Method And Apparatus For Cooling Liquids

Abstract

A process for cooling a fluid circulating in contact with one face of a heat exchange wall, by cooling the other face of the wall with a cryogenic liquid sprayed under pressure and by means of residual gases resulting from the evaporation of this liquid. An exchanger for carrying out this process includes a tube placed in a heat-insulated enclosure, in which circulates a fluid to be cooled and on the walls of which a cryogenic liquid is sprayed. The exchanger includes means for placing a cryogenic liquid under pressure and a flow-regulating valve in the pressure line controlled by a thermostat located at the exchanger outlet.

Description

The object of the present invention is to provide a process for cooling a fluid by means of atomized cryogenic liquids and to provide exchanger devices, and installations equipped with these exchangers for putting this process into practice.

By "cryogenic liquids" are meant gases liquefied at low temperature under atmospheric pressure, e.g., liquid nitrogen or liquid carbon dioxide.

One of the aims of the present invention is to use, as source of heat, the heat of vaporization of a cryogenic liquid and the heat of reheating of the gases coming from the vaporization of this liquid, with a good thermal yield.

Another aim of the present invention is to regulate precisely the outlet temperature of the cooled fluid by acting on the flow of the cryogenic liquid.

Another aim of the present invention is to be able to cool industrially a fluid to very low temperatures and in a large range of temperatures, e.g., between 0.degree. and -190.degree. , using liquid nitrogen.

Another aim of the present invention is to use as a cold-producing agent, a liquefied neutral gas, e.g., nitrogen or carbon dioxide, which eliminates the risks of contamination of the cooled fluid as a result of an accidental contact with the cold-producing agent. This advantage is important for the cooling of certain fragile products, e.g., physiological liquids, food products, pharmaceutical products. In the chemical industry or in the field of atomic energy, an accidental contact in an exchanger between the cooled fluid and the fluids that are usually used to transport the heat may involve chemical reactions of even explosions. For this reason, double-tube exchangers which have a poor efficiency must be used or else an intermediate fluid which does not run the risk of reacting with the fluid to be cooled following an accidental contact must be cooled in a first exchanger, and then this intermediate fluid must be used in a second exchanger. The use of a liquefied neutral gas as a heat producing agent according to the present invention permits eliminating these expensive precautions.

According to the process of the present invention, a fluid circulating in contact with one face of a heat-exchange wall is cooled by placing the other face of this wall in contact with a sprayed or atomized cryogenic liquid and with the gases from the vaporization of this liquid.

The spraying of the cryogenic liquid, i.e., the atomization into a mist of fine droplets, is obtained by projecting the liquid under pressure through small-diameter orifices. The wall to be cooled is placed opposite these orifices and the liquid droplets strike this wall, in contact with which they evaporate.

The atomization increases the contact surface of the liquid and the wall, in such a way that the evaporation is instantaneous and it becomes possible to rapidly modify the exchanged calorific power by regulating the liquid flow. The temperature of the cooled fluid can, therefore, be regulated with precision. The cold gases from the evaporation of the cryogenic liquid flow away at the contact of the exchange wall by absorbing calories for their reheating. The gases flow away counter-current to the liquid to be cooled, so that one recovers a considerable part of the large calories or heat corresponding to the heating of the gases between the evaporation temperature of the liquid and a temperature close to the inlet temperature of the fluid to be cooled.

According to one characteristic of the present invention, an exchanger for putting the process into industrial practice is composed of elements mounted in parallel, each element being constituted of three concentric tubes. The central tube is connected by one of its ends to a source of cryogenic liquid under an effective pressure between 2 bars and 10 bars. The other end of the tube is plugged and a part of the periphery of the tube is pierced with small-diameter orifices through which the cryogenic liquid is sprayed. The middle tube is the exchanger tube. It is open at one of its ends for the escape of the residual gases. The exterior tube is insulated and the fluid to be cooled circulates between the two ends in a direction opposite to the direction of outflow of the residual gases. The exchanger tube is made of a good heat-conducting metal, e.g., copper or stainless steel. Preferably, fins of the same metal, in helical shape, are fixed to the interior and exterior walls of the exchanger tube. To improve the atomization, each orifice of the central tube may be equipped with an atomizing nozzle of a conventional configuration.

According to another characteristic of the present invention, a second type of exchanger for putting the described process into industrial practice is composed of a cryostat, i.e., a double-wall chamber with vacuum in between. The vacuum is at least 10.sup.-.sup.2 torr, and preferably 10.sup.-.sup.4 torr. The cryostat is, preferably, the super-insulated type, i.e., its interior wall is lined with a shield reflecting radiated heat. The cryostat is made of two independent parts, coupled water-tight, using two flanges and a collar.

The cryostat comprises a central cavity communicating with the outside through a neck. Along the axis of this central cavity is arranged a pipe line, plugged at one of its ends, the other end being connected to a source of cryogenic liquid under pressure. A part of the length of this pipeline is pierced with small-diameter orifices through which the cryogenic liquid is atomized.

The fluid to be cooled circulates in a coil arranged in the cryostat cavity.

According to one characteristic of this exchanger, the coil is constituted of two parts in series. The first of these parts, in the direction of flow of the fluid, is formed of separate single turn coils arranged in an annular space delimited by the inner wall of the cryostat and a circular screen. The residual gases from the vaporization of the cryogenic liquid circulate in this space, between the turns of the coil, before escaping through the neck of the cryostat.

The second part of the coil is formed of contiguous turns, arranged in a layer around the part of the central pipe line pierced with atomizing orifices. The atomized cryogenic liquid is projected against the half-periphery of the coil aimed toward the center and evaporates in contact with the coil.

According to one characteristic of the present invention, the surface of the first part of the coil is between 40 and 60 times the total surface of the second part of the coil.

According to another characteristic of the present invention, a device for cooling a fluid according to the described process comprises in combination, a supply reservoir containing a cryogenic liquid, a small-capacity auxiliary buffer-reservoir, means for transferring the cryogenic liquid into the buffer-reservoir, means for keeping the cryogenic liquid in the buffer-reservoir at a constant pressure between 2 bars and 10 bars, a cooler-exchanger of one of the types described in the preceding, a heat-insulated pipe line connecting the buffer-reservoir to the central piping of this exchanger, and a flow-regulating valve of a type capable of operating at the temperatures of the cryogenic liquid placed on this pipe line whose opening is modulated by an electronic device with derivative action, proportional and integral, controlled by a temperature sensitive element placed on the coil at the outlet of the exchanger.

There is provided means for keeping the cryogenic liquid under pressure in the buffer-reservoir as, for example, a cryogenic pump immersed in the supply reservoir and controlled automatically by a pressure sensitive device placed in a buffer-reservoir.

The various characteristics and advantages of the present invention will appear in the following description of several methods of realization of the objects of this invention given by way of example, without limiting character, with reference to the attached drawings wherein:

FIG. 1 is a central sectional view of an exchanger element in accordance with this invention;

FIG. 2 is a central sectional view of an exchanger placed in a cryostat; and

FIG. 3 is a schematic view of a cooling system incorporating the present invention.

The exchanger element represented in FIG. 1 is composed of three concentric tubes. The central tube 1, of small diameter, is connected at the lower end thereof to a source of cryogenic liquid under pressure, as indicated by the arrow. It is pierced, over at least a part of its length, with small diameter orifices 4 through which the cryogenic liquid is sprayed. The upper end of the tube 1 is closed.

The middle or central tube 2 may be denominated as an exchanger tube formed of a good heat-conducting metal. The tube 2 is closed at its lower end 6. The upper end 7 of the tube 2 is open to exhaust the residual gases resulting from evaporation of the cryogenic liquid.

Fixed to the interior and exterior walls of central tube 2 are fins 10 made of a good heat-conducting metal and preferably having a helical configuration. The purpose of these fins 10 is to increase the exchange surface, to retard the rate of escape of the residual gases and to increase the length of travel of the fluid to be cooled.

An outside tube 3 is disposed about the above described elements and is covered on the outside thereof with a heat insulator 11. The outer tube 3 is closed at both ends. The upper part of tube 3 is connected to an inlet line 8 for entrance of the fluid to be cooled. At the lower part of tube 3, the cooled fluid passes out through a pipe line 9. The operation of this exchanger element is as follows.

The cryogenic liquid is introduced into the tube 1 and is projected onto the inner walls of tube 2 as a mist of fine droplets. It evaporates in contact with these walls, obtaining its heat of evaporation from the fluid to be cooled through the wall of the exchanger tube 2. The residual gases of the cryogenic liquid become heated by circulating in contact with the wall of the exchanger tube. The circulation of the gases and the fluid to be cooled occurs counter-current. When more refrigeration power is required than can be provided by one exchanger element, several elements are connected in parallel; the central tubes being connected to a common collector. Likewise, the connections for intake 8 and exit 9 of the fluid to be cooled are connected to two collectors. A flow-regulating valve is placed on the cryogenic liquid inlet pipe line and its opening is modulated by an electronic device controlled by a temperature sensitive device placed on the outlet of the cooled liquid.

Reference is now made to FIG. 2 representing a further embodiment of this invention incorporating an exchanger of a preferred type having heat insulation which considerably reduces heat losses through the walls. A cryostat, of a conventional design is shown to be constituted of two walls, an external wall 12 and an internal wall 13 between which is created a vacuum between 10.sup..sup.-2 torr and 10.sup..sup.-4 torr. This cryostat is of the super-insulated type, i.e., the outside face of wall 13 is covered with a reflecting screen 14 intended to stop heat radiation. This cryostat has the general shape of a cylindrical bottle equipped with an open neck and is formed of two parts, each forming a water-tight enclosure. These two parts are joined through the intermediary of two flanges 15 and 16 welded to the two walls. The flange 16 carries a groove 17 in which is placed a toric joint 18. The two clamps are held in water-tight contact by a collar 19 which engages the external conical face of the flanges.

An adsorbent material 20 is placed between the two walls in contact with the inner wall. The function of this material is to maintain the vacuum, adsorbing the molecules from the degassing of the walls.

Two valves 21 and 22 separately extend through the outer walls of each of the parts of the cryostat to avoid bursting of the latter in case of an abnormal increase in the pressure. These valves at the same time serve to connect a vacuum pump for evacuating the cryostat. Circular hoops 23 are arranged about the periphery of the cryostat. The bottom of the cryostat is convex and through its periphery rests on a flat support 24.

A pipe line 27 is arranged along the axis of the cryostat. It is closed at the lower end thereof. It is connected by its upper end to a source of cryogenic liquid under pressure and is heat-insulated. For example, the pipe is constituted, from the top down to a horizontal screen 28, by a double-wall tube with a vacuum in between.

The part of pipe line 27 located below the screen 28 is pierced with small diameter orifices 29 through which the cryogenic liquid is sprayed.

The fluid to be cooled enters the cryostat through the neck by a pipe line 30 and exits through a pipe line 31. Three coils are arranged in an annular space delimited by the inner wall of the cryostat and by a cylindrical screen 33. These three coils are connected in parallel by a collector 32 to the pipe line 30. The three coils are arranged along three levels of different diameter. The separate turns do not touch. The residual gases from the evaporation of the cryogenic liquid circulate, rising between the turns, and escape through the opening of the neck.

Each of the three coils is extended by a second part formed of joined turns 34 arranged along helices of the same diameter, which encircle the lower end of the tube 27 pierced with atomization orifices. At the upper part, the three coils are joined in parallel by a collector 35 to the outlet 31. A temperature sensitive device 36, e.g., a thermocouple, is placed on collector 35 which corresponds to the zone where the temperature of the fluid to be cooled is the lowest.

The coil section is chosen as a function of the flow of the fluid to be cooled and the number of coils mounted in parallel, so that the fluid circulates in a turbulent mode inside the coil at a speed greater than 2 meters per second.

With such an exchanger operating with liquid nitrogen one has an exchange coefficient, per square meter of wetted surface of the coil and per hour, equal to 2,200 kilocalories in the zone operating in the gaseous phase, and equal to 22,000 kilocalories in the zone placed in the liquid phase. In this zone, the effective exchange surface of the coil is equal to half the total surface, since only one half of the tubes receives the projections of liquid droplets. Experience shows that the best general yield is obtained when the total refrigerating power of the exchanger is divided into two equal parts between the gas and liquid phase zones. The surface of the coils placed in the annular space operating in the gaseous phase is equal to about 50 times the total surface of the coil placed inside the screen 33. In practice, the ratio of the surfaces will be between 40 times and 60 times.

The section of passage of the gases around the turns of the coil in the annular space is calculated in such a way, that the gases circulate in a turbulent manner at a speed greater than 6 meters per second for a normal operating condition of the exchanger. A stack of perforated screens 37 is placed across the neck to support the tubes and to retard the speed of escape of the residual gases.

The provision of the cryostat in two parts allows placing the interior equipment in the lower part of the cryostat which is then capped by the upper part. Maintenance operations can be carried out inside the cryostat without destroying the vacuum in the two halves of the latter.

FIG. 3 represents schematically the assembly of a cooling installation in an application specific to the clarification of wines. The installation unit is constituted by a supply reservoir 44 containing liquid nitrogen at atmospheric pressure. In this nitrogen is immersed a pump 45, of a known model, suitable for operating at the temperature of -195.degree. .

This pump delivers the liquid nitrogen to a buffer-reservoir 46 of small capacity. A pressostat or pressure responsive device 47, placed on this reservoir, controls the start and stop of pump 45 and maintains in the reservoir an effective pressure on the order of 5 bars.

The buffer reservoir 46 is connected to the central pipe line of exchanger 42 by a heat-insulated conduit, e.g., a double-jacketed tube with a vacuum in-between. A flow-regulating valve 48, of a type that can operate at the temperature of liquid nitrogen, is placed in this pipe line. The opening of this valve is modulated by a device 49 controlled by the temperature responsive device 50 placed in the exchanger. The temperature responsive device is a type that converts the temperature to voltage e.g., a thermocouple. The device 49 is an electronic device which compares this voltage with a reference voltage proportional to the outlet temperature desired, which has been set. The voltage difference is amplified. If the valve 48 is pneumatic, the potential difference acts on a pressure converter with derivative action, proportional and integral, which modulates the opening of the regulating valve 48. A branch of valve 48, equipped with an electrovalve 51, is provided for placing of the exchanger into rapid operation.

The device represented by FIG. 3 is an application to the clarification of wines. It is known that the tartaric acid contained in wine precipitates when the temperature falls. In the case of sparkling wines, in particular champagne, since the bottles can no longer be unstoppered before consumption, it is necessary to cool the wine in the neighborhood of the freezing temperature before the bottling. The wine, at ambient temperature, is contained in a container 40. It is pumped by a pump 41 and, after having passed through the exchanger, it is collected in a container 43.

The regulating device 49 is set at a temperature very slightly higher than the freezing temperature of the wine, e.g., one-tenth of a degree higher. The experiments carried out show that a very stable temperature of the wine is obtained at the outlet of the exchanger, the variations being less than one-thirtieth of a degree.

The advantages of the present invention are as follows: The cost of installation of a cooling device is reduced with respect to that of existing refrigerating machines.

Maintenance costs are reduced.

It is possible to cool to very low temperatures, and the same device permits operating in a very broad temperature range, e.g., from 0.degree. to -190.degree., with liquid nitrogen. This advantage is important to research laboratory equipment. It allows subjection of a circulating fluid to substantial thermal shocks.

By using as cryogenic liquid neutral liquefied gases, such as nitrogen or carbon dioxide, any fluid whatsoever to be cooled can be caused to pass directly into the exchanger without risking contamination of the latter as a result of an accidental contact with the cooling agent. The refrigerating powers per unit of surface are high, which permits realizing exchangers of small size. The installations are noiseless and can be placed directly on the operating sites.

As a result of the rapidity of the heat exchanges in the liquid phase zone, it is possible to affect the outlet temperature of the fluid very rapidly by regulating the flow of cryogenic liquid. The result of this is the possibility for very precise temperature control which permits specific applications, such as e.g., the clarification of wines.

When the exchanger is charged with liquid nitrogen, it is possible, for certain applications, such as e.g., the cold preservation of food products or physiological liquids, to use residual gases as the neutral atmosphere in which these cooled products are placed, which improves their preservation.

Tests conducted with an exchanger of the cryostat type, charged with liquid nitrogen at an effective pressure of 3 kilos per square centimeter and cooling a fluid to the temperature of -20.degree., have shown that the refrigerating yield was 80 kilo-large calories per liter of liquid nitrogen.

Claims

I claim:

1. A heat exchanger comprising a cryosat having a central cavity open at one end thereof, a pipe line disposed axially of said cavity and closed at the end opposite said opening, said line being connected at the other end to a source of cryogenic liquid under pressure, said line being pierced over a part of its length with small diameter orifices through which the cryogenic liquid is sprayed, an exchanger coil in which the fluid to be cooled circulates and disposed in the central cavity of said cryostat around said pipe line in sufficiently close proximity with a portion of said pipeline for cryogenic liquid sprayed from said orifices to impinge upon said coil as a liquid for evaporation thereat, and means directing evaporated cryogenic liquid along a further portion of said coil for maximized cooling of fluid circulating through the coil.

2. A heat exchanger according to claim 1 further including means defining an inner chamber and a surrounding communicating annular chamber, said coil being formed of two parts in series, said first part disposed at the end where the fluid to be cooled arrives and formed of physically separated turns arranged in said annular chamber, and a second part disposed in said inner chamber in connection with an exit pipe and formed of contiguous turns of the same diameter arranged around the end of the central pipeline pierced with atomization orifices whereby said liquid evaporates upon striking said second coil part to form cryogenic gas that flows into said annular chamber over said first coil part.

3. A heat exchanger according to claim 1 in which the coil is constituted of two parts in series, a first part being disposed at the end where the fluid to be cooled arrives, and formed of spaced-apart turns arranged in an annular space which is delimited by the inner wall of a cryostat and a cylindrical screen and a second part being disposed at the end where the cooled fluid leaves and formed of joined turns of the same diameter arranged around the central pipe pierced with atomization orifices, the diameter of the coil being such that the fluid to be cooled circulates in a turbulent regime and the surface of the first part of the coil is between 40 times and 60 times the total surface of the second part of the coil.