supply ducts will provide a crude indication of system performance when compared with
start up evaluations. For local exhaust systems, the throat suction method applied to
exhaust hoods or face velocity and the static pressure differentials for air cleaners and
fans will suffice in confirming that the system is performing satisfactorily.
The throat suction method will provide valid information, unless:
(a) The hood entry has been modified or damaged,
(b) There are obstructions ahead of the point of measurement, or
The system has been modified.
(2) A reduction in throat suction or face velocity can provide valuable
information, such as an indication that there has been:
(a) An accumulation of material in an elbow, branch, or main, thus
clogging or restricting airflow. Build-up in elbows results from impaction, while build-up
in straight runs results from insufficient conveying velocity or from overloading the
(b) A change in blast gate settings, if the system is balanced using
(c) Additional branches and hoods added to the system. "Adding on" to
a system is always a real temptation; however, it is not sound economics when it
renders the whole system deficient.
(d) Excessive build-up on the filter. It is best to monitor filter build-up
by attaching a static pressure measuring device across the filter.
(e) Reduced fan output resulting from belt slippage, damaged, or worn
rotor, or build-up on the fan blades.
c. Data Handling and Recording. The sketch of the system made at start-up
or for the initial evaluations survey must be recorded and filed in such a manner that
future air-flow surveys can be conducted in a similar manner. The frequency of airflow
surveys can be determined only by such conditions as:
(1) Nature of the materials being controlled. The more hazardous the
materials, the more frequently the system should be checked.