WO2000037865A1

WO2000037865A1 - Method and installation for making a water/ice mixture

Info

Publication number: WO2000037865A1
Application number: PCT/NL1999/000790
Authority: WO
Inventors: Pieter Jellema; Jelle Luutzen Nijdam
Original assignee: York Refrigeration ApS; DOOMERNIK BV
Current assignee: Johnson Controls Denmark ApS; DOOMERNIK BV
Priority date: 1998-12-21
Filing date: 1999-12-21
Publication date: 2000-06-29
Anticipated expiration: 2001-06-21
Also published as: NL1010857C2; AU1898800A; EP1144926A1

Abstract

The invention relates to a method for making a water/ice mixture, wherein the method comprises: feeding an aqueous fluid at a temperature which essentially corresponds to the freezing point of said aqueous fluid from a source through a closed vessel (7) in which a pressure prevails which essentially corresponds to the saturation pressure of the fluid at that freezing point, at least partially allowing the aqueous fluid to evaporate in said closed vessel (7), heat of evaporation being withdrawn from said aqueous fluid, and discharging the aqueous fluid from the closed vessel (7).

Description

Title: Method and installation for making a water/ice mixture

The present invention relates to a method for making a water/ice mixture, wherein the method comprises: - feeding an aqueous fluid at a temperature which essentially corresponds to the freezing point of said aqueous fluid from a source through a closed vessel in which a pressure prevails which essentially corresponds to the saturation pressure of the fluid at that freezing point, at least partially allowing the aqueous fluid to evaporate in said closed vessel, heat of evaporation being withdrawn from said aqueous fluid, and discharging the aqueous fluid from the closed vessel.

What is achieved by this method is that ice is formed in an efficient manner in a stream of aqueous fluid. In the case of relatively pure water, the saturation point of a stream of water at 0 °C will be at 6 mbar. If a pressure of approximately 6 mbar will be built up in the closed chamber and water is fed in at a temperature of 0 °C, ice formation will take place in the stream of fluid as a result of the evaporation which takes place.

This method is disclosed, inter alia, in US Patent 4 845 954. The latter US patent discloses a method and an installation for making a water/ice mixture wherein various interacting vacuum chambers are linked to one another. Water at a temperature which essentially corresponds to the freezing point of water is fed from a storage vessel containing a water/ice mixture to one of the vacuum chambers. The water is fed along the wall of the chamber to a discharge and then returned to the storage vessel for the water/ice mixture. While a water/ice mixture is being formed in this way, fresh water is fed to one of the other chambers in order to de-ice this. The water that is produced during the de-icing operation will be pre-cooled by the de-icing and can also be fed to one of the other vacuum chambers in order to be converted into a water/ice mixture.

A first significant disadvantage of the method according to the prior art is that it is not simple to achieve the correct process conditions to guarantee optimum production of a water/ice mixture. The aim of the present invention is to provide a method and an installation with the aid of which a water/ice mixture can be made efficiently.

Said aim is achieved in the present invention in that the aqueous fluid is discharged from the closed vessel via a holding tube, in which pressure build-up and buffering can take place, the instantaneous pressure in the holding tube being measured and the quantity of aqueous fluid which is fed to the closed vessel being regulated on the basis of the pressure measured in the holding tube.

The intention is that the method according to the present invention can be carried out for a specific period. Whilst carrying out the method it will be advantageous to be able to regulate the conditions in the closed vessel as well as possible. By now feeding the fluid from the closed vessel via a holding pipe in the direction of the storage vessel, numerous characteristics of the operating conditions in the closed vessel can be established. Thus, it is, for example, possible to establish that when the pressure in the holding tube rises a relatively large quantity of fluid flows from the closed vessel. The feed line can then be pinched together to some extent so that the discharge from the closed vessel will be limited to some extent after a slight delay. As a result the pressure measured in the holding tube will drop back. The converse is, of course, also possible.

According to the invention it is furthermore possible that the aqueous fluid is fed through the closed vessel with the aid of a channel that is separate from the outer wall of the vessel.

As a result of the fact that there is a separation between the fluid stream and the wall of the vacuum vessel, the efficiency of the production of the water/ice mixture is further improved. Much less heat exchange takes place between the stream of fluid and the environment since the stream of fluid does not come into contact with the wall of the vessel. Consequently the wall of the fluid vessel can be kept relatively thin. The wall of the vessel does not have to be provided with an insulating layer on the grounds of insulation requirements. The wall thickness is determined solely by strict technical considerations.

The method according to the present invention is further improved in thai that portion of the aqueous fluid.which evaporates in the closed vessel is sublimed or condensed with the aid of cooling means, the heat of sublimation or heat of condensation being removed from the closed vessel at the same time or thereafter.

With the aid of, for example, cooling means, the amount of heat which is removed from the stream of fluid by the evaporation of a portion of the stream of fluid can be removed from the closed vessel by sublimation and/or condensation of the water vapour.

According to the present method it is possible for the aqueous fluid to be fed to said closed vessel from a storage vessel for a water/ice mixture, said aqueous fluid being discharged from the closed vessel to the same storage vessel.

The water/ice mixture is fed from the storage vessel itself to, for example, a cooling installation in a process in the processing industry to serve as coolant. The storage vessel can, in principle, be designed to operate under atmospheric pressure. Only a portion of the aqueous fluid that is held in the storage vessel is fed to the closed vessel. The storage vessel itself can consequently be of relatively simple construction. Feed lines and discharge lines can also be fitted without complications. Moreover, it is possible to drill through the wall of the storage vessel to provide, for example, a passage for a shaft. If the storage vessel itself were to form the closed vessel these possibilities could not be implemented without complications.

The aqueous fluid is fed from the storage vessel to the closed vessel. The aqueous fluid will at least partially freeze in the closed vessel, with the result that the ice fraction in the aqueous fluid increases. The aqueous fluid is then discharged again to the storage vessel, with the result that the ice fraction in the storage vessel also increases.

According to the present invention it is possible for the closed vessel to be periodically connected to a thawing vessel containing a relatively warm aqueous fluid.

In this context it is possible for the discharge from the closed vessel then to be connected to the same thawing vessel.

It is also possible for the thawing vessel then to be placed in open communication with the storage vessel for the water/ice mixture. As a result of these measures it is possible to allow sublimed and condensed water vapour which has deposited on the cooling means in the closed vessel to melt at least partially and to be able to remove this from the closed vessel. Because at least a portion of the fluid which flows from the closed vessel is fed back to the thawing vessel the temperature thereof will fall. During a complete operating cycle, a low pressure is first produced in the closed vessel and the cooling means in the closed vessel are then connected. Aqueous fluid is then fed to the closed vessel. The fluid will partially evaporate in the vessel and then condense or sublime on the cooling means. Subsequently, during de-icing, relatively warm fluid, such as warm water, is fed to the closed vessel. The condensed/sublimed fraction will thaw and at least part of this will be fed with the relatively warm fluid to the thawing vessel. During this operation the quantity of water in the storage vessel can have decreased to some extent. By placing the thawing vessel (the temperature of which has fallen as a result of de-icing the closed vessel) in open communication with the storage vessel the fluid level in the storage vessel can be raised on the basis of the law of communicating vessels.

The installation according to the method is further improved in that, while making a water/ice mixture, liquid coolant which has condensed in a condenser in the cooling installation is fed through a heat exchanger, fluid from the thawing vessel being fed through the heat exchanger at the same time. With this arrangement it is advantageous that the fluid from the thawing vessel is withdrawn close to the bottom of said thawing vessel, the fluid being fed back essentially to the top of the thawing vessel after it has left the heat exchanger.

By means of this measure it is possible to supercool liquid coolant from the cooling installation with the aid of a heat exchanger, by making use of the cold that is stored in the thawing vessel. The energy efficiency of the cooling installation will increase by supercooling the liquid coolant which is fed from the condenser of the cooling installation. As a result of this rise, any fall in the energy element of the cooling installation, which is the result of warming fluid in the thawing vessel by means of an external heat source or by heat of condensation of the cooling installation, is largely cancelled out.

The present invention also relates to an installation for carrying out the method according to the invention. Said installation comprises: a storage vessel for the ice/water mixture, a vacuum vessel in which a relatively low pressure can be built up, the vacuum vessel having a feed line for aqueous fluid, which feed line is connected to the storage vessel. The installation according to the invention is characterised in that the feed line of the vacuum vessel adjoins a channel in the vacuum vessel, which channel is at least open on the upper side, and in the vacuum vessel adjoins a discharge line for the vacuum vessel, which discharge line is connected to the storage vessel for the water/ice mixture.

The channel in the vacuum vessel can be used to transport the aqueous fluid through the vacuum vessel. The channel will be positioned at an angle to the horizontal. This angle is, for example, 4°. The channel can be open on the upper side. This has the advantage that when de-icing the vacuum vessel ice can drop down into the channel and can be discharged from the vacuum vessel via the channel with the aid of the discharge line. The discharge line, the wall of the vacuum vessel, can be provided with a sight glass. For maintenance work and the like it is advantageous if the vacuum vessel can also be opened.

Said installation is further improved in that one or more cooling tubes are arranged in the vacuum vessel, which cooling tubes include means for connecting the cooling tubes to a feed and a discharge for a coolant.

According to the invention it is also possible for the cooling tubes to be provided with one or more ribs.

As a result of the presence of the ribs the outer surface of the pipes of the pipe bundle is increased. As a result of ice growth on the outside wall of the pipe bundle, the heat transfer between the coolant on the inside of the pipes and the water vapour which has to freeze on the pipes will decrease during use of the vacuum vessel. What can be achieved by increasing the outer surface of the pipes of the pipe bundle is that the vacuum vessel can be used for a relatively long period before the ice that has grown on the outside of the pipes has to be removed.

With the aid of the cooling pipes, the water which has evaporated from the stream of fluid can be effectively condensed and/or sublimed on the outside of the cooling pipes.

According to the invention it is furthermore possible that the installation comprises a vessel for a relatively warm aqueous fluid, which vessel has on the one hand to be connected to the feed line for the vacuum vessel and, on the other hand, has to be coupled to the discharge line from the vacuum vessel.

The mode of operation of the vessel containing relatively warm water has already been described above with reference to the method.

According to the invention it is furthermore possible that a shut-off valve is installed in the feed line of the vacuum vessel and that a holding tube is provided in the discharge line from the vacuum vessel, means for measuring the pressure in the holding tube being arranged in the discharge line, at the location of the holding tube, which measurement means are effectively connected via a control device to the shut-off valve in the feed line for the vacuum vessel.

The operating conditions in the closed vessel can be adjusted to the optimum with the aid of said holding tube and the associated measurement means.

According to the invention it is furthermore advantageous that the storage vessel is provided with a perforated plate or grating, the feed line and the discharge line for the vacuum vessel being arranged on either side of said grating.

The presence of the grating makes it possible that only water is removed from the storage vessel. Ice which has formed in the interim remains behind in the storage vessel. The water is then discharged from the storage vessel, at least partially converted to ice. On balance this will mean that as a result the percentage of ice in the storage vessel can be increased to any desired value. In order to obtain a water/ice mixture having an ice fraction of, for example, 20% it can suffice to feed water/ice mixture having an ice fraction of, for example, 10% to the storage vessel. As has been stated, the ice fraction can be increased by removing only water at the underside of the grating. The invention is further improved in that the outflow opening of the feed line for the vacuum vessel has a guide element for the aqueous fluid, which guide element is provided with a fabric strip which joins onto the channel in the vacuum vessel.

With the aid of this element the aqueous fluid can be distributed in a thin fluid film over the channel.

Furthermore, according to the invention it is possible that means for mixing the water/ice mixture are arranged in the storage vessel for the water/ice mixture.

Because, as has been indicated above, the storage vessel can be essentially under atmospheric pressure, fitting a mixer in the storage vessel, for example via a passage for a shaft, can be carried out without further complications.

Furthermore, it is possible to use said installation for storing relatively warm fluid, the installation being provided with a cooling installation, provided with a condenser, and a thawing vessel being arranged in the installation. The installation according to the invention is improved in that the installation includes a heat exchanger having a first feed line which is connected to the condenser of the cooling installation and having a second feed line which is connected to the thawing vessel. The mode of operation of the heat exchanger has already been described above with reference to the method claims.

The present invention will be explained in more detail with reference to three figures, in which:

Figure 1 is a schematic view of the installation for making a water/ice mixture according to the present invention.

Figure 2 shows the vacuum vessel with the aid of which water/ice mixture is produced in the present invention.

Figure 3 shows a detail of the outer surface of the cooling tubes which are used for removing the heat of evaporation and heat of solidification from the vacuum vessel. Figure 1 shows, diagrammatically, the installation 1 with the aid of which a water/ice mixture can be produced according to the present invention. The installation 1 comprises a storage vessel 2. A mixture of water and ice is held in said storage vessel 2. Said mixture will have an ice fraction of, for example, 20%. The water/ice mixture is outstandingly suitable for numerous cooling applications. The water/ice mixture can be discharged from the storage vessel 2 in the direction of a cooling installation with the aid of a line 3. After carrying out the cooling task, the water/ice mixture is fed back in the direction of the storage vessel 2 via the line 4. A stirrer 5 is installed in the storage vessel 2 to keep the water/ice mixture well mixed. A grating 6 is arranged beneath said stirrer 5. Said grating 6 can be used to separate water and ice from one another. This means that only water, and no ice, is present beneath the grating 6, whilst the water/ice mixture is retained above the grating.

The installation 1 further comprises a vacuum vessel 7. The vacuum vessel 7 is shown in more detail in Figure 2. Water from the storage vessel 2 can be fed from beneath the grating 6 to the vacuum vessel 7 via a line 8. A channel 9, which is positioned at an angle to the horizontal, is arranged in the vacuum vessel 7. From the left-hand side in the figure the water can be fed to the channel and will flow to the lower right-hand section of the channel 9. On the right-hand side, the water flows back in the direction of the storage vessel 2 via a line 10.

Cooling tubes 11 , in which, for example, ammonia evaporates, are arranged in the vacuum vessel 7. The ammonia can be supplied and discharged with the aid of the device 12. Furthermore, the vacuum vessel 7 is provided with means for withdrawing non-condensable gases, such as noble gases, under suction. This device is indicated diagrammatically by 13.

The discharge line 10, which is fixed to the right-hand side of the channel 9, has at least one section that is positioned at an angle and that is indicated by 14. The section 14 forms a holding tube, in which pressure build-up and buffering can take place. A pressure gauge 15 is fitted at the bottom of said holding tube. From said pressure gauge the water is fed via a line 10 in the direction of a centrifugal pump 17, with the aid of which the water can be pumped back via the line 10 to the storage vessel 2.

The installation 1 further comprises a thawing vessel 18 for relatively warm water or thawing water. A certain quantity of water at a relatively high temperature of, for example, 20 °C, is held in said thawing vessel. The water in said thawing vessel is used for periodic thawing of the cooling tubes or the pipe bundle 11 in the vacuum vessel 7. The thawing vessel 18 is connected with the aid of a line 19 to the feed line 8 which runs to the channel 9 in the vacuum vessel 7. Moreover, the return water from the vacuum vessel 7 can be transported back to the thawing vessel 18 via the line 20. Furthermore, the thawing vessel 18 and the storage vessel 2 are coupled to one another via a line 21 which can be shut off.

The mode of operation of the installation 1 according to the present invention is as follows. A very low pressure, which pressure corresponds to the saturation pressure of the fluid from the installation, is applied in the vacuum vessel 7. In the case of water this pressure is 6 mbar. The pressure of 6 mbar is equal to the saturation pressure of water vapour at 0 °C. The shut- off valves 31 and 32, which have been fitted in the lines 8 and 10, are opened. Water at a temperature of approximately 0 °C is fed from the storage vessel 2 via the line 8 in the direction of the channel 9. The water will flow from a metering opening (see Figure 2) to the right along the channel 9 through the vacuum vessel 7. As a consequence of a very low pressure in the vacuum vessel 7, the water will try to evaporate. This means that heat of evaporation is withdrawn from the stream of water. As a result of the withdrawal of said heat of evaporation from the stream of water, ice formation will take place in the water flow. In this way ice is effectively produced using a relatively low amount of energy. The water/ice mixture which is formed in this way in the vacuum vessel 7 will flow via the channel 9 in the direction of the discharge line 10. The heat of evaporation which is removed from the stream of water over the channel 9 is removed from the vacuum vessel 7 by means of the pipe bundle 11. The water vapour condenses and freezes or sublimes on said pipe bundle 11. When a maximum permissible ice layer thickness has formed on the pipe bundle 1 1 the bundle is thawed. Thawing of the pipe bundle 11 is described below.

When forming ice in the stream of water in the channel 9 an ice fraction of 0 to 10% is preferably formed. The reason for not wishing to have an ice fraction of more than 10% is that a water/ice mixture containing 10% ice can still be pumped with the aid of a centrifugal pump. Such a centrifugal pump 17 is incorporated in the return line 1 which feeds the water/ice mixture back to the storage vessel. The stream of water is fed to the vacuum vessel 7 with the aid of a control valve 34 which is incorporated in the feed line 8 which joins the storage vessel 2 to the vacuum vessel 7. The water is drawn towards the vacuum vessel 7 via the line 8 as a consequence of the vacuum which prevails in the vacuum vessel 7. The control valve 34 is controlled by means of pressure measurements. Firstly the pressure which prevails in the vacuum vessel 7 is measured. This pressure measurement is carried out by the pressure gauge 35. The pressure that is measured in the vacuum vessel 7 will also prevail in the discharge line 10. Following a free fall, the water is fed through said discharge line 10 to a holding tube 14. Said holding tube 14 serves as a buffer line. Pressure build-up can take place in the holding tube 14. The pressure prevailing in the holding tube 14 at a given point in time is measured with the aid of the pressure gauge 15. If it is found that the pressure measured with the aid of the pressure gauge 15 is becoming too high, too much water is being discharged through the discharge line 10. This means that the feed of water via the line 8 to the vacuum vessel 7 must be reduced with the aid of the shut-off valve 34. If the pressure measured by the pressure gauge 15 becomes too low, the water feed via the line 8 can be increased. In this way correct metering of the water to the channel 9 with the aid of the line 8 can be guaranteed while the installation 1 is in operation.

The water/ice mixture is pumped from the holding tube 14 with the aid of a centrifugal pump 17 in the direction of the storage vessel 2. As has been stated, the ice fraction will be at most 10%. For various applications it is desirable that the ice/water mixture that is fed to a cooling application via the lines 3, 4 has an ice fraction of approximately 20%. This can be achieved by the presence of the perforated base or the grating 6. The ice will remain behind above the grating, whilst water can be fed to the vacuum vessel 7 from beneath the grating. As a result of the separation of the ice and the water by means of the grating 6 a relatively high ice fraction can be obtained in the storage vessel 2, whilst a water/ice mixture having a maximum ice fraction of approximately 10% is supplied from the feed line 10.

To ensure that well-shaped ice crystals are produced in the stream of water in the channel 9, a seeding agent can be added to the aqueous fluid in the system. Said seeding agent ensures that there are numerous seeds for ice formation during crystallisation, so that no ice floes are produced in the stream of water, but rather a large number of ice crystals. The presence of these ice crystals promotes the cooling properties of the water/ice mixture to be made. If relatively pure water is fed to the vacuum vessel 7, this water will be at a temperature of approximately 0 °C. The saturation pressure of the water vapour from this water at 0 °C is 6 mbar. In this case the pipe bundle will be kept at a temperature of a few degrees below 0, for example -2 or -5 °C. The temperature range in which ice formation takes place can be shifted as desired by, for example, adding salts to the water which give rise to a lowering of the freezing point.

As has been stated, the heat of evaporation which is given off by the stream of water which runs through the channel 9 is removed with the aid of the pipe bundle 1 1. Water vapour will condense or freeze or sublime on the outside of said pipe bundle 11.

The pipe bundle has to be de-iced after some time. The thicker the ice layer the lower will be the heat transfer between the cold medium on the inside of the pipe bundle and the water vapour on the outside of the pipe bundle. In general it can be stated that heat transfer is an order of magnitude greater in the case of a clean pipe bundle than when there is an ice deposit on the tubes. In the case of a clean bundle the heat transfer can be 5000 W/m K, whereas in the case of an approximately 2 mm thick ice deposit the heat transfer can be only 500 W/m K.

In the case of de-icing the pipe bundle 1 1 the following procedure is followed. For de-icing the valves 30 and 33 are opened and the valves 31 and 32 are then closed. This means that water is fed to the vacuum vessel 7 from the vessel containing relatively warm water. On arrival in the channel 9 this water at a relatively high temperature will start to evaporate and condense on the ice layer that has formed on the pipe bundle 1 1. This will cause the ice on the pipe bundle 1 1 to start to melt and drop down into the channel. This mixture of relatively warm water with lumps of ice contained therein is fed via the shut-off valve 33 to the thawing vessel 18. As a result of the thawing process the temperature in the thawing vessel 18, which was, for example, 20 °C prior to thawing, will be able to fall to a level a few degrees above 0, for example 5 °C.

While de-icing the vacuum vessel 7 it is possible to adjust the temperature of the water which is fed to the vacuum vessel 7, for example with the aid of the three-way mixing tap 36 which is located by the thawing vessel 18. With the aid of this three-way mixing tap it is possible, for example, to admix relatively cold water to the relatively warm water that is fed to the vacuum vessel 7.

The water level in the storage vessel 2 will have fallen somewhat after an entire procedure, from start-up of the production of vacuum ice to thawing of the pipe bundle 11. This fall is caused by the water vapour which has frozen on the outside of the pipe bundle 11 and which is fed to the thawing vessel 18 during the thawing process. At the end of the thawing process this reduction in the water level in the storage vessel 2 can be made good again by opening the shut-off valve 38 in the line 21. On the basis of communicating vessels, the raised level in the thawing vessel 18 will drop back and the level in the storage vessel 2 will rise. It will also be possible for water to be lost during the entire process. Means for topping up with water are therefore also provided in the installation 1. These means are indicated diagrammatically by reference numeral 40.

If the thawing vessel 18 is heated by an external heat source or by heat of condensation from the cooling installation, this gives rise to a loss in cooling capacity and thus to a loss in the energy efficiency of the cooling installation. Cold thawing water at approximately 0 °C is present in the thawing vessel 18 after thawing. In order to prevent the abovementioned loss in cooling capacity and the reduction in the energy efficiency of the cooling installation, a heat exchanger 73 is incorporated in the installation according to the invention, as shown in Figure 1. Said heat exchanger 73 is used as a fluid supercooler. The fluid supercoolcr can be used for cooling liquid coolant from the cooling installation 12. During the freezing process liquid coolant that has condensed in the condenser 74 can be fed through the heat exchanger 73. At the same time water is pumped back, with the aid of the pump 75, from the thawing vessel 18, from the bottom thereof via the heat exchanger 73, to the top of the thawing vessel 18. By making use of supercooling with the aid of the heat exchanger 73 it is possible to compensate virtually completely for the loss in cooling capacity and thus the fall in the energy efficiency, as mentioned above, because liquid supercooling from the condensation temperature of the liquid coolant to, for example, 3 °C will cause the energy efficiency of the cooling installation to rise.

This effect can be even further improved if the water is carefully withdrawn from the thawing vessel. The reason for this is that in this case as little mixing as possible of cold and warm water takes place in the thawing vessel 18. In this context it is possible to fit one or more perforated plates in the thawing vessel 18. These perforated plates are indicated by the reference numerals 71 and 72 in Figure 1. As a result of the presence of the perforated plates the relatively warm water that is fed to the top of the thawing vessel 18 will not mix with the cold water essentially at the bottom of the vessel because of the differences in density. Thus it is possible to withdraw the cold water from the vessel without mixing until all of the water in the thawing vessel 18 has warmed up.

As has already been stated above, it is advantageous to add a seeding agent to the stream of water which is fed to the vacuum vessel 7. Said seeding agent can be, for example, NaCl, pekasol or sugar. These seeding agents can be supplied in a quantity by mass of, for example, 1%. If a small quantity of seeding agent, for example in the form of NaCl, has been supplied to the water, ice can be made at a relatively high temperature of approximately -2 to -4 °C. As has been stated, it is possible to make ice at a lower temperature of, for example, -5 to -10 °C by supplying seeding agents or salts which cause the freezing point to fall.

Figure 2 shows an illustrative embodiment of the vacuum vessel 7 that can be used in the installation 1 according to the present invention. The vacuum vessel 7 comprises a pipe bundle 11 positioned horizontally. It is possible, for example, to feed ammonia through this pipe bundle 11 , which ammonia will evaporate in the pipe bundle 11. The vacuum vessel 7 also comprises means 13 for discharging non-condensable gases, such as inert gases. A channel 9 through which the water can be fed is arranged in the vacuum vessel 7. The water is supplied with the aid of a feed line 8. As has been stated, the feed can take place as a result of the fact that a very low pressure prevails in the vessel 7. The channel 9 will be positioned at an angle to the horizontal. This angle can be, for example, 4°. The channel 9 will be open on the upper side. This has the advantage that when the pipe vessel 1 1 is de-iced the ice can fall down from the pipe bundle 11 into the channel 9 and can be discharged from the vacuum vessel 7 with the aid of the discharge line 10. The discharge line 10 is joined with the aid of a flange connection 50 to the lowest section of the channel 9. The wall of the vacuum vessel 7 can be provided with a sight glass. Two sight glasses 51 are shown in Figure 2. For maintenance work and the like it is advantageous if the vacuum vessel 7 can be opened. A door 52 is therefore provided in the right-hand side shown in the figure. Said door 52 is attached to the wall of the vacuum vessel 7 with the aid of a hinge 53. A handle 54 is fitted for opening the door 52.

Ice can be formed relatively efficiently with the aid of the vacuum vessel 7 according to the present invention. One of the reasons for this is also that the stream of water which is fed through the channel 9 does not come into contact with the wall of the vacuum vessel 7. This means that the heat exchange between the stream of water in the channel 9 and the ambient air is very slight. This also has the result that the wall of the vacuum vessel 7 can be kept relatively thin. The wall 7 does not have to be provided with an insulating layer on the grounds of insulation requirements. The wall thickness is determined by strength engineering considerations.

It can be seen from Figure 2 that the feed line 8 opens at the top thereof into a "letterbox-like" feed opening 55. What is achieved with the aid of this letterbox-like feed opening is that a thin film of water can be fed into the channel 9. The stream of water is not fed to the channel from the feed line 8 in waves or the like but relatively continuously.

Figure 3 shows a cross-section of one of the tubes of the pipe bundle 1 1. he outside of the pipes of the pipe bundle 11 can have been provided with ribs 60. The outer surface of the pipes of the pipe bundle 11 is increased as a result of said ribs 60. As has already been indicated above, the heat transfer between the coolant on the inside of the pipes and the water vapour which has to freeze on the pipes will decrease as a result of ice growth on the outside wall of the pipe bundle 1 1. What can be achieved by increasing the outer surface of the pipes of the pipe bundle 1 1 with the aid of the ribs 60 is that the vacuum vessel 7 can be used for a relatively long period before the ice that has grown on the outside of the pipes 11 has to be removed.

If the vacuum vessel which is shown in Figure 2 is used for an application of a 25 kW vacuum installation, a vacuum vessel having a total horizontal length of approximately 3 500 mm can be used. The diameter of the vessel could be 1 100 mm. A pipe bundle where the pipes would have a diameter of 25 mm could be placed in the vessel. The cooling ribs on the outside of the pipes can have a diameter of 45 mm and a thickness of 1 m.

When such a 25 kW installation is in use, a storage vessel with a diameter of 1 000 mm and a height of 1 200 mm can be used. The associated thawing tank has a diameter of 650 mm and a height of 1 200 mm.

The vacuum ice that can be made according to the present invention using the installation in question can advantageously be used for numerous cooling methods and processes. Firstly, the water/ice mixture has a high heat transfer coefficient. One of the reasons for this is the low viscosity of the water/ice mixture. Consequently, the boundary layer which builds up between a transport channel and the liquid can be relatively thin. Secondly, no heat path will be formed in the boundary layer. When heat is supplied to a water/ice mixture the temperature will essentially not increase. However, the ice fraction in the water/ice mixture will decrease.

An ancillary advantage of the use of water/ice mixtures as coolants is that said mixtures are relatively non-hazardous. This is an important advantage in the food industry, for example. The water/ice mixture is also not corrosive.

An ancillary advantage is that the water/ice mixture can serve as a cold buffer. In practice this means that at the points in time when there is a peak demand for cold the ice fraction in the water/ice mixture will decrease. When the demand for cold is somewhat lower again the quantity of ice in the water/ice mixture will be able to increase again. This means, inter alia, that a relatively small cooling installation can be used with buffering taking place in the coolant itself and the installation does not have to be sized for the peak load of cold which is demanded from time to time.

Claims

loClaims

1. Method for making a water/ice mixture, wherein the method comprises: feeding an aqueous fluid at a temperature which essentially corresponds to the freezing point of said aqueous fluid from a source through a closed vessel (7) in which a pressure prevails which essentially corresponds to the saturation pressure of the fluid at that freezing point, at least partially allowing the aqueous fluid to evaporate in said closed vessel (7), heat of evaporation being withdrawn from said aqueous fluid, and - discharging the aqueous fluid from the closed vessel (7), characterised in that the aqueous fluid is discharged from the closed vessel (7) via a holding tube, in which pressure build-up and buffering can take place, the instantaneous pressure in the holding tube being measured and the quantity of aqueous fluid which is fed to the closed vessel (7) being regulated on the basis of the pressure measured in the holding tube.

2. Method according to Claim 1, characterised in that the aqueous fluid is fed through the closed vessel (7) with the aid of a channel that is separate from the outer wall of the vessel (7).

3. Method according to Claim 1 or 2, characterised in that that portion of the aqueous fluid which evaporates in the closed vessel (7) is sublimed or condensed with the aid of cooling means (11), the heat of sublimation or heat of condensation being removed from the closed vessel (7) at the same time or thereafter.

4. Method according to Claim 1, 2 or 3, characterised in that the aqueous fluid is fed to said closed vessel (7) from a storage vessel (2) for a water/ice mixture, said aqueous fluid being discharged from the closed vessel (7) to the same storage vessel (2).

5. Method according to one of the preceding claims, characterised in that the closed vessel (7) is periodically connected to a thawing vessel (18) containing a relatively warm aqueous fluid.

6. Method according to Claim 5, characterised in that the discharge from the closed vessel (7) is then connected to the same thawing vessel (18).

7. Method according to Claim 5 or 6, characterised in that the thawing vessel (18) is then placed in open communication with the storage vessel (2) for the water/ice mixture.

8. Method according to one of the preceding claims, wherein the closed vessel (7) is cooled with the aid of a cooling installation (12) in which a liquid coolant is circulated by pumping, characterised in that while making a water/ice mixture, liquid coolant which has condensed in a condenser (74) in the cooling installation (12) is fed through a heat exchanger (73), fluid from the thawing vessel (18) being fed through the heat exchanger (73) at the same time.

9. Method according to Claim 8, characterised in that the fluid from the thawing vessel (18) is withdrawn close to the bottom of said thawing vessel (18), the fluid being fed back essentially to the top of the thawing vessel (18) after it has left the heat exchanger (73).

10. Installation (1) for the production of an ice/water mixture, wherein the installation (1) comprises: a storage vessel (2) for the ice/water mixture, a vacuum vessel (7), in which a relatively low pressure can be built up, the vacuum vessel (7) having a feed line for aqueous fluid, which feed line is connected to the storage vessel (2), characterised in that the feed line (8) of the vacuum vessel (7) adjoins a channel (9) in the vacuum vessel (7), which channel is at least open on the upper side, and in the vacuum vessel (7) adjoins a discharge line (10) for the vacuum vessel (7), which discharge line (10) is connected to the storage vessel (2) for the water/ice mixture.

1 1. Installation (1) according to Claim 10, characterised in that one or more cooling tubes (11) are arranged in the vacuum vessel (7), which cooling tubes comprise means for connecting the cooling tubes (11) to a feed and a discharge for a coolant.

12. Installation (1) according to Claim 1 1 , characterised in that the cooling tubes (1 1) are provided with one or more ribs (60).

13. Installation (1) according to Claim 10, 11 or 12, characterised in that the installation (1) comprises a vessel (18) for a relatively warm aqueous fluid, which vessel (18) has on the one hand to be connected to the feed line (8) for the vacuum vessel (7) and, on the other hand, has to be coupled to the discharge line (10) from the vacuum vessel (7).

14. Installation (1) according to one of Claims 10 - 13, characterised in that a shut-off valve (34) is installed in the feed line (8) of the vacuum vessel (7) and in that a holding tube (14) is provided in the discharge line (10) from the vacuum vessel (7), means (15) for measuring the pressure in the holding tube (14) being arranged in the discharge line (10), at the location of the holding tube (14), which measurement means (15) are effectively connected via a control device to the shut-off valve (34) in the feed line (8) for the vacuum vessel.

15. Installation (1) according to one of the preceding claims, characterised in that the storage vessel (2) is provided with a perforated plate or grating (6), the feed line (8) and the discharge line (10) for the vacuum vessel (7) being arranged on either side of said grating (6).

16. Installation (1) according to one of the preceding claims, characterised in that the outflow opening of the feed line (8) for the vacuum vessel (7) has a guide element (55) for the aqueous fluid, which guide element (55) is provided with a fabric strip which joins onto the channel (9) in the vacuum vessel (7).

17. Installation (1) according to one of the preceding claims, characterised in that means (5) for mixing the water/ice mixture are arranged in the storage vessel (2) for the water/ice mixture.

18. Installation according to one of the preceding claims, wherein the installation is provided with a cooling installation (12), provided with a condenser (74), and wherein a thawing vessel (18) for storing relatively warm fluid is arranged in the installation, characterised in that the installation includes a heat exchanger (73) having a first feed line which is connected to the condenser (74) of the cooling installation (12) and having a second feed line which is connected to the thawing vessel (18).