WO2002073094A1

WO2002073094A1 - Filtration arrangement and method for an air-conditioning system

Info

Publication number: WO2002073094A1
Application number: PCT/FI2002/000098
Authority: WO
Inventors: Matti LEHTIMÄKI; Ilpo Kulmala; Vesa MÄKIPÄÄ
Original assignee: Oy Lifa Iaq Ltd.
Priority date: 2001-02-19
Filing date: 2002-02-08
Publication date: 2002-09-19
Also published as: GB2390673B; WO2002073094A9; AU2002231834A1; FI20010314A0; FI20010314A; FI117681B; GB0319232D0; GB2390673A; CN1503890A

Abstract

This publication discloses a filtration system and a method for an air-conditioning system. The air-conditioning system includes at least one fan (2) for creating an airflow, ducts (3) connected to the fan (2) for conducting the airflow onwards, and supply airvalves (4) at the end of the ducts (3) for conducting the airflowinto the room (5). According to the invention, at least a three-stage filter unit (20) is connected to at least one supply air valve (4), which unit in turn includes an electrified aerosol filter(8), an adsorbent filter (9), and a terminal filter (10).

Description

FILTRATION ARRANGEMENT AND METHOD FOR AN AIR- CONDITIONING SYSTEM

The present invention relates to a filtration system, according to the preamble of Claim 1, for an air-conditioning system.

The invention also relates to a filtration method for an air-conditioning system.

Filtration arrangements of this kind are used in connection with centralized air- conditioning systems, in both dwellings and office buildings.

According to the state of the art, filtration takes place either with the aid of large centralized equipment and/or by means of filtration units located in the desired rooms and not connected to the air-conditioning system.

A drawback of the state of the art is that the achievement of the desired filtration result in a centralized system requires a filtration system of such a large size as to be possible in present buildings. The problem is particularly acute in cases where the need for filtration is greatest. While it is precisely in large cities that the quality of the air is poorest, the high price of real estate in such places makes it difficult and expensive to arrange the necessary technical accommodation in existing buildings. Another problem with filtration in a centralized system is the dirtying of the air-conditioning ducts themselves. Dirtying of the ducts that occurs already during installation can cause health detriments when the air-conditioning equipment is used, no matter how efficient the filtration is. Additionally, disturbances in filtration, water damage, or neglecting to service filtration equipment lead to dirtying of the air-conditioning ducts, leading to consequences that no centralized filtration can eliminate, because the air is filtered before being led to the air-conditioning ducts.

On the other hand, room-specific filtration units have the drawback that they do not prevent detrimental impurities from entering the room, but only remove impurities from the air travelling through them. In addition, the cleaning principles of known room- specific filter units are such that they do not remove gaseous impurities very effectively, because in present solutions the gas filter's retention times are too short to achieve efficient filtration. Further, cleaning apparatuses of this type include their own fans, thus increasing both the noise and the price of a cleaning unit.

Replacement air devices also exist that are equipped with filters and their own fans and which are installed, for example, in a ventilation window. These devices have poor small-particle filtration efficiency and do not incorporate gas filtration.

The invention is intended to eliminate the defects of the state of the art disclosed above and for this purpose create an entirely new type of filtration system and method.

The invention is based on using room-specific two or multiple-stage filter units, which are located in connection with a room-specific supply air valve. According to one preferred embodiment of the invention, aerosol filtration takes place using a filter in which a charge is induced in the dirt particles and the charged particles are collected by electrical collector elements.

More specifically, the system according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

The method according to the invention is, in turn, characterized by what is stated in the characterizing portion of Claim 8.

Considerable advantages are gained with the aid of the invention.

Problems do not arise when cleaning equipment according to the invention is connected to an air-conditioning system, because its pressure drop is small. Due to the small pressure drop, the flow equilibrium of the air-conditioning system is not disturbed.

With the aid of the invention, effective room-specific filtration is created, which does not depend on the cleanliness of the air-conditioning ducts. The arrangement can be easily installed in the requisite rooms, allowing resources to always be allocated according to need. For example, sufficient cleanliness of the air in hospital wards can be achieved without expensive investments in a centralized system, or a need for separate cleaning, or for maintaining particular cleanliness in the air-conditioning ducts.

The new filtration solution has significant advantages over present solutions. The table below compares indicatively the properties of the present and the new solutions.

The penetration of the conventional centralized solution is poorer than that of the new filter solution; centralized solutions generally have no gas filtration at all. However, if filtration apparatus with a corresponding efficiency was to be made in a centralized system, it would have the following space requirement and properties:

Thus it can be estimated that additional filters installed in present centralized systems would significantly increase the space required. In addition, the additional filters would cause a pressure drop of c. 600 Pa more than room-specific, low-pressure-drop filters. This would mean having to replace the existing fans with more powerful ones, i.e. larger fans. Simultaneously, the energy consumption and noise level of the fans would increase, as would the need for supply-air cooling.

In electrically boosted filters, a high voltage (in the order of 8 kV) is used. The current, however, is very small, so that the power consumption remains a few watts.

The effective filter components and also the fact that all the supply air that is mechanically brought in is filtered result in a clearly improved cleaning efficiency, compared to room-specific cleaning devices that are not connected to the air- conditioning system. The noise level is also clearly lower, because the filter, which operates without a fan, actually attenuates the background noise of the flow in the air- conditioning duct.

In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.

Figure 1 shows schematically one solution according to the known art.

Figure 2 shows schematically one solution according to the invention.

Figure 3 shows graphically an electrostatic filter's cleaning efficiency filter as a function of particle size.

Figure 4 shows a schematic side view of the construction of a two-stage electrostatic cleaner.

Figure 5 shows a schematic side view of the construction of an electrified fibrous filter. Figure 6 shows graphically an electrified fibrous filter's cleaning efficiency as a function of particle size.

Figure 7 shows a detail of the construction of an electret filter.

Figure 8 shows graphically an electret filter's cleaning efficiency as a function of particle size.

According to Figure 1, in known air-conditioning solutions, the filter 1 is located centrally on the supply air side of the air-conditioning system, for example, before the supply air fan 2. Though the air entering the rooms 5 through the supply air valve 4 is thus pre-cleaned, the impurities of the air-conditioning ducts 3 are transferred unfiltered to the rooms. The air is exhausted through an exhaust valve 6 assisted, if necessary, by an exhaust air fan 7.

In the invention according to Figure 2, at least a three-stage filter unit 20 is situated in the room's supply air valve, which u t includes a particle filter, preferably an electrified aerosol filter 8, an adsorbent filter 9, which is, for example, an activated carbon filter, and a terminal filter, which is, for example, a photo-catalytic filter 10. The filter unit's elements 8, 9, and 10 are designed in such a way that they cause the least possible pressure drop. In addition, the unit 20 is made as easy as possible to install in the supply air valve 4. According to the invention, the positions of at least the particle filter 8 and the activated carbon filter 9 can be exchanged.

In the particle filter 8, the separation capacity is increased by charging the particles with the aid of a high-tension corona discharge and utilizing electrical forces to collect. Due to this, the device also filters extremely well small particles (particle size less than 1 micrometre), which are detrimental to health and which are separated poorly by the aerosol filters presently used in central machines. The use of electrical forces allows the separation capacity to be increased without the pressure drop becoming excessive.

The adsorbent filter 9 removes ordinary VOCs (volatile organic compounds), odours, and ozone. This filter must be changed at regular intervals. The filter has typically a honeycomb construction, which is disclosed in, inter alia, FI patents 103641 and 104154. Due to the honeycomb construction, the pressure drop is extremely small compared to present corresponding present adsorbent filters.

The terminal filter 10 removes such impurities from the air on which conventional adsorbent filters have no effect. These include carbon monoxide and nitrogen oxides.

In the following, various alternatives to the electrified aerosol filter 8 are examined.

The major advantages of electrified aerosol filters 8 are a good filtering efficiency for fine particles and a low flow resistance. The effectiveness of an electrified filter is based on directing electrical forces onto charged particles of dirt. The cleaning efficiency depends on the size of the dirt particle, the flow velocity, the dimensions of the cleaner, the corona current, and the collection voltage.

Figure 4 shows one type of electrical filter, in which a charge is induced in the impurity particles in the incoming flow 11 with the aid of the corona discharge caused by plates 13 and wires 12. The charged particles adhere to electrically charged plates 14, thus effectively removing the larger particles from the exit flow 15.

Figure 3 shows the cleaning efficiency of an apparatus according to Figure 4, as a function of particle size. The different curves also show the effect of the flow velocity. It can be seen from the curves that the slower the flow, the more efficient the cleaning result. On the other hand, the relative cleaning efficiency for large particles increases as the flow velocity increases.

Figure 5 shows an electrified fibrous filter. In the case of the corona apparatus 12 and 13, the solution corresponds to that of Figure 4. In the solution of Figure 5, a fibrous or porous material 20 is used, in which a powerful direct current field is created between a positive potential 16 and a negative potential 17. This is realized with the aid of flat- plate electrodes, in which the potentials 16 and 17 alternate, placed between the filter units 21. Naturally, it is important in terms of the operation of the solution for there to be a sufficient difference in voltage between the surfaces with the potentials 16 and 17, which is typically a difference of several, or several tens of kilo volts. Thus, in order to achieve this target, both potentials can naturally be above, or correspondingly below the earth potential. The solution will also operate without the corona-discharge apparatus 12 and 13, but its cleaning efficiency is greater with it. In the fibrous filter solution, the electric field particularly improves the filtration of particles with a size of less than one micron.

Figure 6 shows the cleaning efficiency as a function of particle size of a simple fibrous filter, an electrified fibrous filter, and a fibrous filter equipped with corona-discharge apparatus. As can be seen from the figure, the filter equipped with electrified filtration and corona discharge achieves a cleaning efficiency of nearly 100 %.

Figure 7 shows schematically the construction of an electret filter. The electret filter is based on fibres 18 with a permanent polar charge. Thus each fibre has areas that are either positively or negatively charged. Therefore this electrified filter construction does not need an external voltage source.

Figure 8 shows the filtering efficiency of this type of filter as a function of particle size. The figure shows separate curves for filter constructions, in which the polar charge has been removed, in which there are polarly charged fibres, and a solution in which a charge is induced in the dirt particles by means of a corona-discharge apparatus.

The terminal filter 10 is, in turn intended for the removal of gaseous impurities, such as carbon monoxide and nitrogen oxides. The filter 10 can comprise, for example, a photo- catalytic filter or an impregnated absorbent filter. In terms of flow technique, the filter can be constructed similarly to the other parts of the filter 20. A photo-catalytic filter is based on cleaning gases by using UV (ultraviolet) light and a suitable catalytic surface. One practicable catalytic material is titanium oxide (TiO₂). A layer of titanium oxide a few microns thick is formed on the filter. The titanium oxide layer can be formed by soaking a substrate in titanium oxide (Dip-Coating), or by forming the titanium oxide by means of a sol gel solution, or alternatively by metallic thin-film growing. The UV light forms OH radicals (hydroxyl radicals) from the incoming moisture and gases, which as such kill bacteria other microbes. In addition, the OH radicals activate the titanium oxide to act as a catalyst, which in turn forms harmless substances catalytically from gaseous impurities. For example, carbon monoxide is converted catalytically into water and carbon dioxide.

The terminal filter can also be an impregnated adsorbent filter, which filters gaseous impurities from the air that the adsorbent filter is insufficient to remove.

The power of UV light inside the filter can be increased by means of mirrors, such as aluminium surfaces. The source of light can be located relatively freely in the filter, typically, however, in the front part of the filter in the direction of flow. The distance between the light source and the catalytic surface can be, for example, 3 - 5 cm.

The filter apparatus can also be equipped with a bypass filter, by means of which air impurities arising in the room, such as smoke smells from preparing food, or tobacco smoke, can be removed when necessary. The bypass flow can be created either by using a separate rotating selector switch, or alternatively automatically with the aid of a smoke or odour detector. The flow created by the air-conditioning machinery 2 can be used to form the bypass flow.

The following gives an example of the dimensioning for a solution according to the invention.

A building, in which there are 200 rooms, each with an airflow of 30 1/s, giving a total airflow of 6 m³/s.

The size of the particle filter 8 is 400 x 500 x 150 mm³. The shape of the filter can vary, the volume being the critical dimensioning factor. The pressure drop of each filter is 10 Pa. The size of the adsorbent filter 9 is 400 x 500 x 150 mm³ (the shape can vary). The pressure drop in the filter is typically < 20 Pa. The volume of the photo-catalytic filter 20 is typically 0.3 m³, the shape of the filter can vary.

According to a preferred embodiment of the invention, the filter unit can be constructed compactly, so that it is easily installed in the supply air valve 4. The valve installation should be as airtight as possible, to achieve efficient air cleaning. The filter apparatus 20 is typically a ready-to-install unit, with a current source for the UV lamps and the necessary equipment for the high- voltage filter. The high voltage can be produced used a transformer and/or a switched-mode power supply. The filter unit 20 can thus preferably be a module that can be bought from a shop, allowing the occupant of the apartment 5 to instal it themselves by mechanically attaching the unit to the wall and connecting the plug to a wall socket.

Claims

Claims:

1. A filtration system for an air-conditioning system, which air-conditioning system includes

- at least one fan (2) for creating an airflow,

- ducts (3) connected to the fan (2) for conducting the airflow onwards, and

- supply air valves (4) at the ends of the ducts (3) for conducting the airflow into a room (5),

characterized in that

- a filter unit (20) of at least two stages, which in turn includes

- an electrified aerosol filter (8),

- an adsorbent filter (9), and

- if necessary a terminal filter, which can be, for example, a photo-catalytic filter (10)

is connected to at least one supply air valve (4).

2. A system according to Claim 1. characterized in that the electrified aerosol filter (8) includes means (12, 13) for inducing a charge in the dirt particles.

3. A system according to Claim 1 or 2, characterized in that the electrified aerosol filter (8) includes collector means (14, 16, 17) for collecting the electrically charged dirt particles with the aid of electrical forces.

4. A system according to any of the above Claims, characterized in that the adsorbent filter (9) is an activated carbon filter.

5. A system according to any of the above Claims, characterized in that the photo- catalytic filter (10) includes a UV source and a titanium-oxide catalytic construction.

6. A system according to any of the above Claims, characterized in that the photo- catalytic filter (10) is the final filter element in the direction of flow.

7. A system according to any of the above Claims, characterized in that there is a bypass duct in the filter element (20) for conducting the air in the room into the filter apparatus when necessary.

8. A filtration method for an air-conditioning system, in which method

- an airflow is created in the room (5) through ducts (3),

characterized in that

- the airflow arriving at the room (5) is filtered in the room in at least two stages

- with the aid of an electrified aerosol filter (8),

- with the aid of an adsorbent filter (9), and

- if necessary, with the aid of an terminal filter (10).

9. A method according to Claim 7, characterized in that, in the electrified aerosol filter (8), an electrical charge is induced in the dirt particles.

10. A method according to Claim 7 or 8, characterized in that the electrically charged dirt particles are collected with the aid of electrical forces.

11. A method according to any of the above Claims, characterized in that an activated carbon filter is used as the adsorbent filter (9).

12. A method according to any of the above Claims, characterized in that a UV light source is used in the photo-catalytic filter (10) and titanium oxide is used as the catalytic construction.

13. A method according to any of the above Claims, characterized in that the photo- catalytic filter (10) is located as the final filter element in the direction of flow.

14. A method according to any of the above Claims, characterized in that the room air is conducted to the filter unit (20) for filtering, with the aid of a bypass duct.