Whether sizing new equipment, replacing old equipment, meeting government recordkeeping regulations, or wanting to improve facility efficiency, learning to measure air velocity is an important aid. Plus, it need not be a daunting task, because air measuring kits are readily available and equipped with all the necessary tools needed to do an accurate and thorough job.
Air Measuring Kits
Most air measuring kits include a magnehelic gauge, a pitot tube, small flexible tubing for connections, small hand tools, a calculator, and some easy-to-use conversion charts to help convert velocity pressure readings (based on different pipe diameters) into feet per minute (fpm) and cubic feet per minute (cfm). In many cases, the kit also will contain:
- 0-3-inch magnehelic gauge.
- 0-10-inch gauge.
- 0-50-inch gauge.
A pressure/vacuum gauge to measure and calculate pressure in pounds per square inch (psi) also may be included in a kit. So, if a dust collection system isn’t working well, an air measurement kit is a good tool and investment to help isolate air velocity problems in conveying pipes.
Reasons for Measuring
There are three major reasons why it’s important to know the air velocities and volumes in a facility.
- Air velocity measurements provide a way to gauge whether equipment is operating efficiently and if more horsepower is being used than needed.
- Maintaining air velocity at optimum levels also helps reduce premature wear and tear on the equipment and offers a means to determine when blowers, cyclones, and filters may need replacement.
- With more government regulations, it has become necessary to provide more detailed information about the facility’s airflows in cubic feet per minutes (cfm), before operational permits can be issued.
Federal agencies such as the Occupational Safety and Health Administration (OSHA) often want figures on how much air in cfm is being moved inside a facility, how well the system is functioning, and how much air is being discharged into the atmosphere.
Basic Tools Required
There are a few key instruments and measuring methods to become familiar with when measuring air velocity.
- Normally, a 3/16-inch hole is drilled into the conveying pipe, where an instrument called a pitot tube can be inserted. This pitot tube (like what is used on the outside of an airplane) is an instrument that measures airflow velocity.
- A magnehelic gauge is attached to measure air velocity pressure. Using a supplied chart, the air velocity pressure can be converted into air velocity (expressed as fpm). At this point, the circumference of the pneumatic pipe can be measured and converted into square feet to represent its cross-sectional area.
The formula to calculate airflow in cfm is simple:
Q = VA, where Q = air volume expressed as cfm, V = velocity expressed as fpm, and A = tube cross-sectional area expressed as square feet. Again, air measuring kits provide simple-to-use tables to make conversions of velocity pressure into air velocity speed expressed in fpm, as well as converting pipe circumference measurements into cross-sectional areas expressed as square inches or square feet.
If the pipe diameter is known, most kits also include a table that provides conversions into square inches and square feet. If preferred, a geometry textbook can be used to perform the calculations long-hand. Digital gauges are available to measure air velocity and pressure; however,
such gauges are intended more for controlled laboratory settings. They are more delicate and subject to damage than magnehelic gauges. Due to their size, drilling a larger hole in the field (e.g., 5/8 inch) into the conveying pipe is usually necessary.
Taking Accurate Measurements
While the tools included in an air measuring kit are important, it also is essential to follow a few guidelines to help ensure that the readings are as accurate as possible:
- Airflow pressure and velocity readings will be more distorted if taken near a damper, a wye branch fitting, or a discharge area in the elbow.
- Generally, a reading near a damper shouldn’t be taken any closer than a distance equal to six diameters of the pipe. So, if the pipe diameter is eight inches, take the reading no closer than four feet from a damper on the pipe.
- This same rule of thumb also applies to areas, where elbows or other pipes may converge. If a reading must be taken near an elbow, try to take it on the upstream side of the elbow. It’s important to avoid areas in the line, where material clogging may occur.
- In choosing an ideal spot to conduct readings, the main goal should be to stay as far away as possible from excessive air turbulence that can distort the measurements. Locating an area in the pipeline where airflow is considered smooth will generate more accurate readings.
- In taking measurements, it’s also a good idea to do several and then use those readings to calculate an average. Taking several readings in different planes of the pipe may not always be possible, but if it can be done to generate an average reading, the result can be more meaningful.
- Avoid taking readings near areas where wye branches of piping may converge.
- Once the gauge is attached to a pitot tube and inserted into a pipeline for taking a measurement, take several readings across the pipe. The airflow tends to move the fastest towards the center of the pipe and decreases very slightly around the outside edge, where more friction occurs. From the center of the pipe, move the pitot tube in increments of approximately an inch, and then let the reading settle down, before having another person record it. Taking measurements in these increments will produce an accurate average airflow figure.
- When a pitot tube is used to take airflow measurements, it’s important to watch for excessive dust that may plug the instrument and generate inaccurate readings. If a reading is taken in a dust line, try to stay on the clean air side between the filter and fan when conducting a measurement.
- If the pitot tube becomes plugged, take it out, and blow through the tube. However, be careful to avoid getting condensation on the inside of the instrument. It must be dry to generate accurate readings. In most air measurement kits, a small pin is included to help clean out the pitot tube. Tip cleaners used for acetylene torches also can be used to help poke out any debris that may be clogging it.
- Before taking any measurements, especially with a magnehelic gauge, recalibrate the instrument to the zero setting while holding it level, so that readings will be accurate. Holding the instrument level always during an actual reading is another important method to follow.