Page 364 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 349

used almost exclusively for application as induced draft is when it’s used to provide air for ash or coal transport
fans and primary air fans for coal pulverizers. You’ll dis- systems which require some rather high differential pres-
cover that most air conditioning and ventilation systems sures. See the following discussion on rotary compressors
use forward curved fans because they are more efficient for more information that would apply to blowers.
at delivering large volumes of air at low differential
pressures. It’s important to note that forward curved Fan and Blower Control
fans have a very stretched curve and it’s not at all un- Control of the flow of gases in systems with fans
common for the motors on those fans to be overloaded if and blowers is typically achieved using devices we call
nothing restricts the air flow. dampers that are a leaky version of valves. Sometimes
Blowers will have radial blades or backward the system uses valves or their equivalent when leakage
curved blades depending on the application and can is not acceptable. Dampers are not the best method for
experience the same problems with surging that was controlling air flow because they are typically made to
discussed with centrifugal pumps. That surging will be inexpensive and there isn’t a linear relationship (see
also occur for the same reasons. It’s seldom encountered controls) between the damper position and air flow. Op-
11
in fan and blower applications but is frequently encoun- posed blade dampers (Figure 10-91) provide a better
tered when compression is involved. relationship than parallel bladed dampers. The different
An important thing to remember about these
fans is that they’re centrifugal devices, the differential
pressure that’s produced by them is a function of the
flow through the fan and the density of the gas flowing
through. When the gas is colder a fan will produce a
higher differential pressure (in terms of inches of water)
and, because it’s moving denser air, more pounds of gas.
When a fan is handling gases at higher temperatures
they will not produce as high a differential and move
fewer pounds of gas. In many cases the motor on an
induced draft fan is not big enough to handle cold air
because the power requirement is significantly higher
when pumping cold air. That’s why you have to be care-
ful when starting a boiler with an induced draft fan to
ensure you do not overload it. Once it’s pumping hot
flue gas the load on it drops off.
That’s also justification for not getting excited
when a boiler can’t produce full load in the summer
time. If an F.D. (Forced Draft) fan is installed to collect
the heated air in the top of the boiler plant it will not
pump as much air in the summer, when the tempera-
tures are about 125°F to 130°F but it will in the winter
when those temperatures are 50° to 60° lower. It is im-
portant to realize that the fan is moving less air (pounds
of it) in the summer to reduce excess air because you
might be running fuel rich. If the boiler is summer tuned
then you’ll find that excess air is higher in the winter be-
cause the air is denser than when the boiler was tuned.

Rotary Blowers
Rotary blowers don’t resemble fans, the same con-
struction is used for compressors and the main reason
for rotary blowers is to produce high differentials that
are necessary for material transport systems. Probably Figure 10-91. Resistance curves & diagrams of parallel
the only time you’ll see a rotary blower in a boiler plant and opposed blade dampers