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There are two efficiencies being considered in the fans.
Note that the fan efficiencies rated on fan data sheet
may be reduced due to poor balance of blade, high tip
clearance, incorrect tract of each blade, dirty surface
of blade, obstructions to air flow, plenum geometry
and other factors which were described in Chapter 2.
Also the effect of fan speed at the constant pitch on
cooling tower and the thermal performance depends previously
on the effect of fan efficiency.
S.P. act. (inch Aq.) x Airflow(ft3/min)
Static Effi. = ----------------------------------------------------------
6356 x HP act. (HP)
T.P. act. (inch Aq.) x Airflow(ft3/min)
Total Effi. = ---------------------------------------------------------
6356 x HP act. (HP)
Where, HP act. = Actual break
horsepower obtained from performance curve.
S.P. Act. = Actual static pressure @ given air density
T.P. Act. = Actual total pressure @ given air density
6356 = Unit Conversion Correction Coefficient
(1" Aq. = 5.2 lb/ft2, 1 HP = 33000 lb-ft/min.
Acc'ly, 5.2 press. (in Aq.) / [33000 x HP act. (HP)]
= 1/6356)
The Hudson fan performance
curves are the result of tests run in accordance with
Fig. 13 of Standard 210-74 "Laboratory Methods
of Testing Fans for Rating" adopted by AMCA (Air
Moving and Conditioning Association, Inc.) The test
conditions to obtain performance curves is attached
hereafter. Actual fan efficiencies will be different
from the test conditions unless the actual environment
is equivalent to the test conditions. There are some
factors to rob the fan system of efficiency. The methods
how to improve them shall be suggested here.
1) Fan System Efficiency
When we design an air moving
device one of the most important tools we use is the
fan performance curve. Using this curve of fan performance
we plot a system resistance line to establish an operating
point at which the fan performance exactly matches the
system requirements. From the operating point we can
define the fan pitch and power requirements. With almost
any fans the pitch can be changed from the original
estimate, if airflow is too low, to a higher pitch and
greater flow. However, if the system efficiency or losses
are not as assumed, more air, horsepower increases by
the cube of the flow is needed. A ten percent increase
in flow requires a thirty-three [HP2 = (Q2
/ Q1)3 x HP1 = 1.13
HP1 = 1.331 x HP1, so 33.1% increase
to HP1) percent increase in horsepower.
Fan performance curves generally
are obtained under ideal, reproducible conditions. The
Engineering Test Lab at Texas A&M's Research and
Extension Center is the only independent test laboratory
in U.S.A. with an AMCA certified wind tunnel. The lab
tests everything from kitchen ventilators to scale model
60 feet diameter fans. The test conditions for high
performance axial fans usually require blade tip clearance
on a five foot model of about 0.04 of an inch with a
large inlet bell conditions as ideal as possible. As
a result of good aerodynamic design and minimized losses,
total efficiencies are generally in the 75 to 85% range.
However, from experience
with many full scale fan tests it is rare that "real
life" performance exceeds 55 to 75% total efficiency.
The difference is in "Fan System Efficiency".
Although the fan efficiency is exactly same, the system
efficiency is greatly different. Sometimes we find its
capability is sadly insufficient, requiring expensive
field modifications. What most likely caused the problem?
Generally, tip losses, reverse flow at the fan hub and
recirculation loss as below figure.
To refresh your memory as
to terminology, the head or total pressure that an axial
fan works against is made up of two components. These
are static pressure which is the sum of the system resistance
and velocity pressure which is a loss associated with
accelerating the surrounding air from zero to the design
velocity. The only useful work done is by the static
pressure component. That is measured in inches of water
and an axial fan normally works in the range of 0 to
2 inches total pressure.
2) System Losses
The holes in the Bucket.
Potential losses in system efficiency occurs in several
areas:
(A) Losses caused by the
fixed system design rather than by variable physical
properties. Once the operating point of the fan is fixed
these losses are built-in and cannot be easily detected
or corrected. They are losses because they rob the system
of potential efficiency. Examples of this type of system
"loss" would be: choice of fan design, fan
diameter selection, fan design operating point.
(B) Losses caused by "variable
environmental properties" would be: lack of fan
hub seals, excessive fan tip clearance, poor inlet conditions
of the fan stack, excessively high approach velocity
to the fan, or random air leaks in the fan plenum. Often
allowance for losses in louvers, bug screens, etc. are
simply omitted in design.
(C) Other performance losses
could occur because of hot air recirculation.
Of the above losses, the
only easily corrected problems would be in category
which we call "variable environmental properties."
In the following discussion category (A)will be covered
in The Fan Itself. Category (B) will be discussed in
The Fan Housing and (C) will be covered under Unwanted
Air Movements respectively in below.
(1) The Fan Itself: The ways
a fan system could be inefficient are sometimes obvious
but most of the time they are not. For instance, the
blade design itself is a major factor. Modern axial
fans are usually made by molding fiberglass or extruding
aluminum. Extruded aluminum blades are by nature always
of uniform chord width while molded fiberglass blades
can have an irregular shape. One of the basic design
criteria for blade design is to produce uniform air
flow over the entire plane of the fan. One of the aerodynamic
principles involved is that the work done at any radius
along the blade is a function of blade width, angle
of attack and tangential velocity squared. The "angle
of attack" in airfoil design dictates the amount
of blade twist required at any particular radius along
the blade.
It follows that as a point
on the blade decreases from tip toward the hub the tangential
velocity sharply decreases and in order to produce uniform
airflow, the blade width and twist must be increased.
If the blade chord cannot be increased in width, the
twist must be increased to compensate. With an extruded
blade the twist is created by mechanically yielding
the blade to a prescribed degree. Due to limits in elasticity
only limited twist can be created. In a molded blade
there is no such limitation to chord width or twist
so the ideal blade can be more closely approached.
The point is, that the blade
design itself can create problems of non-uniform air
flow and inefficiency. Another inherent property of
an axial fan is the problem of "swirl" which
is the tangential deflection of the exit air direction
caused by the effect of torque. The air vectors at the
extreme inboard sections of the blade actually reverse
direction and subtract from the net airflow. This is
a very measurable quantity. Swirl can be prevented with
an inexpensive hub component, which usually covers the
inner 25 -30% of fan dia. The hub seal disc prevents
this and should be standard equipment on any axial fan.
A real example that illustrates
performance differences due to blade shape and seal
disc usefulness is shown in above figure. This data
was obtained by a major cooling tower manufacturer who
carefully measured air flow magnitude and direction
across a blade in a full scale cooling tower. Curve
"A" shows the performance of an extruded aluminum
straight type blade with no hub seal disc. Curve "B"
shows performance of a tapered fiberglass blade with
a seal disc. Both 20 feet diameter fans were tested
under identical loading conditions of horsepower and
speed. Note that significant negative air flow occurs
at approximately the 40 percent chord point on the straight
blade but no negative flow was found with the tapered
blade.
Another component of the
fan system efficiency would have to be the fan operating
point where the system resistance line meets the fan
performance line at the desired air flow defines a fan?
operating point. At this point, the fan? output exactly
meets the air-flow and pressure-drop requirements. Such
a point will be represented by only one specific blade
pitch angle, actual ft3/min and total pressure
air output, and fan rpm.
This would be the particular
blade pitch angle that produces the desired air flow
against the required system resistance. This pressure
capability and flow is a function of the fan tip speed.
For a certain fan speed, only one pitch angle will satisfy
the system design operating condition. This fan operating
point will have a discrete efficiency. However, efficiency
varies as much as 10 - 15 percent from pitch angle to
pitch angle and even along the usable portion of each
pitch. An usable portion of curve means beyond the "stall"
conditions. This "stall" condition is easily
discernible on the fan curve and is analogous to cavitation
on a pump: it consumes a lot of energy but produces
no work.
The most obvious thing to
check pertaining to operating point is whether the fan
is "stalled". If a poorly operating fan is
suspected of stall, try lowering blade pitch and see
if the static pressure (measured with a water manometer)
in the plenum changes. If the pressure does not change,
the fan may be stalled. A stalled fan draws more horsepower
with increasing pitch, but air flow and static pressure
may actually decrease.
To be continued. Please press the next button..... |
