Wayne Jin and I recently had a chance to test the flow versus
damper blade position characteristics for the AHU1 economizer
dampers at the Pacific Energy
Center (PEC). We discovered a few interesting
things, including that theory didn’t match reality, at least for
our test. Over the course of the next few posts, I’ll explore
the test Wayne and I performed, including the theory behind it and
the results. This post will give you some background
information and set the stage for subsequent discussions.
Wayne was one of the student’s in last year’s
hands-on retrocommissioning workshop series at the PEC. The top
photo below is a picture of the AHU we tested, along with the 2nd
group of folks to go through the PEC RCx class. The lower
photo is a picture of the unit’s economizer dampers. The
outdoor air dampers are the vertical sections and the return air
dampers are the horizontal section.
As you can see, they are showing signs of age, including
missing blade seals (note the gap at the top of
the outside-air dampers). The jamb seals are still in
place and serviceable. While these conditions may not be the
norm, I would not be surprised if many of you see dampers in
similar condition in many of the systems you are working on; I know
I do. As a result, you may find that the information that is
about to be presented can be extrapolated and applied to your
projects, especially if you are working to improve the performance
of existing buildings and their systems.
The goal of the test we performed on the AHU1 economizer dampers
included a number of targets:
- Understand how well the minimum position signal approach for
regulating minimum outdoor air flow that was in use at the PEC
- Understand the implication of the missing damper blade seals in
terms of leakage at minimum and maximum outdoor air.
- Characterize the damper performance in terms of flow vs. stroke
and compare it to the theoretical predictions.
The minimum outdoor air issue listed in the first bullet is the
original reason the class targeted the dampers as a
retrocommissioning (RCx) opportunity. Common wisdom is that
for a damper to be able to control flow in a system, it needs to
have a pressure drop that is significant relative to the system it
is controlling. Furthermore, the more significant the
damper’s pressure drop is, the more linear the relationship between
blade rotation and flow will be. These concepts are often
stated graphically in curves similar to the following, which are
pressure drop vs. flow performance for opposed and parallel blade
dampers with different ratio’s of damper pressure drop to system
pressure drop (the ratio is often called alpha).
These curves are published in a number of sources, including the
ASHRAE Handbook of Fundamentals Chapter 15, ASHRAE
Guideline 16 –
Selecting Outdoor, Return, and Relief Dampers for Air-Side
Economizer Systems and Powers Application Engineering Form
AE-24 – Damper Sizing and Selection (sorry, no link for this at
this time). The curves in the picture are reproduced in a
damper sizing spreadsheet that is Chapter 2, Section 18.104.22.168.
Component Specifications of the Control Design Guide, which
is a publicly available resource.
Damper pressure-drop is related to damper velocity and for most
HVAC systems a general rule of thumb is that the velocity through
the economizer dampers will need to be in the range of 1,500 to
2,500 feet per minute (fpm) to achieve an alpha that is
satisfactory in terms of providing a linear flow vs. stroke
characteristic. High alphas, which translate to oversized
dampers, will tend to provide a two position characteristic.
If you look at the opposed blade curve in the picture above, you
will notice that for an alpha of 200, 20% stroke provides nearly
50% flow. For a parallel blade damper under the same
conditions you would get 75% flow! Clearly, theory suggests
that an oversized outdoor air damper using a minimum position
signal strategy to control minimum outdoor air flow has the
potential to over-ventilate and thus, is a good target for a
Come back in a few days and we’ll look at how the PEC
Retrocommissioning class used this theoretical data and rules of
thumb to assess the potetial for improving performance and saving
energy in the economizer serving AHU at the PEC.