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
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.
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.
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.
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.