Sensing Static Pressure
In air conditioning, heating and
ventilating work, it is helpful to understand the techniques
used to determine air velocity. In this field, air velocity
(distance traveled per unit of time) is usually expressed
in feet per minute (FPM). By multiplying air velocity by
the cross section area of a duct, you can determine the
air volume flowing past a point in the duct per unit of
time. Volume flow is usually measured in cubic feet per
minute (CFM).
Velocity or volume measurements can often be used with engineering
handbook or design information to reveal proper or improper
performance of an airflow system. The same principles used
to determine velocity are also valuable in working with
pneumatic conveying, flue gas low and process gas systems.
However, in these fields the common units of velocity and
volume are sometimes different from those used in air conditioning
work.
To move air, fans or blowers are usually used. They work
by imparting motion and pressure to the air with either
a screw propeller or paddle wheel action. When force or
pressure from the fan blades causes the air to move, the
moving air acquires a force or pressure component in its
direction or motion due to its weight and inertia. Because
of this, a flag or streamer will stand out in the air stream.
This force is called velocity pressure. It is measured in
inches of water column (w.c.) or water gage (w.g.). In operating
duct system, a second pressure is always present. It s independent
of air velocity or movement. Known as static pressure, it
act equally in all directions. In air conditioning work,
this pressure is also measured in inches w.c.
In pressure or supply systems, static
pressure will be positive on the discharge side of the fan.
In exhaust systems, a negative static pressure will exit
on the inlet side of the fan. When a fan is installed midway
between the inlet and discharge of a duct system, it is
normal to have a negative static pressure at the fan inlet
and positive static pressure at its discharge.
Total pressure is the combination of
static and velocity pressures, and is expressed in the same
units. It is an important and useful concept to us because
it is easy to determine and, although velocity pressure
is not easy to measure directly, it can be determined easily
by subtraction static pressure from total pressure. This
subtraction need not be done mathematically. It can be done
automatically with the instrument hook-up.
Sensing Static Pressure
For most industrial and scientific
applications, the only air measurements needed are those
of static pressure, total pressure and temperature. With
these, air velocity and volume can be quickly calculated.
To sense static pressure, six types of
devices are commonly used. These are connected with tubing
to a pressure indicating instrument. Fig. 1-A shows a simple
thru-wall static pressure tap. This is a sharp, burr free
opening through a duct wall provided with a tubing connection
of some sort on the outside. The axis of the tap or opening
must be perpendicular to the direction of flow. This type
of tap or sensor is used where air flow is relatively slow,
smooth and without turbulence. If turbulence exists, impingement,
aspiration or unequaled distribution of moving air at the
opining can reduce the accuracy of readings significantly.
Fig. 1-B shows the Dwyer®
No. A-308 Static Pressure Fitting. Designed for simplified
installation, it is easy to install, inexpensive, and provides
accurate static pressure sensing in smooth air at velocities
up to 1500 FPM.
Fig. 1-C shows a simple
tube through the wall. Limitations of this type are similar
to wall type 1-A.
Fig. 1-D shows a static
pressure tip which is ideal for applications such as sensing
the static pressure drip across industrial air filters and
refrigerant coils. Here the probability of air turbulence
requires that the pressure sensing openings be located away
from the duct walls to minimize impingement and aspiration
and thus insure accurate readings. For a permanent installation
of this type, the Dwyer® No. A-301 or A-302 Static Pressure
Tip is used. It senses static pressure through radially-drilled
holes near the tip and can be used in air flow velocities
up to 12,000 FPM.
Fig. 1-E shows a Dwyer®
No. A-305 low resistance Static Pressure Tip. It is designed
for use in dust-laden air and for rapid response applications.
It is recommended where a very low actuation pressure is
required for a pressure switch or indicating gage - or where
response time is critical.
Under field conditions,
air turbulence in a duct or plenum often makes it impossible
to quickly install and align a rigid static pressure sensor
to take accurate readings. Under these circumstances, the
Dwyer® Trail-Tail® Static Pressure Sensor (Fig.
1-F), can be quickly inserted through a small hole in the
duct and will trail into automatic alignment with the air
stream. The pressure sensing holes in this device are thus
presented at a 90° angle to actual air flow assuring
quick, consistent, accurate readings.
Measuring Total Pressure
and Velocity Pressure
In sensing static pressure we make
every effort to eliminate the effect air movement. To determine
velocity pressure, it is necessary to determine these effects
fully and accurately. This is usually done with an impact
tube which faces directly into the air stream. This type
of sensor is frequently called a "total pressure pick-up"
since it receives the effects of both static pressure and
velocity pressure.
In Fig. 2, note that
separate static connections (A) and total pressure connections
(B) can be connected simultaneously across a manometer (C).
Since the static pressure is applied to both sides of the
manometer, its effect is canceled out and the manometer
indicates only the velocity pressure.
To translate velocity
pressure into actual velocity requires either mathematical
calculation, reference to charts or curves, or prior calibration
of the manometer to directly show velocity. In practice
this type of measurement is usually made with a Pitot tube
which incorporates both static and total pressure sensors
in a single unit.
Essentially, a Pitot
tube consists of an impact tube (which receives total pressure
input) fastened concentrically inside a second tube of slightly
larger diameter which receives static pressure input from
radial sensing holes around the tip. The air space between
inner and outer tubes permits transfer of pressure from
the sensing holes to the static pressure connection at the
opposite end of the Pitot tube and then, through connecting
tubing, to the low or negative pressure side of a manometer.
When the total pressure tube is connected to the high pressure
side of the manometer, velocity pressure is indicated directly.
See Fig. 3.
Since the Pitot tube
is primary standard device used to calibrate all other air
velocity measuring devices, it is important that great care
be taken in its design and fabrication. In modern Pitot
tubes, proper nose or tip design - along with sufficient
distance between nose, static pressure taps and stem - will
minimize turbulence and interference. This allows use without
correction or calibration factors. All Dwyer® Pitot
tubes are built to AMCA and ASHRAE standards and have unity
calibration factors to assure accuracy.
To insure accurate velocity
pressure readings, the Pitot tube tip must be pointed directly
into (parallel with) the air stream. As the Pitot tube tip
is parallel with the static pressure outlet tube, the latter
can be used as a pointer to align the tip properly. When
the Pitot tube is correctly aligned, the pressure indication
will be maximum.
Because accurate readings
cannot be taken in a turbulent air stream, the Pitot tube
should be inserted at least 8-1/2 duct diameters downstream
from elbows, bends or other obstructions which cause turbulence.
To insure the most precise measurements, straightening vanes
should be located 5 duct diameters upstream from the Pitot
tube.
How to Take Traverse Readings
In practical situations, the velocity
of the air stream is not uniform across the cross section
of a duct. Friction slows the air moving close to the walls,
so the velocity is greater in the center of the duct.
To obtain the average total velocity
in ducts of 4" diameter or larger, a series of velocity
pressure readings must be taken at points of equal area.
A formal pattern of sensing points across the duct cross
section is recommended. These are known as traverse readings.
Fig. 4 shows recommended Pitot tube locations for traversing
round and rectangular ducts.
In round ducts, velocity
pressure readings should be taken at centers of equal concentric
areas. At least 20 readings should be taken along two diameters.
In rectangular ducts, a minimum of 16 and a maximum of 64
readings are taken at centers of equal rectangular areas.
Actual velocities for each area are calculated from individual
velocity pressure readings. This allow the readings and
velocities to be inspected for errors or inconsistencies.
The velocities are then average.
By taking Pitot tube
readings with extreme care, air velocity can be determined
within an accuracy of ±2%. For maximum accuracy,
the following precautions should be observed:
• Duct diameter
should be at least 30 times diameter Pitot tube.
• Located the Pitot tube section providing 8-1/2 or
more duct diameters upstream and 1-1/2 or more diameters
down stream of Pitot tube free of elbows, size changes or
obstructions.
• Provide an egg-crate type of flow straightener 5
duct diameters upstream of Pitot tube.
• Make a complete, accurate traverse.
In small ducts or where traverse operations are otherwise
impossible, an accuracy of ±5% can frequently be
achieved by placing Pitot tube in center of duct. Determine
velocity from the reading, then multiply by 0.9 for an approximate
average.
Calculating Air Velocity
from Velocity Pressure
Manometers for use with a Pitot
tube are offered in a choice of two scale types. Some are
made specifically for air velocity measurement and are calibrated
directly in feet per minute. They are correct for standard
air conditions: i.e. air density of .075 lbs. Per cubic
foot which corresponds to dry air at 70°F, barometric
pressure of 29.92 inches Hg. To correct the velocity reading
for other than standard air conditions, the actual air density
must be known. It may be calculated if relative humidity,
temperature and barometric pressure are known.
Most manometer scales are calibrated
in inches of water. Using readings from such an instrument,
the air velocity may be calculated using the basic formula:
With dry air at 29.9
inches mercury, air velocity can be read directly from curves
on the following page. For partially for fully saturated
air a further correction is required. To save time when
converting velocity pressure into air velocity, the Dwyer®
Air Velocity Calculator may be used. A simple slide rule,
it provides for all the factors needed to calculate air
velocity quickly and accurately. It is included as an accessory
with each Dwyer® Pitot tube.
To use the Dwyer®
Calculator:
• Set relative
humidity on scale provided. On scale opposite known dry
bulb temperature, read correction factor.
• Set temperature under barometric pressure scale.
Read density of air over correction factor established in
#1.
• On the other side of calculator, set air density
reading just obtained on the scale provided.
• Under Pitot tube reading (velocity pressure, inches
of water) read air velocity, feet per minute.
Determining Volume Flow
Once the average air velocity is
know, the air flow rate in cubic feet per minute is easily
computed using the formula:
Q = AV
Where: Q = Quantity of flow in cubic feet per minute.
A = Cross sectional area of duct in square feet.
V = Average velocity in feet per minute.
Determining Air Volume by
Calibrated Resistance
Manufacturers of air filters, cooling
ad condenser coils and similar equipment often publish data
from which approximate air flow can be determined. It is
characteristic of such equipment to cause a pressure drop
which varies proportionately to the square of the flow rate.
Fig. 5 shows a typical filter and a curve for air flow versus
resistance. Since it is plotted on logarithmic paper, it
appears as a straight line. On this curve, a clean filter
which causes a pressure drop of .50" w.c. would indicate
a flow of 2,000 CFM.
For example, assuming manufacturer's
specification for a filter, coil, etc.:
Other Devices
for Measuring Air Velocity
A wide variety of devices are commercially
available for measuring air velocities. These include hot
wire anemometers for low air velocities, rotating and swinging
vane anemometers and variable area flowmeters.
The Dwyer® No. 460
Air Meter is one of the most popular and economical variable
area flowmeter type anemometers. Quick and easy to use,
it is a portable instrument calibrated to provide a direct
reading of air velocity.
A second scale is provided
on the other side of the meter to read static pressure in
inches w.c. The 460 Air Meter is widely used to determine
air velocity and flow in ducts, and from supply and return
grilles and diffusers. Two scale ranges are provided (high
and low) with calibrations in both FPM and inches w.c.
To Check Accuracy
Use only devices of certified accuracy.
All anemometers and to a lesser extent portable manometers
should be checked regularly against a primary standard such
as a hook gage or high quality micromanometer. If in doubt
return your Dwyer® instrument to the factory for a complete
calibration check at no charge.
TO
AIR VELOCITY FLOW CHARTS
