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Pressure Drop from System Piping 
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Soil depressurization is induced by airflow out of the soil. Airflow is created by a fan which is working against the resistance to airflow of both the subslab material and the piping. In many cases the piping and fittings are the greater resistance. Total system resistance is the airflow resistance of the pipe and all of the elbows. The resistance to airflow in each elbow is typically determined in equivalent straight feet of piping. In other words a sweep 90 degree elbow might have the same resistance to airflow as six feet of straight pipe. If you know the typical equivalent straight pipe footage of each elbow you can add up each type of fitting in a system and multiple the quantity by the equivalent footage for that fitting and add that footage to the straight footage of piping. The approximate equivalent footage of different size fittings is given in first table below. 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. 

Pressure Drop Table for 100 feet of 1.5" to 6" piping at different airflows
In the pressure drop chart above the diagonal lines represent different pipe sizes. The horizontal lines represent the airflow though 100 feet of each pipe size. Where the horizontal CFM airflow lines intersect the diagonal pipe size line represents the approximate amount of pressure drop listed on the bottom horizontal line of the chart. An example is 80 CFM airflow through 100 feet of 3" PVC piping would lose about 1 inch of water column of pressure while the same airflow through 4" piping would only lose 0.3 inches of water column.
In the chart below the maximum airflow is given for 3" and 4" piping depending upon the length of pipe and the type of fan that is used. Note that an RP140 fan (20 watt) connected to 4" piping will move as much air as an HP220 (120 watt) can move through a 3" pipe. Pipe size typically influences high air flow more than fan size.
The next chart shows the airflow difference using 6" pipe instead of 4" pipe.
Pressure Drop Table for 100 feet of 6" piping at different airflows
Pressure Drop Table for 100 feet of 4" piping at different airflows
Pressure Drop Table for 100 feet of 3" piping at different airflows
Pressure Drop Table for 100 feet of 2" piping at different airflows
Pressure Drop Table for 100 feet of 1.5" piping at different airflows

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© 2017 Bill Brodhead Call WPB Office at 610 3468004 for Estimates or Information 