David Sellers

Damper tests provide valuable insights, Part 3; The relationship between damper performance and damper velocities
June 10, 2007

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.

Posted by David Sellers on June 10, 2007 | Comments (0)