The present invention relates to scraped surface evaporators, and in particular to scrapers for use in scraping the surface of the evaporator.

Scraped surface evaporators are a known type of fluid cooler for viscous fluids or for fluids near their freezing point. Highly viscous or frozen material forms on a refrigerated surface and is scraped therefrom and mixed with the main bulk of fluid which has not been refrigerated to the same extent. By this means it is possible to cool and/or emulsify greater amounts of substances than would otherwise be capable of being cooled, because, in the absence of scraping, a thick boundary layer forms on the refrigerated surface as a result of the freezing and/or increased viscosity of the substance at the refrigerated surface. Scraped surface evaporators are used for instance in the production of margarine, the solidification of fatty products, the production of ice cream and in the production of pumpable ice slurries.

Scraped surface evaporators typically comprise a refrigerated drum, the outside of which is refrigerated while the substance to be cooled is passed through the inside of the drum. The inner surface of the refrigerated drum is cooled sufficiently to freeze or solidify material in the region of the inner surface. The frozen/solidified product is then removed from the inner surface by a scraper. The scraper is generally attached to a rotating drive mechanism, located within the refrigerated

drum, such that a scraper blade is moved around the inner circumference of the refrigerated drum, scraping off any product formed thereon. Typically more than one scraper (for example 2, 4 or 8) is used to scrape the inner surface of the refrigeration drum.

It is important that the scraper blade makes a good contact with the drum's inner surface, otherwise the product may not be removed efficiently from the surface. In addition it is desirable to use a scraper blade which is not made of metal, as a metal blade could wear away the inner surface of the drum.

Conventional designs of scrapers use scraper blades made of a plastics material, typically nylon, polypropylene or polyethylene, and which are carried in a metal housing from which integral hinge pins project. The hinge pins locate at the periphery of the driving mechanism, such as a drive drum, which allows a scraper blade to be moved across the inner surface of the refrigeration drum. In order to ensure contact of the scraper blade with the surface to be scraped, a separate spring is provided, which is in connection with the periphery of the driving mechanism and the scraper. The spring in turn acts to bias the blade into contact with the surface.

The above described system works satisfactorily under normal conditions. However, if the flow of material through the drum is interrupted or the refrigeration of the drum is too great, such that a thicker layer of product is formed on the drum's inner surface, the strain on the scraper can cause the hinge pins and/or the spring to break. To repair the scraper, the evaporator must be stopped, resulting in lost production.

It is thus an object of the present invention to obviate and/or mitigate the above disadvantages by providing an improved scraped surface evaporator.

It is a further object of the present invention to obviate and/or mitigate the above disadvantages by providing a scraper for use in such a scraped surface evaporator.

Generally speaking the improvement lies in the discovery of a scraped surface evaporator that utilises a self-biasing scraper that does not require separate biasing springs.

The present invention provides a scraped surface evaporator comprising: a refrigerated drum; drive means within the drum arranged to rotate about the longitudinal axis of the drum; and at least one scraper comprising an elongate member having longitudinally extending leading and trailing edge regions comprising leading and trailing edges and being mounted on the drive means so that the longitudinal axis of the elongate member is substantially aligned with the drum axis and so that a radial biasing force is applied to the member, between said edges, to maintain both the edges in contact with the inner surface of the drum.

Preferably the scraped surface evaporator is of the form in which the outside of the drum is refrigerated and a substance to be frozen or solidified is passed through the inside of the drum and a frozen or solidified product forms on the inner surface of the drum. The product is in turn removed by the leading edge of the scraper being moved over the inner surface.

Preferably more than one scraper is used to scrape the surface, such that in combination, a multiplicity of scrapers provide a means of scraping the entire inner surface. More preferably 2, 4 or 8 scrapers may be used - even numbers more easily providing a balanced arrangement.

The scraper(s) and the drive means may be integral such that the scraper(s) and the drive means are arranged in a single unit which can be removed from the drum and/or replaced in one operation. Alternatively, the scraper(s) may be arranged so as to be releasably secured to the drive means, so that individual scrapers can be removed as and when necessary. For example, the drive means may comprise a strut located at the periphery of the drive means onto which the scraper may be releasably mounted. Alternatively a rod may be associated with the periphery of the drive means, on to which the scrapers may be releasably mounted directly, or by way of connectors mounted thereon. Generally the scraper is hinged to the drive means.

The drive mechanism may be in the form of an inner drum, which is adapted to fit inside the refrigeration drum and arranged such that the scraper(s) mounted thereto contact the inner surface of the refrigerated drum. In another embodiment the drive mechanism may be in the form of a central drive spindle from which several arms radiate towards the surface to be scraped. Typically the spindle may have two or four arms radiating therefrom, but more or less may also be employed. A scraper is in turn mounted onto each arm of the drive spindle.

A single drive mechanism may be used within a drum, or alternatively a plurality of drive mechanisms can be arranged

end-to end, with the plurality of drive mechanisms arranged to be driven by a single motor. For example, a drive rod may pass through all of said drive mechanisms and extend therefrom. A drive chain is then connected between the drive rod and motor in order to affect driving of said drive mechanisms. The scrapers of said drive mechanisms can be longitudinally aligned with one another or optionally offset with respect to each other. If a single drive mechanism is to be used then it is preferable that the length of the elongate member is substantially equivalent to the length of the drum. When more than one drive mechanism, to be aligned end to end, is used, the elongate member may be equivalent in length so that of each drive mechanism. Such that, when end to end said elongate members can contact substantially the length of the drum.

In a further aspect the present invention provides a scraper for use in a scraped surface evaporator, the scraper comprising an elongate member having longitudinally extending leading and trailing edge regions comprising respective leading and trailing edges; the leading edge being adapted for contacting the surface at a first contact position for scraping solidified material off the surface, and the trailing edge being adapted for contacting the surface at a second contact position, spaced from the first position; and further comprising mounting means adapted to enable mounting of the scraper to drive means for driving the scraper over the surface; and wherein mounting the scraper on the mounting means results in a biasing force being applied to the member which is operative in a direction which passes between said leading and trailing edges, such that said leading and

trailing edges are biased into contact with said first and second contact positions respectively.

Preferably the scraper is of a one piece construction, thus removing any requirement to connect separate parts together. This has the advantage of eliminating weak links that can occur at connection points. Furthermore, should a scraper become worn or broken, it can be quickly and easily replaced.

Preferably the scraper is formed of a resilient plastics material, such as nylon, polypropylene or polyethylene. If the scraper is of one piece construction, then the entire scraper can be formed by conventional plastics moulding techniques such as extrusion or injection moulding techniques.

Preferably the leading and trailing edge regions of the scraper are disproportionate in size to each other. More preferably the trailing edge region is longer than, and generally thinner than, the leading edge region. This provides a greater flexibility to the trailing edge region, which can flex and distort due to contact of the trailing edge with the surface. Without wishing to be bound by any particular theory, it is believed that this imparts greater stability to the leading edge region and minimises undesirable movement of the blade away from contact with the surface.

The mounting means may be any suitable means which allows the scraper to be mounted on the drive means. Preferably the mounting means is in the form of a relieved notch, formed in the scraper. The scraper may then be mounted onto a mounting means, such as a rod, on the drive means which is arranged to receive the notch of the scraper such as to allow hinging of the scraper.

If necessary the scraper can be retained thereon by suitable retaining means, such as a clamp or the like. Most preferably the relieved notch is adapted such that mounting on the mounting point does not require additional retaining means. For instance, the notch may be arranged so as to allow the scraper to be sprung or "snap-fitted" into place.

In a yet further aspect there is provided a method of scraping a surface of a scraped surface evaporator, which employs the scraper of the present invention.

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings which illustrate a scraped surface evaporator and a scraper for use in such an evaporator, in accordance with preferred embodiments of the present invention.

Figure 1(a) shows an end view of a prior art scraper connected to the periphery of a driving mechanism and in contact with a surface to be scraped.

Figure 1(b) shows the prior art scraper of Figure 2(a) in perspective.

Figure 2 shows an end view, in cross-section, of a scraped surface evaporator in accordance with an embodiment of the present invention.

Figure 3(a) shows an end view of a scraper in accordance with an embodiment of the present invention, connected to the periphery of a driving mechanism and in contact with a surface to be scraped.

Figure 3(b) shows the scraper of Figure 2(a), disconnected from the driving mechanism, in perspective.

Reference is made firstly to Figures 1(a) and 1(b) which show a prior art scraper 50 which comprises a metal housing 52, through which an integral hinge pin 54 is provided. The housing 52 has a recess 56 into which a scraper blade 58 is received. The scraper 50 is connected to an arm end 60 of a drive spindle, similar in construction to that shown in Figure 2, by the hinge pins 54 being received by a hole formed in the arm end 60.

The blade 58 is biased into contact with a surface 62 by a spring 64. The spring 64 is located around the hinge pin 54 with a spring end 66 of the spring 64 being fixed in position by a spring retaining block 68. A second spring end 72 contacts a side 74 of the housing 52. By the nature of the spring 64, the spring end 72 acts upon the side 74 of the housing 52, so as to bias the housing 52, and associated blade 8 into contact with the surface 62.

Figure 2 shows a view from one end and in cross-section, of a scraped surface evaporator 10 which comprises a refrigerated drum 12. Within the drum 12, at its centre, is a drive rod 14 which carries a drive spindle 16. Drive rod 14 is mounted in bearings (not shown) in the end of the refrigerated drum. The drive spindle 16 has four arms 18 which radiate outwards towards the inner surface 20 of the drum 12. At the end 22 of each arm 18 is mounted a scraper 30. The scraper 30 is mounted to the arm by way of a rod 24 connected to the end 22 of the arm 18.

The scrapers 30 are arranged to contact and scrape the inner surface 20 of the drum (as will be described with reference to Figures 1(a) and 1(b)) such that upon rotation of the drive spindle 16 by the drive rod 14, in the direction of the arrow,

the scrapers 30, scrape the inner surface 20 of the drum 12.

Reference is now made to Figures 3(a) and 3(b) which show a scraper 30 in accordance with an embodiment of the present invention. The scraper 30 is of a unitary construction and formed of a polyethylene plastics material. The scraper 30 comprises a leading edge region 32 and a trailing tail edge region 34. The scraper 30 further comprises a relieved notch 36, which allows mounting of the scraper 30 to be made to a rod 38 located at an end 40 of an arm 42 of a drive spindle (as shown in Figure 1). Movement of the arm 42, by the driving mechanism, in the direction of the arrow, moves the scraper 30 over the surface to be scraped.

The relieved notch 36 has been shaped so that the scraper 30 is sprung or "snap-fitted" into place and there retained. The relieved notch is similar in shape to a keyhole. The notch 36 has a "v" portion 37 and a circular portion 39. The rod 38 is received by the "v" portion 37 and directed towards a mouth 41 of the circular portion 39, which opens up to allow the rod 38 to be received by the circular portion 39. Once the rod 38 is received by the circular portion 39, the mouth 31 closes around the rod 38 and the rod 38 is thus retained by the notch 36. The scraper 30 can if necessary pivot around the rod 38 which is robust and adequately supported.

The leading edge of the scraper 30 is formed into a blade 44 designed to scrape a product off a surface 46. As in the prior art scraper 50, the scraper blade 44 of the scraper 30 of the present invention is biased into contact with the surface, but without the use of separate spring. The scraper 30 is

adapted so that a biasing force applied to the scraper 30 by mounting the scraper 30 on the rod 38 is operative in a direction which passes between the leading and trailing edge regions 32, 34 so that the blade 44 and a trailing edge 48 are biased into contact with the surface 46 at first and second contact points 50, 52 respectively.

The trailing edge region 34 is greater in size than the leading edge region 32. The trailing edge 34 provides an inherent resilient nature to the scraper 30, and is proportioned, such that when the scraper 30 is mounted on the drive rod 38, the trailing edge 34 flexes and distorts due to contact with the surface 46. Any change in a biasing force applied to the scraper 30, such as may occur due to wearing of the scraper blade 44, results in a greater or lesser flexing of the trailing end 34 while ensuring that the blade 44 and trailing edge 48 remain in contact with the first and second contact points 50, 52 respectively. Thus, a somewhat self-adjusting scraper 30 may be provided, which can self-adjust for wear by virtue of altering the flexing of the trailing end 34 in order to maintain contact of the scraper blade 44 with the surface 46. Movement of the leading edge region 32 is minimised by the triangular shape formed between the leading and trailing edge regions 32, 34 and the biasing force applied to the relieved notch 36 upon mounting the scraper 30 to the rod 38, and due to the flexibility of the trailing edge region 34.

A scraper of the form described above has been utilised in a scraped surface evaporator subjected to severe overloads and out of design conditions. This resulted in overload trips of the

driving motor but did not damage the scraper blades in any way.

Furthermore, a scraper according to the present invention has been used in a refrigeration apparatus for producing pumpable ice slurries, and it was found that the output per square metre of refrigerated surface was about double the output which is obtained using a scraper of the design shown in Figures 2(a) and 2(b).

It will be appreciated that the scraper for use in a scraped surface evaporator of the present invention provides a number of advantages over scrapers known in the art. Firstly the scraper does not require the use of a separate spring or integral hinge pins, which can break when a scraper is under stress. Secondly, the scraper of the present invention can be formed of a unitary construction. This has the advantages of ease of manufacture and removes weak stress points, such as at the connection of the hinge pins to the metal housing, of previously known scrapers. In addition replacement of the scraper is simple as the scraper need not be screwed or bolted to the driving mechanism in any way, thus minimising any evaporator "down-time".

It will be readily understood by one skilled in the art that various modifications could be carried out to the scraped surface evaporator and scrapers described herein, without departing from the scope of the present invention.