Abstract:
The present invention relates to a defroster ( 5 - 10 ) for defrosting a gas cooling arrangement ( 4 ) of a freezer, comprising a sound generator ( 5 ) arranged to generate a sound pulse and to subject said gas cooling arrangement ( 4 ) to said sound pulse such that an amount of frost formed on said gas cooling arrangement ( 4 ) is reduced by removing frost formed on said gas cooling arrangement ( 4 ) by means of sound energy in said sound pulse. The present invention also relates to a freezer system comprising such a defroster ( 5 - 10 ) and a gas cooling arrangement ( 4 ). The present invention also relates to a method for defrosting a gas cooling arrangement ( 4 ) of a freezer. Figure elected for publication.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates to a method for defrosting a gas cooling arrangement of a freezer. The invention also relates to a freezer system. The invention also relates to a defroster for defrosting a gas cooling arrangement of a freezer. 
       TECHNICAL BACKGROUND 
       [0002]    On the market today, there exist a number of different kinds of freezers adapted to freeze different kinds of products. One group of such products is e.g. food products, including e.g. bread pieces, minced meat patties, and packed ready-meals. 
         [0003]    One basic kind of freezer blows cooled air past the product to freeze. The air flow may have different intensities. The air flow may be anything from a gentle flow of air to a strong flow of air adapted to break the boundary layer of air around the product. The latter is called impingement freezing. 
         [0004]    In a freezer blowing cold air past the product to be frozen, it is common to use one or more fans transporting the air past the product and past one or more gas cooling arrangements in which the gas is cooled. One commonly used gas cooling arrangement is the use of an evaporator in which a refrigerant is evaporated. The evaporation cools the evaporator and thereby the air passing the evaporator is cooled. The evaporators are often also referred to as cooling battery. 
         [0005]    Often the air contains certain amounts of water vapour which has a tendency to be deposited as frost on the gas cooling arrangement. The deposition of frost reduces the heat transfer between the air and the gas cooling arrangement, thereby reducing the gas cooling arrangements&#39; efficiency in cooling the air passing the gas cooling arrangement. The deposition of frost may also geometrically change the air passages designed for efficient flow of air through or past the gas cooling arrangement, which in turn may increase the air pressure drop over the gas cooling arrangement. 
         [0006]    In order to reduce the amount of frost on the gas cooling arrangement it is known to provide different ways to remove or at least reduce the amount of frost on the gas cooling arrangement. 
         [0007]    Many common methods of removing frost deposits have the disadvantage that the air cooler must be switched off and heated in order to melt the frost. Among prior art methods of this kind one may mention electrical resistance heating, “hot gas defrosting” (in which the flow of refrigerants is reversed in a suitable way so that the evaporator functions as a condenser during the defrosting operation) and water defrosting (in which the cooling-coil battery is heated by overflowing water). In many situations it is not desirable that the cooling battery need to be switched off during the defrosting operation. Moreover, these methods often results in that moisture forms around the cooling batteries, which will quickly result in new frost deposits. These defrosting methods also require considerable amounts of energy which substantially will be lost. 
         [0008]    In addition to the defrosting methods described above there are methods of defrosting air coolers during operation by pouring a suitable chemical, e.g. a glycol/water solution, on the air-cooler. However, these methods have the disadvantage that they are complicated, they require a certain equipment for distilling away the melted frost and they require special arrangements to prevent the chemical splashing onto the product in the form of drops, which is e.g. especially important if the product is food products. 
         [0009]    Another method of defrosting the cooling battery entails the use of a number of air nozzles which is adapted to be recurrently directed towards and brought to sweep over the cooling batteries in order to blow away the frost deposit from the cooling batteries by means of a directed flow of compressed air. One such apparatus is disclosed in U.S. Pat. No. 4,528,820. One drawback with this defrosting apparatus is that the cooling battery and surrounding parts of the freezer must be designed such that the air nozzles can be moved along and get access to all parts of the cooling battery. There is also a risk that the cooling battery is damaged by the flow of compressed air. There is also a risk that the moving components of the air defroster break down or freeze stuck. The air defroster also entails the addition of substantial amounts of air which need to be cleaned and which introduces heat into the freezer. 
         [0010]    Another way of attacking the frost formation problem is to remove the water before it forms frost on the cooling battery. One way according to this principle is to provide an air-dehumidifier generally lowering the content of water vapour in the air. However, this entails a separate apparatus, thereby increasing the cost of the freezer. It may also be noted that the product to be frozen often also adds water vapour to the cooling air being circulated over and over in the freezer. Furthermore, considering that a cooling battery accumulating frost in one sense is a highly efficient dehumidifier, it is difficult and costly to provide a dehumidifier which removes the problem of frost deposition on the cooling battery. 
         [0011]    Another way of removing the water before it is deposited as frost is disclosed in JP 2010-048484. The document discloses a method and a system for removing condensed water droplets from e.g. a fin of a heat exchanger. By removing condensed water droplets before they freeze, frost formation is reduced. A sound generator is used to generate a standing acoustic wave. The standing acoustic wave comprises nodes, having a minimum amplitude, and anti-nodes, having a maximum amplitude. By applying the standing wave to the heat exchanger, condensed water droplets are moved to a position of a node of the standing wave. By doing this the condensed water droplets on the heat exchanger can be moved and thus concentrated to specific locations, i.e. the locations of the nodes. Consequently, portions of the heat exchanger not corresponding to positions of the nodes of the standing wave, will have a reduced amount of condensed water droplets. By moving and removing the condensed water droplets, before they turn into ice, ice formation can be reduced or concentrated to the positions of the nodes of the standing wave. 
         [0012]    Further, in JP 2010-048484, it is proposed to sweep the frequency used to alter the positions of the nodes of the standing wave, and thereby move the water droplets along e.g. a fin of a heat exchanger. Another strategy described, for achieving the same result, is to physically move the sound source to alter said positions of the nodes. A third strategy proposed, is to use an high amplitude sound wave to move said droplets away from the sound source used. This approach is, however, complicated and difficult to realise. First of all you have to find the resonance frequency in order to establish a standing wave. Secondly, one have to move the sound wave nodes sufficiently fast to remove the water droplets before they freeze into ice. Thirdly, if any ice anyhow is formed on the heat exchanger, the resonance frequency is most likely changed and a new resonance frequency must be found to establish the standing wave. Fourthly, one have to perform this operation continuously since water droplets continuously condenses on the heat exchanger. 
         [0013]    Another way of removing water droplets from an evaporator is disclosed in US 2007/0039344. This document discloses a method and apparatus for removing excess moisture from evaporator coils of e.g. an air-conditioning system. The moisture is removed from the evaporator coils by vibrating the coils. The coils may be vibrated by mechanical or acoustic devices such as solenoid plungers and acoustic transducers. However, US 2007/0039344 does not address or even mention a problem with frost deposition. It may be noted that US 2007/0039344 relates to an air-conditioning apparatus in which frost deposition is not an issue at all. 
         [0014]    JP 408327288 discloses a heat pump air conditioning system with two evaporators, one indoor heat exchanger and one outdoor heat exchanger. Two sound generators are used to generate vibrations on the outdoor heat exchanger used as a condenser in an air conditioning system. When the heat pump air conditioning system is run as a air conditioning system, i.e. as a cooling system, the outdoor heat exchanger forms a condenser in the system. A first of the sound generators is attached to the frame to induce vibrations to the fins of the outdoor heat exchanger and the second sound generator is attached to the tubing to induce vibrations to the tubing of the system. The vibrations of the fins will cause the air stream passing the fins to vibrate in order to promote destruction of a boundary layer in order to achieve an improvement in heat exchanging efficiency. It is also briefly discussed how the vibration of the air stream have certain effects during a heating period when the system is run in the opposite direction, in a heating mode. The sound generator is installed in the inflow side of the air in order to add vibrations to the fins of the outdoor heat exchanger and to remove dew condensation and frost promptly. The second sound generator will cause vibrations to propagate and to break the boundary layer between the liquid phase and gas phase of the medium in the tube. This is a complex design with two different sound generators that are run constantly in order to break up boundary layers between different phases of the medium and of the boundary layer of air along the fins of the heat exchanger. 
         [0015]    Thus, today there is no satisfactory solution concerning defrosting of a gas cooling arrangement of a freezer. 
       SUMMARY OF INVENTION 
       [0016]    It is an object of the invention to provide a method for defrosting a gas cooling arrangement or a freezer. A further object of the invention is to provide a freezer system provided with a defroster. A further object of the invention is to provide a defroster for defrosting a gas cooling arrangement of a freezer. 
         [0017]    These objects have been met with a method, a freezer system and a defroster according to the independent claims. 
         [0018]    According to a first aspect of the invention it relates to a method for defrosting a gas cooling arrangement of a freezer, the gas cooling arrangement being adapted to cool a gaseous medium passing the gas cooling arrangement, the method comprising: 
         [0019]    generating a sound pulse; and 
         [0020]    subjecting said gas cooling arrangement to said sound pulse such that an amount of frost formed on said gas cooling arrangement is reduced by removing frost formed on said gas cooling arrangement by means of sound energy in said sound pulse. 
         [0021]    When the gas cooling arrangement is subjected to the sound pulse removing the frost may e.g. be performed by the sound pulse cracking or breaking the frost which then e.g. may fall of the gas cooling arrangement due to the gravity. The sound pulse may e.g. also collapse the boundary layer close to the gas cooling arrangement surface and thereby facilitate the normal airflow of the gaseous medium passing the gas cooling arrangement to blow away frost particles from the gas cooling arrangement surface. This design makes it possible to, in a simple way, make a defrosting action whenever it is considered to be suitable. Once a defrosting action is considered suitable a sound pulse may simply be generated. From an overall design point-of-view, the design with a sound pulse removing the frost is a good solution since the gas cooling arrangement may be designed as a compact unit without having to consider how to provide access to a system with air jet nozzles or the like. This also makes it possible to fit a sound pulse defroster to many of the existing freezers. Moreover, there is no significant risk that the sound pulse damages the gas cooling arrangement. 
         [0022]    The invention is especially suitable for applications where the gas cooling arrangement is adapted to cool a gaseous medium passing the gas cooling arrangement to a temperature of below 0° C. Such applications are especially prone to result in a frost development on the gas cooling arrangement. 
         [0023]    Preferred embodiments of the method appear from the dependent claims. 
         [0024]    The sound pulse may be generated with a frequency below 300 Hz, preferably between 10-250 Hz, more preferably between 150-250 Hz. 
         [0025]    A frequency below 300 Hz is considered suitable since the relatively long wave length of the vibration of the air is considered suitable to break up frost formed on the gas cooling arrangement. At frequencies below 300 Hz the wave length will be more than a meter in length and the sound pulse will propagate past the gas cooling arrangement without being attenuated to any significant extent, i.e. the energy put into the sound pulse will effectively be transmitted to the frost and gas cooling arrangement throughout the gas cooling arrangement. Moreover, the long wave length will decrease the risk of the formation of “dead points” not affected by the sound pulse. 
         [0026]    The sound pulse may be generated with a duration between 1 and 15 seconds, preferably between 1 and 10 seconds, more preferably between 2 and 4 seconds. If the sound pulse is too short there is a risk that the frost will not be removed to satisfactory degree e.g. due to that cracks in the frost will not have time to form and to propagate. On the other hand an excessively long sound pulse will not remove significantly more frost (if any) than a sound pulse of limited duration. Most of the frost will be removed quickly since the sound pulse will effectively introduce a vibratory energy to the air, the frost and the gas cooling arrangement, which energy will quickly break the frost in pieces and from the gas cooling arrangement. 
         [0027]    The sound pulse may be generated at predetermined time intervals. This is a simple way of controlling the defrosting action. Moreover, if the sound pulse is generated using a sound horn which uses compressed air, this predetermined time interval makes the consumption of compressed air well defined. 
         [0028]    The sound pulse may be generated at intervals based on current amount of frost on said gas cooling arrangement. This way it is possible to generate the sound pulses such that the gas cooling arrangement may perform at its best. This may e.g. be done when the amount of frost reaches a certain level where the gas cooling arrangement otherwise would have a heat exchange effectiveness falling below a desired level. This may e.g. be done when the amount of frost reaches a certain level where the frost is especially effectively affected by the sound pulse used. The current amount of frost may e.g. be determined by analyzing an image of the gas cooling arrangement and/or by measuring the differential pressure over the gas cooling arrangement. 
         [0029]    Preferably said intervals are set to be at most ten times an hour. This is considered an upper limit from a working environment point of view. 
         [0030]    The sound pulse is preferably directed towards said gas cooling arrangement. This way the energy of the sound pulse is effectively transmitted to the air, the frost and the gas cooling arrangement through the whole gas cooling arrangement. 
         [0031]    The sound pulse exhibit preferably a sound pressure level (SPL) at said gas cooling arrangement of 100-160 dB SPL , preferably 120-150 dB SPL . This is a pressure level that will effectively remove frost but will still be at an acceptable level outside the housing of the freezer. 
         [0032]    According to a second aspect of the invention it relates to a freezer system comprised in a housing, comprising: 
         [0033]    a gas cooling arrangement; and 
         [0034]    a sound generator arranged to generate a sound pulse and to subject said gas cooling arrangement to said sound pulse such that an amount of frost formed on said gas cooling arrangement is reduced by removing frost formed on said gas cooling arrangement by means of sound energy in said sound pulse. 
         [0035]    When the gas cooling arrangement is subjected to the sound pulse removing the frost may e.g. be performed by the sound pulse cracking or breaking the frost which then e.g. may fall of the gas cooling arrangement due to the gravity. The sound pulse may e.g. also collapse the boundary layer close to the gas cooling arrangement surface and thereby facilitate the normal airflow of the gaseous medium passing the gas cooling arrangement to blow away frost particles from the gas cooling arrangement surface. This design makes it possible to, in a simple way, make a defrosting action whenever it is considered to be suitable. Once a defrosting action is considered suitable a sound pulse may simply be generated. From an overall design point-of-view, the design with a sound pulse removing the frost is a good solution since the gas cooling arrangement may be designed as a compact unit without having to consider how to provide access to a system with air jet nozzles or the like. This also makes it possible to fit a sound pulse defroster to many of the existing freezers. Moreover, there is no significant risk that the sound pulse damages the gas cooling arrangement. By arranging the sound pulse generator and the gas cooling arrangement in an insulated housing, the sound pulse generator may effectively transmit the sound pulse to the gas cooling arrangement and some of the energy of the sound pulse will be the attenuated by the housing. 
         [0036]    Preferred embodiments appear from the dependent claims. The advantages of the respective features have been discussed above. 
         [0037]    The sound generator preferably comprises a sound horn powered by compressed air. A sound horn is capable of producing a sound pulse with a significant sound energy capable of breaking the frost on the gas cooling arrangement. Moreover, compressed air is commonly used for other purposes in e.g. food production plants, i.e. there is most often already a supply of compressed air. 
         [0038]    According to a third aspect of the invention it relates to a defroster for defrosting a gas cooling arrangement of a freezer, comprising: 
         [0039]    a sound generator arranged to generate a sound pulse and to subject said gas cooling arrangement to said sound pulse such that an amount of frost formed on said gas cooling arrangement is reduced by removing frost formed on said gas cooling arrangement by means of sound energy in said sound pulse. 
         [0040]    When the gas cooling arrangement is subjected to the sound pulse removing the frost may e.g. be performed by the sound pulse cracking or breaking the frost which then e.g. may fall of the gas cooling arrangement due to the gravity. The sound pulse may e.g. also collapse the boundary layer close to the gas cooling arrangement surface and thereby facilitate the normal airflow of the gaseous medium passing the gas cooling arrangement to blow away frost particles from the gas cooling arrangement surface. This design makes it possible to, in a simple way, make a defrosting action whenever it is considered to be suitable. Once a defrosting action is considered suitable a sound pulse may simply be generated. From an overall design point-of-view, the design with a sound pulse removing the frost is a good solution since the gas cooling arrangement may be designed as a compact unit without having to consider how to provide access to a system with air jet nozzles or the like. This also makes it possible to fit a sound pulse defroster to many of the existing freezers. Moreover, there is no significant risk that the sound pulse damages the gas cooling arrangement. 
         [0041]    The method, the freezer and the defroster are especially suitable for embodiments where the freezer is adapted to freeze food products. The defrosting action is controlled and it requires no additional media, such as liquids or other gases. 
         [0042]    The method, the freezer and the defroster are considered especially suitable for embodiments where gaseous medium which passes the gas cooling arrangement and which is adapted to cool the product is air. The sound pulse may e.g. be generated by a sound horn to which compressed air is supplied. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    The invention will by way of example be described in more detail with reference to the appended schematic drawings, which shows presently preferred embodiments of the invention. 
           [0044]      FIG. 1  schematically shows a cross-section of a freezer provided with a defroster according to the invention, the freezer being provided with a product path extending in an essentially straight line through the freezer. 
           [0045]      FIG. 2  schematically shows a cross-section of a freezer provided with a defroster according to the invention, the freezer being provided with a product path extending in a helical path inside the freezer. 
           [0046]      FIG. 3 a    and  FIG. 3 b    schematically illustrate the method according to the invention. In  FIG. 3 a    the cooling of the gaseous medium continues during defrosting and in  FIG. 3 a    the cooling of the gaseous medium is temporarily discontinued during the defrosting of the gas cooling arrangement. 
       
    
    
       [0047]    In the drawings the same or corresponding features will be given the same reference numeral. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0048]    As shown in  FIG. 1 , the freezer is provided with an insulated housing  1 , having walls  1   a ,  1   b , a floor  1   c  and a roof  1   d . In this cross-section the front wall and back wall, which lay generally in the same plane as the paper in front of and behind the cross-section, are not shown. A conveyor  2  extends from an inlet opening in the front wall to an outlet opening in the back wall. Air is circulated inside housing  1  using a fan  3 . The air circulated inside the housing  1  is cooled by a cooling battery comprising two gas cooling arrangements  4   a  and  4   b  (denoted in common as  4 ). In this example the gas cooling arrangements is formed of two evaporators. The flow path of the air is generally indicated by the white arrows. The fan pressurises the chamber directly above the conveyor and the air flows generally downwardly towards the conveyor  2 . After passing the conveyor  2  and thereby cooing the product on the conveyor  2 , the air flows to the sides and then upwardly along the sides towards the two gas cooling arrangements  4   a - b . The air passes the two gas cooling arrangements  4  in an upwardly directed generally vertical flow. After passing the gas cooling arrangements  4 , the air is drawn to the centre by the fan  3 . 
         [0049]    As shown in  FIG. 2 , the freezer of this embodiment is provided with an insulated housing  1 , having walls  1   a ,  1   b , a floor  1   c  and a roof  1   d . In this cross-section the front wall and the back wall, which lay generally in the same plane as the paper in front of and behind the cross-section, are not shown. A conveyor  2  extends from an inlet opening in the wall  1   b  two an outlet opening in the wall  2   b  (as indicated by the grey arrows in the conveyor  2 ), or alternatively the conveyor  2  runs in the opposite direction. As schematically shown in  FIG. 2 , the conveyor  2  extends during a major part of its extension in a helical path forming a stack  2   b . The stack  2   b  may be so-called self-supporting where on turn of the conveyor carries the turn immediately above said turn. The stack  2   b  may also be supported partly or fully by a still standing framework or by a rotating drum. Such different kinds of stacks of helically extending conveyors are well-known in the art. Air is circulated inside housing  1  using a fan  3 . The air circulated inside the housing  1  is cooled by a cooling battery comprising a number of gas cooling arrangements  4 . In this example the gas cooling arrangements is formed of two evaporators. The flow path of the air is generally indicated by the white arrows. The fan  3  draws air upwardly to the upper portion of the stack  2   b . The air passes down through the conveyor  2  and is then drawn to the side and upwardly through the gas cooling arrangement  4 . The air passes the gas cooling arrangement  4  in an upwardly directed generally vertical flow. 
         [0050]    The gas cooling arrangements  4  form part of a cooling system which is well-known in the art. Basically the gas cooling arrangements  4  are formed of a great number of tubes or pathways in which a cooling medium is allowed to be evaporated. The evaporation of the cooling medium inside the tubes or pathways requires heat and this heat is drawn from the air passing the gas cooling arrangement. In order to improve the heat exchange with the air, the tubes are arranged in rows and are provided with or attached to fins, or are formed as pathways between plates. 
         [0051]    Irrespective of the exact configuration, a typical gas cooling arrangement, used for cooling a gaseous medium, such as air, passing the gas cooling arrangement, basically forms a lamellar structure with pathways for the gaseous medium through the gas cooling arrangement. The pathways may be closed or may be in communication with each other along their extension through the gas cooling arrangement. As e.g. air is circulated through the gas cooling arrangement, water vapour in the air will condensate and freeze on the gas cooling arrangement thereby forming frost on the gas cooling arrangement. The frost will e.g. increase the pressure drop over the gas cooling arrangement, e.g. due to a decrease in the available flow area and due to deteriorated flow conditions. 
         [0052]    In the gas cooling arrangements  4   a - b  in  FIG. 1 , there is formed a great number of vertically extending pathways for the air to pass through the gas cooling arrangements  4   a - b  in a vertical flow direction. 
         [0053]    The freezers are also provided with a sound generator  5 . The sound generator  5  is adapted to generate a sound pulse in order to subject the gas cooling arrangement  4  to said sound pulse such that an amount of frost formed on the gas cooling arrangement  4  is reduced by removing frost formed on the gas cooling arrangement  4  by means of sound energy in said sound pulse. The sound generator  5  may e.g. be a so-called sound horn which is driven by compressed air. The sound generator  5  is connected to a pressure vessel  6  which in turn is connected to a compressor or a supply  7  of pressurised air. Between the pressure vessel  6  and the sound generator  5  there is arranged a control valve  8  and a pressure regulator  9 . The exact configuration of the valves/regulators may be varied. It is desired that there is one open-close-function allowing/preventing pressurised air from vessel  6  to reach the sound generator  5 . It is also desired that there is a pressure regulation function securing that air of the correct pressure is provided to the sound generator. This pressure regulation may be performed when pressurising the vessel. The pressure regulation may be performed using a pressure regulator between the vessel and the sound generator. 
         [0054]    The sound generator  5  is adapted to generate a sound pulse with a frequency below 300 Hz. According to one embodiment the frequency is between 10-300 Hz. According to one embodiment the frequency is between 10-250 Hz. According to one embodiment the frequency is between 10-150 Hz. According to one embodiment the frequency is between 50-300 Hz. According to one embodiment the frequency is between 50-200 Hz. According to one embodiment the frequency is between 50-150 Hz. According to one embodiment the frequency is between 150-250 Hz. 
         [0055]    The sound generator is adapted to generate a sound pulse with a duration between 1 and 15 seconds. According to one embodiment the duration is between 1 and 10 seconds. According to one embodiment the duration is between 2 and 15 seconds. According to one embodiment the duration is between 2 and 10 seconds. According to one embodiment the duration is between 1 and 4 seconds. According to one embodiment the duration is between 2 and 4 seconds. According to one embodiment the duration is between 1 and 5 seconds. According to one embodiment the duration is between 1 and 3 seconds. 
         [0056]    The sound generator  5  is adapted to generate a sound pulse at predetermined time intervals. According to one embodiment the sound pulse is generated at intervals based on current amount of frost on said gas cooling arrangement. Current amount of frost may e.g. be determined by analyzing an image of the gas cooling arrangement or by measuring the differential pressure over the gas cooling arrangement. According to a preferred embodiment are the intervals set to be at most ten times an hour. According to one embodiment the total duration of the sound pulses accounts to less than 10% of the total time the freezer is running. 
         [0057]    The sound generator is adapted to generate a sound pulse exhibiting a sound pressure level (SPL) at said gas cooling arrangement of 100-160 dB SPL , preferably 120-150 dB SPL . According to one embodiment the pressure level at the gas cooling arrangement is 100-150 dB SPL . According to one embodiment the pressure level at the gas cooling arrangement is 120-160 dB SPL . 
         [0058]    The sound generator  5  is adapted to generate a sound pulse which is directed towards said gas cooling arrangement  4  (as indicated by the black arrows). The sound generator  5  is arranged separately from the gas cooling arrangement  4  and at a distance from the gas cooling arrangement  4 . The sound pulse will travel in the air from the sound generator towards the gas cooling arrangement. In accordance with one embodiment the distance is at least 0.5 m. In accordance with one embodiment the distance is at least 1 m. The distance between the sound generator  5  and the gas cooling arrangement  4  allows the sound pulse to spread out forming a more uniform air pulse passing through the gas cooling arrangement  4 . 
         [0059]    In the disclosed embodiments the sound generator  5  is adapted to generate a sound pulse directed towards the gas cooling arrangements  4 . In the disclosed embodiments the sound generator  5  is adapted to generate a sound pulse travelling in an upwardly generally vertical direction. This direction is the same as the air is travelling through the gas cooling arrangement  4 . According to one embodiment it may be said that the sound generator  5  is adapted to generate a sound pulse travelling in the same direction through the gas cooling arrangement  4  as the direction in which the gaseous medium to be cooled passes through the gas cooling arrangement. If this direction is vertically upward, one advantage is that the pathways allowing the upward flow of air will most likely also allow frost to fall downwardly due to the gravity. 
         [0060]    In a preferred embodiment, the sound generators  5  are of the kind being essentially funnel shaped and being provided with a sound generating membrane  10 . In a preferred embodiment, as shown in the figures, the sound generators  5  are adapted to be placed such that the membrane  10  of respective sound generator  5  is placed on the outside of the housing  1   d  and such that the funnel shaped part of respective sound generator extends into the housing. This minimizes the risk of the sound generator being subjected to frost deposition.