The present invention relates to refrigeration apparatus.

As is known, for many years refrigeration apparatus has made use of halogenated hydrocarbons such as chloro-fluorocarbons, as refrigerant fluids. The use of such refrigerants has become successively prohibited or limited in that the release of such substances into the environment is held to be one of the causes of the destruction of the ozone layer in the atmosphere.

Therefore other refrigerant fluids have been proposed as alternatives, such as hydro-fluorocarbons or hydrocarbons such as isobutane or propane. However, even these substances have some disadvantages. In fact, whilst the first can have significant effects as far as global warming is concerned, the second are substances which are difficult to handle in that they are potentially explosive.

It has therefore also been proposed to use carbon dioxide in that it is a relatively inert substance and does not damage the ozone layer in the atmosphere, and it has a smaller impact on global warming than hydro-fluorocarbons.

Carbon dioxide is normally utilised as a refrigerant in applications intended for air conditioning and in medium temperature refrigeration applications, that is to say between about 00C and about -150C.

The use of carbon dioxide in low temperature applications, that is to say less than about -200C, is limited by the thermodynamic properties of this substance, which make it little suited to refrigerating cycles.

In fact, the traditional compression refrigerating cycles comprise a compression phase at constant entropy, a heat expulsion phase at constant pressure, an expansion/evaporation phase at constant enthalpy, and a heat absorption phase at constant pressure. The heat absorption phase takes place by means of evaporation of the fluid in which, in order to completely utilise the latent heat of evaporation of the refrigerant in an evaporator the evaporation process continues until the vapour becomes dry saturated. In practice, it works in such a way that the vapour is superheated when it leaves the evaporator in order to avoid droplets of refrigerant reaching the compressor. By utilising carbon dioxide as the refrigerant and a single compression stage, if the vapour is compressed starting from temperatures less than about -200C, the temperatures reached at the end of the compression are high, in the region of about 1800C. This involves problems in that these temperatures can cause structural alterations in the lubricant utilised in the compressor.

In the second place, carbon dioxide has a critical point temperature equal to about 31°C, which is distinctly less than the temperatures of refrigerant mixtures normally utilised, and in any event rather low in that it is very close to or even less than the environmental temperature present in the majority of the terrestrial regions. This means that in general a refrigerant cycle with CO2 is trans- critical that is to say the heat expulsion phase takes place at greater temperatures than the critical temperature. The heat expulsion phase thus takes place as a simple cooling of the carbon dioxide gas without the transformation from vapour to liquid taking place. This therefore does not allow utilisation of the latent heat of condensation and the heat expulsion phase takes place at variable temperatures involving thermodynamic losses in the system.

For these reason, refrigerant cycles with carbon dioxide and single stage compression are utilised exclusively for the said air conditioning applications and medium temperature refrigeration, that is between 00C and about -15°C.

In order to avoid the said problems carbon dioxide refrigerant cycles having two compression stages with inter- refrigeration have been developed, which utilise two-stage compressors such as that described in European patent application EP 1 209 361-A. These compressors are, however, somewhat complex and expensive, and are generally less reliable than single stage compressors.

The object of the present invention is that of providing a carbon dioxide compression refrigerant plant for low temperature applications, which will be efficient and of simple construction and relatively low cost.

This object is achieved according to the invention by a refrigeration apparatus having the characteristics defined in Claim 1.

Preferred embodiments of the invention are defined in the dependant claims. A further object of the invention is a low temperature refrigeration process which utilises a refrigeration apparatus according to the invention.

A preferred, but non-limitative, embodiment of the invention will now be described making reference to the attached Figure 1 which illustrates a schematic circuit representation of refrigeration apparatus according to the invention.

According to the invention refrigeration apparatus comprises a first closed circulation circuit 10 for a refrigerant fluid, comprising a single stage compressor 20, a cooler device 30 for extracting heat from the fluid, a lamination device 40 for expansion of the refrigerant fluid, and a heat exchanger 50 for accumulation of heat in the fluid, which are connected to one another in such a way as to form a closed circuit.

By means of the cooler device 30 the fluid at high pressure leaving the compressor 20 discharges heat to the outside. Depending on the type of refrigerant fluid utilised in the first circuit 10 this cooler device 30 can be a condenser or a gas cooler.

The lamination device 40 allows expansion and/or evaporation of the refrigerant fluid after this has yielded heat through the cooler device 30. This lamination device is of conventional type, for example a capillary tube or a lamination valve able to control the flow rate passing through it to maintain a counter pressure, that is to say the pressure on the high-pressure side of the circuit at a predetermined level. This control can take place by means of measurement of temperature and/or pressure on either side of the lamination valve.

The heat exchanger 50, preferably of the counter current type, comprises a piping for the flow of the refrigerant fluid at low pressure coming from the lamination device 40, forming part of the first circuit 10, and a piping for the flow of carbon dioxide, forming part of a second closed circulation circuit 100. The carbon dioxide is utilised as refrigerant fluid circulating in this second circuit 100. The working temperatures in the second circuit 100 are such as to permit, in the heat exchanger 50, the transfer of heat from the carbon dioxide of the second circuit 100 to the refrigerant fluid of the first circuit 10. This transfer of heat carries the refrigerant fluid to a dry saturated or superheated vapour condition, intended to supply the compressor 20.

The second closed circulation circuit 100 comprises a single stage compressor 120, a cooler device 130 for extraction of heat from the carbon dioxide, the heat exchanger 50, a lamination device 140 for expansion of the carbon dioxide, and an evaporator 150 for accumulation of heat in the carbon dioxide, which are connected together in such a way as to form a closed circuit.

By means of the cooler device 130 the carbon dioxide vapour at high pressure exiting from the compressor 120 discharges heat to the outside. This cooler device 130 is formed as a gas cooler able to de-superheat the carbon dioxide vapour. The heat exchanger 50 permits a further extraction of heat from the carbon dioxide vapour causing condensation. The lamination device 140 permits expansion and/or evaporation of the carbon dioxide after this has given up heat through the cooler device 130 and the heat exchanger 50. This lamination device is of conventional type, for example a capillary tube.

By means of the evaporator 150 the carbon dioxide at low pressure coming from the lamination device 140 absorbs heat from an environment to be refrigerated. This transfer of heat takes the carbon dioxide to a condition close to dry saturated vapour, intended to supply the compressor 120.

Preferably, between the outlet of the evaporator 150 and the inlet of the compressor 120 is arranged a regenerator 160. This regenerator 160 comprises a piping for the passage of the carbon dioxide vapour from the evaporator 150 to the compressor 120, and a piping for the passage of carbon dioxide condensate from the heat exchanger 50 to the lamination device 140. The regenerator 160 permits a transfer of heat from the condensed carbon dioxide to the carbon dioxide vapour. This transfer of heat, on the one hand takes the vapour to saturated vapour conditions in such a way as to avoid or at least reduce the presence of liquid carbon dioxide within the compressor 120, and on the other hand causes a further condensation of the carbon dioxide destined for the lamination device 140.

As will be appreciated, the interchange of heat between the two circuits makes it possible to make the second circuit work with carbon dioxide in sub-critical conditions still utilising a single stage compressor. The refrigerant fluid utilised in the first circuit 10 may on the other hand be carbon dioxide and in this case this first circuit 10 will be in trans-critical conditions, or else a hydro-fluorocarbon or a hydrocarbon, in which case the first circuit will be in sub-critical conditions. In this way it is possible to reach low temperature conditions, that is less than about -200C avoiding or at least reducing the disadvantages that would occur by utilising a single carbon dioxide circuit with a single stage compressor. Moreover, the refrigeration apparatus according to the invention has a complexity and an overall cost which is less than those which would be present with a single circuit with a two-stage compressor. With an apparatus according to the invention it is moreover possible to obtain a high coefficient of performance (COP) . In fact, the Applicant has provided apparatus in which the ratio between the energy absorbed by the carbon dioxide in the evaporator and the sum of the works of the compressors is close to 2.

It is intended that the embodiments described here are to be considered as examples of the invention; the invention is however susceptible of modifications relating to shapes and arrangements of parts, constructional details and operation according to the numerous possible variants which will become apparent to the man skilled in the art.