Page 289 - Boiler_Operators_Handbook,_Second_Edition

274 Boiler Operator’s Handbook

140°F is that temperature where you can just barely hold off toward some of the heat transfer surface. Actually
your hand on it for a few seconds) you could argue that very little heat is transferred be-
Now is a good time to point out that heat flows cause the face of the wall and the fire are at almost the
from points of higher temperature to points of lower same temperature, but the truth is it’s radiated back
temperature. If there is no difference, there will be no almost as fast as it’s received.
heat flow. The converse is almost true, if there isn’t any Everything radiates energy, we radiate energy. If
heat flow there can’t be any temperature difference. If you can recall a time when you sat with your back to
we were to put a layer of insulation with a super high a window in the winter time you’ll realize you radiate
resistance to heat flow on the outside of the boiler the heat energy. The heat radiating from you goes right out
refractory, insulation, and casing would get almost as the window into the cold making your back feel colder
hot as the inside of the furnace. That’s why you never than when it faces a wall and most of the heat from you
add insulation to a boiler casing that’s not water cooled is radiated back from the wall. It’s also the reason you
because it will overheat. feel cooler when you go into a parking garage. Even in
If the boiler tubes are coated with fireside deposits the heat of summer those floors and walls are colder
they will get hotter and reflect heat back to the fire to than you are (because they lost their heat overnight) and
reduce heat transfer to the water and steam. If the boiler they absorb more radiant energy than they emit so you
tubes are coated with scale on the water side then the feel cooler. Okay, there are rare times when, after several
tube wall will get very hot because the scale acts like warm days, you enter a parking lot on a cool evening
insulation to block the flow of heat from the tube metal and feel the heat radiating out of the concrete.
to the water. You’ll discover that your boiler loads are a little
Other mechanisms are involved when the scale higher on clear nights because of the black sky effect.
on the water side accumulates and it provides an early Heat radiates from the earth and everything else right
indication of potential failure. If the metal gets too hot it out into space on a clear night so it takes more heat to
will lose its strength and begin to bulge under the force keep the buildings warm. On a cloudy night the clouds
of the boiler pressure. Usually found on the top of fire act like a mirror reflecting the radiant heat back toward
tubes and in the bottom of water tubes where exposed to us so we’re warmer. An important factor in radiant heat
the furnace, bulges are evidence of excessive water side transfer is the emissivity of a substance. It has more to
scale formation. do with the color and finish of a surface than the actual
When the tube metal bulges the hard scale is re- material of construction. White and mirrored objects
leased, breaking away from the metal that’s stretched to have a higher emissivity than black and rough surfaces
form the bulge. Once the scale is broken away the metal so they tend to emit more radiant energy than the black
is exposed to water again, cooling it to stop the growth and rough surface even though they’re at the same tem-
of the bulge. Repeated incidents of bulge formation can perature. Keeping those white rubber roofs clean in the
occur with some of the metal stretched until it is very thin summer and letting them get dirty in the winter will
and its chemical composition changes so the surface be- actually help maintain desirable building temperatures.
comes rough oxidized metal, something we call a blister. As the flue gases leave the furnace they carry the
Sometimes the bulges or blisters can be left in remaining heat into what we call the convection section
place if the processes that promoted scale formation are of the boiler. That’s where convective heat transfer takes
eliminated but blisters should eventually be replaced place so it’s reasonable to call it the convection section.
because the metal is thinner than permitted by code. When we’re dealing with water tube boilers it’s also
Slight bulges, where the tube metal is not distended or called the convection bank. (a bank being a group of
deformed beyond its own thickness, can be left in place. boiler tubes that serve a common purpose) Heat transfer
See Code repairs for replacing bulges and blisters. in the convection section is driven by much lower tem-
Changes in heat conductivity of materials in the perature differences, (typically the flue gas leaves the
path of conductive heat transfer can create conditions furnace at less than 1800°F. 1400°F to 1600°F is a normal
that are inconsistent with the original boiler design to re- range, which is almost half of the 3200°F plus flame tem-
sult in failure. Hopefully you will operate and maintain perature. The temperature difference drops to a typical
your boiler in a manner that doesn’t interfere with the leaving differential of 75°F to 150°F so we need a lot more
design heat flow. heat transfer surface in the convection section of a boiler
As for the radiant energy that hits the refractory to get rid of the 40% that wasn’t transferred by radiant
wall, it’s reflected right back to the flame or is reflected energy in the furnace. Okay, there was some convective