With the increasing prices of energy, more and more attention is paid to the energy content of exhaust gases and the utilization thereof. For this reason, different types of exhaust heat boilers have been developed. The boilers are constructed either for a natural circulation of water or for a forced circulation. At present, e.g. a majority of ships use exhaust heat boilers. Known exhaust heat boilers are either of a fire tube or water tube construction. Boilers of a water tube construction are often preferred because of their better heating surface/weight ratio. The exhaust heat boilers of a water tube construction in use comprise both boilers with smooth tubes and boilers with ribbed tubes for promoting the heat transfer.

Exhaust heat boilers are generally either of rectangular or of cylindrical ^' shape. Exhaust heat boilers provided with smooth tubes are in general cylindrical because the manufacture of helical water tubes is easy. The boiler according to the invention is preferably of this type.

The control of an exhaust heat boiler has presented problems because the need of thermal energy, for example, onboard a ship varies considerably. In addition, also the quantity of exhaust gases varies according to the load of the engine. In order to achieve a sufficient reliability of operation in ship conditions, the demands made on the control properties have been reduced in the constructions presently in use. The control of exhaust heat boilers in present ships is in general arranged in any of the following manners:

The boiler is divided into blocks of which a part can be held dry when it is desired to reduce the output of the boiler. The method suffers from the disadvantage that the control is stepped and thermal expansion causes

stresses in the boiler.

- An alternative is to discharge excessive output through a heat exchanger into the sea. The method is reliable in operation but expensive because additional heat exchange capacity is required.

By-passing of the exhaust gases is used as a third control adjustment method. In other words, only a part of the exhaust gases are fed into the exhaust heat boiler. The by-pass channel of an exhaust heat boiler is generally arranged outside the boiler, and the damper plate is made in two parts whereby one of the plates controls the flow going into the boiler and the other plate the flow going into the by-pass channel. The disadvantage in such a mechanism is the fact that both plates can be closed simultaneously whereby the exhaust channel is unserviceable.

A sound damper is often positioned in the exhaust channel after the exhaust heat boiler and the by-pass channel whereby a sufficient sound damping is guaranteed also when running the exhaust heat boiler. This system suffers from the disadvantages of being expensive, requiring a large space and of being heavy in construction due to a plurality of parts. The object of the invention is to provide a new, simpler and more economical exhaust heat boiler.

Thus, the invention relates to an exhaust heat boiler comprising a convection section containing helical water tubes, and the exhaust heat boiler is mainly characterized in that the convection section is formed as an annular space, that a by-pass channel for exhaust gases is formed in the cylinder surrounded by the annular space, and that the walls of the by-pass channel are lined with a sound absorbing material so that the by-pass channel at the same time functions as a sound damper.

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As compared with known exhaust heat boilers and sound dampers, the space required by the boiler according to the invention is considerably smaller, for example, the height required by the aggregate is about one half of the space required by a boiler and sound damper arranged one after the other. As the by-pass channel and the sound damper are inside the boiler, a considerable reduction in weight is achieved. A spark breaker is used as a support structure, which also reduces the total weight. In the boiler according to the invention a simple flap valve is used as damper plate, which is reliable in operation and has good regulating properties .

In the following the invention is described in more detail with reference to a preferred embodiment illustrated in the accompanying drawing.

Figure 1 is a side view of the boiler. Figure 2 is a top view of the boiler. Figure 3 is a longitudinal section of the boiler. In Figure 1 , reference numeral 1 denotes the outer shell of a boiler. Numerals 2 and 3 denote inlet and outlet openings for exhaust gases . The distributing means of the boiler are denoted by numeral 4 and a safety valve by numeral 5. Fastening lugs of the boiler are denoted by numeral 6 and a pressure gauge by numeral 7. The drive mechanism of a by-pass damper plate 12 is denoted by numeral 3 and gates, for maintenance, on top of the boiler by numeral 9. In the lower cone of the boiler is provided a manifold 10 from which the wash water of the boiler and the water leaking from the convection section flow out.

The convection section 21 containing helical water tubes 11 is seen in Figure 3. The convection section 21 is of annular form whereby an exhaust gas by-pass channel 13 is formed in the cylinder 22 surrounded by section 21. The cylinder 22 is lined with a sound absorbing material 16, for example, glass wool, on top of which a

protecting membrane and a perforated steel plate 14 are arranged. In order to improve the sound damping, a second cylinder 23 with a sound absorbing material 16 and protective covers 15 is arranged inside the cylinder 22.

The by-pass damper plate or flap 12 is mounted in ^" a tube 20 connected to the upper end of the by-pass channel sound damper 13 whereby the shaft of the plate 12 is short and the bearings can be placed outside the boiler.

Blades 17 in the lower end of the sound damper 13 function as spark breakers. The blades 17 are preferably mounted as a support structure for the innermost cylinder, either directly or, according to Figure 3, through the protective part 15; the latter is in most practical embodiments sufficiently sturdy for the mounting purpose. Alternatively, the blades 17 can be mounted to the protective part 14 or to the cylinder 22. An exhaust gas inlet channel .18 terminates in front of the spark breaker 17 and is of a smaller diameter than the sound damper 13 so that a connection 19 is established from the inlet channel 18 to the convection section 21.

The exhaust heat boiler operates in the following manner: When the damper plate 12 is open, exhaust gases pass from the inlet channel through the spark breaker

17 to the combined sound damping/by-pass channel 13 and are discharged through the outlet opening 3. When the damper plate 12 is turned towards closed position, the by-pass flow resistance increases and a part of the exhaust gases are deflected through the opening 1 to the convection section formed by the tube spirals 11. When the damper plate 12 is in the closed position, all exhaust gases pass through the convection section. The size of the opening 19 between the sound damper and the inlet channel is determined on the basis of the resistance of the exhaust channel system and the properties

of the engine. The opening 19 is dimensioned as small as possible so that the flow to the convection section will be small when the by-pass is in open position. Even in the case that the damper plate 12 is entirely closed, a very efficient sound damping is achieved thanks to the changes of the gas flow directions taking place in the lower part of the exhaust heat boiler and the sound damper.

The upper part of the boiler is shaped so that the flow resistance at the junction of the by-pass channel and the convection section is negative, which prevents the flow of exhaust gases in the wrong direction.

Impurities accumulating from the spark breaker 17 and the crust loosened when cleaning the boiler as well as any leak water flow to the bottom cone of the boiler, from which it can be removed through the manifold 10.

It is to be understood that the device according to the invention may vary in details. For example, the spark breaker 17 can be of another type, the inner cone 23 can be omitted from the sound damper 13, or the flow to the convection section can be prevented by a damper plate which closes the flow both in the by-pass and the convection section. The invention may, of course, also be applied to a boiler of a rectangular shape; thus, the concept "annular space" is not limited to a circular ring only.

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