1. Field of the Invention
The present invention relates to a heat exchanger for indirect heat exchange between a heating fluid and a heated fluid by a heat transfer medium which works on the principle of the heat pipe, and more specifically, to a heat pipe type heat exchanger for indirect heat exchange between a heating fluid and a heated fluid, employing double tubes each consisting of an outer tube and an inner tube coaxial with the outer tube, and capable of making a heating fluid flowing outside the double tube and a heated fluid flowing through the inner tube exchange heat through a heat transfer medium sealed in the annular space between the outer tube and the inner tube.
2. Description of the Prior Art
The fast breeder reactor uses liquid sodium as a coolant. The cooling system of the fast breeder reactor consists of a primary cooling system and a secondary cooling system, through which liquid sodium, i.e., a heat transfer medium, is circulated to transfer heat from the primary cooling system to the secondary cooling system. The liquid sodium heated in the primary cooling system by the heat generated by the reactor is transferred to the liquid sodium circulating through the secondary cooling system, and then the liquid sodium of the secondary cooling system exchanges heat with water in a steam generator 1 as shown in FIG. 3. In FIG. 3, indicated at 2 is a tube of the secondary cooling system, and at 3 is a water tube in which water is heated by the liquid sodium to generate steam.
As shown in FIG. 4, the conventional steam generator 1 included in the secondary cooling system generates steam by directly exchanging heat between water flowing through water tubes 3 and liquid sodium 11 flowing outside the water tubes 3 through the walls of the water tubes 3. Accordingly, when a serious accident occurs in case the wall of the water tube 3 is broken due to some cause, it induces the reaction of the chemically active liquid sodium with water, which produces high-temperature and high-pressure.
To obviate such an accident, a heat pipe type steam generator has been proposed. This known heat pipe type steam generator exchanges heat indirectly between liquid sodium and water through a heat transfer medium which functions on the principle of the heat pipe.
FIGS. 5(a) and 5(b) illustrate an exemplary heat pipe type steam generator 1 employing heat pipes. Each heat pipe comprises an outer tube 5 and inner tubes 4 extended within the outer tube 5. The heat pipe is immersed in liquid sodium, i.e., heating fluid. Water, i.e., a heated fluid, flows through the inner tubes 4. A heat transfer medium, such as mercury, is sealed in an evaporating space 6 formed within the outer tube 5. The heat transfer medium is evaporated by the heat of the liquid sodium, and transfers heat to the inner tubes 4 as the vapor of the heat transfer medium condenses over the surfaces of the inner tubes 4 to generate steam by heating the water flowing through the inner tubes 4. The evaporating space 6 is partitioned by baffle plates 7 each having openings therethrough into individual sections to enable efficient heat exchange in every region of the evaporating space 6.
This known heat pipe type steam generator, however, has the following problems. Since the heat exchanging capacity of a single heat pipe is limited, the actual heat pipe type heat steam generator needs to be provided with thousands or tens of thousands of such heat pipes. Since the liquid sodium or water leaks into the evaporating space 6 through cracks in the junctions of the inner tube 4 and the outer tube 5 or the heat transfer medium of the heat pipe degrades due to reaction with the materials forming the inner tubes 4 and the outer tube 5, a heat transfer medium maintenance must be provided. It is necessary to inspect the inner tubes 4 and the outer tube 5 to find damages in the inner tubes 4 and the outer tubes 5 by detecting sodium vapor or steam contained in the heat transfer medium. However, since the conventional heat pipe type steam generator 1 is provided with thousands or tens of thousands of individual heat pipes, the heat transfer medium maintenance system and a tube breakage detecting system must be provided for each one of the thousands or tens of thousands of heat pipes, which increases the cost and failure rate of the heat pipe type steam generator.