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Damper tests provide valuable insights. Seriously--I'm not kidding. Part 2
June 9, 2007
As you will recall from the preceding post, a field test conducted as a part of a retrocommissioning (RCx) workshop at the Pacific Energy Center (PEC) led not only to insights regarding the system under test, it also produced results that did not match up with theory. In this post, we will look at the scoping process the class used that led them to develop and perform the test. You may find these techniques useful as you work on your projects.
Two of the key techniques that the class used were assessing the unit's capacity in terms of air flow, and then using that information to assess the performance potential for the economizer. At first, this may sound like a complicated problem without a lot of instrumentation. But, it’s actually not that difficult to get an idea of what the answers might be by simply using the indicators available in the field and rules of thumb. For instance, a common design rule is to limit face velocities through filters to 500 fpm. And, a credible case can be made for holding velocities at 300 fpm to
minimize pressure drops and conserve energy.
I frequently use two numbers as limits to a "bigger than a bread-box, smaller than an airplane hanger" assessment technique. This establishes a range within which the answer lays. Many times, I discover that I need no more information and can proceed on the basis of such an answer. Or, I may discover that the result is of interest, but I need to know more, so I spend time honing the answer with a more sophisticated calculation strategy.
The picture below shows the filter section in AHU1 at the PEC.
In the field, the filter counts and face velocity rules I mentioned above can be used to assess AHU1's flow capacity as follows.
* Filter area:
12 – 24” x 24” filters = 48 sq.ft. (red area)
4 – 12” x 24” filters = 8 additional sq. ft. (blue area)
Total area – 56 sq.ft.
* Flow rate
At 500 fpm = 28,000 cfm
At 350 fpm = 19,600 cfm
When we contrasted the answer generated by this approach with the actual rated capacity of 26,500 fpm we discovered that the technique had indeed put us in the ball park.
The next picture shows the damper section in the AHU. All four sections of the damper (two vertical outdoor air sections and two horizontal return sections) are the same as the left outdoor air section, which is shown in full in the picture.
By simply making our best guess at dimensions, we can use what we see along with the damper face velocity rule-of-thumb cited in the previous post (Part 1), or 1,500 - 2,500 fpm face velocity for a linear characteristic, and our previous estimate of AHU flow rate to assess the AHU1 damper performance characteristics as follows:
* Estimated damper size
Estimated blade width = 6”
Estimated blade length based on blade width = 42”
Blades per section = 6
Area per section = (6” x 42” x 6) = 1,512 sq. in. = 10.5 sq.ft.
2 sections = 21 sq. ft.
* Estimated face velocity
Previously estimated flow rate
Low end – 19,000 cfm
High end – 28,000 cfm
Velocity = Flow / Area
Velocity at the low end = 19,000 cfm / 21 sq.ft. = 905 fpm
Velocity at the high end = 28,000 cfm / 21 sq.ft. = 1,330 fpm
As a result of this assessment, the class concluded that with damper velocities in the 905 - 1,300 fpm range, the performance of the AHU1 economizer could be marginal, especially as the load dropped off. Because the system it serves is a VAV system, the decrease in flow associated with part-load operation will result in a reduction in velocity through the damper section. Thus, they targeted the economizer section for further functional testing and assessment.
Come back in a few days and we will look at some of the other observations the class made regarding the AHU1 economizer dampers and how these observations further focused their retrocommissioning effort and began to define the outline of the test they would perform.
Posted by David Sellers on June 9, 2007 | Comments (0)
