How much do 90 degree elbows affect air flow?

In reality a 90 degree elbow can have significantly less resistance than 6 feet and dramatically more than 6 feet depending on the shape of the elbow and the velocity of the air moving through the piping.

wpb-radon.com - Pressure Drop from System Piping & Fittings - wpb-radon.com

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The Science of loss of Pressure from Fittings Determining Equivalent Feet of Piping for Different Fittings All pipe fittings create some resistance to airflow which translates into a loss of pressure that is inducing that airflow. The shape and curve radius of a PVC fitting makes a large difference in the resistance it creates to airflow. PVC fittings have different radius turns and shapes, even if two fittings are both 45 or 90 degrees. The resistance is quantified as a pressure drop. To provide a practical unit of resistance the pressure drop induced by the fitting is converted into the equivalent length of piping that would produce the same amount of resistance at the same airflow. As an example, the resistance of one 90 degree elbow is often calculated as having the same resistance as 6 feet of piping. In reality a 90 degree elbow can have significantly less resistance than 6 feet and dramatically more than 6 feet depending on the shape of the elbow and the velocity of the air moving through the piping. In order to better quantify the equivalent length of piping each fitting equals, WPB made direct measurements of fitting and piping pressure drop at varying airflows. Commonly used schedule 40 and schedule 20 (sewer & drain) fittings were installed with 20 feet of piping on one side and 40 feet of piping on the other side. Static pressure probes were installed in the center of a straight run of piping exactly 20 feet apart. At the same time two other probes were used to measure the pressure drop for a fitting with exactly ten feet of piping on either side of the fitting. The difference between these two measurements is the pressure drop of the fitting. This fitting pressure drop difference is divided by the pressure drop of the 20 feet of straight pipe and then multiplied by 20 to obtain the equivalent feet of piping the fitting pressure drop is inducing. The measurement is repeated at nine different velocities and then plotted. The equivalent footage of piping for a particular elbow is included in Figure 5 at a mid velocity of about 75 CFM.

Figure 1 Measuring the pressure drop across two offset fittings.

Radon mitigation installers often need to offset pipe to line it up with the fan or move around obstacles. Offset arrangements were tested in the same manner as single fittings. In most cases the offset pressure drop closely matched the single pressure drop result times the number of fittings used but in other cases the affect was significantly greater than the doubling of the single fitting value. This was especially true with offset hard turn 90 degree elbows. At 75 CFM, a sweep 90 degree elbow is equivalent to 5 feet of straight pipe. A hard turn 90 degree elbow equals 17 feet of straight pipe. Two hard turn 90 degree elbows as displayed in Figure 2 equaled the pressure drop of 52 feet of straight pipe!

Figure 2 Equivalent Feet of Piping for the inlet of a pipe

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Some of the air entering a PVC pipe needs to make a 90 degree turn to enter the pipe. If the opening to the pipe was tapered, the airflow into the pipe would be more laminar and have less resistance. See the tapered setup in Figure 3. The table in the Figure 3 quantifies a typical straight into the pipe opening as equivalent to 20 feet of straight pipe for 3" piping and 25 feet of piping for a 4" pipe. This amount of equivalent piping needs to be added to each calculation of total system piping resistance.

Figure 3 Equivalent feet for different Elbows and Airflow velocity

As air flow velocity in a pipe increases the equivalent pipe length for a pipe fitting also increases. In the fitting chart that displays the equivalent footage of piping for each fitting, 75 CFM of airflow was used as typical system airflow. If typical system airflow is between 25 and 150 CFM the equivalent footage for a 4" sweep 90 degree sch 40 fitting would vary from about 4 feet of piping to about 6 feet of piping.

Figure 4 Equivalent feet of Piping for different fittings

The equivalent length of pipe for these different fittings was based on airflow of 75 CFM and rounded to the closest whole number. Note the difference between a sweep 90 degree fitting and a 90 degree hard turn fitting. This is especially pronounced difference when you use two hard turn 90 degree fittings to make an offset as show in the picture in Figure 5. Each of those 90 degree hard turn offsets add an extra 42 feet of equivalent piping versus using sweep 90 degree turns. The affect is dramatically less with angled 45 degree fittings versus sweep 45 degree fittings but overall this is an un-necessary resistance when 90 degree sweep elbows acould be used that are 4 to 5 times less resistance.The elbows in Figure 5 radon system have the equivalent footage of 130 feet of pipe. Determining Equivalent total Footage of Piping

At the job site, add up the estimated footage of straight pipe. Count the number of 90 degree and 45 degree elbow and offsets that will be used. Multiple the quantity of each fitting by the footage equivalent in the table in Slide 5. Include the pipe opening equivalent footage. Use the total equivalent piping and the anticipated airflow of your system with the pressure drop table for that pipe size in Figure ??

Figure 5 Additional Pressure Drop from Sub-Slab Airflow Resistance Soil depressurization is induced obviously by sucking air out of the soil. Airflow is created by a fan which is working against the resistance to airflow of both the sub-slab material and the piping. In many cases the piping and fittings are the greater resistance. The approximate equivalent footage of different size fittings is given in Figure 5 and Figure 6. This gives the total equivalent straight footage of pipe for a system. If the system has only one suction hole the resistance of all the piping and fittings can be determined by the graphs below. The pressure drop is displayed in the graphs below for 100 equivalent feet of piping. The total equivalent footage (straight pipe footage and all of the equivalent footage of fittings) is divided by 100 and then multiplied by the pressure drop of the piping size and airflow rates of the graph below. Notes: 1) You need to know the speed of the airflow (CFM) to make this calculation. 2) Pressure drop increases with airflow 3) Pressure drop decreases if pipe size increases but airflow stays the same 4) As pipe size increases the equivalent footage of pipe fittings increases 5) Elbows with hard edge turns have more than double the pressure drop of sweep turns. Every PVC fitting has an airflow resistance that is approximately equal to the footage given in the table below. For example each 4" 90 degree sweep elbow adds the equivalent of 6 feet of straight pipe resistance while a 4" 90 degree sharp edged elbow adds 15 feet of equivalent straight pipe resistance.

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