Below is sample calculation for the pressure drop or static pressure drop to cater for fan and airflow calculation.For Standard 2 layers MERV6/7 and MERV14 filters.
Prefilter/ Primary Filter
IF30 24x24x2” (MERV7) 55Pa@ 3400cmh (Rated)
12x24x2” 1700cmh
12x12x2” 850cmh
For Airflow Volume 450cmh, can use 12x12x2” will do.
Initial Pressure Drop: 450/850 * 55Pa = 29 Pa
Final Pressure Drop: min twice of initial >58Pa to 150Pa
Secondary filter
C90 2in 24x24x2" (MERV14) 128pa@3400cmh (2.5m/s)
C90 2in 12x24x2" 128Pa@1700cmh
C90 2in 12x12x2” 128Pa@850cmh
For 450cmh, using C90 2in 12x12x2"
Intial Pressure Drop: 450/850 * 128Pa = 68Pa
Final Pressure Drop: min twice =136Pa or 250Pa
So Total Pressure Drop to cater for fan static = 58+136=194Pa. Typically can use to 250Pa to cater extra.
Thus for 450cmh, if C90 2in 12x24x2" filter is used instead, the initial pressure drop = 450/1700 * 128Pa = 34Pa.
* For Quick Calculation for Ducted FCU, Fresh Air Fan for design engineers and consultants
For design running at 1.8m/s , Pressure drop =1.8/2.5 * 128Pa = <92Pa. Final presure drop can take as twice of initial min 184Pa
at 1.25m/s of 1700cmh for face area 24x24" Pressure Drop = 1.25/2.5 * 128Pa = 64Pa. Final presure drop can take as twice of initial, min 128Pa
By looking at the pressure drop curve is good but if one can notice face velocity which is less than rated/maximum 2.5m/s (3400cmh) for 24x24" face area, the pressure drop is much lesser. Thus using proportional ratio will already cater the extra pressure drop or airflow required if there is unexpected shortfall and it is so much easy.
Secondary filter (option) example
IP90 8P 24x24x22" MERV14 89Pa@3400cmh
For Face velocity 1.85m/s, Air volume 2500cmh, 2500/3400 * 89Pa = 65Pa
Final Pressure Drop: min twice = 130Pa to 250Pa
For Rated velocity or max velocity 2.5m/s, pressure drop should cater for Pre 150Pa and Secondary 250Pa , this applies to AHU and Pau as well. However most AHU and Pau cater nowaday are 300Pa as running/design velocity is typically at <75% or 1.85m/s.
Conclusion:
Using Larger Face Area, increase depth of filter, underrating and using lower Velocity 75%(typical airflow design) of rated velocity 2.5m/s (Rated means maximum) use will reduce the final pressure drop required in fan selection, increase lifespan, lower noise, cater for unexpected final built results.
We will describe below methods in improving the filtration aircon system to mitigage/reduce the risk.
Introduction: Air filters in AHU, PAU. HEPA filters
Most important factor is airchange and fresh air. Second, Better air filters which can increase airchange and or efficiency.
1. Most buildings are equipped with secondary filters MERV13-15 air filters. Research showed the MERV13 filters control and removed the more highly contagious influenza and marked reduction for flu,rhinovirus and TB. Most commercial office AHU are NOT designed and not required to install HEPA filters which drastically reduce airchange, ventilation or fresh air which is most important. Many filters in market claim to be HEPA but infact misinterpreted and not standards tested as "HEPA" as Secondary/Final high efficiency filters like MERV14,MERV16 SemiHEPA.
2. Increase airchange and fresh air which promotes ventilation. This can be done by replacing dirty air filters and shorter replacement frequency. Do not wash the air filters. Fresh air can also be done by opening windows and doors to allow outside air to mix. This is opposite to during Haze which has contaminated air outside. A fan blowing out can be used to exhaust room air for cross ventilation. Filters change to be timely. Do not wait till last minute to change air filters (no air). Always standby or store some air filters.
3. Use Better air filters means higher efficiency, better airchange and lower pressure drop which means higher airchange.
Action Plan Solution: Use of Air filters
1. Secondary filters min MERV13/14 is a must have in the AHU/PAU. C90 Ccell MERV14,15 and C90N, VH10 Vcell MERV16 (99%@1-3um) has higher efficiency and able maintain same airflow. Put back secondary filters if there is already slot.
2. Increase the efficiency of pre-filters. Recommended ES65 IF (MERV11) or ES13 IF (MERV13) or ES14 IF (MERV14) Pleated Box panel filters. They are low cost, interchangeable to existing prefilters and unlikely affect existing airflow except shorter replacement period and can be replaced disposed as sacrificial filter.
3. For FCU or areas without secondary filtration or no slot, C90 2in MERV14 (2,4" thickness), ES13 IF MERV13 or ES14 IF MERV14 (1,2,4" thickness) can be used. Sizes can be customised. Quick short term retrofit for old aircon or ducted FCU with no available slot.
4. ES65 Media Rolls 0.61x180mx3mm thickness (MERV10) can be used quickly to cover AHU. Note the filter frequency may increase.
Carbon filters are not required but it can help control the alcohol or smell. (unlike in haze)
** Do not attempt to use two layer of MERV14 filters, it does not mean high filtration but may decrease airflow, inhibit the airflow and airchange ventilation unless the aircon is designed to handle. It will far be better with higher airchange by use of 1st layer prefilters to capture bigger particles to protect the secondary filters instead.
Secondary, SemiHEPA filters
Most secondary filters like bag filters are (SH) Single header construction which will suit most AHU universal holding frames.
1. Vcell of 12" depth SH Model:VH10 MERV16, V98 MERV15, V90 MERV14
Pros: Higher Airchange, very low pressure drop, very high efficiency
2. Minipleat Ccell 2",3",4" depth/thickness available in Model: C90 of MERV14,15, CH10 of MERV16
No Header/Box (NH): 2" depth , Gi 4", 3"
Single Header (SH): Gi 4" depth
Pros: Slim 2",4" thickness, various sizes
Prefilters or Secondary filters
3. Model: ES65 IF, ES13 IF, ES14 IF ES Pleated 24x24x2" (595x595x45mm)
Depth: 1",2",4" (Box, NH)
can Interchange with all prefilters in AHU, PAU, FCU
Pro: Slim 1-4" thickness, various sizes, long life
Being Manufactured in Singapore factory, we are churning out air filters as we give priority and allocation to priority services, existing customers, hospitals and prevent unnecessary stockpile by individual.
Please be patient to us, Please email to This email address is being protected from spambots. You need JavaScript enabled to view it. to facilitate queries.
Other Information, Efficiency Chart for Removal of viruses
Efficiency increase when air filters are loaded with dust! This natural fundamental filtration mechanism may not apply to other filtration mechanism like electronic filtration. Virus like MS2 phage size is about 0.027um =27nanometers, particle size less than 0.1um(micron) or 100nm, the removal efficiency actually increases. So we expect SemiHEPA MERV16 99%@>1um (1000nm) is equivalent to 99%@<0.03um (30nm).
Removal Efficiency of MS2 phage virus size: 0.027um =27nanometers
ASHRAE 52.2
When loaded w dust
for MS2 Phage
When clean
for MS2 Phage
0.3-1.0um%
1-3um %
(BFE)
3-10um%
Type of products
MERV16
99.6
99.3
>95
>95-#99
>95-99
VH10, CH10
MERV15
99
95
>85
>90-#98
>95-99
V98, IP90,C90/98
MERV14
99
87
>75
>90-#96
>95-99
V90, IP90,C90,IFR90
MERV13
93
70
>50
>85-#92
>90-99
ES13,C80,IP80
MERV11
91
60
>20
>65
>85
ES65 IF
MERV 8
91
57
-
>20-#50
>70
IF40 Pleated
MERV 7
90
50
-
- (>#30)
>50
IF30 Pleated
MERV6-8
79
40
-
-
>35
Prefilter Roll T209,T350
* Ours secondary filters V90,IP90,C90 are tested and expect to filtration efficiency of MERV15.
BFE:Bacterial Filtration Efficiency is typically used for mask and tested lower velocities for reference which expects the size of bacterial or virus due to filtration mechanism increase when mass size decreases below 0.3um or 30nm
# You will expect Higher filtration efficiency for our high quality air filter products as it meets more than the required standard.
HEPA air filters
"True" HEPA(High Efficiency Particluate Air) filters are removal efficiency >99.97%@0.3um also call absolute HEPA. Most consumer market or air purifiers "HEPA" operate at lower airflow, if tested to 2.5m/s, they could be in range of MERV14,15 or 16 air filters. Most commercial office AHU are NOT designed to use HEPA filters and may expect near or ZERO flow when installed, drastically reduce airchange ventilation. The recommended airchange is 12 per hour. Hospitals isolation wards,Biosafety labs usually design negative pressure (min-6Pa), Biosafety Cabinets uses HEPA 99.99% which are properly designed to protect the personnel and environment.
ES65 Media Roll or ES14 IF pleated for Smaller aircons cassette units:
This unique permanent charged electrostatic product in roll form can be cut and used in smaller aircons as flat disposable media, ES65 Media can be used in lower airflow velocity achieves MERV10 like small unit Ducted FCU,Cassette units. Due to high efficiency, they need to be changed often 1-3mths and coil must be clean before use. A plus point is you will expect coil to be clean state after use for longer time.
Information: No products in roll forms for MERV13
Please note there is NO products in roll form for secondary aircon filtration of MERV13, 14 or typically above MERV8. The reason is the design pressure drop is too high and thus must be in panel pleated air filters. Many claim to achieve the MERV13 or 14 efficiencies are usually tested at very low velocities which is different from ASHRAE 52.2 MERV tests at rated face velocity 2.5m/s. Thus they will be irrelevant to use in commerical, industrial aircon and will have much low efficiencies. They often marketed as roll filter media in which they required to be constructed manufacture into pleated, bag, minipleat rigid filters for use in HVAC aircon.
** Note information are provided as accurate of our knowledge and limits any liability or consequences that could arise from using them.
Haze can be caused by dry seasons, spot neighbouring forest fires, industrial pollution. In Singapore outside air is at safe levels, haze is dependent to weather, we will see it being mitigated when it rain which wash down the pollutants and winds which change the pollutants location.
Question: I am concern of the haze and the buildings
As discussed in earlier article, are we concern of haze outside or buildings inside. As the question ask and answer, air that draws from outside air PAU and fresh air intakes will impact the buildings inside. As they are drawing the dust particles whether in be 10um or 2.5um. The necessary can be equipped with MERV13 or MERV14 filters like ES13 IF and C90 Ccell. Most PAU should already have secondary filters which are MERV13 or MERV14 will be sufficient which will clear >95% of particles >1um or 1micron.
Question: Just to check do you have filters in Roll instead of Panel Pleated or Boxes
Filters in roll media are limited technical fundamental with MERV2 to MERV7 except MERV8 Carbon Rolls MERV10 ES65M (disposable higher efficiency but high pressure drop at lower flow only,shorter lifespan)
Only ES13 IF , C90 2in thickness and other secondary filters with MERV13 or MERV13 which are constructed in pleated panel boxes or bag with high media area will be effective for haze particles. They will be able replaceable for most existing fans or aircon commercial systems without affecting airflow.
We are experiencing hazy conditions in region of Singapore and region.Air quality may drop to unhealthy levels again.This article will address of commerical and industrial building aircon, fan and ventilation design. Hope this will help the Facilities, Aircon Technicians, Engineers and Managers.
Introduction: Air filters, Carbon Filters
Haze particles are unburnt particles, soot. It is made of all odour gaseous molecules, very small, fine to very big particles. Thus prefilters MERV6-MERV10 are effective for big particles, the fine or visible or particles can be removed by at least MERV11 >85%@3-10um, MERV13 filters 85%@1-3um. Recommended at least MERV14 90%@1-3um. Absolute HEPA are not required in most cases.When haze particles are controlled, the smell odour will be reduced. Further control can be used with Carbon Filters in rolls or Carbon Pleated in direct replacement of prefilters of 1-4"depth
Please consult the operation team or aircon team which is feasible to implement to prevent infeasible or impratical solutions. Air change or volume is important parameter in ventilation to ensure the ACMV equipement functions properly.
Action Plan Solution
Limit the air intake from outdoor. Close windows and on aircon.
Stay indoors by limiting outdoor activities, wear a face mask to go outdoors if required.
As the souce of contaminated or polluted air comes from outside, for enclosed buildings temporary limit the fresh air intakes, PAU, Doors or windows openings or gaps.
Air Filter Factory -Secondary filters and Carbon filters
We have increasing request from customers during haze as our Singapore factory we will give priority to existing customers and stock limit to required qty only to prevent unnecessary stockpile by individual.
Carbon Filter Roll media is fast, easy and effective to implement by covering Aircon or intake for immediate solution.
1. ZKG carbon rolls 1.2x20mx10mm, 1.2x20mx3mm.
Recommended MERV14 Secondary filters
The indoor air quality shall be affected by aircon and ventilation design, source of contaminants including food, furniture, rubbish etc. The recent ISO16890 to replace EN779 relates the indoor air qualtiy with filtration to ePM2.5, ePM10.0 but different suppliers may present it differently. The comparison to one another is similar and does not make an impact. We may be familiar to ASHRAE MERV (EN779) for faster selections.MERV6 (G3) 30%prefilters and MERV14 (F8) 90%secondary filters for the control of haze particle with recommended optional carbon filters to control smell which also recommended in SS553.
ACMV equipment which mostly already have secondary filters with >MERV13 will be sufficient, Any MERV14 secondary filters will do. Ccell is available in 2" and 4" thickness which can be installed in filter holding frames of AHU.
1. Minipleat Ccells 2" thickness and 4" thickness available in MERV14
MERV14 >90% @1-3um, >95%@3-10um, the best protection against haze.
Relative New ISO 16890 standard which replace the EN779 standard in Europe from Jul 2018 is similar to US ASHRAE 52.2. However, filter specialists who are familiar will notice similaries to the MERV in the 0.3-1.0um, 0.3-1.0um, 1-3um range. The arrestance weight efficiencies which is still relational and useful for Coarse dust or big dust or pre/primary filters. The new info will relate closer to the Building Indoor air quality PM1, PM2.5, PM4, PM10 common terms as we see a global pollution of industrialisation, extreme weather and haze.
The focus is in the secondary filtration which did not change much as the EN779 do address them in the test except to catogerise for wider information. The further categorisation gives more information can be useful but for most common users, familiar MERV13/14 or F7/F8 80-85%/90-95% will be much easier for daily use and implementation.
EN779
ISO ePM1 %
ISO ePM2.5 %
ISO ePM10 %
ISO Coarse %
ASHRAE 52.2 MERV
0.3-1.0um %
1-3um %
3-10um %
Arrestance>10um %
G2
-
-
-
≥70%
MERV 3-4
-
-
-
≥70-75%
G3
-
-
-
≥80%
MERV 5-6
-
-
≥20-≥35
≥80-90%
G4
-
-
-
≥90%
MERV 7
-
-
≥50
≥90%
G4
-
-
>50%
-
MERV 8
-
≥20
≥70
M5
-
-
≥50%
-
MERV 9
MERV 10
-
-
≥35
≥50
≥75
≥80
M6
-
50-65%
>60%
-
MERV 11
MERV 12
≥20
≥35
≥65
≥80
≥85
≥90
F7
50-65%
65-80%
>85%
-
MERV 13
≥50
≥85
≥95
F8
65-80%
>80%
>90%
-
MERV 14
≥75
≥90
≥95
F9
>80%
>95%
>95%
-
MERV 15
≥85
≥90
≥95
E10/11
NA
NA
NA
NA
MERV 16
≥95
≥95
≥95
Coarse dust particles are relative bigger particles >10um like pollen, dust, ash. Hair >40um. Arrestance is based of removal of dust weight which target bigger particles >10um which is heavier in nature.
Conculsion:
For most buildings from commercial to hospitals,clean manufacturing, users, managers,design and build contractors and consultants, the info for recommended indoor air quality filters in buildings or fresh air intake is using F7 MERV13 , F8 MERV14. Remains unchange as design,implemenation and most common to implement in airconditioning as secondary filters.
In most commercial or industrial Aircon/ACMV air filtration, 2 filter layers ir recommended. Pre and Secondary
Cleanrooms, Hospitals or laboratories will require 3 layer which is Pre, Secondary and HEPA
EEOI: Data provided above are for general reference and comparison. We are not responsible for any inaccurate or errors, please refer to the standards for more info.
When we refer to nominal Sizes,we meant the external size of the filter holding frame size. Standardised proper and factory fabricated sizes are 24x24" or 610x610mm and 12x24" or 305x610mm. Most use inches as nominal sizes as they will be shorter easier to read which commonly used in US. For prefilter and secondary filters,common nominal sizes are 24x24", 12x24",20x24",20x20",16x24",16x20". Thickness or depth, 1",2",4",6",12" (15-26"depth for Bag Pocket). For HEPA filters, actual filter sizes in mm is used instead. Manufacturer standard sizes should be used to facilitate inital design, project implemenations, stock and facilitate operation and maintainence, practically benefitting everybody from designers,contractors,workers,purchasers,buyers,admin and operation,users.
Thus the filter size or we mean actual size is usually 12-15mm or 1/2 - 5/8" inches smaller in the width(or length) or height, this will allow tolerances for filter to easily fit into filter holding frames. Nobody want a tight hard time during replacement. Nominal Thickness or depth are typcially like 2"(50.8mm) will have actual size smaller thickness 45mm(-5mm). Varying Aircon filters manufacturer or supplier may have tolerances 5-10mm. Tight tolerances will cause filters to be very difficult to install as various factories may use different size tolerances. Another reason why standard sizes should be used.
Gaskets are to be used to seal gaps on the holding frame size to minimise bypass. In cases of HEPA installations, they should be front loaded and clamp to ensure <0.03% to 0.01% leakage.
SS553 2016 7.5 refers to outdoor air treatment. The new changes are not new to most consultants, designers or engineers in which all PAU are already equipped with 2 layers of air filtration except in addition to cater to all fresh air intakes, ventilation fans(lift shaft /toilet intake) and resizing to cater the 2 layers of MERV6 and MERV14 and prefereable with gases removal (carbon) air filters either as permanent installation or the space and static pressure as required.
It applies to aircon/fans intake of outdoor air only and cater for the filters for them so they can be used when outdoor pollution level is in unhealthy range. The rational is logical as imagine during haze periods, there are fresh air or intake fans introducing the contaminants and polluted air right into buildings which is enclosed. Of course they can be partially or closed during emergency like a fire damper which closes/open when smoke is in. However, in the case the outdoor pollution comes, this might be a challenge to do it because it might not be so accesibile or feasible to seal or close all of them.
During Haze time, most of our customers/buildings should already have MERV13 mechanical air filters should not face any problems except for those cases who intake outdoor air without filtration (lift shaft intakes, fresh air intakes). Also they should have another layer MERV13 at the AHU side in which most outdoor pollution should not be a problem. To meet requirements, you can use the MERV14 instead, the slight increase in pressure drop about 20% is manageable for most systems.
The Estimated 9-10” inline duct depth with housing can cater both filters. Standby Secondary filter can be put in when required. Example, car park ventilation intake should not require these filters in normal circumstances.
Carbon Pleated (act as gas phase filter and also prefilter MERV8) 24x24x2”t
C90 Ccell 90-95% Merv14 24x24x4”thickness (also available in 2” thickness but pressure drop is about 200Pa initial@3400cmh)
1. FC Frame with Synthetic Washable Media 24x24x1" thickness (can put carbon type media when required)
2. C90 Ccell 90-95% Merv14 24x24x4”thickness (also available in 2” thickness but pressure drop is about 200Pa initial@3400cmh)
Initial Pressure Drop: 150Pa @2000cmh (1.5m/s) (system usually running at 70% at same face area 610x610mm)
Design or Recommended Final Pressure Drop: >250-350Pa @ 3400cmh (2.5m/s)
The standard PAU should have enough inline depth space, the most Ideally should be to cater 2-4" prefilter and 12-15” secondary filter slot.(can slot in the filter when required).
