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The following procedures are intended to document the
conventional methods of measuring airflow. We realize
that there are much more highly technical methods available.
We also realize that our members have varied needs,
some simple, some extremely demanding. This treatise
is intended to assist the greatest number of people
to be knowledgeable of several current ways to measure
the air flow from their fans.
There are two basic methods
for air flow testing of cooling towers: Anemometer or
Pitot Tube. The purpose of each is to measure the velocity
at a point. After a number of points are measured to
obtain a representative or average velocity, the total
airflow is calculated using the net discharge area of
the fan. Each of these methods is discussed below.
1) Anemometer Method
The anemometer is basically
a calibrated propeller. As the air turns the propeller,
its speed of rotation is counted mechanically or electrically
measured by a D.C. motor.
(1) The oldest design is
a vane anemometer, which has a mechanical readout of
feet per minute velocity of air which passes through
the anemometer over a period of time, usually a 30 second
interval. In use, the anemometer is held in the air
stream on a pole at the prescribed spot. A timer calls
"start", and the operator pulls a string to
start the anemometer. At 30 seconds, the timer calls
"stop". The operator retrieves the anemometer,
reads the thousands, hundreds and unit counts. He records
the reading, resets the counter and moves to the next
spot. The readings are later adjusted using a calibration
curve and doubled to give a feet per minute reading.
As you can see, this instrument is clumsy and takes
two skilled people to make repeatable measurements.
Note that if a "yaw" measurement was needed,
the operator would have to read or estimate and record
the yaw angle as well as the counter reading for each
spot.
(2) Today there are many
electronic direct reading anemometers available although
accuracy and moisture resistance varies with cost. A
typical instrument displays velocity in ft/min or m/sec
and temperature oF or o C. It
averages 20 readings over 2 second intervals and can
save readings which can be dumped into a standard RS232C
interface of a computer. It is battery powered and an
extension cord is available.
(3) An upscale version would
be a propeller anemometer which has a digital readout
based on an analog D.C. voltage proportional to the
wind velocity. This is an unshrouded propeller with
a tiny generator which can be attached to a pole with
a standard 3/4" NPT pipe thread.
2) Pitot Tube Method
The pitot tube measures velocity
pressure as the difference between total and static
pressure at a point. This pressure is measured with
an inclined manometer or an electronic pressure transducer.
Temperature must also be measured so the density at
each point can be computed. Once density and velocity
pressure are measured at a point the velocity can be
computed by the formula:
V = 1096.2 x C x (VP/D)1/2
Where,
1096.2 = Unit conversion factor
C = Pitot tube calibration coefficient
VP= Local velocity pressure (inches - H2O)
V= Local velocity (ft/min)
D= Local Air Density (lbm/ft3)
For wet tower a special pitot
tube with larger than normal holes in the nose are required.
The purpose of the large holes is to prevent water droplets
from clogging the tube, causing erroneous readings.
There are several types of pitot tubes available, some
very sophisticated of high velocity measurements. For
the general purposes, velocity pressures will typically
be less than 0.5 inches-H2O.
For fan testing in moist
air, the Prandtl type design is recommended. However,
other types could also be adapted for cooling tower
use. A pitot tube developed for cooling tower use should
first be calibrated against a well designed pitot tube
in a dry air situation. This will allow any correction
factors necessary to account for the true performance
of the modified tube to be determined.
(1) Pressure Measurement:
The velocity pressures which must be measured in a cooling
tower application are typically less than 0.5 in-H2O.
Impact and static pressures are somewhat higher and
can occasionally exceed 1.0 in-H2O. Their
pressures fluctuate rapidly due to the turbulence of
the flow and the proximity of the measurement to the
fan blades. This makes some method of dampening advisable.
Any pressure measurement
device suitable to these conditions can be used. A simple
inclined manometer is often used. Pressure gauges and
electronic pressure transducers have also been used
successfully. The pressure measurement device should
be checked against a reliable standard before the test
and at least once during the test.
When pressure measurement
is desired in addition to flow measurement, a system
of valves is often used to connect the various pressure
signals to the measurement device. These valves can
be operated either manually or electrically.
(2) Temperature Measurement:
Air temperature should be measured with an electronic
temperature sensor attached to the pitot tube. Hand
held thermistor types work for this purpose. Air temperature
is measured to determine the density of the air. In
cases where saturation cannot be assumed, it may be
necessary to measure both wet and dry bulb temperature.
In these cases, a wetted wick must be provided for the
wet bulb sensor.
(3) Barometric Pressure Measurement:
Barometric pressure can be measured by any suitable
means. Mercury manometers are often used. Electronic
absolute pressure transducers have also been used.
(4) Other Apparatus: The
pitot tube should be attached to a light aluminum tube
with sufficient strength and length to traverse the
radius of the fan cylinder. This tube is usually supported
by the fan cylinder itself and by a tripod mounted on
the fan deck, the pitot tube should be connected to
the pressure measurement device by flexible hoses. The
temperature sensor must be connected to the output device
by a suitable length of wire. The wire and hose should
be firmly attached to the aluminum tube to prevent their
being entangled in the fan or other cooling tower structure.
3) Where to Measure
Generally there is a precise
distance from the edge of the fan ring each of four
quadrants of the inlet or discharge area where measurements
are taken. A minimum number of points for a fan would
be 21, four points in each of five equal area bands
plus one point at the center. These points are calculated
on the basis of fan diameter and seal disc diameter.
These measurements are all in one "plane".
Where that plane of measurement is chosen is another
problem.
Imagine a fan discharging
air. On the inlet side, the air flow is fairly uniform
except around beams or other obstacles. On the discharge
side the direction is significantly changed and at the
center, airflow becomes negative but is blocked by the
fans seal disc or hub. The air at the blade tips is
still axial but the air vectors near the center of the
fan are progressively "bent over" by the effect
of the torque applied by the fan blades. At the center
of the fan, the air actually wants to reverse direction
180o. This is called the "swirl effect".
The net effect would be turbulence close to the center
of the fan on the discharge side.
Also, consider if the fan
has a short fan ring or a tall velocity recovery stack.
The closer the measurement to the fan the less swirl
effect is seen. For a tall stack, there is a cone of
swirling air which travels at least two fan diameters
before all discharge air becomes uniform (axial) again.
Using a pitot tube close
to the exit or inlet side of the fan may require drilling
four holes approximately 1-1/2" diameter at 90o
through the stack for access. Plug the holes with a
rubber plug or other means after the test is completed.
(1) Yaw: The fan imparts
a horizontal and a vertical component to the air. The
angle produced by these two components is the "YAW"
angle. The vector resolution of these components yield
the actual air velocity. However, to correctly determine
the air flow leaving the tower, only the vertical components
of the velocity need be measured. This can be accomplished
by two methods:
- ● Position the anemometer
horizontally, therefore only the axial (vertical)
component of flow goes through the propeller.
- ● Tie a string or streamer
onto the anemometer rod. The streamer will show the
"local" velocity direction so if you hold
the anemometer perpendicular to the streamer it will
measure the air velocity in it's "yaw" direction.
You then must read the yaw angle from a protractor
on the anemometer handle and record it, along with
the velocity read. The axial or vertical component
will be:
V A = V yaw x Cos q
where,
V A = Axial (vertical) Velocity
V yaw = Velocity measured in the direction
of the yaw angle.
Cos q = Cosine of yaw angle
This adds considerably to the time consumed per test
but does add a few percent accuracy to the results.
This would apply to either an anemometer or pitot
tube if measurements are taken on the discharge side
of the fan. If you can use a pitot tube on the inlet
side of fan (with or without a VR stack) several errors
are avoided.
- ○ Inlet air, close to
the plane of the fan is rather uniform - therefore
no yaw is present.
- ○ The confusion of the
effect of swirl of yaw at the discharge, at the
top of the stack is avoided. The Net Free Area,
free of swirl, is well defined. A "caveat"
would be that you must not measure within 12 inches
of a beam under the fan because of turbulence
which would affect the reading. Better to move
over into clear air.
(2) Recommended Measurement
Locations: First choice (with or without V.R. Stack):
In the fan inlet as close as possible to the fan. Second
choice (Short Fan Ring): Over the fan with an anemometer
or pitot making yaw corrections. Third choice (V.R.
Stack): At the top of the stack, making yaw corrections.
Net Free Area is considered from the edge of the stack
to the edge of the cylinder formed by the seal disc
diameter.
Obviously the location you
choose will be tempered by the tower configuration being
measured, the equipment you have and the time allotted
for the test. If you are just comparing two fans, the
important thing is consistency. Measure them both exactly
the same way. You may or may not choose to measure yaw
angles. You may not have pitot equipment. You may not
be able to drill four holes in the stack to get under
the fan.
Your air flow measurement
may not have to be absolute, but you're looking for
relative differences in performance between fan A and
fan B. All these things can effect your measurement
location and method of measurement.
To be continued. Please press the next button....
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