Natural vs Forced Circulation

[Rpsfinest]

I am studying the difference between natural and forced circulation in large boilers from generating fascilities. I have been reading the requirements for natural circulation and am a bit confused to why certain boilers require boiler circ pumps that seem to meet the requiremnts for natural circulation. The boiler at hand is a CE forced circulation tangentionally fired boiler.

The requirements for forced circulation that I have been reading are:

a temperature difference exists
the heat source is at a lower elevation than the heat sink
the fluids must be in contact wiht one another

All these requirements seem to be met by the boiler at hand, the only answer anyone ever gives me is that the boiler is to large... I was kind of looking for a more technical answer... any help???

the unit is rated 400 MW and 2,700,000 lb/hr

 

[Sharik]

It's not so much the difference in temperature, it's the difference in density between the steam and water which determines natural or forced circulation. This is determined by the pressure. The higher the pressure, the less difference in density between steam and water. Until you reach critical pressure, about 3200 psig when there is no difference. If you have a large 400 MW unit, you are probably running at a high pressure, at least 2500 psig?? The boiler would indeed be too large and too complicated for just natural waterside circulation.

 

[rjoaks]

In addition to Sharik's response there are boilers that may have horizontal tubes or circuits that may be sloped but there may not be enought density difference in the water steam mix to drive the circulation.
Forced circulation is then required to to make the thermocycle function inorder not to burn up the tubes.
The design may be such that forced circulation will reduce the over all size of the unit balanced against the hp reguired by the pumps.
Hope this helps.
rjoaks

 

[davefitz]

There is a simple answer and a complicated answer.

The simple answer is that the boilers are bid competitively , and the cost of a foreced circ boiler may be lower than that of a nat circ boiler in some cases. The forced circ boiler uses smaller furnace waterwall tubes ( 2" OD) and also may tolerate a lower circulation ratio, so the downcomers may be smaller. Compared to a nat circ boiler that uses 3" OD tubes and must strickly limit frictional pressure drop in the downcoomer and risers to ensure adequate circulation occurs. The savings in metal mass can offset the cost of the recirc pump, motor, power supply.

The disadvantages of the forced circ boilers may be (a) the furnace waterwall circuityr has a "negative thermal hydraulic sensitivity characteristic", whihc means teh tubes that abosrb the most heat recieve the least amount of cooling water flow. This could lead to dryout overheat or DNB overheat in cases where the slag coverage is excessive. And (b) there could be postulated a loss in availability due to potential failure of the pump/motor/ power supply.

 

[rmw]

CE tended to use forced circ in their designs more than B&W or FW did at least back in the mid 80's.

rmw