The more resistance in the system, (ducts, filters, coils, dampers, etc.) the less air capacity and lower motor
load, and vice versa, the less resistance the more air capacity and higher motor load.
Vent system sizing eventuates in total resistance at the required capacity, which
are criteria for the fan selection. There is a tendency to select a fan with some spare total pressure over calculated one.
This case is considered in the Graph 1.
The blue lines on the graph are for the designed system, and the red ones - for inappropriate fan-motor selection with
following it consequences.
The design working point 1 with the design system characteristic 0-1, CFM=1a-1 and TP=1b-1,
at 3hp motor capacity and Fan Curve A.
Overestimation of TP=1b-2 at designed CFM=1a-1 creates the imaginary
working point 2 and system 0-2 on the Fan Curve B with motor capacity 10hp.
The real working point 3 of the improperly selected fan is the intersection of the designed
system 0-1-3 with the Fan Curve B. So, the field data is: CFM=3a-3 and TP=2b-3, at 20hp motor capacity.
Deduction.
The selected point 2, estimated over the design point 1, in the field becomes the point 3. The CFM capacity of the
point 3 is larger than in points 1 and 2.
In spite of the resistance of the system in the field (TP in the point 3) is lower than the resistance
(TP in the point 2) of overestimated system the power consumption in the field (point 3) is higher (20hp) than
the power consumption (10hp) of the overestimated system (point 2).
Thus, the increase in energy consumption in the field overheats the selected 10hp motor and causes it to burn.
Note:
A
number of manufacturers do not show the Mechanical Efficiency (Mef) of their fans. The possible reason for it - they do not
want to unveil a low efficiency. Their tables show BHP and you shall run through catalog pages in order to find the lowest
BHP for your system. But it does not mean that you selected fan with sufficient and acceptable MEf. Just the lowest BHP alone
is not an assuring criterion for fan selection. The MEf can be still low. Therefore in order to clarify the picture you have
to proceed with calculation of MEf.
MEf=
power output (air horsepower) / power input (brake horsepower)
Air
horsepower (AHP) is a power which moves the desirable air volume at the desirable pressure. Brake horsepower (BHP) is a power
input the fan requires.
MEf= AHP/BHP = {(CFM x TP) / 6356} / BHP
Now,
when you know the value of MEf, you can be frequently disappointed with the result. In spite of the lowest BHP, selected from
table, the MEf is still disappointedly low.
Conclusion
- The fan oversizing must be avoided.
- The attention-grabbing fact, which must be recognized, is that no matter what methods
have been applied to calculate the vent systems (Equal Friction, Velocity Reduction, Static Regain, or Constant Velocity,
etc.,) the results eventuate in a value of the Total Pressure, not in a value of the Static Pressure.
- Thus, Fan Total Pressure, evidently and logically, is the only proper basis for
the fan selection.
- Fan Static Pressure (Fs) is an unacceptable basis for fan selection. “Most importantly, remember that Ps is a defined term.” (ASHRAE Handbook,
Fundamentals). And “It cannot be read directly from the
calculation sheet." (Industrial Ventilation, SMACNA).
- As the essential measures for energy saving and motor safety, the fan manufacturers
must provide graphs illustrating System / Fan trade-offs: air capacity and total pressure, fan and motor curves, fan
and motor mechanical efficiencies, energy consumption, and noise levels.
