This is the second in a multi-part series on air filtration [read the first part here].

Clean air is not only a right but also a necessity. Absolute air filtration is the main channel through which clean air can be obtained. But it cannot be achieved without making a conscious effort to understand, and put in place, the equipment and the necessary process involved.

The complexity of the process highlights the factors that affect its performance. Earlier, though this aspect had not been given the importance and the attention it deserved, fortunately, there is a recent shift in emphasis towards air filtration. There is, at last, a realisation that indoor clean air is critically important to the well-being of human beings, gas turbine air intakes, and HVAC equipment.

TYPES OF FILTRATION PROCESSES

The first step in the general understanding of the air filtration process is distinguishing between the two types of filtration processes, namely depth, and surface filtration. Fabric and membrane filters are considered surface filters, while fibrous and granular filters fall under the depth filtration category.

Surface filtration:

In surface filtration, large particles are expected to get deposited on the surface of the filtration medium by a sieving mechanism so as to form the so-called dust cake. Thus, surface filters rely on the formation of the dust cake to do most of the filtration action, and therefore, have high pressure drop. (Pressure drop is the measure of airflow resistance through a filter.) In this process, the dust cake layer itself becomes the main filtration segment of the entire filtration process. A detailed knowledge of the properties of the local atmospheric dust is needed to better evaluate the surface filtration process. Pulse filters are a good example of surface filters, and are generally used in gas turbines.

Depth filtration:

Depth filtration relies on capturing the particles within the filter medium. This method of non-cake filtration requires an understanding of the media properties. Depth filtration process allows fibrous filters to have the same or better degree of purification and minimal resistance as that of surface filters.

In this process, glass fibre media is used extensively due to its low pressure drop and better performance at high temperatures. The structure of fibre glass media is delicate, and may not be regenerated either by washing with water and/or through the use of compressed air. Attempting to regenerate a fibrous filter by ejecting the dust out of it will lead to the destruction of the structure of the media, and the dust particles may not be removed completely.

The structure of fibrous filters is illustrated in Figure 1. The porosity of fibrous filters is around 90%, which indicates that the sieving mechanism plays no significant role in the process. This type of filtration is widely used in the HVAC industry.

OPERATING CHARACTERISTICS

When evaluating any of type of air filter, there are three main performance characteristics that should be of primary interest: efficiency, the ability of the cleaner to remove particulates from the airstream and pressure drop. Pressure drop is dependent upon fluid properties, filtration media properties, and the properties of the collected dust. In fact, pressure drop across an air filter varies with its thickness, fibre radius and filter packing density, air velocity and viscosity. The third performance characteristic is the filter life time, which depends upon pressure drop and efficiency.

LEARNING LESSONS FROM DISASTERS

Though air filters are necessary, it appears that we always need an unpleasant event to remind us of the critical role they play. To cite examples from the recent past, the impact of SARS, Anthrax, and H1N1 are engraved in our collective memories, and we are not likely to forget the 2010 volcanic eruptions of Eyjafjallajökull at Eyjafjöll in Iceland. The catastrophic eruption caused massive disruption of air travel across Europe. However, the great lesson learnt from it was that suspended high particle concentrations in the atmosphere affect nearly all human and industrial activities. Had appropriate filter selection been made and proper maintenance measures exercised throughout the year to the required professional degree, air filters would have played a pivotal role in particle removal in protecting human beings and industrial applications during the havoc caused by the volcanic ash.

Frequent sand storms in the Arabian Peninsula are another example of high dust concentration, where they can negatively influence air conditioning equipment and gas turbine performance. Figure 2 illustrates how dust particles settled on the cooling coils of air handling units can reduce their effectiveness.

AIR CONTAMINANTS

The two major classes of air contaminants are particulate and gaseous. The particulate class covers a wide range of particle sizes from dust, which are large enough to be seen by the naked eye, to the sub-microscopic particles that escape most filters. These particulates may originate from natural processes, such as wind erosion, sea spray evaporation, volcanic eruption and metabolism or decay of organic matter.

Figure 3 shows various contaminants that may exist in the atmospheric air. A filter’s effectiveness in removing different sizes of particulates depends upon its filter class.

In the case of power generation, the prime function of air filters is to provide clean air to gas turbines. This can be done by means of high-efficiency filters installed at the air intake. Obviously, the problem begins when particles collide with the turbine blades and cause impediments like corrosion, fouling, and erosion, which are responsible for altering the performance of gas turbines. Erosion, for example, occurs when a particle strikes the blades at a high velocity. This causes a change in the blades’ mass, leading to rotation imbalance in the turbine assembly.

The presence of salt in the air is another factor to be taken into consideration, as it not only corrodes the turbine blade but also the particles settled on the cooling perforations will lead to overheating. Clearly, in such instances, a strong case can be made for installing air filters. They defend gas turbine assembly and help minimise annual shut times for maintenance and, thus, reduce loss in energy output.

It is evident from the examples cited that fine filters are necessary to remove particles from the air stream and lower their concentrations. However, what is really needed is the use of High Efficiency Particulate Air (HEPA) as a final stage of filtration to guard against air contamination. Although there is a great push in the gas turbine market for this, and the industry is aiming to use HEPA class filtration on the air intake of gas turbines, designers might still be reluctant to use them due to concerns about the pressure drop and associated loss in energy output. However, with the launch of the latest intelligent designs of new ultra-low pressure drop HEPA filters, the use of absolute filtration is gaining momentum, as it can provide the higher efficiency desired, at the required pressure drop.

NEED FOR APPROACH RE-ALIGNMENT

It is indeed ironic that a great deal of money is invested in purchasing the most efficient HVAC equipment, but the same degree of emphasis is not placed on investing in high-efficiency filters. There are a few serious questions that need to be asked and answered: Do HVAC designers pay equal attention to the filtration stage of AHUs as they do to the rest of the components? What stops us from installing the highest efficiency of filtration at home, at work and for industrial applications? The climate is changing; shouldn’t our approach to it?

The writer is Regional Director, Middle East, and International Consultant, EMW Filtertechnik, Germany. He can be contacted at iyad.al-attar@emw.de