By Benjamin Brits

As we see the world turn to masks as one of the solutions to a global pandemic, it is compelling to think that this was in fact the first recorded method of air filtration.

It was in the late 1840s when the first air-filtering mask was invented and patented. The design incorporated moistened wool and one-way clapper valves and was able to filter dust particles. Subsequently, the original patent for the gas mask was also lodged in 1849 by the same inventor.

In the 1900s, large-scale air filtration went through the most significant developments as complete HVAC systems were developed, incorporating various methods of air-cleansing.

An example of a galvanised sheet metal primary filter. Image credit: Ulpatek (Airbox)

It was also the period where the high efficiency particulate air (HEPA) filter was first invented – one of this year’s hottest components. HEPA filters were originated by the United States defence force between the two World Wars as a protection mechanism against radioactive chemicals used to build the first atomic bomb. The technology was released commercially after World War II.

Today, having a look at any filter company’s brochure, their range of products really opens up a very large door to how and where filters are used, as well as the development that has occurred in filtration over the years.

For the HVAC sector, air quality has become a much bigger commercial concern than what has been required before as building occupants have developed heightened awareness of air quality in relation to pathogens and pollution, and sick building syndrome and social distancing at workplaces are in the spotlight for design reviews.

On the industrial applications side, processing and manufacturing produce various fumes or gases that need to be collected because this cannot just to go into the atmosphere, so extraction systems are also designed that take processed air to a collection point to be filtered before being released.

So, air filtration can be applied to air into a space, or air out of a space and is therefore intended to contain particulate either in or out, ensuring a healthy environment for the occupants and environment.

Filters and HVAC design

Air filtration naturally depends on the type of HVAC system design and the particular application. The most common commercial, industrial (and medical installations) incorporate air handling units (AHUs) and configurations vary between what the desired outcome will be. This could be as simple as the reduction of dust, to ensuring a complete contamination-free zone.

“The legislation in South Africa regarding filtration requirements is covered in the SANS 0400-1900, and is grouped together with plumbing and electrical. Interestingly, the standard doesn’t indicate a particular value (micron size or particle size) when talking about air quality with in applications, but rather it specifies requirements around tobacco smoke or no tobacco smoke, with x-many air changes required. One question you have to ask yourself is how many systems conform to the regulations in this regard? “Engineers design based on criteria and requirements inside of a building which may not require such intense air filtration or which is just not necessary for the particular application,” says Hennie Verster, marketing manager at Freudenberg Filtration.

Part of the air filtration design process is also the establishment of positive or negative room pressures, as in some instances you want to avoid certain particles entering a space but then you may not want particles to leave a space – this is particularly important in testing laboratories, medical facilities and even production lines where cross contamination can occur between different products in powder forms.

It is important to then understand the types of particles ‘in the air’ that are required to (or you wish to) be filtered out, which include natural sources as well as sources of human origin (not limited to):

Natural sources – Organic and inorganic – generally 1 – 10 μm

  • Organic debris such as leaves or seeds
  • Pollens
  • Fungal spores
  • Hair and skin cells
  • Bacteria
  • Viruses
  • Sandstorms
  • Landscape fires
  • Volcanic activity
  • Erosion
  • Salts
Freudenberg pg5The composition of air: Size comparison of solid and gaseous substances in the ambient air. Image credit: Freudenberg Filtration

Man-made sources – Outdoor and indoor – generally less than 1 μm

  • Combusion process in engines
  • Domestic heating
  • Tyre fumes
  • Air traffic / dumped fuels
  • Gases produced by industrial processes
  • Agricultural pesticides
  • Clothing and carpet particles
  • Mineral fibres
  • Emissions from laser printing machines
  • Cigarette smoke, candles, incense
  • Cleaning products

Again, the starting point of your design would come down to the application and desired outcome. The lists above indicate the types of particles that can be found in an ‘air sample’ to be considered in air quality, however another particular element in filtration in the HVAC sector is the protection of components that may be as important on a site where corrosion is a factor, where sands can penetrate mechanical units or dust can cause short circuiting of electronic components.

The types of filters that are applicable to the HVAC sector generally are:

  • Primary filters. These are your first line of defence and is typically a consumable that is thrown away after being cleaned or washed a couple of times. Primary filters only stop coarse or bigger particles such as leaves or feathers.
  • Secondary filters. These come in either a pocket or a cassette design.
    • Pocket filters are normally used in shopping malls, office buildings and data centre applications. They reach a point of particle saturation, and generally cannot be cleaned as the material deterorates.
    • Cassette filters have a much better dust holding capacity than the pocket filter. Cassette filters can’t be cleaned and must be thrown away when expired.
  • HEPA filters. These filters are typically used in most medical applications, clean rooms and production facilities where pure air is required such as food processing and packaging.
  • ULPA filters. These filters come from the same line of materials to make HEPA filters and as the manufacturer tests the material the sections that are particularly good are allocated to the ULPA range. These are very expensive products.
  • Carbon filters. Carbon filters are capable of handling fumes, smells and gases, and come in two different types: a cassette-type filter whereby the media has been impregnated with carbon allowing this type to be used in a variety of applications. The second type, and for more specialised applications, carbon filters also come in modules that are essentially filled with carbon pellets that absorb all of the odours, gases or chemical to a point of saturation and are then disposed of through incineration.
    • Carbon can be blended to suit particular applications in removing gases, odours or acids, which would be different to normal air ‘smells’. This blend can then be impregnated in the filter media.
Airbox 17An example of a v-compact type activated carbon filter typically used in a public building, airports, workshops, hospitals, food and pharmacetutical industry, industrial and civil air conditioning systems. Image credit: Ulpatek (Airbox)

All filters come in various configurations, sizes, airflow ranges, pressure capacities, particle size capabilities and efficiency ratings. They are also produced in a variety of materials such as galvanised sheet metal, plastic, aluminium and wood (MDF). Custom sized products are also available to be manufactured for special applications.

“Sizes of filters in a design are determined by the amount of air pushed through the system. One filter can only handle so much air at a time before it bursts. Generally one full-sized filter (600 x 600) can handle 1 cubic meter of air per second. Obviously, a 1m diameter fan with a small fiter cannot work and will pull the filter apart. Incorrect sizing can result in major damage to the filters, equipment and AHUs, especially when you are working powerful fans. This element is particularly important because if you spec the wrong product for the system, you pay the price with a destroyed system,” says Rean van Wyk, managing director at Airbox.

What is known to the South African market are filters that are classed according to EN779 standards. EN779 is the ASHRAE standard establish whereby dust was taken out of Las Vegas, in America and analysed. This analysis established how much of each type of particulate was in the dust, but in other countries the dust composition is vastly different.

“A new ISO standard on filter selection was established – ISO1689. The major difference is that we are now no longer looking at a particular dust composition, but rather working on the principle that a particular filter must stop a particular size of particle, looking at the  fractional efficiency of the filter. This is broken down into four categories – Course, 10, 2.5, and 1. As an example an ISO-1 90% rated product needs to filter 90% of all possible air compositions with particles of 1micron or less. The new standard was implemented as of 2018 but has not been well used and quoted to-date,” says Verster.

All about HEPA filters

The idea of any application using HEPA filters is to protect this filter type because they are an expensive product. HEPA filters have typically only been broadly used in the medical, chemical and scientific fields until recently, but as air quality needs change, they have been incorporated into many other application areas.

“Engineers in the industry currently specify that HEPA filters must come with a test certificate, so each HEPA filter is issued with a specific certificate indicating the standard and test data parameters – this is commonly known as an MPPS Test Certificate and is widely considered essential. The certificate states a serial number and indicates the products rating achieved in the testing process – proving the manufacturing was completed correctly. Batch certification is not generally accepted in South Africa,” says van Wyk.

Imported HEPA filters for the South African market primarily come from suppliers in Malaysia, Israel, Turkey, Germany and America and are in fact a very sensitive product. Only a very small surface area on HEPA filter is allowed to be repaired before being classed as un-useable.

HEPA filters are made from glass fibre material that has been pleated – it is supplied in rolls and then it goes into a pleating machine. As it is pleated, glue lines are laid down to hold the material together. Most of the companies around the world manufacture these filters in a clean room where the whole facility is contaminant-free.

UV-C as part of the  air filtration process

UV light is another common term in today’s industry lingo and is a relatively new technology in the HVAC sector for commercial applications. UV or ultraviolet light is produced in three main categories, UV-A, UV-B and UV-C.

UV-C lighting typically found in an air purifier. Image credit: AC Expert

The three types of UV radiation are classified according to their wavelength. They differ in their biological activity and the extent to which they react with living organisms. The shorter the wavelength, the more harmful the UV radiation.

Short-wavelength UV-C is the most damaging type of UV radiation. However, in its natural state it is completely filtered by the atmosphere and does not reach the earth’s surface. UV-C can however be produced on earth and its effect is essentially damage to DNA and RNA, not allowing for cell replication, thus rendering a microorganism or virus inactive, or, in simple terms, killing it.

To-date no documented effects have been made in UV-C’s reaction to any steel or wood components, however the effect on plastics has been questioned considering the normal damage products experience outdoors.

“UV-C lights are considered a combination solution with HEPA filters especially when it comes to viruses and bacteria as the filters catch, and the UV lights kill. So, as a combination they work effectively, and also offer safer handling in maintenance. One very important thing to remember is that when you are considering UV-C in system, the air needs to spend a certain amount of time exposed to the UV-C in order for it to be effective. This is where a lot of confusion can come in when installing UV-C solutions because alone it cannot solve the problem of pathogens. For recirculating, this solution may be viable in future, but effectiveness comes down to airflow or several sections of ducting being fitted with UV-C lighting,” notes van Wyk.

Verster adds, “UV lights are also an expensive product, and effectiveness is directly linked to airflow vs the time of exposure, but there is honestly not enough research to indicate the time/exposure factor and how well this actually works at this stage. This is the same for the coronavirus generally – there are a lot of assumptions and now new research also indicates that dust particles can attach themselves to the virus taking it out of aerosol form. The available information is so vast and different. The only consistent recommendation from various world bodies is to increase ventilation. Although UV-C will kill all pathogens, scholars are also highly concerned with what other effects it can have on humans.”

Freudenberg pg6An example of an air handling unit (AHU) indicating, pocket filter, cassette filter and HEPA filter. Image credit: Freudenberg Filtration

The maintenance debacle with filtration and HVAC

“Maintenance is another big issue in our industry as most people know. Poor maintenance becomes a critical problem especially when the AHU is feeding a line, section or space where clean air is essential. Depending on the particular conditions, filters may become full of moulds or other pathogens on top of the the regular dusts and particles, and the AHU itself can also be neglected to the point where it is falling apart – rendering it useless,” says van Wyk.

Poor or no maintenance can lead to production shutdowns or decontaminations being required and this can be an extremely challenging and a costly excersise when an office building or hotel needs to be taken care of and income is affected. This is the same for industrial processes where equipment’s lifecyle is shortened by excessive corrosive particles being present in the space.

Primary filters, as a first defence also experience the same challenges and need to be well maintained to prevent damage and reduced lifespan of secondary and HEPA filters.

Another challenge is that many clients do maintain the filters, but the whole AHU has been there for 20 years so its like a sieve anyway – it sucks from all of the areas of wear and not through the filters allowing pathogens and bacteria into the space.

Freudenberg pg9
A large-scale application of pocket filters. Image credit: Freudenberg Filtration

Poorly maintained filters also make the other components have to work that much harder – fans work harder, chillers work harder – and this increases operational and replacement costs.

Filters should also no be touched when maintenance is being done because they may be contaminated and need to be incinerated.

“Microbial growth on filter media may cause damage to filters and become a secondary contamination source. Moreover, backdraft during filter replacement also posts health and safety concerns to maintenance workers. Backdraft refers to the threat of secondary contamination to technicians and the environment/site faces once airflow through a duct is suspended in order to replace a filter. What happens is that the filter is full of dust, which is loaded with active bacteria and virusus may colonise on the filter. It makes sense that handling a filter jam-packed with contamination will contaminate the system, air channel, and the circulation air in general,” says Michael Smorenburg, managing director of GermZero South Africa.

“Another very important thing to mention is that you cannot tell if a filter needs to be changed by simply looking at it – so you have to check differential pressures,” adds Verster.

New technology

There are various new mobile filter systems or air purifying units with integrated HEPA filters that come in sizes to cater for small rooms to large industrial workshops. These units will become particularly important for existing buildings.

There is a new coating available for filters in South Africa called ‘Germagic’. It is an antimicrobial coating which gives contact-killing, release-killing and anti-adhesion abilities to HEPA filters. Unlike conventional antimicrobial technologies which solely rely on contact-killing, the coating releases reactive oxygen species in a controlled-manner, allowing for a germicidal effect in the coating’s proximal area. This product can be applied to any filter including new installations, retrofits, and existing filters in an HVAC system.

“The polymer nano-technology in Germagic was developed over a decade ago as a longterm/slow-release disinfectant response to the SARS and MERS outbreaks. The coating in spray form can actually be added to any surface. There is also a filter range that aready comes impregnated with the active ingredients. The filter comes in a pouch and folds out to size of 600mm x 400mm. It can be cut to fit any size of HVAC/filter or grill. Where the spray application is used, we recommend standard PPE and the use of an electrostatic spray system,” says Smorenburg.

HEPAPleat 450
                                                                                        An image showing the pleating process of the glass fibre material. Image credit: Mann and Hummel

Looking to the future

“Existing buildings can’t easily be retrofited with any type of HEPA system because HEPA filters take up a lot of the fan pressure, so current systems would need upgrades through additional or replacement fans or adapted AHUs. Offices with splits or non-ducted casettes will have to rely on mobile air purifyers if air quality is a key requirement. Everything is gong to change in future and the engineer will hopefully rethink designs of big offices and open areas in terms of the HVAC system for future alterations. Currently the regulation in place dictates a certain amount of air changes per hour per space, where every room is different and this needs to be evaluated for HEPA inclusion. I believe HEPA will be a definite need as is seen in overseas countries who have very strict air quality policies. With the addition of HEPA and possibly UV-C lighting this will significantly increase general health for building occupants,” notes van Wyk.

“What I see for the future is that better filtration is going to be dictated by the end users and multiple filter scenarios will need to be implemented. Different designs have implications, so if you previously had a small filter, and introduce all of these pressure drops with additional filters, you have to overcome this. Coil designs will change and fan designs will change. Because the design changes it’s easier to add UV-C lights and so on. AHUs will have to change to match the pressure requirements too. Everyone now wants clean air and trends will have to move towards better filters and removing finer particles out of the air,” says Verster.