Damper tests provide valuable insights, Part 3; The relationship between damper performance and damper velocities

In the last post, we looked at assessment techniques that
allowed us to assess the performance of the economizer dampers in
an air handling unit at the
Pacific Energy Center (PEC) as a part of a retrocommissioning (RCx)
workshop
.  The assessment revealed that the damper
velocities would be 1,400 fpm or less under normal operating
conditions. Based on the theory and rules
of thumb discussed in an earlier post
, the class concluded that
the damper performance would not be very linear and targeted the
economizer for further testing.

Some of you may be wondering why low damper velocities and
non-linear damper performance might be cause for concern. There are
actually several reasons, including minimum outdoor air flow
regulation, controllability, and mixing.

Minimum Outdoor Air Flow Control

There are a number of ways to control  minimum outdoor air
flow in an air handling system.  Larry Felker wrote a
good article on this topic that was published in the April 2002
ASHRAE Journal
if you are interested in learning more. 
For the AHU that the PEC RCx class was working with, there is no
independent minimum outdoor air damper. Instead, minimum outdoor
air flow is controlled by applying a minimum percentage of stroke
signal to the outdoor air and return air damper actuators.
Specifically, a minimum signal of 15% is sent to the damper
actuators any time the unit is in operation. The economizer control
process can drive the damper further open if necessary to achieve
the desired temperature set point, but it is not permitted to close
the dampers past the 15% minimum if the unit is operating. The goal
of the strategy is to achieve 15% minimum outdoor air flow. For
this approach to be successful, the relationship between damper
stroke and damper flow needs to be linear. Based on the rule
of thumb cited previously
, it looked like the damper might not
have a linear flow vs. stroke characteristic and that the minimum
outdoor air flow strategy that was in place at the PEC might
over-ventilate.

Controllability

Economizer processes, by their nature, are inherently
non-linear, involving a number of variables including outdoor air
temperature and flow, return air temperature and flow, and, on a
VAV system, total flow. When all of these variables interact in an
effort to maintain a fixed leaving air temperature in a constant
volume system, the result is as depicted in the graphs
below.
 

As you can see, even in the
best of worlds, there is a non-linear relationship between flow
through the dampers and the amount of damper stroke it takes to
maintain a fixed set point. Using dampers with a non-linear control
characteristic has the potential to make the process more difficult
to control.  (Incidentally, both of the preceding figures as
well as an entire chapter on economizer theory supported by a
number of functional tests and other resources can be found in the
Functional Testing
Guide
. The guide is a web based, publicly available
resource developed with funding from STAC (State Technologies
Advancement Collaborative) and the Department of Energy under the
California Public Interest Energy Research Program (PIER)).

If a process is difficult to control, it has the potential to
become unstable and start to hunt. Because the output of the
economizer cycle is the input to a number of other HVAC processes,

instability in the economizer can rapidly spread to other HVAC
processes
. For instance, if the system has a chilled water coil
immediately down stream of the mixed air plenum, the temperature
leaving the mixed air plenum impacts the heat transfer from the
chilled water coil, which, in turn impacts its discharge
temperature. An unstable entering temperature from the economizer
can lead to an unstable leaving temperature from the chilled water
coil as its control process tries to “chase” the varying load
created by unstable economizer control process. In a VAV system,
this instability can ripple out of the system’s temperature
control processes and
impact the flow control processes
. When the economizer dampers
move, they change the pressure in the mixed air plenum, especially
if they are oversized. If the pressure in the mixed air plenum
changes, the supply fan performance changes, which, in turn,
impacts the static pressure in the supply duct system. 
Typically, this pressure is picked up and used to control the
supply fan speed.  If the action of the economizer
ultimately causes the supply duct static pressure to change, then
the supply fan capacity control loop will react and try to
“chase” the upset initially created by the economizer.

If the supply flow changes, then the pressure relationships
through out the building will be impacted, as will the return fan
control system. The bottom line is that instability in one HVAC
control process can rapidly ripple out through the system, wreaking
havoc on comfort, efficiency, and actuator and valve packing life.
Anything that can be done to minimize the potential for instability
– like, for instance, improving the potential for the
economizer to be stable by properly sizing its dampers – is a
step in the right direction.

Come back in a few days and we’ll explore the reason that
econmizer damper velocity might be important with regard to
mixing.  We’ll also look at a few more observatiosn the class
made regarding the PEC AHU economizer and its ability to meet its
design intent.

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