Air flow sensing device for air conditioning systems

Inaccurate readings of the air flow in an air flow system obtained by an air flow sensor because of an uneven air velocity distribution at the sensor are reduced in accordance with the present invention by providing an air redirecting means in the air flow closely adjacent and upstream from the air flow sensor. The air redirecting means is mounted for movement in response to impingement thereon of an air flow having an uneven velocity distribution so as to redirect the air flow and provide a more representative air velocity condition at the sensor.

BACKGROUND OF THE INVENTION 
The present invention relates to the measurement of the flow of air in an 
air flow system, and is particularly concerned with providing more 
accurate measurement of the flow of air. 
This invention also relates to an air volume regulator for use in an air 
conditioning system, and to providing more effective control over the 
regulation of air flow by obtaining a more accurate measurement of the air 
flow. 
The type of air volume regulator with which the present invention may be 
employed has an air flow sensor mounted in the air flow for sensing the 
velocity of the air flowing through the regulator. The sensor is connected 
to a suitable control mechanism, which in turn, is connected to a damper 
mechanism. The damper mechanism is adjustably positioned in response to 
the air velocity sensed by the sensor so that the air flow discharged from 
the regulator is maintained at a substantially uniform volumetric flow 
rate regardless of variations in the pressure of the air being supplied to 
the air volume regulator. 
Under ideal conditions, it would be desirable that the connecting duct 
supplying air to the air volume regulator be straight for a considerable 
length upstream of the regulator to establish a uniform, symmetrical air 
velocity distribution or profile within the connecting duct. This would 
insure that the air velocity measurement obtained by the air sensor would 
be representative of the air velocity conditions existing throughout the 
cross section of the duct. 
Under the conditions which actually exist in field installations however, 
the connecting duct normally must undergo a number of bends or turns in 
leading from the main air supply duct to the individual air volume 
regulator. These bends and turns frequently produce a distorted, 
asymmetrical velocity distribution within the connecting duct which causes 
the air flow sensor to obtain an inaccurate measurement of the air flow in 
the duct and which results in inaccuracies in the regulation of the air 
flow. 
The prior art has recognized the problem produced by flow disturbances and 
uneven velocity distributions upstream from a fluid flow sensor and the 
difficulty in obtaining an accurate flow measurement. Generally, the prior 
art has addressed this problem by mounting various types of stationary 
flow straighteners upstream from the flow sensor. Stationary flow 
straighteners have been proposed in the form of vanes, tubes, honeycomb 
elements, perforated plates, etc. By way of example, reference may be made 
to Preston U.S. Pat. No. 2,706,409; Sprenkle U.S. Pat. No. 2,929,248; and 
Goulet U.S. Pat. No. 3,981,193. 
The prior types of stationary flow straighteners of which applicants are 
aware have a number of limitations which render them unsuited, or of 
limited usefulness, in air conditioning duct systems, and in particular in 
air volume regulator installations. For example, certain types of 
stationary flow straighteners must be mounted a considerable distance 
upstream from the flow sensor. This requires that the flow straightener 
either be mounted in the duct separately from the air volume regulator, or 
requires an undesirably large size regulator. Many of the known types of 
flow straighteners produce a significant flow obstruction and an 
undesirably high pressure drop which is unacceptable in many air 
conditioning installations. Further, because of the various ways in which 
air volume regulators can be installed and the various bends and turns 
which may be present in the upstream connecting duct, the velocity 
distribution of the air supplied to the regulator is completely 
unpredictable and the known stationary types of flow straighteners cannot 
take into account this unpredictability of the velocity profile. 
SUMMARY OF THE INVENTION 
The present invention provides for obtaining a more accurate reading by an 
air flow sensor when an uneven velocity distribution occurs at the sensor 
by positioning an air redirecting means in the air flow closely adjacent 
and preceding the sensor. The air redirecting means is mounted for 
movement in response to the impingement thereon of an air flow having an 
uneven velocity distribution so as to redirect the air flow and cause the 
air velocity reading obtained by the sensor to be more representative of 
the average air velocity conditions. Thus, regardless of the direction of 
the air flow path upstream from the sensor and the uneveness of the 
velocity distribution resulting therefrom, the movably mounted air 
redirecting means will be automatically repositioned so as to redirect the 
air flow for obtaining a more representative velocity distribution at the 
sensor. 
The air redirecting means is constructed with surface portions laterally 
offset from the sensor in the air flow and oriented for redirecting air 
toward the sensor. Preferably, the air redirecting means is mounted for 
pivotal movement in the air flow and constructed so that impingement of 
the air thereon will pivotally reposition the air redirecting means in 
response to the uneven velocity distribution so as to redirect some of the 
air toward the sensor and to thereby provide a more representative and 
accurate measurement of the air velocity by the sensor. 
The air redirecting means of this invention is designed to be mounted only 
a short distance upstream from the air flow sensor to avoid the need for 
an oversize housing or for separately mounting the redirecting means in 
the duct upstream from the air flow sensor. The redirecting means is 
constructed so as to present a minimum obstruction to air flow to thereby 
avoid causing an undesirable pressure drop in the air flow system. 
In the embodiment of the invention illustrated herein, the air redirecting 
means comprises a pivotally mounted circular ring located in coaxial 
alignment with the sensor and mounted on a pivotal axis extending 
diametrically of the ring. The ring is of frusto-conical configuration 
with the walls thereof converging in the downstream direction and oriented 
for redirecting the air flow impinging thereagainst inwardly and toward 
the sensor. An uneven air velocity distribution impinging upon the ring 
will pivotally reposition the ring so that the portions exposed to the 
higher velocity air will be oriented for redirecting some of the higher 
velocity air toward the sensor. 
The air redirecting means of this invention can be advantageously utilized 
in a number of specific applications in air flow systems where it is 
desirable to obtain a more accurate measurement of the air velocity in an 
air flow system. One particularly useful application of the air 
redirecting means is in an air volume regulator of the type which includes 
an air flow sensor, a damper assembly mounted for adjustably restricting 
the flow of air through the regulator housing, and a control unit 
associated with the sensor and with the damper for effecting adjustment of 
the damper assembly in response to the air flow through the regulator 
housing so as to regulate the air flow and deliver a substantially 
constant volume air flow regardless of fluctuations in the pressure of the 
air being supplied to the regulator. In this type of regulator, inaccurate 
measurement of the air flow by the sensor will result in inaccurate 
control over the volumetric flow rate of the air discharged from the 
regulator. The air redirecting means of this invention provides more 
effective control over the regulation of air flow by obtaining a more 
accurate measurement of the air flow by the sensor.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
Referring now more particularly to the drawings, FIG. 1 illustrates an air 
distribution unit or terminal box adapted for being installed in a central 
air conditioning system for controlling the flow of conditioned air into 
an air conditioned room or zone. The air distribution unit illustrated 
comprises an air volume regulator adapted for providing a substantially 
constant volume flow of air therefrom regardless of variations in pressure 
of the air being supplied thereto. The air distribution unit is 
conventionally mounted in the space above a suspended false ceiling and is 
connected to a main air supply duct by a connecting duct, such as the 
flexible connecting duct 11 illustrated. Depending upon the particular 
installation, the connecting duct may undergo one or more turns or bends 
in leading from the main supply duct to the air distribution unit. For 
convenience of illustration, only a single bend is shown in the connecting 
duct 11 but it will be understood that the number of bends and the 
direction thereof will vary with each distribution unit depending upon its 
location and orientation with respect to the main air supply duct. 
The air distribution unit includes an elongate generally rectangular hollow 
housing 12 having opposing pairs of side walls 13 and an end wall 14 at 
the upstream end thereof. A circular air inlet opening is formed in the 
upstream end wall 14, and a cylindrical collar 15 extends outwardly from 
the end wall 14 in surrounding relation to the opening to form a 
cylindrical air inlet passageway for the air distribution unit. Collar 15 
further serves to facilitate securement of the flexible connecting duct 11 
to the air distribution unit. The downstream end of the housing serves as 
an air outlet opening and is adapted to be connected to suitable ducts or 
diffusers for distributing the air into the air conditioned room or zone. 
As is conventional, the inner surfaces of the walls 13 may be covered with 
a suitable thermal and sound insulating material 16. 
Located between the inlet and outlet openings of the air distribution unit 
is a damper assembly, generally indicated by the reference character 20, 
which extends completely across the path of air flow for controlling the 
amount of air permitted to pass from the inlet to the outlet. 
An air flow sensor, indicated by the reference character 30, is mounted in 
the housing in the path of air flow for sensing the velocity of the air 
flowing therethrough. In the preferred embodiment of the invention 
illustrated herein, sensor 30 is located upstream from the damper assembly 
20 and near the air inlet for sensing the velocity of the air as it enters 
the air distribution unit. However, in certain types of distribution units 
it may be desirable or more convenient to position the sensor at other 
locations, such as downstream of the damper assembly for example. 
The sensor 30 is connected to the input of a control unit 31, which, in 
turn, has its output connected to a motor 32 mounted on the exterior of 
the housing. The actuator arm of motor 32 is connected, through mechanical 
linkage 33, to the damper assembly 20. The sensor 30 and control unit 31 
operate so as to adjustably position the damper assembly 20 in response to 
variations in air velocity sensed by the sensor so that a substantially 
uniform volumetric flow of air is discharged from the air distribution 
unit regardless of variations in the pressure of the air delivered to the 
unit by the connecting duct 11. 
In order to provide a desirably high accuracy of control by the air volume 
regulator, it is important that the air velocity measurement obtained by 
the air flow sensor 30 be representative of the velocity conditions 
throughout the entire flow area where the sensor is located. If the 
velocity distribution is uneven, i.e. asymmetrical at the sensing 
location, then an inaccurate reading of the air flow may be obtained. In 
order to obtain a more accurate air flow measurement by the sensor, an air 
redirecting element, indicated by the reference character 40, is mounted 
closely adjacent and preceding the air sensor 30 in the path of air flow. 
The air redirecting element 40 is mounted for movement and so constructed 
that impingement of the air thereon will adjustably reposition the element 
in response to the uneven velocity distribution so as to redirect some of 
the air flow and provide a more uniform air distribution at the sensor. 
In the preferred form of the invention illustrated herein the air 
redirecting element 40 is in the form of a pivotally mounted annular ring. 
It is positioned in the cylindrical flow passageway defined by collar 15 
and located a short distance upstream of the sensor 30. A rod 41, mounted 
in a vertically extending orientation in collar 15, extends diametrically 
through the ring-like element 40 and serves to pivotally mount the element 
in the cylindrical air inlet passageway defined by collar 15. Tubular 
spacers 42, carried by rod 41 and located on opposite sides of the 
ring-like element 40, serve to position element 40 in spaced relation from 
the walls of collar 15 and centered in the collar in coaxial alignment 
with the sensor 30. 
The ring-like element 40 is of frusto-conical configuration and oriented 
with the walls thereof converging in the downstream direction. The 
semicircular halves of element 40 located on opposite sides of the pivotal 
axis are laterally offset from the sensor on opposite sides thereof, with 
the interior surfaces 40a, 40b of each half being oriented to redirect the 
air impinging thereagainst inwardly toward the sensor. 
Abutment stops 43 are secured to the wall of collar 15 and extend inwardly 
therefrom for engaging the pivotally mounted element 40 and limiting the 
pivotal movement thereof. As best seen in FIG. 4, the abutment stops 43 
permit element 40 to undergo a maximum pivotal movement of only a few 
degrees, preferably no more than about 15 to 20 degrees. 
The air redirecting element 40 should desirably be of a relatively small 
surface area in relation to the flow area of the air passageway where it 
is mounted so as to avoid causing a significant flow obstruction and an 
undesirable pressure drop. Preferably, the surface area of the redirecting 
element should be no more than about one-fourth of the cross sectional 
area of the flow passageway where the redirecting element is mounted, and 
most desirably should be no more than about one-fifth. 
The following table sets forth several exemplary but nonlimiting examples 
of size relationships which have been empirically determined as being most 
suitable for use in the type of air volume regulator construction 
illustrated in this application. Referring to FIG. 4, the dimension A 
represents the diameter of the cylindrical air inlet passageway, and the 
dimensions B and C represent the major and minor diameters, respectively, 
of the air redirecting element 40. All dimensions are in inches. 
TABLE 
______________________________________ 
A B C 
______________________________________ 
4 3 21/2 
6 4 33/8 
8 57/8 43/4 
10 57/8 5 
______________________________________ 
The operation of the air redirecting element 40 will best be understood 
from FIG. 2. As illustrated therein, the connecting duct 11 undergoes a 
relatively sharp bend just upstream of where it is connected to the 
cylindrical collar 15. As the air flowing in the connecting duct 11 
reaches this bend, it will have a tendency to resist the sudden change of 
direction caused by the sharp bend, with the result that a greater 
proportion of the air will flow along the outer, larger diameter radius 
than along the inner, smaller diameter radius. This is indicated 
diagrammatically in FIG. 2 by the arrows. The air velocity distribution or 
profile will thus be uneven, or asymmetrical, as the air enters the air 
inlet passageway and flows past the sensor 30. In the absence of the 
redirecting element, the velocity reading obtained by sensor 30 under 
these conditions would not be representative of the air velocity 
throughout the entire flow area. The redirecting element 40, however, 
serves to collect and redirect some of the higher velocity air toward the 
sensor 30 to thereby obtain a more representative and accurate measurement 
of the air flow conditions. 
As illustrated, the redirecting element 40 has been pivotally moved by the 
air flow and has come into engagement with one of the abutment stops 43. 
This pivotal movement of the redirecting element is brought about by the 
flow of air impinging on the interior surfaces 40a, 40b of each half of 
the element. Since the air flowing against the surface 40a of the 
redirecting element is of greater velocity than the air flowing against 
the opposing surface 40b, the air flow has pivotally repositioned the air 
redirecting element and moved the surface 40a in the downstream direction 
and in an orientation for redirecting the air impinging thereagainst 
inwardly and toward the sensor 30. The surface 40b on the opposite side 
redirects relatively little air toward the sensor since the air impinging 
thereagainst is of relatively low velocity as compared to the opposite 
side, and since it is oriented nearly parallel to the air flowing thereby. 
It will be understood that if the connecting duct 11 were to extend in a 
direction opposite that shown in FIG. 2, the redirecting element 40 would 
automatically be repositioned so as to again redirect some of the higher 
velocity air toward the sensor. In a similar manner, if the connecting 
duct were to extend downwardly, upwardly, or at any angle, the redirecting 
element would be automatically repositioned at the proper angular 
orientation in response to the particular uneven characteristics of the 
velocity distribution at the redirecting element so as to thereby provide 
a more uniform velocity distribution at the sensor. Should the velocity 
distribution of the air be substantially uniform, the redirecting element 
will still serve to redirect some of the air toward the sensor and to 
provide a more representative and accurate measurement of the air velocity 
over the entire cross sectional flow area. 
Referring now more specifically to the construction of the air volume 
regulator illustrated, it will be seen that an inner conduit 17 is 
positioned within the housing 12 in communication with the connecting duct 
11. Conduit 17 is carried by the upstream end wall 14 of housing 12 and 
extends therefrom in a cantilever manner into the interior of the housing 
in spaced relation from the side walls 13 thereof and from the insulating 
material 16. The damper assembly 20 is carried by the inner conduit 17 
adjacent the downstream end thereof and extends entirely across the path 
of air flow through the inner conduit. 
The damper assembly is particularly constructed to minimize any change of 
direction of the air as it passes through the air distribution unit and to 
thus reduce air turbulence and maintain the noise level of the air passing 
through the distribution unit as low as possible. To this end, the damper 
assembly 20 is constructed of a series of side-by-side elongate damper 
blades 21. The blades 21 are pivotally mounted along respective spaced 
parallel axes and interconnected by linkage 22 so that adjacent blades 
pivot in opposite directions and move through a relatively small angle 
from a fully open position wherein the blades are oriented generally 
parallel to one another and to the direction of air flow, to a fully 
closed position wherein the blades have their proximal longitudinal edges 
positioned in abutting relation with one another to obstruct the flow of 
air through the housing. The blades are moved between the open and closed 
position by motor 32. In the illustrated embodiment of the invention, 
motor 32 is of the pneumatic type, but it is contemplated that other types 
of motors, such as electric motors for example, could be used in some 
installations. 
The air flow sensor 30 is mounted by a mounting support 34 approximately in 
the center of the cylindrical air passageway defined by collar 15, with 
the upstream end of the sensor being located at the juncture between the 
cylindrical collar 15 and the upstream end wall 14 of housing 12. The 
sensor has a hollow tubular interior oriented in the direction of air flow 
for receiving a sample of the air flowing along the duct, and with a 
sensing device located interiorly of the tubular portion of the sensor for 
measuring the velocity of the air and providing a control signal to a 
control unit 31. The preferred type of sensor is a pneumatically operated 
device known as a "fluidic flow sensor." This type of sensor is 
particularly suited for measuring relatively low velocity currents of air 
such as are carried by an air conditioning duct. With this type of sensor, 
air velocity is measured as a function of the amount of deflection caused 
by the duct air on a small jet of air directed transversely to the air 
flow in the duct. A suitable commercially available fluidic flow sensor of 
this type is the " Velocitrol" sensor available from Honeywell, Inc. 
Another type of pneumatically operated sensor which may be suitably 
employed in the present invention is the differential pressure type of 
sensor. In this type of sensor velocity is measured as a function of the 
difference between the static pressure and the total pressure in the duct. 
An example of this type of sensor is the Pitot tube. A number of sensors 
which operate on this principle are available commercially. It is also 
contemplated that the sensor may be of the electrically operated type. 
The control unit 31 receives the velocity signal from sensor 30 and 
provides a modulated pneumatic signal for operating pneumatic motor 32. 
Motor 32 opens or closes the damper assembly 20 in response to variations 
in air velocity sensed by sensor 30 so that a substantially uniform 
volumetric flow rate is discharged from the air distribution unit 
regardless of variations in the supply air. Suitable control units of this 
type are available commercially from several suppliers. By way of example, 
a control unit particularly designed for use with the Honeywell 
"Velocitrol" sensor is available from Honeywell, Inc. 
The control unit 31 may be used either with or without a thermostat. When 
used without a thermostat, the control unit operates to maintain the 
volumetric flow rate of the air substantially constant at a regulated 
manually preset set point. If desired, the control unit may be connected 
to a thermostat located in the room or zone being served by the air 
distribution unit. In this installation, the thermostat functions to vary 
the set point of the control unit to thereby increase or decrease the 
regulated flow of air to the room depending upon the temperature 
conditions in the room. 
Under some conditions, particularly in larger air distribution units, 
unusual pressure or flow conditions have been observed downstream from the 
sensor, which conditions sometimes interfere with obtaining an accurate 
reading from the sensor. These conditions are believed to be caused by the 
path of movement of the air toward and through the damper blades. In 
accordance with an ancillary feature of this invention, as illustrated in 
FIGS. 5 and 6, the effect of such abnormal flow conditions downstream of 
the sensor is minimized by positioning a baffle plate 36 a short distance 
downstream of the sensor 30. As illustrated, baffle plate 36 is mounted by 
the supporting member 34 for the sensor, and is positioned a short 
distance, e.g. about one inch, behind the downstream outlet end of the 
tubular portion of the sensor. This baffle member, when used in 
conjunction with the air flow redirecting element 40, further enhances the 
accuracy of the flow measurement obtained by the sensor. 
The foregoing description of illustrative embodiments of the invention has 
shown how the present invention may be used in an air volume regulator for 
providing more effective control over the regulation of air flow by 
obtaining a more accurate measurement of the air flow. The particular type 
of air volume regulator illustrated herein is the preferred type of 
regulator presently contemplated for use with the invention, but it is not 
intended that the present invention be limited to this particular type of 
regulator. It will be understood from the foregoing illustrative 
description that there are a number of other specific applications, 
besides air volume regulators, where the present invention can be 
effectively used for obtaining a more accurate measurement of air velocity 
in an air flow system. 
In the drawings and specification, there have been set forth preferred 
embodiments of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only and not for purposes 
of limitation.