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SEMI S6-93 © SEMI 199 3 5 by use of “ clea n smoke” generated from a deionized water vap or gene rati ng sys te m. 8.3.7.8 Once capture has been v eri fied , me asurements should be t aken at specific open i ngs (make-u …

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SEMI S6-93 © SEMI 1993 4
feet) elevation before comparing them to suppliers’
specified data. Additionally, the supplier-specified data
must be corrected to sea level if it was measured above
600 meters elevation.
8.3.4.1 Flow measurements taken at other than
standard air temperatures (21.1°C (70°F)) should be
corrected to standard conditions.
8.3.4.2 Corrections for altitude and temperature should
use the following relationship:
M
3
/s = (m
3
/s)(294.3/T
K
)(D)
CFM = (cfm)(530/T
R
)(D)
M
3
/s = Standard cubic meters per second
m
3
/s = Measured cubic meters per second
CFM = Standard cubic feet per minute
cfm = Measured cubic feet per minute
T
K
= Measured temperature in degrees K (T
K
= T
C
+
273.2)
T
C
= Measured temperature in degrees C
T
R
= Measured temperature in degrees R (T
R
= T
F
+
459.7)
T
F
= Measured temperature in degrees F
D = Altitude density correction factor
Altitude Density Correction Factor
sea level 1.00
300 m (1000 ft) 0.96
600 m (2000 ft) 0.93
900 m (3000 ft) 0.89
1200 m (4000 ft) 0.86
1500 m (5000 ft) 0.83
8.3.5 The expected temperature of the exhaust stream
(from the supplier’s equipment) at the connection to the
user’s exhaust system should be included in the
specifications. The connecting duct material should be
compatible with this temperature. The normal operating
and maximum high and minimum low temperatures
should be specified.
8.3.6 Static pressure is measured at the same traverse
point as discussed in Section 8.3.1. The equipment
specifications should define the minimum Static
Pressure, at the point of connection, required for proper
equipment operation.
8.3.6.1 The target Static Pressure range at the point of
connection to equipment is -125 to -250 Pa (-0.5 inches
to -1.0 inches of water). This permits flexibility of
equipment placement with minimal need for pressure
boosting devices, such as booster fans.
8.3.6.2 For exhaust Static Pressure requirements lower
than -1.0 inches of water, the equipment supplier must
establish, with the user’s concurrence, the specifications
for a safe booster add-on device. A larger negative
number is a lower exhaust pressure. Normally, users
will install boosters only when their exhaust
distribution system cannot reach the specified static
pressure. As each user’s facility is unique, booster add-
on devices should be separate from the equipment’s
functional design and used only as a last resort.
8.3.7 The physical settings and configuration of the
process equipment when specified flow and pressure
measurements are made is critical to the repeatability of
the measurements.
8.3.7.1 Exhaust enclosures, capture zones, and entry
points in the equipment should isolate the area to be
exhausted from adjacent areas. Internal ducts,
partitions, and guide plates in the equipment should be
leak tight to prevent the release of contamination from
the exhaust stream.
8.3.7.2 To capture, move, or dilute contaminants
effectively within the equipment’s exhaust enclosure,
there must be a continuous supply of make-up air. The
enclosure should be designed such that the make-up air
is drawn from a selected area through designed
openings in the enclosure walls. Care should be taken to
ensure the make-up air does not contain, or potentially
contain, vapors or fumes that could be incompatible
with the exhaust enclosures material of construction,
components, or target materials being exhausted.
8.3.7.3 Minimize the volume of the enclosures where
possible to reduce the load on the exhaust system and
on its companion make-up air system.
8.3.7.4 Enclosures supplied with, or as part of,
equipment should be designed so that exhaust properly
sweeps all potential emission release points.
8.3.7.5 Aerodynamic “dead spots” inside the enclosure
should be minimized.
8.3.7.6 Components that have no potential for release
should be located outside the enclosure, when possible.
Additionally, the handles for any valves located inside
the exhausted enclosure should be positioned outside of
the enclosure to reduce the need to open the enclosure.
8.3.7.7 Verification that the exhaust enclosure
provides the desired capture of contaminants can be
accomplished by using Sulfur Hexafluoride as a tracer
gas (see SEMI F15). Verification that aerodynamic
“dead spots” have been eliminated can be accomplished
SEMI S6-93 © SEMI 19935
by use of “clean smoke” generated from a deionized
water vapor generating system.
8.3.7.8 Once capture has been verified, measurements
should be taken at specific openings (make-up air slots,
access ports, or windows, etc.) to establish the
minimum flow velocity necessary at these specific
openings to achieve capture.
8.3.7.9 Record the position of covers, doors, dampers,
valved openings, etc., and flow restrictions (such as
fluid levels in vessels which exhaust air serves or flows
through) at the time of the ventilation tests so that the
test can be duplicated.
8.3.7.10 The supplier should specify how to duplicate
these conditions at the time of field adjustment and
balance. If the original conditions are not duplicated,
the exhaust losses through the equipment may vary.
The major risk is that the equipment exhaust level will
be set as close to the suppliers’ specifications as
possible and may not provide the safety level intended
during actual equipment operation.
8.3.8 The equipment may require different exhaust
flow or pressure during the process cycle than in
standby or maintenance modes. The supplier should
provide specifications for normal operating mode
demands, peak demand, and any other demand level
required for employee safety. The normal, peak, and
other critical safety condition demands should be
compensated for in both the system design and the field
balance.
8.3.9 Equipment data used to establish exhaust
specifications should reflect the equipment’s
environmental conditions anticipated when installed
(e.g., bulkhead mounted equipment is supplied with
data for a bulkhead installation).
8.3.9.1 Typical user clean rooms are designed using
the “bay” and “chase” concept. The bay is served with
100% ceiling-supplied, HEPA-filtered air. Vertical air
flow below the ceiling is typically 0.5 m/s (100 FPM)
across the entire bay. The chase section is the return air
plenum for the bay air supply system. The typical
pressure differential across the bay wall is 2.5 to 7.5 Pa
(0.01 to 0.03 inches of water), with the lower pressure
on the chase side.
8.3.9.2 Bulkhead or through-the-wall mounted
equipment should tolerate this pressure differential.
8.3.9.3 Cabinet or enclosure exhaust applications
should be designed so that back streaming into the
clean bay or chase is prevented.
8.3.10 The set point tolerance at time of initial
installation should be included in the specification.
Performance stability over time for the exhaust flow
and pressure requirements, needed to support
equipment operations and safe working conditions,
should also be specified.
8.3.10.1 The users’ exhaust distribution systems
typically hold a balance setting ±10% of the set point(s)
over time. Equipment which integrates high or low
exhaust level alarms, requiring tighter stability control,
are special cases and should be identified at the time of
purchase.
8.3.10.2 The supplier-specified set points for Exhaust
Flow, Volume, and Pressure should include a tolerance
of -0% +10%. (This is the common acceptance range
established for exhaust distribution system balance.)
8.3.11 The five methods of exhaust use discussed in
Section 7 drive different priorities when establishing
exhaust specifications.
8.3.11.1 The “initiate motion” and “hold” functions
are either pressure or velocity driven, depending on the
safety equipment selected by the supplier.
8.3.11.2 “Capturing” and “moving” are typically
velocity-dependent.
8.3.11.3 “Dilution” is flow volume dependent.
8.3.11.4 The equipment exhaust specification should
focus on the exhaust attribute (see Section 6) associated
with the exhaust use (see Section 7). If multiple uses
are being met, the supplier should prioritize Pressure,
Flow, and Flow Volume for balancing.
8.3.12 The instruments and practices used for taking
measurements to establish equipment specifications
should be recommended in the specifications and used
in field balance operations.
8.3.12.1 A hot wire anemometer is acceptable for
velocities less than 150 m/s (500 FPM). A low range
differential pressure gauge (0 to 60 Pa or 0.0 to 0.25
inches of water) is acceptable for velocities over 150
m/s (500 FPM).
8.3.12.2 An inclined/vertical manometer or high range
differential pressure gauge is acceptable for measuring
static pressure.
8.3.12.3 Conversion charts should be developed to
indicate what flow volume is present at various points
on the inclined/vertical manometer or differential
pressure gauge.
8.3.12.4 Equipment suppliers should include
recommendations in the equipment specifications for
the proper selection of the pitot tube, static tip, etc., to
use when measuring exhaust flow and pressure.
8.3.12.5 The description, operation, maintenance, and
limitations of these and alternate instruments is covered
SEMI S6-93 © SEMI 1993 6
8.3.12.6 in Chapter 9 of “Industrial Ventilation,” A
Manual of Recommended Practice, 20th Edition,
(Appendix 2, Reference 1). A companion reference is
Chapter 5 of HVAC Systems Testing, Adjusting, and
Balancing, (Appendix 2, Reference 3).
8.3.12.7 As with any test performed to establish or
verify specifications, the measuring instruments used
should be in good working condition and current
calibration.
8.3.12.8 If an air balancing contractor is used, it
should be certified by the Associated Air Balance
Council (AABC) or the National Environmental
Balancing Bureau (NEBB) as meeting their technical
standards for membership.
8.3.12.9 For additional information, see National
Standards for Field Measurement and Instrumentation -
Total System Balance, Volume Two, No. 12173,
(Appendix 2, Reference 6). Also see Procedural
Standards for Testing of Cleanrooms, (Appendix 2,
Reference 4).
8.3.13 The exhaust specification provided to the user
by the supplier should include design information on
the complete equipment exhaust system. A prototype
exhaust data worksheet is shown in Appendix 1. The
supplier should also provide:
8.3.13.1 A scaled drawing of the equipment exhaust
system showing all connections, dampers, monitoring
devices, etc.
8.3.13.2 A diagram of the exhaust connection with
critical dimensions, established set point, and the
location of the traverse point used to develop the
specifications.
8.3.13.3 Any special case exceptions.
8.3.13.4 A list of materials used in the construction of
the exhaust enclosure and materials for components of
the exhaust system.
8.3.14 The supplier should provide the user with a list
of constituents that could be expected in the exhaust
stream when the equipment and the users’ exhaust
system are operating within design specifications.
Information typically required is:
8.3.14.1 Percent by volume.
8.3.14.2 Percent by weight.
8.3.14.3 State of constituent (liquid, gas, or solid).
8.3.14.4 Temperature of the composite exhaust stream
under normal operations.
8.3.14.5 Information should be based on a continuous
twenty-four hour operation of the equipment.
(Extrapolation is generally acceptable if the method
used is well-documented and can be duplicated.)