David Sellers

Controlling the Environment Requires Understanding the Environment
July 16, 2007

In the last post, I presented the results of the economizer damper test I have been
discussing. The findings related to the test results fall into four categories:

The leakage through the outdoor air damper when it is fully closed meets or exceeds the system’s
worst-case minimum outdoor air flow requirement.

There is significant leakage through the return air dampers when they are fully closed and the system is on a 100% outdoor air cycle.

The damper performance is linear in the mid-flow range, despite the predictions I made based on the published damper performance curves.

The initial damper motion from 0% outdoor air towards 100% outdoor air has little impact on the outdoor air flow.

In subsequent posts, I will focus on each of these findings and discuss some of their implications in terms of performance and energy consumption. But first, I think it’s important to recognize that for these findings in particular and for many HVAC system issues in general, the implications of the issues are very much a function of the local climate and the application served. To gain some insight into this, consider the climate snap-shots illustrated in the graph below.

The graph was created by plotting ambient temperature versus hours of occurrence for different locations using bin weather data obtained from NOAA . It’s easy to loose sight of the fact that the climate and its implications on HVAC processes varies from the location where you are currently working. I remember walking out of the construction trailer one July day right after I had moved to Portland, thinking how beautiful it was; crystal clear blue skies, temperatures in the low to mid 80’s in the late afternoon, a light breeze, and low humidity. In hot and humid St. Louis, my former home, I would have been opening up the house and enjoying the cool, pleasant weather. Just as I was thinking this, the two women who kept things flowing smoothly back in the construction office trudged by and both commented about how unbearably hot and humid it was! 

Now that I’ve lived in Portland for a while, I would probably agree with them compared to the more typical summer day.  The point is that it’s easy to become acclimated to the environment where we live, which may or may not be where the project we are working on is located. Plots like the one presented in the picture above are one way to gain some perspective on how a local climate might interact with the HVAC systems and processes you are working on. Consider the following with regard to the illustration:

Note the juxtaposition of climate data with the line that represents 32°F. Water will freeze at or below this temperature, including the water in our HVAC system coils. One of the tricks associated with operating an economizer equipped system is to maintain control of the mixed air temperature in a manner that prevents the introduction of subfreezing air into the system. Economizers operating in Key West, Los Angeles, and Yuma seldom if ever have to deal with this issue. Economizers operating in Portland, Oregon have to deal with it, but only occasionally. Economizers operating in St. Louis and International Falls may spend a significant amount of their time dealing with subfreezing air and the issues it can create.  In International Falls, even a system with modest minimum outdoor air requirements may require preheat to deal with the extreme winter conditions that may be encountered.

Consider the different climates relative to the 75°F line. While not a complete picture with out some indication of humidity, this line does provide some insight for contrasting the indoor environment we are trying to control with the outdoor environment. In Key West, given the humidity, providing an economizer on an air handling system may not be worth the time and effort. In contrast, for environments like Portland, Oregon and Los Angeles, economizers represent an opportunity to save significant energy and the system serving them will operate at or near 100% outdoor air most of the time given the mild climate. The energy savings potential also exists in climates like St. Louis, Yuma, Flagstaff, and International Falls, but economizers in these locations will operate over a wide range of outdoor air flow rates as the ambient temperatures vary.

Generally speaking, systems that must deal with a wide range of ambient temperatures like those seen in International Falls will need to be more robust than those that only deal with mild temperatures concentrated in a narrow range like Los Angeles. A system that operates with no perceived problems in LA may be a nightmare to operate in International Falls. This implies that a system in international falls will require more attention to the details of design, construction, and operation for successful results to be delivered.

It is not unusual for the same problem in two radically different locations to lead to different conclusions and focuses for design decisions and operating and commissioning efforts. In the next post, I’ll look at how an economizer damper that is stuck open results in diametrically opposite issues when it occurs in two different climate zones.

Posted by David Sellers on July 16, 2007 | Comments (0)