Patent Description:
In tumble dryers, the heat pump technology is an efficient way to save energy during drying of laundry. <CIT> shows an example of a tumble dryer with a heat pump system. The air that is used to dry the laundry is circulated in an air flow circuit. In the air flow circuit, hot dry air is blown by a fan into the drying drum for removal of moisture from wet laundry. The warm humidified air leaves the drum and is cooled and dehumidified in an evaporator. The cooled air is then heated in a condenser and blown into the drying drum again. A general problem with tumble dryers is how to improve their efficiency further.

There are more examples of tumble dryers in prior art, a few are listed below:.

It is an object to obtain a tumble dryer comprising a heat pump system, which has an improved performance.

This is at least partly achieved by means of a tumble dryer as defined by claim <NUM>. The tumble dryer comprising a heat pump system and an air flow circuit for circulation of an air flow. The air flow circuit comprises a condenser, a drying drum, a fan arrangement, an air flow duct, and an evaporator. The air flow duct comprises a constriction/diffuser, arranged upstream of the evaporator. The constriction is provided with a plurality of openings, distributed over a cross section of the air flow duct and the constriction creates a pressure drop in the air flow that is circulated in the air flow duct, when the air flow flows from a first side of the constriction to a second side of the constriction. The openings in the constriction are adapted to distribute the air flow over the cross section of the air flow duct.

By arranging a constriction with openings in the air flow duct upstream the evaporator, it becomes possible to allocate the air flow more evenly on a larger part of the evaporator surface. This leads to that a more effective dehumidification of the air flow can be obtained in the evaporator, a shorter drying time in the drying drum, higher performance, and energy savings of the heat pump system.

The openings may constitute at least <NUM>/<NUM> of surface of the constriction.

To obtain the desired air flow distribution across the surface of the evaporator, the percentage of openings may depend on the size of the air flow duct and hence the size of the laundry dryer. However, it may be advantageous if at least <NUM>/<NUM> of the constriction surface may consist of openings to have the desired air flow distribution. The distance between the constriction and the surface of the evaporator is between <NUM>-<NUM>, preferably between <NUM> and <NUM>.

The constriction is a perforated plate. For example, the perforated plate may be a sheet metal, or a metal mesh may be arranged in the air flow duct to provide a constriction. These solutions are simple and inexpensive to manufacture.

The tumble dryer's rotatable drum may comprise a circular rear wall with air inlet openings and a radial cylindrical wall with air outlet openings. A space outside the drum's cylindrical periphery may be configured as a duct leading to a filter placed below the drum. This reduces air flow restrictions, and allows the use of a large filter, even about as wide as the diameter of the drum.

<FIG> illustrates schematically a tumble dryer <NUM> comprising a parallelepiped-shaped housing <NUM>. A door <NUM> is attached by hinges <NUM> to a front wall <NUM> of the housing <NUM> for input of wet laundry to be dried and for output of dry laundry.

<FIG> illustrates a cross section view of a tumble dryer <NUM> comprising a heat pump system <NUM> and an air flow circuit <NUM>. The air flow circuit <NUM> comprises a drying drum <NUM>, a fan arrangement <NUM>, an air flow duct <NUM>, an evaporator <NUM> and a condenser <NUM>.

Wet laundry to be dried is fed into the drying drum <NUM> through a door <NUM> arranged at the front side <NUM> of the tumble dryer <NUM>. Warm, dry air may be used for removing moisture from the wet laundry. An air flow may be circulated in an air flow circuit <NUM>, moved through the circuit <NUM> by a fan arrangement <NUM>. In the shown embodiment, the fan arrangement <NUM> is situated below the drying drum <NUM> but before the evaporator <NUM>. The fan arrangement <NUM> could alternatively be located between condenser <NUM> and the drying drum <NUM>. Due to space considerations in the shown embodiment, the location of the fan arrangement <NUM> below the drying drum <NUM> has been selected.

When the wet laundry is dried in the rotating drying drum <NUM>, moisture is transferred from the wet laundry to the air flow that is circulated in the drying drum <NUM>. The warm, humidified air leaves the drying drum <NUM> and is circulated to the evaporator <NUM> through an air flow duct <NUM>. The indicated air flow duct <NUM> is located in front of the evaporator <NUM> directly upstreams thereof with respect to the direction of the air flow. Needless to say the air flow circuit <NUM> includes a number of ducts and passages to form a closed circuit.

The air flow may be passed through a filter <NUM> for separation of particles or lint from the air flow before it enters the evaporator <NUM>. In the embodiment shown, the filter <NUM> is arranged below the drying drum <NUM> but above the fan arrangement <NUM>, although other positions are possible.

In the illustrated example, a high-capacity tumble dryer mainly intended for professional use or for use in shared laundry facilities. Such tumble dryers may comprise a drum <NUM> with air inlet openings in its circular rear wall and air outlet openings in its radial cylindrical wall, particularly in the front part thereof to provide a process air flow through the drum. This may be combined with a lint removing filter located below the drum, rather than with a filter provided at an outlet located in connection with the front wall door <NUM>. The space outside the drum's <NUM> cylindrical periphery may be configured as a duct leading to the filter <NUM> under the drum <NUM>. To a great extent however, the improvements described herein may also be used in connection with typical domestic tumble dryers intended for use a couple of times per week.

The warm, humidified air is passed through the air flow duct <NUM> into the evaporator <NUM>, which is made up of ducts (not shown) where warm, humidified air and a refrigerant are led alternately in the ducts and the air flow is cooled down and dehumidified through heat exchange to the refrigerant.

The cooled, dehumidified air is then circulated from the evaporator <NUM> to the condenser <NUM>, where the air flow is heated again through heat exchange with the refrigerant.

The warm air flow is then blown into the drying drum <NUM> again. The arrow <NUM> indicates the circulation of the air flow in the air flow circuit.

The refrigerant is lead through a refrigerant circuit, where a compressor <NUM> compresses the refrigerant, and where the refrigerant is condensed in the condenser <NUM> and vaporized in the evaporator <NUM> after passing an expansion valve (not shown).

The heat pump system <NUM>, comprising the evaporator, the compressor <NUM> and the condenser <NUM> and the expansion valve, may be arranged behind the drying drum <NUM> and the fan arrangement <NUM>, along the rear side <NUM> of the tumble dryer <NUM> and may be enclosed with an insulating shell <NUM> with openings to let the air flow in and out.

A constriction/diffuser <NUM> is arranged upstream the evaporator <NUM> in the air flow duct <NUM> as seen in <FIG> and <FIG>. The constriction <NUM> is provided with a plurality of openings <NUM> (cf. <FIG>), distributed over a cross section of the air flow duct <NUM> and thus over the surface of the evaporator <NUM>.

The constriction <NUM> creates a pressure drop in the air flow that is circulated in the air flow duct <NUM>, when the air flow flows from a first side A of the constriction <NUM> to a second side B of the constriction <NUM>. By arranging a constriction <NUM> with openings in the air flow duct <NUM> upstream the evaporator <NUM>, it becomes possible to allocate the air flow on a larger part of the evaporator <NUM> surface.

This leads to a more even distribution of the incoming air over the surface of the evaporator <NUM>, i.e. the air flow variance over the evaporator surface becomes reduced as compared to if an unobstructed air flow duct <NUM> would be used. This in turn leads to a more effective dehumidification of the air flow can be obtained in the evaporator <NUM>, a shorter drying time in the drying drum <NUM>, higher performance and energy savings of the heat pump system <NUM>. The area of the constriction <NUM> may take up the full cross section of the flow duct <NUM>.

In an open duct without such a constriction the air flow would in most cases be concentrated to the area at the center of the duct and consequently to the corresponding center area of the evaporator by a turbulent flow. This would lead to inefficient use of the evaporator and thus less water would be extracted from the air flow. Additionally, if the evaporator has a number of parallel circuit tubes, refrigerant fluid in some tubes may be heated to an insufficient degree such that liquid refrigerant leaves the evaporator, which is undesired. Such problems can be avoided to a great extent with a constriction as described herein.

The distribution of the air flow across the surface of the evaporator <NUM> depends on the distance d between the constriction <NUM> and surface of the evaporator <NUM>. To obtain the desired distribution, the distance d may be between <NUM>-<NUM>. Preferably a distance d between <NUM> and <NUM> is used.

Further, the percentage of openings <NUM> of the constriction <NUM> surface affects the distribution of the air flow over the surface of the evaporator <NUM>. To obtain the desired air flow distribution over the surface of the evaporator, the percentage of openings depends on the size of the air flow duct <NUM> and hence the size of the tumble dryer <NUM>. However, at least <NUM>/<NUM> of the constriction <NUM> surface may consist of openings <NUM> to provide the desired air flow distribution. As an example, for a small tumble dryer, <NUM>% of the constriction <NUM> surface may be openings <NUM> and for a large tumble dryer, <NUM>% of the constriction <NUM> surface may be openings <NUM>. An individual opening may typically be around <NUM>.

The constriction <NUM> with a plurality of openings <NUM> may be designed in different ways. The shape of the constriction <NUM> may have the shape of a rectangular plate to fit in the air flow duct <NUM>. Other alternative embodiments are possible as they fit with the corresponding design of the air flow duct <NUM>.

<FIG> shows one embodiment of a constriction <NUM>, where the constriction <NUM> is a piece of sheet metal, such as stainless steel, with perforations. The perforations being evenly distributed over the sheet metal surface. The perforations can be made by punching of the sheet metal. Typically, the sheet metal piece thickness may be in the range <NUM>,<NUM>-<NUM>, preferably about <NUM>. The perforations may be e.g. circular or almost circular openings e.g. with about <NUM> diameter. The openings may have triangular spacings e.g. with <NUM> cc distances between adjacent openings in a triangular configuration. The entire sheet metal piece may be provided with openings in this way.

<FIG> shows an alternative embodiment of the constriction <NUM>, were the constriction <NUM> is a metal mesh, where metal wires have been arranged to form a grid with evenly spaced openings over the surface of the grid.

Claim 1:
A tumble dryer (<NUM>) comprising a heat pump system (<NUM>) and an air flow circuit (<NUM>) for circulation of an air flow, the air flow circuit (<NUM>) comprising a condenser (<NUM>), a drying drum (<NUM>), a fan arrangement (<NUM>), an air flow duct (<NUM>) and an evaporator (<NUM>), wherein the air flow duct (<NUM>) comprises a constriction (<NUM>), with a plurality of openings (<NUM>), arranged upstream the evaporator (<NUM>), and is adapted to create a pressure drop in the air flow that is circulated in the air flow duct (<NUM>), wherein the drying drum comprises a circular rear wall with air inlet openings and a radial cylindrical wall with air outlet openings, wherein a space outside the radial cylindrical wall is configured as a duct leading to the constriction, and characterised in that the constriction (<NUM>) is a perforated plate.