Damper tests provide valuable insights. Seriously–I’m not kidding. Part 2

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.

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