CIRCULATION

Circulation is the flow of water, steam or both through the tube circuits of a boiler (from boiler drum,boiler down comer, riser water walls).

Adequate circulation is necessary for all the circuits to remove the heat  from the boiler furnace and cool the heating surfaces by carry the heat.

Due to carry the heat from the the metal surfaces the temperature of metals remain cools within a specific limit.

Sufficient circulation in provided in steam water circuit to avoid any salts deposits on the inner metallic surface of boiler.

In a circuit if the fluid enters at the inlet and leaves the outlet to the next circuit it is a once through system.  If part of the fluid from the outlet of the circuit is returned to the inlet of the same circuit it is a re circulation system.  

In a boiler the following circuits always adopt once through system.

1. Economiser
2. Super heaters
3. Re heaters

In these circuits the inlet pressure of the fluid effects the circulation.  The water walls or evaporator circuits of a boiler will have either a once through or re circulation system.  Many boilers for these circuits adopt only re circulation system.

CIRCULATION TYPES :

In the evaporating circuits of a steam-generating unit, the circulation may be produced naturally by the action of the force of gravity, mechanically by means of pumps or by a combination of the two.  Accordingly there are three types of circulation as follows:

•  Natural circulation
•  Forced circulation
•  Assisted circulation.

 NATURAL CIRCULATION :
This is adopted in re circulation systems.  Water from the economiser circuit enters a drum and joins the circulation system.  Water entering the drum flows down through the down comer pipes kept in non heating zone and enters the bottom of the water wall tubes, that is the evaporator section.  As these tubes are heated by furnace radiation, a part of the water is converted to steam and the mixture flows back to the drum.  In the drum the steam is separated.  Remaining water mixes with the incoming water from the economiser and the cycle is repeated.  

This sort of steam generation is adopted to

prevent the dissolved salts in water, precipitating and depositing scale on the surface of tubes when all the water in the tube is evaporated to steam.

Ensure the inner walls of the tubes are continuously wetted with water and to have nucleate boiling throughout the tubes.

The circulation in this case takes place on the thermo-siphon principle.  The down comers contain relatively cold water, whereas the water wall tubes contain steam water mixture, whose density is comparatively less.  This density difference is the driving force for the mixture to over come the frictional resistance.  Circulation takes place at such a rate that the driving force and frictional resistance are balanced.

As the pressure increases the difference in density between water and steam reduces.  Thus the hydrostatic head available will not be able to overcome the frictional resistance for a flow corresponding to the minimum requirement of cooling of water wall tubes.  Therefore natural circulation is limited to boiler with drum operating pressure around 175 kg/cm².

ASSISTED CIRCULATION:

Beyond 175 kg/cm² of pressure in a re circulation system, circulation through the evaporator is to be assisted with mechanical pumps to overcome frictional losses in the tubes.  A pump called Boiler circulation pump will be placed between the drum and water wall tubes inlet to do this function.

In such a system some times to regulate the flow through various tubes in accordance to the amount of heat received orifice plates will be used at the inlet of the tubes.  Such a system is known as Controlled Circulation.

FORCED CIRCULATION :

In the once through systems of sub critical pressure or in boilers operating above critical pressure, the water from the feed supply is pumped to the inlet end or ends of the heat absorbing circuits.  This is called forced circulation.  Evaporation or change of state gradually takes place along the length of the circuit and when evaporation is complete, further progress through the heated circuits results in superheating the vapor.  Conventionally this type of forced circulation requires no steam and water drum.

CIRCULATION NUMBER :

The circulation number is the ratio of the quantity of water-steam mixture flowing through the circuit to the quantity of steam produced in the circuit.  In other words if 10 kg of water is circulated for each kg of steam generated, the circulation number is 10.  The circulation number for a boiler will be decided considering many factors especially to ensure nucleate boiling is maintained for all operating conditions.

Higher circulation number reduces the chance of scale formation in evaporating tubes and DNB but needs more evaporating surface.

The general practice adopted for circulation number in various boilers is:

 i)          Natural Circulation Industrial boiler        10 to 15

ii)         Natural Circulation Utility boiler              7 to 9

iii)        Assisted Circulation boilers                     2 to 4

iv)        Forced Circulation/Once through boiler   1

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