shown in Figure 4.8.2. First, consider the left-hand ?gure. This represents a ?ow circuit,
and the pressure drops related to the piping, ?ttings, valves, and any other ?ow devices found in the
circuit must be estimated using the laws of ?uid mechanics. Usually, these resistances (pressure drops)
are found to vary approximately with the square of the liquid ?ow rate. Typical characteristics are shown
in Figure 4.8.3. Most pumps demonstrate a ?ow vs. pressure rise variation that is a positive value at
zero ?ow and decreases to zero at some larger ?ow. Positive-displacement pumps, as shown on the right-
hand side of Figure 4.8.3, are an exception to this in that these devices usually cannot tolerate a zero
?ow. An important aspect to note is that a closed system can presumably be pressurized. A contrasting
situation and its implications are discussed below.
The piping diagram show on the right-hand side of Figure 4.8.2 is a once-through system, another
frequently encountered installation. However, the leg of piping through “pressure drop 1” shown there
can have some very important implications related to net positive suction head, often denoted as NPSH.
In simple terms, NPSH indicates the difference between the local pressure and the thermodynamic
saturation pressure at the ?uid temperature. If NPSH = 0, the liquid can vaporize, and this can result in
a variety of outcomes from noisy pump operation to outright failure of components. This condition is
called cavitation. Cavitation, if it occurs, will ?rst take place at the lowest pressure point within the
piping arrangement. Often this point is located at, or inside, the inlet to the pump. Most manufacturers
specify how much NPSH is required for satisfactory operation of their pumps. Hence, the actual NPSH
(denoted as NPSHA) experienced by the pump must be larger than the manufacturer’s required NPSH
(called NPSHR). If a design indicates insuf?cient NPSH, changes should be made in the system, possibly
including alternative piping layout, including elevation and/or size, or use of a pump with smaller NPSH
requirements.
The manufacturer should be consulted for a map of operational information for a given pump. A
typical form is shown in Figure 4.8.4. This information will allow the designer to select a pump that
satis?ed the circuit operational requirements while meeting the necessary NPSH and most-ef?cient-
operation criteria.
4-248
© 2000 by CRC Press LLC
Several options are available to the designer for combining pumps in systems. Consider a comparison
of the net effect between operating pumps in series or operating the same two pumps in parallel. Examples
of this for pumps with characteristics such as centrifugal units are shown in Figure 4.8.5. It is clear that
FIGURE 4.8.2. Typical pump applications, either in circuits or once-through arrangements, can be represented as
combined ?uid resistances as shown. The resistances are determined from ?uid mechanics analyses.
FIGURE 4.8.3. An overlay of the pump ?ow vs. head curve with the circuit piping characteristics gives the operating
state of the circuit. A typical velocity head pump characteristic is shown on the left, while a positive-displacement
pump curve is shown on the right.
FIGURE 4.8.4. A full range of performance information should be available from the pump manufacturer, and this
may include the parameters shown.
Page 5


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© 2000 by CRC Press LLC
4.8 Pumps and Fans
Robert F. Boehm
Introduction
Pumps are devices that impart a pressure increase to a liquid. Fans are used to increase the velocity of
a gas, but this is also accomplished through an increase in pressure. The pressure rise found in pumps
can vary tremendously, and this is a very important design parameter along with the liquid ?ow rate.
This pressure rise can range from simply increasing the elevation of the liquid to increasing the pressure
hundreds of atmospheres. Fan applications, on the other hand, generally deal with small pressure
increases. In spite of this seemingly signi?cant distinction between pumps and fans, there are many
similarities in the fundamentals of certain types of these machines as well as with their application and
theory of operation.
The appropriate use of pumps and fans depends upon the proper choice of device and the proper
design and installation for the application. A check of sources of commercial equipment shows that
many varieties of pumps and fans exist. Each of these had special characteristics that must be appreciated
for achieving proper function. Preliminary design criteria for choosing between different types is given
by Boehm (1987).
As is to be expected, the wise applications of pumps and fans requires knowledge of ?uid ?ow
fundamentals. Unless the ?uid mechanics of a particular application are understood, the design could
be less than desirable.
In this section, pump and fan types are brie?y de?ned. In addition, typical application information is
given. Also, some ideas from ?uid mechanics that are especially relevant to pump and fan operation are
reviewed.
Pumps
Raising of water from wells and cisterns is the earliest form of pumping (a very detailed history of early
applications is given by Ewbank, 1842). Modern applications are much broader, and these ?nd a wide
variety of machines in use. Modern pumps function on one of two principles. By far the majority of
pump installations are of the velocity head type. In these devices, the pressure rise is achieved by giving
the ?uid a movement. At the exit of the machine, this movement is translated into a pressure increase.
The other major type of pump is called positive displacement. These devices are designed to increase
the pressure of the liquid while essentially trying to compress the volume. A categorization of pump
types has been given by Krutzsch (1986), and an adaptation of this is shown below.
I. Velocity head
A. Centrifugal
1. Axial ?ow (single or multistage)
2. Radial ?ow (single or double suction)
3. Mixed ?ow (single or double suction)
4. Peripheral (single or multistage)
B. Special Effect
1. Gas lift
2. Jet
3. Hydraulic ram
4. Electromagnetic
II. Positive displacement
A. Reciprocating
1. Piston, plunger
a. Direct acting (simplex or duplex)
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b. Power (single or double acting, simplex, duplex, triplex, multiplex)
2. Diaphragm (mechanically or ?uid driven, simplex or multiplex)
B. Rotary
1. Single rotor (vane, piston, screw, ?exible member, peristaltic)
2. Multiple rotor (gear, lobe, screw, circumferential piston)
In the next subsection, some of the more common pumps are described.
Centrifugal and Other Velocity Head Pumps
Centrifugal pumps are used in more industrial applications than any other kind of pump. This is primarily
because these pumps offer low initial and upkeep costs. Traditionally, pumps of this type have been
limited to low-pressure-head applications, but modern pump designs have overcome this problem unless
very high pressures are required. Some of the other good characteristics of these types of devices include
smooth (nonpulsating) ?ow and the ability to tolerate non?ow conditions.
The most important parts of the centrifugal pump are the impeller and volute. An impeller can take
on many forms, ranging from essentially a spinning disk to designs with elaborate vanes. The latter is
usual. Impeller design tends to be somewhat unique to each manufacturer, as well as ?nding a variety
of designs for a variety of applications. An example of an impeller is shown in Figure 4.8.1. This device
imparts a radial velocity to the ?uid that has entered the pump perpendicular to the impeller. The volute
(there may be one or more) performs the function of slowing the ?uid and increasing the pressure. A
good discussion of centrifugal pumps is given by Lobanoff and Ross (1992).
A very important factor in the speci?cation of a centrifugal pump is the casing orientation and type.
For example, the pump can be oriented vertically or horizontally. Horizontal mounting is most common.
Vertical pumps usually offer bene?ts related to ease of priming and reduction in required net positive
suction head (see discussion below). This type also requires less ?oor space. Submersible and immersible
pumps are always of the vertical type. Another factor in the design is the way the casing is split, and
this has implications about ease of manufacture and repair. Casings that are split perpendicular to the
shaft are called radially split, while those split parallel to the shaft axis are denoted as axially split. The
latter can be horizontally split or vertically split. The number of stages in the pump greatly affects the
pump-output characteristics. Several stages can be incorporated into the same casing, with an associated
increase in pump output. Multistage pumps are often used for applications with total developed head
over 50 atm.
Whether or not a pump is self-priming can be important. If a centrifugal pump is ?lled with air when
it is turned on, the initiation of pumping action may not be suf?cient to bring the ?uid into the pump.
Pumps can be speci?ed with features that can minimize priming problems.
There are other types of velocity head pumps. Jet pumps increase pressure by imparting momentum
from a high-velocity liquid stream to a low-velocity or stagnant body of liquid. The resulting ?ow then
FIGURE 4.8.1. A schematic of a centrifugal pump is shown. The liquid enters perpendicular to the ?gure, and a
radial velocity is imparted by clockwise spin of the impeller.
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goes through a diffuser to achieve an overall pressure increase. Gas lifts accomplish a pumping action
by a drag on gas bubbles that rise through a liquid.
Positive-Displacement Pumps
Positive-displacement pumps demonstrate high discharge pressures and low ?ow rates. Usually, this is
accomplished by some type of pulsating device. A piston pump is a classic example of positive-
displacement machines. Rotary pumps are one type of positive-displacement device that do not impart
pulsations to the existing ?ow (a full description of these types of pumps is given by Turton, 1994).
Several techniques are available for dealing with pulsating ?ows, including use of double-acting pumps
(usually of the reciprocating type) and installation of pulsation dampeners.
Positive-displacement pumps usually require special seals to contain the ?uid. Costs are higher both
initially and for maintenance compared with most pumps that operate on the velocity head basis. Positive-
displacement pumps demonstrate an ef?ciency that is nearly independent of ?ow rate, in contrast to the
velocity head type (see Figure 4.8.2 and the discussion related to it below).
Reciprocating pumps offer very high ef?ciencies, reaching 90% in larger sizes. These types of pumps
are more appropriate for pumping abrasive liquids (e.g., slurries) than are centrifugal pumps.
A characteristic of positive displacement pumps which may be valuable is that the output ?ow is
proportional to pump speed. This allows this type of pump to be used for metering applications. Also a
positive aspect of these pumps is that they are self-priming, except at initial start-up.
Very high head pressures (often damaging to the pump) can be developed in positive-displacement
pumps if the downstream ?ow is blocked. For this reason, a pressure-relief-valve bypass must always
be used with positive-displacement pumps.
Pump/Flow Considerations
Performance characteristics of the pump must be considered in system design. Simple diagrams of pump
applications are shown in Figure 4.8.2. First, consider the left-hand ?gure. This represents a ?ow circuit,
and the pressure drops related to the piping, ?ttings, valves, and any other ?ow devices found in the
circuit must be estimated using the laws of ?uid mechanics. Usually, these resistances (pressure drops)
are found to vary approximately with the square of the liquid ?ow rate. Typical characteristics are shown
in Figure 4.8.3. Most pumps demonstrate a ?ow vs. pressure rise variation that is a positive value at
zero ?ow and decreases to zero at some larger ?ow. Positive-displacement pumps, as shown on the right-
hand side of Figure 4.8.3, are an exception to this in that these devices usually cannot tolerate a zero
?ow. An important aspect to note is that a closed system can presumably be pressurized. A contrasting
situation and its implications are discussed below.
The piping diagram show on the right-hand side of Figure 4.8.2 is a once-through system, another
frequently encountered installation. However, the leg of piping through “pressure drop 1” shown there
can have some very important implications related to net positive suction head, often denoted as NPSH.
In simple terms, NPSH indicates the difference between the local pressure and the thermodynamic
saturation pressure at the ?uid temperature. If NPSH = 0, the liquid can vaporize, and this can result in
a variety of outcomes from noisy pump operation to outright failure of components. This condition is
called cavitation. Cavitation, if it occurs, will ?rst take place at the lowest pressure point within the
piping arrangement. Often this point is located at, or inside, the inlet to the pump. Most manufacturers
specify how much NPSH is required for satisfactory operation of their pumps. Hence, the actual NPSH
(denoted as NPSHA) experienced by the pump must be larger than the manufacturer’s required NPSH
(called NPSHR). If a design indicates insuf?cient NPSH, changes should be made in the system, possibly
including alternative piping layout, including elevation and/or size, or use of a pump with smaller NPSH
requirements.
The manufacturer should be consulted for a map of operational information for a given pump. A
typical form is shown in Figure 4.8.4. This information will allow the designer to select a pump that
satis?ed the circuit operational requirements while meeting the necessary NPSH and most-ef?cient-
operation criteria.
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© 2000 by CRC Press LLC
Several options are available to the designer for combining pumps in systems. Consider a comparison
of the net effect between operating pumps in series or operating the same two pumps in parallel. Examples
of this for pumps with characteristics such as centrifugal units are shown in Figure 4.8.5. It is clear that
FIGURE 4.8.2. Typical pump applications, either in circuits or once-through arrangements, can be represented as
combined ?uid resistances as shown. The resistances are determined from ?uid mechanics analyses.
FIGURE 4.8.3. An overlay of the pump ?ow vs. head curve with the circuit piping characteristics gives the operating
state of the circuit. A typical velocity head pump characteristic is shown on the left, while a positive-displacement
pump curve is shown on the right.
FIGURE 4.8.4. A full range of performance information should be available from the pump manufacturer, and this
may include the parameters shown.
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© 2000 by CRC Press LLC
one way to achieve high pumping pressures with centrifugal pumps is to place a number of units in
series. This is a related effect to what is found in multistage designs.
Fans
As noted earlier, fans are devices that cause air to move. This de?nition is broad and can include a
?apping palm branch, but the discussion here deals only with devices that impart air movement due to
rotation of an impeller inside a ?xed casing. In spite of this limiting de?nition, a large variety of
commercial designs are included.
Fans ?nd application in many engineering systems. Along with the chillers and boilers, they are the
heart of heating, ventilating, and air conditioning (HV AC) systems. When large physical dimensions of
a unit are not a design concern (usually the case), centrifugal fans are favored over axial ?ow units for
HVAC applications. Many types of fans are found in power plants. Very large fans are used to furnish
air to the boiler, as well as to draw or force air through cooling towers and pollution-control equipment.
Electronic cooling ?nds applications for small units. Even automobiles have several fans in them. Because
of the great engineering importance of fans, several organizations publish rating and testing criteria (see,
for example, ASME, 1990).
Generally fans are classi?ed according to how the air ?ows through the impeller. These ?ows may
be axial (essentially a propeller in a duct), radial (conceptually much like the centrifugal pumps discussed
earlier), mixed, and cross. While there are many other fan designations, all industrial units ?t one of
these classi?cations. Mixed-?ow fans are so named because both axial and radial ?ow occur on the
vanes. Casings for these devices are essentially like those for axial-?ow machines, but the inlet has a
radial-?ow component. On cross-?ow impellers, the gas traverses the blading twice.
Characteristics of fans are shown in Figure 4.8.6. Since velocities can be high in fans, often both the
total and the static pressure increases are considered. While both are not shown on this ?gure, the curves
have similar variations. Of course the total ?P will be greater than will the static value, the difference
being the velocity head. This difference increases as the volume ?ow increases. At zero ?ow (the shutoff
point), the static and total pressure difference values are the same. Ef?ciency variation shows a sharp
optimum value at the design point. For this reason, it is critical that fan designs be carefully tuned to
the required conditions.
A variety of vane type are found on fans, and the type of these is also used for fan classi?cation.
Axial fans usually have vanes of airfoil shape or vanes of uniform thickness. Some vane types that might
be found on a centrifugal (radial-?ow) fan are shown in Figure 4.8.7.
One aspect that is an issue in choosing fans for a particular application is fan ef?ciency. Typical ef?ciency
comparisons of the effect of blade type on a centrifugal fan are shown in Figure 4.8.8. Since velocities can
be high, the value of aerodynamic design is clear. Weighing against this are cost and other factors.
FIGURE 4.8.5. Series (a) and parallel (b) operation of centrifugal pumps are possible. The resultant characteristics
for two identical pumps are shown.
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