High efficiency air mixer

An air mixer for an air distribution system for a building includes a set of fixed, substantially parallel partitions arranged in a spaced-apart, side-by-side manner, these partitions forming alternating primary and secondary air passageways. The primary air passageways are open-ended and extend from a front side to a rear side of the mixer. Front end plates extend respectively across front sides of the secondary air passageways and each has elongate edge portions extending along two opposite longitudinal edges thereof. Each elongate edge portion projects beyond the plane defined by an adjacent one of the partitions. Air flow splitters are mounted in the secondary air passageways and each is connected to an adjacent pair of the partitions. These splitters in operation of the mixer turn incoming air flow that enters the secondary air passageways towards the front end plates. Air gaps are formed between the elongate edge portions and front edges of the partitions to enable the air flow in the secondary air passageways to enter the primary passageways where the two air flows are mixed.

BACKGROUND OF THE INVENTION 
This invention relates to air mixers for mixing together two different air 
flows, particularly an air mixer for an air distribution system suitable 
for a building or other similar structure. 
In air handling systems designed for large buildings such as office towers 
and other large structures, there has been a need to mix together at least 
two different air flows before distributing the mixed air flow throughout 
the air ducts of the building by means of a fan. Although a number of air 
mixers have been developed for bringing together and mixing two different 
air streams, often these air mixers are not very efficient and/or they 
require a substantial amount of space in the building in order to function 
properly. The two air streams that often must be mixed in an air handling 
system are generally return air that is coming back from the building 
itself and fresh outside air. In cold weather, the return air will 
normally be quite warm, for example, room temperature, while the outside 
air can often be quite cold. 
In these air handling systems for buildings, air stratification that 
results from the momentum inherent in moving air streams can keep air 
streams of different temperatures from mixing for quite some distance. 
This in turn can cause the air handling system to operate poorly or 
inefficiently and can also result in poor indoor air quality. During the 
winter time, lack of proper mixing of the incoming air streams can result 
in freezing or damage of heating coils that are part of the heating system 
and can generate control sensor errors. During the summer, poor mixing of 
the air streams can result in the lack of proper control of the indoor air 
temperature and can increase the energy consumption of the air 
conditioning system. The heat transfer capacities at the cooling coils are 
based on airflow at uniform temperature and velocity across the coils. A 
non-uniform temperature distribution for the entering air will cause 
reduced heat transfer at the coils and the desired temperature in the 
building may not be maintained. 
Moreover, the problems caused by poor mixing of air streams are becoming 
more serious as the amount of outdoor air is increased in the air 
distribution system. It is noted that government regulations and building 
users are now often requiring a greater amount of outdoor air. An 
increased amount of air is now being required by IAQ standards such as 
ASHRAE Standard 62. 
Various solutions have been proposed in the past to prevent air 
stratification in an air handling system and to prevent the damage that it 
can cause to the system. For example, glycol additives have been used to 
prevent frozen heat transfer coils. Although such additives may prevent 
frozen coils, they do not prevent the problem of reduction in heat 
transfer capacity of the coils due to uneven air temperature of the 
entering air. Dampers and high velocity jets have also been used to help 
in the mixing of two or more air streams but often the use of such devices 
creates unacceptable levels of pressure drop in the system. Specially 
designed air mixers have also been proposed in the past and these can 
improve the mixing of the air streams. However, these known mixers have 
some inherent defects which can be caused by the air streams being forced 
to pass through a narrow cross-section of the mixer. These known air 
mixers generally require more downstream space, can create a non-uniform 
downstream velocity profile and can cause a high pressure drop across the 
mixer. In addition, a non-uniform velocity profile caused by the air mixer 
can generate an extra pressure drop at downstream filter and coil 
sections. 
An early form of air mixer is shown and described in U.S. Pat. No. 
1,395,938 issued Nov. 1, 1921 to P. Barducci. In this mixer, two different 
air streams enter the casing of the mixer at an angle of about 90 degrees 
to one another. A number of boxes are arranged across the width of the air 
duct formed by the casing and these boxes open into an inlet duct at the 
side of the casing. The boxes are arranged side-by-side and are spaced 
apart from each other. All the boxes are provided with mouths that are 
open in the direction of the air flow. A main incoming air flow passes 
between these boxes and creates a suction effect at the mouths of the 
boxes so as to draw air in through the side inlet and into the downstream 
end of the casing where the two air streams are mixed. 
More recent U.S. Pat. No. 5,463,967 issued Nov. 7, 1995 to Airflow Sciences 
Corporation describes a static mixer designed for use with a coal-fired 
power plant. The mixer has a series of parallel walls arranged in 
side-by-side spaced apart relationship to form a series of rectangular 
spaces. The perimeters of these spaces are selectively closed to define 
respective first and second inlets and an outlet. The mixer creates 
interleaving of the two air streams and thus promotes increased 
homogeneity some distance downstream of the confluence of the streams. 
This known mixer also has turning vanes for turning one of the sub-divided 
streams as it passes through the mixer. 
It is an object of the present invention to provide an improved air mixer 
that can help avoid undesirable air stratification in the plenum of an air 
distribution system and that at the same time has low pressure drop. 
It is a further object of the present invention to provide an air mixer for 
an air distribution system that can be manufactured at a reasonable cost 
and that is highly efficient. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention, an air mixer for an air 
distribution system for a building or similar structure includes a set of 
fixed, substantially parallel partitions arranged in a spaced-apart, 
side-by-side manner, these partitions forming alternating primary and 
secondary air passageways. The primary air passageways are open ended and 
extend from a front side to a rear side of the air mixer. Front end plates 
longitudinally across the front side of the air mixer and extend 
transversely and extend respectively across sides of the secondary air 
passageways located at the front side of the air mixer. Each plate has 
elongate edge portions extending along two opposite longitudinal edges 
thereof. Each elongate edge portion projects in a transverse direction 
beyond the plane defined by an adjacent one of the partitions. Air gaps 
are formed between the elongate edge portions and the front edges of the 
partitions to enable the air flow in the secondary air passageways to exit 
therefrom and be mixed in the primary air passageways with air flow 
passing through the primary air passageways from the front side of the air 
mixer to the rear side thereof. 
Preferably a series of turbulence creating plates are mounted in each 
primary air passageway and are distributed across the width of their 
respective primary air passageway taken in a direction substantially 
parallel to the longitudinal edges of the front end plates. 
According to another aspect of the invention, an air mixer for an air 
distribution system for a building or similar structure includes a set of 
fixed, substantially parallel partitions arranged in a spaced-apart, 
side-by-side manner, these partitions forming first and second groups of 
alternating air passageways for first and second air flows with the first 
group of air passageways being open ended and extending from a front side 
to a rear side of the air mixer. The front side provides primary air 
inlets for the first air flow while another side of the air mixer 
extending between the front and rear sides provides secondary air inlets, 
which are provided for the second air flow and lead into the second group 
of air passageways. Fixed front end plates extend longitudinally across 
the front side of the air mixer, extend transverserly and respectively 
over sides of the second group of air passageways located at the front 
side of the air mixer, and are adapted to direct the second air flow into 
the first group of air passageways in the vicinity of the front side of 
the air mixer. The front end plates each have opposite edge portions that 
extend beyond the plane of respective adjacent partitions. During use of 
the air mixer, the second airflow is mixed inside the air mixer with the 
airflow that enters the primary air inlets during the course of flowing 
through the first group of air passageways. 
In the preferred embodiment, turbulence creating strips are mounted in the 
first group of air passageways in order to promote faster mixing of the 
first and second air flows. 
According to a further aspect of the invention, a plenum fan system for 
supplying a mixed air flow to a building or similar structure includes an 
enclosed plenum chamber having a return air inlet, an outside air inlet, 
and at least one mixed air outlet. An air supplying fan is mounted in the 
chamber and has a fan outlet connected to the at least one mixed air 
outlet. Heat exchanging coils are mounted in the chamber between the 
return and outside air inlets and the air supplying fan and an air mixer 
is mounted in the chamber between the return and outside air inlets and 
the heat exchanging coils. The air mixer comprises a set of spaced-apart, 
substantially parallel partitions arranged in side-by-side manner, these 
partitions forming alternating primary and secondary air passageways. The 
primary air passageways are operatively connected at a front side of the 
mixer to one of the air inlets and the secondary air passageways are 
operatively connected to the other of the air inlets. The primary air 
passageways are open ended and extend from the front side of the mixer to 
a rear side thereof. Front end plates extend respectively across front 
sides of the secondary air passageways and are adapted to direct airflow 
passing through the secondary air passageways into the primary air 
passageways. The front end plates have edge portions extending along two 
opposite edges thereof with each elongate edge portion projecting beyond 
the plane defined by an adjacent one of the partitions. During use of the 
system, the two air flows from the two air inlets are mixed while flowing 
through the primary air passageways. 
Preferably the partitions are fixedly mounted in the air mixer and airflow 
vanes extend between and rigidly connect adjacent pairs of the partitions. 
Further features and advantages will become apparent from the following 
detailed description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
An air mixer unit or module is illustrated in FIGS. 2 to 5 of the drawings. 
This air mixer 10 is particularly useful for an air distribution system 
for a building or similar structure. Major components of a plenum fan 
system constructed with the air mixer of the invention are illustrated in 
FIG. 1. It will be understood that plenum fan systems per se are well 
known in the air distribution industry and it is the air mixer aspect of 
this plenum fan system that constitutes the novel component of this 
invention. Illustrated in FIG. 1 is a plenum chamber 12 having a first air 
inlet 14 located at the front side of the air mixer and a second air inlet 
16 located at one side, in this case the top, of the air mixer. Not 
illustrated in detail are chamber sidewalls located at 17 to 19. Also, 
another possible location for the second air inlet is in the floor of the 
plenum chamber, this being indicated in dash lines in FIG. 1. These side 
walls can be insulated, if desired, to reduce the amount of sound 
emanating from the chamber which contains an air supplying fan 20. 
Although a centrifugal fan is illustrated schematically, a plenum or axial 
type fan could also be used with the air mixer of the invention. The fan 
20 has a fan outlet at 22 which is connected to at least one mixed air 
outlet 24 of the plenum chamber. Normally, the plenum fan system will form 
part of an air conditioning and/or heating system for the building or 
structure. In this case, two banks of heat exchanging coils indicated at 
28 can be mounted a short distance downstream from the air mixer 10. These 
banks of coils are mounted in the chamber between the location of the two 
air inlets and the air supplying fan 20. The banks of coils are arranged 
across the height and width of the chamber in a manner so that the mixed 
air flow from the air mixer 10 must pass through these banks of coils to 
reach the inlet of the fan. Preferably there are also mounted in the 
chamber one or more filter panels 26. 
In a standard air distribution system, one of the two air inlets is for 
return air that is coming back to the plenum chamber from the building 
itself while the other air inlet is for fresh outside air. Which air inlet 
is chosen for a particular air flow will depend upon the building layout 
constraints. It will be appreciated that depending upon outside 
temperature conditions, there can be a substantial temperature difference 
between the return air flow and the outside air flow. Normally the return 
air will have a temperature that is close to normal room temperature, for 
example, around 20 degrees C. or 70 degrees F. If winter conditions exist 
outside, the temperature of the outdoor air could be close to or below the 
freezing point. On the other hand, if it is a warm summer day, the outside 
air could have a temperature of 30 degrees C. or more. Obviously, the 
mixture of these two air flows must be warmed by the heat exchanging coils 
(or other means) before the air mixture is distributed back into the 
building by the fan in the winter time. Alternatively, the heat exchange 
coils must cool the air mixture to some extent before it is blown through 
the building by the fan in the summer time. 
Turning now to the construction of the air mixer 10, it is made with a set 
of fixed, substantially parallel partitions or panels 30 that are arranged 
in spaced-apart, side-by-side manner. In the illustrated unit of FIGS. 2 
to 4 there are six of these partitions with the outermost two partitions 
indicated at 30a and 30b in FIG. 3 forming outer walls of the unit. The 
partitions as well as other sheet metal components of the unit in one 
preferred embodiment are made from 18 gauge sheet metal and it will be 
understood that these partitions and their connecting members and panels 
can be connected together in several different well known ways, for 
example, by welding, by screws or by riveting. In order to connect the 
panels or partitions at the various joints, steel angle members cut to the 
required length can be used, again in a manner well known in the 
construction of air handling units. 
The partitions 30 form alternating primary and secondary air passageways 
indicated at 32 and 34 respectively. The primary air passageways 32 are 
open ended and extend from a front side 36 to a rear side 38 of the air 
mixer 10. A side wall 40 is located on one side of the air mixer 10 and 
closes the primary and secondary air passageways on this one side. The 
side wall 40 extends substantially from the front side 36 to the rear side 
38 of the mixer. As shown in FIG. 4, opposite the side wall 40, the 
primary passageways 32 are closed by semi-cylindrical end plates 42. The 
rounded exterior of these end plates helps to direct and split the air 
flow entering the mixer through the side air inlet 16. Also shown in FIG. 
4 are suitable supporting bars 44 that can be rigidly mounted in the 
secondary passageways 34 in order to stiffen and support the partitions to 
which they are attached. The number and location of these bars can vary 
depending on the particular air mixer and the size thereof and it will be 
appreciated that these bars are arranged so as not to interfere 
significantly with the air flow through the secondary passageways. 
Rounded front end plates 46 and 48 extend longitudinally across the front 
side 36 of the mixer and transversely and respectively across sides of the 
secondary air passageways located at the front side of the air mixer. 
These plates help to direct the incoming air flows through air inlet 14 
into the primary passageways 32. Each of the smaller outer plates 46 has 
an elongate edge portion at 50 that extends along a longitudinal edge of 
the end plate, this edge being the inner edge in the illustrated mixer. 
Furthermore, the larger, central end plate 48 has two elongate edge 
portions 52 that extend along opposite longitudinal edges of this plate. 
As can be seen in FIG. 3, the elongate edge portions 50 and 52 project 
beyond the planes defined by respective adjacent partitions 30. Elongate 
air gaps or slots 56 are formed between the elongate edge portions 50, 52 
and front edges of the partitions 30 to enable the air flow in the 
secondary air passageways 34 to exit therefrom and be mixed inside the air 
mixer with the airflow passing through the primary air passageways 32. 
Preferably the front end plates 46, 48 each have a front surface that is 
convexly curved between opposite longitudinal edges thereof. As a result, 
each front end plate 46, 48 forms a concave inner surface 60 which faces a 
respective one of the secondary passageways 34. It will be appreciated 
that the end plates 46, 48 are adapted to direct the air flow passing 
through the secondary passageways 34 into the primary passageways 32 in 
the vicinity of the front side of the air mixer and the concave inner 
surface of these plates helps to direct the airflow smoothly and 
efficiently into the primary passageways. It will thus be seen that during 
use of the air mixer 10, the airflow passing through the secondary 
passageways 34 from the side inlet 16 is mixed with the airflow that 
enters the primary air inlets (located at the front end of passageway 32) 
during the course of flowing through the primary passageways 32. Because 
most of the required mixing takes place in the air mixer itself, very 
little, if any, mixing is required downstream of the air mixer. Thus, the 
air mixer 10 of the invention can be arranged quite close to or adjacent 
to the filters at 26. 
Airflow splitters 64 to 66 are preferably mounted in the secondary air 
passageways 34 and the preferred shape and arrangement of these splitters 
can be seen from FIG. 2. Preferably there are two, three or more of these 
splitters in each of the secondary passageways and, during use of the air 
mixer, they act to turn the airflow that enters through the inlet 16 
towards the front end plates. The splitters in each passageway are 
preferably a series of spaced-apart, bent sheet metal plates that divide 
the secondary air passageway into three or more smaller passageways 70 
that extend from an air inlet side 72 of the mixer 10 to either the single 
air gap or the two air gaps 56 that are located along the front side of 
the respective secondary air passageway. In one preferred embodiment of 
the mixer, the splitters are made from 20 gauge sheet metal and each is 
constructed from an elongate, rectangular plate that is suitably bent to 
form a 90 degree curve approximately. The preferred sheet metal is 
non-perforated sheet steel. The splitters can also be described as airflow 
vanes or air directors. Each is preferably connected along two opposite 
longitudinal edges to an adjacent pair of the partitions 30. The provision 
of the splitters also provides additional support for the adjacent 
partitions. 
It will be further appreciated that the splitters 64 to 66 promote flow 
uniformity from the air inlet 16 through the secondary passageways. The 
provision of these splitters helps to ensure that the airflow passing 
through the gaps 56 is reasonably uniform across the width of the mixer. 
This in turn helps to ensure a more uniform mixture of the two air flows 
exiting from the rear side 38 of the mixer. It should be appreciated that 
such splitters are not always required in an air mixer constructed 
according to the invention. Smaller air mixers may not require any air 
splitters in order to provide proper air mixing. It is preferred that 
larger capacity mixers be provided with splitters such as those shown in 
the drawings. 
In the preferred air mixer 10, a turbulence creating device 80 is mounted 
in each of the primary air passageways 32. The illustrated device includes 
a series of curved, spaced-apart metal plates or deflectors 82 that are 
distributed substantially across the width of their respective primary air 
passageway 32. In other words, these plates 82 are distributed in a row 
extending in a direction substantially parallel to the longitudinal edges 
of the front end plates, 46, 48. In the preferred embodiment, the metal 
plates 82 are integrally formed along a main support strip 84 that extends 
across the width of the air mixer. A relatively short air gap 86 is formed 
between adjacent plates. Preferably the plates are aerodynamically curved 
as shown in FIGS. 3 and 5. Because of their smooth curvature, these plates 
do not significantly reduce the air flow speed in the primary passageways 
but at the same time they create the required turbulence therein to 
provide excellent mixing of the two air flows that enter the passageway. 
As shown in FIGS. 3 and 4, each turbulence device is positioned 
approximately midway between the two parallel partitions forming the 
respective primary air passageway. Preferably the plates 82 are curved 
alternately upwardly and downwardly from a central plane that is parallel 
to the partitions 30. This alternate bending of the plates 82 can be seen 
clearly in FIG. 5. In one preferred embodiment, the metal plates or strips 
82 have a length of 4.5 inches and a width of 2.5 inches. The width of the 
support strip 84 is 1.5 inches and the air gap between adjacent plates is 
2.5 inches. 
The theoretical temperature profiles of a mixer constructed according to 
the invention is shown by the temperature fringe plots of FIG. 6 and FIG. 
7 (from Computational Fluid Dynamics (CFD) software program results). In 
FIGS. 6 and 7 the mixer has three primary passageways 32 and four 
secondary passageways 34. The temperature difference between the return 
air and the outside air stream is 27.degree. C., and the outside air ratio 
is 20%. In an actual air temperature test of a mixer, the temperature of 
the airflow at each of the two air inlets was measured by a single 
temperature sensor while the temperature readings of the mixed airflow 
were taken by seven movable sensors arranged in a straight horizontal line 
across the width of the air mixer. The maximum distance between adjacent 
sensors was 7.5 inches and these sensors were controlled by a computer 
data acquisition system. FIG. 6 is the temperature profile on a transverse 
cross-section of the air mixer that is perpendicular to the direction of 
the air flow entering through side inlet 16 shown in FIG. 1. The 
temperatures are measured under steady state conditions. It is found that 
mixing is almost finished inside the mixer. Near the downstream end, the 
temperature becomes very uniform. Shown on the right side is a temperature 
scale with a range of 27 degrees Kelvin with a number from 1 to 27 being 
assigned to each of the listed temperatures measured on the Kelvin scale. 
Thus, the temperature at various locations in the mixer is indicated by 
the numbers on the drawing on the left side. 
Turning to FIG. 7, this figure illustrates the temperature profile of the 
present air mixer on a cross-section of the air mixer in a direction 
parallel to the direction of airflow entering from the side inlet 16. It 
shows that a preferred temperature profile in the passageways 32 is 
generated, which is helpful to accelerate the mixing over a very short 
distance. As in FIG. 6, a temperature scale is provided on the right side 
with a number from 1 to 27 being assigned to each of the listed Kelvin 
temperatures. Thus, the numbers on the drawing on the left indicate the 
corresponding temperature reading. 
In FIGS. 6 and 7, the short form E+02 stands for an exponential to the 
power of 2 or in other words 10.sup.2. Although the illustrated 
temperature profiles of FIGS. 6 and 7 are only theoretical readings 
provided by the aforementioned CFD software program, the actual measured 
temperatures using the aforementioned sensors were close to the 
thearetical projections shown. 
It will be appreciated that the new air mixer 10 is able to distribute the 
incoming air from a side inlet of the plenum uniformly along the entire 
span of the plenum. With this air mixer, multiple layers of cold and warm 
air streams uniformly distributed across the whole cross-section of the 
air mixer and the use of aerodynamic stirring bars 82 enable thorough 
mixing of two incoming air streams in the mixer. The present mixer takes 
advantage of heat exchange through thin sheet metal, the interaction of 
air streams and the use of aerodynamic stirring bars or plates 82 that 
accelerate mixing over a short distance. There is a relatively low 
pressure drop in the mixer itself and there is no extra pressure drop 
created at the filter and coil sections (because of the uniform downstream 
velocity profile). 
With the use of the preferred air mixer described herein, one can avoid 
undesirable freeze up of heat exchange coils and one is able to achieve 
more accurate temperature control in the air handling system because the 
air streams passing by the temperature sensing points will have a more 
homogeneous temperature. Furthermore, the air mixer can achieve a more 
even velocity profile across the air filters and heat exchange coils and 
this in turn leads to even filter loading and enhanced coil performance 
with a resulting decrease in energy consumption. Also, because of the wide 
effective working range of these air mixers, the user of the air 
distribution system can mix more outside air into the supply air stream in 
order to satisfy increasingly higher IAQ requirements. Because the air 
mixer of the present invention is so efficient, no upstream mixing box is 
required and generally the plenum fan system can be made more compact. 
If desired, the air mixer 10 can be provided with mounting flanges formed 
along the outer edges for the purpose of fixedly mounting the air mixer in 
the plenum chamber or for connecting the air mixer to adjacent, similar 
air mixers. It should be noted that the air mixer 10 can be constructed as 
a module of standard size and these modules can be stacked one on top of 
the other or one beside the other in the plenum chamber in order to create 
a large air mixer of the required size. 
It will be appreciated by those skilled in this art that various 
modifications and changes can be made to the described high efficiency air 
mixer 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.