Page 360 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 345

FANS AND BLOWERS air. A fan curve could be used to calculate the differential
it would produce if pumping water. The problem is the
Fans and blowers are used to move gases (com- denser water would produce so much load on the fan
pressible fluids) around a boiler plant. In many cases I that it would break or the motor overload before it actu-
will use the terms “rotating equipment” or “fluid han- ally pumped any water.
dling equipment” to include pumps, fans, blowers and So, a lot of the rules for pumps apply just as well to
compressors without regard to the fluid or the from of fans and blowers. Oh, there are differences, we’re not as
the equipment because they all do the same thing, move particular about some air leaking out of a fan so there’s
a certain volume of a fluid and add energy to it to permit seldom any kind of shaft seal and, because the density
it to flow through the rest of the system. For every de- of the fluid is so low, fans and blowers can get a lot
sign of pump there is a comparable design of fan, blower larger than pumps in order to handle enough volume to
or compressor. Be sure to look through what I’ve written deliver the pounds of air or other gases that have to be
on pumps; it will improve your understanding of fans moved. The typical application of a fan or blower also
and blowers. doesn’t involve raising the pressure of the fluid to move
Differences in the equipment are related primarily it into a reservoir at a higher pressure; the differential
to the different densities, temperatures, and viscosities pressure in a system at zero flow is typically zero for a
of the fluids the equipment handles and the effect the fan or blower because the pressure at the far ends of the
equipment has on the fluid. Fans and blowers are used system are the same. The system curve always starts at
to move compressible fluids, basically gases, not com- zero differential at zero flow. When it doesn’t, the device
press them. That’s what makes fans and blowers differ is a compressor.
from compressors which we’ll cover a little later.
Even though they aren’t designed to compress a Propeller Fans
gas fans and blowers do manage to compress the fluid This prompts a question, why aren’t there many
slightly. In most cases we ignore the compressive effects propeller pumps in a boiler plant? If you ever bought
because the density of the fluid does not change signifi- gas for a day of running around on the water in a motor
cantly. As the differential pressure of a fan or blower in- boat you would know why; they’re not that efficient.
creases compression becomes more significant. There’s Propeller fans have a niche in the world because a pro-
a very gray line between blowers and compressors with peller can move air effectively as long as it doesn’t have
no clear definition of when, specifically, one becomes the to produce any significant differential pressure. If you
other. A fan, on the other hand, is almost never capable haven’t installed some ceiling fans in your home to take
of compression. advantage of the cooling effect they produce by simply
The difference is principally intent. If we intend moving air in the summer, you should.
to compress the gas it’s a compressor, if we don’t it’s The blades of a propeller fan simply push the air
a fan or blower. As for whether a particular piece of along and add some spin to it (Figure 10-84). Housings
centrifugal equipment is a fan or a blower, that’s also a around the propeller can redirect the flow to eliminate
gray area. A centrifugal pump can pump a gas; it doesn’t
produce much differential but it can do it. If you look at
any centrifugal pump, fan, or blower their construction
is pretty much the same and the dynamics that allows
them to move fluid is the same.
These ‘centrifugal devices’ will all perform ac-
cording to their performance curve regardless of the
fluid that passes through them. The differential pressure
they produce is directly related to the tip speed of the
impeller and the density of the fluid because the impel-
ler vanes throw the fluid and the pressure produced is
related to the weight of the fluid flowing at a velocity re-
lated to the tip speed. You could take a centrifugal pump
curve and realizing the differential head of the pump is
feet of fluid, convert to determine the inches of water
differential pressure it would produce while pumping Figure 10-84. Propeller fan