Sunday, July 26, 2009

Teaching Airflow Measurement

Would you hire an electrician who does not own a voltmeter? Would you trust the circuit diagnosis of a technician who does not use a voltmeter? So why would you trust an air conditioning mechanic who does not own or use any air measurement tools? After all, the first word in air conditioning is air. Getting the airflow right is the first thing we should be doing. If the airflow is not correct, nothing else will be. It is a waste of time to check the charge on a system with poor airflow. Many mechanics test airflow by simply holding their hand over a supply register. While it is not wrong to feel the air, this does not replace a real measurement. Feeling the airflow is no more accurate than charging a unit by feeling the suction line.

I believe the reasons many mechanics do not use airflow measurement tools include not wanting to spend the money for the tools, not wanting to spend the time to take the measurements, and not really knowing how to use the tools. Education can address all three of these concerns. Many of the most accurate air measurement tools are expensive, but relatively inexpensive tools that are accurate enough for general service work are available. We should be teaching our students how to use a range of airflow tools, including less expensive ones that technicians might be more apt to own and use. Digital rotating vane anemometers are available for $150 that can be used to take fairly accurate readings. Very accurate digital manometers are available for under $200. A magnehelic and pitot tube can be purchased for around $100 and they can be used to take accurate readings as well. Most airflow measurements really do not take very long if you know how to take them, especially using the newer digital tools that will perform real time averaging. The trick is showing technicians how to take airflow readings using commonly available tools. I believe offering a series of labs that let students get a feel for using the different tools available will go a long way towards encouraging more use of airflow measurement tools in the field.

Labs that will familiarize students with airflow measurement include measuring:

  • Total external static pressure across a blower using a magnehelic
  • Static pressure drop across a filter using a magnehelic
  • Total pressure, static pressure, and velocity pressure in a section of duct using a magnehelic and a pitot tube
  • Total external static pressure across a blower using a digital manometer
  • Static pressure drop across a filter using a digital manometer
  • Total pressure, static pressure, and velocity pressure in a section of duct using a digital manometer and a pitot tube
  • Average air velocity leaving a supply register using a rotating vane anemometer
  • Average airflow velocity entering a return air grille using a rotating vane anemometer
  • Airflow volume out of a supply register using a flowhood
  • Airflow volume entering a return grille using a flow hood

I realize that every program’s budget might not be able to afford access to all of these tools, but I urge you to consider at a minimum a magnehelic gauge, a pitot tube, and a rotating vane anemometer. Another approach is to ask a technician that owns and uses airflow measuring tools to demonstrate their use to your class. This gives your class exposure to instruments that you might not be able to afford. Outside experts always liven a class up. No matter how clever you are, your students see you all the time. If the visiting technician is a graduate of your program, so much the better. Nothing builds program credibility like having successful graduates come around.

Because we believe the measuring airflow is important, airflow measurement is addressed in several units of Fundamentals of HVAC/R. Unit 56 Fans and Airflow gives a thorough explanation of air movement and duct pressures. Unit 57 Duct Systems and Duct Design discusses the role of duct pressures in designing a duct system. Unit 60 Testing and Balancing Air Systems discusses how airflow is measured and discusses how system performance is adjusted to meet system design. Since measuring system airflow is an integral part of troubleshooting an HVAC/R system, airflow plays a prominent role in several troubleshooting units including Unit 36 Troubleshooting Air Conditioning, Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation, and Unit 48 Troubleshooting Electric Heat. Do your students a favor and introduce them to the art of airflow measurement.

1 comment:

  1. Carter, nice job and good thoughts!
    Seems since you have helped AHRI with their book, perhaps you could get them to insist that their member companies publish the delta P for their AC coils (dry) so that a simple pres diff reading could give you a reasonable total CFM value. Perhaps they (manufacturers) will install 2 air pres taps that will give more repeatable results