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SEMI S6-93 © SEMI 199 3 4 feet) elevation bef o re comparing them to suppliers’ specified data. Additionall y, the supplier-specified da ta must be corrected to sea lev el if it was measured above 600 me ters elevati o n…
SEMI S6-93 © SEMI 19933
4. Air density.
5. Temperature of the exhaust stream when it enters
the exhaust system.
6. Static pressure at the point of measurement.
7. Physical condition of the process equipment at the
time of measurement.
8. Peak, normal, and special exhaust requirements.
9. Equipment environmental conditions anticipated
(installed).
10. Stability and tolerance requirements of the
specifications.
11. Priority of exhaust attributes when the system is
balanced. (What is the primary intent of the exhaust,
secondary, etc.?)
12.Instruments and practices used (and recommended)
for taking measurements.
13. Design information on the complete equipment
exhaust system (including materials of
construction).
14. Constituents of the exhaust stream.
Each of these key elements is discussed in detail in
Sections 8.3.1 through 8.3.14.
8.3.1 The point of measurement for the equipment
exhaust duct should be clearly defined. Information
given to locate the point of measurement should include
a diagram of the exhaust connection with locations of
the traverse points used.
8.3.1.1 Turbulence in the duct at the point of
measurement should be minimized. The measurement
point should be in a straight section of duct. It should be
downstream in the connecting duct past the vena
contracta from the last transition made in the
equipment. It should be far enough from fittings,
dampers, or sprinkler heads to minimize their
interference with the measurements. Normal
recommended practice is 7.5 duct diameters from any
point of connection or fitting. See “HVAC Systems
Testing, Adjusting, and Balancing,” (Appendix 2,
Reference 3).
8.3.1.2 Equipment users typically use a throttling
damper for exhaust balancing where the equipment
drop connects to the rest of the exhaust system.
Balancing measurements should be taken on the
equipment side of these balancing dampers. The
supplier may elect to provide a damper, at or near the
equipment connection point, to be used only as a
trimming device over a narrow performance band
around the equipment flow and pressure specifications.
Features of this damper should be designed to prevent
complete blockage of exhaust flow.
8.3.2 Duct size is the inside diameter of the exhaust
connection from the equipment. The supplier should
provide any other dimensions needed to design the
connection of the exhaust system to the equipment.
8.3.2.1 The duct size of the equipment connection
should fall in the low flow velocity range of the duct
friction table. See “HVAC Systems Duct Design”
(Appendix 2, Reference 2). Low flow velocity ducts
will promote flexibility and ease of interface to exhaust
systems serving multiple equipment. When particles are
to be captured and removed, design the system for the
minimum flow velocity that will ensure capture.
NOTE: Exhaust system designers should always keep in mind
the difficulty that can be experienced in flow velocity
measurements in unduly large ducts. Tool connections should
be sized for accurate flow measurements.
8.3.2.2 The design of the equipment exhaust
connection should allow for long radius connecting
elbows at the point of connection. This will reduce
friction losses. Additionally, the equipment supplier
should avoid configurations that would require an angle
of entrance into the user’s branch ducts of greater than
30 degrees.
8.3.2.3 Duct connection configuration should be such
that liquid spills or releases within the equipment
enclosures will not enter the facility’s exhaust system.
8.3.3 Flow through the equipment enclosure, and the
duct at the point of connection, can be specified in
velocity or volume. Normally, the balancing engineer
will measure Flow Velocity and convert velocity to
Flow Volume using the equation Q = VA. (See Section
6.1.1.)
8.3.3.1 Section 8.3.6 of this document shows the
pressure guidelines for semiconductor exhaust systems.
The equipment supplier should establish, through
testing, the Flow Velocity and Volume required for
efficient operation of its equipment’s exhaust system.
The user and the supplier should agree, before the
purchase of the equipment, on the safety control
devices to be used. Any safety control device requiring
flows or pressures outside these ranges is a special case
and should be resolved at the time of purchase.
8.3.4 Exhaust specifications should be stated in
Standard Air Density (kg/m
3
or lb/ft
3
), as defined in
“Industrial Ventilation,” (Appendix 2, Reference 1). If
the measurements are of non-standard air, state the
correction factor for density. Users should correct field
balancing measurements taken above 600 meters (2000
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