Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 2
Applicable Definitions (Alphabetical Listing)
Term Definition Commentary
Air Changes per
Hour (ACH)
Airlock
Anemometer
A unit of measurement that expresses the ventilation rate for a space (room). Each
ACH represents a quantity of airflow per hour relative to the overall room volume. For
instance, if a room’s dimensions are 12 feet wide by 20 feet long by 10 feet in height,
its volume would be 2,400 cubic feet. One ACH for that room would then equal a
ventilation airflow rate of 2,400 cubic feet per hour. However, since airflow is mostly
expressed in terms of cubic feet per minute (cfm), it’s necessary to divide cubic feet per
hour by 60 (minutes per hour) to find the cfm rate. In this example, 2,400 cubic feet per
hour divided by 60 yields an airflow rate of 40 cfm. Therefore to ventilate a room of
2400 cubic feet at the rate of 8 ACH would require an airflow rate of 40 cfm
× 8 cfm
which equals 320 cfm.
A room entry/exit arrangement using two doors separated by a small vestibule that is
applied when it is necessary to prevent air transfer into or out of a space. The airlock
doors are set up with mechanisms that only allow one door to be open at a time.
Without these interlocking mechanisms, the area does not have an airlock. It can only
be categorized as an ante-room or vestibule or a double entry/exit door.
Airlocks are most often applied to clean rooms to prevent the entry of (non-clean) air
whenever personnel enter or leave the cleanroom. In addition, the airlock vestibule is
typically provided with a high amount of exhaust or clean supply air to prevent
unwanted contamination of the cleanroom space. Airlocks are also applied to biological
and high toxicity laboratories when it is necessary to ensure against having any air
migrate to the area outside the laboratory room.
An instrument to measure airflow velocity, usually at a single point. For ventilation
applications the measurement is usually in feet per minute (fpm) or meters per second
(m/sec). However, since airflow velocity typically has a wide variation across any given
area, it is necessary to take many measurements at different points to obtain an
‘average velocity’. The practice of measuring airflow velocities in a carefully defined
pattern across an area to obtain the average velocity is referred to as a ‘traverse’.
Note that an ACH rate is always based on a
room’s gross (totally empty) volume. As
contents are added to the room (furniture,
fume hoods, people, etc.) actual net room air
volume becomes less and thus a given ACH
rate provides more effective ventilation since it
needs to dilute less room air.
Airlock arrangements are also utilized in the
design of other types of safety equipment such
as a glovebox where materials are moved into
or out by means of special drawer
arrangements that only allow access from one
end at a time. This prevents any toxic air from
inside of the glovebox from escaping during a
transfer of materials.
For test purposes, fume hood or biosafety
cabinet average face velocity is often
measured by performing an airflow
measurement traverse in the Face Opening
with a “hot wire” or “thermo” anemometer. This
type of anemometer can measure very low
airflow velocities with excellent accuracy.