EP3365615B1 - Method and device for operating a refrigeration circuit with a sublimator for carbon dioxide as a coolant - Google Patents

Description

The invention relates to a method for operating a refrigeration cycle with carbon dioxide as a refrigerant, wherein the refrigeration takes place below the triple pressure in a sublimator. The invention further relates to a device for carrying out the aforementioned method.

Carbon dioxide, also referred to as R744 or CO ₂ , is an ideal and widely used natural refrigerant, which is used in the temperature range up to about -40 ° C. The advantageous properties of carbon dioxide as a refrigerant are, in particular, that it is non-combustible and, under normal conditions, also non-toxic. The triple point of carbon dioxide is about -56 ° C and 5.2 bar. Below the triple temperature, it is in solid or gaseous state.

The conventional cold vapor process with other refrigerants, however, usually runs in the two-phase region of the gaseous and liquid state of the refrigerant. In order to achieve temperatures below -56 ° C with CO ₂ as refrigerant, the pressure in the circuit must be lowered below the triple point, whereby at least partially carbon dioxide must be used as a solid. However, the usable temperature level can be made accessible up to about -78 ° C or even lower.

Compared to the conventional cold vapor process, when the sublimation line falls below the triple pressure, the evaporator becomes a sublimator. Due to the sublimation of the carbon dioxide from the solid into the gaseous phase, the useful cooling is created while absorbing heat.

From the use of the sublimation process there are some special features compared to the conventional cold vapor process. If an evaporator flows through the refrigerant in the conventional cold vapor process, the coexistence of liquid and gaseous phase results in only a technically caused pressure loss due to fluid friction. The pressure loss due to fluid friction in the channels of the refrigerant path in the evaporator is relatively unproblematic calculable and the corresponding heat exchanger are well tuned to the respective refrigerant with their dimensions. The pressure loss remains about the usual technical fluctuations in the steady state operation of the system about the same.

However, if the cold is provided by sublimation, technically a different situation arises. If a sublimator flows through in the carbon dioxide cycle below the triple point, coexistence of solid and gaseous phase results in addition to the technically caused pressure loss due to fluid friction, a further, significantly more indeterminate component with influence on the pressure loss. It comes to the attachment of solid carbon dioxide on the walls of the sublimator and agglomeration of oversized CO ₂ -Festsstoffpartikeln.

As a consequence, cross-sectional constrictions or even blockages occur in individual regions of the sublimator, which considerably influence the pressure loss.
Since commercially available heat exchangers are generally multi-threaded, blocking of individual paths can not be ruled out and would considerably influence the continuous operation of a system with a sublimator. Under a multi-pass heat exchanger is a heat exchanger understood that unlike the einpassigen heat exchanger has a plurality of parallel refrigerant channels, which are referred to in their sum as the refrigerant path. The formation of blockages or deposits of solid carbon dioxide should now be circumvented or limited by the cleaning procedure proposed in the invention.

From the DE 38 24 235 C1 a chiller refrigeration system is known, which discloses a servo-controlled evaporator pressure control valve for a refrigeration system.

The DE 27 48 796 A1 discloses a method and apparatus for cooling materials using stored cryogenic cooling, particularly using carbon dioxide as the refrigerant.

In the prior art is from the DE 30 04 114 A1 a method for generating low temperatures with carbon dioxide as a refrigerant known. It addresses the problem of solid agglomeration of solid carbon dioxide by suspending the solid carbon dioxide particles in a non-freezing liquid and circulating it as a suspension.
This prior art has the disadvantage that additional expenditure is required in terms of apparatus and process technology, as well as in terms of energy and costs, to provide the solvent, to dimension the system and to circulate the suspension.

In the prior art are from the DE 2 335 130 A1 Furthermore efforts are known to design components of refrigeration systems with carbon dioxide as a refrigerant, for example with a self-cleaning valve. The strategy is that the deposition of solid CO _{2 are designed} on the valve surfaces by design features that upon actuation of the valve, these surfaces are forcibly mechanically scraped and obstruction of the function of the valves should be prevented.

This prior art has the disadvantage that in terms of components with stationary parts, such as a heat exchanger, this solution concept is not readily adaptable.

Furthermore, goes from the EP 1 939 548 A1 a CO ₂ refrigeration system according to the preamble of claim 11 and a method for operating a CO ₂ refrigeration system, which works with multi-stage compression in the supercritical region with intermediate cooling.

The object of the invention is therefore to provide a method for operating a circuit with a sublimator with carbon dioxide as a refrigerant and a device available that largely prevents or reduces the problem of forming blockages or deposits of solid carbon dioxide in the sublimator.

The object is achieved by an article according to the independent claims. Further developments are specified in the dependent claims.

The object of the invention is achieved in particular by a method for operating a refrigeration cycle with a sublimator for carbon dioxide as a refrigerant, wherein the refrigeration cycle is operated below the triple point. During cooling, the special feature is that in the sublimator the sublimation line of the carbon dioxide is exceeded below the triple point. According to the invention, a cleaning of the sublimator of blockages with solid CO ₂ or deposits of solid CO ₂ by a targeted reduction of the cross section of the refrigerant path after the sublimator. The pressure in the sublimator is raised above the triple pressure and the solid carbon dioxide is converted into liquid carbon dioxide. The blockages and deposits of solid CO _{2 are} dissolved. Thereafter, the refrigerant path is at least partially increased again and distributed the liquid carbon dioxide in the sublimator by the pressure drop and after falling below the Tripeldruckes converted back into solid or gaseous carbon dioxide. The change in the cross section of the refrigerant path is carried out in dependence on control or regulating signals.

The invention is advantageously further developed in that the sublimator is filled discontinuously with liquid carbon dioxide above the triple pressure. By subsequent rapid lowering of the pressure below the triple pressure solid and gaseous carbon dioxide is produced in a uniform distribution in the sublimator. The filling of the sublimator with liquid carbon dioxide is preferably carried out when the cleaning of the sublimator of solid carbon dioxide is carried out by cross-sectional constriction after the sublimator.

Preferably, the reduction of the cross section of the refrigerant path is carried out until the refrigerant path is completely closed, in which case a pressure increase associated with the liquefaction of the solid carbon dioxide can be achieved.

Particularly preferably, the reduction of the cross section of the refrigerant path takes place as a function of a pressure loss limit value above the sublimator.

Alternatively or cumulatively, the reduction of the cross section of the refrigerant path takes place as a function of the compressor suction pressure.

A further advantage is the reduction of the refrigerant path as a function of a limit value for the temperature spread, which can be measured on the air side or on the secondary side via the sublimator.

Furthermore advantageously, the reduction of the cross section of the refrigerant path can be determined as a function of a limit value for the temperature spread on the refrigerant side.

According to an advantageous embodiment of the invention, the reduction of the cross section of the refrigerant path after a predetermined time interval in the manner of a control.

A particularly advantageous embodiment of the invention results from the fact that the reduction of the cross section of the refrigerant path in response to a combination of several of the aforementioned control and regulation signals.

The reduction of the cross section of the refrigerant path can be carried out according to an advantageous embodiment of the invention between 5 and 30 seconds.

The object of the invention is further achieved by a device with which the above-described method can be realized, which is characterized in that a shut-off device for changing the cross section of the refrigerant path is arranged after the sublimator in the refrigerant path. Furthermore, at least one sensor and a control and regulating device for controlling and regulating the shut-off device are provided for regulation and control.

An advantage of the invention is in particular that it is now possible to dissolve solid carbon dioxide deposits in the designed as a sublimator heat exchanger by a cleaning procedure during operation or distribute. By temporally minimal delays of the dissolution process, a substantially continuous cycle operation of a carbon dioxide cycle can be ensured below the triple point with constant heat transfer performance.

According to the concept, the sublimator is continuously supplied with carbon dioxide from the circulation by the upstream throttling element of the circuit. In addition, the short-term supply of the sublimator with CO ₂ at higher pressure can also be realized from another reservoir, for example from other plant zones, which are at a higher pressure level due to the process.

If the flow through the sublimator is selectively stopped or reduced by a shut-off device, for example in the form of a solenoid valve, after the heat exchanger, the pressure in the sublimator rises. If the pressure briefly exceeds the triple pressure of the carbon dioxide of 5.18 bar, all carbon dioxide solids components dissolve immediately due to physical reasons and liquid carbon dioxide forms. Thereafter, the shut-off can be completely or partially reopened, whereby the liquid esters are distributed by the pressure drop and converted back to solid or gaseous CO ₂ and fed to the main stream.

If the filling of the sublimator with liquid or gaseous carbon dioxide is carried out simultaneously or independently of the cleaning procedure for filling the sublimator, then the shut-off device after the sublimator should just be opened so far that the pressure in the sublimator during the filling is still above the triple pressure. The sublimator is filled with refrigerant, similar to a conventional evaporator. After adequate filling, the shut-off device is selectively opened after the sublimator and the pressure in the sublimator is lowered below the triple pressure. As a result, a preferably homogeneous distribution of solid and gaseous CO ₂ is formed in the sublimator, which can be used for providing the cold.

The cleaning procedure preferably takes only a few seconds, so that the effect of the short-term increase in temperature and pressure on the continuous provision of cooling in the sublimator is extremely low. Control technology is the start signal for the cleaning procedure, ie the operation of the shut-off device for reducing or blocking the cross-section of the refrigerant path, by different sizes alone or in combination triggered.

Another advantage of the invention is that with the downstream Shut-off the pressure level in the sublimator can be maintained until complete filling with preferably liquid CO _2, the pressure above the triple conditions and then a rapid pressure drop below the triple conditions is possible. The sublimator is then ideally filled evenly with solid and gaseous CO ₂ .

According to one embodiment, the control and regulating device is regulated via a pressure loss limit value above the sublimator. Further control and regulation parameters are the triggering of the reduction of the cross section of the refrigerant path, the compressor suction pressure, a limit for the temperature spread or the outlet temperature on the air side, as well as a limit value for the temperature spread or the exit temperature on the refrigerant side and the absolute pressure at different points in the sublimator itself.
Alternatively or in combination, the cleaning procedure can be triggered, for example, after a predetermined time interval. The cleaning procedure has a special technical condition, especially with sublimators in multi-pass design. Here, even with blocking or unwanted cross-sectional narrowing of individual passes, there is no other way to free the sublimator of its blockages. However, in order to dissolve and distribute individual solid carbon dioxide deposits or to ensure a homogeneous filling, the use of the cleaning procedure is also advantageous in the case of a passable design of the heat exchanger as a sublimator.

In summary, the concept of the invention consists in briefly triggering an increase in pressure above the triple pressure of carbon dioxide by means of an additional shut-off device in the flow direction downstream of the sublimator according to predetermined control and / or control signals and dissolving or distributing carbon dioxide solid matter deposits formed thereby. The short-term desired increase in pressure can alternatively or cumulatively by supplying CO ₂ from a reservoir with higher Pressure level done.
An advantage of the arrangement of the shut-off device in the flow direction after the sublimator is additionally that the pressure level during filling of the sublimator is still slightly above the triple pressure and only then lowered below the triple pressure.

As a result, a more efficient, continuous heat transfer in the sublimator is possible, and the circuit can operate quasi-continuously without major interruptions. Moreover, there is the possibility to operate several sublimers or sections of the same in parallel and in each case first to fill uniformly and then lower the pressure for cooling in the pressure until the solid CO _{2 is} completely sublimated.

Fig. 1:

Phasendiagramm Kohlendioxid,

Fig. 2:

Ausschnitt aus einem Prinzipschaubild eines Kreislaufes mit Kohlendioxid als Kältemittel und Sublimator,

Fig. 3:

Schema einstufiger Grundprozess eines Kreislaufes mit Kohlendioxid als Kältemittel und Sublimator mit innerem Wärmeübertrager,

Fig. 4:

Schema zweistufiger Prozess.

Further details, features and advantages of embodiments of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings. Show it:

Fig. 1:

Phase diagram carbon dioxide,

Fig. 2:

Part of a schematic diagram of a cycle with carbon dioxide as refrigerant and sublimator,

3:

Schematic single-stage basic process of a cycle with carbon dioxide as refrigerant and sublimator with internal heat exchanger,

4:

Scheme two-step process.

In Fig. 1 is a phase diagram for carbon dioxide shown schematically. The state of aggregation of CO ₂ depends not only on its temperature but also on the pressure. At the triple point all three phases are fixed (f), liquid (fl) and gaseous (g) in equilibrium. For CO ₂ , the triple pressure is 581 kPa and thus far above atmospheric pressure, the associated temperature is approximately - 56 ° C. This is liquid (fl) CO ₂ under normal conditions nonexistent, but only under increased pressure. CO ₂ can be present under normal conditions only as gas or in solid state as dry ice. At room temperature, dry ice passes directly from solid (f) to gaseous (g), sublimating. The line in the phase diagram, which marks the transition from the solid to the liquid state, is also called the sublimation line.

In Fig. 2 is a section of a refrigeration system with a sublimator 1 shown for generating lower temperatures with carbon dioxide as the refrigerant. The sublimator 1 is a heat exchanger, which is specially designed to use the sublimation heat in the transition from solid carbon dioxide to gaseous carbon dioxide. The heat exchanger shown is designed multi-pass, so that there are several parallel channels within the refrigerant path through the sublimator 1 therethrough. The refrigerant path 2 thus branches at the input into the sublimator 1 and is recombined at the output of the sublimator 1. The expansion element 4 for generating the solid carbon dioxide is arranged in front of the sublimator 1 in the flow path 2 and leads to the formation of solid carbon dioxide in the sublimator 1. In the refrigerant path 2 after the sublimator 1, the additional shut-off device 3 is arranged, which with a control, not shown. and control device is connected. Furthermore, in Fig. 2 an optionally usable bypass 6 with throttling device is shown, via which carbon dioxide can be fed to the sublimator 1 at a pressure above the triple pressure in order to accelerate the process of pressure increase in the sublimator 1. A subsequent pressure reduction leads to a conversion of the liquid carbon dioxide into gaseous and solid carbon dioxide.
In Fig. 2 schematically various sensors 5 are indicated in the circuit, which communicate state and process variables of various kinds to the control and regulating device, whereupon from the control device, a corresponding control or regulating signal to the shut-off device 3 for Close or open the same is transmitted.

In Fig. 3 is a single-stage refrigeration system with internal heat exchanger 7 shown schematically. The refrigeration plant is next to the in Fig. 2 already designated components in the flow direction of the refrigerant from the compressor components 9, a heat exchanger 8 for heat dissipation, which is referred to depending on the function and condition of the refrigerant as a recooler, gas cooler or condenser and the inner heat exchanger. 7
If the pressure in the sublimator 1 increases after closing the shut-off device 3 at an insufficient rate, it is likewise possible for the bypass 6 to introduce refrigerant into the sublimator 1. The bypass 6 taps the already present in the circulation higher pressure level on the high pressure side directly. About the inserted in the bypass 6 throttle body, in addition to the expansion valve 4 briefly another cross-section can be opened to supply a correspondingly increased amount of carbon dioxide. The pressure increase in the sublimator 1 is thereby faster. The throttle body in the bypass 6 can preferably be designed so that it allows a release of the opening cross section only to just above the triple pressure. This allows the components on the low pressure side of the circuit to be additionally protected against excessive pressure increase. The required control and regulation of the throttle body in the bypass 6 can preferably also be effected by the relevant state and process variables about the sublimator 1.

In Fig. 4 is a two-stage refrigeration system with internal heat exchanger 7 shown schematically. Accordingly, an additional compressor 9, an additional expansion element 4 for the high-pressure stage, a recooler 8 at medium pressure level and a medium-pressure vessel 10 are provided in the circuit for the compression to medium pressure. Another special feature is that two bypasses 6 are provided for the supply of carbon dioxide in the sublimator 1, wherein a bypass 6 from the medium-pressure level and a bypass 6 from the high-pressure level with the sublimator 1 is connected.

The plant represents a possible, particularly efficient variant for a sublimation cycle with CO ₂ .
In the preferred process can be tapped with the bypass 6 and the medium pressure level of the circuit. This is particularly advantageous in that the relaxation of the carbon dioxide from the high pressure to the medium pressure level has already taken place and the liquid fraction of carbon dioxide thus obtained has already been supplied to the separator. According to a variant of the circuits, not shown, at least two sublimers 1 are connected in parallel. This allows a quasi-continuous operation of the refrigeration system, so that no interruption of the refrigeration occurs during the cleaning procedure.

LIST OF REFERENCE NUMBERS

1

sublimator

2

Refrigerant path

3

Shut-off

4

expansion element

5

sensor

6

Bypass with throttle device

7

Internal heat exchanger

8th

Dry cooler, gas cooler, condenser

9

compressor

10

Intermediate pressure vessel

Claims (12)

1. Method for operating a refrigerant circuit with a sublimator (1) for carbon dioxide as a refrigerant, characterised in that

   - the refrigerant circuit is operated below the triple point pressure and that

   - cleaning of the sublimator (1) to remove blockages with or from deposits of solid carbon dioxide through a reduction of the cross section of the refrigerant path (2) after the sublimator (1) takes place, whereby

   - the pressure in the sublimator (1) is raised above the triple point pressure and the solid carbon dioxide is converted into liquid carbon dioxide and the blockages and deposits are broken up, after which

   - the refrigerant path (2) is at least partially enlarged again and the liquid carbon dioxide in the sublimator (1) is distributed as result of the pressure drop, and once the pressure has fallen below the triple point pressure, is converted back into solid or gaseous carbon dioxide, whereby

   - the change in the cross section of the refrigerant path (2) takes place subject to control or regulation signals.
2. Method in accordance with Claim 1, characterised in that liquid carbon dioxide is channelled into the sublimator (1).
3. Method in accordance with Claim 1 or 2, characterised in that the reduction of the cross section of the refrigerant path (2) takes place until the refrigerant path (2) is closed.
4. Method in accordance with one of Claims 1 to 3, characterised in that the reduction of the cross section of the refrigerant path (2) takes place as a function of a pressure loss limit value above the sublimator (1).
5. Method in accordance with one of Claims 1 to 3, characterised in that the reduction of the cross section of the refrigerant path (2) takes place as a function of the compressor suction pressure.
6. Method in accordance with one of Claims 1 to 3, characterised in that the reduction of the cross section of the refrigerant path (2) takes place as a function of a limit value for the temperature spread on the air side.
7. Method in accordance with one of Claims 1 to 3, characterised in that the reduction of the cross section of the refrigerant path (2) takes place as a function of a limit value for the temperature spread on the refrigerant side.
8. Method in accordance with one of Claims 1 to 3, characterised in that the reduction of the cross section of the refrigerant path (2) takes place according to a settable time interval.
9. Method in accordance with one of Claims 1 to 8, characterised in that the reduction of the cross section of the refrigerant path (2) takes place subject to a combination of several control and regulation signals.
10. Method in accordance with one of Claims 1 to 9, characterised in that the reduction of the cross section of the refrigerant path (2) takes place between 5 and 30 seconds.
11. Device for implementing a method in accordance with one of the preceding claims, having a compressor (9), a heat exchanger (8) for dissipation of heat and also an inner heat exchanger (7) and a sublimator (1), characterised in that a shut-off device (3) for changing the cross section of the refrigerant path (2) is arranged after the sublimator (1) in the refrigerant path (2) and a bypass (6) with throttle valve to the expansion element (4) is arranged in front of the sublimator (1) and that at least one sensor (5) and a control and regulation device for controlling and regulating the shut-off device (3) are provided.
12. Device in accordance with Claim 11, characterised in that several sublimators (1) are arranged connected in parallel in the refrigerant circuit.

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