Natural circulation boiler

A natural circulation boiler is a steam generator of the type water -tube boiler, in which the water - steam cycle is achieved in the heated evaporator part due to the density difference between the water in the downspouts and the water - vapor phase.

Description of current principle

The natural circulation boiler has an upper drum, in which there is a water-steam mixture. Which are going through drum saturated steam is separated from the water and goes to the superheaters. On the bottom of the drum downspouts are used, which direct the water to either a lower drum or in today's constructions to the lower lying collectors. The downpipes are unheated. According to the static column in the downpipes, calculated from the water level in the upper drum, the state of the water is slightly below the boiling point, so that the water in the downcomers no water vapor bubbles are included. Of the underlying distribution devices, the water is routed to the evaporator. This may be or evaporator Scots the tube walls of the boiler. By radiation and convection of the flue gases, the water is heated to saturated steam temperature. The density of hotter water-steam mixture in the heated tubes is significantly less than the density in the case of pipes, so that initiates a flow directed towards the upper drum in the heated part.

The upper drum, usually referred to only as a drum, has the following functions:

• Balancing short-term fluctuations in the steam generation,
• Achieving the longest possible sinking when dining facilities fail,
• Separation of the vapor and deposition of liquid droplets of spray sheets and metal wire deposits,
• Installation of the boiler safety equipment on the water side ( drum safety valve, water level, pressure limiter, water level glass).

If the superheater steam is required, the saturated steam from the drum is fed to the superheaters.

History

The natural circulation boiler as inclined tube evaporator or as an array with upper and lower drum is the oldest design of a water tube boiler, which has been developed at the end of the 19th century. Until the 1960s, the boiler walls were bricked. A major leap forward was the introduction of membrane walls with fin tubes. The heating surface and boiler efficiency could be significantly improved. This boiler type is also called vertical tube boiler. If the boiler only consists of a train, receives the evaporator, superheater and economizer, then one speaks of the tower boiler. This design has the advantage that the thermal stress can be better mastered for an elongate boiler body.

Construction

Natural circulation steam generators consist of four main components:

• Upper drum (3) - it serves as a water reservoir and finds in her the steam -water separation ( " phase separation " ) instead. It is located above the combustion chamber ( b ), and if the go down pipes (4) and risers ( 6). The drum is from the feed pump via the feedwater line (1 ) is filled with feed water. Most of the water is still through the economiser ( feedwater ) (2) is directed to preheat the water and make better use of the heat content of the flue gas.
• The water flows from the downcomers (4) to the distribution header ( 5).
• The distributor manifold ( 5) has the task of ensuring a uniform distribution of the water fed to the riser pipes (6). It is sometimes referred to as " lower drum ".
• As riser tubes, the tube walls are referred to (6 ), which limit the combustion chamber ( b ) or are incorporated as Scots in the combustion chamber. By firing in the combustion chamber, these pipes and let the water inside the tubes to be heated. The density of the water within the tubes is lower due to the higher temperature and the vapor content at the same pressure as the density of the water in the downcomers. Inside the riser tubes there is a mixture of water and steam, as by heating the part of the water is evaporated. The riser tubes open at the upper end of the drum, so that the cycle is closed.

Pros and Cons

The advantage of the natural circulation principle is that as opposed to forced circulation and forced circulation boilers there is no pump required. For a circulating pump that is at least one spare unit must be available to dissipate the residual heat in a pump failure can.

If the volume of the upper drum is sufficiently dimensioned to a natural circulation boiler to provide enough water for the removal of residual heat at a lockout available, then the boiler is self-confident; that is, there are no emergency feedwater or - circulation devices required. The relatively simple structure and easy operation of the system are reasons why the natural circulation principle continues to be used preferably at medium system.

In the design of the heating surfaces must be taken to ensure that all connected evaporator tubes are adequately applied in all operating conditions with water. Therefore, a hydraulic balancing may be required by the installation of chokes. It must be used in general, larger pipe dimensions, as in boilers, which are operated in forced circulation. Therefore, certain limitations in the design of natural circulation boiler must be observed. Thus, the outgoing of the lower lying collectors heated pipes must be laid always rising to produce no vapor bubbles, which set the natural circulation overridden.

Boiler tubes must be dimensioned such that, under full load is evaporated in a current not more than 30 % of the water. Otherwise, the steam content in the tubes is so high that no continuous film of water is maintained on the pipe walls. There is then the danger of overheating, there is substantially degraded by the steam inside the heat transfer film; This is called DNB.

Natural circulation boiler must be equipped with an overhead drum. Since the natural circulation boiler can operate due to its operating principle, just below the critical point and also a sufficient density differences between water and steam is necessary, up to konstruktiert only for operating pressures of about 200 bar natural circulation boiler. Furthermore, the density difference between water and steam is not sufficient in order to achieve on the one hand a sufficient number of rotations in the evaporator and on the other hand, a sufficient separation of the water from the steam. Completed projects with a superheater outlet pressure of 190 bar are for many years in operation. For large steam power plants ( power> 150 t / h) once-through and forced circulation boiler is preferred to use

• Because at higher pressures and higher efficiencies are achieved
• The boiler can be controlled better and
• Fewer restrictions in the design are taken into account.