David Sellers

Damper tests provide valuable insights, Part 4; Mixing and seals
June 21, 2007

In the previous post, we discussed why low damper velocities and non-linear damper performance might be cause for concern with respect to economizer damper performance. In that post, we looked at minimum outdoor air flow regulation and controllability. Here, we will take a look at mixing, which is also an important consideration, especially in cold climates.

Economizer damper velocity impacts the ability of the outdoor air and return air streams associated with an economizer process to mix. Higher velocities translate to more momentum, allowing one air stream to penetrate the other and promote mixing. ASHRAE Research Project RP-1045 - Verifying Mixed Air Damper Temperature Control and Air Mixing Characteristics documented this in a series of tests that looked at different mixed air plenum configurations. A summary of the results can be found in the 2004 ASHRAE Journal article titled Thermal Mixing of Outdoor and Return Airflows in Typical Air-Handling Units.  (For what it's worth, my own personal feild experience also validates this concept;  frequently, the hard way, via frozen coils and nuisacne freezestat trips.)  Mixing is more of a concern in climates with extreme weather conditions than in the San Francisco climate, where the air handling unit we were testing is located, so the class elected not to focus on this issue. But, even if you work in mild climate, it is good to be aware of the relationship between economizer damper performance and mixing because you never know where your career might take you.

Damper Seals

The picture below illustrates the condition of the damper seals on one of the outdoor air damper sections in the PEC AHU and is typical of all of the other supply and return air damper sections.

Low leakage dampers are typically equipped with seals on the jamb where the damper shafts penetrate the frame (commonly referred to as jamb seals) as well as blade seals along the edges of the blades where they overlap. Jamb seals are typically inverted, U shaped pieces of stainless steel with the open end of the U resting on the jamb and the bottom of the U rubbing on the damper blade. They are installed so that the damper blade compresses the U shape slightly, and, as a result, they provide a sealing effect along the short edge of the damper blade as it rotates around the shaft centerline (If that made sense to you, then you can thank Ms. Wright, who, in my college technical writing class, made us write a paper describing the operation of a paperclip). The shiny strip on the jamb seal in the picture was created by the action of the damper blade edge rubbing on the bottom of the U shaped jamb seal as the blades rotated.

Blade seals come in a number of designs. You can get a pretty good overview of the available options by taking a look at Greenheck’s blade seal replacement instructions. In any case, when the class inspected the PEC AHU economizer dampers, they discovered that there were no blade seals, despite the fact that the shop drawings indicated that the dampers originally had them. In all likelihood, years of operation had resulted in the degradation and eventual failure of the original seal, a theory that was confirmed by closer inspection which, revealed the remnants of rubber blade seals at various locations on the damper blades.

The missing blade seals had performance implications on a number of fronts. The one that most folks (myself included) jump too is the implication when the dampers are fully closed to outdoor air. Specifically, missing blade seals imply that significant leakage might occur, even if the dampers are fully closed. In turn, this implies several things.

Infiltration

The air handling unit might be a source of infiltration when it is off line since the unsealed cracks provide a pathway for stack effect and wind pressure to move air through the building via the air handling unit and its distribution system. 

Over Ventilation

The system may over-ventilate if the dampers can not throttle the outdoor air flow rate to the required minimum due to the lack of blade seals and the resulting gaps between the blades.

Limited Economizer Effectiveness

Less obvious, but perhaps more significant in the mild San Francisco environment is the fact that the leakage through the return damper compromises the effectiveness of the economizer. Specifically, warm air leaking through the return dampers when the unit is on a 100% outdoor air cycle has the potential raise the mixed air temperature and limit the benefit of the “free cooling” provided by outdoor air that is at or below the required discharge temperature.

In the next post, we’ll take a look at the test procedure the class devised to assess the issues they identified when they inspected the PEC AHU dampers. After that, we will (finally) take a look at the surprising and unexpected results of their test.

Posted by David Sellers on June 21, 2007 | Comments (0)