The present invention relates to a method of operating an assembly of heat exchangers for subchtical and transcritical conditions, by initially arranging at least two heat exchangers in parallel for the subchtical condition, and to an assembly of heat exchangers. BACKGROUND OF INVENTION

In a conventional refrigeration system, heat release from the refrigerant is based on condensation of the refrigerant. The temperature is a critical point, which being constant during condensation. Operating an assembly of heat exchangers below the critical point is defined as subcritical mode. It is previously known to arrange heat exchangers in parallel at such subcritical mode.

However, in heat pump and refrigeration applications using CO2 as refrigerant there is a need to operate in transcritical mode, i.e. above the critical point as well as below the critical point. Transcritical refrigeration systems with CO2 as a refrigerant are well known in the art. The critical temperature of CO2 is 31.0 ⁰ C and the critical pressure is 73.8 bar. At higher temperatures and pressures no clear distinction can be drawn between liquid and vapour, and CO2 is said to be in the so-called super-critical fluid region. The thermal conditions for these two operation modes are dramatically different. During transcritical mode the flow rates of the cold side, typically brine or water, are much lower than during subcritical mode. Thermally the process on the hot side of the refrigerant is also very different. In transcritical mode large temperature drops is required with close approach at pinch point and outlet. All together this calls for two different designs of the heat exchanger to be able to operate the system in an optimal way.

Considering the temperature difference needed in a heat exchanger, i.e. approximate 10 ⁰ C, the upper limit for heat release based on condensation of CO2 will be around 20 ⁰ C ambient temperature. Below this temperature, the CO2 stays below the critical point and the refrigeration system operates in subchtical mode. For refrigeration systems used in supermarkets, the ambient temperature will exceed 20 ⁰ C during the summer in a large part of the world. At these temperatures, cooling of the CO2 is a single-phase cooling, namely a gas cooling. CO2 is above the critical point at the high pressure side of the system, and the refrigeration system operates in transchtical mode.

The efficiency and the cooling capacity of the refrigeration system are lower in transcritical operation than in subcritical operation. It is an important disadvantage of known CO2 refrigeration systems that they have a lowered performance at elevated ambient temperatures above approximately 20 ⁰ C, i.e. when a high performance is actually desired. It is an object of the present invention to provide a transcritical refrigeration system with improved

performance during transcritical operation.

DISCLOSURE OF INVENTION

It is an object of the present invention to constitute a solution to the problem, of the contradictory requirements for heat exchanger design for gas coolers and condensers. Instead of finding a design for a heat exchanger that typically is determined by subcritical conditions and then use it for transcritical operation one may use multiple heat exchangers in the system.

According to a first aspect of the present invention, these objects are achieved by arranging at least one heat exchanger at a transcritical condition in series with the other heat exchangers, and arranging an inlet and an outlet at opposite ends of the assembly, and switching the heat exchangers between being arranged in parallel to being arranged in series by closing a first conduit, connecting said inlet to a first duct of each heat exchanger, after the first heat exchanger and between every second heat exchanger, and a second conduit, connecting said outlet to a second duct of each heat exchanger, between the other heat exchangers.

The method include the use of multiple heat exchangers in parallel during condensation and then change to use them in serial or a combination serial and parallel during transcritical operation.

This improves system efficiency substantially in transcritical mode as the thermal length and heat transfer increases and thus the outlet temperature of the refrigerant can be lowered.

In addition, switching the heat exchangers between being arranged in parallel to being arranged in series by closing a first conduit, which connecting said inlet to a first duct of each heat exchanger, after the first heat exchanger and between every second heat exchanger, and a second conduit, which connecting said outlet to a second duct of each heat exchanger, between the other heat exchangers.

By providing said heat exchangers with a dual-circuit for heat transfer between two essentially liquid media, such as a refrigerant and brine, it is an advantage of switching each circuit between being arranged in parallel to being arranged in series.

By this the flexibility is increased and makes it possible to optimize the performance of the system both for subcritical as well as transcritical mode.

Another aspect of the invention is an assembly of heat exchangers having an inlet and an outlet at opposite ends of the assembly, a first conduit connected to said inlet and to a first duct of each heat exchanger and a second conduit connected to said outlet and a second duct of each heat exchanger, characterised in that a valve being located in the first conduit after the first heat exchanger and between every second heat exchanger and in the second conduit between the other heat exchangers, wherein the heat exchangers being arranged in parallel having all valves in open position and in series having all valves in closed position. BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.

Figure 1 a shows a schematically view of an assembly of heat exchangers according to a first parallel arranged operating condition according to the present invention.

Figure 1 b shows a temperature/position chart for the operating condition according to figure 1 a.

Figure 2a shows a schematically view of the assembly of heat exchangers according to a second serial arranged operating condition according to the present invention.

Figure 2b shows a temperature/position chart for the operating condition according to figure 2a. DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 a and 2a shows an assembly 1 of heat exchangers 2. The heat exchangers 2 each have a dual-circuit for heat transfer between two essential liquid media, such as a refrigerant and brine. However, the present invention is also applicable in heat exchangers with only one liquid media. The assembly 1 of heat exchangers 2 having an inlet A, e.g. from a compressor (not shown) in a refrigerant circuit, and an outlet B, e.g. to an expansion valve (not shown), at opposite ends of the assembly 1. The assembly 1 having a corresponding inlet C and outlet D for the brine circuit at opposite ends of the assembly 1. In addition, the assembly 1 having a first conduit 4 connected to said inlet A and to a first duct 5 of each heat exchanger 2, and a second conduit 6 connected to said outlet B and a second duct 7 of each heat exchanger 2. Further, a valve 3 being located in the first conduit 4, after the first heat exchanger 2 and between every second heat exchanger 2, and in the second conduit 6 between the other heat exchangers 2, wherein the heat exchangers 2 being arranged in parallel having all valves 3 in open position, as shown in figure 1 a, and in series having all valves 3 in closed position, as shown in figure 2a. In figure 1 a the heat exchangers 2 are arranged in parallel for the subchtical condition, i.e. at a temperature below the condensing condition of the refrigerant. The heat transfer is shown in figure 1 b, wherein the upper curve corresponds to the temperature drop from inlet A to outlet B of the refrigerant, which during condensation having a more or less constant temperature, and the lower curve corresponds to the temperature rise from inlet C to outlet D of the brine. In figure 2a the heat exchangers 2 are arranged in series with each other at a transcritical condition, i.e. at a temperature above condensing condition of the refrigerant. The heat transfer is shown in figure 2b, wherein the upper curve corresponds to the temperature drop from inlet A to outlet B of the refrigerant, and the lower curve corresponds to the temperature rise from inlet C to outlet D of the brine. The heat exchangers 2 are switched between being arranged in parallel to being arranged in series by closing valves 3 arranged alternating in a first conduit 4, connected to a first duct 5 of each heat exchanger 2, between each second heat exchanger and in a second conduit 6, connected to a second duct 7 of each heat exchanger 2, between the other heat exchangers 2.

The brine circuit (not shown) having a corresponding conduit 8 and a conduit 9, communicating with the inlet C and the outlet D, respectively, and valves 10. The brine circuit may likewise be switched between being arranged in parallel to being arranged in series. The valves 10 being located in the conduit 8, after the first heat exchanger 2 referred to the inlet C and between every second heat exchanger 2, and in the second conduit 9 between the other heat exchangers 2, wherein the heat exchangers 2 being arranged in parallel having all valves 10 in open position, as shown in figure 1 a, and in series having all valves 10 in closed position, as shown in figure 2a.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example only one circuit of the dual-circuit heat exchanger may be operated according to the present invention.