Industrial Ventilation
Simple Tools for Finding Problems in LEV Systems
Terms and Units Used in this Article Inch wg: Units of air pressure measured using a water manometer where “wg” stands for “water gauge.” Today, most measurements are taken using mechanical or electronic devices but the pressure units remain in “inches of water” in the U.S. SPh: Hood Static Pressure, inch wg.—the static pressure in the ductwork about 4-6 duct diameters downstream from the hood in a straight section of duct. FTP: Fan Total Pressure, inch wg.—the static pressure provided by the fan to move air and overcome losses. Q: Air flowrate in cfm, cubic feet per minute. LEV: Local exhaust ventilation.
Let’s say you’re an IH in a metals shop. To comply with ANSI Z9.2-2012, Fundamentals Governing the Design and Operation of Local Exhaust Ventilation Systems, you’ve been asked to establish real-time airflow monitoring of an LEV system used to control emissions and exposures at a hand grinding table like the one shown in Figure 1. How would you go about doing this? Readers who find industrial ventilation (IV) intimidating should know this truth: while IV is technical and detailed, it is not beyond the capability of any industrial hygienist. (See the sidebar "Eight Things to Know about Industrial Ventilation" at the end of this article for several other truths about IV.) As I’ll show, the installation of static pressure (SP) measurement “taps” will allow you to diagnose and troubleshoot problems in the LEV system. For easy reference, definitions of terms and units used in this article appear in the sidebar above. While the details of this scenario describe the monitoring of one simple LEV system, the approach can be adapted to any LEV system. SYSTEM SPECS Your first task is to obtain the plans and specifications for the LEV system and study its layout, which is shown in Figure 2 (the hood is on the bottom left). Ductwork carries contaminated air past a shut-off gate (or damper, used during maintenance to stop airflow through the hood), into and out of an air cleaner, and then past a duct flex-coupling into the fan. A stack of the same diameter as the duct exhausts air to the atmosphere. The original design and operating criteria show that the required airflow rate (Q = 1,200 cfm) is created at the hood when static pressures (SP) are established in the ductwork as follows (note that these are absolute values):
  • Point 1: SP = 1.25″ wg (near the hood with the shut-off damper wide-open)
  • Point 2: SP = 1.45″ wg (near the entry to the air cleaner)
  • Point 3: SP = 2.45″ wg (just after the exit from the air cleaner)
  • Point 4: SP = 2.65″ wg (near the entry to the fan)
  • Point 5: SP = 0.30″ wg (near the outlet from the fan and entry to the exhaust stack)
Next, you ask for static pressure taps and manometers to be installed at each of the five measuring points. Installation is simple and costs about $500. (Mechanical SP taps are quite inexpensive; more expensive options include automated and digital monitoring and control systems.) You ask a foreman to check and record each SP measurement value every day. If any value changes by more than 5 percent, the foreman is to contact an IH at corporate headquarters.
Tap on the tables and images in this article to open a larger version in your web browser.
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