Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 14
Term Definition Commentary
Microenvironment
Macroenvironment
Space
Pressurization:
Stack Velocity
Stack Exit Velocity
Threshold Limit
Value
(TLV
®
)
These terms are mainly associated with animal housing facilities. A microenvironment
usually refers to small ventilated cages (shoe box size) for small animals – mostly mice.
The macroenvironment typically refers to the overall room which contains the individual
ventilated cages. The microenvironment (temperature, humidity, ventilation rate, etc.)
can be quite different from the room macroenvironment.
A ventilation technology that controls movement of air between spaces by adjustment
of the mechanically driven air flows to and from the spaces. Pressurization is used to
oppose migration of
air contaminants between spaces and for air conditioning purposes
related to the differing temperature and humidity in the various spaces. In laboratory
design, controlling contaminants is the main purpose.
The speed of the exhaust air while still inside of an exhaust stack. The term ‘stack
velocity’ should not be used interchangeably with ‘stack exit velocity’ (see definition
below). Stack velocity should normally be limited to no more than 2400 fpm (12.2
m/sec) which enables droplets of condensing exhaust vapors to trickle down the inside
of the stack and be channeled to a drain near the base of the stack. This will prevent
condensation droplets (which may be highly corrosive) from being entrained in the
exhaust air that is discharged from the stack.
The speed at which the exhaust air leaves the stack. Stack exit velocity should be at
least 3000 fpm (15.3 m/sec) to ensure the exhaust air is dispersed sufficiently high into
the outdoor air to prevent re-entrainment into fresh air intakes nor pose a hazard to
nearby buildings. A higher stack exit velocity (than the ‘stack velocity’ inside the stack)
is normally attained by adding a conical shaped top onto the stack.
The values for airborne toxic materials which are to be use as guides in the control of
health hazards and represent time-weighted concentrations to which nearly all workers
may be exposed 8 hours per day over extended periods of time without adverse
effects. Threshold Limit Values for toxic substances that have been established by the
American Conference of Governmental Industrial Hygienists (ACGIH). TLV-TWA (Time
Weighted Average) values are the maximum allowable average exposure values for an
8 hour workday and a 40 hour work week. TLV-STEL (Short Term Exposure Limit)
values are typically higher but are for a total of 15 minutes per day and no further
worker exposure to the substance at any level for the remainder of the day. TLV-C
(Ceiling) is the absolute concentration level that should never be exceeded.
Note: For additional information on space
pressurization and detailed explanations of
pressurization concepts, controls and related
parameters refer to the Room Pressurization
Control Application Guide (125-2191).
Do not confuse Stack Velocity with Stack Exit
Velocity. Stack Velocity is the exhaust air
speed while it is still inside of the exhaust
stack. Stack Exit Velocity is the exhaust air
speed at the point where the exhaust air has
just left the stack. A significantly higher Stack
Exit Velocity is desirable in order to propel the
exhaust air as high as possible into the
outside air for better dispersion.
American Conference of Governmental
Industrial Hygienists (ACGIH) defined this
term and registered it.