Note: Descriptions are shown in the official language in which they were submitted.
1317503
-- 1
This invention relates to air distribu-tion
systems, in particular apparatus for extracting air
from a main supply duct to a branch duct.
It is well known to dis~ribute air in a building
from a main air supply duct to various branch ducts
throu~h openings in the wall of the main duct which
enter into the branch ducts. The volume flow rate of
air throu~h the branch is determined to some extent by
the static pressure in the main duct and the ~low
resistance of the branch. Because the branch opening
is flush with the wall of the main duct in the
commonly used distribution systems, the dynamic
pressure of the air flow in the main duct does not
assist the flow rate in the branch duct.
Attempts have been made to control or reduce the
level o~ noise created by such air distribution
systems. With the aforementioned configuration, the
noise level at the start of the branch duct is
generally the same as the noise level in the main
duct, the noise being caused primarily by the air
supply fan used in such systems. It is known to use a
silencer at the exit of the fan in the main duct to
reduce the noise level. Silencers have also been
employed at the inlet to the main supply fan. In
order that the silencer will not unduly affect the
operation of the system, its use must result in a
- 2 1317503
low pressure drop and its total open area must be
sizable. Thus, the silencer must be relatively large.
Because of this, known silencers can be costly and can
require a large amount of space in the building.
U.S. patent 4,418,788, issued December 6, 1983 to
Mitco Corporation, describes a branch take-off and
silencer for an air distribution system. The
apparatus includes a static pressure regain section
and a channel section adapted for coupling the input
duct to an output duct and branch ducts. The inner
surface of the wall of the regain section and that of
the outer wall of the channel section form a
continuous curve which results in smooth changes in
air flow velocity in order to provide efficient
1~ conversion of velocity pressure to static pressure. A
major difficulty with such an apparatus is that, due
to the round cross-section of the take-off passageway,
such an apparatus is difficult to manufacture,
particularly if maximum efficiency is to be obtained.
Also, asp2c~s of this Xnown design are not
particularly helpful in reducing the noise level in
the system or in the branch ducts.
U.S. patent 4,319,521, issued March 16, 19~2 to
Mitco Corporation, describes an air distribution
system that includes a mixing plenum for receiving and
mixing ou-tside and re-turn air. There is an input flow
13175~3
concentrator, an integral silencer disposed within and
coupled to the mixing plenum and this device is
adapted to establish a substantially axially
symmetrical flow path for air from the plenum to an
output port. A fan is coupled to the output port to
drive the air through the main duct for dis-tribution.
The path defining walls of the concentrator are
lined with acoustically absorbent material.
The present invention provides an improved branch
take-off air flow device for use in an air
distribution system, which device is not unduly
difficult to manufacture and which has improved sound
attenuating capabilities.
One version of the branch take-off air flow
device disclosed herein is for use downstream of an
axial fan having a central hub and a rouncl fan outlet.
This take-off device includes an elongate air flow
defining member located centrally in the main air
passagewa~ and extending in the axial direction. The
provision of this generally round member provides
improved air flow characteristics to the branch
take-off device and greater sound attenuation.
Accorcling to one aspect of the invention, a
branch take-off air flow device for use in an air
distribution system that includes coaxial input and
output ducts and one or more branch ducts angularly
1 31 7503
oEfset from the input and output ducts has a static
pressure regain section having an air passageway with
an input port of a size substantially the same as the
outlet of said input duct and an output opening.
There is also a take-off section having a central
passageway and one or more take-ofE passageways, the
central passageway having an OlltpUt port oE a size
substantially the same as an inlet of the output duct.
Each oE the take-off passageways is generally
rectangular in transverse cross-section and is deEined
by inner and outer walls with the outer wall being a
continuation of a wall defining the output opening of
the regain section. The inner wall has a relatively
thick, rounded leading edge where the take-off
passageway commences. An elongate airflow defining
member is located centrally in both the passageway of
the regain section and the central passageway of the
take-oEf section and extends in the axial direction.
This member has a generally round, transverse
cross-section which is substantially uniform in the
regain section. The diameter oE this member in the
take-off section is e~ual to or less than its uniform
diameter in the regain section. The airflow definlng
member extends axially past the rounded leadin~ edge
where the take-off passageway commences.
B
1317503
According to another aspect of the invention, a
branch take-off air flow device is provided for use in
an air distributlon system downstream oE an axial Ean
having a central hub and a fan housing with a round
air outlet, the system including an output duct
located downstream oE the device and one or more
branch ducts angularly offset from the centre axis oE
the fan and the output duct. The device includes a
static pressure regain section having an air
passageway extending therethrough, this passageway
having an inlet substantially the same in size as the
round air outlet of the Ean. There is also a take-off
section including a central air passageway extending
therethrough and one or more take--off passageways. The
central passageway has an output port oE a size
substantially the same as an inlet oE the output duct.
An elongate air flow defining member is located
centrally in both the passagewav of the regain section
and the central passayeway of the take-off section and
extends in the axial direction. The member has a
generally round transverse cross-section with a maximum
diameter equal to or less than the diameter oE the hub
of the fan. This member is generally cylindrical in the
regain section and extends axially past an inlet or
inlets of the one or more take-off passageways.
~B~
1317503
Preferably the ai.r flow defining member has a
metal exterior skin that is perforated wlth numerous
holes distributed over its surface and is filled with
sound absorbing material surrounded by this skin.
Preferably the inner wall of the take-off section
is filled with sound absorbing material that extends
to a leading edge of the wall located where the
take-off passageway commences.
Further ~eatures and advantages of the present
branch take-off air flow device will ~ecome apparent
from the following detailed description taken in
conjunction with the accompanying drawings wherein:
Figure 1 is a front elevation, partly in
cross-section, illustrating a vertical up-blast air
distribution system includin~ a branch take-o:Ef air
flow device constructed in accordance with the
invention;
Figure 2 is a schematic view in elevation of an
air distribution system havin~ two or more branch
take-off air flow devices;
~ 7 ~ 13175~3
Figure 3 i5 an elevational view of a vertical
down-blast air distribution system showing two branch
take-off air flow devices;
Figure 4 is a perspective view of a tubular frame
for a static regain section;
Figure 5 is a top view of a static pressure
regain section for a branch take-off air flow device;
Figure 6 is a bottom view of the regain section
of Figure 5;
Figures 7 and 8 are long side and short side views
respectively of the regain section of Figure 5;
Figure 9 is a cross-sectional view taken along
the line IX-IX of Figure 11 of a take-off section of
an air flow device;
Figure 10 is a side view of the take-off section
of Figure 9, which section has three take-off
passageways;
Figure 11 is a top view of the take-off section
of Figures 9 and 10;
' 20 Figure 12 is a cross-sectional elevation of the
take-off section taken along the line XII-XII of
Figure 11;
1317503
Figure 13 is a perspective view oE the tubular
frame work for the take off section shown in Figures
9 to 12;
Figure 14 is a perspective view illustrating the
sheet metal construction of a single take-off
passageway;
Figure 15 is a schematic illustration in
cross-section of a branch take-off air flow device
constructed in accordance with the invention;
Figure 16 is a schematic illustration in
elevation of a branch take-off air flow device with
one only of the branches being illustrated in dashed .
lines;
Figure 17 is a schematic sectional detail of the
curved portion of a take-off passageway; and
Figure 18 is a sectional. detail of the take-off
section illustrating the significant dimensions of the
take-off passageways.
~; 20 The bottom portion of a vertical up-blast air
distribution system is illustrated in Figure l. The
illustrated system has an air inlet silencer indicated
generally at lO having four rectangular inlets 12
adjacent to each other and arranged at 90 degrees to
one another. The inlet silencer feeds air to
- 1 31 7503
an axial fan 14 having a round central hub 16 and a
round fan housing 18 which, in the illustrated
version, is mounted on wheels or rollers 20 so that
the fan unit can be rolled out on roll-out rails 22
for maintenance or repair purposes. Preferably the
fan 14 is of the type wherein the pitch of the vanes
is controllable and there are an integral pitch
actuator, pilot positioner and external blade pitch
indicator of known construction. The impeller blades
of the fan are preferably of aerofoil section cast
aluminium alloy and are mounted on thrust bearings
with grease retaining features. The fan is provided
with a motor 2~ which can be an electrical
squirrel-cage induction type. The entire fan assembly
is mounted on spring isolators 26 in order to isolate
. 15 fan vibration. The fan unit has a round air outlet 28
whieh is connected by a suitable flexible connection
to a first braneh take-off air flow device 30
(hereinafter sueh air ~low devices shall be referred
to as SRT's whieh is an acronym for
silencer/riser/take-of~ module). The first SRT 30
is loeated downstream from the axial fan 14 and, in
the embodiment of Figure 1, is located in the vertical
shaft 32 in the region of the first floor level of
the building indicated at 34. The air distribution
system ineludes an output duct 36 located downstream
of SRT 30 and typically this output duct comprises
another SRT as illustrated in Figures 2 and
- lo - 13175~
3. The air distribution system also includes branch
ducts angularly offset (normally at a 90 degree angle)
from the centre axis of the fan 14 and the output duct
36. The branch ducts 38 illustrated in Figure 1 feed
fresh air to the first floor level 34 to the extent
required by the designer of the system. Return air is
typically returned to the basement level through the
annular passageway around the SRT's, which passageway
is indicated at 40 for the first floor level and 42
for the second floor level.
With the inlet silencer 10 of ~igure 1, there are
typically provided four filter sections 4~ in order to
filter the incoming air in a known manner. Also, if
desired, cooling coils can be incorporated into the
system by installing them vertically on each of the
four open sides of the inlet silencer. As such
cooling coils form no part of the present invention,
further description of their construction is deemed
unnecessary at this time.
Figure 2 illustrates a second SRT mounted
immediately above the first SRT 30. The second SRT by
means of its take-off passageways feeds air to the
second floor level of the building. The second SRT 46
and subsequent SRT's are similar to the initial SRT 30
except that there is typically no elongate air flow
defining member or "bullet" 43 provided in the main
1 3 1 75 03
air passageway extending through the SRT. The purpose
and function of the air flow defining member 4~ in the
first SRT is described hereinafter but gerlerally such
a member is only required in the SRT located
immediately downstream of the axial fan in order to
provide smooth and efficient air flow therethrough and
sound attenuation. ~lso illustrated in Figure 2 is a
wall opening or vent 50 located in the basement level
of the bullding for permitting outside air to flow
into the room containing the inlet silencer 10. This
outside air mixes with the return air indicated by the
arrows R.
Figure 3 of the drawings illustrates an air
distribution system wherein the inlet silencer 52 is
located in the top floor of the building or on the
roof. The illustrated system is a vertical down blast
system wherein the fresh air flows downwardly from an
axial fan 54 to an initial SRT 56 and then to a second
SRT 58. The initial SRT 56 iS connected to at least
three branch ducts indicated at 60, 61 and 62. It
will be appreciated by those skilled in the art that
the number of branch ducts connected to each SRT can
vary from a single branch duct only to as many as four
or more.
- 12 -~317533
The SRT's constructed in accordance with the
invention are a combination of two major sectlons, the
first being a static pressure regain section indicated
at 80 in Figures 4 to 8 and a take-off section 82, one
version of which is illustrated in Figures 9 to 12.
The regain section 80 is connected to the take off
section and is located immediately upstream therefrom
as indicated in Figure 1. The construction of the
regain section for the initial SRT 30 will now be
described in detail with reference to Figures 4 to 8. .
It will be appreciated that the regain sections in the
second and subsequent SRT's are of similar
construction except that they generally do not have
the central airflow defining member 48. The regain
section 80 has an air passageway 84 extending the
entire height of the section. This passageway has an
input port 86 that is round and that has a size
substantially the same as the fan outlet or input duct
28 (see Figure 1). The air passageway has a
rectangular output opening 88 which opens into the
main passageway of the take-off section 82.
The regain section 80 can be constructed using a
tubular frame structure such as that shown in Figure
4. This frame structure includes a bottom rectangular
131~5~3
frame 90 and a smaller upper rectangular frame 92.
These two frames are connected by four upright and
sloping frame members 94~ Mounted on the upper
rectangular frame 92 and welded thereto is a top panel
96 cut to form the circular input port. Welded to the
bottom rectangular frame 90 is a steel bottom panel 98
cut to form the rectangular output opening 88. On -the
outside of the regain section, extending between the
rectangular frames are steel side panels 100 as well
as steel end panels 102.
The central air flow defining member or bullet 48
. is secured centrally by four radially extending
connecting panels or ribs 104. Also extending between
the top and bottom panels are inside wall panels 106
which are shaped to provide a smooth and gradual
transition from the circular input port 86 to the
rectangular output opening 88. A suitable connecting
flange 108 extends upwardly a short distance from the
top panel 96 and extends around the circular port ~6.
The regain section 80 is adapted to transfer most
of the input air flow to a central passageway of the
take-off section where it passes to the main output
port and a minor portion of the air flow is
transferred to the take-off passageways. With the
described configuration of the regain section, the air
flow velocity decreases as the flow passes from the
- 14
1317503
input port 86 to the take-oEf section, resulting in a
static pressure gain. It will be particularly noted
that the air flow defining member 48 has a generally
round transverse cross-section with a maximum diameter
preferably equal to the diameter of the hub 16 of the
fan. This provides a smooth, straight flow of air
from the fan into the air distribution system and
helps in the reduction of noise creation in the fan
region. ~s explained hereinafter in connection with
the take-off section, both the air flow defining
member 48 and the walls surrounding the air passageway
are filled with acoustically absorbing material and
the metal sheet forming the member 48 and the sheets
forming the inside walls of the regain section are
perforated for sound attenuation. In other words, the
metal surfaces in contact with the air flow are all
perforated.
Turning now to the construction of the take-off
section 82 shown in Figures 9 to 12, the illustrated
version has three take-off passageways 110 to 112.
~lso~ the aforementioned air flow defining member 48
extends through this take-off section being a
continuation of the member extending through the
regain section. Again, it will be appreciated that in
the second and subsequent SRT's there is no air flow
defining member 48 in thetake-off section.
- 15 - 1317503
Figure 13 illustrates the tubular framework that
can be employed to construct the take-off section 82.
This framework comprises a large rectangular bottom
frame 114 and a somewhat smaller upper rectangular
frame 116. Extending vertically upwardly from the
bottom frame 114 are four straight, short frame
members 118 and pairs of these are connected by
horizontal frame members 120. The rectangular areas
122 formed by the rame members 118 and 120 provide
the locations for the rectangular output ports of
take-off passageways 110 and 112. To provide support
for the upper ends of the take-off passageway ducts,
there are two parallel frame members 124 and a longer
frame member 126 that is connected to the ends of
members 124 and to the rectangular frame 116. Further
support is provided by two parallel internal frame
members 128 that are connected to the bottom frame
114. Connected to the upper frame 116 is a
rectangular top panel 130 which defines a rectangular
opening 132 having the same dimensions as the output
opening 88 of the regain section.
The take-off section 82 includes a central
passageway 134 through which the aforementioned air
flow defining member extends. This central passageway
has an output port 136 of a size substantially the
same as an inlet of the output duct 36 (see Figure 1).
- 16 ~ 1317503
Preferably, a connecting flange 138 is provided at the
output port to provide a means for connecting the
adjoining duct work.
The preferred construction and cross-section of
the sheet metal walls forming each take-off passageway
is shown clearly in Figure 14. Each of these take-off
passageways is generally rectangular in transverse
cross-section which makes them relatively easy to
manufacture using standard sheet metal technlques.
Each is defined by an inner wall 140 and an outer wall
142 as well as sidewalls 143 and 144 which connect the
inner and outer walls. As shown in Figures 9 and 12,
the inner wall preferably has a thick, rounded leading
edge 146 located where the take-off passageway
commences. It has been found that this type of
leading edge provides improved noise reducing
characteristics, particularly under a variety of air
flow conditions and it is an improvement from a sound
attenuatingstandpoint over a leading edge that is a
thin flat sheet or sharply pointed. In a particularly
perferred embodiment, the inner wall in the region of
the leading edge has a thickness of about 3 1/2
inches. This thicknass permits the inner wall 140 to
be insulated with sound absorbing material.
Preferably both the inner and outer walls are
insulated with this sound absorbing material 150
- 17 ~ 1317503
and preferably this material is covered by a
perforated metal sheet i.e. mild steel or stainless
steel, forming the surface of the wall adjacent to the
air flow in the 5RT. These perforations are indicated
by the dash lines outlining the configuration of the
sheets in Figures 10 and 13. The steel is perforated
with circular openings so that preferably more than
about 33~ of the area is open. As indicated above,
the preferred form of sound attenuating material is a
loose fiber material having a density in the range of
0.8 to 1.2 pounds per cubic foot. The preEerred
material is a fiberglass mat with a special covering
so as to provide zero erosion of the material at 6000
feet per minute air flow. Such material is sold under
the brand name Knauf ~uctliner M sold by Knauf Company
of Shelbyville, Indiana, U.S.A.
Also indicated in Figures 9 and 12 is the
preferred configuration of each take-off passageway in
axial cross-section. Each pas~ageway preferably has a
relatively long straight first portion 152 and an
outwardly curving second portion 154 downstream from
the first portion. As explained in greater detail
hereinafter, the second portion 154 preferably has a
gradually increasing transverse cross-sectional area
in the direction of air flow.
The take-off section includes a front side panel
156 having suitable connecting flanges for securing
the panel to the framework of Figure 13. There is
also an outwardly sloping rectangular panel 158
connected to the panel 156 and positioned directly
above the output end oE the take-off passageway 1l1.
A vertical rear panel 160 is connected to the two
upright frame members 162 and the interconnecting
- 18 ~ 1 31 7 5 03
frame that forms part of the upper frame 116. There
are also narrower side panels lG4 that are connected
to the tubular framework. of Figure 13 and that extend
between the front and rear side panels. There are
also of course internal perforated panels 166 which
define the surface of the central passageway in the
take-off section. These panels connect at the top to
the aforementioned leading edge 146 on those sides of
the device that have take-off passageways.
It should also be noted from Figure 14 that the
straight first portion 152 of each of these
passageways initial tapers inwardly at the sides of
the passageway in the region indicated at 170, that is
the sheet metal side panels 143 and 144 converge in
the downwardly direction. Prior to the commencement
of the curved second portion, the passageway becomes
unlform in width, that is the side panels 143 and 144
extend in parallel planes. By this relatively simple
arrangement, the width of the take-off passageway is
gradually reduced to the width of the branch duct.
In order to design a preferred form of branch
take-off air flow device constructed in accordance
with the invention, reference will be made to Figures
15 to 18 which indicate certain dimensions of the air
flow device that are either known or given or can be
calculated as indicated below. For purposes of the
present discussion, the following lettering will be
- 19
1 3 1 7503
used to indicate the stated dimension or quantity:
DESCRIPTION OF Q~ANTI'rY OR
DIMENSIOII INDICATED BY THE
5LETTER LETTER _ _ _ _
Dl Fan Outer DIA.
D2 Fan Hub DIA.
QF System Volume Flow
l Branch #l Flow
Q2 Branch #2 Flow
H1 Branch #1 Exit Height
Ll Branch #l Exit Width
H2 Branch #2 Exit Height
L2 Branch #2 Exit Width
`.
;'
.
~ 1317503
The system volume flow is a given quantity that
is based on the si~e oE the building, the number of
floors and the rate of fresh air flow into each floor
desired by the architect or engineer. The flow of air
required through each branch duct is also a known
quantity being based on similar factors and
calculations. The size of the fan used in the system
including the outer diameter and the hub diameter D
and D2 are also known quantities once the desired
fan unit has been selected based on the total volume
of air flow required for the system. The heights and
widths of the branch ducts are also known quantities
as each branch must satisfy certain maximum dimension
requirements of the building and must be able to
provide the required air flow. t~ith these known
quantities it is then possible to calculate the
velocity of Elow wherein VF is the velocity of air
flow at the fan exit, Vl is the velocity of air flow
in branch duct 1 and V2 is the velocity of air flow
in branch No. 2.
VF QF
~ /4 (Dl - D2 )
Hl x Ll
V2 = Q2
H2 x L2
- 21 - 1 31 7503
Upon calculating these velocities, it is then
necessary to check the following:
VF ~ 1.3 x t1AX [Vl, V
If this equation is not satisfied then it is necessary
to request or obtain new inputs. It is then necessary
to set the value VI which is the air velocity at the
top or upstream end of the take-off section 82.
VI is set by the fol.lowing equation in order to
ensure optimum regain of between 60% and 80% in the
flow of air downwards in the air flow device.
VI = 1.5 x ~IN [Vl, V2]
One should then check the followlngO
VF C 1.3
VI
If this requirement is satisfied then VI is
redefined as follows:
VI = VF
1.3
- 22 ~ 1 31 7503
The following equa-tions are then solved wherein
the variable Z is such that 0.75 ~ Z ~ 1.334 and
G is the width of the take-of passageway as shown
in Figure 18 and W is -the width of the take-off
passageway at the bottom of the straight section.
WI = { Z
L = W
1.0 Z
Gl Ql Tw = 4 inches
L x V
I I Tc = 3.5 inches
G2 = Q2
~
Wl = Hl X V
W2 = H2 X V2
VI
The following equations are then solved for each of
the two branches
FOR BRANCH #l
h 1 = Hl x ~ ~ l.S 1 )
V
sl ~LI ~ Ll) x tan 60
~ 23 ~ 1 3 1 7593
FOR BRAN~H #2
c2 ~2 x (1 ~ 1.5 (V~))
VI
hS2 = (LI - L2) x tan 60
In these equations for each branch, hc
represents the height of the curved portion of the
take-off duct and hs represents the height of the
straight portion of a take-off ducto
The total design height HL of the take-off
section is determined by the following equation:
H = tMAX ~(hcl ~ h51) ~ (hc2 s2
This equation is based on the fact that the
designer selects the maximum of the two calculated
heights for use in the actual unit.
It is now possible to solve the following four
equations with the dimension WmaX being indicated in
Figure 18 and the dimension Wo being -the width oE
the central air passageway at the outlet (see Figure
15~.
~l = W0 _ [ ~ -(~c + G
25 2 2
maxl ~ ~ HL ~ + Tc + 2.5 Hl (Vl )
~7 = W0 -~ WI - (Tc + G2
2 L 2
Wmax2 ~ 0 - ~.~c2 ) 1 ~ 2 ( _ )
- 24 ~ 1 31 7 5 03
In these equations the ~ symbol represents an
intermediate parameter used to determine the maximum
width of the take off unit.
The following H values are then calculated to
determine the actual HL to be used in constructing
the take-off section:
HL2 = (LI L
.
0.8
HL3 ~ /2 ~I/2 ( 1 )~ 3
0~176
HL4 - ~WO/ - ~WI/ ~ (G2 ~ Tc)~ 3
0.176 .
1/2
HL5 = A)
;~
where A = LI x ~WI - ( Gl + G2 )~ 2
- 25 - 1 31 7 5 03
The dimensions Ll and Lo are shown in Figure 16 of
the drawings. The dimensions ~O and WI~ Gl and
Tc are shown in Figure 18 as is the dimension G2
used in the third equation above. The dimension D2
is the diameter of the fan hub as indicated above.
The value HL is set as the maximum of the five
calculated values HLl, HL2, L3 L4 L5
following design checks should also be run to
determine that the indicated requirement is met and,
if any of these requirements are not satisfied, new
input figures should be used:
1 ~ VI ~1.5
Vl
__
V2
0.75 ~ WO f 1.334
26 - l 31 7503
Gl> Tc
G 2 ~> Tc
WI ~ CG1 + 2TC + ~ O
WI [G2 + 2TC + ~i O
2 2 ~
1 ~ AR C 1.4
where AR = Lo x WO
~ L LI X ( WI ~ (G1+ G2 ~ 2TC)~
- 27 -1317503
Referring now to the curvature of the downstream
portion of the take-off passageway, which curvature is
detailed in Figure 17 of the drawings, it will be
noted that the inner surface 190 of the outer wall has
a uniform radius of curvature RI which curvature is
equal to 1.5 times the width Wl of the respective
take-off passageway at the downstream end of the
straight first portion 152, Wl being measured
perpendicular to the centre line of the take-off
section (see Figure 18). Because of its uniform
curvature, this inner surface is relatively easy to
construct. The outside radius Ro identified in
Figure 17 varies along the curved surface and is
determined by the following equation:
Ro (O) = 2.5 Wl + ~/90 x Hl - Wl
It will be appreciated that the above equations
and calculations have been provided for an SRT having
two take-off passageways but similar equations and
criteria can be used to design and construct an SRT
having three or four take-off passageways.
Preferably the exterior walls of the SRT should
have a thickness of at least 4 inches so that they
will contain adequate acoustic fill to provide good
low frequency sound attenuation. The preferred
- 28 L317503
thickness for the inner walls forming the take-off
passageways is about 3 1/2 inches. If the dimension
of the inner wall exceeds this thickness by a
significant amount, there will be an unnecessary
interference with the smooth flow of air through the
unit.
It will be understood that the function of the
upstream end of the SRT, that is the regain section,
is to carefully slow the air flow thereby reducing
energy losses and to change the transverse
cross-section from circular to rectangular. The shape
transition simplifies the design and reduces the cost
of the downstream portion of the SRT, that is the
take-off section. It will be further noted that all
of the air flow paths have a smoothly increasing
cross-sectional area again reduclng the energy losses
in the system.
In addition to providing a smooth air flow in the
region of the fan discharge, the air flow defining
member or bullet 48 in the initial SRT because it is
filled with sound absorbing material improves the
acoustic absorbtion of this SRT without an undue
increase in energy loss.
With respect to the rounded leading edge of each
inner wall at the commencement of the take-oEf
passageway, this edge has the advantage oE increasing
- 29 - 1 31 7503
the acoustic absorption of both the take-off
passageway and the central passageway and, in
addition, it provides improved off-design aerodynamic
performance. In practice, in commercial buildings the
distribution of air changes with the daily solar cycle
and with changes in office space use. The rounded
leading edge may provide lower energy losses (as
compared to known designs) during times when the flow
distribution has significantly deviated from the
design conditions.
It will be appreciated by those skilled in the
art that various modifications and changes can be made
to the described branch take-off air flow devices
without departing from the spirit and scope of this
invention. Accordingly, all such modifications and
changes as fall within the scope of the appended
claims are intended to be part of this invention.
