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Filtration levels in buildings

One of the most important points for achieving and maintaining adequate indoor air quality is its correct filtration. This article addresses the regulations and relevant aspects to be taken into account in the  design and specification of filtration systems in the building sector.

Classified environments, which are subject to more specific filtering levels, will not be covered, and to which STULZ has already dedicated and will dedicate specific publications by sector and type of application.

1.- APPLICABLE REGULATIONS AND DEFINITION OF FILTRATION LEVELS

The mandatory regulations governing air quality, from the point of view of the design and operation of air conditioning systems, is the Regulation of Thermal Installations in Buildings (RITE). Currently in force under RD 1027/2007 but with the latest modification in 2021 with the appearance of RD 178/20211 .

This regulation addresses the need for filtering, and the type of filters to be used, in the IT 1.1.4.2.4. about “Minimum ventilation outdoor air filtration”.

In addition to the RITE, since it is referenced in it, the applicable standard is UNE-EN ISO 16890:1:2017 on air filters used in general ventilation - Part 1: Technical specifications, requirements and  efficiency classification based on particulate matter. (ISO 16890-1:2016)

 

HOW IS THE FILTRATION REQUIREMENT MADE?

In the technical instruction itself (IT 1.1.4.2.4. Minimum filtration of outside ventilation air) it is required that the outside ventilation air must be introduced duly filtered into the buildings. In addition, it establishes the minimum filtration classes to be used, depending on the quality of the outside air (called ODA) and the quality of the inside air required (called IDA). Specifically, the requirements are regulated in table 1.4.2.5 of the regulation. It is reproduced below.

 

Air quality (Table 1.4.2.5 of RD 178/2021)

EXTERIOR CAI INTERIOR CAI
  IDA 1 IDA 2 IDA 3 IDA 4
ODA 1 F9 F8 F7 F5
ODA 2 F7 + F9 F6 + F8 F5 + F7 F5 + F6
ODA 3 F7 + GF* + F9 F7 + GF + F9 F5 + F7 F5 + F6

* GF= Gas (carbon) filter and/or chemical or physical-chemical filter will be necessary in case ODA 3 is reached due to excess gases.

 

HOW TO INTERPRET THE REQUIREMENT? DETERMINING FILTER EFFICIENCY

As can be seen in the table above, the requirement for the “quality” of the filtration is categorized with an alphanumeric code. Although the current standard is UNE EN ISO 16890:12 These references are based on EN 779:2013.

This standard set an average filtration capacity measured in three levels: For the so-called "coarse dust" corresponding to particles larger than 10 μm (equivalent to 0.01 mm), class G was established. Classified from G1 to G4, according to a certain average arrestance (Am). Arrestance is the capacity of a filter to retain particles in the air, but it refers to the capacity to retain larger particles and is measured as a percentage.

Saying an arrestance of 50% indicates that the filter is capable of retaining 50% of the particles of a certain size. It is important to note that arrestance is not the same as  efficiency, since efficiency also considers the capacity to retain smaller particles. In the associated tests, the difference between the weight of the particles emitted by the test source and the weight retained by the filter gives the average arrestance (Am) and is expressed in % (this is  the so-called “gravimetric efficiency”).

Based on the above we would have four types of filters:

▶ G1: Particle retention ranges from 50% to 65%.

▶ G2: Particle retention ranges from 65% to 80%.

▶ G3: Particle retention ranges between 80% and 90%.

▶ G4: retention of up to 90% of air particles.

For the so-called “fine dust” the F filters were used. For particles larger than 2.5 μm (equivalent to 0.0025 mm and designated ePM2.5, where “e” is the efficiency) the M5, M6, F7, F8 and F9 groups were used.

To have a certain efficiency in retaining fine particles suspended in the air smaller than 1 micron (equivalent to 0.001 mm) and designated ePM1 from the F7 filtration class.

Unlike the G filters, these filters are tested with an aerosol spray that sprays particles of approximately 0.4 microns in size. The average filter efficiency (Am) is calculated based on the filter's ability to stop these 0.4 micron particles and is expressed in % (in the so-called opacimetric efficiency).

Five new classes are therefore created based on the following classification:

▶ F5: with an efficiency between 40% and 60%.

▶ F6: with an effectiveness between 60% and 80%.

▶ F7: with an effectiveness of between 80% and 90%.

▶ F8: with an efficiency between 90% and 95%.

▶ F9: with guaranteed efficiency of up to 95%.

 

Filter classification under EN779 standard

Group Class Pressure Drop
final (test)
in pascals
Arrestance
average (Am)
synthetic dust
10 μm %
Average efficiency
(Em) with
particles
of 0.4 μm %
Min. effectiveness
with particles
of 0.4 μm
Big
Particles
G1 250 50≤Am<65 - -
G2 250 65≤Am<80

-

-
G3 250 80≤Am<90 - -
G4 250 90≤Am - -
Medium
Particles
M5 450 450 40≤Em<60 -
M6 450 450 60≤Em<80 -
Thin
Particles
F7 450 450 80≤Em<90 35
F8 450 450 90≤Em<95 55
F9 450 450 95≤Em 70

Summary table extracted from EN779 standard

But now, filter manufacturers and air conditioning equipment manufacturers are obliged to inform and comply based on the UNE-EN ISO 16890:1:2017 standard referenced in the RITE as  explained above.

 

WHAT IS THE RELATIONSHIP BETWEEN UNE-EN ISO 16890:2017 AND UNE 779:2013 IN TERMS OF REGULATORY COMPLIANCE?

The UNE-EN ISO 16890:2017 standard defines the testing requirements for air filters used in general ventilation. It came into force in 2017 and replaces the aforementioned UNE EN779 in  both Europe and the United States.

The standard allows for a more precise selection of air filters for HVAC systems and a better understanding of the size of air particles actually retained by the filter of the sizes discussed.

Depending on the size of the particles, the UNE EN ISO 16890 standard groups air filters into four categories, which are those already mentioned, namely, ePM1, ePM2.5, ePM10 and coarse dust.

As explained above, the effectiveness of a filter is measured by the percentage of particles of the target particle size that it retains, which must be greater than 50%. That is, a particle filter that  retains more than 50% of ePM10 will be classified as an “ISO ePM10” filter, and so on.

Based on the above we have the following equivalences: with what is requested in the RITE:

 

Equivalences with RITE UNE EN 779:2013 vs. UNE EN 16890

UNE EN
779:2013
UNE EN 16890:2017 Average efficiency  
filter class ePM1 ePM2,5 ePM10 Coarse
G1 - - - -
G2 - - - 30%-50%
G3 - - - 45%-65%
G4 - - - 60%-85%
M5 5%-35% 10%-45% 40%-70% 80%-95%
M6 10%-40% 20%-50% 60%-80% >90%
F7 40%-65% 65%-75% 80%-90% >95%
F8 65%-90% 75%-95% 90%-100% >95%
F9 80%-90% 85%-95% 90%-100% >95%

Comparative table between UNE EN 779:2013 and UNE EN 16890

 

Technical sheets with filtration characteristics

Technical sheets with filtration characteristics

TECHNICAL SELECTION OF FILTERS

As can be seen, there is a direct correspondence, but now much more effective information is provided on the behavior of the filter.

This detailed information will be provided by STULZ in the  technical specifications sheet of the unit, when collaborating in the design phase of the air treatment unit according to the requirements and typology of the installation / project.

AHU CONFIGURATION

In addition to this regulatory information, STULZ will provide the pressure drop of the clean filter at maximum clogging. To monitor this, we offer you the possibility of incorporating pressure taps, a local pressure gauge, a pressure switch or a differential pressure probe, among other field elements and components, when configuring the unit.

These devices facilitate maintenance and allow the monitoring of filters, as well as their preventive maintenance established in IT 03 of the RITE.

2.-DESIGN CONDITIONS OF THE AIR TREATMENT UNIT BASED ON FILTRATION NEEDS.

Given the above, how should I configure my air handling unit to meet regulatory criteria?

The answer lies in the regulations themselves, which establish clear criteria for the location of the filters in the unit and their relationship with other components.

Specifically:

  • The minimum efficiency and filters are those set out in table 1.4.2.5 above.
  • Prefilters (G Filters) are required to keep the components of the ventilation and air handling units clean. These prefilters will be installed at the outside air inlet, as well as at the return air inlet.
  • Final filters will be installed after the treatment section and, when the premises are particularly sensitive to dirt (premises where contamination by mixing particles must be avoided), after the supply fan, ensuring that the air distribution over the filter section is uniform.
  • Filter sections of class G4 or smaller for indoor air categories IDA 1, IDA 2 and IDA 3 will only be permitted as additional sections to those indicated in table 1.4.2.5.
  • Heat recovery devices must always be protected with a filter section, the class of which will be that recommended by the manufacturer of the heat recovery unit; if there isno recommendation, they will be at least class F6.
  • In renovations, when there is not enough space for the installation of the air treatment units, the final filter indicated in table 1.4.2.5 will be included in the heat recovery units.

CONCLUSIONS

In short, the filtration needs, both normative and recommended in different standards, require compliance with certain criteria of efficiency in design and a positioning in the equipment in which STULZ can be of great help given its extensive experience and  quality of equipment and components.

STULZ also has a wide variety of accessories that facilitate the operation and maintenance of the filters housed in the air treatment units.

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