Particulate Filter Chemical Filter
Particulates
Gaseous
Contamination
Molecular filtration
'. Air Filtration at olecular level
Air filtration, as commonly understood and depicted in air conditioner
advertisements and the print media, is about the poor quality of
air around us, which mainly pertains to filtration of particulate
contaminants. This article, however, will discuss various aspects of air
filtration at the molecular level, i.e. filtration and removal of gaseous
contaminants. Air tilt ration at the molecular level is often reterred to
as gas phase tiltration.
Today, when technologies and equipment, eSIJecially in mission critical
facilities, require a clean environment to function at their optimum, an
understanding at molecular phase tiltration is essential.
This article will also discuss the concept, need, technology trends and
equipment tor molecular phase tiltration.
Introduction - the Basics
Molecular Phase Filtration is the filtration of gaseous
contamination of the molecular scale size.
Particulate Air Filtration is the filtration process that removes
solid particulates from the air.
ASHRAE Standard 52.2 classifies particulate filtration as:
• Pre-fi ltration (G class) trom MERV 1 to MERV 8,
• High Efficiency Particulate Air filters (HEPA) (H class). and
• Ultra Low Particulate Air filters (ULPA) (U class).
While HEPA fi lters can control contamination up to 0.3 micron
(~m), ULPA filters can even control up to 0.1 2 ~m.
The various filters described above do take care of unwanted
pa rticulate contamination in the air. However, it is a very big
cha llenge to control contamination of matter smaller than a
particulate size of 0.1 2 ~m and physically arrest them.
In-depth knowledge of adsorption by desiccants would help us
to control contamination in such critical applications. Adsorption
by various desiccants of matter (gases) having molecular diameter
of a fraction of 0.1 ~m in their micro/meso pores (diameter
Unwanted gas
H,S 2.6 34.08 Yes Yes (Hydrogen Sulfide)
1 1.8 64.07 Yes Yes
3 SO, 1.8 80.07 Yes Yes (Sul fur Trioxide)
4 HF
!Hydrogen Fluoride)
1.8 10.01 No Yes
5 NO, 1.3 46.01 Yes Yes (Nitrogen Dioxide)
6 NH, 1.6 17.03 Yes Yes (Ammonia)
CI, 3.1 70.9 Yes Yes IChlorin e)
8 (Oz0o,n e) 1.6 48 No Yes
9 3.2 36.47 Yes Yes
CnHn/VOC
10 (Hydrocarbons/ Volatile Organic 4·4.9 Yes Yes
Compounds)
between 0.2 to 1.0 nano meter (nm)) is the sta rting point of
filtration of gaseous contaminants.
w'·mi-
Limit of ULPA litters _ INSECTICIDE ouST
'§ii'Wifil"
GAS MOLECULES
0.0001 0.001 0.D1 0.1 " 100
PBflicio SiLo, Microns
Figure 1: Airbol'1le contamination: particulates vs. gaseous
• Combustion of fossil fuels • Wood pulping
• Drains
• Sewage treatment
• land filled sites
• Sulfuric acid lIlanufacturing
• Auto emissions
• Fossi) fuels
• Microbiological activities, etc. • Electric
• Oil and
fuels -.
• Drains • Sulfuric acid manufacturing
• Auto emissions
• Tobacco etc.
• Combustion of fossil
fuels
• Drains, etc.
fucls
• All process industries
• Auto emissions
• Fertilizer plants
• Chlorine manufacturing
• Refuse decomposition • Aluminum manu facturing
• Cleaning products, etc. • Paper mills
etc.
• Atmospheric photochemical
processes mainly involve
flilrogen oxides
• Auto emission
• Electrostatic lilters
• Auto emission
• Fossils fuels combustion • Oceanic processes
All petrochemical and fertilizer industries
Paper mill s, etc.
Seplember 2018 Air Conditioning and Refrigeration Journal 67
Air Filtration at Molecular Level
Need for Molecular Filtration
Many of the unwanted gases that contaminate and can cause
serious damage resulting in huge losses are:
i. Odorous,
ii. or corrosive,
iii. or both.
These gaseous contaminants are potentially very harmful to
humans as well as to equipment, especia lly in an environment of
controlled areas housing sensitive equipment like servers in data
centers.
A. Some of the environment ally condition ed areas where
odorous gases cause loss of productivity are:
(a) Animal research faci lities
(b) Autopsy rooms in mortuaries and hospitals
(c) Cal l centers near a landfill area, like Mindspace in Malad,
Mumbai or near an open sewage line, like in Naida.
B. Some of the environmentally conditioned areas where corrosive
gases are a cause of down time of process industries are:
(a) Petrochemical industries
(b) Fertilizer industries
(c) Paper and pulp industries
(d) Medium size server rooms
(el Mission critical facilities, e.g. large size data centers
As shown in Table 1, gases typically have molecular diameter in
the range of 0.0002 to 0.001 ~m. The unit used for measurement is
Angstrom (A) (1 micron ; 10,000 Angstrom).
This artic le will give more insight into the adverse effects of
corrosive gases and potential fil tration options currently available
in the market.
Removing Corrosive Gases
As shown in Figure 3, the process of filtration through
adso rption and neutraliza tion through chemica l reaction is
commonly known as Chemisorption. The air filtration systems
remove corrosive gases through the process of adsorption and
neutralization.
Adsorption with Chemical Neutralization/Oxidation
• The process is specific and depends on the chemical nature of
both the media and gas
• The process is instantaneous and irreversible
• Converts harmful gases to harmless so lids
Major Elements to Remove Corrosive Gases
(i) Granular Media Filter
It is a combination of desiccants impregnated with chemica ls
like:
• Activated alum ina impregnated with KMnO"
• Activated carbon and activated alumina impregnated with
KOH
• Activated carbon alone
• Activated carbon impregnated with H3P04
(iiI Honeycomb Chemical Filters
These are desiccant honeycomb matrix filters impregnated
with a choice of oxidizing agents and/or alkaline/acidic solutions
like:
• Desic ca n t honeycomb matrix ba sed chem ical filters
impregnated with KMnO,
• Desicca nt honeycomb matrix based chemical fi lters having
both metal silicate and activated carbon impregnated with
KOH
• Desiccant honeycomb matrix based chemical filters
impregnated with H3P04
Figure 4 traces how the various types of fi lters from carbon
media to honeycomb chemical filters have evolved.
Evolution of Gas Phase Filtration Technology
Stage 1· Stage 3
Figure 4: Evolution of gas pllQse filfral'iol1 technologies
Classifying Reactive Environments
International Society of Automation (lSA) had defined
severity levels on account of unwanted gases in instrumentation
and control rooms way back in 1985.
Keepi ng in view the implementation
of Restriction of use of Haza rdous
Sub st an ces (ROHS) u nder th e
directive from the European Union
as per 2002/95/EC repla cing lead
(being carcinogenic) with si lver, and
electronic circuits getting furth er
miniaturized, led to ISA revising the
1985 Standard in 2013, which is as
per Table 2.
Figure 3: Chemisorption process continued on page 70
68 Air Conditioning and Refrigeration Journal September 201 B
Air Filtration at Molecular Level
continued from page 68
Table 2: Class ification of reactive eliviroll1nelli"S
Class Saverity
level
Angstroms IA) per 30 days
Comments
Copper Corrosion Silver Corrosion
GI Mild <300 A <200 A
Corrosion is nol a factor
in electronic equipment
.,ii i
G2 Moderal. 300 A - 999 A
Eliect of corrosion is
measurable and may be
a factor in electronic
iii
G3 Harsh 1.000 A - 1,999 A
High probability thaI
corrosive attacks wi ll occur;
should prompt further
evaluation and result in
environmental controls
GX Severe > 2000 A
Only specially designed
and packaged equipment is
to survive
The ISA standard also defines in terms of gaseous concentration
leve ls as per Table 3.
1hble 3: Contaminant concentrations versus severity levels
Concentration of Gases Un ppb) as per ISA 71 04: 2013
G2 {Moderatel G31Harshi
H,s <3 <10 <50 >50
SO, < 10 < 100 < 300 >300
el, < I <2 < 10 > 10
NO, < 50 < 125 < 1250 > 1250
Hf < I <2 <10 > 10
NH, < 500 < 10000 < 25000 > 25000
0, <2 < 25 < 100 > 100
Measuring Severity Levels in Corrosive
Environments
Typically, there are two types of measurement methods:
1. Corrosion Classification Coupons (CCC)
Corrosion cla ssification coupons have two pure metal strips of
silver and copper.
These coupons are placed in the room, where environment
severity has to be measured, for a period of 30 days.
The thickness of the layer of corrosion that forms on metal
strips determines the severity level as per ISA 71.04: 2013
Standard.
2. Real Time Atmospheric Corrosion Monitors
These instruments help to access severity levels on real time
basis. The rea l time measurements in typica lly 24 hours are
extrapolated for 30 days to know the severity levels as per the
ISA standard.
In addition to severity leve ls due to airborne gaseous
co ntaminants, these in struments also measu re room
temperature, RH and optionally the differential pressures, to
give the complete corrosion parameters.
Rea l t ime atmospheric co rros ion monitors can be further
classified in two technologies:
• One is based on Quartz Crysta l Microbalance (QCM), which
measures the rate of increase of corroded metal sensors
mass.
• The other determines the rate of electrical resistance
increase of corroded metal strips.
What are the types of equipment available for
removing corrosive gases?
Equipment for removal of unwanted corrosive gases are
broadly cla ssified as under:
• Thin Bed
For recirculation of air to clean within an enclosed space.
• DeepBed
These are generally designed to clean fresh air inducted into
the controlled space for pressurization.
The above two concept categories can involve both types of
chemical filters:
• Granular type
• Honeycomb type
ri~ .!· ! ~:U~
--,
Figure 5: Deep bed alld thill bed
10 Clean air out
Stage III
Final-Filter
(up to 5 micron)
Stage II
After-Filler
(5-20 micron)
Stage tv
Honeycomb Chemical Filler
SI8ge III
Honeycomb Chemical Filler
Supply air motor assembly
Stege II
Stage I
Pre-Filler
(up to 20 micron)
Honeycomb Chemical Filter •••• !!II Siage I
Honeycomb Chemical Filter lor
various gases and foul odour
Figure 6: intemal view of tile system
Impure air Inlet with
optional manual damper
continued on page II
= 70 Air Conditioning and Refrigeration Journal Seplember 2016
Air Filtration at Molecular level
continued from page 70
Particulate
Pre-Filter
Gas Filter
Media Cassettes
Particulate
Final Filter ..
DEEP BED SYSTEM
Fig"re 7: Working principle of rleep berl (Inri thin berl
Installing the Equipment
Gas phase fi ltration systems are typically installed in three
ways:
1. Re-circulation option (as shown in Figure 8),
2. Pressurization option (as shown in Figure 9), and
3. Re-clrculation + pressurization option (as shown in Figure 10).
Press.
Air -+---+ I Recirc. L
Unit .-
t Room
Return
Air
Room
Supply
Air
A/C Unit
Figure 8: He-CII'CLt/CIlIO/! apI/oil system used for se/vcr/ data cCllte,.
application
-
Room
Supply
AIC Unit Air Room
Clean Return
Press. Air i Air
Press.
~ Unit
Dirty
Out Air
FlgW'C 9: PresSlIl'IZatlO1! optlOll system used for deallll1gjrcsh ai,.
Precautions for Ensuring Proper Filtration
1. Room should be reasonably airtight
2. Pressurize the room and try to maintain minimum positive
pressure of 2.5 mm
Particulate
Pre-Filter
Particulate
.. Final Filter
THIN BED SYSTEM
I Recirc. ~
Unit
t Room Room
Return Supply
Air Air
'.
• Press. I AlC Unit I Dirty Unit
Out Air
Figure 10: Re-circu/atiol1 + pres..Hlrizal iOIl syslem type
3. Continuously monitor air quality in a controlled environment
and at equipment outlet
4. Regular equipment maintenance services
5. Avoid acidic or chlorinated agents for cleaning
6. RH-temperature sensors interlocking with BMS to cut human
intervention
Conclusion
Server rooms, data centers and mobile/base switching
centers are mushrooming in urban areas. Knowledge of filtration
at the molecular level (more commonly referred to as gas phase
filtration system) helps in keeping such facilities with minimum
downtime.
With the increase in automation in process industries, the
need for protecting th eir control rooms against corrosion from
unwanted gases using gas phase filtration has become the need
of the hour.
Particulates
Gaseous
Contamination
Molecular filtration
'. Air Filtration at olecular level
Air filtration, as commonly understood and depicted in air conditioner
advertisements and the print media, is about the poor quality of
air around us, which mainly pertains to filtration of particulate
contaminants. This article, however, will discuss various aspects of air
filtration at the molecular level, i.e. filtration and removal of gaseous
contaminants. Air tilt ration at the molecular level is often reterred to
as gas phase tiltration.
Today, when technologies and equipment, eSIJecially in mission critical
facilities, require a clean environment to function at their optimum, an
understanding at molecular phase tiltration is essential.
This article will also discuss the concept, need, technology trends and
equipment tor molecular phase tiltration.
Introduction - the Basics
Molecular Phase Filtration is the filtration of gaseous
contamination of the molecular scale size.
Particulate Air Filtration is the filtration process that removes
solid particulates from the air.
ASHRAE Standard 52.2 classifies particulate filtration as:
• Pre-fi ltration (G class) trom MERV 1 to MERV 8,
• High Efficiency Particulate Air filters (HEPA) (H class). and
• Ultra Low Particulate Air filters (ULPA) (U class).
While HEPA fi lters can control contamination up to 0.3 micron
(~m), ULPA filters can even control up to 0.1 2 ~m.
The various filters described above do take care of unwanted
pa rticulate contamination in the air. However, it is a very big
cha llenge to control contamination of matter smaller than a
particulate size of 0.1 2 ~m and physically arrest them.
In-depth knowledge of adsorption by desiccants would help us
to control contamination in such critical applications. Adsorption
by various desiccants of matter (gases) having molecular diameter
of a fraction of 0.1 ~m in their micro/meso pores (diameter
Unwanted gas
H,S 2.6 34.08 Yes Yes (Hydrogen Sulfide)
1 1.8 64.07 Yes Yes
3 SO, 1.8 80.07 Yes Yes (Sul fur Trioxide)
4 HF
!Hydrogen Fluoride)
1.8 10.01 No Yes
5 NO, 1.3 46.01 Yes Yes (Nitrogen Dioxide)
6 NH, 1.6 17.03 Yes Yes (Ammonia)
CI, 3.1 70.9 Yes Yes IChlorin e)
8 (Oz0o,n e) 1.6 48 No Yes
9 3.2 36.47 Yes Yes
CnHn/VOC
10 (Hydrocarbons/ Volatile Organic 4·4.9 Yes Yes
Compounds)
between 0.2 to 1.0 nano meter (nm)) is the sta rting point of
filtration of gaseous contaminants.
w'·mi-
Limit of ULPA litters _ INSECTICIDE ouST
'§ii'Wifil"
GAS MOLECULES
0.0001 0.001 0.D1 0.1 " 100
PBflicio SiLo, Microns
Figure 1: Airbol'1le contamination: particulates vs. gaseous
• Combustion of fossil fuels • Wood pulping
• Drains
• Sewage treatment
• land filled sites
• Sulfuric acid lIlanufacturing
• Auto emissions
• Fossi) fuels
• Microbiological activities, etc. • Electric
• Oil and
fuels -.
• Drains • Sulfuric acid manufacturing
• Auto emissions
• Tobacco etc.
• Combustion of fossil
fuels
• Drains, etc.
fucls
• All process industries
• Auto emissions
• Fertilizer plants
• Chlorine manufacturing
• Refuse decomposition • Aluminum manu facturing
• Cleaning products, etc. • Paper mills
etc.
• Atmospheric photochemical
processes mainly involve
flilrogen oxides
• Auto emission
• Electrostatic lilters
• Auto emission
• Fossils fuels combustion • Oceanic processes
All petrochemical and fertilizer industries
Paper mill s, etc.
Seplember 2018 Air Conditioning and Refrigeration Journal 67
Air Filtration at Molecular Level
Need for Molecular Filtration
Many of the unwanted gases that contaminate and can cause
serious damage resulting in huge losses are:
i. Odorous,
ii. or corrosive,
iii. or both.
These gaseous contaminants are potentially very harmful to
humans as well as to equipment, especia lly in an environment of
controlled areas housing sensitive equipment like servers in data
centers.
A. Some of the environment ally condition ed areas where
odorous gases cause loss of productivity are:
(a) Animal research faci lities
(b) Autopsy rooms in mortuaries and hospitals
(c) Cal l centers near a landfill area, like Mindspace in Malad,
Mumbai or near an open sewage line, like in Naida.
B. Some of the environmentally conditioned areas where corrosive
gases are a cause of down time of process industries are:
(a) Petrochemical industries
(b) Fertilizer industries
(c) Paper and pulp industries
(d) Medium size server rooms
(el Mission critical facilities, e.g. large size data centers
As shown in Table 1, gases typically have molecular diameter in
the range of 0.0002 to 0.001 ~m. The unit used for measurement is
Angstrom (A) (1 micron ; 10,000 Angstrom).
This artic le will give more insight into the adverse effects of
corrosive gases and potential fil tration options currently available
in the market.
Removing Corrosive Gases
As shown in Figure 3, the process of filtration through
adso rption and neutraliza tion through chemica l reaction is
commonly known as Chemisorption. The air filtration systems
remove corrosive gases through the process of adsorption and
neutralization.
Adsorption with Chemical Neutralization/Oxidation
• The process is specific and depends on the chemical nature of
both the media and gas
• The process is instantaneous and irreversible
• Converts harmful gases to harmless so lids
Major Elements to Remove Corrosive Gases
(i) Granular Media Filter
It is a combination of desiccants impregnated with chemica ls
like:
• Activated alum ina impregnated with KMnO"
• Activated carbon and activated alumina impregnated with
KOH
• Activated carbon alone
• Activated carbon impregnated with H3P04
(iiI Honeycomb Chemical Filters
These are desiccant honeycomb matrix filters impregnated
with a choice of oxidizing agents and/or alkaline/acidic solutions
like:
• Desic ca n t honeycomb matrix ba sed chem ical filters
impregnated with KMnO,
• Desicca nt honeycomb matrix based chemical fi lters having
both metal silicate and activated carbon impregnated with
KOH
• Desiccant honeycomb matrix based chemical filters
impregnated with H3P04
Figure 4 traces how the various types of fi lters from carbon
media to honeycomb chemical filters have evolved.
Evolution of Gas Phase Filtration Technology
Stage 1· Stage 3
Figure 4: Evolution of gas pllQse filfral'iol1 technologies
Classifying Reactive Environments
International Society of Automation (lSA) had defined
severity levels on account of unwanted gases in instrumentation
and control rooms way back in 1985.
Keepi ng in view the implementation
of Restriction of use of Haza rdous
Sub st an ces (ROHS) u nder th e
directive from the European Union
as per 2002/95/EC repla cing lead
(being carcinogenic) with si lver, and
electronic circuits getting furth er
miniaturized, led to ISA revising the
1985 Standard in 2013, which is as
per Table 2.
Figure 3: Chemisorption process continued on page 70
68 Air Conditioning and Refrigeration Journal September 201 B
Air Filtration at Molecular Level
continued from page 68
Table 2: Class ification of reactive eliviroll1nelli"S
Class Saverity
level
Angstroms IA) per 30 days
Comments
Copper Corrosion Silver Corrosion
GI Mild <300 A <200 A
Corrosion is nol a factor
in electronic equipment
.,ii i
G2 Moderal. 300 A - 999 A
Eliect of corrosion is
measurable and may be
a factor in electronic
iii
G3 Harsh 1.000 A - 1,999 A
High probability thaI
corrosive attacks wi ll occur;
should prompt further
evaluation and result in
environmental controls
GX Severe > 2000 A
Only specially designed
and packaged equipment is
to survive
The ISA standard also defines in terms of gaseous concentration
leve ls as per Table 3.
1hble 3: Contaminant concentrations versus severity levels
Concentration of Gases Un ppb) as per ISA 71 04: 2013
G2 {Moderatel G31Harshi
H,s <3 <10 <50 >50
SO, < 10 < 100 < 300 >300
el, < I <2 < 10 > 10
NO, < 50 < 125 < 1250 > 1250
Hf < I <2 <10 > 10
NH, < 500 < 10000 < 25000 > 25000
0, <2 < 25 < 100 > 100
Measuring Severity Levels in Corrosive
Environments
Typically, there are two types of measurement methods:
1. Corrosion Classification Coupons (CCC)
Corrosion cla ssification coupons have two pure metal strips of
silver and copper.
These coupons are placed in the room, where environment
severity has to be measured, for a period of 30 days.
The thickness of the layer of corrosion that forms on metal
strips determines the severity level as per ISA 71.04: 2013
Standard.
2. Real Time Atmospheric Corrosion Monitors
These instruments help to access severity levels on real time
basis. The rea l time measurements in typica lly 24 hours are
extrapolated for 30 days to know the severity levels as per the
ISA standard.
In addition to severity leve ls due to airborne gaseous
co ntaminants, these in struments also measu re room
temperature, RH and optionally the differential pressures, to
give the complete corrosion parameters.
Rea l t ime atmospheric co rros ion monitors can be further
classified in two technologies:
• One is based on Quartz Crysta l Microbalance (QCM), which
measures the rate of increase of corroded metal sensors
mass.
• The other determines the rate of electrical resistance
increase of corroded metal strips.
What are the types of equipment available for
removing corrosive gases?
Equipment for removal of unwanted corrosive gases are
broadly cla ssified as under:
• Thin Bed
For recirculation of air to clean within an enclosed space.
• DeepBed
These are generally designed to clean fresh air inducted into
the controlled space for pressurization.
The above two concept categories can involve both types of
chemical filters:
• Granular type
• Honeycomb type
ri~ .!· ! ~:U~
--,
Figure 5: Deep bed alld thill bed
10 Clean air out
Stage III
Final-Filter
(up to 5 micron)
Stage II
After-Filler
(5-20 micron)
Stage tv
Honeycomb Chemical Filler
SI8ge III
Honeycomb Chemical Filler
Supply air motor assembly
Stege II
Stage I
Pre-Filler
(up to 20 micron)
Honeycomb Chemical Filter •••• !!II Siage I
Honeycomb Chemical Filter lor
various gases and foul odour
Figure 6: intemal view of tile system
Impure air Inlet with
optional manual damper
continued on page II
= 70 Air Conditioning and Refrigeration Journal Seplember 2016
Air Filtration at Molecular level
continued from page 70
Particulate
Pre-Filter
Gas Filter
Media Cassettes
Particulate
Final Filter ..
DEEP BED SYSTEM
Fig"re 7: Working principle of rleep berl (Inri thin berl
Installing the Equipment
Gas phase fi ltration systems are typically installed in three
ways:
1. Re-circulation option (as shown in Figure 8),
2. Pressurization option (as shown in Figure 9), and
3. Re-clrculation + pressurization option (as shown in Figure 10).
Press.
Air -+---+ I Recirc. L
Unit .-
t Room
Return
Air
Room
Supply
Air
A/C Unit
Figure 8: He-CII'CLt/CIlIO/! apI/oil system used for se/vcr/ data cCllte,.
application
-
Room
Supply
AIC Unit Air Room
Clean Return
Press. Air i Air
Press.
~ Unit
Dirty
Out Air
FlgW'C 9: PresSlIl'IZatlO1! optlOll system used for deallll1gjrcsh ai,.
Precautions for Ensuring Proper Filtration
1. Room should be reasonably airtight
2. Pressurize the room and try to maintain minimum positive
pressure of 2.5 mm
Particulate
Pre-Filter
Particulate
.. Final Filter
THIN BED SYSTEM
I Recirc. ~
Unit
t Room Room
Return Supply
Air Air
'.
• Press. I AlC Unit I Dirty Unit
Out Air
Figure 10: Re-circu/atiol1 + pres..Hlrizal iOIl syslem type
3. Continuously monitor air quality in a controlled environment
and at equipment outlet
4. Regular equipment maintenance services
5. Avoid acidic or chlorinated agents for cleaning
6. RH-temperature sensors interlocking with BMS to cut human
intervention
Conclusion
Server rooms, data centers and mobile/base switching
centers are mushrooming in urban areas. Knowledge of filtration
at the molecular level (more commonly referred to as gas phase
filtration system) helps in keeping such facilities with minimum
downtime.
With the increase in automation in process industries, the
need for protecting th eir control rooms against corrosion from
unwanted gases using gas phase filtration has become the need
of the hour.
