In my business field, many customers inquire about HVAC or general ventilation filters. These air filters have a wide range, usually including panel, bag, and box filters, and the filtration efficiency ranges from G4 to H14. So, this is a relatively extensive range, including most forms of existing air filters.
Decades of experience have given me a lot of knowledge about these filters. I have mentioned some of them in previous articles, such as their working principles, testing methods, etc. In this article, I will give several tips, including practical applications, based on the situations of many buyers.
1. Pick an air filter that has a big filtering area
I have to say, this is a practical tip that many users and buyers often ignore.
The part of the air filter that catches dirt is called the “filtration area.” Except for a few G3 and G4 filters, the filtration area is usually several to a hundred times larger than the front side of the filter where air enters.
Dust mainly gathers on the side where air enters the filter. A more extensive filtering area means the filter can hold more dust and last longer. With a larger filtration area, airflow moves slower through the filter, reducing resistance.
Increase the lifespan of the air filter
Expanding the air filter’s area is the best method for prolonging its lifespan.
Based on my experience, when filters have identical structures and filter materials, increasing the filter area by 50% can extend the filter’s lifespan by 70% to 80% if the final resistance remains the same. Doubling the filter area can extend its lifespan to approximately three times its original duration.
Adding more filter material will raise the air filter price, but the increased lifespan outweighs the price hike. Moreover, with a more significant filtering area, the initial resistance decreases, leading to potential energy savings in the air conditioning system.
Factors to consider when increasing filter filtration area
Certainly, when enlarging the filter area, it’s essential to consider both the filter’s structure and the conditions at the installation site.
For instance, with pocket filters, you can raise the filtration area by adding more filter bags or increasing their length. For air filters featuring separators or mini pleats, consult with the manufacturer about reducing the spacing between separators to add more pleats to the filter.
Opt for air filter types in new projects that can hold more filter material.
Hint: The filter’s efficiency isn’t greatly influenced by the size of the filtration area. However, selecting a filter with a larger filtration area remains cost-effective for the end user.
2. Identify filtration performance by comparing fiber diameters
In the filtration process, fibers serve as barriers to trap dust. Thinner fibers mean more fibers per unit volume, leading to higher filtration efficiency.
Airflow moving around the fibers consumes energy, which is reflected in the resistance of the filter material to airflow.
Coarse fiber material resists airflow more than fine fiber material, even if both materials have the same filtration efficiency.
Added advantages of fine fibers
Besides being trapped by fibers, dust can also be captured by previously trapped dust. As a result, the dust loosely accumulates on the fiber surface, forming a “dendritic structure,” where the fibers act as “stems” and the dust as “branches.”
With numerous fibers closely spaced together, the branch-like structure formed by dust is robust, reducing the likelihood of dust being blown away and causing secondary pollution.
Usually, the greater the number of fibers, the more dendritic structures can form, allowing more dust to be trapped per unit area, thereby extending the air filter’s lifespan.
Filtration efficiency and resistance
From a logical perspective, the filter material’s fibers should be as fine as possible while still strong enough to withstand airflow resistance.
Fine fiber filter material exhibits high filtration efficiency compared to filter material of the same thickness and fluffiness. Additionally, fine fiber filter material typically has a greater dust-holding capacity.
Given identical efficiency and structure, two filter materials made of different fibers will result in the fine fiber filter material having lower resistance.
3. The HVAC system itself needs good filters
In past years, air conditioning designers have chosen filters based on users’ environmental needs. Today, there’s a growing recognition that quality filters also play a vital role in protecting the air conditioning system.
Relying solely on low-efficiency air filters can lead to numerous issues with HVAC systems, such as:
——Temperature and humidity sensors may fail;
——Pipes with moderate temperature and humidity are ideal breeding grounds for microorganisms due to dust accumulation.
Due to dust accumulation, many central air conditioners’ performance declines significantly after one or two years. The leading cause is low filter efficiency.
Air conditioning cleaning costs
In developed nations, central air conditioning systems using F5 efficiency filters usually necessitate cleaning every 5-8 years, while those equipped with F7 efficiency filters may go over 30 years without requiring cleaning.
Installing a high-efficiency filter may entail some initial cost, but this expense is minimal compared to the losses incurred from using a low-efficiency filter. The potential loss in the air conditioner’s lifespan due to dust accumulation far exceeds the cost of employing a high-efficiency filter. Additionally, the cleaning expenses associated with a low-efficiency filter will likely surpass the cost of using a high-efficiency filter.
The filter efficiency should be at least F6-F7 on an excellent air conditioning system.
4. Effect of airspeed on air filter
In many instances, lower wind speeds result in better air filter performance.
Mechanism
The diffusion (Brownian motion) of small-size dust is evident. At low wind speeds, airflow remains within the filter material for extended periods, allowing dust to encounter obstacles. As a result, filtration efficiency is higher.
Based on my experience, decreasing the air speed by half for HEPA filters reduces dust transmittance by nearly tenfold, while doubling the wind speed increases transmittance by an order of magnitude.
Traditionally, for large dust particles primarily influenced by inertial mechanisms, as per conventional theory, when airspeed decreases, the likelihood of dust-fiber collisions decreases, resulting in a corresponding decrease in filtration efficiency.
However, this effect is not readily apparent in practice because, at lower wind speeds, the fiber’s ability to bounce back against dust is diminished, making it more likely for dust to adhere to the fibers.
Airspeed and resistance
Higher wind speeds result in increased resistance. If the final resistance determines the filter’s lifespan and the wind speed is high, the filter’s lifespan will be shorter.
For regular users, it’s challenging to observe the influence of airspeed on filtration efficiency, but it’s much simpler to notice its effect on resistance.
HEPA filters usually experience airflow speeds between 0.01 and 0.04m/s. In this range, the filter’s resistance increases proportionally with the volume of filtered air.
For example, consider a HEPA air filter sized at 484mm x 484mm x 220mm. At its rated air volume of 1000m³/h, the initial resistance is 250Pa. If the actual air volume decreases to 500m³/h, the initial resistance can be reduced to 125Pa.
In air-conditioning units, general ventilation filters typically experience airflow speeds ranging from 0.13 to 1.0m/s. The resistance and air volume relationship in this scenario isn’t linear but follows an upward arc. When the air volume increases by 30%, the resistance may rise by as much as 50%.
Ask the air filter supplier for a resistance curve if filter resistance is critical.
5. Can the filter catch bacteria?
My answer is an air filter to capture bacteria under certain conditions.
The filter treats microorganisms suspended in the air as particulate matter and eats them all. The filter only ensures that it blocks airborne microorganisms like ordinary particulate matter.
To prevent the growth of microorganisms, it’s essential to avoid having nutrients above and below the filter, and the environment where the filter is installed should not be excessively humid. Ongoing advancements introduce new filter materials with sterilization capabilities, suggesting these sterilizing functions are valid.
Filter materials coated with bactericidal substances can target specific microorganisms. However, in most cases, the filter operates with accumulated dust. Despite the potent sterilization capabilities of new filter materials, it’s challenging to dislodge the dust on the filter for sterilization purposes.
If there’s a genuine need to eradicate microorganisms mixed in the filter dust, appropriate measures must be taken.
6. Fireproof and flammable
In my experience with customers, some prefer fireproof filters, while others specifically ask for filters that are flammable. Each customer has their own reasons for their preference.
Hepa filters used in clean rooms should be fireproof. For this reason, the primary raw materials of HEPA filters should be non-combustible. The corresponding test standards in the United States stipulate that when tested with an open flame, the air filter should not burn or emit smoke or should burn slightly and emit only limited smoke.
For this reason, traditional HEPA filters with wooden frame structures and paper separators are no longer suitable for today’s clean projects.
Air filters in central air conditioning and centralized ventilation systems need to be replaced frequently, so trash becomes a big problem.
Considering environmental protection and reducing garbage disposal costs, users require combustible filters.
There are precise specifications regarding flame retardancy, fire resistance, and temperature tolerance in commercial settings, spanning from non-combustible to combustible materials.
7. Washable and disposable
Most air filters utilized in general ventilation and clean rooms are disposable. They are either not designed to be cleaned at all, or cleaning them is not cost-effective.
F6 and above filters cannot be washed
High-efficiency filters usually work in specific environments. Even if the air filter is not damaged after cleaning, do not wash it because there are currently no tests or instruments proving that high-efficiency filters can still ensure filtration efficiency after cleaning.
Ventilation filters with efficiencies above F6 typically have fiber sizes ranging between φ0.5 and φ5mm. These fibers are not robust and cannot withstand rubbing. As a result, air filters above F6 are mostly disposable.
In fact, you can tell if the filter material is cleanable just by looking at it.
G2-G4 filters are washable
Traditionally, cleaning air filters involves washing with water and rubbing with hands. Therefore, the filter material of the cleanable filter should be strong, and the fiber should be thick (direct inspection ≥φ10mm).
Most of these filter materials are low-priced, non-woven fiber felts. They are ideal materials for manufacturing G2-G4 efficiency filters. Certainly, it’s also essential to assess whether the other materials of the filter are waterproof.
There is no doubt that nylon mesh and metal mesh can be washed.
Last Summary
Some customers like to call ventilation filters HVAC filters, but I think the two are the same because both have similar requirements for air filters.
In fact, what I wrote in this article also applies to other fields because, generally speaking, industrial production has much higher standards for filters than civilian use.
