Field of invention

Prior art

It is known to provide cooling to a cooling room and to a freezing room provided with each their cooling evaporator and using a plurality of compressors and a working medium cooler inserted in the circuit after the compressors. The known systems are not energy efficient and take up more space than necessary. This is a problem when cooling plants of this kind are used onboard ships, where the use of space is overly expensive.

US 2017/0122624 A1 discloses a multi-stage system for cooling a refrigerant, which utilises known components such as evaporators, compressors, and gas cooler. However, it does not solve the above- mentioned problems.

Thus, there is a need for a method and an apparatus which enables less spacious devices and devices that may be regulated and controlled more closely in accordance with load shifts. There is also a need for an alternative to prior art cooling plants. Also, there is a need for cooling plants, which will fit in well on a ship and use seawater as working medium coolant even when the working medium is C02.

Summary of the invention

The objects of the present invention are achieved by a method as defined in claim 1 and by an apparatus having the features as defined in claim 7. Preferred embodiments are defined in the dependent subclaims, explained in the following description and illustrated in the accompanying drawings. A method which is adapted for ensuring a predefined temperature in a freezing room with a freezing evaporator and in a cooling room with a cooling evaporator is provided whereby a working medium in gas and/or liquid form is circulated in a circuit and exposed to working conditions in the following units: a) the cooling evaporator, wherein the working medium is evaporated at a cooling evaporator pressure, b) the freezing evaporator, wherein the working medium is evaporated at a freezing evaporator pressure, c) a working medium gas compressor unit, which comprises a first compressor wherein the evaporated working medium gases from the freezing evaporator is pressurized. According to the invention, the compressor unit comprises a second compressor wherein the working medium exiting from the first compressor and the working medium which exits the cooling evaporator are conjointly received and compressed to a final output pressure, and further a working medium cooler is provided wherein the pressurized working medium gas from the second compressor is cooled down whereby the working medium exiting the working medium cooler is received at a working medium receiver tank which is connected to the cooler and freezing evaporators. By these measures the method may easily be adapted to a particular size of a freezing and/or a cooling room.

It is also particularly easy to provide a further cooling at the receiver tank such that it may be possible to receive either transcritical or subcritical working medium fluid in the tank, and still ensure, that it is cooled blow the subcritical temperature. By the insertion of the receiver tank, the working medium cooler inserted after the second compression step, is allowed to deliver the working medium in either transcritical condition or in subcritical condition as mainly liquid.

It is preferred to use C02 as the working medium, and in this case, it shall depend on the temperature of the cooling medium in the working medium cooler, and the load from the evaporators whether a transcritical or subcritical temperature is reached, and in case seawater is used as the cooling medium, it will be difficult to stay below the transcritical temperature of the C02 when the seawater reaches temperatures of between 26 and 28 degrees Celsius. With the used construction, this poses no problem as the required further cooling is easily provided in the receiver tank.

It is preferred that in the first compression step and in the second compression step one, two or more individual compressors are used, and that there are more compressors assigned to the second compression step than to the first compression step.

This allows for a more nimble control of the compression steps, as individual compressors in each step may be switched off or on to accommodate shifts in needs for cooling down cooling storage or freezing storage rooms or to accommodate shifts in a cooling down capacity of the working medium cooler due to shifts in temperatures of a cooling medium such as water used in the working medium cooler.

Not just on/off switching is an option, but also well-known regulations for rotation speed of each compressor is available, and thus a very accurate and fast regulation is possible with the compressors.

The method includes the steps that in the working medium receiver tank, a cooling circuit further cools down the working medium received from the working medium cooler, and that the cooling circuit at an input end receives the joint working medium stream which exits the first compressor and the cooling evaporator and that the cooling circuit at an output end serves the working medium, which exits the cooling circuit, at the second compressor.

In this way, it is assured, that all of the working medium, which exits the first compressor step and exits the cooling evaporator is allowed to pick up energy in the receiver tank cooling circuit. This ensures, that at least a fraction of a transcritical working medium in the receiver tank is cooled down to a subcritical temperature.

It is further remarked that any condensed cooling medium liquid in the line from either cooling room evaporator or cooling medium exiting the first compressor step shall, by way of the further added heat energy in the receiver circuit tank, be evaporated. This protects the second compression step from receiving any trace of liquid at its input end.

In an embodiment, the receiver tank is also connected to the working medium stream by way of a receiver vapor exit pipe, which is connected to the output side of the first compression step conjointly with the output from the cooling room evaporator. Preferably a motor driven receiver tank bleed off regulation valve is provided in the receiver vapor exit pipe, whereby the regulation valve is adapted to be guided by the temperature of the working medium as sensed by a receiver tank supply line temperature sensor, whereby the temperature of the working medium which exits the high pressure working medium cooler is recorded. In this way, working medium may be allowed to escape the receiver tank and expand to a lover temperature in the receiver vapor exit pipe, and be guided into the cooling circuit of the receiver tank and hereby accomplish a further cooling down of the possibly transcritical working medium received here from the working medium cooler. According to the method the pressurized gas from the output at the second compressor feeds into the working medium cooler, and enters a high pressure circular cylindrical tank into the spaces between straight cooling pipes arranged in an array in the high pressure tank whereby cooling water is circulated within the cooling pipes and wherein the working medium is supplied to the tank at a pressure PI, and wherein cooling water se supplied to the pipes at a pressure P2, and wherein PI is higher than P2, preferably up to between 30 and 120 bars higher. The high pressure difference between the cooling side and the pressurised working medium side in the cooler is a challenge, but the high pressure tank is preferably a circular cylindrical tank and by ensuring that the cooling water pipes are aligned with the tank cylinder axis, a cooling medium such as water may be circulated through the pipes, while the high-pressure, and hot working medium is allowed to flow past the pipes in spaces provided between the pipes. This design is especially beneficial in order to accommodate the high pressure difference between working medium and cooling water while it, at the same time, facilitates easy cleaning of the cooling water pipes.

The method further prescribes that the cooling water enters through a manifold provided at both a first end and a second end of the circular cylindrical tank, such that the inside of the straight cooling pipes between the first and the second end may be cleaned by mechanical means at each end of the cylindrical high pressure tank, and such that cooling water, preferably sea water may be circulated through the pipes.

The entrance of the cooling water through a manifold shall ensure, that the pipes between the two manifolds may be cleaned. Also, the manifolds shall ensure, that the high pressure of the working medium gas is maintained inside the cylindrical high-pressure tank. It is preferred, that a closure is provided and placed distally from the cooling pipe manifolds at both ends of the cylindrical tank. The closure is adapted to be removed, such that the cooling water pipes may be accessed at both ends of the circular cylindrical tank. This allows simple mechanical means such as brushes or high-pressure cleaning hoses to be pushed through each pipe.

It is preferred that at one end of the circular cylindrical tank, the closure comprise an entrance opening at a lower part thereof and an exit opening at an upper part thereof, whereby the entrance opening connects exclusively to manifold a part, which connects to a range of pipes placed in the lower half of the circular cylindrical tank, and that the exit opening connects exclusively to a manifold part which is in connection with a range of pipes placed in the upper half of the circular cylindrical tank, and further the tank is placed with its longitudinal cylindrical axis in a horizontal direction. At the closure at an opposed end of the cylindrical tank, the pipes placed in the lower half are allowed fluid flow connection to the pipes placed in the upper half of the tank in the area between the manifold and the closure.

This allows a semi counterflow operation of the cooler, as the hot and/or transcritical gasses from the second compression step are entered into the cylindrical tank through a vertically arranged entrance tube provided at an uppermost ridge of the tank, and allowed to exit the tank trough a vertically arrange exit tube at a lowermost ridge of the tank.

An impinge plate may be arranged in the tank below the entrance tube.

Thus, the gasses shall flow in a generally downward direction while the cooling water in the pipes shall flow firstly inside the lower set of pipes, and then inside the upper set of pipes and thus flow in an overall up ward direction.

According to the method the first compression step and the second compression step are performed by rotary compressors and at all compressors, lubrication oil bleeds off at a controllable amount at a high pressure such as at the output pressure of the respective compressor is performed.

The rotary compressors are both very compact and may also be controlled very precisely and thus are easy to adapt the presented job of ensuring the working medium compression. The bleed off of oil at a high pressure is a measure, whereby oil may be maintained in the compressor and at a well-controlled level, as oil, which has been bled off is piped to the suction or inlet end of the compressors, as is well known in the art.

It is preferred that at each rotary compressor at the first compressor and at the second compressor the flow of lubrication oil bleeding off from each compressor is regulated by a motor driven valve provided in a bleed oil pipe according to a temperature of the lubrication oil at the high pressure side of the motor driven valve and according to a temperature of the lubrication oil at the low pressure side of the motor driven valve.

In this embodiment of the invention, a motor driven valve is provided in the oil flow from the compressor to the intake end thereof, and the oil temperature is measured in front of the valve and after the valve, and the measured temperatures are used to set or regulate the bleed off rate through the valve. In this way, the temperature of the lubrication oil inside the compressor may be regulated, and the lubrication oil flow through the compressor is also kept within acceptable boundaries. This is particularly important in case compressors which regulate the rotation speed are used, such that the oil bleed off amount may be arranged to follow load or temperature conditions at the individual compressor.

The cooling room evaporator and freezing room evaporator and the compressors and coolers may be arranged onboard a ship.

The ship environment is particularly challenging, as the available space on a ship is very costly, and thus real estate use for secondary items such as cooling shall be kept as low as possible. Also, it is to be observed, that a high reliability is a must onboard a ship, as spare parts and repair is often times not readily available. The method according to the invention will be particularly well suited for use onboard a ship.

The invention also provides a cooling plant which is adapted to ensure a predefined temperature in a freezing room by use of a freezing evaporator and in a cooling room by use of a cooling evaporator wherein a working medium compressor unit comprises two compression steps which steps both include the use of rotary compressors and wherein: a) a cooling evaporator is provided, which evaporates the working medium liquid at a cooling evaporator pressure, b) a freezing evaporator is provided, which evaporates the working medium liquid at a freezing evaporator pressure, and c) the working medium gas compressor unit comprises a first compressor wherein the evaporated gases from the freezing evaporator is pressurized.

According to the invention it is further preferred that the working medium gas compressor unit comprises a second compressor wherein the working medium exiting from the first compressor and the working medium from the cooling evaporator are conjointly received and compressed to a final output pressure, and that, d) the working medium gas which exits the second compressor is served at the working medium cooler, and that e) the working medium exiting the working medium cooler is received at a working medium receiver tank which is connected to the cooler and freezing evaporators.

This cooling plant allows for the plant to work with the working medium in both sub-critical and trans-critical condition, which is most important in case waters, on which a vessel which carries the plant, are used for the cooling of the working medium cooler. Especially the varying temperatures of such waters may be a problem, which usual cooling plants are ill prepared to deal with.

It is preferred that a receiver tank evaporator exit pipe is provided between the inside of the receiver tank and the exit pipe of the first compressor, whereby the pipe allows vapours from within the receiver tank to escape and expand to reach a pressure commensurate with the pressure at the exit of the first compression step.

This measure may cool down the contents of the receiver tank when needed.

It is advantageous that the working medium receiver tank includes a cooling circuit adapted to further cool down the working medium received from the high pressure working medium cooler, and that the cooling circuit at an input end is connected to the joint working medium stream in receiver cooling circuit supply pipe, which exits the first compressor and which exits the cooling evaporator and that the cooling circuit at an output end is connected to receiver cooling circuit exit pipe, whereby the working medium which exits the cooling circuit is served at the second compressor.

Through these measures it is further ensured, that the working medium received in the receiver tank may be safely turned into a liquid, in the cases, where a transcritical temperature is obtained in the working medium which exits the working medium cooler.

Whenever a bleed off of this nature is called for, the work load on the second compression step shall rise, and a higher exit temperature or an increased flow of the working medium exiting the second compression step is to be expected, which again leads to an increase of the energy to be transferred to the cooling water in the high pressure cooler.

For the control of the escape of the working medium, a receiver tank bleed off regulation valve which is motor driven, is provided in the receiver evaporator exit pipe whereby the valve is guided by the measured temperature of the working medium supplied to the receiver tank by a receiver tank supply line temperature sensor.

The advantage of this arrangement is that the temperature of the vapor in the cooling circuit in the receiver tank may be controlled by a bleed off of working medium at high pressure.

The working medium exiting the receiver tank through the receiver tank bleed off regulation valve is introduced into the cooling circuit at a lower pressure and thus at a lower temperature.

It is preferred that the cooling circuit within the receiver tank comprises a simple spirally wound pipe, in which the working medium of the cooling plant is circulated.

The plant may further comprise a working medium cooler which has a circular cylindrical high pressure tank with a first end and a second end and having a multitude of straight cooling pipes arranged parallel to a cylinder axis of the high pressure tank between the first end and a second end whereby further space between the pipes is provided and adapted to receive the pressurized working medium from the working medium compressor unit at a pressure 90 bar or higher, while the pipes are adapted to receive the cooling water at a pressure of no more than 2 bar.

The cylindrical shape of the working medium cooler and the use of pipes with a circular cross section enables the use of a very high pressure difference between the working medium, which is hot, and the cooling medium, which may be water, such as seawater. The cooling water may have a temperature as high as up to 35 degrees Celsius.

It is further preferred that a manifold is provided at both a first end and a second end, of the high pressure tank such that the straight cooling pipes between the first and the second end may be cleaned by mechanical means at each end of the cylindrical high pressure tank, and that cooling water, preferably sea water may thus be circulated through the pipes.

The manifolds are adapted to allow access to the inside of the pipes, which makes their cleaning a lot easier. This arrangement allows for a very simple cleaning action of the pipes, as at both ends, the manifolds are provided which shall allow the pipes to be cleaned. This is especially important whenever un-filtered water is used, and such water is thus applicable as cooling water, and both salt, water and sweet water may be used directly from the sea/lake or river on which a ship is navigating.

A closure, which has a diameter in accordance with the high-pressure tank diameter is provided at both ends at a low pressure side of the respective manifolds.

If growths such as mussels or other shellfish is to be avoided, a flow rate within the pipes is to be maintained high. However, deposits are not easily avoided such as in the areas outside of the manifolds, and thus these areas should be easily accessible for cleaning purposes.

In an embodiment the pipes are arranged in two layers in the high pressior tank, which is arranged with its cylindrical axis in a vertical direction, such that a lover set of pipes arranged in the lower half of the tank receives cooling water which flows in the pipes from a manifold at a first end of the tank to a manifold at an opposed end. At the manifold of the opposed end, the water flows upward, to gain access to a second set of pipes, which are arranged in the upper half of the cylindrical high pressure tank, whereby the cooling water shall flow in the opposite direction in the upper pipes to reach the manifold at the first end of the tank, to gain access to an outlet opening for cooling water. The outlet is placed above an inlet for the cooling water.

In a further embodiment, the pipes in the cylindrical tank are divided into four sub-groups, such that a first subgroup receives water at the inlet, and at the opposed end the manifold connects this first subgroup of pipes with a second subgroup of pipes, which directs the water back to the inlet end, where the water at the manifolds low pressure side gains connection with a third sub-group of pipes, which once again feeds the water through the cylindrical tank to the opposed end, where the water finally, again at the low pressure side of the manifold is connected to a fourth sub-group of pipes which runs through the length of the cylindrical tank to exit at the manifold and connect to a water outlet from the cylindrical tank heat exchanger. This arrangement is known as a four-way heat exchanger.

It shall depend on the average temperatures of the waters on which a ship is sailing whether the four-way or the two-way heat exchanger is preferred as the one works better at rather high temperatures of the cooling water, where as the other performs better at lower water temperatures.

It is preferred that the first compressor and the second compressor each comprises rotary compressors of the type which bleeds a controlled amount of lubricating oil off at a high pressure.

Such compressors are commercially available in many variants. And thus, there are many commercially available options for the provision of the first and the second compressor step. It is preferred, that there are always more individual compressors at the second compressor step, such that this step is easy to regulate according to heat spending at the cooling room, which may wary over the day, and also according to the temperature of cooling medium supplied to the high pressure working medium cooler which may vary over time in accordance with water temperatures of the waters on which a vessel carrying the cooling plant is traveling.

In an embodiment of the invention, there is one compressor at the first compression step, and two compressors at the second compression step.

Preferably a first temperature sensor is provided in an oil bleed output line and a second temperature sensor is provided in the oil bleed output line after a bleed off control valve and further the bleed off valve is a motor driven control valve which is adapted to be regulated according to temperature readings of the first and the second temperature sensors.

The provisions of the sensors shall ensure, that the bleed off rate of the oil from each compressor is regulated according to the temperatures recorded before and after an oil bleed off valve provided in an oil return pipe, which is adapted to feed the lubrication oil back to a suction manifold. It is noticed, that some working medium vapour may escape with the oil, and that this working medium shall expand according to the lower pressure at the low pressure side of the oil bleed off valve, and thereby a not insignificant temperature difference may be realized over the valve.

It is preferred that at the entrance of the first compression step and at the entrance of the second compression step, a suction manifold is provided, which allows oil to be introduced evenly into the working medium stream entering each compression step.

In an embodiment an oil separator is provided in the exit of working medium from the second compression step, and the oil which is separated from the high pressure working medium, shall be piped back to the manifolds and distributed to the first and the second compression steps through an oil feed line adapted therefor.

Regulation of the oil flows to the two manifolds from the oil feed line may be provided.

The cooling plant may be provided onboard a ship. A ship with such a cooling plant may navigate at any part of the world and use seawater as the cooling medium to transport the excess heat from cooling and/or freezing rooms away.

Description of the Drawings

The invention will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative of the present invention. In the accompanying drawings:

Fig. 1 shows a schematic view of the cooling plant,

Fig. 2 shows a working medium cooler and Fig. 3 is an enlarged sectional vies of a detail of a working medium cooler.

Detailed description of the invention

Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, a cooling plant 2 of the present invention is illustrated in Fig. 1.

In Fig. 1 and the piping connections and the main elements of a cooling plant 2 is disclosed. The plant 2 comprise the following main components: A freezing evaporator 100 which is adapted for evaporation of a working medium liquid, supplied in a freezing evaporator supply pipe 52, and a cooling evaporator 102, which is adapted to evaporate the liquid working medium supplied in a cooling evaporator supply line 51. At both evaporators 100, 102 there is provided a fan at the evaporator, such that cooled down air, may be supplied inside the cooling room (not shown) and freezing room (not shown) respectively. From the freezing room evaporator 100, a freezing evaporator exit pipe 53 is adapted to conduct evaporated working medium to a first step compressor 20. The first step compressor 20 supplies pressurized working medium to a second step compressor 30. The compressors 20, 30 together comprise a working medium compressor unit 10. Each of the first and the second compressor may comprise one, two or more individually driven compressors, and in the disclosed embodiment, the first compressor 20 is a single compressor unit, and the second compressor 30 comprises two individual and identical compressors 30 inserted in parallel between the first compressor 20 and a high pressure working medium cooler 12.

The high pressure working medium cooler 12 is adapted to receive the working medium from the second compressor 30 and is working as a heat exchanger, in which the pressurized and thus heated working medium is cooled down using a cooling medium.

The cooling medium in the presented embodiment is water, and preferably sea, lake og river water on which a vessel provided with the cooling plant navigates.

From the high pressure working medium cooler 12, the pressurized, but now cooled down working medium is collected in a receiver tank. The receiver tank 16 will usually comprise condensed and liquid working medium at the bottom thereof and above the liquid there will be pressurized working medium vapor.

The working medium fraction above the condensed and liquid working medium may be transcritical, which may be the result of a higher temperature of the cooling water circulated in the high pressure working medium cooler 12. In this case, additional cooling is carried out by either an expansion and bleed off of vapor through receiver vapor exit pipe 55 and/or through cooling by the circulation of working medium in cooling circuit 16. The bleed off of working medium from the receiver tank 14 is guided by a receiver tank bleed of regulation valve inserted in the receiver vapor exit pipe 55. The valve is regulated according to the temperature of the working medium exiting the high pressure working medium cooler.

Further valves and regulations may be provided by use of a range of further valves possibly regulated and controlled automatically. The valves and possible electronic control system is not disclosed, as this is well known in the art.

The receiver vapor exit pipe 55 connects to the receiver tank 14 at an upper part thereof. A pipe is provided, which may guide the liquid from the tank 14 to the cooling evaporator supply line 51 and to the freezing evaporator supply line 52 to ensure, that there is always a supply of liquified working medium at the two evaporators 100, 102.

At each compressor 20, 30 a bleed oil return line 25 shall be arranged, such that the lubrication oil, which exits each compressor is retuned to a manifold of the second compression step 28 and the manifold of the first compression 26 step respectively. In the oil return lines 25, between the high pressure oil bleed output 40 and the respective manifolds, an oil bleed off control valve 34 is provided, and in front of each of the valves 34 there is a first temperature sensor 36, which measures the temperature of the oil exiting each compressor unit. Also, after each valve 34 in the oil lines 25, there is inserted a second temperature sensor 38. The signals from the two temperature sensors 36,28 are used in a control module (not shown) in order to regulate the position of each of the oil bleed off control valves 34. This allows the oil bleed off to be regulated according the working conditions of the respective compressors.

It is noticed that the working medium which enters the second compressor step shall be piped through the cooling circuit 16 in the receiver tank 16, as the receiver cooling circuit supply pipe 56 collects the working medium which exits each of the first compressor 20, the cooling room evaporator 102 and the working medium in the receiver vapor exit pipe 55. At the output of the cooling circuit 16, the receiver cooling circuit exit pipe connect to the two second compressor step compressors 30.

An oil separator 22 is provided at the high-pressure output line of the second compressor step 30 and the oil, which is separated shall be piped back through oil feed line 24 to each of the manifolds 26, 28 of the two compressor steps. The working medium cooler 12 is disclosed in Fig. 2. It comprises a circular cylindrical high-pressure tank 4, with a longitudinal centre axis arranged in a vertical position. The tank has a first end 6 and a second end 8, and at the first end 6 a manifold is provided 5 (see Fig. 3) and a similar manifold 5 is provided inside the tank at the second end 8. Each manifold comprises a plate, which is arranged perpendicular to the centre axis of the tank 4. The manifold plates 5 are resistant to the high-pressure conditions, which will reside inside the tank between the plates.

Further, straight cooling pipes 7 are arranged to connect the distal surfaces of the manifold plates 5 and extend inside the tank parallel to the centre-axis thereof. Cooling water may be circulated inside the pipes 7 at low pressures, while a very high pressure is maintained in the space between the pipes 7 inside the tank 4.

At each their distal side of the manifold plates 5, closures 6.1, 8.1 of the tank 4 are arranged and bolted to tank flanges as is known in the art. Theses closures 6.1, 8.1 may be removed, and the pipes 7 may be cleaned by usual means, such as by use of a high pressure hose with a cleaning tip or by brushes arranged on long shafts.

The closures 6.1 at the first end 6 of the high-pressure tank 4 comprises an inlet 60 for cooling medium, preferably water, and an outlet 62 for the cooling medium. The inlet 60 connects to a range of pipes 7 through a manifold part, whereby the pipes 7 are provided in the lower half of the tank 4, whereas the outlet connects to a manifold part, which is connected to a range of pipes 7 arranged in the upper half of the tank 4. At the closure 8.1, the pipes at the lower half of the tank gain access to the pipes at the upper half of the tank, in the section between the manifold and the closure 8.1. Cooling water shall thus enter the inlet for cooling medium 60 at the first end 6 and flow through the pipes in the lower part of the tank, then flow upwards at the second end 8, and flow back to the first end in the pipes at the upper half of the tank 4 and exit the tank through the outlet 62 for cooling water. This arrangement of inlet, outlet and manifolds is referred to as a two-way heat exchanger, as the pipes are arranged to transport the cooling water from end to end of the cylindrical tank twice. It is however possible to subdivide the pipes into 4 subgroups and allow the water to travel four times from end to end inside the cylindrical tank. Such an arrangement is referred to as a 4-way heat exchanger.

In Fig. 3 an example of a pipe 7 and the manifold 5 in which it is inserted is disclosed. As seen the pipe 7 has smooth inner surfaces, and an exterior surface, which has been rolled or machined to comprise parallel furrows and ridges 71 arranged circumferentially around the pipe in the area thereof which shall be exposed to the hot C02. This surface structuring shall give the exterior surface of the pipes an extraordinarily large surface area, and thereby enhance the heat transfer from C02 to water flowing inside the pipe.

The manifold 5, which is also partially disclosed in Fig. 3 comprises a layer of corrosive resistant material 72 at the side turned toward the water, but is otherwise made from a stronger material such as a high grade steel. The manifold shall comprise up to 40 holes with pipes 7 mounted therein, and thus a high strength is required of this particular piece considering the high pressure difference between the C02 and the water side of the manifold. The pipes 7 shall also be made of material, which will not corrode when exposed to the seawater on the inside.

The working medium of the plant, which arrives from the second step compressors 30 at high temperature and at a high pressure is entered into the tank 4 through a vertically arranged entrance tube 64 at an uppermost ridge of the tank 4. An exit tube 66 is provided below at a lowermost part of the tank 4. An impinge plate 68 is provided inside the tank 4 below the entrance tube 64 to ensure, that the working medium which enters the tank 4 shall be spread to the sides and not travel directly down between the horizontally arranged tubes 7.

It is noticed that entrance tube 64 shall not be placed directly above the exit tube 66 but the two shall always be shifted sideways with respect to each other.

In the two-way heat exchanger, the working medium shall travel generally downward between the pipes 7, as the water shall travel generally upwards from the lower placed inlet 60 to the outlet 62 placed over the inlet. The flow in the heat exchanges is not a real counterflow, but some of the counterflow advantages may be gained by the arrangement of the pipes as explained.

List of reference numerals

2 -Cooling plant

4 -High pressure tank

5 -Manifold

6 - First end of high pressure tank

6.1 -Closure at first end of the tank

7 -Straight cooling pipes

8 -Second end of high pressure tank

8.1 -Closure at the second end of the tank

10 -Working medium compressor unit

12 -High pressure working medium cooler 14 -Working medium receiver tank

16 -Cooling circuit

20 -First compressor

22 -Oil separator

24 -Oil feed line

25 -Bleed oil return line

26 -Manifold of first compression step

28 -Manifold of the second compression step 30 -Second compressor

32 -Receiver tank bleed off regulation valve

33 -Receiver tank supply line temperature sensor

34 -Oil bleed off control valve

36 -First temperature sensor

38 -Second temperature sensor

40 -High pressure oil bleed output

50 -Working medium pipes

51 -Cooling evaporator supply pipe

52 -Freezing evaporator supply pipe

53 -Freezing evaporator exit pipe

54 -Cooling evaporator exit pipe

55 -Receiver vapor exit pipe 56 -Receiver cooling circuit supply pipe

57 -Receiver cooling circuit exit pipe

58 -Working medium input pipe

60 -Inlet for cooling medium 62 -Outlet for cooling medium

64 -Entrance tube

66 -Exit tube

68 -impinge plate

71 -tube ridges 72 -manifold cover material layer

100 -Freezing evaporator

102 -Cooling evaporator