Page 288 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 273

the basic elements of a boiler plant to provide a basic transport to get the heat from one spot to another. In
understanding of the systems and equipment. Hope- your home the furnace or boiler heats air or water which
fully an operator can append this information with the is then moved (blown or pumped) to the room you’re
contents of the instruction manuals to develop a full in and heats the air in the room which then heats you.
working knowledge of his or her boiler plant. There are two types of convection heating, natural and
forced. Forced convection is the result of a fan, pump or
blower forcing the movement of the fluid over a heated
HEAT TRANSFER IN BOILERS surface where it picks up heat then on to another surface
where it gives up that heat.
An understanding of heat transfer is a fundamen- If you’re sitting in a house with a radiator next to
tal requirement for a boiler operator because a lack of the wall that radiator is heating the air around it and the
understanding of heat transfer can result in the opera- air gets lighter (less dense) as it expands from the heat-
tor’s death; it’s that simple. The energy transferred in ing so it rises up in the room like a lighter than air bal-
a little 100-horsepower boiler is about eight times the loon. When it reaches the ceiling it starts to cool because
amount it takes to power an automobile at sixty miles it’s giving up heat to the ceiling and it’s pushed aside
per hour. Screw up to get that energy going in the wrong by hot air following it. When the air reaches a cooler
direction and you’re inviting an accident that can only outside wall it gives up more heat, shrinks to become
be compared to eight or more cars running you over, all denser, and drops to the floor then travels back to the ra-
at the same instant! diator. That’s natural convection heat transfer. All these
There are three ways that heat is transferred, con- methods of heat transfer occur in a boiler.
duction, radiation, and convection and all three means The modern boiler with its water cooled walls
occur in a boiler. Conductive heat transfer is the flow absorbs about 60% of the heat from the burning of the
of heat through a substance molecule by molecule. A fuel using radiant energy. That heat travels in the form
molecule is the smallest piece of a substance that we can of light waves from the glowing hot fire directly to the
get without destroying it’s identity. The heat is absorbed boiler tubes, in water tube boilers, or furnace tube in fire
by one molecule which passes it onto the next and so tube boilers. The reason so much heat is transferred is
on. The best example I know of conductive heat transfer due to the low resistance to the radiant heat flow from
that you can readily understand is toasting marshmal- the fire to the tubes. Though not quite as hot as the sun a
lows. Of course toasting marshmallows is done best fire is an awful lot closer so there’s a lot of heat flowing
over a campfire and I love campfires; if you haven’t had there. You can feel the radiant heat of a fire if you can
these experiences go out and do it so you can learn to open up an observation port to look in. Once it hits the
love them too. fire side of the tube the heat is transferred by conduction
You should remember the time when you and to the water side of the tube and by convection to form
some friends were toasting marshmallows and you got hot water and steam.
stuck with the fork without the wooden handle. As your Conductive heat transfer to the boiler water and
marshmallow was toasting you could feel the metal get steam is limited to the flow through the boiler metal
hot in your hand. The metal over the fire was heated and itself. The steel parts of a boiler are selected for their
that heat was conducted up the metal of the fork to your ability to transfer heat with as little temperature differ-
hand. You should also remember those cold nights at the ence as possible. The outside of a water cooled tube is no
campfire when the front of you was hot and your back more than 60 or 70 degrees hotter than the inside both
was cold so you stood up and turned so the heat from because the heat is passed through the tube easily and
the fire would warm your back. You were using the radi- because the heat is drawn off the tube by the water and
ant means of heat transfer. steam rapidly.
The sun is another good example, when you’re lay- Other parts of a boiler count on poor conductive
ing there on the beach you are soaking up heat from the heat transfer to protect them from the heat of the fire.
sun. It’s almost 93 million miles away with mostly space Refractory material not only can withstand high tem-
(nothing) between it and us but the heat is getting here. peratures it’s a poor conductor of heat. When it’s backed
Radiant heat transfer is the flow of heat energy by light up with some insulation the outer surface of the boiler’s
waves that can penetrate empty space and the air above metal casing is less than 140°F which is the maximum
us but is absorbed by solid and liquid in its path. temperature that should be allowed. (anything hotter
The last means, convective heat transfer, uses a will give someone a serious burn in a matter of seconds,