Page 361 - Boiler_Operators_Handbook,_Second_Edition

346 Boiler Operator’s Handbook

times the differential in inches divided by 6356 and,
since the differential was zero, the horsepower require-
ments as they sat there on the floor should be negligible.
Since I had the instruction manual in hand and it clearly
stated that the fan had to be installed and the differential
has to be at least 80% of the design value he agreed to
have an electrician check the motor current. The motor
current was three times nameplate rating and that’s why
they kept burning them up.
It was later, when I examined one of my engineer-
ing books, that I discovered the reason for the problem.
The differential pressure in the horsepower formula is
total pressure, a combination of static and velocity pres-
sure differences. Those fans had no static difference but
the velocity pressure was there and a lot higher because,
without the static resistance, the fan could force more air
through to produce a higher velocity and, therefore, a
higher velocity pressure. The increased flow and velocity
pressure added up to produce the high horsepower that
overloaded the motor. This is a lesson for testing any elec-
trical device, that contractor had simply wired the fans to
a welding connection in the plant. No starter, no overload
devices, is it any wonder he was burning up motors?
Figure 10-85. Propeller fan housing with flow re- Fans, like pumps, have a theoretical horsepower.
directed From the story I just told you know that it’s the total
pressure across the fan that has to be used. The formula
some spin and make them more efficient (Figure 10- is cfm times total pressure divided by 6356. If all you can
85). Propeller fans are primarily limited to ventilation measure is the differential you can calculate the velocity
services in a boiler plant although they were used in the pressure. Divide the cfm by the area of the fan discharge
middle of the last century for forced draft and induced to get velocity then look up the velocity pressure. Add
draft service when differentials were low. velocity and static pressure differentials to get total pres-
Some key things to know about propeller fans sure. Don’t have a table? The velocity is the capacity in
include the fact that they readily overload their motors cfm divided by the area of the outlet. Divide the velocity
if the system doesn’t produce the design resistance. I re- by 4005 and multiply the result by itself to get velocity
member visiting a job site in a synthetic fiber plant where pressure. Add it to the static to get total pressure.
a contractor had several propeller fans simply sitting on Many fans and blowers are belt driven. The use of
the floor and running with temporary wiring. We were belts will allow an engineer to pick a fan for optimum
informed that they were testing the fans because the mo- speed for a given application because any speed can be
tor on one of them failed and now they’re finding more established by the proper mix of motor speed and size
of them are failing. Luckily I was wise enough to pull of sheave (those pulleys the belts run on). In some cases
one of the instruction manuals out of the envelope at- one of the sheaves is adjustable to permit field adjust-
tached to a lifting eye and read enough to learn the fans ment of the speed. All these features are very valuable
were designed for a two inch differential. The drawings for HVAC equipment where the flow is constant and
showed the installation would produce that but the fans the fan can be tuned to achieve the precise required
sitting on the floor just blowing air were operating with flow without chewing up added energy with dampers.
no appreciable differential. I suggested they quit testing They’re not as valuable in a boiler application where the
them immediately because they were destroying them air flow is varied.
by overloading them. Another advantage of belts is they can slip on start-
In turn I was lectured by one of the contractor’s up to reduce the startup load on the motor, something
engineers that they couldn’t possibly be overloading to let go until you’ve checked the instruction manual.
because fan horsepower is equal to the capacity in cfm Belts are typically provided to the degree that one belt