INDUCTION SUPPLY AIR TERMINAL UNIT WITH INCREASED AIR INDUCTION RATIO,
METHOD OF PROVIDING INCREASED AIR INDUCTION RATIO
The present invention relates to an induction supply air terminal' device
where primary air flow is used to induce a secondary air flow with increased air
5 induction ratio.
An induction supply air terminal device essentially comprises of a primary
supply air chamber, mixing chamber and at least one heat exchanger. From the
primary supply air chamber the primary air is supplied out via one or several nozzles
into a mixing chamber. The secondary air is conducted into the mixing chamber
10 through a heat exchanger, where this secondary air can be heated or cooled. The
primary supply air induces secondary air and they both mix in the mixing chamber.
This mixed air is then conducted into the air-conditioned room space.
The present invention provides such an induction supply air terminal device,
wherein the air induction ratio between the primary air and the secondary air is
15 increased without compromising on equipment capacity, or resulting in enhanced
energy costs, or outside (primary air flow) requirements.
'Room space air handling solutions often comprise supply of air via a cooling
or heating or chilled beam. In such a chilled beam, the supply air is supplied to the
room, while a certain room air volume is sucked in through induction effect into a
20 mixing chamber through a heating or cooling coil and is thereby heated or cooled
therein, and then mixed with the supply air and circulated back into the room.
Chilled beams are components of air treatment systems used for cooling,
heating, or ventilation purposes. Cooling beams or heating beams or chilled beams
as they are generally referred to, provide several advantages for spaces of
25 designated volumes in that the cooling or heating capacity can be satisfied by
different modes such as supply of cold or hot water piped to the chilled beam rather
than by requiring air handling units to handle the entire cooling or heating load.
Chilled beams can be either passive or active, depending on the nature of the
convection process that is adopted. Passive chilled beams adopt a natural convection
30 process where the air treatment device is provided in a box that is recessed or hung
from a ceiling. In active chilled beams, ventilation air is introduced into the
pressurized chamber, also referred to as plenum or supply air chamber, and then
I through small air nozzles in order to enhance the natural convection of air.
I An important consideration in any chilled beam system is that the moisture
content of the room air must necessarily be below dew point conditions. This is
5 important to avoid the condensation in the chilled beam or water pipes surfaces.
Dew point conditions are typically determined based on the coldest temperature on
the surface of the chilled beam. Internal latent load is removed through ventilation
only if the primary air is sufficiently dry and also present in high volume. Traditional
dehumidification technology required a stipulated minimum required ventilation
10 rate in order to keep the moisture level of indoor air at a desired level since moisture
removal was limited in these technologies. Improvements in dehumidification
technologies in air handling units have meant that' greater dehumidification of air is
possible, thereby lowering the minimum required ventilation rate even further or as
may be mandated by code or design.
15 An active chilled beam's cooling capacity is based on the amount of room air
(secondary air) circulating through the heat-exchanger. This secondary air volume is
dependent on the induction ratio of nozzle and the primary air volume. Now when
primary air volume can be reduced, the induction ratio has to improve in order to
keep the secondary air volume and thus the cooling capacity the same.
20 The following table exemplifies some of the challenges/issues in increasing
air induction ratios:
To have the highest possible induction with lowest possible primary air flow
rate is beneficial in terms of HVAC-system energy use. The induction ratio should be
25 the highest possible with the lowest possible primary air flow and shortest possible
induction length.
HPQ DEb.W% % G - E l % - L Q 1 5 10::9 %
3
Internal moisture
load
(kg/s)
55
55
55
55
55
Required
primary air
volume
(I/s)
45.8
22.9
15.3
11.5
9.2
AHU capacity of
moisture
removal
(g/kg)
1
2
3
4
5
Primaty air
volume as per
EN 15251
standard
(I/s)
15.0
15.0
15.0
15.0
15.0
Required
secondary air
volume in
chilled beam
(I/s)
60
60
60
60
60
Primary air
volume as per
ASHRAE 62.1
standard
(I/s)
8.0
8.0
, 8.0
8.0
8.0
Required
induction ratio
(primary :total)
1:2.3
1:3.6
1:4.9
1:6.2
1:7.5
In current products, induction ratio is controlled by changing the nozzle size.
Smaller nozzles have respectively higher induction ratios due to higher perimeter
length compared to same total area of nozzles with bigger diameter. When nozzle
becomes bigger, the air jet diameter in discharge slot becomes bigger and therefore
5 the minimum distance between nozzles also increases. This limits the number of
nozzles per linear length of beam. Respectively, with small nozzle the maximum
primary air volume is limited based on the chamber pressure. Another concept to
increase induction ratio is to shape the nozzle such that with same face area the
perimeter length of nozzle is longer. This can be achieved by shaping the nozzle as
10 flower instead of a circle. Third method is to have a venturi in the mixing chamber.
As can be seen, several methods have been proposed in the art to enhance or
increase air induction ratios. Some of the solutions include modifying the nozzles or
holes intended for pass-through of supply air.
15 These solutions provided for in the art, include variations in the designs of
the nozzles or holes through which the primary air passes, exits and where the air
flow after these holes makes the condition for the re-circulating room air to reach a
mixing zone where both air flows are brought together prior to flow out into the
room. The flow out from the pressure chamber is controlled by a number of holes or
20 nozzles which are configured to different forms.
This type of device typically has several nozzles to induce the secondary air
flow. These nozzles can be either holes, slots, punched collars, conical shaped???), or
any other shape. In case of multiple nozzles, they may be arranged in such a manner
that they form one or several elongate row. Smaller nozzles have higher induction
25 ratio, but also smaller primary air flow rate at any given static chamber pressure. The
size of the nozzle is selected in order to supply the required primary air flow at a
given primary air chamber pressure.
An induction supply air terminal device is used with various primary air flow
rates, therefore the same device may comprise of bigger and/or smaller nozzles or
Face area (mm2)
Perimeter (mm)
Single nozzle
4mm
12.6
12.6
Flower
5.9 mm
12.6
22.3
Cluster
4x2mm
12.6
25.2
nozzles with adjustable face area for setting the desired supply air flow. Common foi
the solutions is the ratio between the primary and secondary air quantities is
controlled so that the desired primary air flow and cooling/heating capacity is met.
Examples of known solutions are described in WO 98/09115, where the induction
5 supply air terminal device includes a primary air chamber where several nozzles or
discharge opening exists.
EP 1 188992 A2 with characterized discharge holes (nozzles here)comprises
of two groups (7,8) that are laterally directed in different directions. These consist of
two elongated slots (13, 16) equidistantly placed and having adjustable area for
10 setting the desired supply air-flow.
Likewise, WO 2011/040853 A1 with characterized discharge holes of different
sizes are comprised in different groups. At any given point of time each group can
have only one active discharge hole, wherein these active discharge hole in each
group are of similar characteristic and equidistantly placed from the active hole in
15 the adjoining group. It is used to regulate the primary air-flow rate.
WO 96/28697 and EP 0 813 672 B1-describes a nozzle with scallop-shaped
outlet edge. This has an effect on reducing noise output from the nozzle and
improves mixing of primary and secondary air flow thereby increasing the rate at
which the primary air flow can induce the secondary air flow. In this example the
20 preferred nozzle shape has a perimeter to cross-sectional area ratio that is equal to
or greater than 1.3 times the perimeter to cross-sectional area ratio for a circle of
the same area.
While smaller nozzles have bigger induction ratio, the smaller face area
means that they are not able to supply as much primary air as bigger nozzles and
25 therefore also the amount of induced secondary air flow is smaller. Reducing the
distance between the nozzles (d) to a value smaller than the diameter of the air jet
(h), results in reduced induction length (I) and thereby reduced secondary air flow.
Another method to induce a higher level of secondary air flow is to use a
venturi inside a mixing chamber. The venturi increases the secondary air flow when
30 its neck size is equal to that of the diameter of the of the air jet. It is also seen that
when the air jet central line velocity is higher, the effect of the venturi is better.
Therefore, with the small air flow rate, the optimum location of the venturi is closer
EPO BELHI 1 6 - 8 1 - 2 0 1 5 10: 3&
to the nozzle than that with the higher air flow rate. Based on varying needs, this
induction supply air terminal device can be used with different air flow rates and
therefore with adjustable venturi location.
As an example EP 0 813 672 B1 describes an induction supply air terminal
5 device with a mixing chamber comprising of a fixed venturi having generally a
circular cross-section of varying diameter along its length.
OBJECTIVES OF THE INVENTION
Optimal venturi location and diameter is dependent on the primary air
volume and nozzle size. Different combinations give different jet sizes in the venturi
10 neck. If the neck diameter is too small or too big compared to the jet diameter or is
located in the non-optimum distance from the nozzle, as it may be a case with fixed
venturi, it is not effectively increasing induction or may even reduce it.
The present invention firstly increases the induction near the nozzle due to
having many smaller nozzles (c1uster)with higher perimeter area compared to face
15 area supplying the air from the pressurized plenum to a mixing chamber. Secondly,
the adjustable venturi allows to locate the venture neck optimally and therefore
further increase the induction near the discharge opening. This combination gives
the highest induction ratio and therefore allows the design of an active chilled beam,
where lower primary air volume gives the required cooling capacity per linear meter.
20 BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The state-of-art operation of device and innovation is described in the
enclosed drawings, wherein
Figure 1 describes the operation principle of an induction supply air terminal
device.
2 5 Figure 2 shows the operation principle of nozzle in case of different nozzle
distances and assuming that primary air flow rate (4) and nozzle (5) size and shape is
the same in all nozzles.
Figure 3 presents a principle of nozzle (bigger and smaller nozzle) operation
as well as array of clustered nozzles, their induction ratios, required primary air flow
30 rates and amount of induced air (numbers are only indicative to describe the
principle).
Figure 4 describes the principles of innovation i.e. multiple nozzle cluster
I P O BERHI 8 5 - O l - % Q % ! S 10: S6E
Figure 5 presents examples of arrays of multiple nozzle clusters.
Figure 6 describes the operation principle of induction supply air terminal
device with a venturi.
Figure 7 describes an adjustable venturi neck based on the primary air
5 volume (qv), induced secondary air volume and nozzle surface area (A) to achieve
optimum velocity (v) in the venturi neck.
Figure 8 describes an adjustable venturi arrangement wherein different
optimally shaped and sized elements are utilised either alone or in groups to create a
venture neck.
10 Figure 9(a) describes a device provided with solely a cluster nozzle
arrangement.
Figure 9(b) describes a preferred embodiment of the invention wherein the
combination of a cluster nozzle arrangement with a fixed venturi is provided.
Figure 9(c) describes a preferred embodiment of the invention wherein the -
15 combination of a adjustable venturi with a single nozzle is provided.
Figure 9(d) describes a preferred embodiment of the invention wherein the
combination of a cluster nozzle arrangement with a adjustable venturi is provided.
SUMMARY OF THE INVENTION
The present invention relates to an induction supply air terminal device
20 where primary air flow is used to induce a secondary air flow wherein the nozzles are
provided in the form of a cluster arrangement, comprising one or more clusters of
three or more nozzles each. The clusters can be arranged according to predetermined
patterns depending on the pattern of air induction that is desired.
The present invention also provides an induction supply air terminal device
25 equipped with an adjustable venturi, where both the distance and the neck size can
be adjusted based on the primary air volume and the nozzle surface area.
In another embodiment, the present invention also provides an induction air
terminal supply device where the primary airflow is used to induce a secondary air
flow wherein the nozzles are provided in the form of a cluster arrangement
30 comprising one or more clusters of three or more nozzles each, and therein a
venture device is provided to enhance the flow of secondary air. The venturi can be
either a fixed venturi or an adjustable venturi.
IPa oELnx 1 ~ 0 1 - 2 ~ ai..ms: s%
DETAILED DESCRIPTION OF THE INVETNION
In the present invention, the nozzle arrangement comprises a cluster of small
nozzles instead of being placed equidistant in an elongate row. The clusters can be
formed of different patterns as is depicted in Figure 5. In this case air jets from a
5 cluster of multiple nozzles create multiple air jet zone of length (II). These multiple
air jets converge into a single air jet at a distance 11, forming into a single air jet zone
of length (I2). The distance (dl) between an array of nozzles in a cluster is smaller
than the distance (dz) between two clusters of nozzles.
The resultant induction ratio of an air jet created by an array of multiple
10 nozzles in a cluster is bigger compared to the induction ratio of an air jet of single
nozzle with the same face area as of the clustered nozzles together.
A cluster can have an array of nozzles starting from 3 in number to more,
based on the required surface area to be catered to.
The secondary air flow induced by primary air flow from a single nozzle of
15 surface area equivalent to that of an array of multiple nozzles in a cluster is smaller
than the secondary air flow induced by the same amount of primary air flow from a
cluster of multiple nozzles.
Accordingly, the present invention provides an induction supply air terminal
device that comprises of primary supply air chamber (I), at least one mixing
20 chamber (2) which opens into the air-conditioned room space, at least one or no
heat exchanger(3) The device is provided with an array of multiple nozzles in a
cluster (5) that supplies primary air flow (4) into at least one mixing chamber (2) to
induce a secondary air flow (6) heated or cooled as it flows through a heat exchanger
(3) and conducted into the mixing chamber (2), wherein both this primary supply air
25 (4) and secondary air (6) mix, whereby this mixed air (7) is then conducted into the
air-conditioned room space (8) with an increased induction ratio.
In one embodiment, the array of multiple nozzles in a cluster can have three
or more number of nozzles.
<" In another embodiment, the nozzles in a cluster can be circular, rectangular,
30 elliptical or scalloped in shape.
In yet another embodiment of the invention, the nozzles in a cluster can be
holes or punched collars in a sheet metal plate or conical nozzle that is fixed over the
opening in the sheet metal.
In another embodiment of the invention, the nozzles in a cluster can be
5 either made of metal (steel or aluminium), plastic or rubber.
In a preferred mode of the invention, the air jets from a cluster of multiple
nozzles create multiple air jet zone of length (Il) that converge into a single air jet at
a distance 11, forming into a single air jet zone of length (Iz).
In a further embodiment, the distance (dl) between an array of nozzles in a
10 cluster is smaller than the distance (d2) between any two clusters of nozzles.
In the embodiment where air induction ratio is enhanced by use of an
I adjustable venturi, whether used in combination with a single nozzle or multiple
nozzles in clusters or otherwise, the resultant induction ratio of an air jet created by
a nozzle or an array of multiple nozzles in a cluster in combination with a venturi is
I 15 larger than when compared to the induction ratio of an air jet resulting from nozzles
I alone.
Turning now to figure 6, the location (x) of the venturi (9) is based on the
central line velocity (v) in the venturi neck and the diameter (h) of the air jet.
Therefore, with the smaller exit velocity (v,) in the nozzle, the venturi neck shall be
20 nearer to the nozzle than it is with higher exit velocity (v,) in the nozzle. This exit
velocity (v,) depends on primary air flow rate (4) and the face area of the nozzle(s).
The central line velocity (v) is dependent on the exit velocity (v,) in the nozzle and
the secondary air flow (6). At the same time the neck diameter (y) of venturi needs
to be set equal to the diameter (h) of the air jet at the same location (x).
2 5 Referring now to Figure 8, the venturi (9) consists of two different optimally
sized and shaped elements that can be used singly or together to create the venturi
neck (9). The core part of the venturi (9a) creates the basic venturi neck (9) for bigger
mixed airflows (7). The reduction part (9b) of the venturi (9) is optimally shaped so
that when two of them are installed parallel they both reduces the size of the venturi
30 neck (y4cy3) and shifts the distance of neck(x4cx3) nearer the nozzle (5). Reduction
parts (9b) can be installed in opposite directions to create the medium size neck
(y4cyScy3) and/or to change the course of mixed air flow jet (7). The core part (9a)
I P O B E L H I 1 6 - 0 1 - Z B l 5 18 %I
and reduction part (9b) of the venturi are both removable and re-installable. Both
the core part (9a) and reduction part (9b) of the venturi can be made from solid
material, be hollow, inflatable or formed from a sheet metal plate.
In the embodiment comprising use of a venturi device whether in
5 combination with a solo nozzle or nozzle clusters, the induction supply air terminal
devicethat comprises of primary supply air chamber (I), at least one mixing
chamber (2) which opens into the air-conditioned room space, at least one or no
heat exchanger (3)) single or an array of multiple nozzles in a cluster (5) that
supplies primary air flow (4) into at least one mixing chamber (2) to induce a
10 secondary air flow (6) heated or cooled as it flows through a heat exchanger (3) and
conducted into the mixing chamber (2)) wherein both this primary supply air (4) and
secondary air (6) mix, whereby this mixed air (7) is then conducted into the airconditioned
room space (8), wherein an adjustable venturi (9) is provided to
increase the secondary air flow rate (6).
15 In one embodiment, the location (x) of the venturi (9) is based on the
optimum central line velocity (v) in the venturi neck, which depends on the primary
air flow rate(4))the face area of the nozzle(s) and the secondary air flow (6).
In another embodiment, the neck diameter (y) of venturi is set equal to the
diameter (h) of the air jet at the same location (x).
20 In another embodiment, the location (x) of the venturi (9) and/or the neck
diameter (y) of the venturi (9) is adjusted manually or automatically using an
actuator. In another embodiment, the venturi (9) shape and type can vary - solid,
inflatable or bent metal/plastic sheet fixed at one end and with an adjustable
another end.
25 It is observed through experiments carried out that there is a definite
enhancement in the air induction ratios using the various arrangements embodied in
the invention, viz.
(a) a cluster nozzle arrangement with a fixed venture; .
(b) an adjustable venture with a single nozzle
(c) an adjustable venture with a cluster of nozzles
(d) chilled beams provided with each of the above combinations.
This data is summarised in the Table below:
I P Q DELHI 1 6 - O P - E O % 5 B E 3 : 3 P
10
Single +venturi
4 mm
12.6
12.6
62 Pa
Face area (mm2)
Perimeter (mm)
Pressure
Distance
0 mm
20 mm
40 mm
60 mm
Single nozzle
4mm
12.6
12.6
70 Pa
80 mm 1.1 1.06 1.13 1.17
Flower
5.9 mm
12.6
22.3
63 Pa
Jet's total air volume (I/s)
Cluster
4x2mm
12.6
25.2
58 Pa
0.1
0.42
0.68
0.91
0.1
0.44
0.66
0.87
0.1
0.52
0.74
0.95
0.1
0.31
0.56
0.92

We claim:
1. An induction supply air terminal device that comprises of primary supply air
chamber (I), connected with at least one mixing chamber (2) which opens
into an air-conditioned room space (8), at least one or no heat exchanger(3)
provided connected with each said mixing chamber (2)) wherein an array of
multiple nozzles is provided on one surface of the primary supply air chamber
(1) in the form of a cluster (5) to supply primary air flow (4) into at least one
mixing chamber (2) to induce a secondary air flow (6) heated or cooled as it
flows through a heat exchanger (3) and conducted into the mixing chamber
(2)) wherein both the primary supply air (4) and secondary air (6) mix,
whereby this mixed air (7) is then conducted into the air-conditioned room
space (8) with an increased air induction ratio.
2. A device as claimed in claim 1 wherein the array of multiple nozzles in a
cluster comprises three or more number of nozzles.
15 3. A device as claimed in claim 1 or 2 wherein the nozzles in a cluster are
selected from circular, rectangular, elliptical and scalloped shape nozzles.
4. A device as claimed in claim 1 or 2 wherein the nozzles in a cluster comprise
holes or punched collars in a sheet metal plate or conical nozzles fixed over
an opening in a sheet metal plate.
20 5. A device as claimed in claims 1 to 4 wherein the nozzles in a cluster are made
of metal, plastic or rubber.
6. A device as claimed in any preceding claim wherein the cluster of multiple
nozzles form a multiple air jet zone of length (Il) through air jets, said zone
converging into a single air jet at a distance Il forming into a single air jet
zone of length (I2).
7. A device as claimed in any preceding claim wherein the distance (dl) between
an array of nozzles in a cluster is smaller than the distance (d2) between any
two clusters of nozzles.
8. A device as claimed in any preceding claim wherein additionally a venturi
device is provided disposed of in the air jet zone at a predetermined distance
from the cluster nozzle array.
9. A device as claimed in claim 8 wherein the venturi is a fixed venturi.
EPO D E L H I % G - o l - B Q 1 5 %El131
12
10. A device as claimed in claim 8 wherein the venturi is an adjustable venturi.
11. A device as claimed in claims 8 to 10 wherein the location of the venturi is a
function of the optimum central line velocity in the venturi neck, in turn
depending on the primary air flow rate, the face area of the nozzle(s) and the
secondary air flow.
12. A device as claimed in claims 8 to 11 wherein the neck diameter of venturi is
set equal to the diameter of the air jet at the same location.
13. A device as claimed in any of claims 10 to 12 wherein the location of the
venturi and/or the neck diameter of the venturi is adjustable manually, or
10 automatically by an actuator.
14. A device as claimed in any of claims 8 to 13 wherein the venturi is selected
from a solid or inflatable venturi, or a venturi with a bent metal/plastic sheet
fixed at one end and an adjustable another end.
15. An induction supply air terminal device comprising of primary supply air
15 chamber (I), at least one mixing chamber (2) which opens into an airconditioned
room space (8)) at least one or no heat exchanger (3), one or
more nozzles provided on said primary air supply chamber (1) to supply
primary air flow (4) into said at least one mixing chamber (2) to induce a
secondary air flow (6) that is heated or cooled as it flows through a heat
exchanger (3) and conducted into said mixing chamber (2), wherein both this
primary supply air (4) and secondary air (6) mix, whereby this mixed air (7) is
then conducted into the air-conditioned room space (8)) wherein an
adjustable venturi (9) is provided to increase the secondary air flow rate (6).
16. A device as claimed in claim 15 wherein the location of the venturi is a
2 5 function of the optimum central line velocity in the venturi neck, in turn
depending on the primary air flow rate,. the face area of the nozzle(s) and the
secondary air flow.
17. A device as claimed in claims 15 or 16 wherein the neck diameter of venturi is
set equal to the diameter of the air jet at the same location.
30 18. A device as claimed in any of claims 15 to 17 wherein the location of the
venturi and/or the neck diameter of the venturi is adjustable manually, or
automatically by an actuator.
I P Q DELWP 1Ei-0P-LC315 XQ:31
13
19. A device as claimed in any of claims' 15 to 18 wherein the venturi is selected
from a solid or inflatable venturi, or a venturi with a bent metal/plastic sheet
fixed at one end and an adjustable another end.
20. A device as claimed in any of claims 15 to 19 wherein the nozzles are present
as a cluster of nozzles in an array.
21. An induction supply air terminal device comprising of primary supply air
chamber (I), at least one mixing chamber (2) which opens into an airconditioned
room space (8), at least one or no heat exchanger (3), an array of
multiple nozzles is provided on one surface of the primary supply air chamber
(1) in the form of a cluster (5) to supply primary air flow (4) into said at least
one mixing chamber (2) to induce a secondary air flow (6) that is heated or
cooled as it flows through a heat exchanger (3) and conducted into said
mixing chamber (2), wherein both this primary supply air (4) and secondary
air (6) mix, whereby this mixed air (7) is then conducted into the air-
15 conditioned room space (8), wherein an adjustable venturi (9) is provided to
increase the secondary air flow rate (6).
22. A device as claimed in claim 21 wherein the location of the venturi is a
function of the optimum central line velocity in the venturi neck, in turn
depending on the primary air flow rate, the face area of the nozzle(s) and the
20 secondary air flow.
23. A device as claimed in claims 21 or 22 wherein the neck diameter of venturi is
set equal to the diameter of the air jet at the same location.
24. A device as claimed in any of claims 21 to 23 wherein the location of the
venturi and/or the neck diameter of the venturi is adjustable manually, or
2 5 automatically by an actuator
25. A device as claimed in any of claims 21-24 wherein the venturi is selected
from a solid or inflatable venturi, or a venturi with a bent metal/plastic sheet
fixed at one end and an adjustable another end.
26. A device as claimed in claim 21 to 25 wherein the array of multiple nozzles in
30 a cluster comprises three or more number of nozzles.
27. A device as claimed in claim 21 to 26 wherein the nozzles in a cluster are
selected from circular, rectangular, elliptical and scalloped shape nozzles.
PPO O E L H I 1 6 - 8 1 - 2 Q P 5 l O : y
28. A device as claimed in claim 21 to 26 wherein the nozzles in a cluster
comprise holes or punched collars in a sheet metal plate or conical nozzles
fixed over an opening in a sheet metal plate.
29. A device as claimed in claims 21 to 28 wherein the nozzles in a cluster are
made of metal, plastic or rubber.
30. A device as claimed in any preceding claim 21 to 29 wherein the cluster of
multiple nozzles form a multiple air jet zone of length (Il) through air jets,
said zone converging into a single air jet at a distance Ill forming into a single
air jet zone of length (I2).
10 31. A device as claimed in any preceding claim 21 to 30 wherein the distance (dl)
between an array of nozzles in a cluster is smaller than the distance (d2)
between any two clusters of nozzles.