Page 291 - Boiler_Operators_Handbook,_Second_Edition

276 Boiler Operator’s Handbook

and water in a boiler to operate it without damaging it. water surface along with each pound of steam that’s
If circulation is interrupted for more than a few seconds formed.
all the water will boil away in areas of high heat transfer Recall that in the boiling pot of water you saw lots
and, only able to heat the steam, metal temperatures will of round bubbles? In among all of them was a lot of
shoot up and the boiler will fail. water. A sphere (bubble) occupies 52.36% of a cube that
To be certain you understand what boiler water would have sides equal to the diameter of the sphere so
circulation is and how it works I’ll use some simple even if every steam bubble was touching another one
examples and develop them to the more complex provi- only slightly more than half of the volume of the rising
sions. If you’ve never watched a pot of water at what steam and water mixture would be steam. In our pot on
we call a rolling boil on the stove take a break and go do the stove the steam occupies 26.8 cubic feet per pound
it; you’ll waste a little energy but the lesson is worth it. and water occupies 0.01672 cubic feet per pound (see
Those of you who already have can read on. steam tables, in the appendix.) If the volume of the pot
Notice how rapidly the steam bubbles and water was one cubic foot we could calculate the weights of
moved in that pot? At a nominal one atmosphere, where steam and water if all the bubbles were touching each
water boils at 212°F the volume of steam is 1,603 times other. The steam would weigh 0.01954 pounds (0.5236
3
3
greater than the volume of the same weight of water so ft ÷ 26.8 ft /lb) and the water would weigh 28.498
3
3
the weight of the steam is about six ten thousandths of pounds ({1-.5235}ft ÷ 0.01672 ft /lb). The weight ratio
the weight of an equal volume of water. Try to push a of water to steam would be 1,458 pounds of water per
balloon full of air down into a bucket of water to get an pound of steam (28.498 ÷ 0.01954).
idea of the force created by the difference in density. I won’t apologize for the math, it’s just adding
If you manage you’ll get your feet wet because the subtracting and dividing and I believe it’s necessary
water in the bucket will be displaced by the balloon and because without supporting math most operators re-
come splashing out. The steam forming in that pot of fuse to believe that the rate the water circulates inside
boiling water would blow all the water out of the pot if the boiler is hundreds of times greater than the rate of
it were not for the fact that it rises to the surface of the steam flowing out the nozzle. The ratio gets smaller as
water and breaks out so rapidly. The steam bubbles have pressures increase, if you would like to know what the
to move fast to get out of the water without displacing ratio would be for your operating pressure all you have
it completely. If you get the pot boiling too fast the level to do is substitute the volumetric values for your operat-
will rise and the water will spill over the top anyway. ing pressure from the steam tables into those formulas.
That’s despite the fact that some of it is converting to Of course you have to admit that the bubbles aren’t
steam so there’s always less water in the pot than when touching each other so there’s a lot more water flowing
you started. around than this calculation would indicate.
Watching the pot you can see that the water is Now that you have a good mental picture of the
circulating, water and steam bubbles rise up, the steam water and steam rising in a pot on the stove let’s translate
separates and goes into the air, and the water that came that to the inside of a boiler. A firetube boiler might have
up with the steam returns to the bottom of the pot, usu- a pattern like that of Figure 10-1. It’s more complicated
ally in the middle but not always and not consistently. than that because the amount of heat transfer changes
Being much heavier than the steam the water manages from the front of the boiler to the rear. In the typical scotch
to find its way down with a force comparable to the one marine boiler the water rises around the furnace over the
that you had to use to get the balloon down in the wa- entire length and drops at the sides to varying degrees
ter. It will tend to go where the velocity of rising steam and considerably against the front tube sheet.
bubbles and water is lowest. Water tube boilers have circulation patterns that
The water in a boiler has to move around, or cir- vary considerably with the boiler design and the firing
culate, just like it does in the pot on the stove in order rate. The typical example shown for circulation in a wa-
to let the steam out of the boiler. Enough water has to ter tube boiler is that shown in Figure 10-2. The water
flow with the steam to carry the solids dissolved in the and steam rises in the tubes that receive the greatest
remaining water and keep them dissolved or they will amount of heat because more steam bubbles are in that
drop out on the heat exchange surfaces to form scale. water. Water along with a little steam that is generated
Luckily water is highly cohesive (it sticks to itself) and drops in the tubes that receive less heat.
tries to hold itself together around those steam bubbles The tubes where water and steam flow up toward
so there are many pounds of water circulating up to the the steam drum are called “risers” and the ones where