EP0276810A2

EP0276810A2 - A method of and an air distribution means for introducing supply air into a room space

Info

Publication number: EP0276810A2
Application number: EP88101101A
Authority: EP
Inventors: Ingmar Erik Rolin; Jouko Kalevi Savela; Jyrki Olavi Pitkänen; Tord Henry Holmlund; Seppo Juhani Leskinen
Original assignee: UK Secretary of State for Defence; ABB Flaekt AB; Flaekt AB
Current assignee: ABB Technology FLB AB
Priority date: 1987-01-27
Filing date: 1988-01-26
Publication date: 1988-08-03
Anticipated expiration: 2008-01-26
Also published as: DK164374C; DK34988D0; FI870345A; EP0276810A3; EP0276810B1; DE3872532D1; FI79608C; FI79608B; DE3872532T2; NO165464B; NO880341D0; FI870345A0; DK34988A; ATE78087T1; DK164374B; NO880341L; NO165464C

Abstract

A method of introducing supply air into a lower portion of a room space (3) directly to an occupied zone (7) by means of at least one elongated air distribution means (1) provided with a plurality of outflow openings (2) for supply air. In order to blow supply air into the occupied zone without draught and without drawing impurities from the upper portion of the room space, supply air is blown from the air distribution means at a high speed but through outflow openings so small that the speed of the air jets (A) from the openings is reduced essentially over a short distance (X) and the entrance of secondary air (B) from the room space into the air jets is prevented in the vertical direction of the room space over said distance (X) and at least at one end of the air distribution means. The air distribution means (1) is provided with a plurality of small outflow openings (2) the distance of which from each other in the longitudinal direction of the air distribution means is great as compared with the size of the outflow opening for enabling secondary air to flow from the room space in between the outflow openings, and to mix with the air jets from all sides.

Description

This invention relates to a method of introducing supply air into a room space, wherein air is introduced into a lower portion of the room space directly to an occupied zone by means of at least one elongated air distribution means comprising a plurality of outflow openings for air.
Air flows occurring in room spaces are controlled by ventilation, that is, by introducing clean outdoor air into the room space through special fixed louvers, valves, or the like air distribution means, and by discharging air from the room by means of special exhaust air means, which air has become warm or excessively moist or which has been contaminated by impurities formed in the room space.
In a so called mixing ventilation, air is introduced into a room space through air distribution means in the form of one or more jets reaching far into the room space in a desired direction, generally to the upper portion thereof. Due to induction, the jets draw indoor air therewith and are mixed with the indoor air so that the entire room space is finally filled with a relatively homogeneous mixture of indoor and outdoor air, the temperature, impurity content, humidity, etc., of the mixture being nearly the same throughout the room.
The mixing ventilation has certain disadvantages. The air flow emerging at high speed from the air distribution means may reach too far or it may hit a flow obstacle, such as a light fixture, so that the air flow changes its direction, thus causing draught in zones occupied by people (so called occupied zones). In addition, the introduction of outdoor air into the upper portion of a room space causes the impurities and excessive heat which have risen by themselves to the upper portion of the room space to be carried back to the occupied zone by the outdoor air flowing to said zone. In fact, instead of the whole room space, the outdoor air could be introduced into the occupied zone only.
For the elimination of these disadvantages, a so called displacing ventilation has been developed according to which outdoor air is introduced directly into the occupied zone of the lower portion of the room space by means of air distribution means. Thus the air is introduced where it should, so that the excessive heat and impurities which have risen to the upper portion of the room do not return to the occupied zone.
The displacing ventilation, however, has some major disadvantages. In order to prevent the formation of draught due to the introduction of air directly into the occupied zone from the air distribution means, the speed of the air has to be kept at a very low value. This is also necessary in order to prevent the air jet from taking with it air from the upper portion of the room due to induction, which would bring the impurities and excessive heat into the occupied zone. Therefore, the speed of the air flow from supply air means operating on the displacement principle is very low, generally from 0.5 to 1.5 m/s.
However, the low speed causes the loss of some important properties of the air distribution means. First, it is not possible to control the air flows in a room space by means of the air distribution means, because the kinetic energy of the air jet is insufficient to put large masses of air in motion. Second, the temperature of the room space cannot be affected by the displacing ventilation due to the low mixing ratio of the displacing air distribution, that is, the small ratio between the secondary air flow and the air flow from the air jet. If the supply air is slightly warmer than the room air, it is exposed to thermal forces which cause it to flow upwards into the upper portion of the room space. Correspondingly, if the supply air is clearly colder than the room air, the flow drops to the floor level and causes draught. Consequently, it is not possible to heat or substantially cool the room air by means of the displacing ventilation.
The object of the present invention is to provide a method which avoids the above disadvantages and enables the introduction of warm as well as cool supply air directly into the occupied zone without draught, so that the temperature of the room space can be affected. This object is achieved by means of a method according to the invention which is characterized in
- that the air is blown at a high speed through a plurality of small outflow openings in such a manner that the speed of air jets from the outflow openings is reduced to a small value over a short distance, and
- that secondary air contained in the room space is prevented from being mixed with the air jets from an undesired direction of the room space over said distance at least at one end of the air distribution means.
The method according to the invention is based on the idea that a high speed of supply air is utilized in displacing ventilation while the introduction of supply air is arranged to take place under such conditions that the speed of the air jet is reduced over a short distance, whereby the secondary air is essentially prevented from entering the air jet from undesired directions along this distance. This kind of ventilation method enables large masses of air to be put in slow flow motion in a desired part of the room space, and the flows in the room space can be controlled irrespective of thermal flows or other similar undesired flows. The result is that all the air contained in the room space is not mixed and that the impurities and excessive heat contained in the air in the upper portion of the room space are not carried to the occupied zone. Further, no draught is caused in the occupied zone. The method enables the introduction of both warm and cool air directly into the occupied zone without draught.
The invention will be described in more detail in the following with reference to the attached drawings, wherein

Figures 1 and 2 illustrate schematically one preferred embodiment of an air distribution means according to the invention in a side view and a cross-sectional view, respectively,
Figures 3 to 5 are side views of alternative embodiments of the air distribution means, and
Figure 6 illustrates the operating principle of a ventilation system according to the invention.

Figures 1 and 2 of the drawings show a preferred embodiment of an air distribution means. The air distribution means is formed by an upright air duct 1 the length of which is essentially greater than the diameter thereof, or in the case of a rectangular cross-section, greater than the larger side dimension. A great number of nozzles 2 or similar holes, slits, or the like outflow openings for air is provided in the duct wall. The nozzles are not positioned over the whole shell surface of the duct but only over a prede termined portion of its circumference, i.e. in a mixing part 1a. Instead, a strip-like secondary part 1b extending longitudinally of the duct does not have any holes provided therein.
In this embodiment, the air duct is mounted beside a wall 4 of a room space 3 at some distance from a floor 5. The lower end of the duct is closed, and an annular baffle plate 6 is mounted at the upper end of the mixing part 1a. The mixing part opens towards an occupied zone 7 of the room space.
The upper end of the air duct is connected to a supply air distribution network not shown for the introduction of supply air into the air duct therethrough so that the air flows out through the nozzles at high speed, e.g. 2.5 to 10 m/s. Due to induction, air jets A from the nozzles draw from all sides secondary air B from the room space, so that the secondary air is mixed with the air jets emerging from the nozzles. Because of the great number of the nozzles and the small diameter thereof, the mixing as well as the reduction in the speed of the air jets takes place over a short distance. Assuming that the diameter d of the nozzle is e.g. 5 mm, and the distance over which the speed has dropped to 1/20 of the nozzle speed is e.g. 50 times the diameter of the nozzle, the speed is reduced from a value 8 m/s to a value 0.4 m/s over a distance of 250 mm.
The above figures are mere examples and depend not only on the above factors but also on the mutual distance of the nozzles, the nozzle length, the ratio of the areas of the mixing part and the secondary part, etc. However, they are illustrative of the most important factors connected with the behavior of the air jets and the mixing of the secondary air, and prove that the method according to the invention enables the introduction of air directly into the occupied zone without draught.
The high-speed air jets take therewith plenty of secondary air so that they move large masses of air, generally at least ten times the amount of the supply air flow. In addition, these air flows are directed by the jets as desired. Thereby the effect of undesired thermal flows and the like remains insignificant.
Due to the underpressure prevailing in the air duct, the nozzles of the air duct take secondary air from all sides, i.e., there prevails an underpressure throughout the whole mixing part. In the mid portion of the duct the underpressure sucks secondary air essentially as shown in Figure 2. At the ends of the duct, the nozzles, however, also suck secondary air in the direction of the axis of the duct due to the underpressure prevailing in the area of the mixing part. If this happens, the air flow from the duct will be throttled and the speed of the air will remain too high. In addition, the secondary flow would bring impurities from the upper portion of the room space.
The baffle plate 6 is provided for the prevention of any undesired vertical flows of secondary air, whereby the size of the plate is chosen so that the speed of the air jets beyond the plate is reduced to at least 1/10 of the nozzle speed. The underpressure causing axial flow of secondary air has thereby been reduced to about 1/100, so that it is no longer able to effect any noticeable flow in the direction of the axis of the duct. By virtue of the efficient mixing, the baffle plate may be relatively small. In the above case, a projection X extending 200 mm from the surface of the duct is sufficient. The lower end of the duct is positioned sufficiently close to the floor of the room space to be able to prevent the formation of undesired vertical flows of secondary air at the lower end of the duct.
In the embodiment of Figure 3 of the air distribution means, the entrance of secondary air from below and from above into the mixing part is prevented by means of the baffle plates 6.
The baffle plates 6 for the secondary flow can be used for the control of the air flows especially when the temperature of the supply air differs considerably from the room temperature. The embodiment shown in Figure 4 is to be used when the temperature of the supply air is lower than the temperature of the room air. The upper baffle plate prevents the entrance of secondary air within the area of the mixing part 1a, so that the air jets are directed in the horizontal direction. On reaching the outer edge of the baffle plate, the temperature of the supply air has risen close to the temperature of the room space, so that the thermal forces are able to bend the air jets only slightly downwards. At the lower end of the duct, the air jets obtain secondary air also from below, which tends to bend the flow upwards against the direction in which the thermal forces are acting. If the distance from the floor is suitable, an air flow opposite in direction to the main flow is formed above the floor level, which prevents the main flow from dropping to the floor level and thereby the formation of floor draught. By means of this arrangement it is possible to introduce, directly to the occupied zone, supply air the temperature of which is more than 10°C lower than the temperature of the room space.
It is preferable that the baffle plate is displaceable with respect to the air duct, so that it is possible to introduce into the room space either warm or cool air by means of the air distribution means. The baffle plate is displaced to the upper portion of the duct when the supply air is cool, and to the lower portion of the duct when the supply air is warm.
In the embodiment shown in Figure 5, the air duct 1 is mounted in a horizontal position, and a baffle plate 6 is fastened to the upper edge of the duct in parallel therewith. Such a structure is particularly suited for the introduction of cool supply air into large hall spaces.
To ensure that all the nozzles obtain secondary air from all directions is equally important as the prevention of the entrance of secondary air from directions disadvantageous for the flow as a whole. This is illustrated in Figure 6, which is an enlarged view of a portion of the mixing part 1a. It shows schematically the main flow of secondary air between the nozzles with the arrows B; the flow of the air jets emerging from the nozzles with the arrows A; and the flow of secondary air drawn with the air jet with the arrows C. It appears from the figure that the longitudinal dimension a of the duct should be sufficiently large as compared with the diameter d of the nozzle in order that the air jets A in the mid portion of the mixing part also obtain sufficiently secondary air C and the mixing is efficient, i.e. that the speed of the air flow decreases rapidly, the temperature differences are levelled out, etc.
If the dimension a is too small, the secondary air B from the sides of the mixing part bends the jet air flow A strongly as shown in Figure 2, and temperature differences causing thermal flows remain in the mid portion of the mixing part. It usually suffices that the ratio a/n × d is greater than 1.5, wherein n is the number of nozzles in one row in the direction of the width of the duct. The ratio varies slightly with the variation of the dimension a or d. In view of the flow B from between the nozzles, it is preferable that the nozzles are positioned in straight rows which may also extend obliquely with respect to the direction of the width, as shown in the figures. If this is not the case, it is more difficult for the secondary air flow B to enter the mid portion of the mixing part.
The distance b between the nozzles is not equally important. In principle, the dimension b may be 0, whereby the nozzles are replaced with a continuous slit. Thereby the secondary air flow C, however, can enter the air flow of the nozzle from two directions only, so that the mixing is diminished and the speed of the air jet is decreased more slowly, i.e., the so called throw distance is extended. It is to be preferred that the openings of the nozzles are round and the mutual distance thereof in the direction of the width b of the duct is larger than 3d. The mixing is thereby efficient; a great amount of secondary air is carried along; and the equalizing of the temperatures as well as the reduction in the flow speeds take place over a short distance.
In order that sufficiently secondary air could be introduced everywhere over the mixing part from the desired directions, the secondary part of the air duct, that is, the part without any holes, has to be sufficiently large, preferably cover at least 1/6 of the mantle surface of the air duct, in which the nozzles are positioned.
The drawings and the description relating thereto are only intended to illustrate the idea of the invention. In their details, the method and the air distribution means according to the invention may vary within the scope of the claims. So the air duct can be provided with two baffle plates 6 pivotably mounted thereon which are turned, depending on the difference between the temperatures of the supply air and the room air, alternately in parallel with the duct into a position in which they do not prevent the vertical flow of secondary air. The inner diameter of the nozzles is preferably no more than 10 mm.

Claims

1. A method of introducing supply air into a room space, wherein air is introduced into a lower portion of the room space (3) directly to an occupied zone (7) by means of at least one elongated air distribution means (1) comprising a plurality of outflow openings (2) for air, characterized in
- that the air is blown at a high speed through a plurality of small outflow openings (2) in such a manner that the speed of air jets (A) from the outflow openings is reduced to a small value over a short distance (X), and
- that secondary air (B) contained in the room space is prevented from being mixed with the air jets from an undesired direction of the room space over said distance (X) at least at one end of the air distribution means.

2. An air distribution means for the introduction of supply air into a room space, comprising a tubular air duct (1), a wall of which is provided with a plurality of outflow openings (2) for air, characterized in
- that the outflow openings (2) are dimensioned so small for a high air speed at the openings that the speed of the air jets (A) from the outflow openings is reduced to a small value over a short distance (X),
- that the length of the air duct (1) is considerably greater than the width of the duct;
- that the distance (a) of the outflow openings (2) from each other in the longitudinal direction of the air duct is great as compared with the dimensions (d) of the outflow opening; and
- that the air duct (1) is provided with at least one flow baffle plate (6) which prevents the mixing of secondary air (B) contained in the room space with the air jets (A) from an undesired direction of the room space over said speed reduction distance (X).

3. An air distribution means according to claim 2, characterized in that the air duct (1) is dimensioned for an outflow speed of supply air of at least 2.5 m/s.

4. An air distribution means according to claim 2 or 3, characterized in that the air outflow openings (2) are nozzles the inner diameter of which is no more than 10 mm.

5. An air distribution means according to claim 3 or 4, characterized in that the speed of the supply air and the diameter of the outflow opening (2) are chosen so that the speed of the air jet (A) is reduced to at least about 1/10 of said speed over the distance (X) which is at the most about 50 times the diameter of the outflow opening.

6. An air distribution means according to claim 3 or 4, characterized in that the distance (a) of the outflow openings (2) from each other in the longitudinal direction of the air duct (1) is at least 1.5 × n × d, wherein n is the number of the openings in one row extending in the longitudinal direction of the air duct, and d is the diameter of the opening.

7. An air distribution means according to claim 3 or 4, characterized in that the mantle surface of the air duct (1) forms an unperforated longitudinal strip (1b) which adjoins a surface area (1a) provided with the outflow openings (2) and extends in the longitudinal direction from one end of said area to the other.

8. An air distribution means according to claim 7, characterized in that the unperforated surface strip (1b) covers at least 1/6 of the mantle surface of the air duct (1).

9. An air distribution means according to claim 2 or 8, characterized in that the baffle plate (6) is displaceable in the longitudinal direction of the air duct (1) to different places of the surface area (1a) provided with said outflow openings (2).