Proper air filtration is a key element in any maintenance program for a building’s heating, ventilating and air conditioning (HVAC) system. While air-filtration systems were first developed to protect the HVAC system’s coils, ductwork, and other components from dust and dirt, they have evolved to the point where today they serve as the front-line defense in maintaining a building’s indoor air quality (IAQ).
While engineering and maintenance managers generally recognize the relationship between air filtration and IAQ, the energy-conservation implication of selecting the best-suited air filter for a particular application often is overlooked. Instead, too many managers focus on the filter’s price to reduce operating expenses.
In reality, the cost of the energy used by the HVAC system due to the filter’s impact far outweighs the cost of the filter itself — typically by a factor of five to ten, depending on the application and the type of filter.
A filter installed in an HVAC system offers resistance to the air the system supplies to the conditioned space. The higher the filter’s resistance, the more energy the system’s fan requires to supply the proper air volume.
This disparity creates a problem for managers because the need to maintain an acceptable level of IAQ requires a filter that can efficiently remove particles from the airflow. But in general, filters that more efficiently remove dust and dirt drive up the levels of resistance to airflow, causing higher requirements for fan energy.
Compounding this problem is the fact that today’s HVAC systems must supply more airflow as a result of both increased outdoor air demands and the need to maintain a higher minimum airflow volume. These higher airflow volumes cause greater pressure drops across the filter system, resulting in increased fan-energy requirements to maintain the flow rate.
Reducing the resistance of the air filter will decrease fan energy requirements, but reduced filter resistance generally translates into lower arrestance rates for dust and dirt and, as a result, poorer IAQ.
The situation often becomes even more interesting. The longer a filter remains in service, the more it becomes loaded with dust and dirt particles. These particles further resist airflow through the filter, resulting in greater pressure drops. In constant-volume HVAC systems, this additional resistance decreases airflow in the system. In heavily loaded filters, it is possible this increased resistance results in airflow rates below acceptable minimum values, again harming IAQ.
In variable-volumes systems, the additional resistance caused by a loaded filter often means the system must increase the fan’s speed to maintain the desired airflow rates. Not surprisingly, this increase in fan speed results in increased energy use.
This need to balance acceptable IAQ levels and a fan’s energy use was not a problem before IAQ and energy costs became issues in managing facilities. Fortunately, managers have a range of options in selecting HVAC filtration systems and can select one that meets both goals.
For example, the most common filter design traps particles between fibers by a process known as straining or impingement. Another design uses an electrostatic charge to capture and hold particles.
Other designs use disposable or rechargeable containers to capture vapors and gases. Managers also can specify ultraviolet-light-based systems to reduce the concentrations of fungi, bacteria, and viruses in the air stream.
Selecting the most appropriate system requires an understanding of the types of contaminants found in a particular facility and the acceptable level of IAQ for the particular application.
The first step in evaluating filter options is to understand the types and sizes of airborne contaminants found in an application. Most particulate contaminants in a typical building consist of dust and dirt particles, smoke, mists, and bacteria. The dust and dirt particles typically range from 10 microns in diameter to several thousand microns. Common bacteria range from 0.1 microns to 10 microns. Smoke and mists typically range from 0.01 microns to 10 microns.
Managers need to look for sources that generate contaminants both inside and outside a building. Internal sources include processes, office equipment, chemical cleaners, and off-gassing from building materials. Sources of contaminants brought into the building from outside include Dumpsters, loading docks and nearby roads.
It is best to eliminate as many sources of contaminants as possible before looking for ways to filter them with the HVAC system. Relocated equipment, dedicated exhaust systems, alternative cleaners and materials, and relocated outdoor air vents all can reduce the level of contaminants introduced into buildings.
Once managers have identified the contaminants, they need to determine the filter’s efficiency. The higher that efficiency, the more contaminants it will remove. Higher efficiency also means the filter can remove smaller particles.
While it would be nice to remove all contaminants, it simply isn’t practical because higher-efficiency filter systems generally mean higher filter and energy costs. For most commercial and institutional applications, a filtration system efficiency of 25-40 percent is sufficient.
Specialized applications, such as clean rooms and surgical centers, will require efficiencies of 95 percent or higher. The important goal in terms of IAQ and energy efficiency is to match the filter’s efficiency to the application’s needs.
The increased need to reduce fan-energy requirements while maintaining acceptable IAQ has resulted in the introduction of a number of new filter types. These filters offer improved performance at a reduced pressure drop.
For example, synthetic filter media are replacing traditional cotton/poly media in many applications. Synthetic media offer higher filter efficiency, while minimizing pressure drop across the filter. By using synthetic media filters, managers can maintain or increase the provided level of IAQ without increasing fan-energy requirements.
Synthetic media also will absorb less moisture than cotton/poly media. Moisture on conventional HVAC system filters creates a condition suitable for the growth of bacteria, which ride the air flow across the filter media into the conditioned space. Because they do not absorb and hold moisture, synthetic media reduce the chances the filters themselves will contribute to poor IAQ conditions in the building.
Synthetic filter media also resist shredding, reducing the number of filter particles carried by the air stream into the coils and ductwork. Synthetic materials also can be manufactured with a varying density structure that results in improved efficiency over the life of the filter.
Traditional cotton/poly filter media have a woven structure to improve filter strength and performance. By contrast, synthetic filter media can be manufactured in a non-woven structure, resulting in media that can provide specific performance characteristics.
Another new-generation filter based on synthetic materials uses electrostatically charged fibers. These fibers attract and capture particles, improving filter efficiency — particularly with respect to small particles — without increasing the pressure drop across the filter.
Ultraviolet-light filtration systems, installed downstream of the HVAC system’s coils, can reduce biological growth within the system or kill biological contaminants drawn into the HVAC system. Ultraviolet-filtration systems offer minimal resistance to airflow in the system and, as a result, do not impact fan-energy use.
The systems do require energy to power the lamps, but the power requirement is small in comparison to fan-energy requirements. Since the systems do not filter particulate matter from the air stream, they work in conjunction with a conventional filtration system.
Gas-absorption systems can be used in applications where harmful gases or vapors are present. These systems use disposable or rechargeable carbon or charcoal pads installed in racks mounted downstream of a conventional filtration system. Most of these filters can remove vapor and gas particles as small as 0.0003 microns.
By properly selecting and maintaining their HVAC system filters, managers can achieve improved IAQ without increasing the system’s energy requirements.
James Piper is a national facilities management consultant based in Bowie, Md. He has more than 25 years of experience in institutional and commercial facilities.
Filter Focus: To Replace or Not?New technology is helping maintenance and engineering managers determine the best time to replace existing air filters. In the past, most managers scheduled filter replacement based on time in service, a visual inspection of the filter media, or measured pressure drop across the filter bank. But time in service and visual inspections do not provide the level of reliability and accuracy required to determine the need for filter change. As a result, filters either were replaced too soon, increasing filter costs, or too late, reducing air flow and increasing fan-energy requirements in variable-volume systems. Measuring pressure drop across the filter bank was reliable in constant-volume systems. But in variable-volume systems or systems equipped with variable-frequency drives, constantly changing fan speed to match airflow to building requirements results in constantly changing measured pressure drops across the filter bank. New-generation monitoring systems measure both the pressure drop across the filter bank and the velocity of the air through the filter, providing a more accurate assessment of the filter’s condition. — James Piper, P.E.
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