TECHNICALAREA

The present invention is based on a filtering device for process air in industry. The invention is especially based on a device for filtering particles in a flow of process air. In particular, the invention is based on a filtering device containing a labyrinth for the deposition of particles.

TECHNICAL BACKGROUND

A common procedure for separating particles from an air flow is to decrease the air speed in an air chamber. This causes the particles to fall downwards ("sediment") under their own weight. Another common procedure is to have the air flow rotate in a cyclone. Through centrifugal force, this causes the particles to be pushed outwards and then sediment. The air flow in the middle of the cyclone becomes free of particles and the filtered air is taken from here and carried onwards. These procedures do not work so well with very small particles such as dust.

Dust is more difficult to deflect from the air flow and is usually filtered using mechanical filters. A typical filter for this purpose comprises a mat of long fibres that are locked to each other in such a way that a number of channels of interlinked cavities are formed. Clean air can thus pass through these channels while dust lodges in the cavities. Dimensioning parameters for achieving the correct degree of filtration are: mat thickness and density; and, fibre thickness. Contaminated air is caused to pass through a number of such fibre mats, the small particles fastening therein. These mats can be designed as cassettes or bags. However, they all have the disadvantage that the filter eventually becomes clogged. To loosen the particles, the filter must then be shaken or exposed to an opposing air flow (preferably pulsating). Sometimes, the filter must quite simply be replaced.

A further filtering method is an impactor. This uses a process in which particles are deposited because, owing to their mass and inertia, they cannot follow the lines taken by a gas as it flows around an obstacle. In an impactor, an aerosol flows through a nozzle that is directed towards a flat plate. Aerosol here means a gas that contains particles ranging from 0.001 micrometres to over 100 micrometres in size. The impactor can be made to filter particles of different sizes by correctly dimensioning: the speed of the gas; the nozzle; and, the distance between the plate and the nozzle. A common development of the impactor is a labyrinth in which the aerosol is forced to pass along/through a number of sharp bends, the particles being deposited on the labyrinth's walls.

A filtering device for air-oil mixtures is already known through SE 518 368

(Lindblom). The filtering device, which is self-draining and washable, has the job of providing filtering throughout continuous operation. For this purpose, the filtering device has a cassette with a folded/corrugated filter material that is parallel with the air flow.

Intended to extend the interval between services, a filter unit with several filter elements is already known through WO03/028851 (Lindblom). The known filter unit comprises an expansion chamber and a filter section with a number of filter cassettes (arranged standing vertically in series) that contain fibre mats. In the known unit, the contaminated air first passes through an expansion chamber where the speed of the particles is lowered so that, owing to gravity, the large particles fall downwards. The contaminated air is then turned upwards and forced to pass a number of serially arranged filters comprising fibre mats that, in the direction of the air flow, are of an increasing density.

A device for enlarging small, sticky particles is already known through EP0564914 (Behr). The device comprises a number of layers of corrugated plates/foils arranged at an angle to each other. This creates channels that cross each other. As the contaminated air passes through the channels, a number of vortices are set up. These vortices result in the sticky particles carried in the air being deposited on the corrugated plates.

Production premises for die casting are amongst the dirtiest and most difficult to keep clean. Die casting machines are large and often poorly enclosed. This allows contaminated air to leak from the manufacturing process and contaminate the general air in the premises. These machines create large quantities of a very special smoke that cannot be filtered from the air using conventional methods. The particles are relatively large (5 - 10 μm) and very sticky. This means that filters designed for dry particles and those designed for wet particles both work unsatisfactorily here. The service lives of the filters on the market are short. This gives high maintenance costs. As the maintenance costs are high because of the unsatisfactory performance, many casting shops choose not to clean their process air at all. This leads to a poor work environment. There are also emissions of contaminated air to the surroundings.

Environments similar to those in die casting shops can also be found in other manufacturing processes (e.g. hot pressing) and processes that involve cold- hammering.

Getting rid of the trapped particles in some way or other is the basis of the filter equipment for the type of very large quantities of air contaminants created here. Owing to the very large quantities of generated particles, filters very soon get clogged if there is no form of draining, cleaning or washing. There are currently systems for cleaning filters for dry particles. There are also wet particle filters that drain off the trapped particles. For sticky particles that resist cleaning or draining by conventional systems, there are currently no good systems.

EXPLANATION OF THE INVENTION The purpose of the invention is to indicate ways for bringing about a method and a device for filtering out sticky particles from process air.

Said purpose is achieved by: the invention's use of a filtering device having the special characteristics set out in the characterisation section of independent claim 1 ; and, by a procedure having the special characteristics set out in the characterisation section of independent procedure claim 9. Advantageous designs are set out in the characterisation sections of the dependent claims.

As per the invention, the filtering device includes parallel filters of which a first filter filters the process air while a second filter, arranged in parallel with said first filter, undergoes a cleaning process. In one design of the invention, both filters are equivalent and include an impactor. In one design, the impactor includes at least one labyrinth. The contaminated air is forced to make several changes of direction at high speed. This causes the largest and heaviest particles to hit the labyrinth's walls and lodge there. On a weight basis, the greatest quantity of particles lodges in this stage. If counting the number of particles, a large number of small particles pass through this stage. Because this stage traps a very large quantity of sticky particles, it also quickly becomes clogged. For this reason, the invention provides a method for cleaning, draining and/or washing the filter stage.

In one design, the cleaning process includes a heat source. The heat source can include radiant heat, hot air or microwaves. In one design, the cleaning process includes mechanical cleaning in the form of a scraper.

In accordance with a first aspect of the invention, the purpose is achieved by a filtering unit that includes: a closed casing with an inlet for receiving a contaminated air flow; an outlet; a first filter that includes at least one impactor; a second filter arranged in parallel with the first and containing at least one impactor; and, between the first and the second filter, a damper device for switching the air flow between the first filter and the second filter. In one design, each impactor includes at least one labyrinth. In one design, the filtering unit includes a heat source. In one design, the heat source includes a radiant heater. In one design, the heat source is parallel with the labyrinth.

In accordance with a second aspect of the invention, the purpose is achieved by a procedure for cleaning an air flow containing sticky particles, said procedure using a filter device that contains a first filter and, arranged in parallel with this, a second filter, the air flow being caused, in a first period, to flow through the first filter while the second filter undergoes a cleaning process and, in a second period, to pass through the second filter while the first filter undergoes a cleaning process.

DETAILS OF THE FIGURES

The invention is explained more closely through design descriptions that refer to the attached drawings set out below.

Fig. 1 is a filtering unit as per the invention containing a first and a second filter, a damper and a first and a second heat source.

Fig. 2 is an advantageous design of the filtering unit.

DESCRIPTION OF DESIGN EXAMPLES

The filtering unit as per figures 1 and 2 comprises an enclosure with an inlet (1) for receiving a contaminated air flow and an outlet (3) for the cleaned air. The filtering unit comprises a first and a second chamber (14), each of which has a labyrinth device (2) for cleaning the through flow of air. The labyrinth device is designed as a cassette with a number of plates or aluminium profiles that form a system of sharp bends for the deposition of contaminants in the air flow. Each chamber contains its own heat source (4) for heating the labyrinth device so that the viscosity of the deposited particles (13) is reduced and the particles run out of the cassette. The filtering unit also contains a damper device for directing the air flow to either the first or the second labyrinth device. Figures 1 and 2 show how the damper device can be arranged in an open position (11) or a closed position (12). In the design shown in figure 2, the labyrinth device includes a drain (5) for draining off the filtered, hot, sticky particles (13).

Because the particles are sticky, they remain in the labyrinth. Consequently, the latter soon becomes clogged and airtight. In a very short space of time, no air can get through. When the labyrinth becomes airtight, the damper device is used to switch the air flow to the other chamber. Switching between the chambers can be manual or automatic. In automatic switching, an electrically or pneumatically controlled damper is used. This receives a signal from a pressure sensor that senses how much air is passing through the active labyrinth. When this quantity of air is below a set level, the air flow switches from the first to the second chamber.

Parallel with the air flow being filtered in the second chamber, the dirty and airtight labyrinth in the first chamber undergoes a cleaning process. In a first method, the labyrinth is cleaned by a heat source in the form of thermal rays from an IR heater. At room temperature, the viscosity of the sticky particles is very high (they do not run). If they are heated, the viscosity falls. The particles can be made liquid and, in this way, made to run out of the labyrinth.

In a second method, the labyrinth is cleaned by a heat source (4) in the form of hot air. This works in the same way as the IR heater. In a third method, the labyrinth is cleaned by a heat source (4) in the form of microwaves. This works in the same way as the IR heater. In a fourth method, the labyrinth is cleaned mechanically using, for example, one or more scrapers. In a fifth method, the labyrinth is cleaned by spraying a liquid on the labyrinth. The liquid "loosens" the particles, thereby lowering their viscosity.

The "loosened" sticky particles can be led either directly out of the chamber into a suitable container or to the chamber's outlet (13), from where they can run further to the bottom of a subsequent filter unit.

The described cleaning methods can also be combined. The quantities of air passing through the filter unit are very large. This air has a cooling effect. Consequently a lot of energy would be required to heat a labyrinth if the air was passing through it. For this reason, a parallel filter is provided so that one filter filters the air while the other undergoes a cleaning process. With mechanical cleaning as per method four, and with spraying as per method five, the labyrinth device can be cleaned at the same time as air passes through. However, the set-up is more efficient with the air flow shut off.

One or more further filter stages are provided to deal with the particles that pass through the labyrinth. This/these filter stage/stages can comprise fibre beds, centrifugal filters, electrostatic filters and/or rotating separators.

A final filter stage can be used to ensure that no particles pass through the filter unit. This filter stage could thus comprise a so-called HEPA filter (class H filter) or similar.

By providing parallel filters in such a way that one filters while the other is cleaned, continuous filtering of the air flow can be achieved. This results in a dramatic reduction in the cost of filtering process air in die casting shops. This filter concept is based on filtering the contaminated air in several stages, the clean air that leaves the filter unit being completely free of particles.