Cryogenic Cooling Method and Installation Using Liquid CO2 and Employing Two Exchangers in Series

Description

The present invention relates to the field of cooling processes employing CO₂.

The use of CO₂ in such cooling processes is, as is known, very advantageous, since this fluid has a solid phase at −80° C. at atmospheric pressure, thereby allowing dry ice to be used in certain applications, this ice being very effective, especially as far as localized cooling without refrigeration equipment is concerned. Solid CO₂ has many applications, such as for example dry ice bags, which are loaded into containers for transporting food or pharmaceutical products, or which indeed may be used to keep meals cool in the field of aerial transportation.

However, when this gas is used in (“indirect injection”) heat exchangers, typically tube or finned heat exchangers, this advantage turns into a drawback because the untimely appearance of solid CO₂ (carbon snow) in an exchanger rather rapidly leads to the latter becoming blocked.

Therefore, to prevent this drawback of transition into the solid phase, it is sought to prevent the solid phase of CO₂ from appearing, and conditions are therefore preferred that make it possible to keep the CO₂ in liquid or gaseous form in the entire exchanger.

In order to exchange heat while ensuring the CO₂ remains in its gaseous or liquid phase (vaporization of the liquid CO₂ without running the risk of forming solid CO₂), the pressure in the tube must be kept above the theoretical pressure of 5.18 bar corresponding to the triple point pressure of this fluid. In practice, the system is restricted as it were to a pressure of 6 to 7 bar, thus ensuring a safety margin of 0.82 to 1.82 bar.

Whereas the sublimation temperature of solid CO₂ at atmospheric pressure is −80° C., keeping the pressure in the exchanger at 6 bar relative increases the vaporization temperature to about −50° C.

However, carrying out the heat exchange at 6 bar and not at atmospheric pressure slightly decreases the cooling capacity of the CO₂. Specifically, when a kilogram of CO₂ is taken out of storage, for example under standard conditions such as 20 bar absolute/−20° C., and enters into an exchanger, it releases 277.97 kJ/kg if it is discharged at −50° C. in gaseous form at 6 bar relative, whereas the same amount of CO₂ releases 292.6 kJ/kg when it is discharged at −50° C. at atmospheric pressure, i.e. an increase of 5%.

By way of example, CO₂ discharged at 6 bar after passing through an exchanger is used in applications such as refrigerated trucks but also in freezing tunnels or chambers. In these applications, a heat exchanger is supplied with liquid CO₂ which, by evaporating in this exchanger, extracts heat from the medium to be cooled and thus produces the desired cooling. Cooling of the products to be cooled is achieved via heat exchange with the internal air of the tunnel, chamber or truck, via blowing means associated with each exchanger.

It will therefore be understood that it would be advantageous to be able to provide a technical solution allowing heat exchange in a tube or finned heat exchanger (indirect exchange), at exchanger temperatures that are nonetheless low (typically −50° C.) without of course running the risk of forming snow and losing the cooling capacity of the CO₂ following its expansion from 6 bar to atmospheric pressure.

As will be seen in more detail below, the present invention provides a new exchange solution, the main features of which may be summarized as follows:

• • the solution proposed here relates to the exchanger configuration adopted, the exchanger used, which may for example be a tube or finned heat exchanger, consisting of two exchangers connected in series;
  • the first exchanger can be supplied with liquid CO₂ (for example under standard conditions such as −20° C./20 bar), the liquid encountering, before it enters the first exchanger, a thermostatic regulator or a temperature probe/regulator/valve assembly, or any other means allowing the flow rate of the CO₂ entering the 1st exchanger to be matched to the cooling requirements in question, i.e. allowing the “excess” temperature, or, in other words, the temperature difference between the temperature corresponding to the saturation vapor pressure (for example at 6 bar, −53.1° C.) and for example −50° C., which corresponds to 3.1° of excess, to be controlled;
  • the pressure in the first exchanger is kept above 5.18 bar relative, i.e. the temperature of the phase change of the CO₂ (the formation of snow thus being prevented), the pressure is, for example, kept at 6 bar absolute by virtue of a back-pressure regulator or a pressure sensor/regulator/valve assembly, placed at the outlet of the first exchanger;
  • this arrangement makes it possible to ensure that the CO₂ in the first exchanger is only present strictly in its gas/liquid two-phase form without at any point the conditions used allowing a solid to form;
  • the minimum temperature obtained in this first exchanger is then about −50° C.; and
  • according to one embodiment, the back-pressure regulator at the outlet of the first exchanger is preceded by a phase separator in order to prevent any liquid from exiting the first exchanger.

The back-pressure regulator and the outlet of the first exchanger will possibly be installed in an upper part of the complete installation to prevent liquid from escaping, but configurations where the two exchangers are located at the same level may also be envisioned.

The optional presence of the aforementioned separator increases the reliability of the system. It prevents liquid from reaching the back-pressure regulator and therefore snow from forming and causing a blockage at this point.

• • In summary, the liquid CO₂ vaporizes in the first exchanger and the gas formed in this exchanger, for example at 6 bar, is released in the second exchanger;
  • this second exchanger is at atmospheric pressure (or in any case at a pressure below the triple point of the fluid), the gas then expands, after entering this second exchanger at 6 bar (or more generally at the pressure maintained in the first exchanger), to atmospheric pressure (or in any case to a pressure between the triple point of the fluid and atmospheric pressure) which causes cooling, namely to a temperature typically between −60° C. and −70° C.;
  • and therein lies the advantage of the present invention, since the cooling produced by expansion to atmospheric pressure is used by the second exchanger, and thus all the energy contained in the CO₂ is used.

The present invention thus relates to a process employing liquid CO₂ as a cryogenic fluid for cooling products, this process being an “indirect injection” cooling process where the liquid CO₂ is transferred to a heat exchanging system where it evaporates, the removal of heat from the products occurring by an exchange between the atmosphere surrounding the products and the cold walls of the heat exchanger, this process being noteworthy in that the exchanging system consists of two exchangers connected in series, the first exchanger being kept at a pressure above the triple point pressure of CO₂, whereas the second exchanger is itself kept at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

The present invention also relates to an installation for cooling products using liquid CO₂, the installation employing an “indirect injection” process, and comprising:

• • a heat exchanging system through which the liquid CO₂ can pass; and
  • blowing means associated with the heat exchanging system, able to bring the atmosphere surrounding the products into contact with cold walls of the heat exchanging system,

the installation being noteworthy in that the following measures are implemented:

• • the exchanging system consists of two exchangers connected in series;
  • the installation comprises, upstream of the inlet of the first exchanger, a means able to adjust the CO₂ flow rate and to control the excess temperature thereof relative to the corresponding temperature at saturated vapor pressure, such as a thermostatic regulator or a temperature probe/regulator/valve assembly;
  • the installation comprises a means for keeping the pressure in the first exchanger at a pressure above the triple point pressure of CO₂, preferably a back-pressure regulator or a pressure sensor/regulator/valve assembly; and
  • the second exchanger is at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

The installation therefore comprises, if required, a means for keeping the pressure in the second exchanger at atmospheric pressure or at a pressure between the triple point of the fluid and atmospheric pressure.

Other features and advantages of the present invention will become more clearly apparent from the following description given by way of completely nonlimiting illustration with regard to appended FIGS. 1 and 2, which are partial schematic representations of installations according to the invention, FIG. 3 showing the temperature profile expected in the exchanger assembly on the CO₂ and heat-transfer fluid (air) sides.

The following elements, and therefore the course followed by the CO₂, in its various phases, in the installation, may be seen in FIG. 1:

• • the first exchanger is able to be supplied with liquid CO₂ (for example under standard conditions such as −20° C./20 bar), the liquid encountering, before it enters the first exchanger, a thermostatic regulator (downstream of point 1) or any other means allowing the flow rate of the CO₂ reaching the first exchanger to be matched to the cooling requirements in question, i.e. to control the excess temperature, or, in other words, the temperature difference between the temperature corresponding to the saturated vapor pressure (for example 6 bar, −53.1° C.) and for example −50° C., which corresponds to 3.1° C. of excess temperature.

Downstream of point 2 the fluid enters into the first exchanger.

• • a pressure higher than 5.18 bar relative is maintained in this first exchanger, temperature of the phase change of CO₂ (thus allowing the formation of snow to be prevented), by virtue of the back-pressure regulator placed at the outlet of this first exchanger in the figure (back-pressure regulator placed between points 3 and 4);
  • this arrangement makes it possible to ensure that the CO₂ in the first exchanger is present only in a strictly liquid/gas two-phase form, without at any point the conditions used allowing a solid to form;
  • the minimum temperature obtained in this first exchanger is then −50° C.;
  • in the embodiment illustrated here, the back-pressure regulator at the outlet of the first exchanger is preceded by a phase separator (between points 3′ and 3), in order to prevent any escape of liquid from the first exchanger. In this embodiment, the back-pressure regulator and the outlet of the first exchanger are installed in an upper part of the complete installation, to prevent escape of liquid.
  • at the outlet of point 4 and therefore of the back-pressure regulator, the gas enters into the second exchanger, which it exits at point 5.

The table below collates the thermodynamic properties of the fluid at various points in FIG. 1, and makes it possible to show unambiguously the advantages of the invention in terms of cooling efficiency. The table especially illustrates a number of temperature conditions, at the outlets of the exchangers.

In addition, to clearly show the benefit of the present invention, the energy efficiency of a system not employing the invention and a system employing the present invention are compared, in the case where the final temperature in the exchanger is −25° C. and in the case where the final temperature in the exchanger is −5° C.

Considering the first case (the final temperature in the exchanger being −25° C.)

• • for a system employing a single exchanger: 1 kg of CO₂ releases 457−154.5=302.5 kJ;
  • for a system employing two exchangers according to the invention: 1 kg of CO₂ releases 464.5−154.5=310 kJ; i.e. a 2.5% increase in energy.

In the second illustrative case, where the final temperature of the exchanger is −5° C.:

• • for a system employing a single exchanger: 1 kg of CO₂ releases 474.6−154.5=320.1 kJ;
  • for a system employing two exchangers according to the invention: 1 kg of CO₂ releases 480.8−154.5=326.3 kJ; i.e. a 1.9% increase in energy.

The temperature profile expected in the exchanger on the CO₂ and heat-transfer fluid (air for example in an application such as the transportation of frozen products) sides, shown in FIG. 3, demonstrates that the present invention also has a beneficial impact on the temperature profile in the exchangers: the fact that the second exchanger stage is at atmospheric pressure makes it possible to take advantage of a cryogenic effect as the curves in FIG. 3 show.

If FIG. 1 showed a first embodiment of the invention, FIG. 2, for its part, shows another, which will not be described in greater detail here. As will become clear on examination of the figure, it illustrates a variant employing:

• • upstream of the first exchanger, an assembly consisting of a calibrated orifice and a temperature-controlled valve, rather than a thermostatic regulator; and
  • at the outlet of the first exchanger of the installation, a pressure sensor/regulator/valve assembly, rather than a back-pressure regulator.