Boiler (power generation)

A boiler is a closed heated vessel or pressure piping system, which serves the purpose of water vapor ( bar absolute p> 1.013 ) or to produce hot water at temperatures above 100 ° C for heating and operating purposes of a higher than atmospheric pressure.

If the boiler to generate steam used, then one speaks of a steam generator. Depending on the use of saturated steam or hot steam is generated in a steam boiler.

• 6.1 blowdown and boiler water monitoring
• 6.2 quality rules
• 6.3 Operating rules

Dimensions

The size of steam boilers is far stretched. They range from small boilers in the household ( steam cleaners, steam irons ) to tower steam boilers in steam power plants with a height up to 155 m with a steam output of up to 3600 t / h

The dimensions for example, be in the progressive, powered by lignite block K of the power plant Niederaussem:

Design parameters at 274 bar and 580 ° C ( steam ) or 600 ° C ( after secondary steam reheater ).

Heating

The heat can be supplied by firing with gaseous, liquid or solid fuels, or the use of waste heat, the heating by solar radiation focused, nuclear gap heat or by electrical heating.

Waste heat produced in chemical reactions or the melting of glass and metal. Using the power-heat coupling, the waste heat from gas engines or gas turbines ( CCGT ) is used in a Abhitzewärmeübertrager to generate steam or hot water.

The fired plants are used as fuels coal, oil, natural gas and, increasingly, biomass and waste. For solid fuels, a distinction is pulverized combustion, grate and fluidised bed.

Designs

In the design, a distinction between the quick steam generators, water-tube boilers and water tube boilers. To produce low amounts of steam and electrically heated steam boilers are used. A special feature is still the moving locomotive boiler to mention.

Locomotive Boiler

The locomotive boiler was originally a riveted construction which has been optimized for mobile use. It must be generated in a limited space as possible, a high steam production. The locomotive boiler has a water-cooled firebox. The walls of the firebox are stabilized with studs on the outer sheath. At the outlet of the firebox of the so-called extended boiler connects through which the flue gas is passed into flue pipes. It usually consists of a plurality of mutually riveted or welded boiler shots. To increase the efficiency of the steam engine superheater coils have been used in high-powered locomotive steam boilers, which are ushered as a U- tube into the flue pipes. The operating pressures of locomotive boilers are usually at 12 to 16 bar ( Germany ) or from 14.1 to 21.8 bar ( USA). Higher operating pressures have not been successful.

New boilers, which were used after the 2nd World War, have been carried out mostly due to the advancement of welding technology as a welded structure.

Steam output: up to 22 t / h ( Germany, Boiler Series 45, so-called splinter classes are not taken into account); to about 45 t / h (USA, boilers of various series )

Shell boilers

A distinction is made between:

• Flame-tube smoke -tube boilers
• Fire-tube boiler
• Fire tube boiler
• Rolling boiler.

This boiler type has a cylindrical shell. The fire-tube boiler has been further developed from the shell side heated roller boiler of the 19th century, which has a more efficient heat transfer to the result. With the introduction of welding the flame-tube smoke -tube boiler was. The flame tube received a reversing chamber, through which the flue gas stream is directed into the downstream flue gas pipes. The design now mainly used is the flame -tube smoke- tube three-pass boiler. It has a flame tube and two flues, and thus a front and back reversing chamber. The train called a continuous heating surface between two diversions of the flow path.

The smoke tube boiler is a heat recovery steam generator. The boiler is connected in a furnace, for example, a firebox, a furnace or a gas turbine.

The water-tube boiler is used for small to medium steam capacities and vapor pressures. The pressures are limited because of the coat with diameters of up to 4 m shall be designed for the pressure. Performance limits are as follows:

Vapor pressures: up to 32 bar, at pressures up to 40 bar maximum capacity of 4 t / h is possible.

Steam output: up to 28 t / h ( A flame tube ), 55 t / h (two- liners ).

The entire welding work on the boiler body can be made ​​at the manufacturer. For smaller and medium-sized boilers and the insulation and the installation of the equipment from the manufacturer may take place, so that at the site, only the connection must be made to the components of steam boiler plant and steam to the load circuit.

Water-tube boilers

When water tube boiler is in contrast to the shell boiler, the water in the pipes. This boiler type is used at higher steaming rates and pressures. The water -tube boiler is also used in the solid combustion since the combustion chamber as opposed to the flame tube can be designed as desired by the arrangement of tube walls. In the flue gas soot blowers can be retracted to clean the heating surfaces in very dusty.

The following variants are among the most water -tube boilers:

• Natural circulation boiler
• Forced circulation boilers and
• Once-through boiler.

The heating surfaces are touched by the hot exhaust gas and take the heat. In the field of combustion (high heat flux ) and thermally or by abrasion / corrosion atmospheres refractory linings are in front of the tube walls is introduced or the surfaces are studded and lined with rammed earth. To improve the heat transfer in the evaporator coil coatings are hooked. Since the steam generated by water tube boiler is often utilized for the steam turbine, the steam needs to be overheated. Superheater to be suspended in the flue gas flue gas temperature in the medium. In the last train of the feedwater heater ( economizer ) is most often used. The residual heat can ( LuVo ) are used to heat the combustion air still in the air preheater. The exhaust gas is then supplied to the exhaust gas purification.

The static stability of this construction is by welding the tubes with a flat iron reaches ( picture), which thereby simultaneously is tight with respect to the flue gases.

Steam Generator

The Steam Generator is a water -tube boilers for smaller capacities for the production of wet or saturated steam. The heating surfaces consist only of a helically wound tube assembly. The burner is disposed on the axis of the heating spiral. Amount of fuel and the delivery rate of the pump are adjusted so that the wet steam is generated with low residual water content. In the vapor line, a water separator is often disposed in order to achieve almost saturated steam conditions. Since the Steam Generator has no storage capacity, only those steam consumers should be connected that require a uniform amount of steam. The advantage of the quick steam generator is based on the lower price compared to the flame -tube smoke- tube boiler and the short start-up time from a cold to operating condition. The power limits are vapor pressures: up to 32 bar at a steam output up to 2 t / h

Electric steam boiler

The generation of steam in electric steam boilers can be done in two ways:

• The use of heating elements which are immersed in the water space. The heating is carried out by the ohmic resistance of the heating coils.

• In the water compartment three against the sheath insulated electrodes (three-phase ) are immersed. The boiler water acts as an electrolyte, and the heating is carried out by the ohmic resistance of the water. It must be used in the case of salt-containing boiler water in order to achieve a sufficient conductivity. This arrangement is, however, rarely used.

Electric steam boilers are used when only small amounts of steam are required or only irregularly steam is needed (eg for test facilities ). Another reason for the use of electric steam boilers can represent official requirements regarding emissions.

The steam output electrically generated a boiler is usually well below 1 t / h The boiler usually consist of cylindrical shells with dished bottoms. The heating elements are inserted into a blind flange and sealed and bolted to a flange connection of the boiler.

Sterilizers in hospitals or laboratories are often supplied with steam from electric boilers, which are integrated to save space in the enclosure of the plant. The electric steam boilers can also use the steam iron or steam-heated devices for cleaning purposes or for solving wallpapers are attributed.

Nuclear power plants

In a boiling water reactor nuclear power plant with the steam generated in the reactor pressure vessel.

In nuclear power plants with pressurized water reactor, the steam heat exchangers is generated, in which by means of forced passage of the heated primary water in the reactor pressure vessel, the feed water is vaporized in the secondary circuit. In nuclear power plants of Siemens AG / Kraftwerk Union steam generators occur with standing U-tubes, in Mülheim -Kärlich ( BBC/BBR- Babcock - Brown Boveri reactor GmbH) were standing straight-tube steam generator in use.

Generating steam in a T- s-diagram

The Ts diagram shows the typical changes of state of water and steam in a steam boiler with superheater ( pressure losses are neglected ):

( Energetic considerations: see steam power plant)

Use of steam boilers

Steam boilers are especially used in applications where steam and hot steam is required. So steam boilers are used in the energy industry in the form of power plant steam generators to generate electricity. In addition to the various areas of application in industry, for example in heating systems or in the production of steam boilers are used in agriculture for the vapors ( soil disinfection ) for soil sterilization.

Safety of steam boilers

Blowdown and boiler water monitoring

In the boiler, especially in the steam generator, accumulate with time salts, since only chemically pure water escapes through evaporation or evaporation from the tank and this water is loaded with salts, must be replenished. These salts must be removed by flushing again. It may cause corrosion and fouling.

Perform the pads on the smoke pipes or the flame tubes first to a deteriorating heat transfer and associated energy losses, to high salt content results in the boiler to the " foaming ", comparable to the cooking potatoes, ie saline boiler water can with the steam are entrained and lead in the subsequent steam pipes and system components to corrosion. The pads to build on the extent inadmissible that the heat transfer from the heating surfaces is hindered in the boiler water, it comes to overheating of the heating surfaces, which can lead to burn out and thus also to the boiler explosion. The consequences are similar to a lack of water damage.

Preventive protect the boiler against such damage caused by the use of automatic Absalzregelungen. In this case, electrolytic conductivity measuring systems to monitor the electrical conductivity of the boiler water continuously. If the defined limits are exceeded, water is discharged by means of a continuous blowdown valve. In the systems of two- and four- electrode systems are distinguished, similar to the two-and four -wire circuit. Suited the two-electrode measurement for monitoring of clean boiler waters in a preferred conductivity range from 0.5 to 1000 ĩS / cm, you use the four-electrode measurement especially where can be expected depending on the boiler water constituents with deposits and deposit formation. In the two- electrode measurement resistance- coverings go directly into the measuring result, that is, it is feigned a lower conductivity. Component tested systems recognize this fact and go into the fault message. Simple systems simply display only a lower conductivity. In the four- electrode measurement method, however, polarization effects can be excluded on the measurement result and also dirt or deposit formation compensated largely by the separation of live and used for the measurement electrodes. The type-tested systems have automatic temperature compensation, that is, the increase in conductivity due to temperature increase is compensated automatically and permanently.

In the boiler sedimented particles can be removed by blowdown.

Quality regulations

Steam boilers are the risk of overheating equipment in the sense of 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 have been complied with. The manufacturer must affix the CE mark and issue an EC declaration of conformity.

Harmonised product standards for boilers are:

• EN 12952-1 to 17: Water-tube boilers
• EN 12953-1 to 14: Shell boilers
• EN 14222: Stainless steel shell boilers

In applying these standards, the manufacturer can assume that it satisfies the fundamental safety requirements of the Directive ( presumption of conformity).

Basis for the demand for reliable conductivity measurement systems can be found in the TRD regulations 604 The requirements are a prerequisite for a type test, one finds, for example, in the VdTÜV " water monitoring devices 100 ."

However, other standards and regulations are applied. However, the manufacturer must prove that he fulfills the essential safety requirements of the Pressure Equipment Directive.

Operating rules

Steam boiler or steam boiler systems have a high risk potential due to their high stored energy and their high internal pressure. They therefore belong to the systems requiring monitoring after the Industrial Safety Ordinance. These provisions are

• To examine the boiler before commissioning by an approved inspection body
• Obtain a permit to operate this facility to the competent authority
• In certain periods recurring inspections ( internal and external audits, strength testing, functional testing of safety devices ) to be performed by an authorized body.

In most cases these systems may also be operated only by qualified personnel, heating guide, boiler attendant or heater.