Technical Field The technical concepts of the"Steamblock"boilers have for long been characteristic for installation of the same heat transfer surface elements, including flame tube and smooth smoke tubes, which resulted in solutions that enable a comparatively low-intensity heat transfer from the flue gases to the operating medium and consequently result in less cost-effective solutions with relatively low efficiency.

Background Art The known concept of"Steamblock"boilers for power generation have been in use for quite a long period with a comparatively small improvement. From the design standpoint, these boilers are horizontal cylindrical vessels, partly filled with water that is heated with flue gases and evaporated; generated steam is either conveyed from the vessel top in saturated condition or brought to specially designed reheaters.

The water is heated on the heat transfer surfaces submerged in water, which consist of flame tube and smoke tube. As a rule, this concept is used for gas and/or liquid fuel fired boilers. The flue gases flow in two or three flue gas passes, the first pass being the flame tube with a burner assembled at its front end. Diverting of flue gases between the first and the second pass, and between the second and third pass is achieved with the so called turning chambers. The connecting channel between the first and the second flue gas passes is submerged in water in the cylindrical boiler body or outside the cylinder. For construction reasons, the working pressure of the generated steam is as a rule around 13 bar, max. 20 bar, and the generated steam is generally used for the process and heating purposes. The boiler construction and pressure of the generated steam determine the boiler efficiency factor since the complete water volume in the boiler is actually at the saturation temperature for the generated steam pressure. The boiler efficiency is generally determined by the flue gas temperature at the boiler outlet; due to the boiler construction, this temperature is commonly by 20° to 30°C above the boiler water temperature. Recently, in order to increase the boiler efficiency, the flue gases have been additionally cooled in the external feedwater heater (the so called economizer) which has resulted in the efficiency increase by 2-4%.

Disclosure of the Invention The primary objective of the invention is to increase the heat transfer intensity by its design, install larger heat transfer surfaces into the available volume, decrease the number of flue gas passes and increase the boiler efficiency. In comparison with the state-of-the-art, these requirements are realized by introduction of the second flue gas pass taking the form of a flue gas duct fitted with finned water tubes and the elements which turn the water flow through the boiler in the direction opposite to the flue gas flow, which makes the subject boiler a return-flow device.

Brief Description of the Drawings The enclosed drawings are an integral part of the invention description, which best illustrate the invention and facilitate explanation of the basic principles of the invention.

Fig. 1 assembly drawing of the longitudinal section of the water-tube boiler with flame tube The drawing shows fitted stay tubes 14, baffle plates 11 and 12, inlet 15 into the flue gas duct 7, and the tubes 9 fitted in the duct.

Fig. 2 assembly drawing of the cross-section of the water-tube boiler with flame tube and flue gas duct The drawing shows the flue gas duct 7 cross-section, fitting of the finned tubes 8 and the position of the baffle plates 11 and 12, and the interrelation among the flue gas duct 7, flame tube 5 and the connecting channel 6.

Fig. 3 an example of a possible design of the flue gas duct 7, trapezoidal in section, and fitting of finned tubes 7 and shortened tubes 16.

Fig. 4 shows fitting of the shortened tubes 16 and the inclined side 17 of trapezoidal flue gas duct 7 with the indication of tube 16 supply.

Fig. 5 layout showing assembly of tube 9 and membrane (tube-fin-tube) elements 25 at the combustion products inlet from the connecting channel 6 into the flue gas duct 7. The duct 7 and connecting channel 6 stiffening is also shown.

Detailed Description of Minimum One Invention Realization Method Below, a detailed description is given of the design concept the realization of which leads to the invention realization.

The steam boiler consists of the cylindrical shell 1, front 2 and rear 3 heads, filled with water to the specified level above which is the steam space 4. Combustion of fuel (light distillate fuel oil and/or gas) happens in the cylindrical flame tube 5 which is usually flat or corrugated,

and submerged in the boiler water space. The flame tube 5 ends in the connecting channel 6 where the combustion products change flow direction and turn by 90°C to leave the connecting channel and enter into the flue gas duct 7 of the second flue gasses pass. The boiler is of flue gas double-pass design where the first pass consists of the above flame tube 5 and the second pass of the flue gas duct 7. Interrelation between the flame tube 5 and flue gas duct 7 is as a rule such that they are placed side by side and parallel to each other, i. e. each occupies one of the elongated boiler halves. The flue gas duct 7 might be of rectangular section (Fig. 2) or of any other shape. The flue gas duct 7 is a duct open in one end for release of flue gases and closed with the bottom plate 24 in another end (Fig. 5), with the lateral opening 15 for inlet of the flue gases from the connecting channel 6 into the flue gas duct 7. An example of another duct design option is given in Fig. 3, where the flue gas duct 7 is trapezoidal in section. The flue gas duct of the second flue gas pass is fitted with vertical finned tubes 8 through which the water is supplied into the boiler from the bottom tube end, heated by the flue gases while passing through the tube, to enter the water space through the tube top end. The tube size, pitch, number and finning characteristics are the known design elements and they are determined by thermodynamic and aerodynamic calculations. When the flue gas duct is trapezoidal as in Fig. 3, the finned tubes 8 of equal length are added shorter tubes 16 the length of which is determined in accordance with the design requirements. The lower end of the shortened tubes 16 is fitted into the inclined side of the duct 7 as shown in Fig. 4. The plate 18 is perpendicular to the shortened tube 16 and it forms, together with the plate 19 welded on the wall 17 of the duct 7, tube supply chamber 18.

This chamber is closed with triangular plate 20 on both ends. The tube 16 is welded on plate 18 and supplied with the boiler feedwater by way of the opening 21 in the flue gas duct wall 17 with diameter larger than the tube 16 diameter. The tube 16 supply chamber length along the lateral plate 17 may differ and encompass a number of tubes; on principle, it may be a complete tube bank between the stay tubes 14. Consequently, the number of openings 21 is adequately increased so that their surface area as a rule exceeds the diameters of all tubes 16 supplied from the subject chamber. The flue gas temperatures in the connecting channel are 1,000-1,200°C, depending on the thermodynamic characteristics of the combustion products and configuration and size of the flame tube 5 and the connecting channel 6. Depending on the combustion products temperature in the zone of the highest temperatures in the flue gas duct 7, different concepts may be used for vertical tubes 9, such as the tubes finned with refractory material or the tubes with studs welded longer or shorter round iron elements (the so called studded tubes) or all-welded plates to form membrane (tube-fin-tube) elements (Fig. 5). The Fig. 5 shows

method of fitting of tubes 9 into the flue gas duct 7 when the membrane (tube-fin-tube) elements 25 are used. The membrane (tube-fin-tube) elements 25 consist-of two, three or more smooth tubes interconnected with a metal strip along the entire length of the tube fitted inside the flue gas duct 7. The membrane (tube-fin-tube) elements 25 on the flue gas inlet into the flue gas duct 7 are fitted perpendicularly to the duct 7 centerline. In duct 7, the flue gases are turned by 90°, and the membrane (tube-fin-tube) elements in the turning chamber are fitted at 45 ° to the flue gas duct 7 centerline; downstream the flue gases turning by 90°, the membrane (tube-fin-tube) elements are fitted in parallel with the flue gas duct 7 centerline, as shown in Fig. 5. Fitted in this way, the membrane (tube-fin-tube) elements 25 act as the baffle plates which reduce the flow resistance. In such a design, the flue gas cooling with tube bank 9 starts already after the flue gases from the connecting channel 6 are turned into the flue gas duct 7; that, along with the increased heat transfer surface of the tube 9 initiates intensive cooling of the flue gases by convective heat transfer. Within the zone with combustion product temperatures below 800°, only finned tubes are fitted with the sections which ensure cooling of the combustion products in the second pass to 130-150°C at the boiler outlet into the outlet chamber 10. The flue gases cooling to the temperatures below the boiler working medium temperature is ensured with building in of the baffle plates 11 and 12 which turns the boiler into the return-flow heat exchanger. The baffle plates 11 and 12 are fixed on the flue gas duct 7 along its entire length, i. e. from the front head 2 to the flue gas duct 7 end. The baffle plate 11 must be higher than the highest boiler water level. The baffle plates 11 and 12 are fitted on the flue gas duct above the duct 7 vertical wall, so that the entire boiler water space is separated into two parts where the flame tube 5 as the heat transfer surface is placed in one half and the flue gas duct 7 with all the fitted tubes in the other half. These two boiler water spaces are connected by the area between the connecting channel 6 and the rear head 3. The boiler is supplied with feedwater by way of the tube 13 fitted under the flue gas duct 7, by the front head 2. In this way, the feedwater is circulated and heated in the tubes 8 and 9, and in forced circulation from the feedwater tube 13 to the rear head 3, to finally flow into the boiler water space near the flame tube 5. Since the combustion product temperatures are the highest at the flame tube 5 and the connecting channel 6, it is the point of the most intensive evaporation and the loss of the water volume in this space enables the water inflow from the flue gas duct 7 water space.

According to its stress-and-strain characteristics, the flue gas duct 7 has unfavorable design shape because of the boiler pressure which might reach up to 25 bar. The horizontal sides of the flue gas duct 7 are restrained with tubes 8 and 9, vertical sides are restrained with

regularly arranged bracing tubes 14, which are fitted at the angle of 5-10° to the horizontal and cooled with the boiler water and their arrangement is determined by the stress and strain analysis.

Vertical wall of the duct 7, opposed to the inlet 15 from the connecting channel, is restrained with vertical plates 22 which are lengthwise welded to the flue gas duct 7. The flat sides of the connecting channel 6 are restrained in the same way, with welded plates 23.

The described construction enables cooling of the combustion products under the saturation temperature for the boiler working pressure without the external water heater and with only two flue gas passes, which all together results in the boiler efficiency increase by 4 to 5 %.

This will be achieved provided the deaeration temperature is 105 °C. The boiler construction does not include external turning chambers, which facilitates the boiler manufacture and reduces the material and labor. The combustion products exhaust on the front side enables the simplest design of the combustion air heater that enables further cost effective increase in the boiler efficiency, particularly in gas-fired power plants.

Application of the Invention The professionals will recognize that the subject invention may be realized by a number of reworks and modifications within the field of the technical and technological development of the construction, such as is current practice in the boilermaking, while the idea, spirit and the scope of the invention remain unchanged.