- 14 February 2007 -

 

Upgrading indoor air filtration: The right decision, the right time

How do you know when to upgrade filters responsible for indoor air quality - and which ones to choose? Marty Schloss, KBD/Technic takes us through a decision making solution that makes the process cleaner for everyone.

Traditionally, HVAC system filters in manufacturing plants were designed to provide only the minimum filtration necessary to protect cooling or heating coils from dust or contaminant build-up. As Indoor Air Quality (IAQ) has become an important issue for both general production areas and associated office space, plant managers increasingly are seeking to upgrade their existing filters without adding significant operating and replacement costs. This cost-effective solution has gained particular attention recently due to advancements in air cleaning technology and new products coming to market.

This article provides a three-step decision process designed to guide plant managers in the analysis and selection of an upgraded filtration scheme: 1) requirements definition, 2) air cleaner selection, and 3) life cycle cost analysis. Without proper planning and analysis, there can be considerable waste - both in the initial installed cost of the system, as well as over the operating life of the equipment when excessive power and maintenance costs accumulate. (Note: Specially designed HVAC systems - such as clean rooms for electronics or pharmaceutical plants - have been designed with these process requirements in mind and, as a result, are excluded from this analysis.)

Initial HVAC System Assessment

Before spending time and money on the analysis of an air cleaner upgrade, an initial assessment of the total HVAC system design is recommended to determine if the contemplated upgrade to the air cleaner will achieve the desired objectives. For example, upgrading the air cleaner may not provide the desire results if another part of the system is not properly designed or has inadequate capacity. In some situations, maintenance on the existing air cleaning device - including seals, retainers, mounting racks, plates, equipment casings, and access doors - may be sufficient to provide the desired increase in the air cleaning efficiency of the system.

Decision Process

The decision process for the evaluation of HVAC air cleaner performance and life cycle costs is similar to the evaluation of process filtration systems. The analysis of the advantages of an air cleaner upgrade includes defining reasonable IAQ goals for the areas, selecting the appropriate air cleaner type and efficiency, and a life cycle cost analysis.

Step 1: Defining IAQ Filtration Requirements

Although employee comfort has always been a driving requirement in HVAC system design, reducing employee exposure to contaminants and manufacturing quality issues now have been moving to the forefront in the HVAC system design criteria.

The proper selection of the air cleaners can reduce the concentration of particulates being introduced into the space by diluting the concentration of contaminants and possibly reducing employee particulate exposure. Also, the air cleaners can remove particulate that can cause product contamination or cross contamination.

Quantifying the type, size and concentration of contaminants to be removed is the first step in the decision process. The removal of particulate from the air stream requires a different approach from the removal of odours and vapours. Typically HVAC filters are designed for particulate removal and will not remove most odours. With recent advancements in media technology, however, filter media is now available with carbon impregnated or other compounds for odour removal. This article is focused on particulate or "dust" removal. Dust is classified as solid particles smaller than 100µm (micrometers) with fine particulate smaller than 2.5 µm.

Some of the common particle sizes are human hair (100 to 150 µm), common pollens (15 to 25 µm), observable dust in air (>10 µm), atmospheric dust (0.3 to 3 µm), bacteria (1 to 5 µm), copier toner dust (0.4 to 1 µm), tobacco smoke (0.1 to 1 µm) and viruses (<0.1 µm).

The size of the particulate to be removed is an important consideration in the selection of the filter type because the removal efficiency can vary widely on the different sized particles.

The concentration of the containment affects the amount of particulate not removed by the filter and introduced into the space, as well as the frequency of filter replacement or cleaning cycle.

Step 2: Air Cleaner Selection

After the design goals have been set, the next step is to select the proper air cleaner and determine the type of filter or air cleaning device required. The three variables to consider are:

. Efficiency (measure the ability of the filter to remove particles from the airstream);

. Resistance to airflow (the static pressure drop across or air pressure differential across the filter at the rated air flow);

. Dust holding capacity (the amount of particulate the filter will hold when it reaches the maximum air pressure differential the filter is designed for).

Air Cleaner Efficiency

Consider efficiency at the desired particle size and distribution. A filter may be 75% efficient at removing 10 µm sized particles but only 20% efficient at removing 3 µm sized particles.

Determine what tests were used to measure the efficiencies. For example, an arrestance test is based on the weight reduction of all size dust particles in the test dust by the filter and is suited to distinguish between many types of low to medium efficiency filters. The dust spot test measures the ability of a filter to reduce soiling of fabrics and building interior surfaces and is suited more useful for higher efficiency filters. A more recent air cleaner rating standard "Minimum Efficiency Reporting Values" (MERV) has been developed to more accurately rate air cleaners throughout their operating life.

Compare the initial or "clean" efficiency with the final efficiency. In the case of media filters the collected dust cake on the filter increases the filtration efficiency. Electrostatic air cleaners lose efficiency as the collection plates become dirty.

Resistance to Airflow

The filter is rated at initial clean air pressure differential and final air pressure differential. The system fans will be required to produce full design air volume at the final air pressure differential for the system to operate correctly. The fan manufacturer is a good resource to assist with this analysis.

Each filter is rated and tested for design airflow and based on the inlet size of the filter (design face velocity). It is important to determine the filter bank size required for the system airflow based on these values to determine if additional space is required or if the new media will occupy the same space as the existing filters.

An important concept to remember is that the energy operating costs go up with the increased air pressure drop. A filter with an air pressure drop of 200 Pascals will require twice the power of a filter with an air pressure 100 Pascals.

Dust Holding Capacity

The dust holding capacity in conjunction with the airstream particulate determines the operating life of the filter from clean to replacement or cleaning. A filter with twice the dust holding capacity as another will require replacement or cleaning 50 percent less often.

When the objectives require a high level of filtration on 3 µm and smaller particles, a less efficient prefilter in front of the final filter is advisable to eliminate the larger particulate from the airstream before the final filter this will increase the life of the more expensive final filters. This prefilter may require additional space, air pressure drop, and increased energy and maintenance costs.

Air Cleaner Types

There are many types of air cleaning devices available for HVAC applications including viscous impingement panel media filters, dry extended surface media filters, electrostatic filters, minipleat style media filters, renewable media filters, and membrane filters. For a quick broad comparison the following is a list of commonly used media type filters commonly used for HVAC systems utilizing the MERV rating system. On the MERV scale a MERV 1 is lowest efficiency to a high of MERV 20 for a HEPA or ULPA filter.

MERV 1 to MERV 4

. Applications with a typical controlled contaminant of above 10 µm (residential and minimum filtration)

. Disposable fiberglass or synthetic panel filters with arrestance efficiencies below 80%.

. Washable filters with aluminum mesh or foam rubber panel filters.

. Extremely low cost, extremely low pressure drop (25 Pascals), and low efficiency.

MERV 5 to 8

. Applications with a typical controlled contaminant of 3 to 10 µm (commercial and industrial workplaces).

. Disposable fiberglass or synthetic panel filters with arrestance efficiencies of 80% to >90%. This type of filter is the most used in HVAC systems in operation today.

. Cartridge filters, viscous coated cube or pocket filters, synthetic media.

. Pleated filters which are disposable, extended-surface, 25mm to 125mm thick with cotton/polyester blend material, cardboard frame.

. Low cost, low pressure drop (180 Pascals), and medium efficiency.

MERV 9 to 12

. Applications with a typical controlled contaminant of 1 to 3 µm (better commercial and industrial workplaces and some hospital laboratories). Efficiencies above 95% arrestance.

. Bag Filters with ultrafine fiberglass or synthetic media, 300 to 900mm deep, 6 to 12 pockets.

. Box filters, rigid type cartridge filters 150 to 300mm deep using air laid or wet laid media.

. Medium cost, medium pressure drop (350 Pascals), and medium/high efficiency.

MERV 13 to 16

. Applications with a typical controlled contaminant of 0.3 to 1 µm (hospital inpatient care, general surgery, smoking lounges, superior commercial and industrial workplaces.). Efficiencies above 98% arrestance.

. Bag Filters with ultrafine fiberglass or synthetic media, 300 to 900mm deep, 6 to 12 pockets.

. Box filters, rigid type cartridge filters 150 to 300mm deep using air laid or wet laid media.

. High cost, high pressure drop (600 Pascals), and high efficiency.

MERV 17 to 20

. Applications with a typical controlled contaminant of <0.3 µm (clean rooms, pharmaceutical manufacturing, orthopedic surgery). These filters are extremely efficient HEPA and ULPA filter which have very high cost, very high pressure drop (1250 Pascals), and extremely high efficiency.

Step 3: Life Cycle Costing

The final step in the analysis is to determine the life cycle cost of the proposed changes including first cost, replacement costs, and energy operating costs.

First Cost

Because the upgrading of the air-cleaning scheme can involve changes to equipment, it is important to include the following costs in the analysis:

. First cost of the new filtration equipment including required modifications to the HVAC equipment to accommodate a longer or larger face area filter bank. This may require transitions from the existing equipment modules to the new module. It is important that the air be straight entering the air cleaning module for proper operation.

. Modifications or replacement of HVAC fans or fan motors in response to increasing the filtration efficiency often results in increased air resistance and additional air pressure drop on the fans. The fans must be analyzed to ensure that they have adequate pressure capacity and motor horsepower to provide for the increased pressure requirements. Motor wiring and starters may need to be upgraded as well.

. First cost of any additional floor space requirements for equipment modifications or relocations.

. First cost of utility modifications or additions to accommodate the new air cleaning scheme.

Replacement Cost

All air cleaners require periodic cleaning or replacement. Media filters require replacement when they have reached their maximum design air pressure drop or dust holding capacity. Electrostatic air cleaners require cleaning to maintain peak efficiencies. The costs associated with these functions include:

. Replacement costs of new media and the associated labor and downtime required to perform the maintenance. Some filters may require replacement every week or two, others may last for a year.

. Energy operating costs associated with the equipment due to the resistance to airflow of the filtration equipment.

. Disposal costs of used filters.

These can be partially offset by:

. Productivity gains or health savings from improving the work environment.

. Reduced maintenance costs due to cleaner air passing through coils or cleaner workspaces will be cost savings.

Operating costs

The resistance to airflow or air pressure drop resulting from the air cleaner must be generated by the HVAC fans. A formula for calculating the energy consumed by the fans due to the filters is:

Fan Electrical Cost = (Hours of operation x Air Flow x Air Pressure Drop x Cost/kWh)

(Fan Mechanical Efficiency x Constant x 1000w/kWh)

Where:

Fan Electrical Power Cost = Cost per year

Hours of operation = Total operating hours in a year

Air Flow = Flow in Cubic meters of air per second

Air Pressure Drop = (Final - Initial air pressure drop)/2 in Pascals

Cost/kWh = Cost for the average kilowatt of energy for the plant

Fan Mechanical Efficiency = Assume 80% for most HVAC fans or fan manufacturer information.

Conversion Constant for standard air = 1 in SI units

Typical energy operating costs

The table above is based on 10m3/second airflow, 3000 operating hours per year, 80% fan mechanical efficiency, and assumed average pressure drops for the different filter types.

Summary

Indoor Air Quality (IAQ) has moved to the forefront as a major design factor for HVAC systems. The selection of air cleaning devices in the HVAC equipment can meet many IAQ requirements if reasonable IAQ filtration goals are established and matched with the appropriate air cleaner type and efficiency, without sacrificing first cost considerations and long term operating costs.

Continuing advances in new air cleaning devices, filtration media, and technologies are directed at offering the greatest filtration efficiency at the lowest air pressure drop to minimize operating costs.

About the author:

Marty Schloss PE is General Manager Southeastern Operations of KBD/Technic, Inc (a CECO Environmental Company) Greensboro, NC, USA. He is a professional engineer, with 28 years experience in the design of HVAC system and pollution control systems in foundries, textile, steel, and metal fabrication plants.