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.
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.
Fan Total Pressure, inch wg.—the static pressure provided by the fan to move air and overcome losses.
Air flowrate in cfm, cubic feet per minute.
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.
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.
Figure 1.
Local exhaust hand-grinding table hood.

Figure 2.
LEV system schematic with hood at bottom left.
The tap at Point 1 measures SPh, the Hood Static Pressure. This is the amount of static pressure required to move a specific air volume flowrate (Q) of air through the hood, overcoming any losses and accelerating the air to duct velocity. SPh is related to Q through a squared relationship. Table 1 shows the approximate relationship of chang
es in Q for percentage changes in SPh. (For changes of 40 and 50 percent, the first figure for Q relates to decreased SPh and the second figure relates to increased SPh.)  Having reviewed the specs, you know that the original air volume flowrate (Q1) is 1,200 cfm at the original Hood Static Pressure (SPh1) of 1.25 inch wg. If SPh is reduced by some malfunction in the LEV system to (for example) 0.88 inch wg, or about 30 percent, using Table 1 you’ll know that the new Q2 is reduced about 15 percent to 1,000 cfm (rounded for significant figure rules).
Table 1.
Change in Q Resulting from Percent Change in SPh at a Hood
Table 2.
Typical Static Pressure Changes for Each of Eight Potential Problems
Using the SPh tap at Point 1, you can determine in real time whether the required flowrate exists at the hood, which is your primary concern as an IH. The other taps allow you to identify the potential causes of flowrate changes at the hood. (Remember, IHs must be
problem cause finders
as well as
problem detectors
.) If any of the measurements at Points 2 through 5 change more than about five percent, the air flowrate at the hood will likely change. Below are short explanations of the significance in pressure changes at these points (note that changes in static pressures of < 5 percent at any point in the system are common and can usually be ignored):
  • The SP difference (also known as “drop”) between Points 1 and 2 is the amount of SP required (or “lost”) to move the air from the hood to the entry of the air cleaner. This SP loss “pays for” friction loss and any other static pressure losses through elbows, across the damper housing, and so on.
  • The SP difference between Points 2 and 3 is the amount of SP required to move the air through the air cleaner during normal “air scrubbing” operations.
  • The SP difference between Points 3 and 4 is the amount of SP lost to move the air from the air cleaner, through the duct, past the flex coupling and into the fan.
  • The sum of the absolute static pressures at Points 4 and 5—usually designated the “Fan Total Pressure,” or FTP—represents the total amount of SP required to move the air through the entire IV system and the fan. (Fans are chosen based on required Q and FTP.) Interestingly, an increase in FTP without any change in fan speed suggests a
    of airflow through the fan, while a decrease in FTP suggests an
    of airflow through the fan. (My
    Industrial Ventilation Workbook
    provides a full explanation of this phenomenon, in addition to equations and practical applications of industrial ventilation. Future columns in The Synergist will also provide additional IV tools for IHs.)
  • The static pressure at Point 5 is the amount of static pressure required to move the air from the fan outlet to the atmosphere through the exhaust stack and any rain protection devices.
Any of several common problems could affect the performance of the LEV system at the hand grinding table. The descriptions below discuss the typical changes in static pressures at Points 1 through 5 in response to specific problems.
Problem A: airflow partially blocked between Points 1 and 2.
In a metals shop, blockage could be due to a partially closed damper, a dented duct, or settled materials in t
he ductwork. Instead of creating the desired airflow at the hood, the static pressure is used to overcome the additional losses in this section of the duct.
Problem B: a hole in the duct or flex coupling between Points 3 and 4.
Static pressure is not able to reach the hood and results in lower flowrates in ductwork upstream of a hole.
Problem C: fan speed reduction.
Common causes of reduced fan speed are slipping pulley belts when new belts stretch after installation, and poor maintenance that allows pulley sheaves to move closer together. Slower fan speeds result in less static pressure and lower air flowrates at the hood.
Problem D: clogged or blocked air cleaner.
Overloaded filters will increase static pressure losses and reduce airflow rates.
Problem E: blockage in the exhaust stack.
A blockage could be caused by a dent, snow in the stack, or a bird’s nest built in a rain cap.
Problem F: increased fan speed.
This results in a higher static pressure being made available to the system and, therefore, higher pressures throughout and higher volume flowrates at the hood. (In some cases higher flowrates can adversely affect emission capture and containment characteristics at the hood. They can also increase motor operating costs, which are related to flowrate through a third-power relationship.)
Problem G: a difference in a single manometer reading.
Since non-negligible changes in pressure will affect manometer readings at all five points, a change at just one point (especially at Point 1) suggests that the manometer is malfunctioning—a problem requiring inspection and maintenance—or that data has been incorrectly recorded.
Problem H: blockage in duct between Points 3 and 4.
Possible causes of blockages include settled material in the duct, a dent in the ductwork, or the pulling of flexible ductwork into rigid ductwork. Blockages increase losses of static pressure at that point.
Table 2 is a visual guide to how each of these problems affects the various static pressures. You’ll have this table on hand to help you troubleshoot the LEV system if the SP value
s change significantly.

About six weeks after installation of the taps, you get a message from the shop foreman: the SP values have changed. Table 3 is a comparison of the original static pressures with the new values.
Table 3.
Comparison of SP Values
Table 4 helps you see that the changes match those of Problem A, a blockage between Points 1 and 2. You text the foreman and ask him to check the damper and ductwork between the hood and the air cleaner for a misadjusted damper, settled materials, dents, or other blockages in the duct. Twenty minutes later, the foreman replies that a new worker had partially closed the damper to “reduce the noise.” With the damper re-opened, all the values have returned to normal. Congratulations: you’ve solved the problem quickly and protected employee health without even leaving your office—although now you may need to investigate a potential noise problem.
is an IH engineer with broad experience in ventilation used for emission and exposure control. He is author of many books, distance-learning and onsite training courses and current chair of the ANSI Z9.2 and Z9.10 subcommittees. His full bio is found
. He can be contacted at
Eight Things to Know about Industrial Ventilation (IV)
“Industrial” in IV has the same meaning as “industrial” in IH and covers ventilation control of all employee occupancies.
Every human occupancy requires ventilation to control odors and the buildup of hazardous concentrations of air contaminants (which will occur in every occupancy in the absence of ventilation).
IV is not rocket science. It is technical and copious in its details, but the rudiments can—and should—be learned by every IH.
Every chemical exposure complaint/problem/issue involves IV, either as part of the cause or part of the solution.
IV is composed of two main parts: local exhaust ventilation (LEV) and dilution ventilation (DV).
Only fresh, clean “outside air” (OA) provides dilution of air contaminants. Untreated returning (recirculated) air (RA) does not provide dilution ventilation. (It can enhance air mixing.)
IH tradition and various IH codes and standards require the use of “engineered controls” like IV before the primary use of administrative and personal protective controls.
Almost all IV standards and codes require monitoring of the ventilation system. (This article provides one way IHs can easily monitor the flowrate at a hood in an LEV system.)
Consulting Table 1, you determine that at a reduction of SP by about 50 percent, the air flowrate at the hood was probably reduced by about 29 percent to Q = 850 cfm—obviously, a problem with potential to adversely affect the health of workers at the grinding table. FTP, the sum of SP at Points 4 and 5, had increased, also suggesting reduced airflow through the fan and system. Next, you create Table 4, which indicates whether SP has dropped, increased, or stayed the same at and between various points in the LEV system.
Table 4.
Changes in SP at and between LEV Taps
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Tap on the tables and images in this article to open a larger version in your web browser.