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SEMI S2-0703a E © SEMI 1991, 2004 38 manifold b oxes, and seco ndary gas panel enclosures, are typically six-sided ful ly enclosed enclosures with access panels/doors on at least one side. These ventilated enclo sures ar…

SEMI S2-0703a

A2-4.1.6 Exhaust volume settings should consider

laminar air flow volumes and be balanced to minimize

turbulence and to ensure capture.

A2-4.1.7 The station design should consider airflow

patterns in the operating zone to minimize turbulent

horizontal airflow patterns into and across the work

deck.

A2-4.1.8 Additional considerations to reduce exhaust

demand include providing covered tanks, and recessing

tanks below deck level.

A2-4.2 Control Specifications

A2-4.2.1 Wet station specifications are complicated by

the fact that wet stations generally do not have an easily

definable face velocity to measure. A number of

methods have been used and are all acceptable if used

consistently and provided documentation indicates

chemical containment meets the 1% of the OEL at

distances beyond the plane of penetration at the exterior

of the wet station.

A2-4.2.2 Maintain an average capture velocity of 0.33

to 0.50 m/s (65–100 fpm) immediately above a bath.

A2-4.2.3 Calculate the total exhaust volume

requirement by determining the total volumetric flow of

laminar air hitting the deck and increasing this value by

20 to 25%.

A2-4.2.4 For some wet stations that are partially

enclosed from the top, an artificial plane opening

(“face”) can be defined where the downward laminar

air flow penetrates the capture zone (at “face velocity”)

of the wet station. Depending on the hood design and

laminar air flow provided, average face velocities can

range from 0.20 to 0.50 m/s (40 to 100 fpm). The

measurement location can greatly influence the

measured face velocity; therefore, this method should

be supplemented with at least one of the preceding

methods for greater accuracy and reproducibility at the

user’s facility.

A2-4.3 Recommended Measurement/Validation

Method

A2-4.3.1 Confirmation of capture using vapor

visualization.

A2-4.3.2 Confirmation of laminar flow of make up air

into the station using vapor visualization.

A2-4.3.3 Tracer gas testing may be used as

supplemental verification, provided an emission rate

can be accurately defined.

A2-5 Supplemental Exhaust

Supplemental exhaust, if not designed into the

equipment, can be provided by a flexible duct with a

tapered hood. This can be placed in the work area to

remove potential contaminants before they enter the

breathing zone. Supplemental exhaust is frequently

used during maintenance or service.

A2-5.1 Design Recommendations

A2-5.1.1 Retractable or movable non-combustible flex

ducting for easy reach and placement within 150 to 300

mm (6 to 12 inches) of potential emissions to be

controlled.

A2-5.1.2 Manual damper at hood to allow for local

control, i.e., shut off when not required.

A2-5.1.3 Tapered hood with a plane opening as a

minimum. The additional use of flanges or canopies to

enclose the process will result in improved efficiency.

A2-5.2 Control Specifications

NOTE A2-1: This is one equation that is most commonly

used. Other equations may be appropriate; see also ACGIH

Industrial Ventilation Manual, and Semiconductor Exhaust

Ventilation Guidebook.

A2-5.2.1 A minimum capture velocity of 0.50 m/s (100

fpm) is required at the contaminant generation point for

releases of vapor via evaporation or passive diffusion.

Ventilation should not be relied upon to prevent

exposures to hazardous substances with release

velocities (e.g., pressurized gases). For a plane open

ended duct without a flange, the air flow required at a

given capture velocity can be calculated by:

Q = V(10X

+ A)

Where: Q = required exhaust air flow in m

/s (cfm)

V = capture velocity in m/s (fpm) at distance X from

hood

A = hood face area in square meters (square feet)

X = distance from hood face to farthest point of

contaminant release in meters (feet). NOTE: This is

only accurate when X is within 1.5 diameters of a round

opening, or within 0.25 circumference of a square

opening.

A2-5.3 Recommended Measurement/Validation

Method

A2-5.3.1 Measurement of capture velocity at farthest

point of contaminant release. Measurements taken with

a velometer or anemometer.

A2-5.3.2 Confirmation by visualization check of

capture using vapor capture testing.

A2-6 Equipment Gas Panel Enclosures

Equipment gas panel enclosures, also known as gas

boxes, jungle enclosures, gas jungle enclosures, valve

SEMI S2-0703a

manifold boxes, and secondary gas panel enclosures,

are typically six-sided fully enclosed enclosures with

access panels/doors on at least one side. These

ventilated enclosures are designed to contain and

remove hazardous gases from the work area in the

event of a gas piping failure or leak. Gas panel

enclosures are typically of two types, those requiring no

access while gas systems are charged, and those that

must be opened during processing while gas systems

are charged. There is also a distinct difference in

control specifications for those with pyrophorics or

other flammables vs. other HPMs, specifically in the

control of pocketing.

A2-6.1 Design Recommendations

A2-6.1.1 Compartmentalize potential leak points.

A2-6.1.2 Minimize the total size of the panel and its

enclosure.

A2-6.1.3 Minimize size and number of openings.

A2-6.1.4 Minimize static pressure requirements of the

enclosure; control has been shown to be achievable

with –1.3 to –2.5 mm ( –0.05 to –0.1 in.) w.g.

A2-6.1.5 Design for sweep. Minimize the number and

size of openings. Seal unnecessary openings (e.g.,

seams, utility holes).

A2-6.1.6 Where routinely used access doors are

required:

 Make the access door as small as practical.

 Place the openings to the enclosure in the access

door to minimize air flow requirements.

 Provide baffles behind the door to direct leaks

away from the door and openings.

 Compartmentalize the enclosure so that access to

one area does not affect air flow control in other

areas.

A2-6.2 Control Specifications

A2-6.2.1 Exhaust volumes as low as 4–5 air changes

per minute or less can be specified and meet the S2

criteria in Section 23.5 if the design principles listed

above are considered when designing equipment and

enclosures.

A2-6.2.2 Where there is potential for chemical

exposure during access which can be controlled by face

velocity, the enclosure should also provide a minimum

face velocity of 0.36 to 0.76 m/s (70 to 150 fpm) when

open. Face velocity should not be relied upon to control

emissions from a pressurized fitting.

A2-6.2.3 Enclosures for pyrophoric or flammable

gases should be designed to ensure adequately uniform

dilution (i.e., prevent “pocketing”) and to prevent

accumulation of pyrophoric and flammable gases above

their lower explosive limit. Uniform dilution can

generally be verified through exhaust vapor

visualization techniques. Ventilation flow rate should

be adequate to maintain concentrations below 25% of

the lower explosive limit for the gas with the lowest

LEL that is used in the enclosure. This can generally be

verified using engineering calculations to verify

dilution, and vapor visualization to verify mixing.

A2-6.3 Recommended Measurement/Validation

Method

A2-6.3.1 Preferred validation by tracer gas testing per

SEMI F15.

A2-6.3.2 Additional confirmation by visualization

check of air flow, mixing and sweep using smoke or

vapor testing.

A2-6.3.3 Measurement of average face velocity at

inlet(s), opening(s), or routinely used access doors.

Measurements should be taken with a velometer or

anemometer. For larger openings, multiple

measurements are taken in a grid, at least 10 to 40 per

square meter (1–4 per square foot) of open area. Useful

equation: V = 4.043 (VP/d)

0.5

, where V = velocity in

m/s, VP = velocity pressure in mm H

O, and d =

density correction factor (unitless).

A2-7 Equipment Exhaust Ventilation

Specifications and Measurements

A2-7.1 Specifications for equipment exhaust should be

provided by the supplier and define:

A2-7.1.1 The control specification or standard for the

hood or enclosure, i.e., face velocity or capture velocity

if applicable.

A2-7.1.2 The airflow in the duct required to maintain

the control volume or flow required. Measurements

should be made using the ACGIH pitot traverse method

described below.

A2-7.1.3 The location where the Pitot traverse

measurement in the duct was made.

A2-7.1.4 Static pressure requirements.

A2-7.1.5 Coefficient of Entry (C

) (see definitions and

Section 22.3).

A2-7.1.6 Hood Loss Factor (K or F

) (see definitions

and Section 22.3).

A2-8 Duct Traverse Method

A2-8.1 Because the air flow in the cross-section of a

duct is not uniform, it is necessary to obtain an average

by measuring velocity pressure (VP) at points in a

number of equal areas in the cross-section. The usual

SEMI S2-0703a

method is to make two traverses across the diameter of

the duct at right angles to each other. Reading are taken

at the center of annular rings of equal area. Whenever

possible, the traverse should be made 7.5 duct

diameters downstream and 3 diameters upstream from

obstructions or directional changes such as an elbow,

hood, branch entry, etc. Where measurements are made

closer to disturbances, the results should be considered

subject to some doubt and checked against a second

location. If agreement within 10% of the two traverses

is obtained, reasonable accuracy can be assumed and

the average of the two readings used. Where the

variation exceeds 10%, a third location should be

selected and the two air flows in the best agreement

averaged and used. The use of a single centerline

reading for obtaining average velocity is a very coarse

approximation and is not recommended. If a traverse

cannot be done, then the centerline duct velocity should

be multiplied by 0.9 for a coarse estimate of actual

average duct velocity. Center line duct velocity should

not be used less than 5 duct diameters from an elbow,

junction, hood opening, or other source of turbulence.

A2-8.2 For ducts 150 mm (6 in.) and smaller, at least 6

traverse points should be used. For round ducts larger

than 150 mm (6 in.) diameter, at least 10 traverse points

should be employed. For very large ducts with wide

variation in velocity, 20 traverse points will increase the

precision of the air flow measurement.

A2-8.3 For square or rectangular ducts, the procedure

is to divide the cross-section into a number of equal

rectangular areas and measure the velocity pressure at

the center of each. The number of readings should not

be less than 16. Enough readings should be made so the

greatest distance between centers is less than 150 mm

(6 in.).

A2-8.4 The following data are required:

A2-8.4.1 The area of the duct at the traverse location.

A2-8.4.2 Velocity pressure at each point in the traverse

and/or average velocity and number of points measured.

A2-8.4.3 Temperature of the air stream at the time and

location of the traverse.

A2-8.4.4 The velocity pressure readings obtained are

converted to velocities, and the velocities (not the

velocity pressures) are averaged. Useful equation: V =

4.043 (VP/d)

0.5

, where V = velocity in m/s, VP =

velocity pressure in mm H

O, and d = density

correction factor (unitless). Some monitoring

instruments conduct this averaging internal to the

instrument.

A2-8.5 Flow measurement taken at other than standard

air temperatures should be corrected to standard

conditions (i.e., 21C [70F], 760 mm [29.92 in.] Hg).