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SEMI S2-0703a E © SEMI 1991, 2004 36 there is a failure, and then sligh tly increasing the flow rate until the test is successful, can be used to help minimize air flow specifications. A2-2.4.4 Chem ical air or wipe m on…

SEMI S2-0703a

Hood Type Recommended Test Methods Typical Design and Test Exhaust

Parameters (See NOTE 1.)

References

Parts-Cleaning

Hoods

Primary: face velocity, vapor

visualization

Supplemental: tracer gas, air sampling

0.40–0.64 m/s (80–125 fpm) face

velocity

ASHRAE Standard 110

SEMI F15

ACGIH Industrial

Ventilation Manual

Pump and

Equipment

Exhaust Lines

Primary: static pressure

Supplemental: tracer gas

–6 to –25 mm (–0.25 to –1.0 in.)

O static pressure

125% maximum volume flow rate

from pump

ACGIH Industrial

Ventilation Manual

SEMI F15

Glove Boxes Primary: static pressure, tracer gas

Supplemental: vapor visualization, air

monitoring

No consensus for a reference at the

time of publication of this guideline.

ACGIH Industrial

Ventilation Manual

SEMI F15

Drying/ Bake/

Test Chamber

Ovens

Primary: static pressure, tracer gas

Supplemental: vapor visualization, air

monitoring

–1.3 to –2.5 mm (–0.05 to –0.1 in.)

O static pressure

SEMI F15

ACGIH Industrial

Ventilation Manual

Spin-Coater

(cup only)

Primary: vapor visualization,

velometry

Supplemental: air sampling

(see SEMI S2 Sections 23.5.1–3) ACGIH Industrial

Ventilation Manual

Supplemental

Exhaust

Primary: capture velocity, vapor

visualization, air sampling

0.50–0.76 m/s (100–150 fpm)

capture velocity

ACGIH Industrial

Ventilation Manual

NOTE 1: All measurements should be within ± 20% of average for face velocity, ± 10% of average along the length of each slot for slot velocity,

and ± 10% of average between slots for slot velocity.

A2-2 Exhaust Optimization

A2-2.1 Exhaust optimization is the use of good

ventilation design to create efficient equipment exhaust.

The design and measurement methods discussed below

confirm that equipment exhaust is acting as the

manufacturer intended. This information is not meant to

prohibit alternate methods of achieving or verifying

good ventilation design. References for ventilation

design are included at the end of this Appendix.

A2-2.2 Design Recommendations

A2-2.2.1 Equipment exhaust design can attempt to

reduce inefficient static pressure losses caused by:

friction losses from materials; openings, and duct

geometries (elbows, duct expansions or contractions);

turbulent air flow; fans; internal fittings such as blast

gates and dampers; directional changes in airflow.

A2-2.2.2 Other good design principles can include

minimizing distance between the source and hood, and

reducing enclosure volumes.

A2-2.2.3 For non-chemical issues such as heat from

electrical equipment, heat recapture rather than exhaust

may be appropriate.

A2-2.2.4 The possible impact of highly directional

laminar airflow found in most fabs should be

considered when designing equipment exhaust.

A2-2.3 Recommended Equipment Controls — The

location of internal blast gates or dampers inside

equipment, and their appropriate settings, should be

clearly identified. The number of equipment dampers

and blast gates should be minimized. Gates/dampers

should be lockable or otherwise securable. Static

pressure or flow sensors installed on equipment by the

manufacturer should have sufficient sensitivity and

accuracy to measure exhaust flowrate fluctuations that

place the equipment out of prescribed ranges.

A2-2.4 Recommended Measurement/Validation

Method — Measurements should be made to identify

optimal exhaust levels and confirm that safety and

process requirements are being addressed. The

manufacturer should be able to identify any critical

equipment locations for chemical capture, and quantify

appropriate exhaust values. Multiple validation/

measurement methods may be needed.

A2-2.4.1 Measurements should be done after

equipment components are assembled.

A2-2.4.2 Computer modeling can be done to predict

exhaust flow and hazardous material transport in

equipment by solving fluid mechanics conservation of

energy and mass equations. Modeling can be used in

the equipment design stage or to improve existing

equipment. Computer models should be verified

experimentally, using one or more of the methods

discussed below.

A2-2.4.3 Tracer gas testing provides a method to test

the integrity of hoods by simulating gas emission and

measuring the effectiveness of controls. Testing until

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.

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