A customer once mentioned that because he ignored the return air filter, his filtration and ventilation system did not achieve the expected results and even caused customer complaints. So, he had to spend extra money to upgrade the return air filter.
I can’t help but think that in many commercial environments or clean room filtration projects, a good return air filter must be installed. Only in this way can a successful ventilation filtration system be ensured and the desired indoor air quality be achieved.
What is the return air filter?
Return air filters are installed in return air vents in commercial environments such as office buildings, airports, or movie theaters, as well as in industrial clean rooms.
Its function is to filter pollutants in the polluted air discharged from the room when the ventilation system runs. This is especially important in high-level clean environments.
Many factors affect indoor air quality. The optimal design of the system, fresh air volume, equipment performance, etc., can all significantly impact air quality.
Controlling every aspect of air circulation is necessary to improve indoor air quality.
The filtration function of the return air filter is a vital link that has been ignored.
Some common return air filter settings and issues
At present, the return air outlet and its filtration in air conditioning and purification places usually have the following common forms: the first is to add a layer of nylon mesh to the return air outlet; the second method is to add a filter made of multi-layer metal mesh to the return air outlet. The third type of return air outlet does not have any filter.
The method without filtration has been proven to be desirable in practice. Not only can it not meet the functional requirements, but some of the return air vents can also become passages for small animals and insects and a place for randomly discarding debris.
Can we use nylon mesh as the return air filter?
In common semi-centralized air conditioning systems with fan coil units, a single layer of nylon mesh basically cannot filter.
When the particles and fiber pollutants in the air circulate to the fan coil surface cooler, they are stuck to the surface cooler by the condensed water and some drip into the condensate pan with the water. These pollutants become a breeding ground for bacteria to adhere and multiply.
Dust deposited on the surface cooler can also reduce the heat exchanger’s efficiency and the capacity of the entire air conditioning system.
After some air conditioning systems have been running for a period of time, the air blown out by the air conditioner has an unpleasant smell when it is turned on, and the temperature cannot be adjusted to the design temperature on time.
After analysis, contamination of the surface cooler and reduced heat exchange performance were the direct causes, and cleaning the surface cooler was extremely difficult.
Can we use a metal mesh filter as the return air filter?
Some users believe that metal mesh is high in strength, durable, washable, and can block large particles and some insects.
I want to say that it is difficult to thoroughly clean the fibrous pollutants produced by bedding and carpets in hotels, office buildings, and hospital wards. Therefore, using a metal mesh filter as a return air filter is generally not recommended.
What kind of air filter do you choose for the return air filter?
There is no doubt that a fiber material filter is a suitable choice, which can provide relatively high filtration efficiency and ensure breathability and low resistance. In addition, its price is common and will not bring additional cost pressure.
How do you choose a return air filter for the clean operating room？
Given the cost and effectiveness of electrostatic and ultraviolet sterilization, physical barriers—air filters—are our first choice in surgical clean rooms.
The filtration level of the filter installed in the fresh air positive pressure and return air sections is very particular. Too high or too low will affect not only the sterilization efficiency but also the cost.
The counting efficiency of particles >0.5um is at least 60%, medium efficiency filter F5-F7, return air is one of the sources of pollution in the system, so the filter efficiency should be higher.
The return air outlet louver blades should be vertically adjustable blades.
How do you choose a negative-pressure operating room return air filter？
HEPA filters must be installed at the entrance to the air exhaust outlet on the ceiling of the negative pressure operating room and at the entrance to the indoor return air outlet, and a check valve should be installed at the exhaust outlet.
A sealing valve is installed at the return air inlet.
How do you choose a return air filter for an industrial cleanroom?
In some cases, regulations mandate the addition of a return air filter. For example, an air filter should be installed at the return air outlet of a dust-producing workroom to prevent dust from accumulating in the return/exhaust duct. The air outlet filter level is determined according to product requirements.
Adding a primary G4 filter (non-woven filter) to the return air outlet effectively increases its resistance, making it easier to maintain the pressure difference between clean rooms and between clean areas and non-clean areas.
It has better compliance with regulations (the regulations require that in the same space system, the resistance of each air outlet should be roughly the same to avoid changes in indoor pressure. Otherwise, a room pressure difference adjustment device should be used.)
At the same time, it can filter some large dust particles, especially after the clean room equipment is overhauled. This layer of protection is effective for the subsequent air cabinet filters (I believe many people who have done clean projects know this); of course, it can also increase the aesthetics of the clean room.
Last summary
Return air vents are often inconspicuous, and return air filters are even easier to overlook. But they are essential in achieving and maintaining overall indoor air quality. From another point of view, return air filters are also very critical for the stable operation and maintenance of ventilation and air conditioning.
