shown for general ventilation systems. Figures 4-1 through 4-3 represent a
simplification of this concept. Sketches of systems should be considered as part of the
permanent record on which future changes in the system may be recorded.
(2) Specific air-flow measurements. Measurements in terms of airflow,
velocity, and static pressure must be made to determine that the system is adequately
balanced and performing to the design basis. These measurements include:
(a) Air flow in cfm at hoods (throat suction method), branches and
mains (Pitot tube), and up and downstream of fan (Pitot tube).
(b) Static pressure measurements at hoods, up and downstream of the
air cleaner, and up and downstream of the fan.
(c) Supply, capture, face, and transport velocities at diffuser outlets
(supply velocity), face or opening of hood (face velocity), velocity at point of contaminant
release (capture velocity), branches, and mains (conveying velocity).
(d) Fan performance in terms of fan speed in rpm and horsepower
(BHP) is calculated using cfm (Q), total pressure (TP), and mechanical efficiency (ME)
of fan. This operation is occasionally performed by a preventive medicine specialist.
NOTE: The measurements obtained should agree within 10. If they do not, the system
will have to be modified until it meets design criteria.
(3) Records. The locations of the measurements must be identified on the
sketch and a record kept for future comparisons.
(4) Other checks. Local exhaust systems are installed for the singular
purpose of removing some contaminant from the work environment. Visualization
techniques using smoke tubes or candles can be most helpful in verifying that the
system exerts a sphere of control over a sufficient area to prevent excessive exposures
to operating personnel. Air examination for specific contaminants is also recommended
to verify that the system will control contaminants to levels known to be safe. Air
samples taken in the breathing zones of operating personnel will be most helpful in
assessing the adequacy of contaminant control. Photographic records of smoke tests
and the results of evaluation tests should be maintained for future reference.
b. System Operation vs System Start-Up. Once systems are started up and
determined to perform satisfactorily, the degree of evaluation can be reduced as long as
good records of start-up or initial conditions have been made. Experience with airflow
systems clearly indicates that periodic surveys are required to assure that system
performance is adequate. Operating personnel cannot be relied upon as an "indicator"
of system performance. Also, ventilation systems are rarely an integral part of the
operation in terms of quality and production, and all too often receive inadequate
maintenance. For most systems, simple velocity measurements at exhaust hoods and