The structure and type of water wall tube

The structure of the water wall tube

The water-cooled wall tube usually adopts seamless steel pipe and inner threaded pipe with an outer diameter of 45~60mm, and the material is 20# high-quality boiler steel (20G). Small-capacity boilers widely use light-tube water-cooled walls (as shown in Figure 1), which are vertically arranged parallel to each other along the four walls of the furnace, the upper end is connected to the upper header or steam drum, and the lower end is connected to the lower header. The tightness of the arrangement is usually expressed by the ratio s/d (relative pitch) of the distance s between the centerlines of two adjacent water wall tubes (pitch between tubes) to the outer diameter d of the tubes. When the relative pitch s/d value is small, the tubes are arranged densely, the heat radiated between the tubes to the furnace wall and reflected from the furnace wall to the back of the tubes is less, and the utilization rate of metals is poor, but the protection effect on the furnace wall is better; on the contrary, when the s/d value is large, the arrangement is sparse and the metal utilization rate is high, but the protection effect on the furnace wall is poor. Considering the protection of the furnace wall, the relative pitch of most boilers adopts s/d=1.1~1.2, which is also to use light furnace wall or tube-laid furnace wall as much as possible, so as to reduce the total material cost of the boiler.

 The structure and type of water wall tube
Figure 1 – Water wall of medium pressure pulverized coal boiler

Large-capacity boilers all use membrane water-cooling walls to make the furnace air tight. Most of the membrane-type water-cooling walls are welded together with internal screw or smooth tube and fins (flat steel bars). As shown in Figure 2, there are three welding methods for the membrane water wall. Finned tubes can be rolled in steel mills or welded from steel pipes to flat steel. At present, with the improvement of welding and manufacturing technology, boiler manufacturers weld the membrane-type water-cooling wall in sections into tube rows when leaving the factory, and splicing each section of the tube row into a whole piece of water-cooling wall for hoisting at the installation site, which can save installation time and easily ensure installation quality.

The structure and type of water wall tube
Figure 2 – Three typical membrane water wall structures

Types of water wall tubes

With the difference in the capacity, form and parameters of the boiler, the form of the water wall tube is also different. Water-cooled wall tubes can be divided into three types: light-tube water-cooled wall tubes, pin-type water-cooled wall tubes, and membrane-type water-cooled wall tubes.

①Light tube type water-cooled wall tube. The light tube type water wall tube is composed of general boiler steel tubes, as shown in Figure 3. The denser the tube arrangement, the better the protection effect of the furnace wall. Lightweight refractory materials and thermal insulation materials are widely used for furnace walls. These materials can be built into furnace walls, and can also be laid on water-cooled wall pipes to form pipe-laying furnace walls for easy installation. Small-capacity, medium and low pressure boilers mostly use light-tube water-cooled wall tubes.

The structure and type of water wall tube
Figure 3 – Light tube type water wall tube

②Pin type water wall tube. A lot of round steels with a diameter of 6~12mm and a length of 20~25mm are welded on the outside of the light tube type water wall tube to form a pin type water wall tube. The refractory plastic can be laid and fastened with the pins, so that the heat absorption of the water-cooled wall tube can be reduced and the temperature in the furnace can be increased. Due to the large number of pins, the welding workload is large and the quality requirements are high, pin-type water-cooled wall pipes are generally used in special areas such as the sanitary burning belt of solid-state slagging pulverized coal furnaces, the slag pools of liquid slagging pulverized coal furnaces, and the cyclone barrels of cyclone furnaces.

③Membrane water wall tube. The membrane-type water-cooled wall tube is composed of many finned tubes welded in sequence in the longitudinal direction to form a whole heating surface, so that the inner wall of the furnace is tightly surrounded by a layer of water-cooled wall membrane.

The advantage of the membrane type water-cooled wall tube is good air tightness; the outer side of the tube panel only needs to be coated with a thin thermal insulation material, and the high temperature flue gas of the furnace does not directly contact the furnace wall, which is beneficial to prevent slagging; the tube panel can be fabricated in the factory. Sheet prefabricated, easy to install on site. Large-capacity, high-temperature and high-pressure boilers mostly use membrane water-cooled wall tubes.

There are two types of finned tubes: rolled finned tubes and welded finned tubes.