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Every modern Heating Ventilation and Air Conditioning (HVAC) system is faced with a common question, “when is the right time to replace the air filters”? In the past this question may have been inconsequential, but increasingly the right answer to this question determines the ability to provide indoor air quality, utilize energy efficient technology, and provide an accurate building automation system.
How We Do It
Every modern HVAC system relies on a method for monitoring, detecting and maintaining the air filter system. Without an accurate and reliable means to monitor and detect air filters it is difficult to assure that the proper level of indoor air quality is maintained. The methods used to monitor and maintain air filters are varied. Some installations utilize differential pressure gauges across the filter bank. The filters are replaced when a relative differential pressure reading for a dirty filter is reached. In other facilities trained technicians periodically visually examine filters to determine when they are due for replacement. Still others replace filters on a specified schedule without regard for their condition. Those who have adopted differential pressure to monitor their air filtration system often use the “rule of thumb” that filters are replaced when the final pressure drop is double the initial pressure drop. While this method gives some relative measure of filter obstruction it often does not provide the degree of accuracy that is expected in building controls systems. In constant airflow HVAC systems monitoring air filters with differential pressure is limited by the fact that fans tend to push less air as the filter becomes clogged. Unless there is a means to monitor and increase fan speed as the filter becomes loaded, it is difficult to determine when the final pressure drop is reached in these systems. Building management recognizes that as the requirements for air quality increases an air filtration monitoring system that accurately monitors air filters is required.
Current Demands for Energy Efficiency
As we look to providing more efficient and environmentally friendly buildings, reliable air filtration becomes a key element. We expect buildings to be designed not only with good functionality, but also with a standard of performance that complies with the norms of the Building Management System (BMS). Increasingly building automation is tasked with implementing energy technologies that provide building efficiency and sustainability. One technology proven to provide greater energy efficiency is the use of variable frequency drive (VFD) electric motors. Adoption of this technology is one of the most effective strategies for reducing energy consumption in heating and cooling. Studies by the U. S. Department of Energy have shown dramatic reductions in energy consumption with the use of VFD technology. As an example, a textile plant in cooperation with Pacific Gas and Electric replaced constant speed ventilation fans with VFD fans. The results of this effort were that they achieved energy savings of 59%*. However, a significant constraint upon adoption of this technology in the HVAC industry is that VFD’s produce changes in differential air pressure across the air filters. The traditional method for monitoring air filters is to measure by this means. These changes in pressure completely fool traditional air filter monitors. Consequently, the adoption of this energy saving technology is inhibited by the inability to detect filter status in variable airflow conditions. The building automation industry recognizes the gap between this proven energy efficient technology and the expectations of accurate filtration monitoring for the BMS.
Another air filtration technology that brings greater energy efficiency is adoption of low-pressure filters into the HVAC system. Many filter manufacturers produce low pressure air filters. These newer low-pressure filters lessen the “drag” of the system and improve efficiency as less fan energy is required to ventilate the building. To properly adopt this technology the reduced pressure of such a system must be incorporated into the design and operation of the system. Here as in the case of the variable frequency fans, the prevailing technology of using differential pressure does not necessarily provide accurate monitoring and detection of air filters.
The modern BMS has unparalleled abilities to monitor and control a multitude of environmental elements such as temperature, humidity, airflow, and energy usage. To a remarkable degree air filtration touches upon these environmental factors. The amount of heating or cooling is determined by how much air is able to pass through the filtration system. Obstructed filters allow less air circulation meaning it takes longer to heat and cool. Keeping the HVAC system cleaner with proper air filtration methods improves other efficiencies such as dehumidification, overall indoor comfort and lessens the accumulation of mould and bacteria within the building environment. Adequate ventilation is determined by the amount of air able to pass through the filter bank. Engineering more energy efficiency into air handler units (AHU) is determined by newer strategies such as adopting VFD’s and low-pressure air filters into the HVAC system. For all these strategies to work properly they must have accurate BMS control. Until recently there were few advances in air filtration technology able to provide accurate monitoring, detection and maintenance of the air filtration system. Every building management team is tasked with utilizing the technologies that give them the greatest ability to fully exercise the vast capabilities of their automation system, yet the monitor and control of the air filtration system has remained somewhat more of an “art” than science. The economic consequences of the status quo are that real costs cannot be fully accounted.
Advances to Air Filtration Monitoring
To fill this gap between the capability of automation systems and the ability to provide accurate air filtration status, an innovative air filtration monitoring system utilizing microprocessor technology was developed by Precision Air Technology Inc. This advanced approach to filter monitoring has the ability to accurately sense the resistance of a dirty filter in both constant and variable airflows. Unlike the conventional method of monitoring air filters that uses differential pressure to detect a dirty filter this device measures both pressure and velocity and conditions them through a microprocessor. The process begins with the push button calibration of a clean set of filters. The alarm resistance is command programmable to 1.5, 2.0 and 2.5 times the initial filter resistance.
As the resistance increases alarms are sent in the form of a dry contact signal and LED display signalling that the filters are due for replacement. The limitations imposed by the traditional methods of monitoring the air filtration system are eliminated with this advancement to building automation. With this added tool building automation systems can more fully integrate the function of air filtration into their building networks and control systems.
The microprocessor-based series 3200 Filtrometer air filter monitor has the following features:
Microprocessor technology for precise measurement of the status of particulate air filters in both constant and variable airflows.
Calibration and command of air filtration monitoring is accomplished with push button control.
Air filter alarm for replacement is adjustable.
Powered by 24 VAC it is compatible with other low voltage systems.
For local display there is an LED that displays both filter status as well as the replacement or dirty filter alarm.
Additionally there is a dry contact relay, 4- 20 mA sink output, and analog signal outputs.
It is easily installed between the fan and filter bank in both original and retrofit applications.
*Web site reference: U.S. Department of Energy-Energy Efficiency and Renewable Energy Industrial Technologies Program-Best Practices. Project Profile: Industrial Technologies, Best Practices, Motors, Fans, and Pumps. Industry: Cotton Fabric, Facility Ventilation and installation of variable frequency drives. Nisshimbo Industries reduce electricity demand from 322 kW to 133 kW.
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