Quantitative Troubleshooting of Industrial Exhaust Ventilation Systems

Abstract

This article proposes two troubleshooting tools that may allow precise and accurate assessment of changes to ventilation systems of any type. Both are useful in discovering and quantifying most modifications that affect the distribution of airflows among the branches and static pressures throughout the system. The approaches are derived from energy balance considerations, using power loss coefficients (X) computed for any contiguous section of the system from the duct velocities and static pressures measured at that section's inlets and outlets. The value of X for a given portion of the system should be nearly constant with changes in airflow and with modifications to other portions of the system. A substantial change in the value of a coefficient determined from “before” and “after” measurements can be attributed only to a modification in that portion of the system. That responsiveness to local modifications and insensitivity to changes elsewhere in the system—including gross changes in fan performance—make X coefficients a valuable troubleshooting tool.

Static pressure ratios within a given branch are functionally related to ratios of X coefficients. Therefore, they vary with modifications to the branch and are highly insensitive to changes outside that branch. Unlike X coefficients, determination of static pressure ratios does not require velocity traverses, making them faster and easier to determine than X values. On the other hand, values of X are more universally applicable and have direct physical significance. Use of both static pressure ratios and X coefficients are described in a suggested troubleshooting procedure.

Systematic measurement errors have surprisingly little impact on the usefulness of values of X or static pressure ratios. The major impediment to using either tool is the necessity for “baseline” measurements, which are often unavailable. On the other hand, a baseline for future comparisons can be created piecemeal, beginning at any time and extending over any period of time. Guffey, S.E.: Quantitative Troubleshooting of Industrial Exhaust Ventilation Systems. Appl. Occup. Environ. Hyg. 9(4):267–280; 1994.