Each equipment manufacturer provides very specific engineering data that contains the answers to questions you need answered to solve problems you face in the field daily. Let’s take a look at how fan tables can reveal the source of system defects that are invisible unless you know how to plot fan airflow.
Every piece of air moving equipment is shipped with fan tables that will allow you to determine the fan airflow in less than five minutes. Determining fan airflow requires three simple steps:
- Determine the fan speed setting
- Measure the total external static pressure
- Plot the fan airflow on the manufacturer’s fan tables.
Determine Fan Speed Setting
Most residential equipment has a fan speed setting device. When working with commercial equipment, you will need to measure the RPM of the fan.
Residential equipment fan speed is usually set by one of two methods. Variable speed fans often use dip switch settings to adjust fan speed and CFM. Constant speed fans have usually have a circuit board, spade connectors or a harness with different colored wires plugged in to set the fan speed.
The key to knowing how to set fan speed is in the specifications or installation instructions. Often a table or schematic is provided that clearly indicates which dip switch settings or wire configurations will result in the desired fan speed.
With commercial system, the table often requires a fan RPM measurement be taken. Fan speed is then adjusted by manipulating the diameter of adjustable pulleys. As the diameter of the pulley is changed, the fan speed responds.
As the adjustment is made the fan speed changes; however, how much the airflow changes depends on the static pressure the fan sees
Total External Static Pressure
To plot airflow on most fan tables, one more bit of information is needed in addition to the fan speed — total external static pressure.
Total external static pressure is the resistance to airflow that is imposed on a fan by the air distribution system and/or by a coil in the system that is remote or not shipped with the equipment. An air filter, other than the one shipped with the equipment also adds to the total external static pressure.
Total external static pressure is measured using a manometer and a pressure hose. Two 3/8-in. test holes are drilled into the air stream where air enters and exits the air moving equipment. The pressure travels through the hose into the manometer. The two pressures are then added together to determine the system’s operating total external static pressure.
There isn't time in this article to describe the test procedure, but I’d be happy to send you a free step-by-step procedure if you’ll send me an email. See the end of the article for my contact information.
Plot the Fan Airflow on the Fan Tables
Once you have the fan speed and the measured total external static pressure you are ready to plot fan airflow. Using the example fan table below, (Courtesy of Ducane) let’s plot the fan airflow of this fan.
Say the design airflow is 1,200 CFM, the fan speed is set on high, and the measured total external static pressure is .50-in. wc. Circle high under blower speed and circle 0.5 under external static. Now draw lines from the fan speed and the static pressure until the lines intersect. The fan speed is 1,235 CFM. Compare the 1235 CFM to the required CFM of 1200 to find you’re in good shape in the fan airflow department.
Now try to plot the airflow using the fan table above. Here are your test conditions:
- Medium speed fan setting
- Total external static pressure of .90-in. wc.
- What is your fan airflow?
Now compare the 710 fan CFM to the required airflow of 1,200 CFM for this system and how are you doing? No wonder the temperature rise is so high, the heat exchanger is trashed, and you have had to replace the compressor twice. You can now see why rooms are uncomfortable and your customer’s utility bills are so large.
Where to Find Fan Tables
Most manufacturers’ data is right on your smart phone if you’re connected to the internet. Just download a bar code app and scan the equipment label. Often the engineering data is only a click or two away. You can also Google the model number to locate fan tables.
Of course, fan tables are shipped with equipment, so now you have a reason to keep the paperwork. You can also collect the data digitally on your iPad, or collect paper copies of the models you work on most in a three-ring binder.
Once you recognize the value of fan tables, you won’t be caught without access to this information again.
So next time you service or install equipment, plot fan airflow. It is so simple and allows you to find problems with the systems you work on every day of your career if you will just take a few minutes to add this to your routine.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free procedure on how to measure total external static pressure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.
