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Damper tests provide valuable insights. Seriously--I'm not kidding. Part 1
May 31, 2007

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 worked.
• 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). 

(insert Opposed blade damper curves.jpg and Parallel blade damper curves.jpg)

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 2.4.3.2. 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 retrocommissioning effort.

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.

Posted by David Sellers on May 31, 2007 | Comments (3)