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SEMI S6-93 © SEMI 199 3 2 (inside th e duct). The unit of measure is pascals (Pa, Newtons per s q uare meter (N/m 2 )) or inches of wa ter. 5.10 traverse measurements — Mu l t ip le air flow meas ure ments ta ken at poin…
SEMI S6-93 © SEMI 19931
SEMI S6-93
SAFETY GUIDELINE FOR VENTILATION
NOTICE: This guideline does not purport to address
all of the safety issues associated with its use. It is the
responsibility of the user of this guideline to establish
appropriate safety and health practices and determine
the applicability of regulatory limitations prior to use.
NOTE: As listed or revised, all documents cited shall be the
latest publications of adopted standards.
1 Introduction
The equipment supplier should design an internal
equipment exhaust system that is efficient when
connected to potential users’ exhaust systems. Seldom,
if ever, are users’ systems designed for a specific piece
of process equipment or set of process equipment. Most
users’ exhaust systems are designed and installed to
industry-accepted exhaust principles.
2 Purpose
This document is intended to help the semiconductor
equipment supplier design equipment exhaust systems
to a common set of performance criteria as well as to
provide assistance to both users and suppliers in the
understanding of exhaust requirements for equipment
systems. It is not intended to establish design
specifications. The supplier will still own, and be
responsible for, the information it specifies. This
guideline has six concurrent performance-related
objectives.
2.1 Ensure hazardous gases, fumes, and vapors are
controlled (during normal operations of equipment and
facilities services) such that concentrations present in
the work room air should be less than 1.0% of the
Threshold Limit Value (TLV), as established by the
American Conference of Governmental Industrial
Hygienists (ACGIH) or Permissible Exposure Limit
(PEL) as published by the appropriate regulatory
agency, whichever is lower.
2.2 Ensure exhaust is provided within specifications to
support the proper operation of equipment.
2.3 Optimize the user’s exhaust system(s).
2.4 Use the most cost effective methods to interface
with user’s exhaust systems.
2.5 Provide a platform for communicating “special
case” exhaust requirements.
2.6 Establish guidelines for measurement and
communication of equipment exhaust requirements.
3 Scope
This guideline applies to all semiconductor equipment
that incorporates exhausted enclosures that are intended
to be connected to user facilities’ exhaust system(s).
This document is written with the assumption that the
users’ exhaust distribution systems will be designed
with central fans, ducting, and, where applicable,
abatement equipment. Where users use other types of
exhaust systems, equipment suppliers should acquire
exhaust specifications from the users. Whether
equipment is connected to a central exhaust system or
to one which serves only that piece of equipment, this
guideline should be applied.
4 Referenced Documents (s ee Appendix 2)
5 Terminology
5.1 balancing — Adjustments made after the
ventilated equipment and the ventilation system are
installed to assure that airflow to each piece of
ventilated equipment is within design specifications.
5.2 capture — To entrain undesirable elements (gases,
fumes, vapor, and particles) in the exhaust stream for
removal.
5.3 dilute — To reduce the concentration of
undesirable elements to acceptable levels. Dilution is a
function of flow volume.
5.4 flow velocity (V) — The average speed at which
the effluent stream travels through the exhaust duct. It
is measured in meters per second (m/s) or feet per
minute (fpm).
5.5 flow volume (Q) — The volumetric flow rate of
the effluent stream passing a given location in the
exhaust system per unit of time. It is measured in cubic
meters per second (m
3
/s) or cubic feet per minute (cfm).
5.6 initiate motion — To use an exhaust stream to start
contaminants (or machine parts) moving from a rest
position.
5.7 long radius elbow — An exhaust duct elbow that
has a center line radius 1.5 or more times the duct
diameter.
5.8 set point tolerance — The range (+/-) of static
pressure within which an exhaust enclosure will
perform efficiently and effectively.
5.9 static pressure (SP) — The measure of differential
pressure across the duct wall to the ambient pressure
SEMI S6-93 © SEMI 1993 2
(inside the duct). The unit of measure is pascals (Pa,
Newtons per square meter (N/m
2
)) or inches of water.
5.10 traverse measurements — Multiple air flow
measurements taken at points of equal separation, in a
matrix pattern, along the face plane of an exhaust duct
or opening in an exhaust enclosure.
5.11 vena contracta — A point in a duct where the
diameter of the air stream is smaller than the diameter
of the duct.
6 Attributes of Typical Semiconductor
Facilities’ Exhaust Systems
6.1 A typical semiconductor facility’s exhaust system
has three measurable working elements: (1) flow
velocity, (2) flow volume, and (3) pressure.
6.1.1 Flow Volume is related to the Flow Velocity by
the equation Q = VA. In this equation, Q = volumetric
flow rate, V = average velocity, and A = duct cross
sectional area (m
2
or ft
2
).
6.2 There are three duct pressure measurements
possible: (1) static pressure, (2) velocity pressure, and
(3) total pressure.
6.2.1 In exhaust distribution systems with central fans
and abatement equipment (typical of semiconductor
facilities), exhaust duct pressure (static) upstream of the
fan is less than the ambient pressure outside of the duct.
6.2.2 While Velocity Pressure (VP) and Total Pressure
(TP) provide a more reliable measure of Flow than
Static Pressure, VP and TP are normally not specified
for semiconductor facilities’ exhaust systems; therefore,
they will not be discussed in these guidelines. If more
information is desired on VP or TP, it can be found in
“Industrial Ventilation” (Appendix 2, Reference 1).
7 Exhaust Usage Typical of Semiconductor
Operations
Semiconductor equipment typically uses exhaust to
perform one or more of the following tasks:
7.1 Capture — Capture should occur as close to the
source as practical to prevent exposure of personnel or
products to the undesirable elements.
7.1.1 Capture is a function of Flow Velocity. Chapter
3 of “Industrial Ventilation,” A Manual of
Recommended Practice, 20th Edition, (Appendix 2,
Reference 1) outlines the design principles and required
velocity calculations for proper capture of process
emissions.
7.2 Moving is also a function of Flow Velocity. The
exhaust requirements specified by the equipment
supplier should be adequate to maintain motion of
undesirable elements once they are captured in the
exhaust stream.
7.3 Dilution is a function of Flow Volume. Dilution
may also provide reactive elements to undesirable
elements entrained in the exhaust stream (for example,
air and silane).
7.4 Holding — The movement of the exhaust stream is
often used to activate and hold safety control devices,
such as vane switches or differential pressure gauges in
the active state. Such safety control devices should be
used to sense adequate exhaust pressure or flow to
ensure safe operation of the process equipment.
7.5 Initiation of Motion — The use of exhaust to
intiate motion in the semiconductor industry is a special
case application and should be resolved at the time of
purchase. Most semiconductor-related gases, vapors,
fumes, and fine particles (less than 20 microns in
diameter) move with the air in which they are mixed.
8 Key Elements of Exhaust Specifications
8.1 SEMI S2 establishes specific requirements for
exhaust enclosures of equipment that use hazardous
materials. Access to enclosures serving hazardous
materials requires special design. If there is a potential
for exposure of personnel to hazardous materials when
the access hatch is open, an average face velocity
(measured at the hatch opening) sufficient to capture
the hazardous materials should be maintained in the
hatch opening. (SEMI S2 also has recommendations for
exhaust monitoring and alarms. S2 should be fully
understood before designing exhaust systems for
hazardous materials.)
8.2 Properties of chemicals (density, vapor pressure,
boiling point, flammability, etc.), the state of the
materials within the exhausted enclosure (solid, liquid,
or gas), and conditions such as temperature and
concentration will determine the final design and
exhaust specifications of the equipment enclosure.
Equipment suppliers should use available reference
books and Material Safety Data Sheets (MSDS) to
obtain information on chemical properties. The
equipment’s process requirements will dictate the state
and condition of the material.
8.3 When developing specifications for equipment
exhaust systems to be given to the equipment user,
equipment suppliers should incorporate all of the key
elements that will provide clear and accurate
information. These key elements are:
1. Point of measurement.
2. Duct size at the point of measurement.
3. Flow through the duct.
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