Abstract:
A through-flow freezing cylinder is disclosed comprising on its outer surface a set of substantially parallel tangential cooling fins and a set of substantially parallel grooves intersecting the set of cooling fins, preferably at a right angle.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a freezing cylinder for use, for instance, in an apparatus for the production of frozen ice cream. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the production of edible ice cream products, it is well-known to use so-called through-flow freezers. Often, such freezers comprise a freezing cylinder through which the ice cream mass is transported, typically by pumping or by means of a conveyor screw. The freezing cylinder is being cooled from its outside using a liquid coolant such as, for instance, ammonia (NH 3 ), carbon dioxide (CO 2 ) or some sort of Freon, such as R404a. During the freezing process, the coolant absorbs heat energy from the outer cylinder surface mainly due to a phase-shift from liquid to gaseous phase of the coolant, whereby tiny gas bubbles containing evaporated coolant are formed on the outer cylinder surface. 
         [0003]    The cooling efficiency of such a system depends on a number of factors. One important factor is the area of the cylinder surface to be cooled, which can be increased by well-known means, such as the use of cooling fins extending from the surface. However, also other factors affecting the evaporation rate, the size of the bubbles and the rate at which the bubbles leave the cylinder surface through the still liquid coolant are important for the efficiency of the cooling system. This is due to the fact that bubbles staying at the cylinder surface form an insulating layer between the cylinder surface and the liquid coolant, thus reducing the transportation of heat energy from the cylinder surface to the liquid coolant. The relation is relatively straightforward: The more the bubbles adhere to the surface of the freezing cylinder, the more the surface is covered with insulating air bubbles, and the more the heat transfer coefficient is reduced. These properties relating to the formation and the removal of the gas bubbles depend mainly on the properties of the liquid coolant and on the structure of the cylinder surface. 
         [0004]    The surface structure may be improved for a better cooling efficiency by means of so-called nano coating. However, this is a very expensive way of obtaining a better cooling efficiency. 
         [0005]    EP 0670461 A1 discloses a freezing drum, in which a cylindrical freezing chamber is coaxially positioned inside a cylindrical external jacket, wherein a spiral-shaped path is provided in the hollow space between the jacket and the chamber, through which path a thermal exchange fluid circulates from an inlet to an outlet. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a more cost-efficient way of optimising the surface of a freezing cylinder for obtaining a significantly better cooling efficiency of a through-flow freezer comprising such a freezing cylinder. 
         [0007]    The present invention relates to a through-flow freezing cylinder comprising on its outer surface a set of substantially parallel tangential cooling fins and a set of substantially parallel grooves intersecting the set of cooling fins. 
         [0008]    Contrary to the disclosure of EP 0670461 A1 as described above, the use of the present invention does not involve a thermal exchange fluid circulating along a predefined path in a hollow space between a freezing chamber and an external jacket. Rather, a thermal exchange fluid in the form of a liquid coolant must simply be placed to be in contact with the outer surface of the freezing cylinder as described further below. 
         [0009]    This means that there are no requirements relating to the shape of an external jacket, in which the freezing cylinder may be placed or to the positioning of the freezing cylinder in relation to such a jacket. 
         [0010]    It has surprisingly been found that by structuring the outer surface of the freezing cylinder with a set of parallel grooves intersecting a set of parallel cooling fins, the freezing efficiency of a through-flow freezer comprising the freezing cylinder can be increased by up to about 20% without increasing the size of the freezer. 
         [0011]    In an embodiment of the invention, the grooves intersect the cooling fins at an angle of more than 30°, preferably at an angle of more than 60°, most preferably at a substantially right angle. 
         [0012]    It has been found that the best results are obtained if the angle of intersection between the grooves and the cooling fins is not very acute. 
         [0013]    In a preferred embodiment of the invention, the cooling fins and the grooves are oriented tangentially and axially, respectively, in relation to the freezing cylinder. 
         [0014]    For production purposes, this configuration of the cooling fins and the grooves requires a relatively simple set-up of tools, for instance for machining the grooves. 
         [0015]    In an embodiment of the invention, the cooling fins have a substantially triangular cross-sectional shape. 
         [0016]    In an embodiment of the invention, the maximum height of the cooling fins is between 0.5 mm and 10 mm, preferably between 1 mm and 5 mm, most preferably between 2.5 mm and 3.5 mm. 
         [0017]    In an embodiment of the invention, the mutual distance between the centrelines of two neighbouring cooling fins is between 0.5 mm and 10 mm, preferably between 1 mm and 5 mm, most preferably between 2.5 mm and 3.5 mm. 
         [0018]    In an embodiment of the invention, the maximum width of the cooling fins is between 0.5 mm and 10 mm, preferably between 1 mm and 5 mm, most preferably between 2.5 mm and 3.5 mm. 
         [0019]    In an embodiment of the invention, the mutual distance and the maximum width of the cooling fins are substantially the same. 
         [0020]    In an embodiment of the invention, the grooves have a substantially rectangular cross-sectional shape. 
         [0021]    In an embodiment of the invention, the mutual distance between two neighbouring grooves is between 1 mm and 12 mm, preferably between 2 mm and 10 mm, most preferably between 5 mm and 7 mm. 
         [0022]    In an embodiment of the invention, the maximum width of the grooves is between 0.2 mm and 4 mm, preferably between 0.5 mm and 2 mm, most preferably between 0.8 mm and 1.2 mm. 
         [0023]    In an embodiment of the invention, the maximum depth of the grooves is between 0.2 mm and 6 mm, preferably between 0.5 mm and 6 mm, most preferably between 2 mm and 3 mm. 
         [0024]    It has been shown that the optimum freezing efficiency is obtained by choosing the properties of the cooling fins and the grooves within the above-specified ranges. 
         [0025]    In an embodiment of the invention, the grooves are machined. 
         [0026]    Machining the grooves (and often also the cooling fins) rather than casting a complete freezing cylinder with cooling fins and intersecting grooves is advantageous in that it results in a relatively rough surface containing numerous tiny burrs, which have shown also to contribute to the optimisation of the cooling efficiency of the system. 
         [0027]    In preferred embodiments of the invention, the through-flow freezing cylinder is made from nickel, brass, stainless steel or black steel. 
         [0028]    Making the freezing cylinder from nickel is advantageous in that nickel has high specific heat conductivity. 
         [0029]    If brass is in contact with ammonia, which is one of the preferred cooling media in through-flow freezers of the type comprising freezing cylinders like the one of the present invention, the ammonia will react with the copper in the brass alloy, which can destroy the structure made from the brass. However, if another cooling media, such as carbon dioxide or Freon, is to be used, the freezing cylinder may advantageously be made from brass, which is substantially less expensive than nickel. Like nickel, brass has high specific heat conductivity. 
         [0030]    The freezing cylinder may also be made from stainless steel, which is less expensive than nickel and brass. However, the use of stainless steel results in a lower freezing capacity due to the relatively poor heat transfer capabilities of this material. 
         [0031]    Due to the low price of black steel, it is also considered to make the freezing cylinder from this material. This, however, requires a coating of the inner side of the cylinder wall for hygienic reasons. 
         [0032]    In some embodiments of the invention, the freezing cylinder will be coated on the inner side of its wall with a layer of hard chrome or the like in order to make it more wear-resistant. 
     
    
     
       FIGURES 
         [0033]    A few exemplary embodiments of the invention will be described in the following with reference to the figures, of which 
           [0034]      FIG. 1  illustrates schematically a cross-section of a through-flow freezer comprising a freezing cylinder according to an embodiment of the invention and 
           [0035]      FIG. 2  illustrates schematically the structure of an outer surface of a freezing cylinder according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]      FIG. 1  illustrates schematically a cross-section of a through-flow freezer  1  in which a freezing cylinder  3  according to an embodiment of the invention is placed inside an outer cylinder  2 . The distance between the walls of the two cylinders  2 ,  3 , respectively, will often be relatively small, such as between 10 mm and 40 mm. 
         [0037]    The outer surface of the freezing cylinder  3  is provided with a number of tangentially oriented cooling fins  4 , which are intersected by a number of longitudinally oriented grooves  5 . The mass  6  to be frozen, for instance edible ice cream mass  6 , is transported through the freezing cylinder  3 . Inside the freezing cylinder  3 , one or more scrapers (not shown) scrape the layer of frozen ice cream mass  6  continuously formed on the inner surface of the freezing cylinder  3  off the cylinder wall. 
         [0038]    A liquid coolant  7  is placed in the cavity between the inner surface of the outer cylinder  2  and the outer surface of the freezing cylinder  3  for cooling the freezing cylinder  3  and its contents  6  by absorbing heat energy from the freezing cylinder  3 , mainly due to a phase-shift from liquid to gas phase of some of the coolant  7 . 
         [0039]    The freezing cylinder  3  is typically made from nickel, brass, stainless steel or black steel depending on the type of coolant  7  with which it is meant to be used. 
         [0040]    Brass is an alloy of copper and zinc. Ammonia reacts with copper resulting in the formation of cracks in brass structures in contact with ammonia. Eventually, such structures may collapse. Therefore, the use of ammonia as coolant  7  generally requires that the freezing cylinder  3  is made from nickel or steel. Brass, which is less expensive than nickel and still has high specific heat conductivity, can be used if another coolant  7  than ammonia such as, for instance, carbon dioxide or Freon is to be used. Generally, the cooling effect obtainable with carbon dioxide is about 25% higher than the one obtainable with ammonia. However, the pressure required is also higher for carbon dioxide than for ammonia. 
         [0041]    The structure of the outer surface of the freezing cylinder  3  is important for the adhesion of the gas bubbles to the freezing cylinder  3  surface and, thus, for the cooling efficiency of the through-flow freezer  1 .  FIG. 2  illustrates schematically the structure of an outer surface of a freezing cylinder  3  according to an embodiment of the invention in which a plurality of parallel and tangentially oriented cooling fins  4  are intersected by a number of parallel and longitudinally oriented grooves  5 . 
         [0042]    Numerous investigations have revealed that the cooling efficiency can be optimized by choosing the height of the cooling fins  4  to be about 3 mm, the width and depth of the intersecting grooves  5  to be approximately 1 mm and 2.5 mm, respectively, and the mutual distance between two neighbouring grooves  5  to be between 5 mm and 7 mm. 
         [0043]    Optimisation investigations relating to the height of the cooling fins  4  alone, i.e. without using any intersecting grooves  5 , have shown to result in an increase of the cooling efficiency and, thus, of the freezer capacity of up to about 5% without increasing the size of the freezer  1 . 
         [0044]    However, the addition and optimisation of intersecting grooves  5  surprisingly turned out to lead to more significant increases of the cooling efficiency, namely up to about 20% as compared to through-flow freezers known from the art using freezing cylinders having cooling fins of a height of approximately 1.5 mm. 
         [0045]    The investigations have shown that similar increases in evaporation and, thus, in cooling efficiency are obtained, whether the coolant  7  used is ammonia, carbon dioxide or nitrogen, and whether the freezing cylinder  3  is made from nickel, brass or stainless steel. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1 . Through-flow freezer 
           2 . Outer cylinder 
           3 . Freezing cylinder 
           4 . Cooling fin 
           5 . Groove 
           6 . Mass to be frozen 
           7 . Liquid coolant