Air Filtration at Molecular level

Particulate Filter Chemical Filter

Particulates

Gaseous

Contamination

Molecular filtration

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 Filtration.

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,

? Medium filtration (F class) from MERV 9 to MERV 16.

The above classification caters more to domestic, commercial

and industrial needs. However, for clean rooms and super clean

rooms, we need to install

? 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-

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.

We hope this article would help in better understanding of the

basics of air filtration at the molecular level and the need for gas

phase filtration. This article, however, only gives an overview of the

dynamics involved. ?

72 Air Conditioning and Relrigeration Journal Seplember 2018