Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SLIDING GATE TERMINAL UNIT
FOR AIR HANDLING SYSTEM
TECHNICAL FIELD
The present invention relates to a terminal unit for
use in an air handling system and through which conditioned
air flows to a space to be air conditioned and wherein the
volume of air entering the space is controlled by the terminal
unit which can divert some of the air to a by-pass outlet.
BACKGROUND ART
Terminal units, also referred to in the art as by-pass
boxes, are known and are usually of rectangular shape. They
have an inlet at one end and an outlet at another end and
conditioned air normally flows straight through the box. A
by-pass opening may be provided in a top or a side wall of the
box, and a diverter element, such as a pivoted blade, is
positioned within the box to deflect all or some of the
conditioned air entering the inlet to the said by-pass outlet
upstream of the outlet. The underlying principle of the air
handling system in which by-pass boxes are used is that under
all operating conditions, the pressure delivered by the supply
fan not decrease and should ideally remain constant. This is
due to the fact that in low cost air handling systems, the
supply fans used are usually of the forward curve type. These
fans have the inherent characteristic of compensating for a
drop in system pressure by increasing the air volume at a
rapid rate with the result that the power required to drive
the fan increases. This then implies that the by-pass box
must be able to cycle the supplied air between the space and
the by-pass outlet without creating any changes to the air
flow at the by-pass box inlet at all positions of the diverter
blade throughout this cycle.
The underlying problem with existing by-pass units
arises when the diverter blade or plate is at or near the
halfway position. In duct systems, pressure is required at
the entrance to the system to overcome frictional losses of
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the air as it moves through the duct system. The quantity of
pressure required to move the desired amount of air through
the ducts varies with the square of velocity of the air in the
ducts. To reduce the quantity of air in a given duct system
while maintaining the same pressure at the entrance, it is
necessary to increase the resistance to the flow of air in the
duct system, i.e., reduce duct size, add restrictions such as
dampers, lengthen the duct, etc. For example, let us examine
three duct systems: one from the supply fan to the inlet of
the by-pass box, one from the inlet of the by-pass box to the
conditioned space and one from the inlet of the by-pass box
through the by-pass outlet and a manually adjustable balancing
damper. Let us suppose for case of argument that through the
use of the manually adjustable balancing damper the frictional
losses are equal in the two latter duct systems in this
example. As the diverter blade moves the conditioned air from
one of these two duct systems to the other, the velocity in
one duct will drop as the other will increase from zero. In
the mid-cycle position of the diverter blade, the air velocity
in each duct system will be half of its previous maximum:
there are now two outlets for the same quantity of air. With
the reduction in air velocity, the frictional losses have been
reduced and less pressure is now required to deliver the air
to the conditioned space and the by-pass outlet. In response
to the change in pressure requirements, the velocity through
all three duct systems will then increase until the loss of
pressure has been compensated for. This translates into an
increase of the airflow at the supply fan. In the case where
several by-pass boxes in the system are subjected to the same
conditions, the increase in airflow at the supply fan may
become more than the supply fan motor can handle.
In known by-pass boxes on the market today, supply fan
motors must be oversized to protect against the above
eventuality and use more power than would be theoretically
necessary if no variations in system pressure were created by
the by-pass box. In prior art, several shapes and methods of
mounting the diverter blade have been proposed with little or
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no success in overcoming this problem. In addition, they may
create additional problems. In its simplest form, the
diverter blade takes the form of a plain rectangular plate
that is pivoted on one edge and mounted with the pivot axes
close to the wall of the duct adjacent to the by-pass outlet
opening. The blade travels between a position in which it
closes the opening so that all of the air flows straight
through the duct, and a position in which the blade blocks the
outgoing air so that the air is totally diverted by the blade
into the by-pass opening. Typically, an actuator is mounted
externally on the duct and coupled to the shaft for turning
the shaft between its two extreme positions under the control
of a thermostat in the space to be conditioned. Since
pressure is required to move the air from the by-pass box to
the conditioned space, it exerts a force perpendicular to the
blade that tends to rotate the blade to its extreme positions.
The velocity of the air impinging against the blade also
exerts an additional pressure and thus a force that tends to
rotate the blade out of the air stream. Typically the forces
associated with the pressure to overcome the system frictional
losses is two to five times the pressure generated by the air
velocity. Two problems may arise: 1. The actuator must
resist these forces and must supply a relatively high torque
to move the diverter blade at or near the extremes of its
travel. Relatively powerful actuators must therefore be used
with their adjacent additional cost. 2. When the pressure
losses downstream from the unit are relatively high, the
diverter blade will have a tendency to pulsate as it
approaches the extremes of its travel: the pressure
differential across the blades is at or near its maximum. In
some cases, the diverter blade may flex and snap closed over
the opening, causing an audible noise. Also, with the
diverter blade at mid-cycle, the supply fan will sense a
reduction in pressure and the related problem as described
previously.
In an alternate attempt to address these problems, it
has been proposed to relocate the pivot axe to the center of
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the rectangular blade and extend it perpendicular to the
airstream through the center of opposite faces of the
rectangular duct. In this configuration, part of the blade is
above the pivot shaft and an equal part below so that the
turning effect on the shaft imposed by the pressure and air
velocity impinging on one part of the blade is counteracted
and ideally balanced by the pressure and air velocity that
impinges on the other part of the blade. While this blade
arrangement avoids the imposition of high torque loads on the
blade pivot shaft, an auxiliary blade must be provided to
close the by-pass outlet in the duct when all of the air is to
flow straight through. Normally, the auxiliary blade is
pivoted through the center or less ideally to the edge of the
by-pass opening and out of the main air flow through the duct,
and the auxiliary blade is coupled to the main diverter blade
by a linkage so that the auxiliary blade is opened and closed
automatically in response to the turning of the main diverter
blade under the control of the actuator. This arrangement not
only introduces additional components and, therefore, cost and
attendant service difficulties, but the problem of the
pressure drop at mid-cycle is still present. In the case
where the auxiliary blade is pivoted at its edge, the actuator
must also overcome the additional load of the pressure acting
against the blade.
In an attempt to eliminate the additional auxiliary
blade, it has been proposed that the blade have an angled
shape with an intermediate blade portion selected so that, in
the diverting position, the blade presents to the incoming
air, surface portions of substantially similar area disposed
on respectively opposite sides of the shaft and, in the
straight-through position, a portion of the blade closes the
by-pass outlet. While this arrangement will ideally eliminate
the forces attributed to the air velocity, the higher forces
due to pressure acting perpendicular to the blade are not
balanced since the surface portions of the blade as seen by
this pressure are not equal on both sides of the pivot shaft.
This proposal is also subject to the pulsation and flexing
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problems noted for the edge pivoted diverter blade and only
slightly addresses the problem of the drop in pressure at mid-
cycle.
SUMMARY OF INVENTION
It is a feature of the present to provide a variable
air volume terminal unit for an air handling system which
substantially overcomes the above-mentioned disadvantages of
the prior art.
Another feature of the present invention is to provide
a variable air volume terminal unit for an air handling
system, and wherein the unit is provided with an air divider
gate which is slidingly displaceable transverse to the air
flow path to deflect air entering an inlet of the unit to one
or both of an outlet opening or a by-pass opening, and wherein
the actuator device used to displace the divider gate is
subjected to a substantially constant force during its
displacement of the divider gate.
Another feature of the present invention is to provide
a variable air volume terminal unit for an air handling system
wherein the unit is comprised of a housing having an inlet for
receiving forced conditioned air, a main outlet and a by-pass
outlet and an air divider gate which is slidingly displaceable
transverse to an air flow path defined between the inlet and
the outlet openings and wherein substantially constant inlet
air flow conditions are maintained during the displacement of
the divider gate.
According to the above features, from a broad aspect,
the present invention provides a variable air volume terminal
unit for an air handling system comprised of a housing having
a conditioned air inlet for receiving forced conditioned air
in the housing. The housing also has a main outlet opening
and a by-pass air outlet opening. An air flow path is defined
between the air inlet and the outlet openings. An air divider
gate is slidingly displaceable transverse to the air flow path
to deflect air entering the inlet to one or both of the outlet
openings. The divider gate has an outlet obstructing rear end
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and an upstream deflecting face. Linkage means is connected
to the divider gate. An actuator device is connected to the
linkage means to displace the divider gate across the air flow
path to simultaneously open one of the outlet openings and
close the other in predetermined proportions as desired while
maintaining a substantially constant total outlet open area as
well as increasing the pressure drop through the unit at or
near mid-cycle so as to substantially compensate for the
reduction in pressure created by the low velocities in the
outlets and thus maintaining substantially constant inlet air
flow conditions, while the force required to displace the
divider gate by the actuator device remains substantially
constant. The upstream deflecting face is a triangular wedge-
shaped face and has an apex ridge facing the inlet. The apex
ridge extends vertically of the outlets. The outlet openings
are substantially equal in cross-section and are positioned
side-by-side and equidistantly spaced from a central flow axis
of the inlet. The housing is a rectangular housing with the
inlet being disposed substantially central to a front wall of
the housing. The tracks are secured to a respective one of
opposed walls. The divider gate has a rectangular rear end
which is guidingly secured at a opposed edges by bearing means
slidingly engaged between the tracks. The outlet openings are
dimensioned to be totally obstructed by the rear wall when the
divider gate is disposed in obstructing alignment therewith.
BRIEF DESCRIPTION OF DRAWINGS
A preferred embodiment of the present invention will
now be described with reference to the accompanying drawings,
in which:
FIGS. lA to lC, 2A to 2C, and 3A to 3C are schematic
representations of divider gates of the prior art;
FIGS. 4A to 4C are schematic representations of the
operation of the divider gate of the present invention;
FIG. 5 is a fragmented perspective view showing a
preferred form of construction of a variable air volume
terminal unit in accordance with the present invention; and
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FIG. 6 is a similar perspective view of the air
divider gate of the present invention showing the air flow
paths.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figs. lA to lC are schematic representations of a
simple form of a divider blade or gate of the prior art
wherein the gate 10 is mounted on a pivot 11 secured adjacent
a by-pass opening 12 provided in a side wall or top wall 13 of
a terminal unit (not shown). As the diverter gate 10 is
pivoted downwardly, as shown in Fig. lB, some of the
pressurized incoming conditioned air, indicated by arrows 14,
is diverted to the by-pass opening 12 and the remaining air
flows through to the main opening 15. As can be seen, the
incoming air stream 14, which is under pressure, will exert a
perpendicular force against that blade to tend to rotate the
blade to its extreme by-pass position, as shown in Fig. lC.
In order to resist this force, the actuator, which operates
the pivotal gate, must supply a relatively high torque.
Therefore, it is necessary to have powerful actuators, as
previously described, and these are very costly.
Figs. 2A to 2C are further representations of another
type of prior art divider gate wherein the pivot axis 16 has
been relocated to the center of the blade 10'. Although this
arrangement avoids the imposition of high torque loads on the
blade pivot shaft 16, an auxiliary gate 17 is required to
close the by-pass opening 12' when all of the air is to flow
straight through the duct to the outlet opening 15', as shown
in Fig. 2A. The auxiliary gate 17 is coupled to the diverter
blade 10' by a linkage 18. This type of an arrangement
requires additional components which results in added costs
and maintenance, and furthermore, the actuator must also
overcome the additional load of the pressure acting against
the auxiliary blade.
In an attempt to eliminate the auxiliary blade, a
diverter blade having a specific shape has been proposed, and
such is described in U.S. Patent 5,044,402 which issued on
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September 3, 1991 and illustrated by Figs. 3A to 3C. It
discloses an angled shaped blade with an intermediate blade
portion. When the angled blade 10" is pivoted to by-pass air
to the by-pass opening 12" it can be seen that the blade
presents to the incoming air flow 14" substantially similar
surface areas 19 and 19' on opposite sides of the pivot shaft
20. In the straight-through position, as shown in Fig. 3A,
the portion 21 of the blade closes the by-pass outlet 12".
Since the incoming air pressure, indicated by arrows 14", act
perpendicular to the blade 10", the forces are not balanced,
as the surface portions of the blade are not equal on both
sides of the pivot shaft when the blade is displaced. This
causes additional torque for the drive motor.
Referring now to Figs. 4A to 4C, and Figs. 5 and 6,
there will be described the variable air volume terminal unit
25 of the present invention. As herein shown, the unit is
constituted by a substantially rectangular housing having a
front wall 26, a rear wall 27, a bottom wall 28, a top wall
29, and opposed side walls 30. An inlet 31 is provided in the
front wall 26 connected to a main conduit (not shown) of an
air handling system via a pipe or a duct coupling 32 to admit
conditioned air under pressure into the housing 25. The
housing 25 is also provided with a main outlet 33 provided to
one side of the rear wall 27 and a by-pass outlet 34, herein
shown as provided in the side wall 30. The by-pass outlet 34
could also be provided in the top wall 29 or bottom wall 28,
and it usually redirects air into an air return conduit (not
shown) of the air handling system or in a ceiling plenum. A
means of varying the by-pass opening to permit the balancing
of the air flows in the respective outlet is shown as a
sliding plate 51. A divider wall 35 separates the main outlet
33 from the by-pass outlet 34.
An air divider gate 36 is slidingly displaceable
between a pair of guide tracks or channels 37 secured
transversely in the housing 25 and disposed transverse to the
air flow path entering the inlet opening 31. As herein shown,
the divider gate 36 is secured to a linkage mechanism or link
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arm 37' which is actuated by an axially rotatable drive shaft
38 operated by an actuator device 39 which may be an
electrically operated drive motor or a pneumatic drive unit.
As illustrated more specifically in Figs. 5 and 6, the
divider gate is of substantially triangular shape cross-
section and defines an upstream deflecting face formed as a
triangular wedge 40 having opposed angulated deflecting faces.
The apex 41 of the triangular wedge 40 faces the incoming air
flow path 4Z, as better illustrated in Figs. 4A to 4C. The
main outlet 33 has an entrance 33' formed between the tracks
37 to one side of the triangular divider gate 36 and the by-
pass outlet has an entrance 34' on the opposite side of the
divider gate.
As can be seen, the divider gate rear end 43
constitutes an outlet obstructing rear end of the gate.
Extension flanges 44 are provided along the top and bottom
edge of the rear end and are guidingly received within the
guide tracks 37. The flanges 44 constitute bearing means
which slide within the tracks 37.
A vertical slide track 45 is secured across the rear
end and receives a follower member 46 secured to an end of the
link arm 37'. As the actuating rod 38 is axially rotated as
shown by the arrow 47, the link arm follower member 46 will
move up and down the slide track 45 in the directions of
arrows 48 to displace the deflector along its guide tracks 37,
transversely to the incoming air flow and in the direction
indicated by arrows 49. Because the deflector gate 36 always
faces the incoming air flow, which is at substantially
constant pressure and guided transversely thereto, a
substantially constant resistance is present and consequently
the torque force required to displace the divider gate by the
actuator device 39 is also substantially constant. The
actuating rod 38 is supported stationary between a bracket 50
and the rear wall 27 of the housing. It is pointed out that
other suitable linkages and drives may be provided to displace
the divider gate 36, and the invention should not be
restricted to the linkage mechanism herein shown.
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The diverter gate 36 and its rear end 43 are
dimensioned to be approximately equal to the size of the
outlet entrance openings 33' and 34' so that it can totally
block the main outlet entrance 33' or the by-pass outlet 34',
or portions of both. As the divider gate is displaced across
the air flow path, it simultaneously opens one of the
entrances to the openings and closes the other in equal
proportions with the total outlet surface area (the
combination of both outlets) remaining substantially constant.
Also, the force, due to the velocity of the air impinging
against the divider gate at the extremes of its travel, will
be reduced, since only half of the gate is within the air
flow, and as the gate moves to an intermediate position, the
deflector equalizes the velocity forces by splitting the air
flow, as shown by flow lines 60 and 61 in Fig. 6. The partial
obstruction created by the deflector gate, as the gate
approaches the mid-cycle, tends to counter-balance the problem
of the lower pressure requirements of the outlet duct
associated with their reduced duct velocities, thus
maintaining approximately constant inlet air flow conditions,
as previously mentioned.
It is within the ambit of the present invention to
cover any obvious modifications of the example of the
preferred embodiment described herein, provided such
modifications fall within the scope of the appended claims.
