Patent Publication Number: US-6907744-B2

Title: Ice-making machine with improved water curtain

Description:
REFERENCE TO EARLIER FILED APPLICATION 
   The present application claims the benefit of the filing date under 35 U.S.C. §119(e) of Provisional U.S. patent application Ser. No. 60/365,435, filed Mar. 18, 2002, which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to ice-making machines, and particularly to cube ice-making machines that have a vertical ice-forming mold and a water curtain to direct water cascading down the surface of the ice-forming mold back into a water sump. 
   Automatic ice-making machines have become widely used to make ice on the premises where it is used. The food service industry in particular uses such ice-making machines. For example, a restaurant needs ice to put in drinks served as part of a meal. In addition, ice is often used to cool food items. In a beverage dispenser, ice may be dispensed into a cup and may also be used to cool a cold pate that in turn cools beverage components that are dispensed and mixed in valves mounted on the dispenser. 
   The demand for ice at many eating establishments is hardly constant. Instead, demand peaks at meal times. Most ice-making machines are therefore mounted on ice collecting and storage bins. The ice machine can then run constantly and build up a reserve of ice. Often the reserve is built up over night, and ice machines are purchased by their size, based on the expected total daily demand for ice. 
   Cube ice makers and flake ice makers are both in common usage. However, cube ice is generally more desirable for cooling beverages. A common design for a cube ice-making machine includes a vertical ice-forming mold. The mold has dividers that create individual pockets. When the pockets are sufficiently filled with ice, the control system for the machine switches into a harvest cycle. The ice cubes are released from the mold. The dividers may be sloped downward toward the open front so that the ice cubes slide out of the ice-forming mold under the influence of gravity, and into the ice collection bin. 
   The ice-making machine also includes a sump located beneath the ice-forming mold, a water distributor above the ice-forming mold, and a pump to pump water from the sump up to the distributor. The water cascades down over the surface of the ice-forming mold. A part of the water freezes into the pockets and the rest runs off the surface of the ice-forming mold. A water curtain is placed adjacent to the ice-forming mold so that any splashing water is directed back into the sump. The bottom edge of the water curtain is bent to reach back under the ice-forming mold. This allows the front edge of the sump to be spaced behind the front of the ice-forming mold. With this design, the unfrozen water can return to the sump, but ice can fall straight down out of the ice-forming mold and into the collection bin. 
   The water curtain is typically suspended from pivots or hinges located near the top of the water curtain. The shape of the water curtain and location of the pivots are such that the center of gravity of the water curtain causes the sides of the water curtain to stay closed against the ice-forming mold frame while the machine is making ice. However, during the harvest cycle the water curtain can swing away as the ice is released from the ice-forming mold. 
   A thin bridge of ice forms over the dividers and between the individual cubes of ice. Most automatic ice-making machines allow for adjustment of the duration of the freeze cycle, which thus controls how thick this ice bridge becomes. A common control technique is to mount an ice thickness sensor so that as the ice bridge gets thicker, water running over the surface of it will contact a probe, directing the machine to automatically go into a harvest cycle. A thick ice bridge has the benefit that it helps in the harvest cycle, when water stops cascading over the front of the ice and the ice-forming mold is heated. A thick ice bridge allows the entire slab of interconnected ice cubes to be released at once. On the other hand, with a thin ice bridge, individual cubes have to each melt and drop out of their pockets, and adjoining cubes cannot help pull all of the ice out at once. 
   While thicker ice bridges have some benefits, there are also some drawbacks. Because ice is an insulator, the efficiency of the freezing operation decreases as the ice bridge builds, since the heat is commonly transferred out of the back of the ice-forming mold by serpentine refrigerant coils forming the evaporator section of a refrigeration system. Most importantly, many end users do not want thick ice bridges, because the slabs of ice cubes do not break into individual cubes as easily, and chunks of ice cubes frozen together are hard to dispense, scoop or fit into a cup. 
   One other common feature found in automatic ice-making machines is that they are designed to automatically shut down when the ice collecting bin is full. These automatic ice machines then operate around the clock, unattended. The result is hopefully a bin that is constantly full of ice, but not a machine that keeps making and harvesting ice when the bin is already full. 
   A common technique for shutting down the ice-making machines when the bin is full is to place a sensor, such as a magnetic reed switch, near the water curtain, and put a magnet on the water curtain. The reed switch can then determine whether the water curtain is closed. This reed switch has two uses. First, when the water curtain closes, the machine can automatically switch back into an ice-making mode from a harvesting mode. Second, if ice has built up in the bin such that the slab of ice being harvested does not fall all of the way past the bottom edge of the water curtain, the water curtain will remain open, and the reed switch will not close until ice no longer holds the water curtain open. 
   It is desirable that the compressor of the refrigerator system not stop and start every time ice is harvested. Therefore, it is typical to let the compressor continue to run unless the water curtain remains open for a set period of time. In prior art ice-making machines, this period of time was often set at 7 seconds. Normally, the water curtain would open and close in much shorter than 7 seconds during a typical harvest cycle. However, if the bin is full, the ice cannot fall out of the way and ice remains in the way of the water curtain to keep it from closing for more than 7 seconds. In this situation, the machine would sense a “bin-full” condition and shut down until the water curtain closed again, which could happen if the ice in the bin was removed or if it melted to the point that the pile of ice no longer supported the ice holding the water curtain open. 
   There have been instances when a false “bin-full” signal is generated and the ice machine shuts off even if the ice bin is not full. If it stays off for a prolonged period of time, it seriously reduces the amount of ice produced by the machine. Some end users have a high demand for ice, and when an ice machine shuts down without the bin being full, the user quickly calls and reports a malfunction. What may be worse, if the machine has shut down and no one notices it, such as over night, employees came in to work expecting to have a ready supply of ice and find the bin only partially full. By then, the machine may be running again, and the cause of the problem is therefore not discernible. However, in these instances, the end user again is not happy with the equipment. Therefore, there is a strong desire to prevent a false “bin-full” shutdown on the part of ice machine manufacturers. 
   Solutions to the problem have been tried in the lab, but when the solution was implemented in the field, the problem reoccurred. One piece of equipment that was prone to this problem was brought back from the field to be studied. It was determined that for some reason, cubes of ice were getting caught between the bottom edge of the water curtain and either the bottom of the ice-forming mold or the top edge of the water sump. This ice would hold the water curtain open until it melted sufficiently that it dislodged itself. Even with the ice machine being shut down, this might take several hours because the bin and ice machine are insulated and the ice in the bin keeps the temperature inside the ice-making compartment relatively cool. Of course, this situation did not occur on every harvest cycle, but it did happen frequently enough that it was a serious problem for the end user. Most ice-making machines of this same model were not being complained about. However, the number of complaints with respect to this model of machine, which had two evaporators and two ice-forming molds (a dual evaporator machine), was higher than with other models of machines. 
   Unfortunately, dual evaporator machines are often put into establishments where there is a high demand for ice because these machines can generally make twice as much ice per day as a comparably-sized single-evaporator machine. Thus, in the very place where it is least desirable to have a machine with a false “bin full” condition, machines most prone to this problem were being installed. Also, because both evaporators are on the same refrigeration system, if the problem occurred with either ice-forming mold, both evaporators quit freezing ice. As a result, there was an urgent need to find a solution to the problem, preferably one that could be used to retrofit ice machines already manufactured that exhibited this problem. 
   Another problem that has been encountered arises from the fact that most water curtains are fairly large pieces of plastic. They can be assembled by gluing smaller pieces together, but most economically they are made using vacuum forming or injection molding techniques. With such large sheets of plastic it is very difficult to get them perfectly flat once they have cooled after being molded. If the water curtain is not flat, one side may not be as close to the ice-forming mold as the other. This is known as “racking,” and can let water spray come out of one side. Also, if the water curtain is racked, the slab of ice may hit one side of the water curtain first, causing a high load on the hinge pin on that side and premature failure of that hinge pin. Further, if the ice curtain is twisted too much, the side with the magnet used to operate the reed switch may be open even though the other side is closed, shutting down the machine; or it could be the other way around, with the side having the magnet being closed even though the other side of the water curtain is open because the ice bin is full on that side. Ice then continues to be made even though the bin is full on one side, and water going down the face of the ice-forming mold may fall into the bin rather than being directed by the bottom of the water curtain back into the sump, resulting in either water falling on and freezing the cubes of ice in the bin together, or wet ice in the bin. Also, for those machines that do not add water during the freeze cycle, and go into harvest when the water level drops to a predetermined point, the loss of water will result in less ice being made in each cycle. 
   BRIEF SUMMARY OF THE INVENTION 
   After investigation, it appeared that the ice slab on the dual evaporator machine seemed to release more quickly from the top of the ice-forming mold than from the bottom. Also, the problem seemed to occur mainly because the ice slab was not remaining intact as it fell into the ice bin. This seemed to happen either because the ice slab, which first hit against the bottom side of the water curtain as it fell, broke apart on this impact, or because the slab hitting the angled bottom side of the water curtain caused the water curtain to rapidly swing away. When it swung back, it would hit the slab of ice that was still falling and break the slab apart. Fragments of the broken slab would then get turned sideways and caught between the bottom of the water curtain and either the sump or some other portion of the ice machine. Especially in the dual evaporator machines, the space for the water curtain to swing open was quite limited, and often the curtain would strike an object inside the compartment and rebound into the falling slab of ice. Even in other machines where the ice slab comes out from the top and the bottom at the same time, it has been formed that the false “bin full” signal sometimes occurs because the slab is breaking apart before clearing the bottom of the water curtain. Again, this could be due to the slab hitting the bottom side of the curtain as it falls, either breaking the slab or causing the curtain to rapidly swing away and rebound back into the falling slab. 
   Many different solutions were suggested, many of which were not very practical, or were expensive, or could not be used in a retrofit situation. One suggestion was to add two rounded sections to the intersection between the inside surface and bottom side of the water curtain, in an effort to make the slab&#39;s contact with the water curtain less violent. While this made some improvement, there was another improvement that was found to reduce the problem of false bin full shutdown as to be nearly non-existent. It was found that by configuring the inside surface of the water curtain so that the ice slab contacted the curtain and pushed it open from near the top, or at least near a mid-portion of the water curtain, the bottom edge of the water curtain could be moved completely out of the way before the ice slab fell, so that the bottom of the slab did not contact the bottom edge of the water curtain. In its most preferred form, this was done by adding two ribs of the correct shape and dimensions to the inside surface of the water curtain. 
   Further, it has been discovered that by adding other structural elements on the face of the water curtain between the ribs that tie into the side walls of the ribs, the rigidity of the water curtain was greatly improved. This prevents the water curtain from racking. The increased rigidity helps the water curtain to remain square to the ice forming mold, preventing splash out on one side, and the problem of the reed switch being closed even when the ice bin is full on one side, thus reducing the problems caused by racking. Further, with the increased rigidity, even if the falling ice strikes one side of the water curtain first, the load is more uniformly carried between the hinge pins. 
   Thus, in a first aspect, the invention is an improved ice-making machine having a substantially vertical ice-forming mold for freezing cubes of ice, a water distributor for distributing water so as to cascade over a front surface of the ice-forming mold and a hinged water curtain with a bottom edge for directing the cascading water into a sump, with the hinge allowing the water curtain to swing out of the way so that a slab of ice cubes harvested from the mold may fall past the sump and into an ice collecting bin. The improvement comes from providing the water curtain with an inside surface (adjacent to a front surface of the ice-forming mold) configured such that as the slab of ice cubes is released from the mold during a harvest cycle, ice in the slab contacts the inside surface and gently forces the water curtain to open to a point where the bottom of the falling slab of ice cubes will not contact the bottom edge of the water curtain. 
   In a second aspect, the invention is a water curtain for an ice-making machine comprising one or more ribs formed on the water curtain so as to contact a slab of ice as it is released from an ice-forming mold, the ribs having a sufficient height so that when the water curtain is in place on an ice-making machine, the slab of ice will contact the ribs to force the water curtain out away from the ice-forming mold so that the bottom edge of the water curtain is not underneath the slab of ice. 
   In a third aspect, the invention is an ice-making machine comprising: a) a water system including a pump, a sump, a substantially vertical ice-forming mold having a back surface and an open front surface to form a slab of ice, a distributor for distributing water pumped from the sump over the front surface of the ice-forming mold, and a water line interconnecting the pump and the distributor; b) a refrigeration system comprising a compressor, a condenser, an expansion device, an evaporator with refrigerant channels formed in a serpentine shape in thermal contact with the back surface of the ice-forming mold, and interconnecting lines therefore; and c) a water curtain having a bottom edge, the water curtain being positioned adjacent the front surface of the ice-forming mold so as to direct water cascading over the open front surface of the ice-forming mold into the sump, the water curtain being hinged so as to swing away from the ice-forming mold during harvest of an ice slab from the ice-forming mold, the water curtain having an inside surface configured so as to contact the slab of ice as it is released from the mold, the position of the hinge and the configuration of the inside surface cooperating so as to cause the bottom edge of the water curtain to gently swing out to a point where the bottom of the slab of ice can pass between the sump and the bottom edge of the water curtain without striking the bottom edge of the water curtain as it falls. 
   In a fourth aspect, the invention is a method of producing and harvesting ice from a cube ice-making machine into an ice collecting bin so as to reduce the chance that the ice-making machine will shut down before the ice bin is full, comprising the steps of: a) forming cubes of ice from water cascading down over a substantially vertical ice-forming mold having a back surface, an open front surface and lateral and vertical dividers that form individual pockets in which individual cubes are frozen, with an ice bridge formed between cubes of ice and over the dividers on the open front surface of the ice-forming mold to constitute a slab of ice cubes, with water not being frozen being directed into a water sump by a bottom edge of a water curtain, the water curtain being adjacent the open front surface of the ice-forming mold; b) halting the flow of water and heating the ice-forming mold in a harvest cycle to release the slab of frozen ice cubes, and c) using the upper portion of the slab of cubes to contact ribs on the inside surface of the water curtain to push the bottom edge of the water curtain away from the water sump enough so that the slab of ice released can fall past the bottom edge of the water curtain without the bottom of the slab contacting the bottom side of the water curtain. 
   In a fifth aspect, the invention is an improved water curtain for an ice-making machine that can be mounted so as to have an inside surface that catches splashes of water flowing over an ice forming mold and directs the water into a sump, the improvement comprising a pair of generally vertical ribs on the inside surface of the water curtain and an additional structure also on the inside surface extending between and tying the ribs together. 
   The ribs that were added to the inside surface of the water curtain preferably start at a point midway down in the top half of the water curtain and have a downwardly inclined top surface, with a generous radius connecting this top surface to the remainder of the rib. Because the curtain is held open from the top, the ice slab, even with a fairly thin ice bridge, can drop into the ice bin without interference and being broken by the water curtain. The control system of the ice-making machine is also preferably modified to increase the set period of how long the water curtain may be open before the machine shuts down, because with the ribs the water curtain begins to open as the ice first starts to release, and frequently the ice curtain is open for longer than 7 seconds during a normal harvest cycle. Downwardly pointing chevrons or other structure molded into the inside surface of the water curtain and extending between the ribs ties the ribs together and adds rigidity to the water curtain to prevent racking. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These benefits of the invention, and the invention itself, will be best understood in view of the attached drawings, in which: 
       FIG. 1  is a perspective view of a preferred ice-making machine in accordance with the present invention, with its front face and top panel removed. 
       FIG. 2  is a front elevational view of the water curtain as it was designed prior to the modification of the present invention. 
       FIG. 3  is a side elevational view of the water curtain of FIG.  2 . 
       FIG. 4  is a side elevational view of a water curtain used on a different prior art ice-making machine. 
       FIG. 5  is a front elevational view of the preferred water curtain used on the ice-making machine of FIG.  1 . 
       FIG. 6  is a side elevational view of the water curtain of FIG.  5 . 
       FIG. 7  is a top plan view of the water curtain of FIG.  5 . 
       FIG. 8  is a cross-sectional view taken along line  8 — 8  of FIG.  5 . 
       FIG. 9  is a cross-sectional view of the ice-making machine of  FIG. 1  showing a slab of ice being harvested, with the ice machine sitting on an ice collecting bin. 
       FIG. 10  is a schematic diagram of the refrigeration system used with the ice-making machine of FIG.  1 . 
       FIG. 11  is a front elevational view of a second preferred water curtain of the present invention. 
       FIG. 12  is a cross-sectional view of a second ice making machine showing a slab of ice being harvested and a cross-sectional view of the water curtain taken along line  12 — 12  of FIG.  11 . 
   

   DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS OF THE INVENTION 
   While the present invention was developed initially for a dual evaporator ice-making machine, it has application on other ice-making machines having substantially vertical ice-forming molds. Especially as such machines are made more compactly, the distance that the water curtain has to swing out of the way in a harvest mode will become more limited. The preferred embodiment of the invention as discussed herein is applied to a dual evaporator cube ice-making machine based on the Model QYDUAL4C ice-making machine sold by Manitowoc Ice, Inc. of Manitowoc, Wis. Since many of the aspects of that machine are not changed in making the present invention, they are not discussed further herein. Suffice it to say, the Model QYDUAL4C ice-making machine uses cool vapor defrost technology, disclosed in U.S. Pat. No. 6,196,007, incorporated herein by reference. The ice-making machine has a refrigeration system  300  shown in FIG.  10 . The system is housed in two separate units, a condensing unit  306  and an ice-making unit  308 , usually separated by a roof  304  of a building. The refrigeration system  300  includes a compressor  312 , a condenser  314 , an expansion device  326  and an evaporator  328 . In the dual evaporator machine shown, there are two expansion devices,  326   a  and  326   b , and two evaporators,  328   a  and  328   b . The evaporators each have refrigerant channels in the form of tubing  26  ( FIG. 9 ) formed in a serpentine shape in thermal contact with the back surface of the ice-forming mold, and interconnecting lines. 
   In the preferred embodiment, the condensing unit  306  also includes refrigerant line  313  between the compressor  312  and the condenser  314 , another refrigerant line  315  correcting a head pressure control valve  316  to the condenser  314 , and a bypass line  317  between the head pressure control valve  316  and the compressor  312 . The preferred condensing unit  306  also includes an accumulator  332  including a J-tube  335 , a fan cycling control  352 , a high pressure cut out control  354  and a low pressure cut out control  356 . 
   In the ice making unit  308 , two thermal expansion valves  326   a  and  326   b  are used, feeding liquid refrigerant through lines  323   a  and  323   b  to evaporators  328   a  and  328   b , respectively. Each is equipped with its own capillary tube and sensing bulb  329   a  and  329   b . Likewise, two solenoid valves  336   a  and  336   b  are used to control the flow of cool vapor to evaporators  328   a  and  328   b  through lines  333   a  and  333   b . This allows the two evaporators to each operate at maximum efficiency, and freeze ice at their own independent rate. Of course it is possible to use one thermal expansion valve, but then, because it would be very difficult to balance the demand for refrigerant in each evaporator, one evaporator (the lagging evaporator) would not be full when it was time to harvest the other evaporator. 
   Having two separate solenoid valves  336   a  and  336   b  allows one valve to be closed once ice has been harvested from the associated evaporator. When it is time to harvest, solenoid valves  336   a  and  336   b  will open, and cool vapor from receiver  318  will be permitted to flow into lines  333   a  and  333   b  and into evaporators  328   a  and  328   b . Both evaporators go into harvest at the same time. However, once ice falls from evaporator  328   a , the valve  336   a  will shut, and evaporator  328   a  will be idle while evaporator  328   b  finishes harvesting. With valve  336   a  shut, cool vapor is not wasted in further heating evaporator  328   a , but rather is all used to defrost evaporator  328   b . Of course, the reverse is also true if evaporator  328   b  harvests first. 
   The preferred ice making unit also includes a check valve  358  on liquid line  319  into receiver  318 . The receiver includes inlet  320 , liquid outlet  322  and vapor outlet  334 . A hand shut off valve  360  may be included, along with drier  324 , liquid line solenoid valve  362  and refrigerant lines  321 ,  325  and  331 . 
   The ice making unit  308  also houses the water system of the ice-making machine. As shown in  FIGS. 1 and 9 , this includes a pump  20 , and two (labeled a and b) of each of the following: a sump  30 , a distributor  32 , and an ice-forming mold  40 . A water line  34  interconnects the pump  20  with each of the distributors  32   a  and  32   b . The distributors  32  distribute water pumped from the sump over the front surfaces of the ice-forming molds  40   a  and  40   b.    
   The ice-forming molds  40  are substantially vertical in their preferred orientation. The preferred molds are made from copper pans with a flat back surface and an open front surface. The tubing coils  26  of the refrigeration system are soldered to the back surface, as seen in FIG.  9 . The molds  40  also preferably contain lateral and vertical dividers that form individual pockets (not shown). The lateral dividers  46  are preferably sloped downwardly so that the slab of ice slides out of the pockets under the influence of gravity. 
   The water curtains  50   a  and  50   b  are shown closed in FIG.  1  and on the right half of FIG.  9 . In this position each water curtain  50  directs water cascading over the open front face of the ice-forming molds into its respective sump  30 . As shown in  FIGS. 5-8 , the water curtain  50  includes side portions  56  with side edges  58  that are designed to contact the frame on the sides of the ice-forming mold  40 . These keep the water from splashing out the sides. The water curtain  50  has a bottom side  52  that bends back under the ice-forming mold and terminates in an edge  54  that extends under the ice-forming mold and directs the cascading water into the sump  30 . The bottom side  52  is sloped downwardly. 
   The water curtain  50   a  is shown open in the left half of  FIG. 9 , with a slab of ice cubes  48  being harvested and opening the water curtain according to the present invention. This is possible because the water curtains  50  are hinged to swing about an axis near the top of the water curtain. This is preferably accomplished by molding a hole in the top of each of the sides  56  of the water curtains and mounting a pin  59  in the holes (FIG.  8 ). These pins  59  fit into holes  16  ( FIG. 9 ) in tabs extending from the frame surrounding the ice-forming molds  40 . This hinge arrangement allows the water curtain  50  to swing away from the ice-forming mold  40  during the harvest cycle so that the ice cubes harvested from the mold may fall past the sump  30  and into ice collecting bin  14  through openings in the bottom of the ice making unit and the top of the ice collecting bin  14 . 
   What has been described so far is also applicable to the water curtain  70  ( FIGS. 2 and 3 ) as originally designed for and used on the Model QYDUAL4C ice making machine, as well as on other water curtains used for many years on other ice making machines, such as the water curtain  80  ( FIG. 4 ) used on the Model QY1304A Manitowoc ice making machine. These prior art water curtains  70  and  80  also had a figure X design  72  molded into the plastic making up the main surface of the water curtain, to add rigidity and strength, as is well known in the art of plastic molding. However, other than the figure X design  72 , the inside surface of the water curtain  70  was generally flat. Up at its top, the water curtain  70  was molded with a section  74  permitting room for the ice thickness sensor that sits at the top of the ice-forming mold. This sensor includes a probe that is contacted by the water when the ice cubes freezing in the pockets and the bridge between cubes and over the dividers has grown to the desired thickness. 
   As can be imagined from looking at  FIG. 3 , when the slab of ice was harvested, it would slide out of the ice-forming mold and fall down until it hit the bottom side  76  of the water curtain  70 . This bottom side  76  was sloped. As the ice slab hit the bottom, the impact would force the water curtain to swing outwardly. As noted above, however, unfortunately many times the force of the impact would also cause the ice slab to break up. The slab of ice was designed to be broken up as it entered the ice bin as it would strike deflectors  18  ( FIGS. 1 and 9 ) suspended from the bottom of the ice making unit in the upper portion of the ice collecting bin  14 . 
   Also shown in  FIGS. 2 and 3  is the magnet  78  that is used to detect when the water curtain is open. A similar magnet  88  ( FIGS. 5 and 6 ) is found on the improved water curtain  50 , to perform the same function. 
   The main difference between the prior art water curtain  70  and the improved water curtain  50  is the addition of two ribs  60  on the inside surface of the improved water curtain  50 . The profile for these ribs is best seen in FIG.  8 . The figure X design  62  is also reduced in size to fit between the ribs  60 . The section  64  is still provided at the top to give clearance for the ice thickness sensor. 
   With the addition of the ribs  60 , the inside surface of the water curtain  50  is configured such that as a slab of ice cubes  48  is released from the ice-forming mold  40  during a harvest cycle, ice in the slab contacts the inside surface and forces the water curtain to open to a point ( FIG. 9 , left side) where the bottom edge  54  of the water curtain will not contact the bottom of the falling slab of ice cubes  48 . The position of the pin  59  forming the hinge axis and the configuration of the inside surface cooperate to cause the water curtain to open so that the bottom edge of the water curtain is out of the way and the slab of ice can pass between the sump  30  and the bottom edge  54  of the water curtain  50 . 
   While two ribs  60  are depicted, the number of ribs may be varied, and the size may be varied as well, so long as they provide the inside surface of the water curtain with sufficient points of contact. If one rib were used, it would preferably be placed in the center of the water curtain  50 . The benefit of using two ribs  60  is that they can be spaced toward the outer sides portion  56  of the water curtain and provide spaced points of contact. Although narrower ribs will work, it is preferable that the ribs are each wider than individual cubes of ice. Typically, the ice cubes will be ⅜, ⅞ or 1⅛ inches (1.0, 2.2 or 2.9 cm) wide, depending on the ice-forming mold used and the spacing between the vertical dividers. The preferred arrangement is two ribs  60  each 1¼ inches (3.2 cm) wide. This will allow the ribs to contact at least one, and usually two or three of the ice bridges over the vertical dividers. 
   As shown in  FIG. 9 , the ribs  60  extend up to a height below the height of the top of the ice-forming mold. Also, the ribs  60  have a top portion  65  ( FIG. 8 ) that is tapered downwardly and outwardly from the inside surface of the water curtain  50 . The remainder of the rib  60  is preferably generally parallel to the edge  58  of the water curtain designed to contact frame of the ice-forming mold. In this fashion the rib is also generally parallel (over most of its length) to the front face of the ice-forming mold, as seen on the right side in FIG.  9 . 
   The distance that the rib  60  extends toward the ice-forming mold  40  is important, as this determines whether the water curtain will open sufficiently far. This distance is a function of a number of factors, such as the depth of the ice-forming mold, the thickness of the ice bridge, the position of the hinge axis, and the length that the bottom edge  54  extends back into the sump area. In the preferred embodiment, at its greatest height the rib extends to within about {fraction (5/16)} to {fraction (11/32)} inch (0.8 to 0.9 cm) of the slab when the slab is frozen sufficiently thick to be harvested. Preferably the rib will extend at least 1⅛ inches (2.9 cm) from the inside surface of the water curtain at this point, and more preferably about 1{fraction (3/16)} inches (3.0 cm). 
   It is preferred that the top section  65  joins the rest of the rib  60  with a rounded profile  63  having a radius of at least one inch (2.5 cm), and more preferably at least 2⅜ inches (6.0 cm). Since this is the point where the slab of ice first contacts the ribs  60 , a generous radius prevents the slab from getting hung up at this juncture. It is preferred that this tangent point, where the rib  60  contacts the ice slab, be at least 3 inches (7.6 cm) from the axis of the pins  59 , and more preferably about 5 inches (12.7 cm). At this distance there is a sufficient torque arm to force the water curtain open. However, it is believed that if the point of contact is below the top half of the slab for the fairly tall evaporator in the Model QYDUAL4C ice-making machine, and possibly even below the top third of the slab, the slab may be prone to break apart on contact with the ribs. 
   In the preferred water curtain  50 , the distance from the pivot axis of the pins  59  to the point of contact of the ice slab is about 5 inches (12.7 cm). The distance from the pivot axis to the bottom edge is about 23 inches (58.4 cm). Thus the distance that the slab moves the rib will be multiplied by a factor of 23÷5 in determining the distance that the bottom edge  54  is moved. In the preferred ice-making machine the pockets have a dept of about ⅞ inch (2.2 cm), and the ice bridge is about ⅛ inch (0.3 cm) thick. Thus the ice slab will have a thickness of about 1 inch (2.5 cm). When the ice slab has moved out about {fraction (5/16)} of an inch (0.8 cm), it will contact the ribs  60 . As the ice slab moves out the remaining {fraction (9/16)} inch (⅞-{fraction (5/16)}) (1.4 cm (2.2-0.8)), before the ice slab is released, the bottom edge  54  will be moved just over 2.5 inches (6.4 cm). This allows the bottom edge  54  to clear the drop zone before the ice slab is released. 
   At the bottom of the ribs  60 , the bottom side  52  of the water curtain is preferably formed with rib extensions  68  as shown in FIG.  8 . These rib extensions  68  form a smooth transition with a radius  61  of about 1¼ inches (3.2 cm) with the rib  60 . 
   A prior art water curtain  80  ( FIG. 4 ) was produced with ribs  82  and rib extensions  84 . The rib extension  84  were initially placed on the water curtain  80  to provide a more gentle slope in the area where the ice slab contacted the bottom side of the water curtain  80 . In this model of ice machine, the slab of ice is rather large, and the striking of slab on the bottom side of the water curtain was quite violent. The rib extension helped to give a more gentle opening of the water curtain. In this water curtain, rather shallow ribs  82  were used just to provide rails that would contact the top of the slab as it fell down. As seen in  FIG. 4 , these ribs  82  did not extend far enough to contact the ice slab until after it was already released from the mold. The ribs  82  did not contact the slab and force the curtain open, but rather guided the slab as it fell after the curtain was opened by the slab contacting the bottom side  86  of the water curtain  80 . As noted above, when this design of a water curtain was initially tried as a fix to the QYDUAL4C machine having a frequent false “bin full” condition, the prior art design did not sufficiently correct the problem. It is somewhat of a coincidence that a solution to the problem was embodied in the preferred ribs  60 , yet the shallow ribs  82  were found on prior art water curtains. 
   The control system of the present invention may be modified to make the set period longer before the machine shuts down to take into account how long the water curtain is normally open. A set point of 10 seconds or greater is preferred, and a set point of 20 seconds or greater is more preferred. In a most preferred embodiment, the set point may be about 30 seconds. Of course, in a retrofit situation, it would be best if the original 7 second set point could continue to be used. This may be possible on some models of ice machines, or with further refinement of the rib design and other water curtain dimensions. 
   The retainer clip  19  that supports the back of the sump trough can be seen in FIG.  9 . Even though it appears that this is directly under the slab of ice  48 , it is actually set back into the machine at a depth that it does not interfere with the falling ice. However, it includes a defector to help keep any ice cubes from falling on and damaging the edge of the water sump. 
   While sloped horizontal dividers are preferred, it is also possible to use other means to help release the ice from the ice-forming mold. For example, a mechanical pusher could be used, or a pressurized fluid could be introduced between the back of the ice cubes and the pockets in which they are formed, as disclosed in the U.S. patent application Ser. No. 10/236,488, filed Sep. 6, 2002, which is hereby incorporated herein in its entirety. 
   While the preferred embodiment uses ribs  60 , there are other ways that the inside surface of the water curtain  50  could be modified so that the face of the ice slab contacts the water curtain to make it open before the ice is fully released from the ice forming mold. It is contemplated that bumps instead of ribs could be placed on the surface. A horizontal rib or series of horizontal ribs may be utilized. The entire surface could be modified to bring it closer to the frozen ice, rather than just adding one or more ribs. 
   The preferred embodiment of the present invention has the benefit that the ribs actually act to hold the top of the ice slab from falling out as far as it might otherwise do if the ribs were not present. This keeps the ice cubes from getting wedged into the pockets and thus having to be melted more to be released. Also, because the water curtain is started to be pushed open gradually as the ice starts to be released, it is not opened an excessive distance, and therefore does not rebound or swing shut causing the bottom edge  54  to hit into the slab  48  with enough force to break the slab. Thus, even if the ribs  60  are not large enough to hold the curtain open until the ice slab is all the way past the bottom edge  54 , there is not as much of a potential for the ice slab  48  to get broken up, and a portion of the slab getting wedged in between the bottom edge  54  and the sump  30 , causing a false bin-full condition. 
   Another embodiment of an improved water curtain  150  is shown in FIG.  11 . The water curtain  150  is shown mounted in an ice making machine  110  in FIG.  12 . Just as in the ice making unit  308 , the ice machine  110  includes an ice forming mold  140  and a sump  130 . The sump  130  is secured to the rest of the machine  110  with mounting tabs  132  on both sides of the machine, though only one of the mounting tabs  132  is seen in FIG.  12 . It will thus be understood that tabs  132  are to the side of the ice-forming mold  140  and thus do not interfere with the ice falling from the mold during harvest. 
   The water curtain  150  is designed for ice-making machine  110  which does not have as tall of an ice-forming mold as that shown in FIG.  9 . In this shorter machine, it has been found that the slab of ice cubes  142  releases more uniformally top-to-bottom than did slab  48 , which tended to tip out from the top. Also, it was discovered that if the ribs  160  extended too far inwardly from the inside surface of the water curtain, and were placed too high, the ice slab could contact the ribs before the slab was free of the ice pockets, but without a sufficient torque to enable the water curtain to swing open. As a result, the ribs would hold the slab of ice until it melted sufficiently, extending the harvest time. However, if the ribs do not extend sufficiently close to the ice forming mold, when the ice contacts the water curtain it will not make it open sufficiently far that the bottom of the slab can clear the bottom edge of the water curtain. If the rib extends outward from the inside surface of the water curtain sufficiently, it may be placed lower on the water curtain so that the moment arm is sufficient for the slab to cause the curtain to swing open and the curtain may still open enough for the slab to clear the bottom edge. Thus as discussed above, the height of the rib  160  and the distances between the pivot axis of the water curtain hinge and the point where the top section  165  of the rib contacts the slab of ice is important, and may require testing on individual ice machines to optimize the rib design and obtain a proper balance of forces. 
   The water curtain  150  includes another improvement to aid in the rigidity of the water curtain. Additional structure, in the form of three downwardly pointing chevrons  172 ,  174  and  176  are molded into the water curtain so as to extend inwardly, the same as ribs  160 . The chevrons extend between the ribs  160  so that the ends of the chevrons  172 ,  174  and  176  tie into the inside wall of the ribs  160 , as shown in FIG.  12 . By having the additional structure tie into at least one of the side walls of each of the ribs, the ribs  160  (which are large structural elements) are tied together so as to provide rigidity to the water curtain. The figure X  62  of the water curtain of  FIG. 5  is on the outside of the water curtain and does not tie into the side walls of the ribs  60 . A figure X design could be used as additional structure, instead of the chevrons, but it would need to be on the inside of the water curtain and extend further so that it was tied into the ribs. 
   Instead of the three chevrons shown, a number of other structures could be molded into the space between the ribs  160 . For example, fewer or more chevrons could be used; “W” shapes, curved structures such as half or quarter circles, straight across and lattice structures, and even a snowflake design could be used. Also, rather than two ribs with the additional structure extending between the ribs, other designs could be used. For example, if only one vertical rib were used in the center of the water curtain, one or more chevrons could be added so that their points tied into the ribs. The additional structure could also extend beyond the ribs. 
   The preferred additional structure has a depth of at least {fraction (3/16)} inch (0.48 cm). For example, the chevrons  172 ,  174  and  176  are each about ¼ inch (0.64 cm) deep. 
   One benefit of the downwardly pointing chevrons is that water flowing down the inside face of the water curtain tends to track towards the center of the water curtain. 
   It will be appreciated that the addition of some other process steps, materials or components not specifically included will have an adverse impact on the present invention. The best mode of the invention may therefore exclude process steps, materials or components other than those listed above for inclusion or use in the invention. However, the described embodiments are to be considered in all respects only as illustrative and not restrictive. Other changes could be made without detracting from the invention. For example, lateral ribs could also be added to the water curtain just outside the width of the ice-forming mold. These ribs would then guide the slab as it falls so that it would not shift to one side. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.