CRYOGENIC COOLING COMPOSITION AND METHOD

Abstract

A cooling composition including a mixture of solid particles of CO.sub.2 and liquid nitrogen, wherein the content of solid particles of CO.sub.2 is between 70 and 85% by weight and the solid particles of CO.sub.2 have a diameter of less than or equal to 50 m is provided.

Claims

1.-4. (cancelled)

5. A cooling composition comprising a mixture of solid particles of CO.sub.2 and liquid nitrogen, wherein: the content of solid particles of CO.sub.2 is between 70 and 85% by weight and the solid particles of CO.sub.2 have a diameter of less than or equal to 50 m.

6. A process for producing a cooling composition as defined in claim 5, comprising: a) forming the solid particles of CO.sub.2 comprising the expansion of CO.sub.2 gas; and b) mixing the particles of CO.sub.2 and liquid nitrogen.

7. A process for cooling an element to be cooled, using a cooling composition as defined in claim 5, comprising the following successive steps: a) stirring the composition at a speed of less than 1 revolution per second, c) immersing and maintaining the element to be cooled in the composition, wherein throughout the duration of step b): the stirring of step a) is maintained, and the proportion of liquid nitrogen in the composition is measured and is kept constant to within plus or minus 5% by the addition of liquid nitrogen.

8. The process according to claim 7, wherein step c) is carried out at a pressure of between 1 bar absolute and 10 bar absolute.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0036] FIG. 1 is a graphic representation of the heat flow a function of the difference in temperature at the liquid/solid interface for a brass bar immersed in liquid nitrogen as known in the art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] A cooling composition comprising a mixture of solid particles of CO.sub.2 and liquid nitrogen is provided, wherein the content of solid particles of CO.sub.2 is between 70 and 85% by weight and the solid particles of CO.sub.2 have a diameter of less than or equal o 50 m.

[0038] The cooling composition is preferably produced by means of a process including a step of forming the particles of CO.sub.2 including the expansion of CO.sub.2 gas, preferably in an expansion cone; and a step of mixing the particles of CO.sub.2 and liquid nitrogen.

[0039] These particles either can be dispersed in liquid nitrogen with slight stirring or the liquid nitrogen is poured onto the particles contained in a container. It should be noted that the order of implementation does not affect the size of the CO.sub.2 particles obtained.

[0040] In the cooling composition, the liquid nitrogen must completely wet the mass of the particles.

[0041] The amount of liquid nitrogen relative to the amount of solid CO.sub.2 should be as close as possible to the amount necessary for: [0042] the liquid nitrogen to wet all the solid CO.sub.2 particles and [0043] there to be a sufficient excess of liquid nitrogen to prevent very fast drying of the solid CO.sub.2 mass (paste) which occurs during the first seconds of quenching of the object to be cooled and which is difficult to compensate for by compensatory injection of liquid nitrogen at the surface.

[0044] In this configuration, the solid CO.sub.2 particles cooled at the temperature of the liquid nitrogen are the main vector of the heat exchange participating in the heat exchange by the direct solid/solid contacts and minimize the heating effect. This cooling composition shows a heat exchange capacity that is greatly increased compared to liquid nitrogen under the same conditions.

[0045] The cooling composition according to the invention makes it possible to obtain a heat exchange coefficient which is equal to or >230 W.Math.M.sup.2.Math.K.sup.1 in the heating zone, that is to say approximately twice that of liquid nitrogen under the same conditions, and which can range up to 210 W.Math.M.sup.2.Math.K.sup.1 depending on the conditions in the nucleate boiling zone, that is to say 10 times that of liquid nitrogen under the same conditions.

[0046] This cooling composition is sufficiently fluid and manipulable to constitute immersion baths for deep cooling of metals, plastics, food products, plant and human tissues. This involves a very low temperature cooling, known as deep freezing. The composition is transferable and pumpable by the usual means for the transfer of cryogenic fluids.

[0047] A subject of the present invention is also a process for cooling an element to be cooled, using a cooling composition as defined in Claim 1, comprising the following successive steps: [0048] a) stirring the composition at a speed of less than 1 revolution per second, [0049] c) immersing and maintaining the element to be cooled in the composition, with throughout the duration of step c): [0050] the stirring of step b) is maintained, and [0051] the proportion of liquid nitrogen in the composition is measured and is kept constant to within plus or minus 5% by the addition of liquid nitrogen.

[0052] By virtue of the cooling process according to the invention, cooling at cryogenic temperature of the element to be cooled is made possible.

[0053] Preferably, step c) is carried out at a pressure of between 1 bar absolute and 10 bar absolute.

[0054] It should be noted that the cooling time depends on the size of the element to be cooled, its shape, the type of material and also its temperature. It can be said that under the same conditions and for one and the same object, the gain in cooling time to reach a target temperature obtained by implementing the process according to the invention is at least 30%.

[0055] By way of example, for a bar with a diameter of 40 mm and a height of 100 mm, made of brass (70% Cu/30% Zn), the bar must be immersed for approximately 3 minutes and 30 seconds so that its surface temperature (measured using a Pt100 thermal probe at 3 mm from the edge of the bar) goes down from 13 C. to 196 C.

[0056] It should be noted that the stirring of the composition enables the particles to be maintained in homogeneous suspension.

[0057] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.