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SEMI S2-0703a E © SEMI 1991, 2004 37 A2-4.1.6 Exhau st volume settings sho uld consider laminar air flo w volumes and be balance d to minimize turbul ence and to ensure c apture. A2-4.1.7 The stati on design should co ns…

SEMI S2-0703a

there is a failure, and then slightly increasing the flow

rate until the test is successful, can be used to help

minimize air flow specifications.

A2-2.4.4 Chemical air or wipe monitoring can be used

to confirm that chemical transport is not occurring into

unintended areas of the equipment.

A2-2.4.5 Velocity profiling will confirm expected

airflows, the direction of flow, and the effect of

distance.

A2-2.4.6 Vapor visualization will confirm expected

airflows, the direction of flow, and the effect of

distance. Vapor visualization is the observation of

aerosols (e.g., aerosols generated by using water, liquid

nitrogen, or dry ice) so that exhaust flow patterns can

be observed. Smoke tubes or aerosols may also be used,

however they can produce contamination.

A2-3 Chemical Laboratory Fume Hoods, Parts

Cleaning Hoods

Lab fume hoods and part cleaning hoods are designed

to control emission by enclosing a process on five sides

and containing the emission within the hood.

A2-3.1 Design Recommendations

A2-3.1.1 Fully enclosed on five sides, open on one side

for employee access and process/parts placement and

removals.

A2-3.1.2 Front (employee access side) should be

provided with sliding door and/or sash.

A2-3.1.3 Minimize size of the hood based on process

size.

A2-3.1.4 Minimize front opening size based on size of

process and employee access needs.

A2-3.1.5 Ensure hood construction materials are

compatible with chemicals used.

A2-3.2 Control Specifications — Face velocity is the

specification generally used with hoods open on only

one side.

A2-3.2.1 Generally acceptable laboratory fume hood

face velocities range from 0.40 to 0.60 m/s (80–120

fpm) with no single measurement  20 % of average.

0.64 to 0.76 m/s (125–150 fpm) is recommended for

hoods in which carcinogens or reproductive toxicants

may be used.

A2-3.2.2 Velocities as low as 0.30 to 0.40 m/s (60–80

fpm) can be effective but require no cross drafts or

competing air movement in the work area.

A2-3.2.3 An average face velocity of 0.50 m/s (100

fpm) is generally found to be acceptable in most

applications.

A2-3.2.4 Face velocities of 0.64 to 0.76 m/s (125 to

150 fpm) may be required when a lab hood is installed

in an area with laminar air flow.

A2-3.2.5 Face velocity above 0.76 m/s (150 fpm)

should be avoided to prevent eddying caused by a lower

pressure area in front of an employee standing at the

hood.

A2-3.3 Recommended Measurement/Validation

Method

A2-3.3.1 The preferred method is measurement of

average face velocity and hood static pressure.

Measurements are taken with a velometer or

anemometer. Multiple measurements are taken in a

grid, at least 10 to 40 per square meter (1–4 per square

foot) of open area, in the plane opening of the hood.

This allows representative, evenly spaced

measurements to be taken (see also open-surface tanks).

A2-3.3.2 Additional confirmation by visualization

check of containment using smoke or vapor testing.

A2-3.3.3 ASHRAE Method 110, or equivalent (use

appropriate sections), for tracer gas testing of lab hoods

may be used as a supplemental verification provided

that an accurate emission rate can be defined.

(ASHRAE 110 lists 3 tests: “as manufactured,” “as

used,” and “as installed.” The “as manufactured” test is

the test that is used most frequently.)

A2-4 Wet Stations

Wet stations are slotted hoods designed to capture

laminar air flow while also capturing wet process

emissions from the work area. Wet stations can be open

on the front, top and both sides (it is usually preferable

to enclose as much as possible).

A2-4.1

Design Recommendations

A2-4.1.1 Slots should be provided uniformly along the

length of the hood for even distribution of airflow.

A2-4.1.2 Additional lip exhaust slots should be

provided around tanks or sinks to control emissions.

A2-4.1.3 The plenum behind the slots should be sized

to ensure even distribution of static pressure. These

slots should be designed to ensure adequate airflow is

provided by the side slots, and to minimize turbulence

that could reduce exhaust performance.

A2-4.1.4 Velocity along length of slot should not vary

by more than 10% of the average slot velocity.

A2-4.1.5 Additional use of end or side panels/baffles

can reduce negative impact of side drafts.

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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

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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