Note: Descriptions are shown in the official language in which they were submitted.
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1 Background
Centrifugal fans are well known to those skilled in
the art. The use of a centrifugal fan by utility companies in
connection with supply of air to boilers is not effectively
competitive with axial flow fans. ~hile a centrifugal fan has
a higher efficiency at the maximum boiler load, the efficiency
falls off drastically, as compared with an axial flow fan,
due to the fact that flow control with a centrifugal fan is
~ attained by adjustment of dampers. In an axial flow fan, adjust-
ment of load is attained by varying the pitch of blades. In
a typical installation, with each of a centrifugal fan and an
axial fan operating at 75% boiler load, the efficiency of the
centrifugal fan would be approximately 45% while the efficiency
of the axial fan would be approximately 80%.
The present invention is directed to apparatus and
method for enabling a centrifugal fan having dampers to operate
at a higher efficiency as compared with an axial fan having
variable pitch blades.
Summary of the Invention
In practicing the method of the present invention, air
flow is controlled by providing a centrifugal fan having a fan
impeller and dampers, and driving the impeller by a motor speed
control. Control of air flow is attained in two different ways
depending upon whether the air flow is above or belo~ a predeter-
mined crossover point. When air flow is above the predetermined
crossover point, air flow is controlled only by changing the
velocity of the fan impeller. When air flow is below the crossover
point, air flow is controlled only by adjustment of the dampers.
It is an object of the present invention to provide a
centrifugal fan control system whereby the efficiency of the cen-
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trifugal fan will exceed the efficiency of a comparable axial
flow fan.
It is an object of the present invention to increase
th~ efficiency of a centri~ugal fan by using two different
ways of varying air flow depending u~pon whether air flow at
any given time is above or below a predetermined crossover
point.
It is an object of the present invention to substantially
increase the efficiency of a centrifugal fan air ~low system
in a manner which is simple, inexpensive and reliable while
providing for load management, multiple fans in the system,
system upset, etc.
In accordance with one broad aspect, the invention relates
to a method of controlling air flow comprising: (a) providing
a centrifugal fan with dampers or vanes upstream from a fan
impeller, driving said impeller by an electric motor having a
speed control circuit which includes means for converting slip
losses into primary power coupled to said motor, (b) controlling
the air flow from said centrifugal fan in response to a
flow demand signal only by changing the velocity of the fan
impeller when the flow is in excess of the flow correspond-
ing to a predetermined crossover point, (c) providing said
dampers or vanes with an operative range of adjustment so
that the dampers or vanes are fully open at said crossover
point, and (d) controlling air flow from said centrifugal
fan in response to a flow demand signal where the flow rate
is below that corresponding to said crossover point only by
changing the posit:ion of said dampers or vanes while main-
taining the speed of said impeller dampers or vanes while
maintaining the speed of said impeller constant and corres-
ponding to the impeller speed at said crossover point.
In accorclance with another aspect, the invention relates
to a centrifugal fan with dampers and a motor driver impeller
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the improvement comprising: (a) a speed control circuit for
the impeller motor, sai~ circuit incluaing means for converting
slip losses into primary motor power, (b) means for changing
air Slow ~rom the fan by only changing the speed of the fan
impeller when air flow is above a predetermined crossover
speed at which the dampers are fully open and for only
adjusting the dampers when air flow is below the crossover
speed while maintaining the impeller speed at said crossover
speed.
Other objects will appear hereinafter.
For the purpose of illustrating the invention, there
is shown in the drawings a form which is presently preferred;
it being understood, however, that this invention is not
limited to the precise arrangements and instrumentalities
shown.
Figure 1 is a graph of pressura versus air flow volume
for a conventional prior art centrifugal fan with inlet vane
control.
Figure 2 is a graph similar to Figure 1 but for a
conventional axial flow fan with variable pitch blades.
Figure 3 is a graph of pressure versus speed for the
system of the present invention.
Figure 4 is a diagrammatic simplified circuit for
controlling slip losses of the centrirugal fan drive motor in
accordancP with the present invention.
Figure 5 is a graph of speed versus flow.
Figure 6 is a comparative graph of a fan system in
accordance with the present invention as compared with axial
flow fans in tenns of total fan efficiency versus rate of flow.
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1 Figures 7A and 7B are block diagrams setting forth a
sequence of steps when practicing the present invention~
Figu}e 8 is a schematic control diagram.
Figure 9 is a diagrammatic graph of static pressure
versus flow.
Referring to the drawings in detail, wherein like num-
erals indicate like elements, there is shown in Figure 1 a typ-
ical pressure versus volume graph for a conventional centrifugal
fan with inlet vane control. A similar graph for an axial fan
~ith variable pitch blades is shown in Figure 2. The ~ajor dif-
ference between the two types of fans is that the efficiency
curves for a centrifugal fan are Perpendicular to the system
resistance line while those for an axial fan are almost parallel
to the system resistance line. As a result thereof, at a typical
boiler load of approximately 75~, the efficiency of a centrifugal
fan system is approximately 45% while the efficiency of an
axial fan system is approximately 80~. Hence, an axial flow
fan is well known to be more efficient than a centrifugal flow
fan except at maximum air flow volumes where the centrifugal
fan is more efficient.
As shown in Figure 4, the fan 10 is provided with a
drive motor 12. The motor 12 is driven by an al-ternating current
supply and may be a wound rotor type motor with an external
speed control of conventional design. Although a multiple phase
winding system is l:ypically employed, for the purpose of sim-
plicity only a sinqle phase is shown in Figure 4. Thus, the
rotor of motor 12 is connected through a rectifier 14 to a
high inductance coil or choke 16. The choke 16 smooths out the
current. The coil 16 is connected to a silicon control rectifier
18 which is also known as a SCR. The SCR 18 is fired by fire
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1 control circuit ~0 to convert the rectified rotor energy to
alternating current which is then coupled to the power supply
side of the motor 12. Hence, rotor ,slip losses are converted
into primary power for the motor 12 thereby substantially
increasing its efficiency.
Load Management
when the fan 10 is provided with the circuitry illus-
trated in Figure 4, it is now possib:Le to change the efficiency
curve of a centrifugal fan. In Figure 3, there is illustrated
a pressure versus flow curve for a centrifugal fan in accordance
with the present invention. The operating portion of the graph
in Figure 3 with respect to a centrifugal fan of the present
invention is designated as O-A. Maximum flow occurs at point
A wherein the fan impeller is operating at 100~ of speed. If
it is desired to decrease the flow output of the fan 10 from
point A, to a figure which is between point A and a crossover
point designated as B, such change in air flow output is at-
tained only by changing the velocity of the fan impeller which
does not affect the efficiency of the fan. For purposes of
illustrating the invention, the crossover point is designated
as being 70~ of fan speed with the damper fully open. Between
points A and B, the dampers are fully open. At air flow rates
below point B, change of flow is attained only by adjusting
the dampers from a fully open position to a fully closed posi-
tion while fan speed remains constant at said crossover point
speed.
The above discussion with respect to crossover speed
is diagrammatically illustrated in Figure 5. As shown in Figure
5, the crossover point B may actually be a range with initiation
of the change being based on air flow which has been converted
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1 to a signal having a value between 0 and 10 volts. The use of
a crossover range avoids huntin~ or instahility.
In Figure 6, there is illustrated a graph of total
~an efficiency versus rate o~ Elow for the centrifugal fan
of the present invention as compared with commercially available
axial flow fans. In Figure 6, commercially available axial flow
fans have an efficiency versus rate of flow which falls within
the hatched zone 20. As shown in Figure 6, a conventional cen-
trifugal fan has an efficiency versus rate of flow defined by
the line A-C where rate of flow is controlled only by adjusting
the outlet dampers and is defined by the line A-D where rate
of flow is adjusted by varying the inlet dampers. ~s shown in
Figure 6, efficiency remains constant when rate of flow in ac-
cordance with the present invention between points A and B is
attained only by adjustment of the speed of the fan 10. Below
the crossover point B, the efficiency versus rate of flow is
defined by the line B-E. Along the line B-E, rate of flow is
varied only by changing the position of the dampers. It will
be noted that at any particular rate of flow in Figure 6 the
efficiency of the fan 10 of the present invention as defined
by the line ABE is greater than the efficiency for an axial
flow fan as defined by the hatched zone 20.
Adjustment of dampers is accomplished by a flow de-
mand signal from the boiler or other process associated with
the present invention in the same manner as was accomplished
heretofore. However, in accordance with the present invention,
a logic system is interposed between the demand signal and
the fan 10 to decide whether adjustment should be by speed
control or by damper control. Thus, the logic system first
determines whether the fan 10 is operating at a rate of flow
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1 which is above or below the crossover point B. If the rate oE
flow is above point ~, the logic system modulates the demand
signal to attain the desired change solely by varying the speed
of the fan 10. If the then existing rate of flow is below point
B, the logic system modulates the demand signal to attain the
desired rate of flow by damper adjustment while fan speed re-
mains at the speed corresponding to the crossover point.
Referring to the schematic diagram in Figure 8, it will
be noted that the diagram includes the AFC (automatic fan con-
troller~ e~uipment segregated from the equipment located in the
fan drive control. A flow demand signal 30 is coupled by normally
closed contacts to a controller 32 which converts the voltage
signal to a signal representative of flow demand rate. The flow
demand rate is coupled to a comparator 34 which compares the flow
demand ra~e signal with a crossover set point signal. Comparator
34 generates an output signal which is desi~nated as AFC speed
control signal, which in turn is coupled to a rate controller
38. The controller 38 integrates or smooths the AFC speed con-
trol signal to prevent sudden extreme changes in motor speed
which may be outside motor capacity. The controller 38 is coupled
to a fire control circuit 40. See Figure 4. ~ tachometer speed
signal of the motor 12 is likewise coupled to the fire control
circuit 40 which controls the firing of the SCR 18 to bring the
motor 12 to the crossover speed.
~he siclnal from comparator 32 is also coupled to the
comparator 36 which generates a damper position control signal
depending on whether the fan speed is above or below the cross-
over point. The comparators 34 and 36 are interlocked by an
overlap so that the damper position control signal will only be
generated when the fan speed is below the crossover point and
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1 so that the speed control signal from comparator 34 will remain
at the crossover speed when actual fan speed is below the cross-
over point. The damper position cont:rol signal from comparator
36 is coupled through a rate controller 42 to the mechanism
13 for control o~ the dampers. Rate controller ~2 integrates
or smooths the damper position control signal to prevent sudden
extreme changes in damper position which may overload the damper
drive ~echanism.
Switches are provided in the circuitry to facilitate
manual operation as well as automatic operation. Also, switches
are provided as shown at the top of Figure 8 to Eacilitate
modiEication of the speed control signal at a fast or slow rate
in an increasing direction or decreasing direction and override
all other signals. When a drive for a fan is started in the
automatic mode from zero speed, it accelerates to minimum oper-
ating speed. If the flow demand is not satisfied but is calling
for a decrease in speed, the AFC speed control signal stays
at the minimum operating level. If the flow demand signal calls
for greater flow, the damper position signal increases until
the 100% open position for the damper is reached. Above that
level, the damper position signal stays at the 100% level and
the fan driven speed synchronizes with the flow demand signal
30.
Multi Fan System
If a particular flow control system involves a plur-
ality of fans, cornparator 3~ must be capable of recognizing
a demand signal which exceeds the rate of flow of a single fan
and thereafter sp:Lit the demand signal so as to brinq a second
fan into operation. When a second fan is brought into operation
in parallel with the first fan, the logic system splits the
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l demand signal equally so that the first fan has a rate of flow
which is decreased until both fans operate at approximately
the same rate of flow depending upon the pressure of the system
downstream from the fans. When the first fan rate of flow is
decreased, such decrease is attained as desscribed above first
by varying speed of the first fan to the crossover point B and
thereafter varying damper positions.
System Upset
With the advent of large boiler systems, and the re-
strictions im osed by pollution control standards, a typical
boiler system is provided with an upstream forced draft fan
and a downstream induced draft fan so that the boiler can oper-
ate with a balanced draft situation. A balanced draft situation
can be upset for a variety of reasons such as an inadvertent
opening of a breaker, a loss of fuel supply, and the like. If
there is a boiler upset, the operating conditions can soon
reach a situation which results in pressure of the system ex-
ceeding the structural strength of the system. When a boiler
upset occurs, it is essential that air flow be adjusted rapid-
ly.
In order to rapidly adjust air flow, the present in-
vention is particularly suited for this problem in that air
flow can be rapidly decreased from 100% to 70% merely by a speed
change of the fan. In this regard, motor 12 is provided with
a dynamic brake 22. The brake 22 is a small D-C excitation
system to apply D-C amps to the winding of motor 12 to generate
torque in a braking mode. Brake 22 will facilitate a decrease
in 300 rpm in 15 seconds whereas a conventional centrifugal
fan may take hours to accomplish a similar decrease in speed.
In a fan designed with a maximum speed of 700 rpm, a decrease
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1 of 300 rpm is the e~uivalent of a decrease in ~0% of the head.
The system of the present invention enables one to
tailor the operating conditions so as to provide a limit line
in a controlled manner as opposed to the stall line in a con
ventional axial fan which is uncontrolled.
As shown in Figure 9, there is schematically illus-
trated a graph o~ static pressure versus flow. Outside the oper-
ating line representing the expected spectrum of operation,
there is provided a warning line 44 and an action line 46. Each
of tAe lines 44, 46 is a limit line. The limit line for a cen-
trifugal fan is an artificial limit replacing a real limit on
an axial fan. On an axial fan, the stall line has been proposed
as a means for preventing the development of excessive pressures
which could exceed the structural integrity of the system result-
in~ in damage that is referred to as an implosion. The condition
of stall on an axial fan is unstable and can result in surges
as well as mechanical damage. i`
In the system of the present invention, there is
created a limit line whereby upset conditions which could lead
to an implosion are monitored and corrective action initiated
without causing unstable operation and with a response time
equal to or better than that capable of being obtained on an
axial fan~
The illustration in Figure 9 is purely exemplary as
to the ~ocation of the limit lines 44, 46. Thus, the lines 44,
46 could be horizontal. The only limi-tation is that they be
above the operating curve for the fan. Fan speed and damper
position are known factors The pressure is a factor which is
capable of being measured and will vary with the flow rate.
The actual static pressure as measured may be below the limit
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1 line 44. If so, no action is taken. If the actual measured
static pressure equals or exceeds that of the warning line 44,
at any particular flow rate, a contiact switch is closed or in
some other manner a warning signal is generated to warn the op-
erator. If the actual static pressure measured, at any particular
flow rate, exceeds the action line 46, then another contact
switch is closed or in some other manner a signal is generated
to indicate to an operator the corrective action that is neces-
sary to prevent an implosion. In the alternative, the action
signal may automatically shut down the system.
Referring to Figure 8, the signal from controller 32
is also communicated to calculator 48. Calculator 48 is pro-
grammed with the operational curve of the fan and determines
what the maximum desired pressure should be at the particular
flow rate being handled by the fan. The calculator 48 is coupled
to a pressure comparator 50. The signal from calculator 48 is
divided into a signal representative of actual measured pressure
communicated thereto by comparator 52. If the resultant signal
is equal to or greater than .9, for example, this indicates
that the system is operating at or above the limit line 44 and
the warning signal is generated. If the resultant signal is
equal to or greater than 1, this indicates that the fan is op-
erating at or above the limit line 46 and the action signal
is generated.
The fan pressure signals of all parallel fans are
compared with a set point pressure differential signal. See
Figure 8. The set point differential signal 53 is coupled to
the differential comparator 54. The calculated and actual
pressures for each of the fans are likewise coupled to the com-
parator 54. The comparator 54 ascertains the pressure differ-
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1 ential between desired and actual for each fan and then compares
that differential with the set point differential signal 53.
When the set point differential signal 53 is equal or greater
than 75% of the differential pressure for a particular fan,
a warning signal is generated. If the set point differential
signal 53 is equal to or greater than the differential pressure
for a particular fan, an action signal is generated. In this
manner, means is provided for detecting the fact that one fan
of a fan system is acting improperly and corrective action may
be taken whereby that particular fan is taken out of action
or otherwise is adjusted by appropriate controls to bring it
back into line with the other fans.
Figures 7A and 7B illustrate a block diagram of a se-
quence of steps which are followed when putting a fan on line.
The operator will press a start signal contact. Each fan has
a pressure signal. The pressure signals are compared to deter-
mine the maximum pressure signal. If the system is on manual
operation, the sequence immediately jumps to position B. If
~he system is not on manual operation, the closed position of
the dampers is detected and speed is increased.
Actual and desired pressures are determined and com-
pared by comparator 50. Depending upon the ratio of the actual
pressure to the desired pressure, no signal, or a warning signal,
or an action signal may be generated as described above. Also,
the pressure differential between fans of the system is compared
by comparator 54 with a resultant no signal, or warning signal,
or action signal as described above.
The speed control signal may be changed if there is a
rate demand signal superimposed thereover. Various contacts
are provided to facilitate a rate demand signal change, fast
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or slow, while increasing or decreasing. Suitable rate demand
contacts would be provided for slow motor speed, fast motor
speed, slow damper, fast damper, slow damper - slow motor speed,
fast damper - fast motor speed, and :East damper - slow motor
speed.
If the speed control signal is less than the cross-
over speed point or range, the sequence reverts to point A.
If the speed control signal is equal to or greater than the
crossover speed, the next step in the sequence is to stop in-
creasing the speed control signal. If the flow demand signal30 is not satisfied, controller 36 opens the dampers wider.
If the flow demand signal is satisfied, and the damper position
is at 100%, then fan speed increases so that the speed control
signal i5 synchronized with the flow demand signal 30. If the
damper control signal is not at 100% value, it is adjusted
to attain the 100% value. Thereafter, the speed control and
the damper control signal continue to drive the controls to
maintain the system in balance as described above.
Thus, it will be seen that the centrifugal fan
system of the present invention provides for load management,
paralleling of fans in a system, as well as system upset
controls.
The present invention may be embodied in other spec-
ific forms without departing from the spirit or essential at-
tributes thereof and, accordingly, reference should be made
to the appended claims, rather than to the foregoing specific-
ation as indicating the scope of the invention.
Various ~elements identified in Figure 8 of the drawing
are commercially available devices and preferably are the devices
set forth hereinafter which are available from the General
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Electric Company as part of their "Directomatic II" system.
Elements 32, 38, 42, the speed fast rate control, the speed
slow rate control, the damper position fast rate control,
and the damper position slow rate control are principally
time accelerating (rate) function generators sold as GE Model
IC 3600-AFGH. Elements 34 and 36 are primarily an amplifier
such as GE Model IC 3600-AOAL. Element 52 is preferably a gate
expander such as GE Model IC 3600-LLXA. Element 50 is preferably
a divider circuit such as GE Model IC 3600-AAMA. Element 54
is preferably an amplifier such as GE Model IC 3600-AOAL
~ith a voltage sensitive relay circuit such as Model IC 3600-
AVFA.
