Note: Descriptions are shown in the official language in which they were submitted.
This invention relates -to a method and apparatus for
de-termining the fluid flow in a Eluid distribution systemn
This application is a division af application Serial
No. ~45,275 filed Jan. 13, 19~4.
The method and apparatus of the present inven-tion is
particularly suitable for use in determining the rate of flow of
air in a variable air :Elow fan system which has a variable speed
fan, such as an air conditioning system.
The method and apparatus of the present invention is
also suitable for use in deterrnining the rate of flow of liquid
in a variable liquid flow pump system which has a variable speed
pump .
PRIOR ART
The need for a simple, inexpensive and direct method of
indicating flow in an air circulating system or a liquid
circulating system is well known.
In air circulating systems such as air conditioning
systems the rate of flow is usualiy measured by installing a duct
traverse consisting of a series of flow tubes with a pitot tube
in each flow tube. The signal from each flow tube is averaged,
and summed. ~ince this signal varies as the square of the flow,
the square root must be taken. This output may then be used to
indicate air flow.
The problem with this approach is that each stage of
signal processing introduces errors. Also, the most economical
computation systems are pneumatic. Manufactu.rers of this type of
system claim an accuracy of about ~ or - 7~ with a newly
calibrated system. The reason for this inaccuracy is the problem
of obtaining accurate signals from a duct traverse. The accuracy
will deteriorate further if the computation circuits drift with
age or if the pitot tubes become clogged i.n use. Substituting
electronic computation circuits for pneuma-tic ones will result in
less drift, but the maximum error oE about ~or- 7% may not be
appreciably decreased.
SU~MARY_OF INVENTION
Using the characteristic curves of a fluid circulating
system and fluid delivery device I have developed a simple and
accurate method of generating a rate of :Elow signal which is
proportional -to the flow rate in a fluid distribution system and
which is ba.sed on a signal which is proportional to the speed of
-the :Eluid circulating device.
In addition I have designed a simple and accurate rate
of flow si.gnal generating means for generating a rate of flow
signal which is proportional to the flow rate in a fluid
distribution system and which is based on a signal which is
proportional to the speed of operation of the fluid circulating
device.
My me-thod and control system is much simpler than that
previously available and has a greater accuracy wi-th essentially
no deterioration in accuracy with -time.
According to one aspect of the present invention there
is provided a control system for an air-conditioning system for
conditioning an enclosed air space which has, a supply fan
adapted to supply a variable volume of air to the conditioned
space, a return fan adapted to return a variable volume of air
from the conditioned space to the supply air fan, comprising;
i) control means adapted to monitor and/or indicate the
volume of air supplied by the supply fan and to adjust the volume
of air returned by the return fan to track the volume of air
supplied by the supply Ean,
ii) return fan controller means for offsetting the
tracking of the volume of air returned by the return fan with
respect to the volume of air supplied by the supply fan whereby
the air pressure within the conditioned space may be adjusted to
achieve any required balance with the external air pressure.
_~¢~
According -to yet another aspec-t of the present
invention there is provided in an air-conditioning system having
a variable alr volume system for cond.itioning an enclosed air
space, a variable air volume supply fan, a variable air volume
return fan, the improvement of a control system compri.sing; a
controller for con-trolling the volunne of air GUtpUt of -the return
fan in response to a control signal, and control signal
generating means for generating a control signal which is a
function of the volume of air supplied by the supply fan, said
control signal generating means comnnunicating with said
controllerl at least one supplementary air exhaust system for
exhausting air from said enclosed air space, actuator means for
selectively activating and de-activating each of said
supplementary exhaust systems, and adjustments means for
adjusting said control signal in response to the activating and
de-activating of each supplementary air exhaust system whereby
the volume of air re-turned by the return fan is adjusted such
that the ratio of the combined return air volume plu5 exhausted
air volume to supply air volume is maintained substantially
constant for all conditions of each exhaust fan.
PREFE~RED EMBODIMENT
The invention will be more clearly understood with
reference to the following detailed specification read in
conjunction with the drawings wherein:
Figure 1 is a diagrammatic represen-tation of a building
having an air conditioning system of the type -to which the
present invention relates~
Figure 2 is a block diagram illustrating a control
system suitable for use in association with variable speed fans
of an air conditioning system of -the type illustrated in Fig. 1.
Figure 3 is a diagram illustrating a variable air
volume controller suitable for use in controlling the capacity of
the supply fan and controlling the operation of the return fan of
an air conditioning system.
Figure 4 is a diagram illustrating a curve fitter
suitable for use in the controller of Fig. 3.
Figure 5 is a diagram illustrating a response curve of
a typical controller system.
Figure 6 is a diagram of a system for generating an
offset signal to the controller when air exhaust systems are
provided in the air space which is to be conditioned.
Figure 7 is a graph showing percentage fan capacity
versus fan speed of a typical supply fan.
Figure 8 is a diagram illustrating a typical system
curve of an air conditioning instalation, and
Figure 9 is a diagram illustrating a typical fan curve
of a variable speed fan upon which the plot of the system curve
of Fig. ~ is superimposed.
The System Curve Equation (Pressure/Flow) which is
characteristic of the duct system and which relates the total
pressure in a system such as a fan system to the rate of flow of
air in the system is well known.
Similarly fan curves (Pressure/Flow/Speed) which relate the speed
of -the fan to the pressure drop and to the flow rate of the fan
have been available to fan users for many years. ~he fan
s
curves and computer print-outs which provide similar information
are provided by fan manufacturers.
I combine the information which is available in the
System Curves, which are characteristic of the system, and the
information which is available in the air circulating device
curves, which are characteristic of the air circulating device,
to determine Combined Air Circulatin~ Device/System
Characteristics of a Air Circulating System.
I can, for e~ample, combine the information which is
available in the System Curve of an air circulating system, which
is characteristic of the system, and the information which i5
available in the Fan Curves, which are characteristic of the fan,
to determine Combined Fan/System Characteristics of an air
conditioning system or the like.
I can also combine the information which is available
in the System Curve of liquid pumping system, which is
characteristic of the system, and the information which is
available in the Pump Curves, which are characteristic of the
pump, to determine Combined Pump/System Characteristics of a
liquid pumping system.
As previously indicated it is well known that the
conditions under which air flow takes place in an air
conditioning duct is subject to the following equation:
dP = Clq -~ C2q + C3q EQUATION 1
Where Cl, C2, C3 are constants
- q = air flow in suitable units
dP - pressure drop of duct system in suitable
units.
EQ~AI'ION 1 breaks up the pressure drop into components
due to totally turbulent flow depicted by the q2 term; due to
laminar flow depicted by the ql term; and a component
independent oE Elow depicted by the q term. (Note q = 1).
Experience indicates that in the range of operation of most air
conditioning systems of less than 50~ capacity reduction
(turndown) and low settings of the discharge pressure controller,
the laminar flow component is usua:Lly negligible. If we assume
this term to be zero, then EQUATION 1 simpliEies to the
following:
dP = Clq + C3q EQUAT:[ON 2
In air conditioning systems where the laminar flow
component is not negligable, it must, of course, be taken into
account by solving EQUATION 1.
On the other hand in a system where the pressure is not
controlled and laminar flow does not exist in the normal
operating range of the fan (i.e. C2 = 0); then Equation 1 may
be further simplified as follows;
dP = Clq + C2q + C3q EQUATION 1
dP = Clq + 0 + 0
dP = C q EQUA~ION 3
3~
CURVE FITTER
A suitable curve Eitter is illustrated in Fig. 4 of the
drawings.
As previously indicated the fan speed and air flow in
an air conditioning system may be expressed by the Equation 1 as
follows:
dP = C1~2 + C2ql -~ C3q
Therefore; 0 = Clq -~ C2q + C3q - dP (EQUATION 4 ~
The theoretical relationship between the speed of a fan
~ and the pressure rise across a fan and the relationship between
the speed of a fan and the air flow of a fan is well known;
i.e. Sl = ~1
and -I = Pl
S22 P2
Where S is the speed of the fan in RPM.
Therefore; 1 2 -1
s22
and when the speed is at a maximum then
-2 = K where K is a constant
S2
Therefore; dP = KS in EQUATION 4
0 - Clq ~ C2q ~ C3q - KS
The curve fitter 53 (Fig. 3) is designed to be
programmed to solve EQUATION 4. With reference to Fig 4 of the
drawings it will be seen that the curve fitter 53 comprises a
potentiometer 1 and a squaring amplifier 2 which are connected to
a multiplier 3 which is in turn connected to a summer 4. A second
potentiometer 5 and the line 15 (Q) are connécted to a multiplier
6 which is in turn connected to summer 4. Static pressure line 17
is also connec-ted to the summer 4. A squaring amplifier 7 and a
potentiometer 8 are connected to a multiplier 9. The outputs of
the summer ~ and multiplier 9 are connected to the inputs of a
difference amplifier 11 which is in turn connected to integrator
13.
In use this curve fitter is programmed by dialing ln
the value of the constants Cl, C2 and K to the potentiometers
1, 5 and 8 respectively. The multiplier 3 receives signals C]
and Q and multiplies the values to provide an output signal
ClQ . The multiplier 6 receives signals Q and C2 and
generates an output signal C2Q to the summer 4. The signal C3
is fed directly to an input of the summer 4. The summer 4 sums
these signals to provide an output signal (ClQ + C2Q +
C3) which is fed to the difference amplifier 11. Similarly the
signal KS , which is the output from the multiplier 9, is fed
to -the other input of the difference amplifier 11.
If the two input signals to the difference amplifier 11
are equal, then the output signal from the difference amplifier
11 is zero, then the output of the integrator 13 is constant and
this signal can be interpreted as a measure of the rate of flow
of air through the air conditioning system. If the two input
signals to the difference amplifier 11 are not equal then the
output signal from the difference amplifier 11 is non zero. This
error signal is integrated by integrator 13 to provide a feedback
which will adjust the inputs to the summer 4 until the balance is
restored in the difference amplifier 11, which has the effect of
adjusting Q to equal the actual rate of flow signal. Thus when
the signal Q, which is based on the speed of the fan, is stable,
it is proportional to the actual rate of flow of air.
EXAMPLE 1
In a typical air conditioning application, where the
pressure is not controlled and laminar flow does not exist the
ducting system is designed by an engineering contractor. The
engineering contractor also specifies the fan to be used in
association with the system. Typically, the designer of a
control system may be advised that the fan which is to be used is
designed to deliver 61,200 cEm at 5" w.g7
~ ith this information, I can proceed to design a
control system which will have a substantially greater accuracy
and stability when utilizing a variable speed fan to control the
rate of flow oE air.
In this example7 where the value of "q" (rate of flow)
is high and where the pressure is not controlled and laminar flow
does not exist, EQU~TION 3 may be used to establish the System
Curve.
At maximum air flow the system is designed to convey
61,200 CEM of air when overcoming a duct pressure of 5" WG.
therefore:
in EQUATION 3, when dP = 5", we have;
5 = Cl (~1,200)2,
therefore: Cl = 5 2
(61,200)
Therefore Equation 3 provides:
dP = S 2x q
(61,200)
3~'oJ
Thus, air flow in this type of duct system is in
accordance with the Equation 3, which can now be solved Eor flow
rates within the stable operating conditions of the system such
as 100~ to 40~ in 10% increments.
For example when 90% flow rate required;
dP = 5 2x (.9 x 61,200)2
(61,200)
= 4.05
With this information the System Curve A of Fig. 8,
(Pressure/Flow) may be plotted or a System Table (Pressure/Flow)
may be tabulated.
The following TABLE 1 is a typical System Table which
can be prepared by repeating the foregoing at 10% increments in
reduction of air flow capacity.
TABLE 1
AIR FLOW (C.F.M.) PRESS~RE (ins.W.G.)
ACT~AL %
61,200 100 5
55,080 90 4-05
48,960 80 3.2
42,840 70 2.45
36,720 60 1.8
30,600 50 1.25
24,480 - 40 0.8
I am now in a position--to plot the System Curve A
(Fig 8) on the Fan Curves (Fig 9) of the selected fan and the
point at which the System Curve intersects the speed curves
(Fig 9) is noted and tabulated to provide Table 2 as Eollows;
~C~3~ ~
TABLE 2
AIR FLOW (C.F. M. ) PRESS~RE FAN SPEED
ACTUAL ~ (ins.W.G.) (rpm)
61,200 100 5 587
55,080 90 4.05 523
48,960 80 3.2 ~62
42,840 70 2.45 40g
36,720 60 1.8 351
30,600 50 1.25 289
24,480 40 0.8 231
With this information, I can now obtain a Curve C
(Fig 7) of air flow/fan speed for the condition where the
pressure in the duct is not controlled. Thus, I can establish
the speed at which the fan should run in order to provide any
required percentage of flow capacity when the pressure in the
duct system is not controlled.
In practice the fan curves which are provided by the
fan manufacturer may not accurately reflect the conditions which
prevail in a particular instalation. In these circumstances it is
necessary to take readings of the actual air flow in the duct of
the system in order to determine Points Pl and P2' to obtain a
modified Curve C'(Fig. 7). This is inforrnation which is commonly
provided by the System Balancing Contracter. These measurements
provide the fan speed and air flow at maximum and minimum
conditions which can be used to plot Points Pl and P2' of Curve
C' of Fig 7. Thus Curve C' of Fig 7 is a typical "Actual System
Fluid Flow Curve" and Curve C is a "Theoretical System Fluid Flow
Curve". The Curve C' and the Curve C may be substantially
identical in well designed fluid circulating systems.
I know that the limi-ting case of the path Pl-P2' is a
straight linel an~ further that a further Point 3 cannot be above
this straight line (Pl-P2'~, although it may be below the
straight line (Pl-P2'). If the third point (Point 3) can be
obtained from the Balaneing Contracter it would greatly increase
the aeeuraey oE path Pl-P2'.
The theoretieal relationship between the speed of a fan
and the pressure rise aeross a fan and the relationship between
the speed of a fan and the air flow of a fan is well known;
- 12
i.e. RPM~
RPM2 q2
and RPM P
1 ~ = r,l
RPM2 r 2
Thus, as a Eirst approximation I can determine the
relationship between the air flow output of a fan and its speed
(Curve C' Fig 7) which is the "System Fluid Flow Curve" from
which the "System Fluid Flow Data" may be obtained. This
relationship does permit me to utilize the speed of the fan to
generate a signal which is proportional to air flow. In my
control system I provide a curve fitter, which may be in the form
of an amplifier or a digital computer or the like which I program
with the System Fluid Flow Curves or System Fluid Flow Data which
I have previously developed as described above. I am then able to
input a signal to the curve fitter which is proportional to the
speed of the fan which is processed by the curve fitter to
generate an output signal which is proportional to the actual
rate of flow in the system.
~ y system may also be employed in an air or liquid
circulating system of a building which has a building autotnation
coï.l21ter to monitor and control the operations of these systems.
This type of computer may be progra~ned to accept the speed input
signal from a fan or pump and to calculate air flow (C.F.M.) or
liquid flow (G.P.M.).
The foregoing Example relates to a system in which the
pressure in the system is not controlled.
If a fan is attached to an air conditioning system
consisting of filters, coils, ducts, terminal boxes, dampers,
etc., air flow will take p]ace at the point where the System
Curve intercepts the Fan Curve. If the system is a constant
volume system, there would be one fan curve depicting the
head-flow condition at a particular constant speed. In a Variable
volume instalation there are many system curves between the curve
depicting minimum flow and the curve depicting maximum flow.
Each t:ime a damper changes position, resulting in
a difEerent pressure in the duct, flow would be affected. Since
13
air Elow can only ta~e place at the point of intersection of the
fan curve and the system curve, if the duct resistance changes
the System Curve will change. Hence, there can be ~any System
Curves within the definable limits of maximum and minimum flow.
If the fan is to vary capacity, there would likewise be
an infinite number o fan characteristic curves between minimum
and maximum capacity; one for each speed. The fan system
operation is somewhere in the region defined by the minimum
capacity-flow and the maximum capacity-flow condition which is
determined by the system demand. That is to say the minimum
capacity flow is that which results when the load demand is at a
minimum and the maximum capacity-flow results when the load
demand i~s at design conditions.
If the system curve follows EQ~TION 1 or the
simpli~ied EQUATION 2, then an accurate graph of fan speed versus
air flow may be obtained. In variable speed systems, a signal
proportional to speed is easily obtained. Since this graph is
linear for at least part of its range, it can be used to read out
air flow directly within the linear range. The ~ero Elow offset
is the value of the measured pressure in the duct. This is
easily obtained from the static pressure controller s~ecifically
from the pressure sensor output after signal conditioning or from
the set point circuit. Therefore, an inexpensive and accurate
method of indicating air flow can be obtained.
EXAMPLE 2
In a typical air conditioning application, the ducting
system is designed by an engineering contractor. The engineering
contractor also specifies the fan to be used in association with
the system. The System Balancing Engineer will provide the data
which includes the actual pressure and speed of flow at the
specified maxirnum and minimum flow rates. Typically, the
designer of a control system may be advised that the fan which is
to be used is designed to deliver 61,200 cfm at 5" w.g. and the
pressure in the duct system is to be controlled to 1 1/4" w.g.
When such a system has been balanced the Balancing Engineer may
typically report that, at the actual minimum flow the fan must
deliver 25,000 c.f.m. of air at 2.1" w.g. while rotating at 335
r.p.m.
2~'
14
With this information~ I can proceed to design a
control system which will have a substantially greater accuracy
and stability when utilizing a variable speed fan to control the
rate of flow of air.
In this example, where the value of "q" (rate of flow)
is high, EQUATION 2 may be used to establish the System Curve.
At the condition where no air flows through the duct,
the pressure at the inlet to the duct will equal the control
pressure~ namely 1 1/4" w.g.;
therefore:
in EQUATION 2, when dP = 1 1/4" and q = 0, we have;
1 1/4" = Cl (0) + C3 ,
therefore: C3 = 1 1/4 (i.e. the setting of the
pressure controller)
If we now assume the air flow to be 61,200 cfm, at a
pressure (dP) oE 5" w~g., we find that Equation 2 provides-
5 = C1 (61,200)2 + 1 1/4 or;
Cl = 5 - 1 1/4 = 3O75 2
(61,200)2 (61,200)
Thus, air flow in the typical duct system is in
accordance with the Equation 2, which can now be solved Eor flow
rates within the the stable operating conditions of the system
such as 100~ to 40% in 10% increments.
Using this equation, we can therefore develop System
Curve B (Fig 8) which provides an indication of pressure (dP)/air
flow rate (q).
As discussed above, the 100~ air flow capacity is
predetermined, namely 61,200 cfm at 5" w.g.
Using Equation 2, we can now determine the required
pressure drop for various reductions in air flow as for example,
90% to 40~ in 10% increments.
At 90~ air flow capacity, q = (.9 x 61,200) cfm,
thereEore: clP = 3 75 x ( 9 x 61,200)2 ~ 1 1/4
(61,200)2
= 3.75 x o.g2 ~ 1 1/4
= 4.3
The following TBLE 3 can be established b~ repeating
the foregoing at 10% increments in reduction of air flow
capacity.
TABLE 3
AIR FLOW (C.F.M.) PRESSURE (ins.W.G.)
ACTUAL %
_~ _ . __
61,200 100 5
55,080 90 4-3
48,960 80 3.65
42,840 70 3.09
36,720 60 2.60
30,500 50 2.2
24,4B0 40 1.85
We now know the pressure rise across the fan which is
required in order to achieve 40% to 100~ air flow capacity at 10
i ncrements .
The manufacturer's fan curves relate the fan speed, air
flow and pressure drop of the selected fan. It follows that if
we plot the air flow and pressure drop from Table 3 on the Fan
Curves of the selected fan, we can obtain a reading of Fan
Speed/Air Flow. That is to say if the system curves are
superimposed on the Fan Curves, the point of intersection is an
indication of the required fan speed.
In a typical example where the fan is a variable speed
fan ~odel 1320 manufactured by Canadian Blower/Canada Pump
Limited, which will deliver 61,200 cfm at 5" w.g., the following
fan speeds are obtained by plotting the air flow and pressure
drop from Table 3 as aforesaid.
] 6 s~
TABLE_
AIR FLOW (C . F. M, ) PRESStlRE FAN SP~ED
ACTUAL % __ ( ins . W. G . ) ( .r~m) _
61,2~0 100 S 587
55,080 90 ~3 538
48,960 ~0 3.65 ~90
42,840 70 3.09 443
36,720 60 2.60 399
30,600 50 2.2 35
24,~80 40 1.85 302
With this information, we can now obtain a graph (Curve
D Fig 7) of air flow/fan speed as a first approximation for the
condition where the pressure in the duct is controlled at 1 1/4
w.g. Thus, we have established a speed at which the fan should
run in order to provide any required percentage of flow capacity
when the pressure in the duct system is maintained at 1 1/4'l.
If, for example, the system is operating at 60%
capacity and is delivering 36,720 cfm at a fan speed of 399 rpm.,
and it is determined that the flow rate should be reduced to 503,
the controller will reduce the speed of the fan to 358 rpm.,
which provides a flow rate of 30,600 cfm., without any
appreciable change of static pressure in the duct system while
the pressure across the fan is reduced from 2.60l' w.g. to
2.2" w.g.
It will, however, be understood that the curve D is a
curve which is a reflection of a theoretical system. In
practice, however, the balancing engineer will advi~e the actual
conditions which prevail at maximum and minimum flow and may also
provide particulars of the conditions which prevail at one or
more intermediate flow condition.
~t the lower Elow rate, the curve D may not be
sufficiently accurate to provide the level oE control that is
required. I can improve the accuracy of the system by developing
a curve D' in the Eollowing manner. The balancing engineer has
advised as previously indicated, that at the actual minimum flow
the fan must deli.ver 25,000 cfm at 2.1l' wg when rotating at 335
rpm. With this i.nformation, I am able to plot point 4 on Figure
and point 4 will be a point on the minimum flow system curve.
It follows that point 1 which is an indication oE the ma~ximum
r3~7
17
flow condition and the point ~ lie on the modified system curve
applicable to the air conditioning system of this example~ In
the limiting condition, the modified system curve may be
considered to be a straight line such as that identified as the
second approximation system curve E. If the conditions which
prevail at an intermediate flow configuration, is available from
the balancing engineer, a third point 5 may be determined from
which a third approximation system curve F may be established.
We can now plot the third approximation system curve F on the fan
curves of Figure 9 in order to obtain a further table similar to
table 4 from which we can in turn plot a further curve D'' on
Figure 7 which provides a more accurate system fluid flow curve.
By programming the dual input curve fitter with the
curve D'', I can then obtain an output which is an accurate
measure of the flow rate in the system for all flow rates.
In many air conditioning systems, it may be necessary
to make provision for adjustment of the constant pressure
setting. That is to say there may be conditions under which it
is necessary to change the constant pressure setting in order to
permit the system to operate effectively. If it is necessary to
change the constant pressure of the systems previously described
so that the duct is controlled at, for example, l l/2" wg, we
will have to modify the third approximation system curve F by
such an amount that at O air flow, the fan will develop l l/2" wg
pressure. We can then plot this modified system curve F on the
fan curves and this results in a new system fluid flow curve D'''
as shown in Figure 7. This curve D''' is similar to curve D''
except that it i5 rotated about the point l. In this manner, the
curve fitter can be adjusted to accommodate any change in duct
pressure by adjusting the setpoint oE the control.
If the characteristics of the duct system remain
unaltered, a variation in air flow demand may be achieved merely
by reducing the speed of the fan to that required to provide the
air flow rate as determined by the various curves of Fig. 7.
In rnost air conditioning systems, however, the ducting
system includes a plurality of adjustable flow restrictors at the
18
outlet into the space which is to be conditioned. These
restrictors are thermostatically controlled from within the space
which is to be conditioned such that they may be activated to
increase or reduce the size of the air flow passage oE the
ou-tlets from the system thereby to adjust the flow rate of the
system. If, for examplel the restrictors are activated to reduce
the flow rate, this will result in an increase in static pressure
in the system. The static pressure sensor will detect this
increase and will generate a signal which is indicative of the
actual pressure in the system. It is then necessary to reduce
the speed of the Ean in order to return the pressure in the
system to the original 1 1/4" w.g. pressure.
With reference to Figure 1 of the drawings, the
reference numeral 10 refers generally to a building in the form
oE a multi-storey structure which has a fan rooln 12 from which
conditioned air is discharged through a duct 1~ from which a
plurality of branch ducts 16 extend at various levels through the
building.
As shown in Figure 2 of the drawings, a supply fan 18
is connected to the duct 14. A variable speed motor 20 is
connected to the fan 18 and is powered from a suitable power
source. A variable speed controller 22 is connected to the motor
20. In addition, a return fan 19 is connected to the return air
duct 15. A variable speed motor 21 is connected to the Ean 19
and is powered from a suitable power source. A variable speed
controller 23 is connected to the motor 21.
A sensing tap 26 is located in the supply duct 14 at a
distance L from the discharge end of the supply fan 18 which
preferrably less than 5D wherein D is the largest duct diameter
downstream of the fan.
The location of the sensing tap 26 is oE considerable
importance. In order to keep the fan system operation steady,
stable and accurate, it is more important to keep the fan from
operating in its unstable range than it is to satisfy the duct
system requirements for good overall control. I have been able
to achieve this objective by locating the static pressure sensing
tap 26 as short a distance downstream from the fan as is
19
practical. Ideally, the static pressure sensing probe 26 would
be placed at the supply fan discharge flange, however, this is
impractical since turbulence is too great at this point leading
to erroneous readings. Effectively, the same result may be
obtained by locating the static pressure sensing probe 26 within
approximately 1 to 5 duct diameters downstream Erom the fan
discharge. It will be noted, however, that the static pressure
sensing probe may be located at any position of convenience along
the discharge duct upstream of the first branch line 16. The
location of the static pressure sensing probe in this manner is
contrary to the conven-tional practice in the air-conditioning
industry. This is undoubtedly a carryover from the variable
inlet vane control technologies and methods. Conventional
thinking is that it is the space which is to be conditioned that
must be satisfied and therefore the sensor should be located
about 2/3rd's of the way along the longest duct run. While it
-nay be diEficult to argue against this reasoning from an
intuitive point o view, in practice it is virtually impossible
to determine where such a location may be~ The duct systems
usually run from the Ean discharge down one or more shafts in a
building with multiple take-offs on each floor with the result
that it is difficult, if not impossible, to determine the optimum
location for the pressure sensing probe within the building.
The main reason Eor selecting the 2/3 location, however, relates
to the control system stability of variable inlet vane control
systems and is a compromise between satisEying the longest duct
run and the practicality of identifying such a location. It has
been generally believed that if the sensing point or tap is
sufficiently far away from the fan, the problems associated wi-th
the usual inlet vane controls do not show up.
The supply fan controller 30 (Fig.2) receives a signal
from the sensor 26 through a line 28 which may be in the form of
a plastic tube or the like which conveys a pneumatic signal to
the controller 30. The controller 30 generates an output signal
102 which is directed to the variable speed controller 22 which
adjusts the speed of the supply fan motor 20 as required in use.
The supply fan controller 30 will now be described with
reference to Figure 3 of the drawings.
As shown ln Figure 3 o~ the drawings, the line 28
cornmunictes with a transducer 40 which serves to convert the
pressure signal to an electrical signal. The electrical signal
is fed to a signal conditioner 42 which provides a signal having
an output o~ the order oE 0 to 5 volts or 0 to 10 volts. This
signal is fed to an adjustable filter 44. The filter 44 is
adapted to overdampen the signal which it receives from the
conditioner 42 so that its output signal will change
progressively to a steady state over an extended time period to
ensure that a substantially stable air pressure is maintained in
the air duct system 14 in use. This overdamping feature will be
described hereinafter with reference to Figure 5 of the drawings
in more detail.
The output signal from the filter 44 is directed to an
(optional) integrator 64 and the integrated signal is directed to
a driver 47 which in turn produces an output signal which is a
direct reading of the pressure in the duct 14. The output signal
of the (optional) integrator 64 is also directed to the
controller 46 which communicates through line 48 with a driver
50. The driver 50 communicates with the supply fan speed
controller 22 through line 102. A set point potentiometer 57
also communicates with the controller 46. The controller 46 is
programmed to generate a signal output which is proportional to
the required speed of the supply fan. The signal is directed to
the driver 50 and then to the supply fan variable s~eed
controller 22 which is in turn connected to the motor 20.
The pressure signal in the line 202 which is derived
from the pressure in the supply duct as previously described, is
directed to the curve fitter 53. The curve fitter 53 is
programmed wi-th the appropriate fan/system curves for the supply
fan which is used with the system and acts much in the manner of
a look-up table similar to the graph illustrated in Figure 7, so
as to provide an output signal which is directly proportional to
the capacity output o~ the fan. As shown in Figure 7 oE the
s~t~
21
drawings, this curve may not be linear. I have Eound that the
signal which is proportional to the static pressure can be used
to locate the intersection of the fan curves and the Y axis
(speed). The curve fitter 53 is a device which receives the
speed signal from line 101 and the pressure signal from the
filter 44 and is programmed to provide an output signal which is
a reading oE the supply fan capacity. Thus, it will be seen that
the controller 30 provides three outputs, one oE which is a
pressure indicator, the other of which is a supply fan speed
control signal and the other of which is a supply fan air flow
output signal. The controller 30 has two inputs which are the
pressure signal input from the line 28 and the speed input from
the variable speed controller by way of line 101.
A further output signal from the controller is conveyed
through line 108 to the controller 104 which is the return fan
capacity controller. This signal is the same signal as that
which is the indication of the capacity output of the supply fan.
This signal is transmitted to a signal translator 220 (Fig.3)
which in turn feeds the controller 246 which drives the driver
247 which provides an output signal in line 103 which is the
return fan variable speed control signal.
A manual Building Pressure Compensation Adjustment, to
componsate for the "stack effect" or the like, in the form of a
potentiometer 270, is provided for the purposes of adjusting the
set point of the signal translator 220 to compensate for the
stack effect will be described hereinafter. A further outside
temperature sensor 273 may be alternately provided which serves
to monitor the outside temperature and also serves to provide an
offset signal through a signal conditioner 275 which has a range
potentiometer 277 and a zero set potentiometer 279. A fume hood
sensor circuit which will be described hereinafter with reference
to Figure 7, communicates with the signal translator 220 through
sensing line 290.
The controller 104 receives an input speed signal in
line 104 from the return fan variable speed controller 23. This
signal is received by a signal translator 281 and is processed by
22
a curve fitter 253 which functions in the same manner as the
curve fitter 53 of controller 30. The curve fitter 253 does not,
however, have an lnput from t'le pressure sensor because as shown
in Figure 8 of the drawings, the curves for the return fans are
linear and originate at the origin of the X and Y axis. The
speed input signal is, however, transmltted to the controller 246
through the line 283. The control:Ler 246 receives the supply fan
capacity signal as a set point signal, the return fan capacity
signal and is pro~rammed to provide the appropriate return fan
variable speed output signal to the variable speed controller 23
along the line 103.
The output signal frorn the curve fitter 253 is
proportional -to the return fan air flow output and provides a
direct reading thereoE by measur:ing the signal on line 106.
In use, a reduction in pressure in the duct 14 is
detected by the sensor 26 which directs a signal to the
controller 30 which is activated to direct a signal to the
variable speed controller 23 to increase the speed of the motor
20. The controller 30 provides an output signal to the
controller 104 which is an indication of the supply fan air flow
output and the controller 104 matches this output with the
required return fan capacity and generates a signal to the
variable speed controller 23 which serves to drive the variable
speed return fan at the speed requirod in order to maintain a
balance in the system so that the roturn Ean capacity is
effectively matched with the supply fan capacity. Various other
parameters are monitored as previously described so that the
speed of the return fan motor may be modi-fied depending upon
outside air temperature, manual stack efEect control and
auxiliary air venting systems such as fume hood venting systems.
Various modifications of the controller of Figure 3 will be
apparent to those skilled in the art. For example, an integrator
62 may be used in place of the filter 44 or an integrator 64 rnay
be used in addition to the Eilter 44. If the integrator 64 is
used in addition to the filter 44, it is arranged in series with
the filter 44 and may be located beEore or after the filter 44.
The integrator 62 or 64 may serve the same function as the filter
Eor periodic disturbances, that is to say the integrator 62 and
64 may serve to integrate out periodic Eluctuations in the signal
prior to transmission of the signal to the controller.
As previously indicated, the input signal to the return
fan capacity controller 104 is the signal conveyed by the line
108 from the controller 30. It will be understood that the
return fan characteristics are considerably different to those of
the supply fan with the result that before passing the signal in
the line 108 to the controller 246, it is necessary to transpose
or condition this signal be means of a conditioner 220 so that it
varies in accordance with the output curve oE the return fan
rather than the output curve of the supply fan. This signal is
transmitted through the line 228 to provide the set point signal
of the controller 246. The controller 246 and driver 247 operate
in the same manner as the corresponding components of controller
30. Thus, it will be seen that t'ne output signal of the
controller 104 which is directed to the variable speed controller
23 -through line 103 serves to control the operation of the return
fan in a manner to insure t`nat the capacity of the return fan
closely matches the capacity of the supply fan.
OFFSET TRACKING OF THE RETURN F'AN
In order to overcome the difficulties associated with
the building pressure or "stack effect" previously described, I
provide a manual Building Pressure Compensator potentiometer
(SEC) 270. This signal is conditioned by a signal conditioning
or translating device 220 and is then fed to the controller 246
through the line 228. The signal generated by the suilding
Pressure Compensator is received by the controller 246 and serves
to offset the tracking of the return fan by an amount equal to
approximately half the rotation of the potentiometer. A 50%
rotation of the potentiometer would track the return fan as if
the Building Pressue Compensator was not in the circuit.
Rotating it to one extreme would decrease the return fan tracking
and rotating it to the other extreme would increase the return
fan tracking.The former wi11 result in the return o~ less air
than would be delivered by the supply Ean resulting in more air
2~
being available to pressurize the building, in winter, to
compensate for the higher outside air pressure. This would
eliminate the door operating problem and whistling around the
building shafts. ~otating it to the other extreme would increase
the amount of air being returned. This would reduce the problem
of air exfiltrating from the builcling and would eliminate the
door operating and whistling problems in the summer.
The ~uilding Pressure "stack effect" problem is caused
by a temperature difference between the constant building
temperature and the variable outside air temperature. This
problem is overcome by providing an automatic suilding Pressure
Compensator. This device is illustrated in Figure 3 of the
drawings and includes an outside air temperature sensor 273 which
provides a signal which is conditioned by a signal conditioner
275 which has a range adjustment potentiometer 277 and a zero set
potentiometer 279. The output signal from the signal conditioner
275 may be fed to the translator 220 through line 288 in the same
manner as the signal of the manual potentiometer. This signal
has the ability to automatically adjust the tracking range of the
return fan to minimize space pressure fluctuations in the same
manner as the manual potentiometer (SE~).
A further signal may be used for the purpose of
offsetting the controller 246. This signal is supplied through
line 290. This signal can be used to compensate for air which is
exhausted from the air conditioned space such as by way o~ the
fume hood exhaust fans which are widely used in laboratories. A
typical ~ume hood control system is illustrated in Figure 6 of
the drawings wherein an auxiliary contact 301, 302, 303, 304, and
305 is provided and a line pair which communicates with each fume
hood exhaust fan motor starter. Potentiometers 310, 311, 312,
313, and 314 are provided, one associated with each fume hood
motor starter. The potentiometers 310 to 31~ are wired in series
and fed from a constant current source 320. The potentiometer
values are adjusted to be analogous to the air exhausted by the
fan with the result that when the fan is started by the occupant,
the auxiliary contacts associated with the fan short out that
resistance and a voltage change proportional to the air exhausted
by that fan is obtained. The signal translator 322 translates
this signal to a signal which can be fed through line 290 to the
signal translator 220. It will be appararent that by this
system, i-t is possible to keep the total air exhausted plus the
returned air about the same as the air supplied by the supply
fan. There may be a slight offset either negative or positive
depending on whether it is desirable to keep the space negative
or positive with respect to the outside. This system is
particularly well suited to solve the serious air control
problems which plague variable volume laboratory air conditioning
systems. It will be understood that this system is not
restricted to fume hood exhaust systems but is applicable to any
system in which conditioned air is discharged from the space by
way of a separate exhaust system.
OVER-DAMPING OF CONTROL SIGNAL
According to general control theory, the optimum
control system is one that is optimally dampened, i.e. a system
wherein the output amplitude cycling falls within a decaying
envelope within approximately 3 excursions. In this regard,
reference is made to Figure 5 of the drawings which illustrates
optimal damping of the amplitude oE the conventional control
signal illustrated by the line 70. The signal 70 is normally
dampened within the decay envelope illustrated by the lines 72
and 74 within approximately 3 excursions.
In air-conditioning systems, any changes that take
place in the system loading are mainly due to the sun shining in
windows, due to people moving into and out oE the space to be
conditioned or due to lights and heat producing equipment such as
computer terminals or the like being turned on or off. If, for
example, a computer is running and producing heat, when it is
turned off, it functions as a heating element. The residual heat
reduces very slowly. In addition, there is the thermal mass of
the building structure that dampens all air-conditioning load
changes. By the time the terminal air conditioning boxes
collectively react to these load changes, and they should
collectively react slowly, sometimes minutes will pass.
Frequently the load changes in one part oE a system are cancelled
by load changes in another part of the system which are nearly
identical in magnitude but opposite in direction. For example,
26
the load resulting from the movement of direct sunlight around a
building in the course of a day can result in changes in the load
applied to various parts of a building while the total load may
remain substantially the same over the full day.
Conventionally, control system response times are
measured in very small fractions of a second, frequently in milli
or micro seconds. However, air-conditioning load changes take
place in much longer time periods as for example, periods as long
as several minutes or more. When these changes take place, the
control system must be sensitive enough to sense them and to
react accordingly. I have recognized these characteristics and
for this reason I provide a control system in which the control
signal is over-dampened, thereby to increase the controller
sensitivi-ty. Over-damping is achieved by the filter 44
previously described so as to generate a signal to t'ne controller
46 which follows a curve like curve 76 illustrated in Figure 5 of
the drawings.
The slow system response which I am able to generate is
important for other reasons. Terminal boxes connected to the
duct system, are usually under the control of a room thermostat.
Such a thermostat will allow a typical box to pass just the
correct amount of air to the space to satisfy the load. The
correct amount of air depends on the pressure upstream of the box
remaining constant. If -the fan capacity control system which
tries to keep this pressure constant is "live" and allowed to
cycle, causing the duct pressure to vary, the box will allow more
or less air than desirable to pass to the space causing
temperature fluctuations. These fluctuations would be sensed by
the thermostat which would try to correct them. The net result
of this scenario is that the two control systems fight each other
and the whole air-conditioning system would become unstable. The
over-damping of the fan capacity controls helps to prevent this
from occurring and greatly assists in providing stable controls.
As previously indicated in Figure 2 of the drawings,
the sensed variable which is detected by the detector 26 is the
fan pressure. Generally, this pressure is not steady enough for
control purposes and the filter 44 which has a long time constant
serves to filter out transients and to allow only actual load
27
changes heavily filtered to pass to the controller ~6~ The time
constant is adjustable with the adjustment providing a range of 0
to 200 seconds. rrhe -time constant may therefore be adjusted
on-site to be a good compromise between the desired system
response time and control system stability.
As previously indicated, integrators 62 or 64 may be
substituted for or used in conjunction with the filter 44. In
control systems requiring fast response times, cycling occurs at
a specific frequency. If the control system is set for the
desired sensitivity and impermissable and excessive cycling
occurs, substituting an integrator for a filter may help to
overcome this difficulty. The integrator must be inserted into
the circuit as illustrated in Figure 3 of the drawings between
the signal conditioner 42 and the controller 46. As the
transmitter senses the system cycling, the integrator will act on
the output. As long as the cycling is of a set frequency, the
integrator will integrate out the fluctuations. The integral of
a symetrical periodic function is zero. Therefore, the
integrator output will consist of the steady state value only as
sensed by the transmitter. It is important that the frequency of
the cycling be measured on the job site and adjusted into the
integrator. An alternate position of the integrator is in the
feedback loop. This device may be used with or wit'nout filter 44
and integrators 62 and 64.
Controller 30 may also be constructed with a
micro-processor replacing some or all signal processing,
conditioning, filtering, integrating, controlling, feedback, set
point and detecting functions by means of software algorithms
contained in the micro-processor's memory.
In air delivery systems having well designed air entry
and exit configurations for the Ean, the accuracy of my method
and control system is derived from the accuracy of the fan curves
which are provided by the manufacturers of the various fans. The
manufacturers taice great pains to assure the accuracy of their
fan curves. It will be apparent that the manufacturer's fan
curves may have l:o be modified for a particular installation if
the inlet and outlet configuration of the fan are significantly
different from those utilized by the Ean manufacturer in
preparing the ideal fan curves.
While some inaccuracy will inevitably be present in my
system this will primarily result from loading of the air or
fluid filters which are employed in my system. However, if the
pressure drop across the filters due to clogging increases by 1
inch W.G. in a fan system selected for 4 inches W.G., the error
only increases by 3%. Consequently the accuracy of my system is
improved by a significant factor. Furthermore, because all
signalling processes are done electronically, drifting is
essentially eliminated and reca~ibration is essentially no longer
required.
Various modifications of the present invention will be
apparent to those skilled in the art. It will be apparent that
the air conditioning control system of the present invention may
be used to advantage in a single storey building in which case it
serves to overcome the difficulties experienced with lateral
rather than vertical air disturbances.
