Page 290 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 275

heat transfer in the furnace but it was minimal compared to other surfaces. Its adhesion is greater than its cohe-
to the radiant heat transfer and, no, there shouldn’t be sion as evidenced by the meniscus (see water analysis)
any measurable flame to boiler conductive heat transfer and I’m sure you’ve noticed that water clings to surfaces
in the furnace because the steel can’t handle those flame so the concept of a film is not difficult to envision. To
temperatures if the flame touches the tubes. improve convective heat transfer the fluid flowing past
I had better mention flame impingement right now the heat transfer surface is made turbulent (all mixed
because that’s when we have conductive heat transfer up and swirling around) to sweep against that film and
from the flame to the boiler tubes. It’s also called flame transfer the heat from the fluid through the film to the
gouging because the tube metal is melted and swept metal. As velocities in a boiler drop, a point is reached
away when flame impingement really happens. You where the flue gases can’t disturb the film, it gets thicker,
must have seen what happens when someone heats and the heat transfer drops off dramatically.
metal to cut it with a cutting torch, that’s flame impinge- When flow is so low that the flue gases simply me-
ment. If you have flame impingement you can see the ander along, like congested traffic where the vehicles in
damage during an internal inspection. the middle can’t get to the sides of the road, a lot of the
The truth is that we seldom have flame impinge- gas leaves without contacting the tubes. It can’t give up
ment problems in a boiler despite many people arguing its heat so it’s hotter, carrying that valuable energy out
that they have it. I have only seen a couple of incidents of the boiler and up the stack.
of true flame impingement in my forty-five years in the Something unique happens to that film on the wa-
business so I refuse to believe anyone’s claim of it until ter side when we’re making steam so heat transfer from
I’ve examined the boiler. It doesn’t happen because the metal to boiling water is a lot greater than heat transfer
flame is cooled so much by radiant heat transfer that it’s to water or steam. If you think about it, it’s easy to un-
normally quenched (below ignition temperature) before derstand. I mentioned it earlier in the chapter on water,
it gets to the tube. steam, and energy. When heat is transferred from the
When I can look into the furnace and see the flame tube to the water to make steam a bubble of steam forms
bouncing off the tubes or furnace wall just like you and it grows to several times the volume of the water
would see water bouncing when a wall is sprayed with a it came from (in the typical heating boiler operating at
water hose that appears to be flame impingement. Even 10 psig the steam expands to 981 times the volume of
then you can examine the boiler and find no damage at the water) so there’s a dramatic movement of the steam
all on the tubes. and water interface. The steam bubble then breaks away
Bulges and blisters (mentioned earlier) are not due from the metal (steam is nowhere near as cohesive as
to flame impingement, they’re due to scale formation. If water) and water rushes in to fill the void. All that ac-
the flame seems to be rolling along the tubes or passing tivity makes steam generation much easier than simply
along them so close that they must be touching we call it heating water or superheating steam and it requires less
“brushing” the tubes and it doesn’t do any damage. heat transfer surface to get the heat through. Similarly
The same thing that helps prevent true damage when getting heat from steam the steam forms conden-
from flame impingement also makes it difficult to trans- sate at almost one thousandth of the volume and more
fer heat by convection. The molecules of air and flue gas steam rushes in to fill that void while the condensate
that are in contact with the tubes stick to the tube and drizzles down the heat transfer surface effectively
each other to form what we call a “film.” It’s a very thin scrubbing it clean.
layer of gas that acts like insulation separating the hot The range of heat transmittance (U) for steam con-
2
flue gases from the tubes. In the course of heat flow from densers is 50 to 200 Btuh-ft -°F (British thermal units per
the flue gases to the water and steam it contributes the hour per square foot per degree Fahrenheit) compared
2
most resistance to heat flow. That film is mainly what to water to water heaters at 25 to 60 Btuh-ft -°F, and su-
9
2
protects the tubes in a furnace from the hot flue gases perheaters have values of 2.6 to 6 Btuh-ft -°F . Also see
in the fire. Otherwise the metal temperature would be the comparison of E.D.R. in the Chapter 1. No wonder
so high that it would melt. The typical boiler steel will steam is an excellent heat transfer medium.
melt around 2800°F and it begins to weaken at tempera-
tures above 650°F. (It actually gets a little stronger as it is
CIRCULATION
heated up to 650°F.)
A film forms on most gas to metal or liquid to In addition to heat transfer a boiler operator has to
metal surfaces to resist heat transfer. Water really sticks have a sound understanding of the circulation of steam