|
1) Introduction
Much confusion exists in
both the cooling tower and the fan industry concerning
field testing. There is no doubt that a need exists
for a simple, accurate, practical method of evaluating
the fan performance since it is a vital factor in overall
tower performance. Several fan manufacturers and several
tower manufacturers have written procedures, and both
CTI in their Bulletin ATP-105 and ASME in their PTC-23
have covered this subject. ASHARE also has a code for
small towers. All of these leave something to be desired.
When a cooling tower fails
to meet performance guarantees, one of the first items
to be investigated is the fan. Truthfully, there are
many other factors of equal or greater importance such
as water distribution, air distribution, amount of surface
and fill, etc. However, the fan flow is something that
can be measured after a fashion in the field, and generally
is a starting point for remedial work. There is no doubt
that this is not an easy task, nor that laboratory accuracy
can be maintained, but a reasonable job is possible.
Here the statement contained in the former fan test
code Bulletin AMCA-110 is subscribed briefly.
"Research undertaken
by the members of the Association in cooperation with
test code committees of the engineering societies thus
far that no accurate or practical method of testing
fans in the field has been developed by means of gas
analysis or by use of the Pitot tube or other instruments.
Since the values obtained from field tests vary so widely
from actual results, a guarantee of the performance
of fans can be made only from laboratory tests conducted
by the manufacturer in accordance with the Standard
Test Code for Centrifugal and Axial Fans."
It is true that performance
measured in the field should not be used to base fan
rating tables, but the industry needs a reasonable method
to evaluate fan performance on a tower, and the accuracy
can be held to reasonable limits.
2) Instruments
(1) Flow: Most authorities
suggest that on an induced draft cooling tower the fan
flow shall be measured on the fan discharge side. The
instrument to be used should be a calibrated, vane-type
anemometer. This type instrument used with a good stop
watch is less sensitive to the disturbing factors found
in field testing of induced draft towers than other
instruments now available. The instrument is accurate
at velocities from 200 - 3200 feet per minute which
cover the range required.
When the anemometer is held
normal to the plane of fan operation, it tends to minimize
the errors due to the angle of the air discharged. For
example, with an angle of yaw of 14o the
anemometer error is only 1%. A table is given under
procedure showing corrections for yaw angles up to 50o.
In addition, at the velocities found in a cooling fan
discharge, the effect of densities other than calibration
density (0.075 lb/ft3) are rather low. The
following table shows the toleration in density ratio
for a 1% variation in a accuracy:
| Velocity
(ft/min) |
Max.
value of d/da for 1% error |
| 300 |
1.05 |
| 600 |
1.10 |
| 1200 |
1.21 |
| 1800 |
1.32 |
(Note: d is ambient density
and da is calibration density.)
An extension rod and a device
for starting and stopping the instrument in addition
to the stop watch, and two men are a must! One man can
use the stop watch and log results while the other handles
the anemometer. The other commonly used flow measuring
devices are not recommended for tests such as these.
The Pitot tube is highly sensitive to yaw angle and
in addition, the small static side holes become filled
with water very rapidly. The hot wire anemometer is
useless in the wet air, as is the velometer. No attempt
will be made to go into the characteristics of all these
instruments. This does not mean that the anemometer
is a perfect instrument as it is slightly affected by
water droplets, and by pulsating air flow. It is the
best available at this time, and is the instrument to
use for this test method.
(2) Static Pressure: The
static pressure or resistance pressure under which the
flow is produced is very important. The values taken
beneath the fan are, of course, negative (lower than
atmosphere) and in most cases are all that need to be
taken. The pressure readings are very low, generally
0.60" water gauge or less, and for this reason
an inclined manometer is suggested. This gauge should
have a zero adjustment, a built-in level, a slope of
at least 10:1 and 0.01" graduation.
A colored gauge fluid of
a density commensurate with the gauge graduations should
be used to give direct readings in inches-water gauge.
Do not use a water filled U-tube. Remember an error
in reading a U-tube of just 1/32" on a 0.40"
measurement is an error of about 8% and cannot be tolerated.
Together with the manometer you will need a probe to
insert through the tower deck. This can be a length
of 1/4" tubing with the end smoothed and free from
burrs. This coupled with about 40 ft of good neoprene
tubing will enable you to leave the gauge standing in
one spot while you insert the probe in various locations.
(3) Power Input to Motor:
Only those applications where the prime mover is an
electric motor are considered here. This will cover
about 95% of the towers involved. With an electric motor,
the manufacturer? curves for efficiency can be used
for the purpose. This means that all we need is to get
the power input to the motor. If possible, a three phase
wattmeter or two single phase wattmeters can be used.
If neither of these is available, then a voltmeter,
ammeter, and power factor meter must be used. All meters
should have an accuracy of 1% of full scale reading
or better. Avoid the mistake of reading only volts and
amperes. The power factor changes radically with changes
in voltages away from the motor rating, particularly
with the new NEMA frame motors. in any case, reading
of volts and amperes only is very poor practice and
leads to high magnitude of error.
(4) Temperature: Mercury
type wet and dry bulb thermometers, with scale divisions
no greater than 1 oF should be used.
(5) Fan Speed: It is generally
very difficult to measure fan speed directly due to
the size of the units and actual physical limitations.
It is recommended that the motor shaft speed be measured
with a tachometer having an accuracy of 1% or better.
Then by using the gear ratio, the fan speed can be calculated.
It is possible to use a strobe light except with some
older models which do not go low enough in RPM and do
not have a good enough light source to make this desirable.
Newer models promise to remedy these difficulties, and
may prove to be satisfactory. They have an accuracy
of 1% of full scale reading.
(6) Barometric Pressure:
In most instances, the local power plant or weather
station can provide an accurate barometer reading for
the test. If this is not available an aneroid barometer,
temperature compensated, with scale readings not greater
than 0.02" can be used.
(7) Fan Blade Angle: This
can be measured at the point on the blade recommended
by the manufacturer by using a bubble protractor and
a straight edge. Care must be taken to establish any
deviation of the fan assembly from level and all blades
should be brought to the same location in the cell for
measurement.
2)Preliminary Observations
Before we discuss the actual
test procedure, it would be well to outline the preliminary
observations and precautions that should be taken.
(1) Wind Conditions: The
wind direction and velocity should be measured using
an anemometer, and noted on the test sheet using a sketch
of the cell to show relative direction. The wind is
the greater external influence on a test such as this
and a test should not be attempted under high wind or
gusty conditions. Tests should preferably be run in
the early morning or late afternoon when the wind is
lowest in velocity and reasonably steady. Good test
results cannot be obtained when the wind velocity is
greater than 75% of the average fan outlet velocity,
and should not be run in any case when the wind exceeds
10 MPH. Also, if the wind shifts direction over 45o
during the time of the flow traverse, the test should
be voided. If gusts occurs of a magnitude of over 5
MPH greater than the average wind velocity during flow
measurement, the test should be avoided.
(2) Fan Inspection: Be sure
that the face of the blade is one the leaving air side.
This precaution may sound absurd, but you can be assured
that some fans have been installed upside-down and running
backwards. Check fan tip clearance to be sure it satisfies
manufacturer? recommendations. This will not effect
the test procedure as such, but if clearances are excessive
you will never be able to approach the tunnel test performance
of the fan with the field values measured.
Make a sketch of the location
of the fan relative to the cell and measure the inside
diameter of the fan stack for at least two points 90o
apart at the plane of flow measurement. Use the average
diameter to calculate the area of the stack. Make sure
the eliminator baffles beneath the fan are not clogged
or broken. If a guard is placed above the fan it is
best to measure the resistance to flow of such a guard,
and if it is left in place you will have to estimate
the static pressure loss to attribute to it in the final
fan analysis.
To be continued. Please press the next button....
|
