WS1: Filters in HVAC Systems
By Ulf Johansson, Innovation Manager – Camfil Farr

The workshop 1 will discuss the role of air filtration to improve indoor air quality and the design of energy efficient air filtration systems. The main focus will be:
• Help the designer and user to understand air filtration,
• How to select air filters,
• Avoid problems associated with hygienic and other conditions at operation of air filters,
• Discuss the future needs for air cleaning systems

Introduction
Air filters for HVAC systems have always been used to protect the ventilation components. A filter can keep fan, ducts and coils clean and avoid increased pressure drop and malfunctions. The past decades have seen a rapid increase in research on the health effects of air pollution. It is now widely accepted that the exposure to urban air pollutions is associated with a broad range of acute and chronic health effects. Especially fine and ultra – fine particles are suspected to have a large negative impact. About 50% of those outdoor airborne pollutants are carried into buildings and will therefore have a major influence on indoor air quality. Buildings supply air quality is often the dominant factor in exposure to air pollution and associated risk to health.

Air filtration has the potential to reduce the outdoor to indoor transport of pollutants, improve the health of occupants and reduce the productivity loss. Regardless of perspective effective particle filtration is anticipated to lead to annual saving significantly exceeding the running costs for the filtration.

Air Filtration in HVAC Systems

However, economic losses resulting from even a small decrease in productivity caused by sensory pollutants emitted from soiled air filters could exceed the economic benefits of filtration. The air filter is one component in a system and it is thus important to design air filters and HVAC systems together to minimize ill health and sensory problems caused by pollution.
The filter and its energy costs are insignificant relative to workers salaries. High indoor air quality (IDA) and high hygienic requirement (high efficient filters replaced two times per year) will cost about 50 Euro per person and year or 1 Euro per week and person.

Air pollutants and indoor air
Outdoor and indoor air pollutants characteristics vary greatly with place, time and local conditions. Huge numbers of particles of different sizes, shapes, concentrations and toxicity are found in the air as well as different gases from harmless to irritating and unhealthy. Airborne impurities consist of different size and composition, from ultrafine or nano particles (less than 0, 1 µm) to fine particles (0, 1 µm to 2, 5 µm) and coarse particles (dust > 2, 5 µm).
Typically we spend 90 percent of our time indoors and breathing healthy indoor air is a human right and everyone associated with a building have a responsibility to work for obtaining acceptable air quality (WHO 2000). Air filtration may improve indoor air quality and occupants productivity as well as reduce the costs associated with building and HVAC cleaning.

Example of particles in exhaust air

Energy

Energy consumption for a fan is linked to the pressure drop of air filters and this obviously contributes to the global climate change. The United Nations roadmaps on cutting emissions and the European Directives on saving energy will therefore encourage measures to reduce the air flow resistance and the amount of energy used by air filters. However, at the same time the delivered air quality should not be compromised by simply lowering the efficiency of air filters. The correct design and good maintenance of air filters is important to minimize the energy consumption without affecting the removal efficiency.

The energy cost to operate the filter can be 80% of the overall cost of the filter. The rest includes filter replacement, labour and disposal costs. Reducing resistance at the required air flow is the most effective way of reducing both the cost and the environmental impact of air filters.

Hygienic consideration of air filters

Supply air filters prevent downstream air handling unit and duct internal surfaces getting soiled and reduce air particles concentrations. Nevertheless, some studies have stated that soiled filters may have a negative effect on both the immediate and longer term perception of indoor air quality.

Recent studies indicate that a combination of particle filters and activated carbon will improve the acceptability of the filtered air. Such filters can replace existing and would have particle removal efficiencies and air flow resistance comparable to standard filters. This would remove sensory offending pollutants and a significant fraction of ozone from the air stream. This would make improvements in air quality with little or no modification to the existing air handling system.

Microorganisms are normally present in the air and the elimination of possible growth sites within the HVAC system is an effective way to control bio – aerosol contaminants. Proper supply air filters effective collect microorganisms are continuously collected in the filters, but they will not survive for long periods in the airstream. However at higher humidity levels there is a risk of microbial growth, which may cause deterioration in the indoor air quality. Therefore is a correct design of the air intake together with the proper air filtration essential to achieve a good indoor environment.

Replacements and disposal

Air filters should be replaced due to hygienic reasons and the services intervals should be 2.000 h for the first filter stage and 4.000 h for the second filter stage. Replacement should be made in autumn after the pollen season and in demanding applications also in spring after the winter heating season to avoid organic odours. The relative humidity should be kept below 80% to avoid microbial growth. Used air filters should be put into a plastic bag, sealed, marked, and classified according to European Directives or national legislation of disposal.

Classification of air filters

The European standard EN 779:2002 includes the measurement of the particles removal efficiency and a classification system based on filters loaded with synthetic (ASHRAE) dust injected at high concentration. For fine (F) filters the classification system used the average of 0, 4 mm particle efficiency measured along the whole artificial clogging process up to very high pressure drops (450 Pa)

The artificial test dust is not representative of the atmospheric dust. Unfortunately there is a big discrepancy between laboratory filter tests and actual operation performance. Several studies have shown that filters with electrostatic charged media lose efficiency when exposed to fine atmospheric aerosols (figure 3).

Due to the difference between laboratory and real operation performance of air filters in HVAC systems it is useful – sometimes mandatory – to test and control the actual removal efficiency of an installation (In Situ tests). The minimum life efficiency (MLE) in an installation is indicated by the discharged efficiency according to EN 779. An efficiency specification of air filters should besides the filter class include the required minimum efficiency of the filter in operation. This is the only efficiency which could be guaranteed and checked in the installation.

Certified filters

Eurovent has a certification program for air filters, which guarantees the air filters to be within the claimed filter class and pressure drop measured in accordance with EN 779:2002. Bu the discharged efficiency has been excluded and is not part of the certification.

The Swedish Technical Institute, SP, has a certification program (with P – marking system), that guarantees the quality of the filter in the laboratory and production as well as in real life. A P – marked F7 filter shall for instance have minimum 50% efficiency of 0, 4 mm particles during a 6 months test with outdoor air.

Workshop 1 “Presentation of the guidebook from REHVA concerning HVAC air filters and future actions on filters” is co-sponsored by Camfil.

Source: REHVA Journal – May 2010

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The Camfil Ltd Team