WO2002066908A1

WO2002066908A1 - System and method in which co2 is used for defrost and as refrigerant during stand-still

Info

Publication number: WO2002066908A1
Application number: PCT/DK2002/000121
Authority: WO
Inventors: Torben Melchior Hansen; Frank Elefsen
Original assignee: Teknologisk Institut
Priority date: 2001-02-23
Filing date: 2002-02-25
Publication date: 2002-08-29
Also published as: DK200100310A; DK174257B1

Abstract

A refrigeration system is disclosed in which CO2 is used as refrigerant and as working medium during defrost. The refrigeration system comprises a distribution system (40) for refrigerants and a refrigeration equipment (44) on a consumption place for cooling effect. In order to provide a system which has the advantageous known from CO2 refrigeration systems and which does not require a separate construction for defrosting, the system is provide with a unit (30). This unit is connected between the distribution system and the refrigeration equipment. The unit is arranged for defrost by providing a high pressure and a corresponding high temperature in a part of the refrigeration equipment (44). The CO2 which has been brought under high pressure would following be led through the defrost pressure controller (3) to the distribution system at a low pressure. Accordingly, only a small part of the system needs to be dimensioned for high pressure.

Description

SYSTEM AND METHOD IN WHICH C0₂ IS USED FOR DEFROST AND AS REFRIGERANT DURING STANDSTILL

Background of the Invention

The present invention relates to a refrigeration system in which CO₂ is used as refrigerant and as working medium during defrost and with a distribution system for the refrigerant and refrigeration equipment on a consumption place for cooling effect.

The invention also relates to a method in which CO₂ is used as refrigerant and as working medium during defrost and where the refrigerant is led through a distribution system and through a refrigeration equipment on a consumption place for cooling effect.

As refrigerant in freezing systems and cold rooms, ammonia is most frequently used in two-stage systems or in small single-stage R22 systems, which no longer is permitted in new systems. The use of ammonia in freezing equipment is a well-tested and reliable technology, however, in certain cases it may be undesirable to have ammonia in workrooms. On board fishing vessels, the installation of mandatory safety equipment results in a considerable increase in the installation costs of the system.

CO₂ is an applicable alternative to R22, ammonia and other refrigerants in freezing equipment and cold rooms both in systems by land and by sea. In addition to an environmental benefit, increased freezing capacity can also be achieved by use of CO₂ and under certain circumstances, the energy consumption as well as the size of the refrigeration compressor system can also be reduced. CO₂, on the other hand, differs significantly from traditional refrigerants in a number of ways, which have to be considered in the design of the freezing equipment and the refrigeration system.

The critical point of CO is at 31°C and at a pressure of 72 bar. These thermodynamic conditions differ considerably from traditionally used refrigerants and this factor greatly influences the selection and design of components and system construction. The pressure at a given temperature is higher than for traditionally used refrigerants such as ammonia, HCFC and HFC. The critical point indicates that CO₂ generally is most applicable for energy purposes for low-temperature applications in cascade with e.g. ammonia or another refrigerant. When designing compressors, heat exchangers, valves, vessels, pipes and flexible tubes, it is however necessary to take the higher pressure into account.

During operation of refrigeration and freezing equipment, the temperature is however so low normally that the pressure is less than 20 bar and standard components can thus be used in such a system.

The higher pressure has however also favourable effects on different properties. The density in the gas phase is far greater and the temperature variation during pressure loss is much smaller for CO₂ than for other refrigerants. In this way, dimensions in equipment on the gas phase can be reduced.

In existing systems, CO₂ can be used in the low-temperature stage by use of two in principle different methods, i.e. as a secondary refrigerant and in a cascade solution which will be explained further in the following.

In existing systems, there are several drawbacks, e.g. during defrost and increasing pressure during standstill.

Evaporators in freezing equipment (e.g. plate freezers or air coolers) have to be defrosted regularly in order to continue to operate in an optimum way.

Hot gas from compressors is normally used with traditional refrigerants for defrost of freezing equipment as compressed gas from the compressors is led to the freezing equipment where the gas typically condenses at temperatures between 20°C and 30°C and in this way melts the frost on the surfaces.

In this way, the energy is provided for defrost from the heat uptake in the other evaporators together with the power input to the compressor during compression. When using CO₂ in cascade systems or as secondary refrigerant, compressed CO₂ gas is not available in the same way at an adequately high condensation temperature and the use of CO₂ has thus been limited so far.

There are alternative solutions in which electrical rods, internal hot liquid (e.g. glycol) or sprinkling with hot water for defrost of CO₂ systems are used.

Moreover, there is knowledge of a method in which CO₂ is used as working medium during defrost. The method works in this way: CO₂ from the pump vessel is pumped up to high pressure (e.g. 65 bar) in an evaporating vessel where it is heated and evaporates by heat exchange with hot gas from the primary refrigerant side (e.g. R22 or ammonia). The hot CO₂ gas under high pressure is led to a refrigeration system, which has to be defrosted, where it condenses and again expands back to the pump vessel.

The method is described further in the patent application GB 2 258 298 A. The method differs in the way that energy is provided for defrost from another refrigerant circuit or a waste heat source, and the high pressure requisite for defrost is established by means of a hydraulic pump working up against the high pressure determined by the saturation temperature in a hot vessel. During standstill, the pressure in the system will rise due to the heat input from the surroundings. At an ambient temperature of 25°C, the saturation pressure is 64 bar. If the refrigerant charge has to be kept in the system at this pressure, it will imply that all components have to be designed for this pressure. For liquid vessels as receiver and low-pressure liquid separator, this will imply an unacceptable high production price, and systems where the temperature is kept at e.g. 0°C or -10°C during standstill are much more attractive.

There is known a method from US patent no. 6,112,532 in which CO₂ is kept in a high-pressure vessel during standstill. The pressure in the vessel is kept down by cooling the liquid internally by means of a refrigerant coil at low pressure. The method uses a high and a low-pressure circuit. The CO₂ charge is mainly kept in the high- pressure part during standstill. At the same time, the consumption places are supplied with cold low-pressure liquid gas during standstill.

It is desired to keep the entire charge at low pressure which exists in the entire refrigerant distribution system and the consumption places.

US patent no. 6,112,532 deals in principle with a coil cooled liquid vessel, which can keep CO₂ under controlled high pressure, which is equal to or higher than the saturation pressure at ambient temperature.

It is desired to keep the entire CO₂ charge at a pressure smaller or equal to the saturation pressure at ambient temperature.

It is the object of the present invention is to provide a system and a method in which CO₂ can be used both as refrigerant and for defrosting, and where the drawbacks of the existing systems are avoided. A further object is to provide a system and a method in which standstill pressure minimisation is possible.

According to the present invention, this is achieved with a system, which is distinctive in in that it comprises a unit, which is connected between the distribution system and the refrigeration equipment, that the unit comprises a high-pressure compressor the suction side of which is connected to the distribution system in order to draw gaseous refrigeant out of the distribution system at a low pressure and the pressure side of which comprises a valve arrangement, which may ensure connection to a first circuit which comprises a part of or the whole refrigeration equipment and defrost pressure controller and which leads CO₂ under high pressure to the refrigeration equipment in order to defrost this, and further through the defrost pressure controller and back to the distribution system at a low pressure which is established in the defrost pressure controller and that the possible other parts of the refrigeration equiment is still coupled to the distribution system and functions at the low pressure.

The method according to the present invention is distinctive in that defrosting is effected by using a primary curcuit in which CO₂ under high pressure is led through the refrigeration equipment as gaseous CO₂ at low pressure is drawn from the distribution system and is compressed in a high-pressure compressor, that the gaseous CO₂ at high pressure is led through a part of the refrigeration equipment in order to defrost this, that said gaseous CO₂ is led through a pressure regulation to be returned to the distribution system at a low pressure and that these steps are repeated for all parts of the refrigeration equipment until it is defrosted.

By means of such a system and such a method, CO₂ under high pressure can be used for defrost and only a small number of system components need to be designed for such a high pressure. At the same time, the advantages of CO as refrigerant can be used and in this way, CO₂ is the only working medium needed in the system/for the method.

What is also new about the invention is the system in particular, which makes it possible to use CO₂ only. The system comprises a unit which makes the system different from existing systems in the way that it handles defrost during operation of freezing equipment (defrost) and that the pressure in the main system at the same time is prevented from exceeding a given maximum (e.g. 25 bar) during standstill (standstill pressure minimisation). Moreover, with a system and a method according to the present invention it is possible to keep the entire charge at low pressure which exists in the entire refrigerant distribution system and the consumption places. Also, the system and the method acording to the present invention keeps the entire CO₂ charge at a pressure smaller or equal to the saturation pressure at ambient temperature.

According to an additional embodiment, the apparatus is also distinctive in hat the valve arrangement also may ensure the connection to a second circuit which comprsies a hot gas cooler/condenser and an expansion valve and which leads CO₂ under low pressure back to the distribution system during standstill so that the unit is arranged for integrated defrosting of the refrigeration equipment and standstill pressure limitation. In a further aspect the unit further comprises a pressure switch being arranged over the compressor and controlling the on/off condition of this in relation to a set point.

According to a specific embodiment, the apparatus is distinctive in the way that the suction side of the unit is connected to the distribution system, mainly to a pump vessel or a receiver.

According to a still further embodiment the apparatus is distinctive in that an expansion valve is arranged in a pipe being connected with the outlet of the refrigeration equipment in order to expand and lead CO₂ under low pressure back to a pump vessel or a receiver.

According to a still further embodiment the apparatus is distinctive in that the compressor (7) is dimensioned for an outlet pressure being larger than 72 Bar, preferably between 75 and 85 Bar and that the defrost pressure controller is dimensioned to keep the pressure above 35 Bar, preferably between 55 and 65 Bar. According to a still further embodiment the apparatus is distinctive in that the distribution system is connected with a primary refrigeration circuit comprising on or more steps and with a primary refrigerant which in a heat exchanger codenses/cools CO₂ . According to a still further embodiment the apparatus is distinctive in that the suction side of the unit is connected to the ditribution system through a pump vessel or a receiver.

According to another embodiment, the method is distinctive in the way that standstill operation is established by using a second circuit in which CO₂ under low pressure is led through the distribution system and/or the refrigeration equipment.

According to a still further embodiment the method is distinctive in that that all CO₂ charge is kept in liquid form in the distribution system which is kept at a low pressure smaller or equal to the saturation pressure at ambient temperature.

According to a still further embodiment the method is distinctive in that the high pressure is maintained above 35 Bar, preferably between 40 and 90 Bar and that the low pressure is maintained below 35 Bar, preferably between 30 and 10 Bar.

According to the invention, the system differs from existing defrost systems in the way that:

• It comprises a unit which can be used as defrost and standstill pressure minimiser in any system designs.

• The unit can be used as plug-on unit for an existing system or built as an integrated part of the entire system.

• CO₂ is used as the only working medium in connection with defrost and standstill pressure minimisation.

• Hot CO₂ gas is produced by means of vapour compression.

• That the energy for defrost is provided by evaporation of CO₂ in the heat exchang- ers at the other consumption places.

• The entire CO₂ charge is kept in liquid form in the low-pressure vessel at a given set pressure smaller than the saturation pressure at ambient temperature. • No high-pressure vessels are required.

• There is a limited extent of the high-pressure side.

• There is no risk of leak of CO₂ under high pressure into the primary cooling circuit. • There is no risk of leak of primary refrigerant into the CO₂ cooling circuit.

Brief description of the drawing

In the following, the invention will be explained further referring to the enclosed drawing in which:

• Figure 1 shows a plan of one embodiment which according to the invention is designed as a pump circulation system,

• Figure 2 shows a plan of another embodiment which according to the invention is designed as a cascade system where the gas is extracted and compressed from a vessel on the low-pressure side of the cascade system (pump vessel),

• Figure 3 shows a plan of a third embodiment which according to the invention is designed as a cascade system where the gas is extracted and compressed from a vessel on the high-pressure side of the cascade system (receiver),

• Figure 4 shows a plan of a fourth embodiment which according to the invention is designed as a direct expansion system, and

• Figure 5 shows a schematic plan of a total system design in principle according to the invention.

In the different figures, identical or equivalent elements will be designated with the same reference designation and for that reason, there will not be given a specific explanation of all elements in connection with each figure.

The unit described below is a plug-on solution, which can be used in both pump circulation systems (figure 1), cascade systems (figure 2 and figure 3) and direct expansion systems (figure 4). CO can be used in the low-temperature stage as a so-called secondary refrigerant which is pumped from a pump vessel (10) to the consumption places where CO₂ evaporates completely, partly or only is heated as shown in figure 1. Depending on the type of refrigerant and levels of temperature, the upper stage can be omitted.

The method requires a traditional refrigeration system (e.g. ammonia), where the evaporator (11) is used for condensing/cooling CO after evaporation/heating in the consumption places. The design of the system in this way implies that only a pump (14) is required to circulate CO₂.

CO₂ can alternatively be used in cascade with another primary refrigerant as shown in figures 2, 3 and 4. Here the low-pressure stage is built around a CO₂ compressor, which condenses against a cascade cooler (20) with evaporating primary refrigerant on the other side.

In figure 2 and 3, CO₂ liquid is expanded from the receiver (21) through the valve (17) to a low-pressure liquid separator/pump vessel (10). During normal operation, the suction valve (18) is open, and the CO₂ compressor (22) removes the evaporating amount of CO . The refrigerant is distributed by means of a pump (14) to the con- sumption places (44), where CO₂ evaporates partly.

In figure 3, CO₂ liquid is expanded from the receiver (21) directly to the evaporators (44) through the expansion devices (19). The principle is called direct expansion and the pressure difference between the high and low-pressure side provides the driving potential during distribution of refrigerant to the consumption places where CO₂ evaporates completely.

Figure 5 shows schematically how a system according to the invention interacts with the other parts of a refrigeration system.

What is meant by the distribution system (40) in this connection is a vessel with an equilibrium between liquid and vapour together with the pipe system through which the refrigerant is distributed to consumption places (44), where the refrigerant evaporates completely or partially. The vessel can be a high-pressure vessel also called a receiver (21), a low-pressure liquid separator with natural circulation (gravitational driven) or a low-pressure liquid separator (10) with pump circulation.

The invention is advantageous, as it can be plugged-on to the distribution system of any system design. The suction side (31) has to be connected on the gas phase of distribution system's liquid vessel, and the outlet side (33) has to be connected on the gas or liquid phase of the distribution system's liquid vessel.

The defrost function requires furthermore a connection of a hot gas pipe (32) on the outlet side of the evaporators (44).

In the shown figure, the high-temperature stage (42) and the heat exchange (43) can be omitted, so that the process is carried out in one stage (41).

Notice that the valve arrangements in figures 1 to 4 are not complete due to the clarity of the description. With the given level of details, the principle should be comprehensible for a skilled person in the area.

The integrated unit (30) delimited by the dotted line in figures 1 to 4 is built around a CO₂ compressor (7), which operates with an outlet pressure of at least 80 bar. The suction side (31) is connected to the refrigerant distribution system of the existing refrigeration system.

The unit consists moreover of a solenoid valve (1) for selection of defrost, a solenoid valve (2) for selection of pressure minimisation, a defrost pressure controller (3), a hot gas cooler/condenser (4), an expansion valve (5) and a pressure switch (6).

In figures 1 to 4, the principle is shown in which CO₂ gas is used as the only working medium. The gas is compressed directly from the pump vessel (10) or from the re- ceiver (21) by means of the CO₂ compressor (7), so that energy is not supplied from the primary side or electrical heat sources to the CO₂ system.

The overall defrost principle appears from figures 1, 2 and 3. During defrost, the valve (1) is opened and the valve (2) is closed. The compressor (7) sucks cold gas from the pump vessel (10) and the pressure on the outlet side is kept constant by means of the controller (3) (e.g. 60 bar corresponding to a condensation temperature of 20°C in the evaporator, which has to be defrosted).

Before defrost takes place, the valve (8) is closed, and at the same time the main valve (9) is opened. The hot gas is led through the check valve (25) to the evaporator, which has to be defrosted, where it condenses. During defrost, the CO₂ vessel (10) can be shut off from the recondensing heat exchanger (11) by means of the valves (12) and

(13).

During defrost, the cold liquid and then the condensate is led through the main valve (9) from where it is expanded down to the pressure in the liquid vessel (10) or the CO₂ receiver (21).

The defrost concept with hot gas supply, condensing and expansion is well-known from traditional refrigerants.

What is new about the described principle is that the defrost takes place with compressed CO₂ and that the number of components, which have to be designed for high pressure with the selected system design, is minimised as it is only the pipeline on the compressor's (7) pressure side (7)-(l)-(2)-(3), the evaporator and pipes between (8) and (9) which have to be designed for the condensation/defrost pressure, e.g. 60 bar, while the rest of the system can be dimensioned for e.g. 25 bar, including the pump vessel (10). The system is shown in details in figures 1, 2 and 3, where there is only shown connection to one single evaporator (44), while the unit (30) in principle can be connected to any number of evaporators (44), which are defrosted by turns as required.

Normally, during defrost of one evaporator, there will be heat load on the pump vessel

(10) from the other evaporators. If there is only one evaporator on the CO₂ side, it will be necessary to supply the CO₂ vessel with an external heat load, e.g. by supplying hot gas from the main compressor system's high-pressure side, the pressure side of the compressor's low-pressure side, hot glycol or electric heat through an external heat coil (15) on the CO₂ vessel (10) in the figures 1 and 2 or the receiver (21) in figure 3.

When defrost has been completed, the compressor (7) is stopped and the valve (1) is closed.

The unit can be connected to any system designs where CO₂ is used. As shown schematically in figure 4, the new unit (30) only has to be connected with a suction pipe (31) on the gas phase of the existing system's distribution system and a return pipe (33).

The distribution system (40) can either be with direct supply of the liquid to evaporators (direct expansion) and in this case, the suction pipe (31) can be connected either on the receiver (21) or on the CO compressor's (22) suction side. In other cases, the distribution system consists of a pump vessel (10), on which the suction pipe (31) also can be connected.

A hot gas pipe (32) is led from the new unit (e.g. 60 bar) to each evaporator (44), which has to be defrosted.

In the described invention, the pressure (e.g. -5°C saturation temperature) will always be lower than the saturation pressure at ambient temperature. The compressor (7) is in this case always used for keeping the pressure below a given set point (e.g. -5°C). The valve (1) is closed and the valve (2) is opened. In this case, the compressed gas is led through a heat exchanger (4) where it rejects energy to the surroundings or to another heat sink and from there, it expands back into the CO₂ vessel (10) through the expansion valve (5).

At low ambient temperature (e.g. below 20°C), what is dealt with is a traditional vapour compression process, but in order to handle the task independent of season, geographic conditions and type of condenser, the process has to operate transcritical (above 31°C, where the condensation no longer takes place).

The compressor is controlled on/off by the pressure switch (6) in relation to the set point. The entire system side with CO₂ is kept at the pressure given by the set point of the pressure switch, however with the exception of the evaporator, which is at ambient temperature. During standstill, the evaporator is kept in equilibrium with the ambient temperature by expanding liquid from the evaporator through the main valve (9) until the evaporator is emptied of liquid. The pressure in the evaporator corresponds to the set point of the pressure switch.

The required energy consumption for maintaining the pressure minimisation will normally be minimal compared to the capacity of the refrigeration system.

Claims

1. A refrigeration system in which CO₂ is used as refrigerant and as working medium during defrost and with a distribution system (40) for the refrigerant and refrigeration equipment (44) on a consumption place for cooling effect, characterised in that it comprises a unit (30), which is connected between the distribution system (40) and the refrigeration equipment (44), that the unit (30) comprises a high-pressure compressor (7) the suction side (31) of which is connected to the distribution system (40) in order to draw gaseous refrigeant out of the distribution system (40) at a low pressure and the pressure side of which comprises a valve arrangement, which may ensure connection to a first circuit which comprises a part of or the whole refrigeration equipment (44) and defrost pressure controller (3) and which leads CO₂ under high pressure to the refrigeration equipment (44) in order to defrost this, and further through the defrost pressure controller (3) and back to the distribution system (40) at a low pressure which is established in the defrost pressure controller (3) and that the possible other parts of the refrigeration equiment (44) is still coupled to the distribution system (40) and functions at the low pressure.

2. Refrigeration system according to claim 1, characterised in that said unit (30) is arranged as a plug-on unit or a integrated part of the entire refrigeration system i pump circulation systems, in cascade systems or indirect expansion systems.

3. Refrigeration system according to claim 1 or 2, characterised in that the valve arrangement also may ensure the connection to a second circuit which comprsies a hot gas cooler/condenser (4) and an expansion valve (5) and which leads CO₂ under low pressure back to the distribution system (40) during standstill so that the unit (30) is arranged for integrated defrosting of the refrigeration equipment (44) and standstill pressure limitation.

4. Refrigeration system according to 3, characterised in that the unit (30) further comprises a pressure switch (6) being arranged over the compressor and controlling the on/off condition of this in relation to a set point.

5. Refrigeration system according to claim 3, characterised in that an expansion valve (5) is arranged in a pipe being connected with the outlet of the refrigeration equipment in order to expand and lead CO₂ under low pressure back to a pump vessel (10) or a receiver (21).

6. Refrigeration system according to any of the preceding claims, characterised in that the compressor (7) is dimensioned for an outlet pressure being larger than 72 Bar, preferably between 75 and 85 Bar and that the defrost pressure controller (3) is dimensioned to keep the pressure above 35 Bar, preferably between 55 and 65 Bar.

7. Refrigeration system according to any of the preceding claims, characterised in that the distribution system (40) is connected with a primary refrigeration circuit comprising on or more steps and with a primary refrigerant which in a heat exchanger codenses/cools CO₂.

8. Refrigeration system according to any of the preceding claims, characterised in that the suction side of the unit is connected to the ditribution system (40) through a pump vessel (10) or a receiver (21).

9. A method in which CO₂ is used as refrigerant and as working medium during defrost and where the refrigerant is led through a distribution system (40) and through a refrigeration equipment (44) on a consumption place for cooling effect, character i s e d in that defrosting is effected by using a primary curcuit in which CO under high pressure is led through the refrigeration equipment (44) as gaseous CO₂ at low pressure is drawn from the distribution system (40) and is compressed in a high- pressure compressor (7), that the gaseous CO₂ at high pressure is led through a part of the refrigeration equipment (44) in order to defrost this, that said gaseous CO₂ is led through a pressure regulation to be returned to the distribution system (40) at a low pressure and that these steps are repeated for all parts of the refrigeration equipment (44) until it is defrosted.

10. A method according to claim 9, characterised in that stand still operation is established by using a second circuit in which CO₂ under low pressure is led through the distribution system (40) and the refrigeration equipment (44).

11. A method according to claim 9 or 10, characterised in that all CO charge is kept in liquid form in the distribution system (40) which is kept at a low pressure smaller or equal to the saturation pressure at ambient temperature.

12. A method according to claim 9, 10 or 11, characterised in that the high pressure is maintained above 35 Bar, preferably between 40 and 90 Bar and that the low pressure is maintained below 35 Bar, preferably between 30 and 10 Bar.