TAB Resources and a Vintage SMACNA Manual
TAB stands for Testing, Adjusting and Balancing. There are a number of TAB related resources available on this page including:
In addition, the presentations from the Pacific Energy Center Design, Performance, and Commissioning Issues classes on Fans, Ducts, Pumps and Pipes contain additional information related to balancing including the concept of system effect, pump testing, how to read pump and fan curves, how a pitot tube works, duct construction information, and pictures of different types of pumps, pipes, ducts and fans. Most of this information will be fund in the Fans and Ducts class, the Pumps and Pipes class and in the VAV systems class slides and related resources.
In terms of waterside balancing resources, the Bell and Gossett training manuals web page includes a number of useful references. In addition, the balancing guide Armstrong publishes to guide technicians in the proper application of their balancing valves, while product specific, also contains very useful information on the topic.
Finally, one of the most useful balancing tests (in my experience), is the simple pump test. All that is required is the pump's performance curve, a gauge and gauge taps at the pump flanges. You will find a description of how to do the test as well as examples of how to apply the test and interpret the results in the Energy Design Resources design brief titled Pumping System Troubleshooting. And if you think the video clip included on this page is exciting, then you will love the pump test video clip on the videos page, where I demonstrate performing a pump test, including real time indications of the kW, differential pressure and pump speed during the test as I throttle the valve.
- A 1967 vintage SMACNA balancing manual, which I suspect was the forerunner of the current NEEB technicians manual. The current NEEB manual has a lot more information and is well worth the money. But if you are just learning about balancing and the basic techniques that are used, this may be just the thing to get you started. Near as I can tell, physics back in 1967 worked about the same as it does now, and balancing is simply a form of applied physics.
- A discussion of velocity profiles and turbulent and laminar flow, including an exciting video clip illustrating both flow regimes.
- A discussion of pitot tube traverses and a number of resources on the topic.
- A paper and PowerPoint presentation about how the disturbance created by the damper in a terminal unit can impact the accuracy of the flow sensor if the damper is immediately down stream of the flow sensor. This phenomenon is not well recognized but causes the flow sensor calibration curve to shift from what the manufacturer indicates.
In addition, the presentations from the Pacific Energy Center Design, Performance, and Commissioning Issues classes on Fans, Ducts, Pumps and Pipes contain additional information related to balancing including the concept of system effect, pump testing, how to read pump and fan curves, how a pitot tube works, duct construction information, and pictures of different types of pumps, pipes, ducts and fans. Most of this information will be fund in the Fans and Ducts class, the Pumps and Pipes class and in the VAV systems class slides and related resources.
In terms of waterside balancing resources, the Bell and Gossett training manuals web page includes a number of useful references. In addition, the balancing guide Armstrong publishes to guide technicians in the proper application of their balancing valves, while product specific, also contains very useful information on the topic.
Finally, one of the most useful balancing tests (in my experience), is the simple pump test. All that is required is the pump's performance curve, a gauge and gauge taps at the pump flanges. You will find a description of how to do the test as well as examples of how to apply the test and interpret the results in the Energy Design Resources design brief titled Pumping System Troubleshooting. And if you think the video clip included on this page is exciting, then you will love the pump test video clip on the videos page, where I demonstrate performing a pump test, including real time indications of the kW, differential pressure and pump speed during the test as I throttle the valve.
1967 Balancing Manual
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As you may have noticed, I like old books from the industry. Actually, I just like old books. Actually, I just like books.
The .pdf below the image is a 1967 SMACNA balancing manual that was given to me by one of my mentors when I first started out and is what I used as a reference book when I was learning how to take measurements out in the field. I pulled it out the other day to scan it because a number of students are trying their hand at taking traverses to do diagnostics and since the physics of it all hasn't changed, I thought it would be a good reference for them in terms of understanding the basic principles. In the image to the left, which comes from the manual, the technician is using an inclined manometer to measure velocity pressures for a duct traverse. The gizmo in his right hand is the a pitot tube and the round thing it is inserted through (along with the others next to it) are special fittings that provide a very professional, leak-tight, access point to a hole in the duct for the tube. On most projects, the technicians use plastic plugs or rubber stoppers, but you can still purchase higher quality, more robust hole covers if you want them from a number of vendors including Young Regulator and Vent Fabrics. | ||
The higher quality products are particularly desirable for higher pressure systems or traverse points that are out in the weather, or if you simply want to look really cool and professional to your client. But there is an amazing array of plastic and rubber plugs as you can see from this entire catalog dedicated to that function.
Flow Profiles, Turbulent Flow, and Laminar Flow
A pitot tube traverse like the one in the image in the preceding section involves taking an array of measurements over the cross section of a duct as a way to determine the total air flow in the duct. The reason for taking multiple measurements instead of one single measurement is that the flow profile in a duct is not perfectly uniform, even if the duct is a long straight run. That is because the air flowing closest to the wall is slowed down more by its interactions with the wall as compared to the air in the center of the duct. The same is true for the flow of a liquid in a pipe.
As a result, the flow profile achieves what is usually termed a "bullet shape". But how "pointy" the bullet is will vary depending on how fast the air is moving and also depending on if the flow is laminar or turbulent flow, as illustrated below.
As a result, the flow profile achieves what is usually termed a "bullet shape". But how "pointy" the bullet is will vary depending on how fast the air is moving and also depending on if the flow is laminar or turbulent flow, as illustrated below.
As indicated in the titles, the length of the arrows represent the average velocity of the air at various points in the duct cross-section. If you looked at the actual path that an air molecule was taking as it moved down the duct, you would find that for laminar flow, the path of the air molecule actually as along a fairly straight line. But for turbulent flow, the air molecules would be swirling around as they moved down the duct and would not follow a straight line at all.
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Most, if not all of us have observed the laminar flow and turbulent flow phenomenon by virtue of the patterns that the smoke coming off a piece of incense or a cigarette makes. In In the exciting video short to the left, if you watch the smoke coming off the incense stick closely, you will see that sometimes, it flows in a line for a while before it breaks into a swirly pattern and disperses.
When the flow is in a straight line, it is laminar flow. When it starts to swirl around, that is the transition to turbulent flow. |
As you can observe, I was able to trip the flow up from laminar to turbulent by blowing in the direction of the incense stick and by waving my hand around to make a breeze.
In either case, the general motion of the air is away from the incense stick. But for the turbulent flow, the actual path of the air molecules is no longer a straight line. As a result, the irreversible energy losses associated with turbulent flow will generally be higher than with laminar flow. But the heat transfer will be better since the air molecules in the center of the duct get some exposure to the perimeter, where the heat is crossing the boundary into the duct. For laminar flow situations, the air molecules in the center of the duct are insulated from the walls of the duct by the other air molecules flowing closer to the duct wall.
In either case, the general motion of the air is away from the incense stick. But for the turbulent flow, the actual path of the air molecules is no longer a straight line. As a result, the irreversible energy losses associated with turbulent flow will generally be higher than with laminar flow. But the heat transfer will be better since the air molecules in the center of the duct get some exposure to the perimeter, where the heat is crossing the boundary into the duct. For laminar flow situations, the air molecules in the center of the duct are insulated from the walls of the duct by the other air molecules flowing closer to the duct wall.
Pitot Tube Traverses
Bear in mind that the image above is one dimensional and ducts (and pipes) are two dimensional. As a result, the flow profile has a curve to it both in the vertical and the horizontal direction.
By measuring the velocity at multiple points in the duct cross section, a pitot tube traverse provides a much more accurate assessment of the flow in the duct as compared to the result you would get if you simply measure the velocity in the center of the duct and assumed it applied to the entire duct cross-section. The layout of the array of points that will be sampled by a pitot tube traverse has actually been the subject of much controversy.
If you look at the recommendations in the 1967 manual, the approach used at that time recommended dividing the duct cross section into a minimum of 16 equal areas and taking a velocity pressure reading in the center of each one, thus the name "equal area method".
By measuring the velocity at multiple points in the duct cross section, a pitot tube traverse provides a much more accurate assessment of the flow in the duct as compared to the result you would get if you simply measure the velocity in the center of the duct and assumed it applied to the entire duct cross-section. The layout of the array of points that will be sampled by a pitot tube traverse has actually been the subject of much controversy.
If you look at the recommendations in the 1967 manual, the approach used at that time recommended dividing the duct cross section into a minimum of 16 equal areas and taking a velocity pressure reading in the center of each one, thus the name "equal area method".
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The velocity pressures are converted to velocities and then the velocities are averaged and applied to the duct cross section using the continuity equation to the left to come up with the flow rate.
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If you consider the fact that there is a velocity profile across the duct, then you will conclude that using the equal area method may misrepresent the actual flow occurring near the duct wall because the velocity gradient is changing more rapidly across the area represented by the traverse point as compared to what is going on towards the center of the duct. To address this a technique was developed that is called the Log-Tchebycheff method, which was adopted by ASHRAE as their recommended approach.
I believe that subsequently, ASHRAE has organized a research project to figure out how to pronounce it. The image to the right, which is taken from the NBCIP Return Fan Capacity Control Guideline, compares the results from both methods as well as illustrating what happens if you average the velocity pressures instead of converting them to velocities prior to averaging. |
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The downloads at the end of this section include a number of articles and papers about pitot tube traverses including an article published by Curt Klaassen and John House that compared traverses taken in the same system at the same location using both methods. It also looks at the impact of the location including 50% effective duct length down stream of the fan vs. 100% effective duct length as well as a traverse taken in the same supply duct but one equivalent duct diameter downstream of an elbow prior to the first branch in the system.
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The Impact of a Damper Located Immediately Down Stream of a Terminal Unit Flow Sensor
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The paper and PowerPoint presentations below document how the upstream flow disturbance created by a terminal unit damper as it throttles can impact the calibration of the flow sensor in the unit. This issue is not well known but has significant implications in terms of accurately measuring terminal unit flow over its operating range.
The phenomenon is very similar to the impact of a rock in a stream on the water upstream of it as illustrated in the video clip to the right. Notice how water piles up into a standing wave upstream of the rocks in the video. |
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