GB2117222A

GB2117222A - Freezing a liquid

Info

Publication number: GB2117222A
Application number: GB08303674A
Authority: GB
Inventors: Peter Harold Gibson; Robert Ian Taylor
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1982-02-10
Filing date: 1983-02-10
Publication date: 1983-10-12
Also published as: GB8303674D0; GB2117222B

Abstract

A continuous, pulsating stream of liquid (e.g., cream) is dispensed from nozzles 12 onto a stream of liquefied gas flowing along downwardly inclined channels provided by a trough 22. The liquid forms itself into discrete bodies, a substantial number of which have a larger cross-sectional area than that defined by the outlet of each nozzle from which they are dispensed. The bodies are carried by the liquefied gas along the channels and at least their peripheries are frozen by the time the bodies reach the downstream end of the channels. The bodies then fall into a rotary drum 28 having slots 30 formed therein to separate the liquefied gas from the bodies. A screw 46 propels the frozen bodies to an outlet 48 leading to a collection device 50. The liquefied gas operated from the bodies is collected in a sump 34 and returned by rotary buckets 70 from the sump 34 to a reservoir 18 which supplies the trough 22 with liquefied gas. <IMAGE>

Description

SPECIFICATION Freezing a liquid This invention relates to freezing a liquid. The term 'liquid' is used herein to encompass emulsions; suspensions; solutions; the liquid phase of substances that are solid at ambient temperature but which melt at temperatures not greatly in excess of ambient; pastes; semi-liquid foods such as cream, yoghurt, cottage cheese and butter; purees; egg albumen; mixtures of albumen and yolk, whole blood; blood bodies; and serums of drugs. It is essential that the liquid has a viscosity which is not so great that it is not able to be pumped or otherwise passed through a dispensing nozzle or orifice.

The invention is particularly concerned with the freezing of liquids that tend to deteriorate, e.g. by virtue of chemical or bacteriological action, if stored, typically for prolonged periods of time, at ambient temperatures. An example of such a liquid is a dairy product such as cream. Much cream is produced on farms or production centres remote from the eventual consumers. Difficulty can therefore arise in keeping the cream fresh while it is being distributed from a place of production to a place of sale. A particularly efficient distribution system is required, and in general, cream has traditionally not been transported over large distances before sale at a retail outlet. It has therefore been proposed to freeze the cream and store the frozen cream in a refrigerator before sale. This has made possible the export of cream from one country to another.For example, it is known to freeze cream in slabs, break the slabs into lumps of more manageable size, package the lumps in the Republic of Ireland and export the resultant product to England.

One disadvantage of such commercially practical cream freezing technology is that after being thawed the cream is not of such high quality as traditional fresh cream. For example, if the cream after being thawed is added to a hot drink (e.g. coffee), it tends to break down and leave an oily of fatty layer on the surface of the drink. This has led us to look for new methods of freezing liquids.

UK patent specification no 1 264 439 relates to a frozen food substance comprising free-flowing discrete particles of egg or semi-liquid dairy product (e.g. cream) wherein said particle is of pop-corn-like form. The substance is produced by causing the egg substance or semi-liquid dairy product to fall into direct contact with a non-toxic, liquefied gas refrigerant having a temperature below 500C and a turbulent surface. The substance sinks and the frozen substance is collected after sinking and is stored in a frozen condition. The liquefied gas is liquid nitrogen.

The process described in UK patent specification No 1 264 439 suffers, we believe, from two disadvantages. First, a product of popcorn-like form is readily crushed owing to its hollow thin-walled structure and there is a tendency for unacceptably large quantities of dust to be produced while the product is being transported to a shop. Second, the process makes poor use of the refrigeration available from a liquefied gas such as liquid nitrogen. The reason for this shall be explained below.

Another process of interest is described in UK patent specification No. 1 376 972. The process it describes is limited to the production of a frozen food substance from eggs. The egg is caused to fall from at least one nozzle into direct contact with a non-toxic, liquefied gas (for example liquid nitrogen) at a temperature below-i 500 F. The flow rate of egg from the or each nozzle is from 1 to 5 Ibs per hour so that the egg enters the liquefied gas from above its surface as discrete globules which are frozen therein to form pellets with the size range 3 mm to 7 mm. The pellets are collected at the bottom of the vessel containing the liquefied gas and removed therefrom. The pellets are then stored in a suitable freezer.

In that pellets, as distinct from a pop-corn-like product, are produced the process described in UK patent specification No 1 376 972 overcomes the first disadvantage associated with the production of frozen egg in the process described in UK patent specification No 1274 439. However, so far as overcoming the second disadvantage is concerned, the process described in UK patent specification No 1 376 972 offers no improvement over that described in UK patent specification No 1 264 439.

As described in our co-pending application 2 092 880 A, the reason for the poor utilisation of liquid nitrogen in the processes described in the aforementioned UK patent specifications is that as the pellets sink (the frozen cream having a greater specific gravity than liquid nitrogen) so there is a rapid fall in their temperature of from about 0 or -200C to well below --1000C.

Accordingly, our co-pending patent application provides a method of producing frozen pellets for liquid, which method comprises the steps of causing drops of liquid to fall onto or into a volume of non-toxic liquefied gas having a boiling point below -300C; allowing the drops to freeze to form buoyant pellets of frozen liquid and separating such pellets from the liquefied gas before they lose their buoyancy. Typically, a flow of liquefied gas is established and the droplets allowed to fall onto the surface of the liquefied gas and then be carried by the liquefied gas to a separator where they are separated from the liquefied gas, the residence time of the pellets in contact with the liquid nitrogen being insufficient for them to lose their buoyancy.

We have now discovered that for a liquid having a given viscosity there is a maximum size to the pellets or solid bodies of frozen liquid that can be produced by this method. This is because evaporated liquefied gas (typically nitroge) forms a gas cushion under each droplet and counteracts the force of gravity. Consequently, surface tension forces exerted by the liquefied gas tend to pull the drops apart such that there is a limitation on the maximum size of drop that will remain stable on the surface of the liquid nitrogen. Mbreover, there is a limit on the quantity of frozen pellets that can be formed per unit time per nozzle from which the droplets are dispensed. For example, we have found that the maximum rate of production of frozen double cream pellets is in the order of 10 cm3 per'nozzle per minute.It is desirable in commercial practice greater to exceed such production rates without having recourse to a freezer with tens of dispensing nozzles.

It is an aim of the present invention to provide a method and apparatus for producing generally spherical or spheroidal bodies of frozen liquid which makes it possible to overcome or mitigate (at least in the example of cream) the limitations mentioned in connection with our aforesaid co-pending application.

According to the present invention there is provided a process for freezing a liquid to form generally spherical (or generally spheroid) frozen bodies of liquid, comprising passing the liquid to be frozen through at least one orifice and causing a pulsating and substantially continuous flow of the liquid to pass into or onto a stream of liquefied gas, having aboiling point below --300C, flowing along a channel, the rate of passage of the liquid through the orifice being sufficiently large for a proportion of the liquid to form into discrete bodies of greater cross-sectional area than the orifice; maintaining the bodies in contact with the liquefied gas for a time sufficient for at least their peripheries to freeze, and collecting the resulting generally spherical or generally spheroid frozen bodies of liquid.

The invention also provides apparatus for freezing a liquid comprising means defining at least one orifice; at least one channel, the orifice defining means being adapted to be positioned above the channel to dispense the liquid to be frozen into the channel; means for creating a stream of liquefied gas along the channel, the liquefied gas having a boiling point of less than minus 300C, means for passing the liquid to be frozen through the orifice and for creating a pulsating and substantially continuous flow of the liquid into or onto the stream, and means for collecting frozen bodies of the liquid from the apparatus, whereby in operation the liquid can form itself into discrete, generally spherical or generally spheroid bodies at least some of which are of greater cross-sectional area than the orifice and the bodies can be maintained in contact with the liquefied gas for a time sufficient for at least their peripheries to freeze.

Preferably, the said channel has a downwardly sloping bottom along which the larger bodies are able to be rolled by the liquefied gas. The slope is desirably relatively gentle, i.e. in the range 1 in 10 to 1 in 60.

The liquefied gas is desirably non-toxic, and need not have a greater specific gravity than the liquid to be frozen. Indeed, we prefer to use liquid nitrogen as the liquefied gas. Even if the liquid to be frozen has a greater specific gravity than liquid nitrogen (as it typically may), gas bubbles collecting on the underside of the liquid initially coming into contact with the liquid nitrogen will tend to cause the liquid initially to float on the surface of the stream of liquid nitrogen.

The orifice is typically defined by a nozzle. The diameter of the orifice may be selected in accordance with the viscosity of the liquid to be frozen. in general, the preferred orifice diameter tends to increase with increasing viscosity. For producing frozen double or whipping cream we have used orifices having a diameter in the range of 1 to 3 mm.

The liquid to be frozen is preferably passed through the orifice by means of a positivedisplacement pump. Such a pump is naturally able to provide a continuous, pulsating flow of the liquid to be frozen. It is, however, possible to use a pump that provides a steady or non-pulsating flow of the liquid and provide a reciprocating or other member that temporarily interrupts or constricts the flow downstream of the orifice at a chosen frequency so as to give a pulsating and substantially continuous flow. Various kinds of positive-displacement pump may be employed in the method and apparatus according to the invention. For example reciprocating piston pumps, diaphragm pumps or lobe pumps may be used.We believe that the more pronounced the pulsations, that is the greater the difference in amplitude between the maximum and minimum widths of the pulsating liquid issuing from the orifice, the more the formation of relatively large bodies of the liquid is facilitated. Further, the formation of relatively larger generally spherical or generally spheroid bodies of the liquid tends, we find, generally accompanied by a greater rate of production of frozen liquid than when substantially all the cream is formed in bodies of particles having a cross-sectional area less than or approximately the same as that of the orifice.In order to produce a suitably pulsating flow of the liquid to be frozen we prefer to use a peristaltic pump. Such a pump also offers the advantage of facilitating hygienic handling of liquid or semiliquid foodstuffs such as cream as there is no direct contact between the moving parts of the pump and the cream.

In use, the nozzle is preferably positioned in relation to the stream of liquefied gas so as to avoid creation of excessive turbulence, that is such turbulence as would inhibit the formation of generally spherical or generally spheroidal bodies of the liquid to be frozen.

Accordingly, in use, the outlet orifice of the nozzle is preferably just (e.g. up to 1 cm) above the surface of the stream of liquefied gas and faces generally towards rather than generally in the opposite direction to the direction of flow of the liquefied gas, the axis of the nozzle typically making an angle of from 10 to 600 with the stream of liquefied gas. Alternatively, the outlet of the nozzle may be positioned underneath the surface of the liquefied gas, although this alternative is not preferred. In order to avoid creating excessive turbulence, it is desirable to provide a substantially laminar flow of liquefied gas along the channel.

It is preferred that the depth of liquefied gas in the channel is maintained in the range 0.5 to 1.5 times (and most preferably 0.6 to 1.2 times) the maximum diameter (i.e. the diameter along the major axis in the case of a spheroid) of the largest spherical or spheroid bodies that are produced.

(For example, the depth may be in the order of 1 cm.) It is then found that the larger size bodies are rolled along the bottom of the channel by the liquefied gas. The bottom of the channel is preferably shaped'so as to facilitate such a rolling action.

The flow rate of the liquid through the orifice is selected in accordance with the invention so as to give relatively large bodies of frozen liquid, i.e.

generally spherical or generally spherical bodies having a diameter (along the major axis in the case of spheroid particles) substantially greater than that of the orifice (which is typically circular).

When using a peristaltic pump to pass double cream continuously through a suitably disposed nozzle having an outlet orifice of a diameter in the range 2 to 3 mm onto the surface of a stream of liquid nitrogen having a depth in the order of 0.5 to 1 cm and flowing along a gently inclined channel, we found there were three distinct phases of operation dependent upon the flow rate of the cream. At the lowest flow rates substantially no frozen spherical or spheroid bodies of cream having a diameter (along the major axis in the case of spheroid particles) greater than that of the orifice were formed. In an intermediate range of flow rates we found that the cream did not break up on the surface of the liquid nitrogen to form spherical or spheroid bodies.

Surprisingly, however, we found that on increasing the flow rate of cream through the nozzle we found that we could again form generally spheroid bodies of frozen cream but this time with a substantial proportion of the bodies having a diameter (along the major axis) of from 0.6 to 0.8 mm. Typically, such bodies can be produced with a cream flow rate in the order of 200 cm3 per nozzle per minute depending on nozzle size, the speed of the liquid nitrogen and viscosity of the cream along other parameters.

Thus, the flow rate of the liquid required in accordance with the invention may be determined by simple experiment.

We also find that when relatively large spheres of spheroids of frozen cream are formed in accordance with the invention, smaller bodies are formed as well. This is not disadvantageous as a range of sizes facilitates the achievement of a high packing density when packaging frozen cream produced by the method accordingly to the invention.

Preferably the speed at which the liquid nitrogen flows along the channel, and the length of the channel are selected so as to avoid freezing the entire mass of the larger bodies of liquid that are formed. The downstream end of the channel is therefore preferably positioned above the inlet of a separator which separates the frozen bodies from the liquefied gas but which allows a sufficient duration of contact between the liquefied gas and/or its cold vapour and the bodies to complete their freezing. Typically, in the separator, the bodies are passed to an outlet in a direction generally opposite to that followed by the liquefied gas as it flows along the channel. The separator can thus be positioned generally below the channel thereby enabling the apparatus according to the invention to be accommodated in a relatively compact housing.

Typicany,the residence time of the liquid in the channel may be selected to be in the order of 3 to 1 5 seconds provided that at least the peripheries of the bodies of the liquid are frozen. Typically, the or each channel may be from one to two metres long and the velocity of the liquefied gas in the range 0.14 to 0.66 metres per second (for double cream we have employed velocities in the range 0.2 to 0.4 metres per second). A suitable liquefied gas flow velocity may be found by simple experiment for a given depth of liquefied gas and given rate of dispensing the liquid to be frozen, amongst other parameters.We have found that the depth of the liquefied gas in the (or each) channel is an important parameter being interrelated with liquefied gas flow velocity and the rate at which the liquid to be frozen is dispensed. Thus, with all other parameters constant, if liquefied gas in the channel is too shallow there will be a build up of liquid to be frozen at the upstream end with consequential formation of relatively long strands or elongate bodies of frozen or partially frozen liquid rather than spherical or spheroid bodies at the upstream and, whereas if the liquefied gas in the channel is too deep a build-up tends to occur at the downstream end as the bodies enter the separator (particularly the preferred kind of separator described hereinabove) with eventual formation of relatively long bodies of frozen cream.Moreover, it is not always possible to make a complete adjustment for excessive depth by decreasing the speed of flow of the liquefied gas as this may result in separate spherical or spheroid bodies of cream agglomerating as the liquefied gas flow along the channel. In general, however, we believe it is possible to operate the apparatus according to the invention satisfactorily if the aforementioned relationship between the depth and the diameter of the largest spherical of spheroid bodies is adhered to.

The separator preferably comprises a perforate rotary drum which in operation permits liquefied gas to fall under gravity into a sump and which is inclined with its inlet end uppermost so as to pass the bodies of liquid to a rotating screw which urges said bodies towards an outlet where they can be collected. The rotating screw preferably urges the bodies up an inclined surface in contact with cold vapour evaporating from the liquefied gas. This arrangement facilitates substantially complete freezing of the spherical or spheroid bodies.

The liquefied gas separated from the said bodies is typically collected in a sump and returned to a reservoir which feed the or each channel by means of a lift pump (for example an Archimedean screw or arrangement of buckets that are moved along a path extending from the sump to the reservoir, the arrangement being such that the buckets scoop liquefied gas from the sump and deposit it in the reservoir).

It is not necessary for the separation of the liquefied gas from the partially or entirely frozen bodies of liquid to be perfect. Indeed, it is sometimes preferred for the perforations or other apertures in the drum to be of a size sufficient for the smallest particles of bodies of frozen liquid to pass therethrough. For example, in the freezing of cream, we prefer to make the apertures of a size that allows particles whose largest diameter is 1 mm of less to pass therethrough and be collected in the sump with the liquefied gas. Thus, such particles are returned to the channels, and in practice a proportion of them agglomerate with larger bodies of cream to form even larger bodies.

If the viscosity of the liquid to be frozen varies significantly with temperature is may be desirable to control the temperature at which the liquid is taken by the pump or other means used to pass it through the orifice. Generally, it is preferred to choose the temperature at the lower viscosity end of the temperature range. For example, in the freezing of, say, double cream, we prefer to supply the cream to the pump at a chosen temperature in the range 13 to 250C, at which temperature its viscosity is significantly less than in the range 1 to 1 00C at which it is typically served.

Before dispensing the liquid to be frozen into or onto the stream of liquefied gas, it is desirable to pre-cool the apparatus by passing liquefied gas through it. It is in particular desirable to maintain the bottom of the or each channel at a temperature not greater than the boiling point of the liquefied gas. If bubbles collect at such surface we find that they tend to hinder the formation of relatively large spherical or spheroid bodies of liquid to be frozen.

Generally, in commercial embodiments of the apparatus according to the invention there will at least three channels extending in parallel with one another, each having its own orifices associated with it.

The method and apparatus according to the present invention will now be described by way of example with reference to the accompanying drawings; in which: Figure 1 is a schematic drawing illustrating means for supplying cream to the nozzles of a cream freezer according to the invention.

Figure 2 is a schematic side elevation, partly in section, of a cream freezer according to the invention.

Figure 3 is a section taken through the line IIIIlI in Figure 2.

Figure 3(a) is a perspective view from above and one side of a bucket in a tipped position, the bucket being as shown in Figure 3.

Figure 4 is a schematic end view of the arrangement of the dispensing nozzles and trough forming part of the cream freezer shown in Figures 2 and 3.

Figure 5 is a schematic side view of the arrangement of the dispensing nozzles and trough forming part of the cream freezer shown in Figures 2 and 3.

Figure 6 is a schema illustrating the reservoir shown in Figures 2 and 3. (Figure 6(a) is side elevation of part of the reservoir shown in Figure 6.) Figure 7 is a schema illustrating the sump shown in Figures 2 and 3.

Figure 8 is a perspective drawing copied from a photograph showing the formation of cream bodies in accordance with the invention.

Referring to Figure 1 of the drawings, a peristaltic pump 2 employs four flexible tubes 4 which at the inlet ends are connected to a header 6 fitted to the outlet 8 of a Pasteuriser 10, and at the outlet ends are four cream dispensing nozzles 1 2 associated with the cream freezer shown in more detail in Figures 2 to 7 of the accompanying drawings.

Referring now to Figure 2, the creaam freezer 14 has a thermally-insulated housing 1 6 through the top of which the nozzles extend in use of the apparatus. Typically, the housing 1 6 comprises inner and outer skins of stainless steel with a suitable insulant (e.g. Perlite) therebetween. The purpose of the insulation is to reduce the rate at which heat would otherwise be absorbed by the freezer 14.

Situated within the housing 1 6 towards the left hand end (as shown) thereof is the reservoir 18 having an outlet in a dam or baffle 20 adapted to feed liquid nitrogen to the upstream end of a trough 22 having four channels 24 for the flow of liquid nitrogen. (The arrangement of the reservoir 18 and the dam 20 will be described in more detail below with reference to Figure 6.) The channels 24 extend generally parallel to one another and are generally downwardly inclined from their respective upstream ends to downstream ends at the right hand ends (as shown) of the housing 16, the slope being in the order of 1 in 35. Extending from beyond the downstream end of the trough 24 is a guide plate 26 which terminates within and near the inlet of a first rotary drum 28.The guide plate 26 has an upper generally vertical portion extending just above the top of the trough and an integral lower portion sloping downwardly to the drum 28. In operation, liquid nitrogen falling from the downstream end of the trough 22 is guided by the guide plate 26 into the rotary drum 28. The rotary drum 28 is generally of frusto-conical shape with its inlet end being narrower than its outlet end and its longitudinal axis being horizontal. There is thus a gentle slope down which the bodies of cream are able to tumble in operation of the cream freezer. The lower or outlet end of the drum 28 has narrow slots 30 formed through it. In operation, this permits liquid nitrogen to fall under gravity through the slots 30 while partially or fully frozen spherical or spheroidal bodies of cream are retained. This enables the cream to be separated from the liquid nitrogen.Located beneath the slotted end of the drum 28 is a downwardly inclined guide plate 32 which is adapted to collect liquid nitrogen falling through the slots 28 in operation of the apparatus. The guide plate 32 slopes towards a sump 34 adapted to collect the liquid nitrogen.

The outlet end of the rotary drum 28 is joined by means of flanges 36 to a second rotary drum 38 which is formed with narrow slows 40 adapted to separate any residual liquid nitrogen from frozen spherical or spheroid bodies of cream and to permit nitrogen vapour evolved from the sump 34 to pass into the interior of the drum 38. A guide plate 42 slopes downwardly from the outlet end of the drum 38 to the sump 34 so as to collect any such liquid nitrogen in operation of the apparatus and guide it into the sump 34. The drum 38 is of generally frusto-conical shape and is positioned with its longitudinal axis horizontal.

There is thus an upward slope along which the frozen bodies of cream are propelled in operation of the cream freezer. The inlet end (the right hand one as shown in Figure 2) of the drum 38 is joined by flanges 36 to the outlet end of the drum 28.

Within the drum 38 there extends a shaft 44 to which a screw 46 is attached or formed integral therewith. The screw 46 is adapted to propel frozen spherical or spheroidal bodies of cream in an upward direction along the surface of the drum 38 to an outlet 48 down which such bodies of cream are able to be fed into a collection tray or device 50 which may extend through the insulated housing 1 6 into the interior of the freezer 14 at a region below the outlet 48.

The sump 34 has located thereabove a spray header 52 connected to a source 59 of liquid nitrogen (typically a vacuum-insulated vessel adapted to supply liquid nitrogen), via a pipe 54 having a flow control valve 56 disposed therein.

The sump 34 has upper and lower level sensing elements 58 disposed therein, the arrangement being that the valve 56 opens automatically on the lower of the level sensing elements 58 becoming exposed thus causing liquid nitrogen to be sprayed into the sump 34 and closes again automatically on the upper one being covered.

Thus causing the supply of liquid nitrogen to the sump 34 to be discontinued. Typically, to effect this, electrical signals are relayed from the sensing elements 58 to a control box 60 mounted on the drum, which control box 60 is adapted to generate signals in response to the sensors so as to open and close the valve 56. For example, the control box 60 may generate electrical signals and the valve 56 may be a solenoid valve, the electrical signals appropriately energizing and de-energizing the solenoid. By such means, the level in the sump 34 can be kept between chosen minimum and maximum values.

The rotary drums 28 and 38 are driven by means of an electric motor 62. There is a belt-andpulley drive 64 that transmits the drive from the motor 62 to the rotary drums 28 and 38. The motor is typically mounted to the outside of the housing 16.

At the outlet end of, but within the rotary drum 38 is a gas outlet 51 communicating with a fan 53 located outside the housing 1 6. Operation of the fan creates a positive flow of nitrogen vapour along the interior of the drums 28 and 38 in the direction of passage of the cream bodies thereby facilitating complete freezing of the cream.

The sump 34 may have at its bottom a drain pipe 66 having a tap 68 therein. The pipe 66 typically extends through the bottom of the housing 16, the tap 68 being located at a position outside the housing 16 so as to permit manual operation.

As shown in Figures 2, 3 and 3(a) a lift pump is adapted to transfer liquid nitrogen from the sump 34 to the reservoir 18, the lift pump including eight generally radially disposed and equally spaced apart buckets 70. Each bucket 70 is pivoted to a rod 73 that extends through diagonally opposite corners at the mouth thereof.

Each rod 73 extends generally horizontally from an elbow-piece 74 that is welded or otherwise fixed to the flange 36. Each bucket 70 is formed with a relatively low wall portion 72 at one end face 77, such portion 72 being provided at an unpivoted corner and being in part bounded by a side wall 76 having a counterbalancing generally triangular baffle 78 integral therewith, said baffle helping, in operation, to direct the liquid nitrogen through the portion or channel 72 when the bucket is tipped.

The pivoting of the buckets 70 is arranged such that, when full of liquid, they do not normally tip to one side or the other spilling liquid nitrogen. In operation, the buckets are rotated along a path which at its lowermost region extends through the sump 34 whereby the buckets are able to scoop up liquid nitrogen from the sump and which at its uppermost region extends over the reservoir 1 8. In order to effect emptying of the buckets, the end face 77 of each bucket preferably has projecting therefrom an external lug 75 carrying a small cylindrical cam follower 78 which, as the buckets are rotated, is adapted to follow a cam surface 80 (omitted for the purposes of clarity of illustration from Figure 2) at an upper region of the circular path (see Figures 3 and 3(a). The cam surface 80 is shaped such that as each bucket 70 travels over the reservoir 1 8 so a moment is applied to the bucket causing it to pivot about the rod 73 such that liquid nitrogen passes through the channel 72 and falls under gravity into the reservoir 1 8. If desired, an overflow pipe 82 may be provided in the reservoir 1 8 and conduct excess liquid nitrogen back to the sump 34. If desired, the overflow pipe 82 may not make a fluid-tight fit of the bottom of the reservoir 1 8 so as to allow a small trickle of liquid to fall from the reservoir 18 into the sump 34 when the reservoir contains liquid.Such an arrangement facilitates cleaning of the apparatus after use, as water is thereby able to pass directly from the reservoir 1 8 to the sump 34, thereby allowing the reservoir to drain itself of liquid. This arrangement of the reservoir is shown in more detail in Figure 6.

As shown in Figure 6, the floor of the reservoir 18 slopes towards the upstream end of the trough 22. The dam or baffle 20 is a plate which has at its bottom a row of generally rectangular slots 101 formed therein (see Figure 6(a)). The slots 101 cooperate with the floor of the channels 24 to permit a laminar flow of liquid nitrogen from the reservoir along the channels 24 to take place. The overflow pipe 82 is positioned such that liquid nitrogen cannot, in operation, flow over the top of the dam or baffle 20. The baffle or dam 20 is typically curved presenting in transverse crosssection a generally concave face to the trough 22.

In order to help provide a laminar flow of liquid nitrogen from the reservoir 1 8 to the trough 22, a perforate plate 103 cooperates with the floor of the reservoir 18. The plate 103 has a multitude of apertures 105, each of a diameter in the order of half an inch, formed therethrough. The plate 103 extends generally parallel to the floor of the reservoir 1 8 and is typically positioned about 1 cm above the floor. In operation, the plate 103 helps to dampen turbulent flow that might be caused by liquid nitrogen pouring out of the buckets 70 (see Figure 3) into the reservoir 1 8.

Referring now to Figures 4 and 5 of the accompanying drawings, the trough 22 has four similar channels 24. Each channel 24 has a curved bottom portion 86 of generally arcuate crosssection integral with inclined generally rectangular sides making an angle of about 450 with the horizontal. The depth of the arcuate portion 86 of each channel is typically in the order of half a centimetre. The length of its longest chord is typically in the order of 1 centimetre.

The channels 24 are open at their tops so that, in operation, there is an interface between the liquid nitrogen flowing along the channels 24 and the gas space thereabove. The nozzles 12 depend from a support arm 90 passing through a slot 92 in the housing 1 6. The support arm 88 is mounted on an hydraulic or pneumatic cylinder 94 (see Figure 4) operabie to raise and lower the nozzles 12. In their lowermost positions, the nozzles 12 terminate in the respective channels 24 of their outlet 96 about half a centimetre above the level of the liquid nitrogen flowing along the channels 24. The nozzles 12 may, however, be retracted from such position (for example, for cleaning) by operation of the cylinder 94 (which is typically operatively associated with the control box 60).

The axes of the outlets 96 of the nozzles 1 2 typically face towards the downstream end of the channels 24 making an angle of approximately 200 with the surface of the liquid nitrogen in the channels. Moreover, the axes of the nozzles at their outlets are desirably coplanar with a perpendicular plane bisecting the respective channels.

A spray tube 98 having spray orifices 100 located along most of its length extends from outside the housing 1 6 into the interior of the drums 28 and 38 and is connectible to a source of water or other cleaning fluid to enable the drums to be cleaned after use.

The control box 60 typically comprises pneumatic, hydraulic electrical or electronic control circuits of a kind well known in the art of cryogenic engineering to enable the following operations to be completed. By operation of a manual switch or push button (not shown) provided on the control box, the motor 62 is energised so as to start rotation of the drums 2 and the buckets 70 and, the valve 56 is opened to initiate liquid nitrogen supply from the spray header to the sump 34 and hence to the reservoir 1 8. The fan 53 is simultaneously energised thereby creating a flow of cold nitrogen vapour along the drums 28 and 38 as aforesaid. The liquid nitrogen flows from the reservoir 1 8 through the dam 20 and along the trough 22. It then flows into the separator falling through the slots in the drum 28.The liquid nitrogen falls through the slots in the drum 28 and collects in the sump 34. The level sensing elements 58 keep the level of liquid nitrogen in the sump 34 between chosen limits by sending signals to the control box 60 to close and open the valve 56 as appropriate. Initially, the apparatus may be at ambient temperature, and thus much of the liquid nitrogen first supplied to the sump 34 and the other parts of the cream freezer 14 will evaporate. Gradually, the rate of evaporation will diminish as the cream freezer cools down. After several minutes of operation, the temperature of the bottom portions 86 of the channel 24 will be lowered to the boiling point of the liquid nitrogen (-1 960C). This may, for example, take 5 minutes.There is thus typically a timer circuit in the control box 60 which a predetermined period after energizing the motor 62 causes the hydraulic cylinder 94 to lower the nozzles 12 into their lowermost positions just above the surface of the liquid nitrogen at the upstream end of the channels 24 and which simultaneously energizes the peristaltic pump 2 to pass cream from the Pasteuriser 10 to the nozzles 12.

The peristaltic pump 2 is set to give a relatively high flow rate therethrough. Typically, for nozzles having outlet of 2 to 3 mm in diameter, the flow rate is at least 1 50 cubic centimetres per nozzle per minute. This is in comparison to a liquid nitrogen flow velocity along the channels 24 in the order of 0.2 to 0.6 metres per second and a liquid nitrogen depth (measured from the bottom of the channels) of 1 centimetre. (The channels are approximately 2 metres long and therefore the liquid nitrogen flow rate is in the order of 1 litre per channel per minute.) Referring now to Figure 8 of the drawings, the cream issues from each nozzle 12 as a continuous and pulsating stream generally circular in crosssection. The pulsation is such that the crosssectional diameter of the crease at successive minima is appreciably less than the diameter of the outlet 96 of the respective nozzle 1 2.

The pulsating stream issuing from the nozzle tends to float on the surface of the flow liquid nitrogen in the channels 24 as a result of gas bubbles collecting underneath the cream. We believe that on the surface of the flowing nitrogen portions of the stream of cream of maximum cross-sectional area advance towards the downstream end of the channels 24 at a different speed from the portions of minimal cross-section.

This differential velocity tends to accentuate the difference in cross-section between maxima and minima and results in the formation of a length of cream having a pronounced head of greater crosssectional area of the respective nozzle outlet 96 and a relatively thin tail adjoining an upstream length of cream in which the accentuation between maximum stream thickness and minimum stream thickness is not so pronounced.

The tail then breaks thereby forming a separate head-and-tail length of cream which soon forms itself into a relatively large body that typically is typically of sufficient size to touch the bottom of the respective channel 24 and be rolled therealong by the flow of liquid nitrogen. It may be that the head touching the bottom of the channel happens before the breaking of the tail takes place. It is to be emphasised however, that the above description of how the relatively large spheroidal bodies of cream are formed may be a simplification of a complex hydrodynamic process and is in no way intended to limit the scope of the invention. When the tail breaks we also believe that relatively small particles of cream typically having a diameter not greater than 1 mm are formed.These smaller particles float on the surface of the liquid nitrogen and are carried with it towards the downstream end of the channels 24. The bodies that are rolled down the surface of the channels 24 by the liquid nitrogen travel more slowly than those which are carried on the surface of the liquid nitrogen.

The velocity of the flow of the liquid nitrogen along the channels 24 is arranged such that by the time the bodies reach the downstream end of the channels 24, they have had sufficient contact with the liquid nitrogen to be frozen at their peripheries but to remain liquid inside.

Typically, the residence time is chosen to be in the order of 5 to 10 seconds. The partially frozen spheroidal bodies of cream, together with smaller bodies (some of which may be partially frozen) are carried by the stream of liquid nitrogen over the outlet end of the channel 24 onto the guide plate 26 and from there into the rotary drum 28. At least a major portion of the drum 28 has slots 30 in it to enable the liquid nitrogen to pass therethrough. This causes the liquid nitrogen to separate from the bodies of cream. The liquid nitrogen collects on the guide plate 32 and flows under gravity into the sump 34. The slots 30 are sized such that some of the smallest particles of cream fall through the slots with the liquid nitrogen.

It is to be appreciated that once the bodies of cream are separated from the liquid nitrogen cooling does not end. The considerable volume of cold nitrogen vapour, and rotation of the drum 28 causes intimate contact between this cold nitrogen vapour and the bodies of cream.

Moreover, operation of the fan 53 causes a flow of nitrogen vapour from the drum 28 into the drum 38 and the gas cooling is continued in the drum 38 as the bodies of cream pass into it from the drum 28. The screw 46 then propels the bodies of cream upwards to the outlet 48 through which they fall under gravity to be collected in the collecting device 50. Any residual liquid nitrogen or small particles of cream fall through the slots 40 under gravity and are collected in the sump 35.

Typically, the residence time of the bodies of cream in the part of the freezer 1 4 intermediate the end of the channels 24 and the outlet 48 is in the order of 10 to 20 seconds. This is generally sufficient to enable even the centres of the largest bodies to be frozen without giving rise to over freezing (i.e. cooling to a temperature below about -300C, when, there is a tendency for substantially all the largest bodies to crack). It is an advantage of the apparatus according to the invention that it is capable of being operated so as to prevent both undercooling of the cream (exhibited by incomplete freezing of the largest bodies) and over-freezing (exhibited by cracking of substantially all the largest bodies). Typically, some cracking of the largest spheroidal bodies may take place as these are relatively dimensionally unstable.

The operation of the lift pump returns liquid nitrogen from the sump 34 to reservoir 12 together with the small particles of frozen cream that have been collected therein. The lift pump employing buckets shown in Figures 2, 3 and 3(a) of the accompanying drawings is found to be particularly advantageous as it does not employ any valves and is therefore not prone to valve failure as it is able to cope with particles or small bodies of frozen cream and as it is relatively easy to clean. The particles of cream returned to the reservoir 1 8 are carried with the liquid nitrogen under the dam 20 onto the flowing liquid nitrogen in the channels 24. At least some of the particles then merge or agglomerate with cream freshly introduced onto the flowing stream of liquid nitrogen through the nozzles 12.This phenomenon helps to give a wide range of varying particle sizes from the smallest of typically 1 mm (depending on the size of the slots 20) up to 8 or 9 mm. Such a distribution in the sizes of the spheroidal bodies that are formed helps to improve the packing density of the resultant frozen cream.

Substantially, all the cream is formed into spheroidal bodies. Typically, at least 25% and generally at least 50% of the bodies of cream that are formed have a diameter along their major axis of at least twice the diameter of the outlets 96 of the nozzles 1 2. We have found it possible to collect a frozen cream product including at least 85% by weight of such relatively large bodies. We have also found it possible to collect a frozen cream product including at least 90% by weight of bodies having a diameter (or length) along the major axis of at least three times the diameter of the nozzle. The frozen bodies of cream that are collected may typically be packaged (e.g. in suitable plastics bags, or containers) and stored in a freezer ready for use.We have found that on allowing the frozen cream to thaw, there is not substantial deterioration and, in particular, when using it in hot drinks, there is no separation of fat from the cream.

The term "generally spherical or generally spheroid bodies" is used herein to mean bodies that include a curved surface or curved edge. As well as regular spheres and spheroids, disc shapes and egg-shapes are included. Moreover, the bodies need not be of a geometrically regular shape. Typically, we find that the bodies produced are irregular spheroids, the curvature of the bodies being less pronounced than in a regular spheroid.

The method according to the present invention is further illustrated by the following example.

EXAMPLE An apparatus substantially as shown in Figures 2 to 7 was used to produce frozen bodies of double cream.

The apparatus employed four nozzles 1 2 each associated with its own channel 24. The channels 24 were each 1 mm 70 cm in length and were downwardly inclined, the slope being 1 in 35.

Each nozzle 1 2 has an outlet with an internal diameter of 2 mm.

The apparatus was first cooled down by initiating a flow of liquid nitrogen therethrough.

The buckets 70 and drums 28 and 38 were rotated at 7 revolutions per minute. When the apparatus had been cooled, the nozzles were lowered into a dispensing position just above the liquid nitrogen level in the channels (about 0.5 mm above), the axes of the nozzles 12 making an angle of approximately 250 with the trough 22.

Pasteurised double cream was fed to the nozzle 12 by a Watson-Marlow 301 four roller peristaltic pump. The pump was operated at 141 6 revolutions per minute and four continuous pulsating streams of cream were produced. Thus, each stream was produced with 354 pulses per minute. The tubing employed in association with the peristaltic pump was of silicone rubber having an internal diameter of 4.8 mm and a wall thickness of 1.6 mm.

Cream was dispensed from each nozzle in a continuous, pulsating stream at a rate of 1 90 cm3 per minute. The velocity of liquid nitrogen flow along each channel was 0.4 metres per second.

The depth of the liquid nitrogen in each channel was 7 mm in normal operation (i.e., with cream being dispensed at the aforementioned rate).

Frozen "generally spheroid" bodies of cream were collected.

An estimate was made of the size distribution of the bodies. According to this estimate, about 95% by weight of the bodies of cream were from 7 to 9 mm long; about 3% in weight from 5 to 7 mm long, about 1% by weight from 1 to 5 mm long, and less than 1% were less than 1 mm long.

The length referred to was the length along the longest dimension of each body.

It is not necessary to employ the fan 53 to create a flow of nitrogen vapour along the interior of the drums 28 and 38. Such flow will tend to take place, the nitrogen vapour being exhausted through the outlet 51, even if fan 53 is omitted. If desired, however, a fan may be employed to extract nitrogen vapour from the vicinity of the tray 50 in the interior of the housing.

Claims

1. A process for freezing a liquid to form generally spherical or generally spheroid frozen bodies of liquid, comprising passing the liquid through at least one orifice and causing a pulsating and substantially continuous flow of the liquid to pass into or onto a stream of liquefied gas, having a boiling point below --300C, flowing along a channel, the rate of passage of the liquid through the orifice being sufficiently large for a proportion of the liquid to form into discrete bodies of greater cross-sectional area than the orifice; maintaining the bodies in contact with the liquefied gas for a time sufficient for at least their peripheries to freeze, and collecting the resulting generally spherical or generally spheroid frozen bodies of liquid.

2. A process as claimed in claim 1, in which at least 85% by weight of the liquid to be frozen is collected in the form of bodies having a diameter (or length) along the major axis of at least twice the diameter of the orifice.

3. A process as claimed in claim 2, in which at least 90% by weight of the liquid to be frozen is collected in the form of bodies having a diameter along the major axis of at least three times the diameter of the orifice.

4. A process as claimed in claim 1, in which the liquefied gas is liquid nitrogen.

5. A process as claimed in any one of the preceding claims, in which the liquid is passed through the orifice by means of a positivedisplacement pump.

6. A process as claimed in claim 5, in which the pump is a peristaltic pump.

7. A process as claimed in any one of the preceding claims, in which at least some of the said bodies are rolled along the bottom of the channel by the liquefied gas.

8. A process as claimed in any one of the preceding claims, in which the depth of the liquefied gas in the channel is in the range of 0.5 to 1.5 times the maximum diameter (or width) of the said bodies.

9. A process as claimed in any one of the preceding claims, in which the liquefied gas and the said bodies are passed from the said channel into a separator including a member having apertures therein to allow the liquefied gas to fall under gravity into a sump.

10. A process as claimed in claim 9, in which the smallest bodies of the said liquid pass through said apertures with the liquefied gas.

11. A process as claimed in claim 9 or claim 10, in which the liquefied gas is returned from the sump to a reservoir which feeds liquefied gas to the channel.

12. A process as claimed in any one of the preceding claims, in which only the peripheries of the largest bodies of the liquid are frozen which such bodies are in contact with liquefied gas flowing along said channel.

13. A process as claimed in claim 12, in which completion of the freezing of said largest bodies is completed in a separator in which the bodies are separated from the liquefied gas.

14. A process as claimed in claim 13, in which said freezing if completed at least in part by contact of the said bodies with cold vapour evolved by the liquefied gas.

15. A process as claimed in any one of the preceding claims, in which the residence time of the said bodies in contact with the liquefied gas in the channel is in the range 4 to 1 5 cms.

16. A process as claimed in any one of the preceding claims, in which there are one or more nozzles through which the liquid to be frozen is dispensed onto the stream of liquefied gas, the or each nozzle having a diameter in the range 1 to 3 mm.

17. A process as claimed in claim 16, in which the rate of passing the liquid through the or each nozzle is at least 1 50 cm3 per minute per nozzle.

1 8. A process as claimed in any one of the preceding claims, in which the liquid to be frozen is cream.

19. A process of freezing a liquid, substantially as hereinbefore described with reference to the accompanying drawings.

20. A process of freezing a liquid substantially as described in the Example.

21. A novel process of freezing a liquid including any one or any combination of the features disclosed herein.

22. Apparatus for freezing a liquid, comprising means defining at least one orifice; at least one channel, the orifice-defining means being adapted to be positioned above the channel to dispense the liquid to be frozen into the channel: means for creating a stream of liquefied gas along the channel, the liquefied gas having a boiling point of less than minus 300C; means for passing the liquid to be frozen through the orifice and for creating d pulsating and substantially continuous flow of the liquid into or onto the stream, and means for collecting frozen bodies of the liquid from the apparatus, whereby, in operation, the liquid can form itself into discrete, generally spherical or spheroid bodies at least some of which are of greater cross-sectional area than the orifice and the bodies can be maintained in contact with the liquefied gas for a time sufficient for at least their peripheries to freeze.

23. Apparatus as claimed in claim 22, in which the said channel has a downwardly sloping bottom along which at least the larger bodies are able to be rolled by the liquefied gas.

24. Apparatus as claimed in claim 22 and 23, in which the orifice is defined by a nozzle.

25. Apparatus as claimed in any one of the preceding claims, in which the orifice has a diameter in the range 1 to 3 mm.

26. Apparatus as claimed in any one of claims 22 to 25, in which a positive-displacement pump is adapted to pass liquid through the orifice and thereby form the pulsating and continuous flow of the liquid.

27. Apparatus as claimed in claim 26, in which the positive-displacement pump is of the peristaltic kind.

28. Apparatus as claimed in any one of the claims 22 to 27, in which the downstream end of the channel is positioned above the inlet of a separator adapted to separate the bodies from the liquefied gas.

29. Apparatus as claimed in claim 28, in which the separator is adapted to separate the liquefied gas and relatively small bodies of frozen or partially frozen liquid from the rest of the bodies of the frozen or partially frozen liquid.

30. Apparatus as claimed in claim 28 or claim 29, in which the separator comprises a perforate (or apertured) rotary drum which in operation permits liquefied gas to fall under gravity into a sump, the apparatus additionally including means for returning liquefied gas from said sump to a reservoir adapted to pan liquefied gas along the channel.

31. Apparatus as claimed in claim 30, in which the means for returning the liquefied gas to the reservoir comprises a lift pump.

32. Apparatus as claimed in claim 31, in which the lift pump comprises a plurality of radially disposed buckets adapted to be moved around a circular path such that, in operation, the bucket scoop liquefied gas from the sump and deposit in the reservoir.

33. Apparatus as claimed in claim 32, in which each bucket is pivoted, and has a cam follower projecting therefrom, the apparatus additionally including a cam surface positioned in relation to the reservoir, such that, in operation, the cam surface and cam follower cooperate together to tip the bucket, whereby liquefied gas is poured from the bucket into the reservoir.

34. Apparatus as claimed in any one of claims 28 to 33, additionally including a fan operable to create a directional flow of cold vapour (evolving from the liquefied gas) along the separator, whereby, in operation, complete freezing of partially frozen bodies entering the separator is able to be achieved.

35. Apparatus for freezing a liquid substantially as herein described with reference to, and as shown in, Figures 2 to 7 of the accompanying drawings.

36. Apparatus for freezing a liquid, including any novel feature or novel combination of features described herein.

37. Frozen bodies of liquid, whenever produced by the method claimed in any one of claims 1 to 21 or using the apparatus claimed in any one of 22 to 36.

38. Frozen cream, whenever produced by the method claimed in any one of the claims 1 to 21 or using the apparatus claimed in any of 22 to 36.