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TitleBoiler Operator's Handbook
TagsBoiler Operation
LanguageEnglish
File Size10.1 MB
Total Pages415
Document Text Contents
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Boiler Operator’s Handbook

Page 207

Plants and Equipment 199

heat transfer in the furnace but it was minimal compared
to the radiant heat transfer and, no, there shouldn’t be
any measurable flame to boiler conductive heat transfer
in the furnace because the steel can’t handle those flame
temperatures if the flame touches the tubes.

I had better mention flame impingement right now
because that’s when we have conductive heat transfer
from the flame to the boiler tubes. It’s also called flame
gouging because the tube metal is melted and swept
away when flame impingement really happens. You
must have seen what happens when someone heats
metal to cut it with a cutting torch, that’s flame impinge-
ment. If you have flame impingement you can see the
damage during an internal inspection.

The truth is that we seldom have flame impinge-
ment problems in a boiler despite many people arguing
that they have it. I have only seen a couple of incidents
of true flame impingement in my forty-five years in the
business so I refuse to believe anyone’s claim of it until
I’ve examined the boiler. It doesn’t happen because the
flame is cooled so much by radiant heat transfer that it’s
normally quenched (below ignition temperature) before
it gets to the tube.

When I can look into the furnace and see the flame
bouncing off the tubes or furnace wall just like you
would see water bouncing when a wall is sprayed with
a water hose that appears to be flame impingement.
Even then you can examine the boiler and find no dam-
age at all on the tubes.

Bulges and blisters (mentioned earlier) are not due
to flame impingement, they’re due to scale formation. If
the flame seems to be rolling along the tubes or passing
along them so close that they must be touching we call
it “brushing” the tubes and it doesn’t do any damage.

The same thing that helps prevent true damage
from flame impingement also makes it difficult to trans-
fer heat by convection. The molecules of air and flue gas
that are in contact with the tubes stick to the tube and
each other to form what we call a “film.” It’s a very thin
layer of gas that acts like insulation separating the hot
flue gases from the tubes. In the course of heat flow from
the flue gases to the water and steam it contributes the
most resistance to heat flow. That film is mainly what
protects the tubes in a furnace from the hot flue gases in
the fire. Otherwise the metal temperature would be so
high that it would melt. The typical boiler steel will melt
around 2800°F and it begins to weaken at temperatures
above 650°F. (It actually gets a little stronger as it is
heated up to 650°F.)

A film forms on most gas to metal or liquid to
metal surfaces to resist heat transfer. Water really sticks

to other surfaces. Its adhesion is greater than its cohesion
as evidenced by the meniscus (see water analysis) and
I’m sure you’ve noticed that water clings to surfaces so
the concept of a film is not difficult to envision. To im-
prove convective heat transfer the fluid flowing past the
heat transfer surface is made turbulent (all mixed up and
swirling around) to sweep against that film and transfer
the heat from the fluid through the film to the metal. As
velocities in a boiler drop, a point is reached where the
flue gases can’t disturb the film, it gets thicker, and the
heat transfer drops off dramatically.

When flow is so low that the flue gases simply
meander along, like congested traffic where the vehicles
in the middle can’t get to the sides of the road, a lot of
the gas leaves without contacting the tubes. It can’t give
up its heat so it’s hotter, carrying that valuable energy
out of the boiler and up the stack.

Something unique happens to that film on the
water side when we’re making steam so heat transfer
from metal to boiling water is a lot greater than heat
transfer to water or steam. If you think about it, it’s easy
to understand. I mentioned it earlier in the chapter on
water, steam, and energy. When heat is transferred from
the tube to the water to make steam a bubble of steam
forms and it grows to several times the volume of the
water it came from (in the typical heating boiler operat-
ing at 10 psig the steam expands to 981 times the volume
of the water) so there’s a dramatic movement of the
steam and water interface. The steam bubble then breaks
away from the metal (steam is nowhere near as cohesive
as water) and water rushes in to fill the void. All that
activity makes steam generation much easier than sim-
ply heating water or superheating steam and it requires
less heat transfer surface to get the heat through. Simi-
larly when getting heat from steam the steam forms
condensate at almost one thousandth of the volume and
more steam rushes in to fill that void while the conden-
sate drizzles down the heat transfer surface effectively
scrubbing it clean.

The range of heat transmittance (U) for steam con-
densers is 50 to 200 Btuh-ft2-°F (British thermal units per
hour per square foot per degree Fahrenheit) compared to
water to water heaters at 25 to 60 Btuh-ft2-°F, and super-
heaters have values of 2.6 to 6 Btuh-ft2-°F9. Also see the
comparison of E.D.R. in the Chapter 1. No wonder steam
is an excellent heat transfer medium.

CIRCULATION

In addition to heat transfer a boiler operator has to
have a sound understanding of the circulation of steam

Page 208

200 Boiler Operator’s Handbook

and water in a boiler to operate it without damaging it.
If circulation is interrupted for more than a few seconds
all the water will boil away in areas of high heat transfer
and, only able to heat the steam, metal temperatures will
shoot up and the boiler will fail.

To be certain you understand what boiler water
circulation is and how it works I’ll use some simple ex-
amples and develop them to the more complex provi-
sions. If you’ve never watched a pot of water at what we
call a rolling boil on the stove take a break and go do it;
you’ll waste a little energy but the lesson is worth it.
Those of you who already have can read on.

Notice how rapidly the steam bubbles and water
moved in that pot? At a nominal one atmosphere, where
water boils at 212°F the volume of steam is 1,603 times
greater than the volume of the same weight of water so
the weight of the steam is about six ten thousandths of
the weight of an equal volume of water. Try to push a
balloon full of air down into a bucket of water to get an
idea of the force created by the difference in density.

If you manage you’ll get your feet wet because the
water in the bucket will be displaced by the balloon and
come splashing out. The steam forming in that pot of
boiling water would blow all the water out of the pot if
it were not for the fact that it rises to the surface of the
water and breaks out so rapidly. The steam bubbles have
to move fast to get out of the water without displacing
it completely. If you get the pot boiling too fast the level
will rise and the water will spill over the top anyway.
That’s despite the fact that some of it is converting to
steam so there’s always less water in the pot than when
you started.

Watching the pot you can see that the water is cir-
culating, water and steam bubbles rise up, the steam
separates and goes into the air, and the water that came
up with the steam returns to the bottom of the pot, usu-
ally in the middle but not always and not consistently.
Being much heavier than the steam the water manages
to find its way down with a force comparable to the one
that you had to use to get the balloon down in the water.
It will tend to go where the velocity of rising steam
bubbles and water is lowest.

The water in a boiler has to move around, or circu-
late, just like it does in the pot on the stove in order to
let the steam out of the boiler. Enough water has to flow
with the steam to carry the solids dissolved in the re-
maining water and keep them dissolved or they will
drop out on the heat exchange surfaces to form scale.
Luckily water is highly cohesive (it sticks to itself) and
tries to hold itself together around those steam bubbles
so there are many pounds of water circulating up to the

water surface along with each pound of steam that’s
formed.

Recall that in the boiling pot of water you saw lots
of round bubbles? In among all of them was a lot of
water. A sphere (bubble) occupies 52.36% of a cube that
would have sides equal to the diameter of the sphere so
even if every steam bubble was touching another one
only slightly more than half of the volume of the rising
steam and water mixture would be steam. In our pot on
the stove the steam occupies 26.8 cubic feet per pound
and water occupies 0.01672 cubic feet per pound (see
steam tables, in the appendix.) If the volume of the pot
was one cubic foot we could calculate the weights of
steam and water if all the bubbles were touching each
other. The steam would weigh 0.01954 pounds (0.5236 ft3

÷ 26.8 ft3/lb) and the water would weigh 28.498 pounds
({1-.5235}ft3 ÷ 0.01672 ft3/lb). The weight ratio of water
to steam would be 1,458 pounds of water per pound of
steam (28.498 ÷ 0.01954).

I won’t apologize for the math, it’s just adding
subtracting and dividing and I believe it’s necessary
because without supporting math most operators refuse
to believe that the rate the water circulates inside the
boiler is hundreds of times greater than the rate of steam
flowing out the nozzle. The ratio gets smaller as pres-
sures increase, if you would like to know what the ratio
would be for your operating pressure all you have to do
is substitute the volumetric values for your operating
pressure from the steam tables into those formulas. Of
course you have to admit that the bubbles aren’t touch-
ing each other so there’s a lot more water flowing
around than this calculation would indicate.

Now that you have a good mental picture of the
water and steam rising in a pot on the stove let’s trans-
late that to the inside of a boiler. A firetube boiler might
have a pattern like that of Figure 9-1. It’s more compli-
cated than that because the amount of heat transfer
changes from the front of the boiler to the rear. In the
typical scotch marine boiler the water rises around the
furnace over the entire length and drops at the sides to
varying degrees and considerably against the front tube
sheet.

Water tube boilers have circulation patterns that
vary considerably with the boiler design and the firing
rate. The typical example shown for circulation in a
water tube boiler is that shown in Figure 9-2. The water
and steam rises in the tubes that receive the greatest
amount of heat because more steam bubbles are in that
water. Water along with a little steam that is generated
drops in the tubes that receive less heat.

The tubes where water and steam flow up toward

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