GOVERNMENT OF INDIA CIVIL AVIATION REQUIREMENTS
CIVIL AVIATION DEPARTMENT SECTION 2 - AIRWORTHINESS
SERIES `I' , PART IV
ISSUE I,DATED 27TH AUGUST '1992
FILE NO. 11-690/92-AI(2) EFFECTIVE : FORTHWITH
Subject :Airworthiness Procedures for clean rooms and
environments for aircraft system/accessories shops.
1. PURPOSE:
1.1 Aircraft Rule 57 requires that every aircraft shall be
fitted and equipped with instrument and equipment including
radio apparatus as may be specified according to the
circumstances under which the flight is conducted.
Sub Rule 2 of Rule 57 further states that such instrument
and equipment shall be maintained in a serviceable
condition. In order to ensure high standard of
maintenance, servicing and overhaul of equipment,
accessories and instrument installed on an aircraft, it is
imperative that various shops associated with these
accessories must be relatively free of dust and other
contaminants.
1.2 The higher reliability requirements specified for aircraft
system components and in particular, those associated with
complex electronic instrumentation and mechanical system,
necessitated the development of techniques for controlling
contamination which in various forms is a common cause of
components premature failure. It has, therefore, become
necessary to apply these techniques to selected areas of
manufacturing and aircraft operating organizations in which
the various processes of manufacture, overhaul and testing
can be carried out under controlled environmental
conditions. Such selected areas are referred to as `Clean
Rooms', the design and construction of which form part of an
independent and highly specialized field of work are
normally specified by the manufacturers of equipment or laid
down in British Standard BS 5295 Parts 1,2 and 3. This part
of CAR lays down the various standards for clean room
conditions and means to control the dust level.
2. INGRESS/PRODUCTION OF CONTAMINATION:
2.1 Air - The air which continually surrounds the components
may be considered as a contamination storehouse containing
dirt and dust particles, organic and inorganic vapours.
2.2 Manufacture - Contaminants are produced during all
manufacturing processes, and particles, such as swarf
resulting from a machining operation, or particles forced
into the surface of a component during a pressing or heating
process, can be of such a nature that their effect can be
immediate or delayed. Depending on the composition of the
particle and component materials, the alloys or compounds
formed by interaction can result in serious loss of a
components structural strength over a period governed by the
rate of diffusion.
2.3 Assembly - During the assembly process the possibility of
introducing contaminants is probably greatest because of
exposure to the highest levels of contaminant sources. In
the soldering process for example, the vaporization of flux
causes particles to escape into the surrounding air which,
on cooling, condense as droplets on a nearby cold surface of
the component. Depending on the location of the particles
and the forces applied to them, they can act as a
contaminant with an immediate or delayed effect.
2.3.1 The use of jointing adhesives can also produce
contamination similar to that of a soldering process.
In addition, vapours can be given off which can migrate
to other portions of an assembly and act as a delayed
action contaminant.
2.3.2 Assembly of components using threaded joints can
produce fibre-shaped fragments or flakes as a result of
an effect similar to wire drawing. For extremely close
fit or for balancing purposes, it may be necessary to
fit individual parts of a component together by
grinding, lapping or honing operations. In any such
operation, contaminant particles can be dispersed in
the atmosphere, suspended in fluids, adhere to the
surfaces of component parts, or become embedded into
the surfaces.
2.3.3 Assembly of components in jigs or while being handled
or supported by tools, may result in deformation of
surfaces and production of contaminant particles. For
example, if during tightening of bolt, slippage of the
spanner jaws occurs, particles are produced from the
bolt head. Particles are also produced from the heads
of bolts or screws and component surfaces during final
tightening.
2.4 Storage and Transit - During the storage period of
assembled components and of associated independent parts,
contamination can occur in several ways notwithstanding the
use of protective covering or containers. Particles from
the air may be deposited as a result of gravitational
settling and also as a result of electrostatic effects.
Improperly cleaned containers or covers may transfer
particles to components, in particular where padded
containers and plastics containers are used. In the first
case, the contours of the container may trap particles which
are not released until the component causes deformation of
the padding. In the second case, plastics containers may
pick up particles from the air due to electrostatic
charging, and hold them until transferred to the packed
component.
2.4.1 Containers which are not hermetically sealed are
subject to a `breathing' cycle as the temperature of
the container varies. During the intake portion of the
cycle, particles in the air surrounding the container
may be drawn into a position where they can contaminate
the component.
2.4.2 The movement of packed containers during transit is
also a source of contamination since it may dislodge
contaminant particles not previously cleaned off, or
create new particles by abrasion.
2.5 Component Cleaning Processes - A cleaning process is
actually a process of transforming contamination from a high
level of concentration to a lower one; therefore, tolerance
levels must be considered relative to the components'
function and required operational accuracy.
2.5.1 The transfer of contaminant particles is dependent on
the methods used in the cleaning process, i.e. whether
wiping or polishing with an absorbent or collecting
material (dry cleaning transfer) or cleaning by means
of a liquid (wet cleaning transfer). Problems exist in
each of these processes.
2.5.2 The ways in which dry cleaning can contaminate include
the following:
a) Removal of fibrous particles from the cleaning
material.
b) The material, after use, may have a particle
concentration sufficiently high so that as much
contamination is left on the component as is
removed.
c) Wiping or polishing action can cause particle
adhesion as a result of electrostatic charges.
d) Particles can be moved about on a component
surface without necessarily being lifted from the
surfaces.
2.5.3 In the wet cleaning process, the contaminated surfaces
are exposed to clean fluid which will wet the particles
and the surfaces. The fluid or the component is then
agitated so as to pull particles from the surfaces.
After a specified period the component is withdrawn and
the surfaces are dried. The ways in which wet cleaning
can contaminate include the following:
a) It is often difficult to obtain clean fluid and to
keep it clean when handling it.
b) Agitation of the fluid is normally done by
ultrasonic means, but there is a possibility of
recontamination if the amplitude of agitation is
not large enough to remove particles an
appreciable distance from the surface of the
component.
c) Often a wet surface may have particles in the
liquid layer that can easily be moved laterally
over the surface but are removed from the liquid
layer only with great difficulty.
d) Until the component is dried, any airborne
particles will collect on the wet surface and
remain.
2.6 Personnel Activity - The activity of personnel is probably
the greatest single cause of contamination which arises from
several sources. The act of walking, or other movements
required at a work bench, produces transient air currents
which redistribute airborne particles and the brushing off
of particles from many surfaces. Another contaminant source
is the shedding of skin and hair particles. The outer
layers of skin flake off almost continuously, the flake rate
and size depending on the amount of abrasion to which the
skin is exposed and its condition.
2.6.1 Exhaled air is another source of contamination since it
contains moisture retaining solid particles and is
usually acidic in nature. Perspiration from the skin
is a similar hazard.
3. CONTROL OF CONTAMINATION:
Control of contamination is effected in two ways by
establishing a clean room which will provide a clean
atmosphere and working conditions and by rigid routines
adopted by personnel to prevent process, transfer and
associated sources of contamination by working within the
area of the clean room.
3.1 The construction of a clean room and its air handling system
(see paragraph 8) must be designed to control airborne
particles over a range of sizes and suited to the nature of
the work performed in the room. Control is accomplished by
filtration of the air entering the room, changing the air to
remove generated particles, designing walls, floors and
furnishings to be resistant to particles generation and
retention, protecting components from impact and settling of
particle and providing additional areas for cleaning of
parts and personnel.
4. SIZE OF CONTAMINANTS:
The degree to which contaminants are effectively controlled
is determined by measurements of the size of particles and
the number in a given volume. The conventional unit of
measurement is the micrometer (mm). In general, the
filtration systems of clean areas are designed to control
particles of 0.5 mm and larger in size.
5. CLASSIFICATION OF AIR CLEANLINESS:
In addition to all principles of airconditioning, certain
specialized cleanliness requirements are defined by
standards which establish classes of contamination levels to
be achieved in the design of a clean room for a specific
task. Classifications relate to the number of contaminant
particles 0.5 mm and larger in size, present in one cubic
metre of air. Four classes of contamination level are
generally adopted and these are shown in descending order of
cleanliness in Table 1. Special classifications may used
for particle count levels where special conditions dictate
their use. A summary of the cleanliness requirements for
some typical products is given in Table 2.
TABLE 1
/-----------------------------------------------------------------------------------------------------------------\
| Controlled | Recommended Air | Recommended | Max. Permitted Number of Particles per cubic |Final |
| Environment | Flow configu- | Periodicity for | micron(equal to, or greater than, stated size)|Filter |
| (Clean room, | rations | Air sampling and |------------------------------------------------|Efficiency |
| work station | | Particle counting | | | | | |% |
| or clean box)| | | 0.5 um | 1 um | 5 um | 10 um | 25 um | |
| | | | | | | | | |
|--------------|-----------------|-------------------|----------|----------|-------|--------|---------|-----------|
| Class 1 | Unidirectional |Daily or continuous| 3,000* | N/A | Nil | Nil | Nil | 99.995 |
| | |by automatic | | | | | | |
| | |equipment | | | | | | |
| | | | | | | | | |
|--------------|-----------------|-------------------|----------|----------|-------|--------|---------|-----------|
| Class 2 | Unidirectional |Weekly | 300,000 | N/A | 2,000 | 30 | Nil | 99.95 |
|--------------|-----------------|-------------------|----------|----------|-------|--------|---------|-----------|
| Class 3 | Unidirectional |Monthly | |1,000,000 | 20,000| 4,000 | 300 | 95,00 |
| | or Conventional | | | | | | | |
|--------------|-----------------|-------------------|----------|----------|-------|--------|---------|-----------|
| Class 4 | Conventional |3-Monthly | | |200,000| 40,000 | 4,000 | 70.00 |
|--------------|-----------------|-------------------|----------|----------|-------|--------|---------|-----------|
| Controlled | Normal | - | - | - | - | - | - | - |
| Area | Ventilation | | | | | | | |
|--------------|-----------------|-------------------|----------/----------/-------/--------/---------|-----------|
| Contained | Unidirectional | To suit required | | 99.997 |
| Work Station | | class and | | |
| | | application | | |
|--------------|-----------------|-------------------|------------------------------------------------|-----------|
| Portable | As selected | To suit required | To suit required class | To suit |
| Clean | | class and | | required |
| Boxes | | application | | class |
\-----------------------------------------------------------------------------------------------------------------/
* Subject to maximum particle size of 5 micron
TABLE 2
/---------------------------------------------------------------\
| Class | Particles/cubic | Product |
| | micron | |
|---------------------------------------------------------------|
| | | Air bearings |
| | | Miniature ball bearings |
| | | Miniature contacts |
| | | Floated gyros |
| 2 | 0.5 um to 10 um | Hydraulic and pneumatic systems |
| | | Optics |
| | | Semi-conductor networks |
| | | Miniature timing devices |
|---------|-------------------|---------------------------------|
| | | Hydraulic and pneumatic systems |
| | | Precision timing devices |
| 3 | 1 um to 25 um | Stable platforms |
| | | Gyros |
|---------|-------------------|---------------------------------|
| | | Ball bearings |
| | | Electronic components |
| | | Engine pumps |
| | | Aerospace instruments |
| 4 | 5 um to 25 um | Printed circuits |
| | | Valves |
| | | Hydraulic and pneumatic systems |
| | | Precision measuring equipment |
\---------------------------------------------------------------/
NOTE :Class 1 is outside the scope of this part of CAR and would
not normally be used.
6. CLASSIFICATION OF CLEAN ROOMS:
The cleanliness achieved by a clean room is dependent on the
air-handling systems' capacity to purge the room of
contaminant particles. This includes not only effectiveness
of the filters and number of air changes per hour but also
the distribution of the air within the room. There are two
main methods of distributing air into clean rooms namely,
conventional clean rooms and unidirectional flow clean rooms
and these also serve as the basic clean room classification.
It is up to the operator to select clean room based on the
recommendation of the Vendors/Manufacturers or he may take
the guidelines from the British Standard Specification BS
5295.
6.1 Conventional Clean Rooms. Conventional clean rooms are
based on recognized air-conditioning techniques. The
conditioned air is highly filtered and distributed through
ceiling-mounted diffuser outlets, and then exhausted from
return airducts located near the floor around the periphery
of the room. In addition to direct emission from the
diffuser outlets, spreading of conditioned air throughout
the room is obtained by secondary mixing of the air caused
by thermal effects of warm and cool air currents. This is
an advantage from the point of view of maintaining
conformity of room temperature conditions, but the
turbulence created gives rise to the problem of contaminant
particles being re-introduced to the airstream.
7. ENVIRONMENT AND COMFORT CONTROL
The temperature, humidity and pressure characteristics of
the air passing through the air handling system(see
paragraph 8) should be controlled to established and
environment suitable for work processes to be carried out in
a clean room and for the comfort of clean room personnel.
7.1 Temperature and Humidity. The selection of temperature and
humidity ranges to be controlled are dependent on the design
of the component or system and the effects on their
functional accuracy under varying environmental conditions.
Normally a suitable temperature for working condition is 20
+2 degree C. Humidity should be controlled and maintained
at a relative humidity of 35 to 50% for all classes of clean
rooms, contained work stations and clean boxes.
7.2 Pressure. Clean rooms are always slightly pressurized in
order to maintain the required outward flow of air under
closed working conditions and to prevent the entry of
contaminant airborne particles when entry ways or doors are
opened.
7.2.1 Unidirectional -flow rooms should normally have an air
velocity of 0.45+0.1 m/s for horizontal flow rooms and
0.30+0.05 m/s for vertical flow rooms. Air pressure for
conventional flow rooms should be such that the number
of air changes, including recirculated air, should not
normally be less than 20 per hour except for class 4
rooms where not less than 10 changes per hour may be
acceptable.
7.2.2 Arrangements should also be made to ensure that excessive
turbulence is not produced and every precaution should
be taken to obviate the possibility of contaminated air
being carried back to the work stations. Contained work
stations and portable work boxes should normally conform
to the requirements of the type of air flow selected.
Air pressure and graduations between successive pressure
areas should not normally be less than 15 pa ( 1.5 mm
water gauge).
NOTE: 25 Pa(2.5 mm water gauge) is normally regarded as
adequate but when selecting the actual pressure, care
should be taken to ensure that leakage is prevented.
8. AIRHANDLING SYSTEMS:
The primary function of an air handling system for any type
of clean room is to control the level of airborne
contaminant particles by constantly filtering and
recirculating the air. The arrangement of a system depends
on whether it is to be a conventional clean room, the basic
form, however, it consists of a fan, ducting for inlet and
exhaust air and an air filtration system. In some
instances, the use of ducting may be minimized by adopting a
false ceiling arrangement and by flowing air through the
plenum chamber formed between two ceilings, and also be
adopting a twin cross-flow system ( see paragraph 6.2.1).
The air is conditioned to the required temperature and
humidity values (see paragraph 7.1) by adopting recognized
air-conditioning principles and by the integration of an
appropriate air -conditioning plant.
8.1 Fans-Fans are usually of the electrically operated type
designed to deliver a constant airflow rate through the
clean room as the filter pressure drop increases. They
should be mounted external to the ducting where possible, to
avoid heat loading of the air and introduction of further
contamination. Care should also be taken to avoid
contamination of the atmosphere by gaseous effluents.
8.2 Ducting-Ducting is constructed from materials which are non-
flaking and corrosion -resistant, stainless-steel and
aluminium being commonly used, or should normally be treated
to prevent the introduction of contaminants from the duct.
8.3 Filtration System-Filtration of airborne contaminant
particles is selected on the basis of cleanliness level
required and, generally a system is made up of two required
and, generally, a system is made up of two principal stages:
pre-filter stage and final filter stage. Pre-filtering is
carried out at the inlet to the air handling system and at
one or more points upstream of the clean room, and final
filtering directly at the inlet to the clean room. The
filters are specifically designed for clean room systems and
are graded at each stage, thus providing control of
diminishing size particles. Filtering action depends on the
particles contacting and adhering to the fibres or
collecting surface of the filter medium which is made from
such materials as glass-fibre and asbestos. The filters
utilized for final filtering are variously known as super-
inspection, absolute or high-efficiency particulate air
(HEPA) filter, and maybe used as individual units or
assembled to form filter bank or module. In the latter
case, each unit is connected to a common plenum chamber
incorporating its own fan. The number of individual units
in a bank is governed by design requirements for the air
handling system.
9. LAYOUT OF CLEAN ROOMS:
The layout of a clean room is governed by many factors
arising principally from the manufacturing processes and
test procedures to be carried out on specific types of
equipment. As a result there are a variety of design and
layout specification to meet the requirements of individual
manufacturers and operators of equipment. In their basic
form, however layouts are directly related to the
unidirectional flow and conventional.
9.1 Unidirectional Clean Rooms. The area devoted to the
facility is arranged in accordance with-the operating
practices common to all clean rooms, i.e. components and
personnel flow progressively from an uncontrolled or dirty'
environment to one in which the desired level of cleanliness
is maintained.
9.1.1 Personal Cleaning. Entrance to the clean room is via a
change room the purpose of which is to decontaminate
personnel without introducing removed contaminant
particles into the clean room. A change room is divided
into three distinct areas; an uncontrolled or 'dirty'
area, a wash-up ( Semi-contaminated) area and a change
(uncontaminated) area. These area are arranged so that
personnel must follow a definite path for entry into the
clean room.
a) In the uncontrolled area lockers are provided for
housing out door clothing such as overcoats and
raincoats, and also shoe cleaning machines.
From the uncontrolled area, entry to the wash-up
area is made via an air shower compartment the
purpose of which is to remove gross contaminant
particles from personnel. The size of the
compartment may be large enough to accommodate
only one person of a group of persons depending
on the number that must enter the clean room in
a given length of time.. The design of the air
shower may vary but in general, it consists of
an air inlet system operated by independent
fans. Air flows through the compartment from
air inlet nozzles or louvers mounted in the
ceiling or in one wall of the compartment. The
entrance and exit doors of the compartment are
interlocked so that only one of them can be
opened at a time. The closing of the entrance
door starts the fan and, until the cleaning
cycle is completed, the exit door remains
locked. The cycle may in some cases, be
interrupted by a safety override system in the
event of an emergency. Air velocities are
sufficiently high to cause flapping of clothing
but without discomfort to personnel.
b) On leaving the air shower, personnel proceed to
the changes area via the semi-contaminated area
in which washing and toilet facilities are
located. These facilities include foot
controlled washstands, liquid-soap dispensing
units and heated air hand-drying machines to
prevent contamination from toweling. A section
of the change area is provided for changing into
special clean room garments(see paragraph 12)
stored in racks or lockers. The entrance to
this section is remove residual contaminant
particles from the undersurfaces of shoes.
Entrance to the clean room after changing is
made via another air shower compartment.
9.1.2 Parts Cleaning. Prior to entry into a clean room, all
parts, tools, equipment, and material must also be
decontaminated and it is therefore necessary to provide
an additional are adjacent to the clean room. The
layout of a parts cleaning room depends largely on the
types of component and the number of work processes
involved. Similarly, the cleaning methods adopted
depend on the type of contaminant, the materials used in
the construction of components, and the level of
cleanliness required. In general, the room is equipped
with the required number of work tables, specialized
equipment, cleaning machines and washing facilities for
personnel.
a) The transfer of cleaned components to the clean room
is effected by means of a pass -through box
forming an air lock in the wall dividing the
appropriate areas. Boxes are provided with
double windows and doors; an interlock system
ensures that only one door can be opened at a
time. In some clean room facilities a pass-
through box maybe of the circular type with a
single opening so that the box must be rotated
through 180 degree to insert or remove a
component. Since the boxes are designed to
prevent a direct opening between rooms, a means
of verbal communication between relevant
personnel must be provided adjacent to the box.
This can be an intercommunication system, a
voice diaphragm, or a specking tube.
9.1.3 Additional Support Rooms- Since unidirectional clean rooms
require more rigid control to prevent contamination
entering, it is usual to make provision for additional
support rooms such as offices, lunch rooms, rest rooms,
etc. The construction of these rooms follows a similar
pattern to that of a clean room (see paragraph 10)
although the air handling system is usually not so
elaborate.
9.2 Conventional Clean Rooms-The use of conventional flow clean
rooms eliminates the necessity for support areas such as air
showers and special changing rooms and, as may be seen from
the typical conventional layout, increased working area is
available and entry procedures are much a simpler. The main
entrance is situated at the air outlet or 'dirty' end of the
room and personnel can pass through this directly from a
locker room and change area. Work benches and equipment are
disposed so that the cleanest operations are carried out
closest to the filter bank forming the end wall while dirty
operations such as soldering, cleaning etc. are performed
toward the outlet end of the room. Parts cleaning and
preparation may be performed in a manner similar to that
adopted for a unidirectional clean room(see paragraph 9.1.2)
or carried out in a parts cleaning room situated within the
clean room itself.
10. CONSTRUCTION OF CLEANROOMS:
The construction of clean rooms involves the application of
specifically developed building techniques, air-conditioning
installation particles and careful selection of construction
materials. This is normally undertaken by a specialist
organization working to the detailed BS 5295 Parts 1,2 and 3
and the specification of a user organization. However, the
following factors are to be taken into consideration while
constructing clean room.
Noise and Vibration shall be minimum but in no case noise
level shall exceed 65 db. Floors, walls, ceilings, lighting
and other utilities shall be of acceptable standard.
11. CLEAN ROOM CONDITION:
Furnishing such as work benches, chairs and containers for
components parts require careful selection, design and
choice of materials for their construction. However the
main structure of work benches and chairs should be of metal
and designed in such a way that contaminant particles cannot
accumulate and adhere to.
12. CLEAN ROOM GARMENTS:
Clean room products can be readily contaminated by particles
from clothing and it is therefore necessary to make
provision for the wearing of protective garments. These
take the form of smocks, overalls, caps and hoods. In
addition, 'boottee' type shoe covers, separate clean room
shoes and gloves must also be provided. The extent to which
all the garments are used depends on the type of clean room,
class of cleanliness to be achieved and the work processes
carried out.
12.1 Design-The garments are of special design to prevent the
transfer of contaminant particles from personnel and at the
same time to provide the maximum of comfort. The materials
from which they are fabricated are usually selected from the
range of available man made fires which exhibit such
properties as non-flammability, limited linting, and
negligible electrostatic generation. These materials are
available under a variety of trade names. Typical design
requirements for clean room garments are given in the
following paragraphs.
12.1.1 Smokes -Smokes should be of simple design, with no pockets
and with a few seams as possible. Seams should leave no
open end of material which might become frayed and give
off lint or loose strands. In addition, seams should be
double-stitched with thread of the same fibre as the
garment. Adjustable neck bands and cuffs should be
provided in preference to collar and loose sleeves and
must provide a snug fit when worn.
12.1.2 Overalls-Overalls should have a full-length zip fastener
with flap front and be provided with adjustable neck
bands and cuffs. If overalls are to be used with shoe
covers, the overalls should fit inside the covers.
Overalls to be used with clean room shoes should be
designed so that the legs of the overalls meet and
slightly overlap and shoes.
12.1.3 Caps - These should be of the style worn in hospital
operating rooms. They should fit snugly around the
head, covering the hair to prevent air particles and
dandruff falling into the clean room area.
12.1.4 Hoods - Hoods should be designed to confine all hair under
them to eliminate contamination by hair particles and
dandruff and to fit snugly inside overalls to provide
complete coverage of personnel; if beards are permitted,
masks must also be provided.
12.1.5 Shoe Covers and shoes- Covers should be worn over normal
shoes and should be high enough to hold the legs of
overalls. Covers should have a reinforced sole and be
of a type which will prevent personnel from slipping and
falling on smooth floors and, for reasons of durability
and economy, nylon is recommended as the achieve optimum
cleanliness, covers should be provided the legs and
above the ankles. As an alternative to shoe covers,
shoes can be issued to personnel for exclusive wear in
the clean room. They should be simply designed,
comfortable, washable and fabricated from materials
which will not shed particles due to abrasion and wear.
12.1.6 Gloves-When there is a risk of contamination from contact
with the hands or fingers, gloves or fingers stalls must
be used. Such coverings should be comfortable and
should enable the user to maintain a delicate finger
touch. If plastics is necessary for the 'touch' portion
of gloves the remainder should be made of a material
that will allow breathing thus preventing overheating of
the hands.
12.2 Garment Storage and Cleaning-When not in use, clean room
garments should not be allowed to come into contact with any
possible contaminant. They should always be stored on
individual hangers in the lockers provided in changing
rooms. Three sets of garments per person should normally be
provided; one set in use, one set being cleaned, and one set
in reserve.
12.2.1 Cleaning of garments is a specialized technique based on
conventional laundering and dry cleaning processes-
Ideally, a laundry should established as a specialized
unit supporting clean room operations, and functioning
under similar conditions of decontamination as a clean
room. T typical unit is divided into three distinct
areas; soiled garment receiving area, washing and dry-
Soiled garments are placed in polythene bags and
transferred to the receiving area through an air lock.
The garments are then emptied into specially built tubs
equipped with the appropriate machines. After cleaning
and drying, the garments are transferred to the third
area for inspection, sampling of contamination level and
packaging and sealing in polythene bags.
13. CLEAN WORK STATION:
These stations are work benches specifically designed to
incorporate their own filtered air supply system. They may
be utilized in a clean room, in addition to benches or
tables based on conventional patterns, or in an uncontrolled
environment.
13.1 The design of work stations has been developed from bench-
mounted dust free cabinets. Although these cabinets provide
low contamination levels, depending on the type of filter,
the problem of contamination while operations are performed
inside arises. Contaminants move about in turbulent air and
find their way out of the cabinet only at random intervals.
Another design is commonly referred to as a 'glove box'. It
utilizes a recirculating air system and although it produces
lower contamination levels than other forms of cabinet, it
has the disadvantage of requiring an operator to work
through arm ports and the attached gloves.
13.2 Work, stations overcome the deficiencies of 'dust free'
cabinets by incorporating an air distribution system which
operates on principles similar to those employed in a
unidirectional-flow clean room (see also paragraph 6.2).
The air distribution system consists of a fan and a pre-
filter mounted below the work surface, and an outlet with a
super-interception filter, mounted so as to produce either a
horizontal flow or a vertical flow over the work surface.
Glass panels form the sides of the work area which on
account of the unidirectional flow technique is open at the
front thus permitting unrestricted movement at the work
surface. Illumination of the work area is provided by
lighting units enclosed i,n the canopy above the work
surface. Individual switches foe lighting units and fans
are located at convenient points as also are the controls
for the various services required for relevant work
processes.
13.3 The selection of a work station best suited to a specific
application involves such factors as type of airflow, size
of work area, space available, and design and performance of
the air distribution system. Units employing horizontal
flow are generally less costly than vertical flow units for
equal size of work area and can usually be provided with
lower overall heights thus making them more suitable
when vertical space is a critical factor. When work
processes require the exhausting of fumes from the work
area, or when recirculation of the air is required, vertical
flow units provide for these functions more easily than
horizontal flow units. Horizontal flow units on the other
hand, provide better 'clean up' of a work area than vertical
flow units of equal size.
13.4 The most important consideration in selecting a particular
size of work station is to ensure that it will provide
unidirectional flow over a work area of sufficient width,
depth and height to accommodate the component being
assembled or tested, and the necessary associated equipment.
If several items of equipment must be sited around a
component, a vertical flow unit tends to produce less
turbulence and moves clean air in the most direct fashion
from the filter to the component. The filters are of a type
similar to those used in unidirectional clean rooms (See
paragraph 8.3).
14 CLEAN ROOM OPERATION:
In addition to the air handling system, the contamination
level in a clean room is kept at an acceptable level by two
other methods, namely limiting the contamination entering
the room and limiting the contamination generated within the
room. Both these methods are controlled to a large extent
by the personnel selected for clean room operations. The
contamination entering the room is limited by the wearing of
proper garments (See paragraph 12) personnel cleaning, parts
and equipments cleaning, etc. The contamination generated
is limited by restricting movement, proper work techniques
etc. It is therefore, necessary to establish routines and
disciplines related to personnel selection, personal
hygiene, entry procedures, and control of working
activities. The extent to which certain of these routines
and disciplines are applicable depends on the type of clean
room; for example, and unidirectional flow clean room
requires more rigid control of entry and clothing procedures
than a conventional clean room due to the air handling
system used (see paragraph 9.1 and 9.2).
14.1 Personnel Selection- The selection of personnel for clean
room duties involves consideration of both physical and
human factors, including manual dexterity visual appearance,
patience, concern for details, attitude towards repetitive
operations, and reaction to the rigid disciplines that
accompany confinement in a controlled environment. Certain
physiological problems must also be considered and some
examples which are detrimental to clean room operations are:
allergies to synthetic fabrics; allergies to solvents used
in cleaning processes; profuse nasal discharge; skin
conditions that result in above normal skin shedding or
flaking and dandruff; high amounts of acid found in the
hands; severe nervous conditions such as itching, scratching
or claustrophobia.
14.2 Person Hygiene- The development of personal hygiene is of a
great importance in clean room operations, not only to limit
contamination of vital components but also to maintain a
healthy working environment. Personnel with colds,
temporary coughing and sneezing should be assigned to
temporary jobs outside the clean room until they are
sufficiently recovered. This also applies to personnel have
received severe sunburn, to prevent peeling skin from
contaminating a component or the surrounding area.
14.3 Entry Procedures- Clean rooms are necessarily restricted
area and entry must only be allowed to personnel assigned to
them. The procedure to be adopted is governed by the type
of clean room. Typical activities associated with entry
procedures are as follows:
a) Removal of outdoor clothing such as overcoats and
raincoats and stowage in the lockers provided in the
'dirty' or uncontrolled area.
b) Checking clothes and shoes for visible contamination
such as mud, dirt, sand etc. Removal of such
contamination.
c) Washing of face and hands using foot-controlled
washstands, liquid soap dispensers and air driers.
d) Passing through air showers and air locks to ensure
adequate air scrubbing.
e) Walking over sticky or tacky mats.
f) Changing into the requisite clean room garments. In
connection with unidirectional flow clean room
operations, changing is done in the uncontaminated
section of the change room adjacent to the clean room.
In conventional clean rooms changing is done in an
area located at the 'dirty' end of the clean room.
14.4 General Rules for Operation. The following are general rules
which should be enforced to assist in the successful
operation of clean rooms.
14.4.1 Personal Activities:
a) Hands should be washed often and fingernails kept clean.
b) The specified clothing should always be worn in the
approved manner.
c) Personal items such as keys, coins, cigarettes, matches,
pencils, handkerchiefs and combs should be deposited
in lockers prior to changing into clean room garments.
Valuable items such as wallets may be carried into a
clean room in jacket or trouser pockets provided they
are not removed inside the clean room.
d) Foodstuff should not be taken into a clean room
e) Smoking is strictly forbidden.
f) The wearing of jewellery such as large rings, bracelets,
watches, necklaces, earrings, lockets etc. should be
avoided.
g) Nervous mannerisms such as scratching the head, rubbing
of hands or similar action should be avoided. h)
Movement of personnel should be avoided.
h) Movement of personnel should be restricted as much as
possible to prevent stirring settled particles on the
clean room floor. This applies particularly to
conventional clean rooms.
i) Solvent contact with hands should be avoided as many
solvents remove natural skin oils causing excessive
skin 'peeling' or flaking.
k) female personnel should not wear or apply fingernail
polish or cosmetics in a clean room.
l) Visitors or clean room maintenance personnel must be
authorized to enter a clean room and must follow the
specified entry procedures.
14.4.2 Work Activities:
a) All tools including personal tool kits should be kept
clean and in good condition and should undergo
cleaning processes in accordance with a periodic
cleaning schedule. Tools not essential to specific
work processes should be excluded from tool kits.
b)Paper materials should not be allowed in a clean room
unless the paper is plastic-coated or covered, sprayed
to prevent linting or is a special limited-linting
paper. Paper should not be subjected to excessive
shuffling, handling, rolling or bending as they can
generate excessive amounts of small particles under
these conditions.
c) Pencils and erasers are not allowed. All writing should
be with ball-point pens.
d) Parts of components should be kept in their individual
containers until ready for assembly. They should not
be left exposed on a work bench or station.
e) Containers and any component parts surplus to
requirements should always be returned to a parts
cleaning area for cleaning and re-issue.
f) Metal objects such as wire clippings and solder splashes
should be deposited in waste boxes at the end of each
process.
g) Where cleaning of parts is to be carried out insides a
clean room, the type of cleaning equipment and its
location within the room should be carefully selected.
15. MAINTENANCE OF CLEAN ROOMS:
In order to maintain clean rooms to the necessary standards,
good house keeping particles and monitoring of the air
handling system are of prime importance. The frequency of
cleaning is usually determined by taking into account the
change in contamination level that can occur due to the
cleaning operation, and the number of air changes per hour.
Monitoring of the air handling system should be carried out
at the time a clean room is put unto initial operation and
at regular periods thereafter, when filters have been
changed, and when it is evident that down-grading of its
operating level is taking place (see table 3)
15.1 Cleaning- Rooms should be cleaned when no work processes are
being performed. Minor dry floor and bench vacuuming can be
done, if necessary during normal room operation if the
equipment and procedures used ensure a minimum of
disturbance to settled particles.
15.1.1 Cellulose mops and sponges can be used with water
which meets specific particle-count requirements.
High-grade plastics buckets which are not subject to
falking should be used. If ladders are required, they
should preferably be of the anodised aluminium type.
The use of detergents should be restricted to those
which produce the minimum amount of residue after
drying. F9or vacuum cleaning, a central vacuum
cleaning system or a specially designed portable
vacuum cleaner should be employed.
15.1.2 Cleaning apparatus and utensils are prevalent sources
of contamination and their movement in and out of
clean rooms should be carefully scheduled. They
should be thoroughly cleaned and vacuumed prior to
their entry.
TABLE 3
/----------------------------------------------------------------------------------------\
|Controlled | Sampling for Particulate | Temperature | Humidity | Air Pressure |
|Environment | Contamination | | | |
|------------|-----------------------------|---------------|-------------|---------------|
| | | | | |
|Class 1 | Daily or continuous by | Continuous | Continuous | Continuous |
| | automatic equipment | | | |
| | | | | |
|Class 2 | Weekly | Continuous | Continuous | Continuous |
| | | | | |
|Class 3 | Monthly | 4-hourly | 4-hourly | Continuous |
| | | | | |
|Class 4 | 3-monthly | 12-hourly | 12-hourly | Continuous |
| | |-----------------------------| |
|Contained | Daily or to suit the | Dependent on use | Not applicable|
|Work Station| product or as Class 2 | | |
| | | | |
|Controlled | Dependent on use | To meet requirements of | Not applicable|
|Area | | personnel and product | |
\----------------------------------------------------------------------------------------/
15.1.3 The responsibility for cleaning work benches or
stations should be delegated to personnel assigned to
the benches to prevent improper handling of components
and equipment by room maintenance personnel.
15.1.3 Inspection, maintenance and testing of air handling
system components should be carried out i,n accordance
with the relevant maintenance instructions at periods
determined by the type of clean room operations, and
when down grading of the contamination level begins to
occur.
15.2 Monitoring of clean rooms-Monitoring refers to the
procedures adopted for checking the factors influencing
clean room environment. Such factors are the level of
contamination, temperature, humidity and pressure. The
exact requirements f9r monitoring and methods to be employed
depend on the type of clean room and classification of
cleanliness level, and are therefore determined on an
individual basis (see table 3).
15.1 Contamination Monitoring_ This is the most difficult
monitoring problem of clean room operation owing to the
variations in contamination level through out a room and
also to the many factors which must be considered in
selecting a specific monitoring technique. Some of the
factors causing variations in contamination level are
filtered air entering a room at one or more locations;
contamination being generated in various amounts throughout
a room; contaminated air exhausted from a room at one or
more locations. The highest level of exhaust locations,
since air from a highly contaminated area may be diluted
with filtered air prior to its being exhausted. Higher and
lower levels of contamination can thus readily exist within
a given room. The areas of most concern are those
immediately surrounding the component of which work
processes are to be carried out.
a) The locations within a clean room at which sampling of
the air is to be taken should be carefully considered in
order to obtain a representative contamination level.
Samples should be taken at identical times or as near as
possible, since contamination level of areas vary at
different periods.
b) Various techniques may be applied to contamination
monitoring and some of those most widely accepted,
together with details of principles as listed in BS 5295
Parts 1,2, and 3.
15.2.2 Humidity Monitoring - This may be achieved by the use of
conventional wet and dry bulb thermometers and
psychometric charts. The thermometers may be
supplemented, if necessary by automatic recording
devices. Humidity can become troublesome if it is
allowed to reach a level where static charges are
generated by personnel or where corrosion may be a
problem. In general, a humidity level of not less than
40% is desired. For those components where humidity
tolerance is critical, special control measures should
be employed.
15.2.3 Pressure Monitoring-A clean room should always be slightly
pressurised and it is therefore necessary to monitor the
pressure difference between the room and its outside
surroundings. Monitoring may be achieved by a simple U-
tube manometer, or a differential pressure gauge
calibrated in mm water gauge.
Premature failure of may instruments, equipments and
accessories have been attributed to ingress of
contaminants during overhaul manufacturing, assembly
stages. It is essential that accessories are tested,
overhauled and assembled in controlled environmental
condition in the various shops in accordance with the
conditions in the preceding paragraphs. All operators
are, therefore to adhere to these standards in their
various shops forthwith and document the procedure so
adopted in their Quality Control Manual.
Sd/-
(S. L. Srivastava)
Dy. Director General of Civil Aviation