Steam and Boilers

[thiru4vino]

Can any body suggest how to derive and relate the pressure drop between downtakers(water) and uptakers(steam and water) and the steam temperature in a boiler that is under natural circulation? If somebody can help by suggesting a book also, it is welcome...Advance thanx...

 

[unclesyd]

I would start with a book "Steam" that can be found at

http://www.babcock.com

 

[athomas236]

In general terms the temperature of steam and water in the evaporative part of a boiler is the saturation temperature corresponding to the drum pressure. This temperature can be obtained from steam tables.

For the downcomers the case is a little more complicated. The flow in the downcomers comes from two sources which are the incoming feedwater at economiser outlet conditions and the recirculated water which will be at saturated conditions. Consequently the enthalpy of combined water in the downcomers can be calculated by heat balance from the mixing of these two streams. The temperature from the steam tables.

athomas236

 

[jehar]

You may find the answer you are looking for at

http://www.HRSGdesign.com,

under thermosyphon circulation. There are example calculations that show the relationship between the downcomers and risers and develops the mechanism that causes the cicuits to circulate.
Jack

 

[davefitz]

The above comments are helpful regarding the theoretical calculation of the cirulation ratio, but the following real life effects will dominate the actual tested cirulaiton of the water thru the downcomers:
a) the vortex and steam entrainment that occurs in the inlet to the downcomer can result in an apparent loss of net downcomer head by about 10-20 ft on large high pressure boilers
b)large boilers have large variations in heat absorption rate around the furnace circumference as wellas as vertically. If the circulation system components ( downcomvers, feeders, unheated risers, headers) are not partitioned and separated properly( ie separate and distinct subsystems for each discrete section of the furnace permiter) then the ciculaiton will suffer at teh most higly heated and also the least heated sections.

 

[athomas236]

When responding to general questions it is always difficult to know how much information to provide. Some times too much information can cloud the main principles.

Having said this davefitz is correct.

The enthalpy of water leaving the bottom of cyclone separators is not exactly equal to that of saturated water. The reason is that separators are not 100% efficient in separating water from steam and steam from water. The water leaving the bottom of separators can contain typically 3-5% steam at normal water levels at lower water levels this can increase to as much as 20-25%.

This factor needs to be taken into account when calculating the enthalpy and density of water to the downcomers because it will affect the enthalpies and hence dryness fractions/ density of fluid in the furnace tubes. These in turn will affect the overall circulation ratio and whether conditions in the furnace tubes are acceptable from a burnout/dryout viewpoint.

It is also true that heat absorptions across the width and up the height of the furnace will affect the will affect the circulation. These variations in heat absorption will depend upon the burner locations and the fuel fired.

For example, for front wall oil-fired units rated at 660MWe we found variations in heat absortion of +/- 30%. We also found variations in spot heat fluxes of +/- 50%. These spot values were used to check against allowable heat fluxes.

Best regards,

athomas236