As is known, the function of a heat exchanger is to transfer thermal energy between two fluids. For example, in the case of the widely used domestic gas boilers, the function of the heat exchanger is to heat water circulating inside it, starting from hot fumes that result from combustion produced by a burner. Said condensation boilers are devised for said purpose in order to exploit both heat that develops following upon combustion and latent heat of condensation contained in the combustion fumes. Heat contained in the fumes can be recovered via a heat exchanger comprising a shell or casing, inside which is set a pipe for circulation of water, against which the fumes are made to flow.
The amount of condensation that is recovered mainly depends upon the temperature of delivery and return of the water in the heat exchanger, with respect to the heating system. Furthermore, to obtain a considerable heat exchange between the fluids inside and outside the pipe of the heat exchanger, it is necessary to have a surface of heat exchange as extensive as possible. For this purpose, the pipe is usually constituted by a tube bent to form a coil, positioned in the casing of the heat exchanger so as to surround the burner.
In order to have available a high surface of heat exchange the coiled pipe must, however, be rather long, and this at the expense of the requirement of rendering the dimensions of the heat exchanger as a whole compact. Said drawback can be reduced by shaping the tube that forms the coiled pipe with a section of passage having parallel plane surfaces. If on the one hand said solution enables a good heat exchange to be obtained even with a coil that is not too long, on the other hand it involves a greater complexity of construction of the heat exchanger, with a consequent increase in its cost.
Another drawback of known heat exchangers is that they must be adequately insulated thermally, for example to prevent excessive heating of the body of the boiler. The need to reduce the temperature of the casing of the heat exchanger in this way involves the use of insulators, which further complicate the construction of the heat exchanger, as well as its disposal at the end of its useful working life.