PARTICULARLY FOR SAFETY PURPOSES DESCRIPTION

The system, described in this document, is mainly aimed at the removal of residual decay heat (typically in a nuclear plant) or of heat produced by a runaway reaction (typically in a chemical plant). The refrigeration system is equipped with a tank or pool (A), generally in communication with the atmosphere, in which a "hybrid" (air and water mixed-exchange) heat exchanger (B) is contained, comprised of vertical pipes equipped with special fins and assembled together. The pool, which is an integral part of the invention, is separated into two distinct volumes, thanks to the presence of an innovative baffle (C) regulating air and water inflow through the heat exchanger. The heat exchanger is contained in one (D) of the two volumes into which the pool is subdivided, whereas in the other one (E) no component is present. Heat to be removed is provided by a fluid (e.g. , water), which outflows inside the vertical pipes and transfers heat to the fluid(s) present in the pool.

Refrigeration systems, adopted for these or analogous purposes for safety duties, are currently equipped with active components (requiring power supply and/or an external signal) or are sized to respond to the first instants of the transient, when power to be removed is at its peak, therefore proving oversized for most of the transient duration. In other solutions it is envisaged that the same systems may be equipped with a pool, in which a replenishment system is provided, for a purpose of maintaining the amount of water, or alternatively a water amount is provided such as to suffice for a preset period of time (e.g. , 72 hours from transient start). The system proposed herein allows to obtain limited overall size, and concomitantly to be capable of functioning for an unlimited time, with no need to provide active components.

In order to increase heat transfer capacity, during the first transient phase the pool can be filled with a fluid (e.g. , water). In the initial condition, when the system is ready for starting, water level inside the pool (F) may be higher than the maximum height of the pipes of the heat exchanger (B) and the separating baffle (C). This solution is chosen, e.g. , to have a natural circulation of water between the two volumes of the pool by exploiting convective motions for heating water present therein. Thus, use of all of the pool water volume is optimized in terms of heat capacity, an important feature, above all in cases in which the pattern of the power to be removed be decreasing over time; in fact, there are obtained a delay in vaporization, therefore in water level reduction, and a lesser power to be removed when the contribution is entrusted to the sole air (fluid to which refrigeration is entrusted in a subsequent phase of the transient).

The two zones of the pool, thanks to the peculiar separating baffle, base their functioning on the principle of communicating vessels. By suitably selecting the height of the separating baffle, the initial water level, the shape, arrangement, width and height of the holes made in the baffle, the cooling system can have a different behavior. The two volumes of the pool are communicating, both below and above the separating baffle. Referring to a situation characterized by a water level above the baffle, transit of water from one volume to the other one occurs thanks to the presence of communication openings between the two volumes. Displacement of warmer water, from the heat exchanger volume (D) to the other volume (E) of the pool, occurs above the separating baffle, a zone in which no obstacle is present. Instead, reverse displacement occurs below the separating baffle (I), besides through the openings on the separating baffle (G) and on the fins (N) of the pipes, enabling a continuous inletting of water inside the heat exchanger volume.

Baffle design is one of the main novelties introduced with this invention: this baffle is nothing but a plate characterized by special openings (G) (e.g. , holes), present exclusively into the bottommost part of the heat exchanger (below a preset height (H)). In the initial phases it is envisaged that the water level be above such openings, so that only water transit is had between the two zones of the pool. In the above-mentioned initial condition it is the sole water which ensures system refrigeration. Water boiling, characterizing the transient phase which onsets as soon as water saturation temperature is reached, causes a continuous reduction of the pool level and, therefore, of the surface useful for heat exchange. When the water level drops sufficiently, exposing the first openings, air present on the liquid surface of the water in the "free" volume enters the heat exchanger volume, between the pipes, contributing to heat removal. In the subsequent stages, when water will be totally vaporized, air will be the sole cooling means.

The heat exchanger, used in this system, is comprised of ducts (e.g. , pipes), connected to each other by fins, so as to create closed channels. In the pipes the fluid to be cooled will flow, in a descending motion, whereas in the channels the pool coolant will flow. The fins are perforated, analogously to the separating baffle. The heat exchanger is capable of releasing heat under different operating conditions (it may release heat, without distinction, to water and/or air) and therefore is defined "hybrid". In the first phases of the transient, in which generally it is necessary to release a greater amount of heat, the pool may be filled with water; on the contrary, during the last phases of the transient the contribution of air as a cooling means becomes ever more relevant, until being the only one. On the basis of the different needs which the system may have to meet, the pool can be empty also under initial conditions (though losing the advantage of high performances characterizing heat exchange with water). The system is ready to function, under any condition, with no need of any outside intervention. Thanks to the specific design, this cooling system proves particularly versatile and can be connected, according to different applicative needs, to various systems. Thanks to the presence or absence of water inside the pool, power values sensibly different from each other can be removed.

The mode by which the cooling circuit is isolated from the system during normal functioning lies outside the scope of the present invention. In this context, it is important to highlight that an amount of "hot fluid" (e.g., water) outflows top to bottom inside pipes of the heat exchanger thanks to the density gradient generated by its cooling, or to other reasons. The initial condition of the reference transient, deemed such as it envelops the totality of possible transients, envisages the pool to be completely filled with water. In such a condition, external air lying above the pool water level (anyhow within the free volume (E)) is prevented from circulating through the heat exchanger.

Heat exchanger functioning is characterized by two different transient phases. The first phase, lasting until water is present in the pool, is in turn characterized by three subphases, whereas the second phase refers to the period of time in which air is the sole useful means for heat dissipation. In the first subphase, pool water is in subcooled liquid conditions and heated up until reaching saturation conditions. In the second subphase, having reached saturation conditions, water boils and its level begins to decrease; air is still prevented from accessing the volume in which the heat exchanger is present. In the third subphase, a condition of "mixed" heat exchange (concomitance of boiling water, steam and air) is had. From the instant in which also air begins to flow about the finned pipes, the "hot fluid" temperature tends to decrease, irrespective of the (decreasing or constant) power pattern to be disposed of. The maximum admissible value for the "hot fluid" temperature reachable throughout the transient (in turn determined by the maximum admissible temperature in the primary system that has to be refrigerated), is a decisive parameter for all system design and, in particular, for defining the height starting from which the openings begin to be present. The functioning conditions of the entire cooling system (such as the hot fluid temperatures) are guided from contour conditions, in this case from pool water/air conditions. The special design of the separating baffle allows air to flow between the heat exchanger pipes, before complete water depletion.

Pool water and "hot fluid" temperatures pattern in the first phases of the transient is generally increasing, until inside the pool saturation conditions are reached. From this instant, until complete vaporization of the pool water, water temperature will be constant, whereas hot fluid temperature will gradually increase because of the reduction in heat removal capacity, due first to a reduction in the heat exchange surface and then to the replacing of boiling water with air (assuming to have as input a constant value of thermal power supplied to the hot fluid). The importance of the openings is evident when comparing the system with a system consisting in a pool having a single volume: should pool water level be too low with respect to the thermal power to be removed, excessive temperatures would be reached inside the pipes, because air could not contribute to heat removal. Thanks to the peculiar heat exchanger, and in particular to the perforated separating baffle (C), this problem is overcome by allowing to limit the maximum temperature of the liquid to be refrigerated (cooled). The separating baffle, one of the key features of this system, has openings (G) enabling air to flow inside all channels before water inside the pool be completely evaporated.

Besides the separating baffle, also the finned pipes of the same exchanger are suitably designed. Each pipe (L), making reference - by way of example - to a rectangular matrix, is equipped with four fins ( ), arranged every 90°, which form, upon assembling the heat exchanger, outflow channels for cooling water and air. It will be appreciated that the number of fins obtained on each pipe could also be greater than four. Starting from a given height, and bottomwise, each fin is equipped with suitable openings (N) for air transit from a subchannel to the other one. In crossing the exchanger, air increases in temperature and its density reduces by following an upward path. The vertical geometry of the openings allows to obtain a continuous increase, with the reducing of the level, in the crossing flow between a channel and the adjacent ones, and moreover affords lesser load losses (and therefore obstacles) to the mixing flow between adjacent subchannels. Of course, according to the specific use of the cooling system, the openings provided on the baffle and fins may be characterized by different geometries (shape, dimensions, distribution, maximum and minimum height).

Pipe arrangement is organized according to a specific matrix, characterized by a certain number of rows (O) and columns (P). The matrix to which reference is made herein is rectangular, as this allows to obtain a mechanical simplification in the assembling stage. For the arrangement of the pipe bundle a series of different geometries can be provided, with analogous functioning (operation) of the system. Load losses, thermal power to be removed and any geometric restriction allow to define, for each specific application, the best configuration: the pipes can be arranged according to a square (rectangular) matrix, a triangular matrix, etc. Given the relevance of the air flow for mid- and long-term heat removal, it is important that all channels of the heat exchanger (B) be lapped by the air flow. In order to facilitate the initial air flow (Q), through the holes of the separating baffle and of the fins, heat exchanger geometry should be characterized by a row/column ratio of much less than one (in the supposition of a rectangular matrix). This contrivance is useful to reduce load losses related to bypass air flow, of course, in order to meet specific restrictions related to the zone of system installation, several solutions may be adopted. The air flow, capable of removing heat inside the heat exchanger, flows through the holes following different paths (R) or (S) according to water level. The above-mentioned paths are followed by the air until complete pool evaporation is reached. In this new condition the air flow can follow path (T), characterized by lesser load losses, which involves the entire surface useful for heat exchange. Once pool water is completely evaporated, air will mainly follow path (T) rather than the bypass paths (R) and (S).

A relevant feature of this invention is the ability to benefit from the high heat exchange coefficients which characterize heat transfer with boiling water; furthermore, through use of air, even if it is possible to remove only a lesser amount of heat, there are no time restrictions. The design of the heat exchanger, sized for the most critical condition, depends on the time pattern of the thermal power to be removed; for nuclear applications, the most critical condition could occur at the instant when there is only air to ensure heat removal. Pool size will be selected according to use and required heat capacity; number and size of pipes and fins will depend on the specific application.

Any modification to pool geometry may be effected in order to increase water volume, and therefore delay pool water depletion. Preferably, such a modification has to be implemented through a widening in the horizontal direction, rather than in the vertical one, of the volume (E) in which the heat exchanger is not contained; in fact, evaporated water being equal, level decrease (and therefore decrease of heat exchange surface) will be lesser.

The geometry of "hot fluid" inlet and outlet collectors, which lies outside the invention proposed with this cooling system, should be such as to minimize both air- side and "hot fluid"-side load losses, as both circuits are based on natural circulation.

The instant invention relates to an innovative refrigeration system aimed at the removal of heat produced by a process, being based solely and exclusively on inescapable natural laws (water vaporization due to heat transfer, liquid circulation due to density gradient). A typical application of this invention relates to safety systems, i.e. systems aimed at the removal of decay heat, in the nuclear field, or at the removal of heat produced in the wake of runaway reactions, in the chemical industry. The means ensuring heat removal and its release to the outside environment, during the first transient phase, is, e.g., water, in liquid phase or boiling, in the subsequent phases of the transient, it is atmospheric air. The cooling system is comprised of a heat exchanger with vertical pipes (pipe number and size depend on the power that has to be removed and on operating (functioning) conditions (temperature of the fluid to be cooled; air temperature, etc.)). The finned pipes of the heat exchanger are connected in parallel, and the fluid to be cooled outflows inside the pipes, bottomwise. The specifically designed fins connect adjacent pipes to each other in order to create channels for venting generated steam, as well as for air outflow, so as to increase heat removal capacity from the external side. The pipe-fins element can be immersed in water in the initial instant when heat removal from the "hot fluid" flowing inside the pipes is started. Under these operating conditions, pool water is heated until reaching saturation temperature; then, by vaporizing, it reduces its volume. When pool water level drops to a sufficient extent, until exposing the openings, external air can enter the volume surrounding the pipes, flowing upwards, into contact with the external surface of the pipes; in this condition, cooling of the fluid internal to the pipes is jointly ensured by air and the remaining boiling water. When water level leaves the pipes completely exposed, pipe cooling is provided by air alone.

This solution:

• In the initial phases, when generally the power that has to be removed is greater, makes use of high heat exchange coefficients and of the high difference in temperature between fluid to be cooled and pool water, characterizing the water heating and boiling phases;

• In the subsequent phases, with a generally lower power to be removed, the unlimited capacity of heat removal, typical of air, is exploited to reach the final purpose of obtaining a passive cooling, without any time limit.

In light of the preferred embodiment of the invention given herein by way of a non- limiting example, the following components have been described:

• Pool (A), subdivided into two different volumes, of which one (D) be provided with a heat exchanger and the other one (E) contain no mechanical component, characterized in that the volume comprising the heat exchanger and the one without mechanical components be kept in communication according to the principle of communicating vessels.

• Pool (A), wherein the volumes into which it is subdivided be more than two in number and the heat exchangers one or more in number, compatibly with the overall number of volumes into which the pool has been subdivided. In any solution there should anyhow be present at least one volume provided with a heat exchanger and at least one volume in which no mechanical component be present.

• Pool (A), wherein the volume comprising the heat exchanger is equipped with one or more heat exchangers.

• Separating baffle (C), ensuring a pool subdivision into two or more volumes, characterized by the presence of through openings (G), in the bottom zone thereof, and beginning from a predetermined height, to ensure communication between one or more volumes provided with a heat exchanger and one or more volumes in which no mechanical component be present.

• Separating baffle (C), in which the openings (G) be oriented along a vertical direction, a horizontal direction, or according to different angles, and/or be present also in the topmost zones of the separating baffle.

• Separating baffle (C), in which the typology and/or the arrangement of the openings (G) be different depending on the position or height.

• Heat exchanger (B), having vertical pipes (or ducts of different geometries), wherein the hot fluid flows on the internal side of the pipes, whereas the coolant be provided at the outside thereof, characterized by pipes (L) equipped with four fins (M) assembled according to a square matrix, to form parallel channels, and whose fins be equipped in the bottom zone with vertical openings, to set adjacent channels in communication.

• Heat exchanger (B), wherein the number of the fins (M) of each pipe of the heat exchanger be in a number different from four.

• Heat exchanger (B), wherein the pipes (L) of the heat exchanger be arranged according to a non-square matrix.

• Heat exchanger (B), wherein the fins (M) thereof be equipped with openings (N) oriented along a horizontal direction or according to different angles, and/or be present also in the topmost zones of the fins themselves.

The present invention has been hereto described with reference to a preferred embodiment thereof. It is understood that other embodiments might exist, all falling within the concept of the same invention, and all comprised within the protective scope of the claims hereinafter.