Filtration Monitoring Lags Behind Advances in HVAC Technology
The last decade has seen great advances in building controls and automation. Today we monitor all facets of the building environment; however, when it comes to air filtration monitoring, we rely upon the same methods employed for decades. The common means of monitoring air filters continues to be by measuring the differential pressure across the filter bank. In addition, many practitioners merely visually inspect the filter to determine when to change a filter. Still others will change the filter on some scheduled replacement whether it is days, weeks, or months. While these methods may be adequate for some, they do not fulfill the promise of advanced building automation and control.
Looking at the dynamics of air filtration, we are really talking about the changing airflow as a filter progresses from a clean to an obstructed state. Air filters are defined by their relative airflow. When we use differential pressure as the means to monitor air filters, we introduce several problems. Pressure across the filter bank is influenced by environmental factors such as changes in atmospheric temperature. Systems using variable speed fans change the relative pressure according to the demand of the building. When monitoring air filters with differential pressure, practitioners must be aware that relatively small changes in air pressure result in large changes in airflow. HVAC systems can appear to have adequate airflow as measured by pressure, yet the system has reached a point where a small change in pressure can result in loss of flow and equipment failure.
Replacing filters by visual inspection has its own set of problems. Visual inspection becomes subjective especially between individuals who might share the responsibility of filter maintenance. The particles deposited on the filter medium can be opaque and thus difficult to detect with the naked eye. Sometimes this method seems more like putting your finger to the wind than a reliable method of maintenance.
While scheduled replacement of filters brings some regularity to maintenance, it does not necessarily translate into an efficient means to monitor, detect, and maintain the air filtration system. The external environment changes with the seasons, the population, as well as economic development. These changes are not accounted for when filters are changed by the calendar. The result is that filters are often changed sooner than required or alternatively, longer than recommended.
Building owners and plant managers will readily admit the replacing air filters is left to varying degrees of estimation-often guesswork. The real question many ask is when do we really know when to change a filter?
The commonly accepted industry "rule of thumb" is that a filter is ready for replacement once it has reached twice the initial pressure drop across the filter bank. In practice, achieving a doubling of the differential pressure is problematical. The statement is true when the air velocity remains constant for both the clean and the dirty filter measurements However, in constant volume system (constant motor speed) the fan often begins to push less air as the filter becomes impeded. As the obstruction increases, there is no commensurate increase in pressure. Thus, in the worst case, achieving a recommended doubling of the pressure may not be reached until the system becomes excessively obstructed resulting in the system not functioning as designed. Similarly, in a variable speed fan system using differential pressure to monitor filter impedance can lead to erroneous results. As the velocities are changed the pressure also changes resulting in erroneous measurement of the filter status.
The Need for Better Filtration Monitoring
Optimum filtration is a necessity for several important reasons:
* Reliably clean air is a resource not unlike water and power.
*. Accurate monitoring eliminates the guesswork of knowing when to replace air filters.
* Proper replacement reduces material and labor costs to monitor detect and maintain the filtration system.
* The automated air filtration system affords greater environmental control eliminating HV AC failure and plant disruptions.
* Air filtration monitoring reduces energy consumption when filters are changed "on time" eliminating drag on HVAC system.
* Air filtration monitoring can assist in reducing energy consumption when higher efficiency filters are used to keep heat exchange elements clean.
All the expensive environmental controls are only as good as the first line of defense, air filtration. While plant operations are wired for all facets of environmental controls, they do not have an accurate air filter monitor to connect to the automation system. The financial costs from inadequate air filtration translate into lost plant efficiency, increased energy usage, and even plant and production disruption.
A new technology developed at Precision Air Technology Inc. is an electronic air filter monitor, called the Filtrometer, which accurately senses the relative airflow of particulate filters in both constant and variable airflow.
The technology is a thermal sensing system consisting of sensing probe and controls unit. The sensing element, in standard HVAC applications, is placed between the fan and the filter bank. Two signals are generated and sent to the controls unit, one related to the velocity of the air through the filter bank and the other signal measures the ambient air conditions,. The monitor requires a one-time calibration of the filter type (efficiency) to the air handler unit. As the filter bank becomes obstructed by particulates the difference between the two signals changes resulting in the dirty filter status. Perhaps the most important feature of the system is its ability to accurately monitor air filters in variable air velocity. The relationship between the signals does not change with velocity changes. The electronics is designed to give virtually the same output whether the velocity is 200-fpm or 2000-fpm. The signal output changes only when the resistance in the filter bank changes as a result of particulates obstructing the airflow.
The air filter monitor features a local LED display as well as a 0-24 volt dry contact relay for remote monitoring. Options include and analog output available for data gathering as well as graphing of the filter status.
The air filter monitor operates accurately over a wide range of air velocities. Tests of the performance characteristics by Systima Technologies Inc., Woodinville, WA, report: "By using differential air velocity measurements, the Filtrometer Air Filter Monitor was able to distinguish an increase restriction in the air flow representative of a dirty filter in and HVAC system. It provided an appropriate higher signal voltage for the "dirty" configuration throughout a range (200-1000 fpm) of airflow velocities." The tests showed that increased airflow restriction, such as a dirty filter,' can be detected long before increases in static pressure reveal a significant blockage.
The advantages of the monitor as a tool for plant automation are several. First automated maintenance reduces the material and labor costs to monitor, detect, and maintain the filtration system. Second, greater environmental control eliminates HVAC failure and resulting plant disruptions. Third, properly maintained air filters reduce unnecessary drag on the HVAC system increasing energy efficiency.