Perform a Traverse
- Step One: Find an acceptable test site in the duct you want to traverse.
- Step Two: Determine test port location and install them with a drill. Step Three: Use a thermal anemometer to measure multiple velocity readings in a grid pattern. ...
- Step Six: Plug the average velocity and duct area (in sq. ft.) into the CFM formula. Voila! ...
What is a traverse HVAC?
0:145:48Performing a Duct Traverse - YouTubeYouTubeStart of suggested clipEnd of suggested clipOften air velocity will not be distributed evenly across a section of the duct. So the preferredMoreOften air velocity will not be distributed evenly across a section of the duct. So the preferred method is to take multiple measurements at different points throughout the cross.
What are duct traverse readings?
To get an average air velocity, a formal pattern of sensing points is taken over the duct cross section. These are known as traverse readings. There are two main patterns for traverse readings: equal-area and log-linear.
What is pitot tube traverse?
It is a Pitot-static tube that mounts in the working section of the wind tunnel, either upstream or downstream of the position of the test model. This allows students to do wake traverses, downstream of a model.
What is a velocity traverse?
A traverse is a series of measurements used to determine the average velocity of air moving through an opening. Air velocity is measured in a grid pattern through a variety of openings.
What is the good rule of thumb in sizing ductwork?
One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.
How far apart should duct take offs be?
two feetIt requires about 24′ or two feet to do this. This is the 'Two-Foot Rule?. Ideally, you should allow at least 24′ between takeoffs and at the end of any duct run.
Why is it called a pitot tube?
The pitot tube is a classic fluid dynamic sensor named for its inventor, Henri Pitot, who in the 18th century developed it to measure the speed of rivers and canals in France.
How do you check air duct flow?
An anemometer, a test instrument that measures air velocity is used to determine the average airspeed in the duct. Then the average feet per minute is multiplied by the area of the duct in square feet to determine the airflow moving through the duct.
Why are pitot tubes still used?
Pitot tubes are critical instruments Measuring airspeed is an indispensable part of flying, and essential for all pilots to keep a gauge of. Pitot tubes provide readings for indicated airspeed (IAS), based on measuring the dynamic pressure.
What is an example of a traverse?
Traverse is defined as something that lies across or crosses over. An example of a traverse is a zigzag route down a steep hill taken by a skier. An example of a traverse is a log laying across a road.
How is traverse point calculated?
In circular stack, traverse points are located at the centre of equal annular across two perpendicular diameters. In case of rectangular stacks, the area divided into 12 to 25 equal areas and centres are fixed for each square. The traverse should be carried out at least on nine squares on at least three lines.
What is the difference between Traverse and traversing?
A traverse is a series of connected lines whose lengths and directions are to be measured and the process of surveying to find such measurements is known as traversing.
How is duct traverse measured?
The preferred method is to drill 3 holes in the duct at 60° angles from each other as shown in figure 1 below. Three traverses are taken across the duct, averaging the velocities obtained at each measuring point. Then the average velocity is multiplied by the duct area to get the flow rate.
How much duct leakage is normal?
According to the EPA, the typical home with a forced air HVAC system loses between 20 and 30 percent of its conditioned air due to duct leakage.
What is traverse distance?
A traverse is a continuous series of connected lines of known lengths related to one another by known angles. The lengths of the lines are determined by direct measurement of horizontal distances, slope measurement, or by indirect measurement using the methods of stadia or the subtense bar.
What percentage of air leakage from ducts is considered acceptable?
In summary, the standard requires that each duct system of 3000 L/s or more be tested to at least 1.25 times its operating pressure, and not leak more than 5% of the design air quantity of the duct system.
What is duct traverse?
Performing a duct traverse is a fundamental skill that air balancers need to master. It’s also a key skill for HVAC technicians who want to measure and improve system performance for their customers. With the different instruments available for performing a traverse, it can be confusing to know when to use which instrument.
Who invented the Pitot tube?
These and other limitations persisted for more than 120 years, relegating the Pitot tube to ‘little more than a scientific toy’ until another French engineer, Henry Darcy, systematically improved the design of the Pitot tube.
Can a Pitot probe measure flow?
A straight Pitot or airfoil probe can also measure flow in both directions, so you’ll know if your measuring a flow profile that isn’t developed enough for an accurate measurement. Thermal anemometers can’t be used for diagnosing your measurements like this.
Can you rotate a Pitot tube?
One aspect to be aware of when using a thermal anemometer is that it won’t indicate flow direction. With a Pitot tube you can rotate the probe to find where the reading is highest, and the position of the probe indicates the direction of the flow.
Do you need a Pitot tube or thermal anemometer?
If you’re a professional balancer working on a wide variety of systems, you’ll probably want to have both a Pitot tube setup and a thermal anemometer. This will allow you to tailor your tool to your specific measurement application, always selecting the instrument that will provide the best outcome.
What is a duct traverse?
Many circumstances warrant measuring air velocities or air flow, and a duct traverse is the most precise method of obtaining that information. A duct traverse consists of a number of regularly spaced air velocity and pressure measurements throughout a cross sectional area of straight duct (see Figure 1).
How many straight equivalent duct diameters are there in a traverse?
The preferred location of the traverse in a supply duct should be in a straight section of duct with 10 straight equivalent duct diameters upstream, and 3 straight equivalent duct diameters downstream of the traverse plane, although a minimum of 5 duct equivalent diameters upstream and 1 duct equivalent diameter downstream may give adequate results.
How many measurements are required for a circular duct?
However, circular ducts require measuring across 3 diameters (Figure 1), with a minimum of 6 measurements per diameter . Multiply the number of points you’ll measure with the figure in the second half of table 1 to determine the measuring position relative to the inner wall of the duct. 1.
How many traversal points are needed for ducts?
For duct sides shorter than 30”, five traversal points must be taken (5 on each side, 5*5=25).
What port to connect a pressure tube to?
Connect the total pressure tube to the 922 “ ” port, and connect the static pressure tube to the “-” port.
Where to position Pitot static tube tip?
Position the Pitot-Static tube tip within the duct at the first traverse point.
How to find velocity of ducts?
For rectangular ducts using the equal-area method, the cross section is divided into small rectangles of equal area. The velocity is taken at the center point of each of these rectangles and then averaged to get the duct velocity.
Where are the reading points in ductwork?
The reading points are the center of each of the rectangles. For round ducts using the equal-area method, the velocity pressure should be taken at the center of equal concentric areas and averaged. In the example shown, two holes are drilled in the centerline of the duct.
What causes turbulence in ductwork?
Fans, corners, duct fittings, tees, and coils all create turbulence in the duct, further changing the velocity over the cross section. Readings should always be taken with at least 8 and 1/2 duct diameters upstream and 1 and 1/2 duct diameters downstream from any device or intrusion that can cause turbulence.
How to calculate velocity with Pitot tube?
If using a Pitot tube, the velocity is directly proportional to the velocity pressure and can be calculated using the formula shown for air with V (velocity), d (density of air in the application), and hv (the velocity pressure from the measuring device).
How to get average air velocity?
To get an average air velocity, a formal pattern of sensing points is taken over the duct cross section. These are known as traverse readings. There are two main patterns for traverse readings: equal-area and log-linear. For rectangular ducts using the equal-area method, the cross section is divided into small rectangles of equal area.
How many reading points for duct?
If the duct diameter is greater than 14 inches, then use 20 reading points; if it is between 10 and 14 inches in diameter, use 16 reading points; and if it is less than 10 inches in diameter, use 10 reading points with half the points in each reading plain. The reading plains do not have to be vertical and horizontal, but they do need to be 90 degrees from each other. Use the formulas shown for the insertion depth of each reading according to the duct radius.
How to find volume flow rate?
From the velocity, it is easy to calculate volume flow rate where flow rate Q is equal to the velocity multiplied by the cross sectional area of the duct or pipe.
What is the easiest duct material to traverse?
The easiest duct material to perform a traverse on is sheet metal. Common duct configurations to traverse are rectangular, round, and oval duct. Each has its own method for a proper traverse that we’ll discuss later. Flexible and fiberglass duct can present challenges if you attempt to traverse them.
What is the second method of measuring ductwork?
The second style is called the “Tchybecheff” method. This method uses a lot of math to calculate test port and traverse sites.
What is traverse in air balancer?
A traverse is a series of measurements used to determine the average velocity of air moving through an opening.
How long is a 12 inch duct?
The ideal length is 10 duct diameters. To give you an idea of how long this is, if you have a 12-inch round duct, it would require a straight length of 10 feet, or 120 inches, to be traversed by the book. It can be hard, if not impossible, to find ducts that meet these requirements, so many simply say it can’t be done.
What instrument do you use to measure traverse?
As your skills increase, explore other test instrument options, such as a manometer and pitot tube or velocity grid, to perform traverse measurements at other locations.
How to find airflow in a duct?
But once you combine it with a simple formula, you can determine airflow. The formula is: area x velocity = cfm.
How many velocity readings are needed for a duct?
It is mainly used when specified by an engineer and requires dedication to master. Once your test locations are set, you’ll obtain between 12-96 velocity readings inside the duct. A good rule to remember is the more readings you take, the greater your chances are for an accurate traverse.
