Water-tube boiler

A water tube boiler is a steam boiler design. It is characterized in that the water is led into the tubes. The water tube boiler can be used as steam or hot water boiler.

General

In the area of gas and oil firing with steam capacities up to 25 tonnes per hour and pressures up to 32 bar only shell boilers are used as the boiler manufacturer can be prepared ready and efficient for new installations. The shell boiler is not suitable for automated combustion of solid fuels, or it must be preceded by a combustion chamber.

For higher pressures, Shell boilers can no longer be used. The tubes in water -tube boilers have much smaller diameter than the casing of Greater kettles, so here suffice substantially lower tube wall thicknesses to accommodate the internal pressure. The water -tube boiler is suitable, with appropriate design of the firebox for combustion of all fuels. In the flue gas soot blowers can be retracted to clean the heating surfaces in very dusty.

Development

The first type of water tube boiler ( water tube boiler original name ) was the inclined tube boiler. The first useful boiler of this type was built by Dr. Alban from Schwerin, who was first and executed as single-chamber (1840 ) later than two-chamber boiler. The tubes of the boiler type are arranged at an angle of approximately 15 °. The tubes are rolled in water chambers on both sides of forged steel. The chambers were provided in relation to any Rohreinwalzung with a lock, in order to clean or replace the tubes. The large chambers had to be reinforced with studs. The chamber formed a component so that the term large-chamber boiler has been used for this type of inclined tube boiler. The flue gases are often redirected more than once by the introduced downstream of the combustion chamber lining, so that the evaporator tubes are as perpendicular as possible to flow out of the flue gases. The chambers are connected to a less heated pipe to the boiler drum. The water flows from the drum through a large-sized pipe connection in the deeper water chamber and is distributed over the evaporator tubes. By the heat transfer from the flue gas stream, the water is heated and partially evaporated and rises up in the evaporator tubes. Of of the higher chamber of the water -vapor stream was fed to the upper steam space of the drum.

1874 had Stephen Wilcox patented the first Sektionalkessel. It puts no continuous water chamber more was used, but the staggered evaporator tubes were connected by a wavy sub-chamber, so getting a tube with the overlying connected. These sub-chambers are connected to the inlet and outlet manifold. The advantage of this arrangement is a higher elasticity and it enabled mass production with individual adjustment of the number of tube rows and thus the steam capacity to the customer's wishes.

The former steam power was 2.4 tons of steam per hour. Essential prerequisite for the further increase in output was the development of the seamless tube by the Mannesmann brothers in 1886.

The development was the vertical tube boiler. The name derives from the almost vertical arrangement of the tubes, which open into a lower and upper boiler. Garbe built in 1904 the first vertical tube boiler with straight tubes, followed in 1906, the Stirling boiler bent tubes, which allowed perpendicular to introduce all tubes in the drum. The lower and upper boilers were riveted components, in which the tubes are rolled. The advantage of the design is the lower area requirement, the kettles over the inclined tube boilers increasing in height. Furthermore, the elaborate chambers with the high number of closures and sealing elements accounted for. Between the rows of tubes, a septum is the flue gas side recessed. The front part of the tube rows is through the hotter flue gases heated more strongly than the rows of tubes on the downstream side of the flue gas. Due to the different heat transfer, the rear rows of tubes formed the downpipes and in the more heated front pipes, the water rose to the upper boiler up and thus generated the natural circulation. However, the water circulation of the first vertical tube boiler was classified as poor of the inclined tube boiler. In the 1910s, the diameter of the evaporator tubes was 80 to 100 mm, the operating pressures were 15 bar and the heating surfaces had a magnitude of up to 350 m². The specific steam generation stood at 12 to 18 kg / ( m² h).

1918 coal dust was used for the first time in continuous operation for the firing of a water tube boiler. In the course of further technical development in 1927/28 operating pressures of 100 bar was reached. A further increase the operating pressures in natural circulation boilers is limited by the increasing thickness of the drums. Consequently once-through boiler has been developed, which do not require the drum.

The technical limitations of steam generation are currently at pressures of 300 in the supercritical region of water and overheating at temperatures of 600 ° C. The steam output of a power plant unit is in the course of development to the current 2,000 t / h increased.

Designs

Forced circulation boiler 1 SPW- pump 2 SPW preheater 3 Evaporator 4 downpipes 5 drum 6 superheater 7 to the turbine 8 Circulation pump

Once-through boiler design: Benson 1 SPW- pump 2 SPW preheater 3 Evaporator 6 superheater 7 to the turbine

Once-through boiler type: Sulzer boiler 1 SPW- pump 2 SPW preheater 3 Evaporator 6 superheater 7 to the turbine 9 Water Separator

Following types are distinguished

• Natural circulation boilers, which are fed with the feed pump with water from the condenser and in which the water circulates between the upper and lower headers in a natural way,
• Forced circulation boiler, where the water circulation is ensured by a circulation pump,
• Once-through boiler in which the water is pumped in one direction through the pipe system.

The metropolitan area of the evaporator is largely self -supporting ( the tube walls consist of tubes welded together ). The rest of the supporting structure of a water tube boiler consists essentially of steel sections with concrete foundations. Alternatively, concrete can be used for the structure. The exterior walls of the structure are thermally insulated and covered with sheets for the weather and noise protection.

Typical pipe dimensions are:

• Preheater: 38 × 3.5 mm
• Evaporator: 60 × 5 mm
• Superheater 32 x 5 mm

Security

Quality requirements

Water-tube boilers are pressure vessels according to the Pressure Equipment Directive 97/23/EC and may only be placed on the market if the manufacturer has demonstrated by a conformity assessment procedures involving a Notified Body that the essential safety requirements of the Directive has been complied with. The manufacturer must affix the CE mark and issue an EC declaration of conformity. Harmonised product standards for water -tube boilers are:

• EN 12952-1 to 17: Watertube boilers

In applying this standard, the manufacturer may assume that it satisfies the fundamental safety requirements of the Directive ( presumption of conformity).