CIRCULATION SYSTEMS
CIRCULATION SYSTEMS
In power plant boiler is used as a steam generator. From boiler high pressure & high temperature steam is generated and used in steam turbine.
In power plant boiler may be drum type or without drum.
Drum type boilers are called as sub-critical boiler because its working pressure is bellow critical pressure while without drum type boiler is called once through boiler because its pressure is higher than critical pressure.
In sub-critical units boiler drum acts as reservoir for water & saturated steam and also provides means and arrangements for separation and purification of
steam.
The term circulation generally refers that "the movement of fluid from boiler drum to the combustion zone (furnace water walls) and back to the drum". In boiler drum steam separator equipment are installed.
Feed water is supplied to the drum via the economizer in heated condition.
when feed water enters in to the boiler drum then it mixed with the drum water which is higher temperature than supplied feed water.
So it is important to supply the feed water continue to the boiler drum because continue from boiler steam is drawn.
Types of circulation
- Natural circulation ; as described earlier.
- Controlled circulation ; This system is akin to "ASSISTED CIRCULATION" system as described above and is generally adopted beyond 180 kg/cm2.
- Combined Circulation ; The system is similar to "Forced circulation" system described earlier.
But in some boilers arrangement is given for re-circulation of water through the furnace tubes at low loads. This protects tubes and used for start up procedures.
A typical operating pressure
for such a system is 260 Kg/cm2.
NATURAL CIRCULATION
A steam generation may be basically classified according to the method employed to establish flow through its evaporator, since for all types flows through the economizer up to the drum are established by feed pump. Natural circulation is the movement of the circulating fluid in conformance with the available differential head and in a boiler, this is due to the difference in densities of the contents of down comers and upriser. The circulation in this case is said to be taking place on Thermo-syphon' principles. As the pressure rises the difference is densities between water & steam (Fig.1.1) and consequently the driving force reduces. Thus the head available will not be able to overcome the frictional resistance required for the flow. Therefore, the natural circulation is limited to boilers with drum operating around 175 Kg/cm2.
In any given natural circulation system, the movement of the steam and water will increase with increased heat input to a maximum value or so called end point, after which further increase in heat absorption will result in a decrease in flow. The general form of the curve is shown in Fig. 1.6. In the process of circulation, following two opposing forces are present:-
(I) The increase in flow results from the increase in the difference of the densities of the respective fluids in down comers and risers caused by the increase in heat absorption.
(II)However, at the same time, the friction and impact losses in the system increase, mainly due to increase in specific volume in the riser circuits. Therefore, when the losses increase as compared to gain due to pressure differential the flow rate will begin to drop.
1.4.2 In natural circulation boilers the objective is to design the circuits in the region of rising part of the curve. In this region, the boiler tends to be self-compensating for the usual heat absorption variations such as:
(i) Sudden over loads
(ii) Soot and other deposits on heating surface.
(iii)Non uniform fuel bed or burner conditions.
(iv)Inability to forecast actual conditions over the operating life time.
Typical flow curves for steam output with increasing circulating flow are shown in Fig.1.7 for a 186 bar natural circulation boiler working in the rising region of operative curves.
Self compensation may take place due to the reason given below:
The heat absorption rates may be somewhat higher than the predicted once because of miscalculation of friction loss, localized hot spots, or other unforeseen circumstances. These reduce the average density of the circuit and thus the flow to the circuit may be more than that calculated such as assisted or controlled self compensation is not available.
One of the characteristics of natural circulation is its tendency to provide the highest flow in the tubes with the greatest heat absorption.
Heat transfer rates between the inside surface of steam generating tube and the boiling water in it are extremely high, and the tube inner wall is normally only a few degrees above the saturation temperature corresponding to the operating pressure.
Tube overheating and failures are almost invariably due to internal deposits that insulate the tube from the cooling effects of the water flowing in the tube. Those failures are mostly in high heat flux zones.
CIRCULATION RATIO
It is essential to maintain a certain amount of flow of water to the steam generating circuits in commensurate with the amount of steam generated from them, in order to prevent `Burnouts' and `On-load Corrosion'. The ratio by weight of the water fed to the steam-generating circuits to the steam actually generated (Kg water : Kg steam ) is called `Circulation Ratio'. Taking circuit shown in Fig. 1.8 as an example for a unit period of time 5 kg water is admitted to risers (which will get converted into mixture of water and steam during its passage through the furnace) and 1 kg of steam is taken out of the drum, the value of circulation ratio will be five. The remaining 4 Kg of water will be recirculated in the system. To compensate for 1 Kg. Of steam taken out, 1 Kg. Of water will have to be added to the drum, which will enter the risers alongwith the water under recirculation.
1.5.2 Circulation ratio to be adopted for a particular design will be influenced by the operating pressure and the available head. The values of these parameters will decide the circulation head
available to produce the driving force available. The curves in graph of Fig. 1.8 show:
(i) Typical design values of circulation ratio as a function of operating pressure.
(ii) With given circulation ratio and uniform heat absorption over height `h' the maximum available
circulation head for overcoming flow resistance
1.5.3 The circulation ratio for various types of boilers are:-
(i) Utility Boilers…. 6 to 9
(ii) Industrial Boiler….8 to 30
Higher circulation provides higher thermal inertia and faster response essential for industrial
boilers.
FORCED CIRCULATION
1.7.1 This is also termed as `Positive Circulation'. In this system circulation pumps are employed to force movement of water through different circuits.
1.7.2 Under certain conditions, forced circulation can be usefully employed for steam generation, particularly when the pressure are very high. This is due to the reason that circulation head caused by density differential is too low to cause effective circulation. Generally positive circulation is resorted above 2650 psi ( 182.7 bars), but certain boilers at lower pressure 1800 PSI (124.1 bares) are also designed on this system to take advantage of the small thin tubes and higher velocities made possible by pumping. The boiler at lower pressures will have the conventional drum. The circulation in such a case may be classified as `Assisted Circulation' or as per the criteria adopted by C.E/BHEL it may be
termed controlled Circulation. The forced circulation also makes it possible to have most optimum utilization of available space.
In the forced circulation system, we have following types of boilers:
a) Once-Through Type…. Water from the feed supply is pumped to inlet ends of the heat absorbing circuits. Evaporation or change of stage gradually takes along the length of the circuit and when the evaporation is complete, further progress through the heated circuits results in superheating the vapor. No steam and water drum is required in this system and is generally applicable above supercritical pressures. Fig. No.1.4 shows the scheme of forced circulation boilers at supercritical pressures.
b) A modification of once through type boiler is that, evaporation is up to partial dryness (9o%) and the water is removed in a separator and the dry steam passed further through the circuit for superheating. The system is used at sub-critical pressures in the higher pressure zone as shown in Fig. No. 1.5
c) Even near on beyond the critical pressures it has been found advantageous to recirculate the water through the furnace tube at low loads. This protects the furnace tubes and simplifies the start-up procedure. A typical operating pressure for such a system is 260 Kg/cm2. This system is called Combined Circulation System' and has been adopted in C.E. Boiler designs.
ASSISTED CIRCULATION, `RECIRCULATING' FORCED CIRCULATION.
In this type provision of steam-water drum is made as in the case of natural circulation boilers and circulating pumps for circulation in various circuits like the forced circulation boilers. This corresponds to `Controlled Circulation System' as per the CE/BHEL classification. In this system of circulation, there is net thermal loss for the boiler unit because of the separate circulating pump. See Fig. 1.10
The flow to individual tubes is controlled by orifice plates to compensate for different positions along the feed headers and different heat absorption.
Though it is possible to adopt natural circulation for boilers working up to the range of 170 to 180 Kg/cm2 but some designers feel it necessary to have assisted circulation even in this or lower ranges. The advantages are:
(i) It is possible to have tubes of smaller diameter for driving up the steam water mixture by making the pumps to do a little more work.
(ii) The provisions of orifices helps to have a more uniform temperature giving another slight saving in tube wall thickness over and above that already obtained by using smaller tubes.
(iii)There is overall reduction in furnace size.
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