Page 363 - Boiler_Operators_Handbook,_Second_Edition

348 Boiler Operator’s Handbook

difference, is one way works better.
You’ll also encounter some definitions on width
and number of inlets. I’m sure you have seen single inlet
fans where air enters one side, but there are also double
inlet fans where air enters both sides. They are defined
by simple abbreviations with SWSI (Single Width, Single
Inlet) being the most common and DWDI (Double Width,
Double Inlet) where the air can enter both sides used in
Figure 10-89. Different shapes of fan blades
many applications from little convectors (those fan pow-
ered heating and cooling units mounted under windows blade will generate an imbalance in the fan wheel that
in many buildings) to large forced draft fans. Please don’t adds load to the fan bearings, a variable shock load. If
ask me to explain the width business, I just look at the fan that’s allowed to happen you can have everything from
and decide whether to call it single or double based on shaft distortion to where the fan wheel hits the inlet bell,
the ratio of wheel diameter to width and that’s all. cutoff or casing. You should clean every fan, or have
Instead of calling the primary rotating element an it cleaned, during the annual inspection. Some forced
‘impeller’ we call it a ‘wheel.’ The term scroll is applied draft fans or what’s below them in the ductwork can’t
to the casing because the radius increases from the cutoff tolerate a water wash so you’ll have to limit cleaning to
to the discharge. A casing is still a casing and many other brushing and vacuuming. Be sure to do the inside of the
labels are consistent with what we use for pumps. The scroll too because the dust is thrown at it.
cutoff is the portion of the scroll that’s closest to the out- Centrifugal fans and blowers are used more than
side diameter of the wheel. It’s where the swirling fluid any other device for moving air. In order to accommo-
in the fan is cut off so it heads out the discharge instead date a variety of applications they are also supplied in
of riding around with the fan wheel. The inlet bell, is a significant variety of configurations. Three principle
that specially formed section that connects the fan inlet variations involve the shape of the vanes or blades in
to the inside diameter of the wheel. Makes sense, be- the fan. A fan is called ‘backward curved,’ ‘forward
cause when you take it out and set it on the floor it does curved,’ or ‘radial’ depending on the shape of the
look like the bottom of a big old church bell. Small fans blades as shown in Figure 10-89. These three shapes
won’t have an inlet bell, only a hole in the casing that produce significantly different fan curves as shown in
faces the wheel. Figure 10-90.
There are some additional gadgets that are not Most applications in a boiler plant use backward
found on pumps because fans usually don’t have seals curved or radial bladed fans because they are more effi-
or packing glands, although they are used on occasion. cient for the operating condition when backward curved
We have ‘heat slingers’ that are like little fan wheels and, in the case of radial blades, do not accumulate
located on the shaft outside the fan to draw cooling air solids on the blades in operation. Radial bladed fans are
over the bearings and protect them from hot gases and
the heat that conducts along the fan shaft. Instead of
strainers a fan will be protected by ‘inlet screens’ which
keep sticks and stones out but not dust.
Dust is, therefore, something an operator has to
keep in mind. Keep it in mind for two reasons; because
it can damage the fan or hinder its performance and that
dust can be converted from large harmless sizes to much
finer particles that are injurious to human health after
they pass through a boiler.
A certain amount of dust will be struck by the
blades on the fan wheel and trapped there, accumulat-
ing until they form a rather thick layer if they aren’t
cleaned. The accumulation will tend to reduce the fan
capacity. The bigger problem, however, is that once it
reaches a certain level it will suddenly start breaking
off. Losing a fair sized accumulation of dust on one Figure 10-90. Fan curves, BC, Radial, FC