WO2012069954A1

WO2012069954A1 - Ult freezer with reduced ice formation

Info

Publication number: WO2012069954A1
Application number: PCT/IB2011/055067
Authority: WO
Inventors: Giovanni Pettirossi; Maurizio Ascani; Daniele Pericolini
Original assignee: Angelantoni Industrie Spa
Priority date: 2010-11-24
Filing date: 2011-11-14
Publication date: 2012-05-31
Also published as: IT1402899B1; ITMI20102179A1

Abstract

A ULT freezer operating at temperatures lower than -50°C for storing biological samples comprises a refrigerated internal cell (11) closed by a vertical access door (12). The cell has, above it, a compartment (15) which contains an evaporator unit (25) for a refrigerating circuit of the freezer, the evaporator unit (25) being provided with electric heating elements (18) for performing defrosting and heat insulation being provided between compartment (15) and cell (11). The compartment (15) is connected at the rear and at the front to the two ends of a circuit for circulating air around the cell (11), the air being forced to pass through the evaporator unit (25) by means of ventilator means (26) contained in the compartment. The air circulation circuit comprises a vertical space (22) arranged between the door (12) and the inside of the cell (11), into which the air flow flowing out at the front from the compartment (15) enters after passing through the evaporator unit (25), a horizontal cavity (16) underneath the cell and a vertical cavity (17) behind the cell for return of the air flow at the rear into the compartment (15). The air flow inside the vertical space (22) generates a thin stream of air from the top downwards, substantially parallel to the door (12). A control device (50) is connected to the heating elements (28) and to the ventilator means (28) in order to activate cyclically the heating elements (28) with simultaneous stoppage of the ventilator means (26) in order to perform cycles for defrosting the evaporator unit (25) while keeping the temperature inside the cell below a predefined maximum value in the region of a working temperature of the cell.

Description

ULT FREEZER WITH REDUCED ICE FORMATION

DESCRIPTION

The present invention relates to a low temperature freezer, in particular of the type for laboratories and/or for storing biological samples, such as blood or blood derivatives. Here "low temperature freezers" is understood as meaning freezers with a temperature of less than -50°C. For example, low temperature freezers for blood derivatives operate generally at temperatures in the region of -80°C or even lower.

The freezers operating at these low temperatures (at least below -50°C) are usually called ULT (Ultra Low Temperature) freezers. These ULT freezers suffer from the drawback that they result in the production of a large and damaging amount of ice in the refrigerated cell owing to the extremely rapid condensation and freezing of the environmental water vapour which penetrates into the cell whenever the door is opened. In particular, this damaging effect occurs mostly in vertical freezers which have a large front door for easy access to the refrigerated cell.

Although this problem is of major importance, hitherto a satisfactory solution for this problem has not been proposed hitherto.

In the prior art relating to freezers and fridges which operate at a much higher temperature (from about zero to a minimum temperature of about, for example, -20 to -30°C) for some time so-called "no-frost" freezers and fridges have been known. These appliances use an automatic defrosting system which performs the periodic heating of the refrigerating circuit evaporator so as to cause melting of the ice which forms on the evaporator owing to its temperature which, during normal operation, is lower than that of the remainder of the refrigerated cell. In certain cases, this measure is supplemented by a circulation of forced air between the cell and the zone where the evaporator is located. For example, similar solutions are described in WO01/46630, US4272969, GB1082764 andTW269856.

Although no-frost systems are fairly common in the case of relatively high cooling temperatures, they are currently not used at temperatures lower than -40°C. In particular, the technical expert considers that these solutions are completely unsuitable for use in low temperature or ULT freezers.

This unsuitability is due mainly to the fact that in the known no-frost freezers the temperature of the contents of the freezer necessarily rises during the defrosting cycles, owing to heating of the evaporator above the freezing temperature of water. While this may be acceptable for domestic freezers or in any case freezers of the type operating at a relatively high temperature (where, among other things, the temperature fluctuation of the evaporator between the working temperature and the defrosting temperature, above the water freezing threshold, is relatively low), a substantial variation in temperature becomes unacceptable in the case of low temperature freezers, for example intended for use in laboratories or for the storage of biological material, such as bags of blood and the like, where upward temperature fluctuations of more than 10 degrees or so may damage the stored products.

It should be considered that, in order to produce defrosting of an evaporator operating at temperatures in the region of -80°C, it is required to heat the evaporator to at least a few degrees above zero, i.e. with a very high thermal delta which may even reach 90°C. The technical expert considers that such a thermal delta inevitably causes an unacceptable instability of the temperature of a ULT cell, with inevitable deterioration of the samples contained inside it. Moreover, during the defrosting cycles, sublimation of the water may occur and cause or alter laboratory products stored inside containers which are not perfectly sealed.

Technical experts have therefore always sought alternative solutions to the problem of ice formation, abandoning completely the idea of a possible solution which uses heating of the evaporator.

An example of a ULT freezer is described in US 7621148. In this patent, in order to reduce the formation of ice, among other things, it is proposed keeping the refrigerated cell at least in overpressure so as to reduce the entry of moist air from the outside.

The known solutions remain unsatisfactory and the disadvantage of ice formation is not adequately reduced, even with the more complex measures which are proposed in the prior art.

The general object of the present invention is to provide a ULT freezer with low ice formation characteristics, while maintaining at the same time a satisfactory low and constant temperature inside the refrigerated cell.

In view of this object the idea which has occurred, according to the invention, is to provide an ULT freezer operating at temperatures lower than -50°C for storing biological samples, comprising a refrigerated internal cell closed by a vertical access door, the cell having, above it, a top compartment which contains an evaporator unit for a refrigerating circuit of the freezer, the evaporator unit being provided with electric heating elements for performing defrosting and heat insulation being provided between top compartment and cell, the top compartment being connected at the rear and at the front to the two ends of a circuit for circulating air around the cell, the air being forced to pass through the evaporator unit by means of ventilator means contained in the compartment, the air circulation circuit comprising, in order, a vertical space arranged between the door and the inside of the cell, into which the air flow flowing out at the front from the top compartment enters after passing through the evaporator unit, a horizontal cavity underneath the cell and a vertical cavity behind the cell for return of the air flow at the rear into the compartment, the air flow inside said vertical space generating a thin stream of air from the top downwards, substantially parallel to the door, a control device being connected to the heating elements and to the ventilator means in order to activate cyclically the heating elements with simultaneous stoppage of the ventilator means in order to perform cycles for defrosting the evaporator unit while keeping the temperature inside the cell below a predefined maximum value in the region of a working temperature of the cell.

In order to illustrate more clearly the innovative principles of the present invention and its advantages compared to the prior art, an example of embodiment applying these principles will be described below, with the aid of the accompanying drawings. In the drawings:

- Figure 1 shows a schematic cross-sectional view of a freezer according to the invention;

- Figure 2 shows a schematic perspective view of the freezer according to Figure 1;

- Figure 3 shows a block diagram of the refrigerating circuit of the freezer according to Figure 1.

With reference to the figures, Figure 1 shows a ULT freezer - denoted generally by 10 - designed in accordance with the principles of the present invention for storing the contents (for example, biological samples, bags containing blood or blood derivatives, etc.) at a low temperature, namely at least below -50°C and, preferably, in the region of -80°C or even lower. The freezer 10 comprises a refrigerated cell 11 which is isolated from the rest of the external environment by means of a vertical door 12 and suitably insulated external walls.

Advantageously, inner doors 13 are provided behind the door 12 for closing the compartments (or drawers) in the cell, creating a cavity between the inner doors and the inner side of the door 12. In the cell usually horizontal shelves or dividers 14 are present.

A top compartment 15 and a horizontal bottom cavity 16 are present above the top wall of the cell and below its floor, respectively, and are connected together at the rear by means of a vertical rear cavity 17 which is situated behind the back wall of the cell. The cell has an inner box-like structure or container 18 which is formed with walls made of non heat-insulating material, for example steel, such that there is a heat exchange between the inside of the cell and the cavities, which are instead thermally insulated with respect to the exterior by the insulated walls 19 of the freezer. The bottom and rear cavities may for example have a thickness of about 30 mm and width equal to the width of the container 18.

The bottom wall of the top compartment 15 is thermally insulated with respect to the underlying cell 11 and contains a heat exchanger unit or evaporator unit 25 which forms the evaporator of a suitable refrigerating circuit (the compressor of which may be advantageously housed in the base of the freezer underneath the cavity 16) and ventilator means 26 advantageously in the form of several ventilators arranged alongside each other in parallel (for example four ventilators) so as to blow an air flow through the exchanger. The ventilators are chosen so as to be suitable for operating at the very low working temperatures. For example, the bearings will have special lubrication grease for the very low working temperatures present in the freezer.

The top compartment and the bottom cavity have respective openings 20, 21 in the front part, close to the door 12, so as to communicate with each other via a space 22 present behind the door. This space 22 may also be in the form of a vertical cavity between the rear surface of the door and the inner doors 13 where present.

The ventilators 26 draw in the air from the rear cavity 17 and convey it (via the exchanger 25) into the front space 22 from where it returns to the rear cavity 17 through the bottom cavity 16, generating an air circulation which cools, passing through the top exchanger 25, and then cools the cell. For better efficiency, the rear cavity 17 may also have, alongside it, additional cavities on the sides of the cell 11, with suitable conveyors for connecting these additional cavities to the bottom cavity with the bottom cavity 16 and the top compartment 15 upstream of the ventilators 26 so as to increase the heat exchange between the cold air and the cell.

The openings 20 and 21 have respective walls or partitions 23, 24 which bound the front part of the compartment and the bottom cavity such that the openings 20 and 21 are directed towards each other. The openings 20, 21 form thin horizontal mouths extending lengthwise parallel to the door so as to produce a flow preferably in the form of a thin stream of air in a plane parallel to the door and immediately behind it. In this way, the stream of air which flows in front of the cell and behind the door generates an air barrier effect along the section which separates the two openings or mouths 20, 21. This can also be clearly seen in Figure 2.

Thus the moving air, in addition to cooling the working compartment of the cell, drawing heat through the non-insulated walls of the cavities, forms a cold barrier when the door is opened. This helps condense the external water vapour before it is able to penetrate into the cell.

Advantageously, for the reasons which will become clear below, the freezer also has a second evaporator 27 of the refrigerating circuit. The evaporator 27 consists of a conventional coil arranged around or inside the container so as to be in thermal contact with its interior. Advantageously, the evaporator 27 may also extend in contact with the side walls of the cell. Advantageously, the evaporator 25 and the evaporator 27 may operate both simultaneously and independently of each other controlled by the control device.

During opening of the door 12 the air barrier comes into contact with the external environment and is therefore "contaminated" by the external moisture. The air barrier however prevents entry of the moisture into the cell and the quantity of water vapour is conveyed by the air flow inside the cavities, so as to then accumulate in the form of frost/ice on the surfaces of the exchanger unit 25, since these are the coldest parts with which it comes into contact. Advantageously, the condensation effect in the air barrier causes the visible formation of mist which immediately makes the user aware of the presence and effectiveness of the barrier. To avoid adversely affecting operation of the freezer, the accumulation of frost/ice on the exchanger must be periodically removed by means of a defrosting operation.

In order to perform this operation, the exchanger unit 25 comprises electric heating elements 28 (for example electric heating resistors) which can be controllably activated during a periodic defrosting cycle where the flow of refrigerating fluid in the exchanger 25 is also interrupted. The elements 28 are of a size such as to be able to raise rapidly the temperature of the exchanger above 0°C.

A collection tray 29 is provided underneath the exchanger 25 in order to collect the water droplets which are produced by defrosting and which are conveyed outside via a small tube 30 for disposal thereof. In order to avoid freezing again the water the surfaces of the tray and the tube may be advantageously heated by means of respective electric heating elements 31, 32 (advantageously electric resistors in this case also). The heat developed by the defrosting elements in the top part of the freezer is insulated from the cell by means of the use of a suitable thermal insulant 33 between the compartment and the underlying cell. In order to reduce the insulant thickness, nanoporous insulating panels may be advantageously used, namely insulating panels characterized by internal porous structures with a nanometric diameter which are arranged as filling inside the ceiling lining of the cell and have a high insulating power. Advantageously, during defrosting, the refrigerating fluid is conveyed through the coil 27 so as to further limit raising of the cell temperature, thus keeping the temperature below a predefined threshold.

Owing to the air barrier which prevents the penetration of moisture into the cell and which condenses it on the exchanger 25 during normal operation, the conventional evaporator 27 is affected much less by the formation of ice than known freezers operating at a comparable temperature. This is true both during a possible operating cycle where the two exchangers are both in operation (for example during recovery of the normal condition after a defrosting cycle) and during operation of the coil exchanger alone 27, while the defrosting cycle of the exchanger 25 is being performed. In fact, in the first case (which may last a relatively long time) the air barrier effect with condensation on the exchanger 25 occurs, while in the second case this effect does not occur (since the air circulation is stopped so as to prevent a heated air flow around the cell), but the operating time in these conditions of the exchanger 27 is relatively limited and the number of times the door is opened during this time period is usually very small, if not equal to zero, while the cell is already devoid of condensable moisture. Advantageously, the top evaporator may also be provided with two input capillaries which can be controllably connected in parallel (for example by means of an electric valve). During normal operation of the machine a single capillary is used; when instead a rapid drop in temperature (for example after defrosting or after prolonged opening of the door) is required, the second capillary is also used so as to double the flowrate of the refrigerating fluid which passes in the evaporator.

Figure 3 shows a possible diagram of the circuit of the freezer according to the invention, in accordance with that already described above. The evaporator unit 25 is therefore supplied via the capillaries 42,43 by means of a selection valve 44. A compressor 41 and a condenser 40 complete the refrigerating circuit. The additional coil evaporator 27 is connected to the circuit by means of a capillary 45 and valves 46, 47 are provided for selecting the insertion of either one or both the evaporators 25, 27 in the circuit. Figure 3 also shows schematically the fans 26 which form the ventilator means for forced circulation of the air and the heating elements 28.

A control device 50 is connected at least to the heating elements 28 and to the ventilator means 26 so as to activate cyclically the heating elements with simultaneous stoppage of the ventilator means in order to carry out the defrosting cycles of the evaporator unit 25. Advantageously, for cyclic activation, the control device also has a timer which starts defrosting at predefined intervals.

The control device (advantageously a suitably programmed microprocessor controller, as may be easily imagined by the person skilled in the art in the light of the present description) advantageously also controls the valves 44, 46 and 47 so as to activate additional cooling (with the two capillaries 42, 43 in parallel) and use alternately or together the two exchangers 25 and 27, as already described above.

Suitable known temperature probes connected to the control device will allow the temperature inside the cell to be controlled and monitored. The control device may also have interfaces for data exchange with external systems for managing several freezers, as used for example in blood banks or the like.

At this point it is clear how the predefined objects have been achieved. A freezer according to the invention, while having a simple and robust low-cost structure, may function at extremely low temperatures (even lower than -80°C) with very few problems in terms of ice formation, even in the case of frequent opening of the access door, and with a suitably constant temperature inside the refrigerated cell in each operating condition.

The temperature inside the cell during defrosting is kept below a predefined maximum value in the region of a working temperature of the cell. Advantageously, raising of the temperature may be kept within a few degrees or, at the most, within ten degrees or so of the normal working temperature of the cell.

Among other things, the use of ventilators which force the movement of the air ensures an optimum thermal uniformity within the freezer in addition to favouring conveying of the water vapour towards the surface of the exchanger.

Obviously, the above description of an embodiment applying the innovative principles of the present invention is provided by way of example of these innovative principles and must therefore not be regarded as limiting the scope of the rights claimed herein. For example, the refrigerating circuit and the shape and size of the freezers may vary depending on the specific preferences and practical requirements. The two exchangers may also have completely separate refrigerating circuits, each with its own compressor and its own condenser. In the case of particularly low temperatures, the refrigerating circuit may be of the two-stage type. Finally, in order to facilitate removal of the ice which might form sporadically in the bottom cavity, a base which can be easily extracted and removed may be provided for this cavity. The position of the additional evaporator 27 may also vary depending on the preferences and/or specific applicational or constructional requirements.

Claims

1. ULT freezer operating at temperatures lower than -50°C for storing biological samples, comprising a refrigerated internal cell (11) closed by a vertical access door (12), the cell having, above it, a top compartment (15) which contains an evaporator unit (25) for a refrigerating circuit of the freezer, the evaporator unit (25) being provided with electric heating elements (18) for performing defrosting and heat insulation being provided between top compartment (15) and cell (11), the top compartment (15) being connected at the rear and at the front to the two ends of a air circulation circuit for circulating air around the cell (11), the air being forced to pass through the evaporator unit (25) by means of ventilator means (26) contained in the top compartment, the air circulation circuit comprising, in order, a vertical space (22) arranged between the door (12) and the inside of the cell (11), into which the air flow flowing out at the front from the top compartment (15) enters after passing through the evaporator unit (25), a horizontal cavity (16) underneath the cell and a vertical cavity (17) behind the cell for return of the air flow at the rear into the top compartment (15), the air flow inside the said vertical space (22) generating a thin stream of air from the top downwards, substantially parallel to the door (12), a control device (50) being connected to the heating elements (28) and to the ventilator means (28) in order to activate cyclically the heating elements (28) with simultaneous stoppage of the ventilator means (26) in order to perform cycles for defrosting the evaporator unit (25) while keeping the temperature inside the cell below a predefined maximum value in the region of a working temperature of the cell.

2. Freezer according to claim 1, characterized in that it comprises a coil evaporator (27) which is placed in thermal contact with the inside of the cell (11) and which is activated by the control device (50) at least for a period during the defrosting cycles so as to keep the temperature inside the cell below a predefined maximum threshold.

3. Freezer according to claim 1, characterized in that a tray (29) for collecting defrosting water with a water discharge pipe (30) is present below the evaporator unit (25), tray and pipe being provided with electric heating means (31, 32) which can be activated so as to keep the water in them liquid.

4. Freezer according to claim 1, characterized in that the heat insulation between top compartment (15) and cell (11) comprises a nanoporous insulating material.

5. Freezer according to claim 1, characterized in that the evaporator unit (25) comprises at its input a first capillary (42) and a second capillary (43) and at least one electric valve (44) connected for operation thereof to the said control device (50) for controlled activation in the refrigerating circuit of one or both the capillaries in parallel.

6. Freezer according to claim 1, characterized in that the said vertical space (22) is bounded towards the inside of the cell by inner doors (13) which close the cell parallel to the door.

7. Freezer according to claim 1, characterized in that above the vertical space (22) there is a vertical partition (20) which defines a mouth or opening (20) directed downwards and extending along the breadth of the door for exit of the air flow from the top compartment (15) and for directing it vertically inside the said vertical space (22).

8. Freezer according to claim 1, characterized in that below the vertical space (22) there is a vertical partition (24) which defines a mouth or opening (21) directed upwards and extending along the breadth of the door for entry of the air flow from the said vertical space (22) inside the bottom horizontal cavity (16).

9. Freezer according to claim 1, characterized in that an additional evaporator (27) of the refrigerating circuit is present in thermal contact with the inside of the cell (11), the additional evaporator being connected so that it can be activated in addition and/or as an alternative to the evaporator unit (25), controlled by the control device.