14 June 2002
that shown to ensure flow balance.
Figure 4-4. Steps in Developing a Reverse Return Piping Layout
The 30 Percent Rule. Flow balance through the collector array depends
on the relative pressure drop associated with the different piping branches of the array.
The change in pressure along any flow path is a measure of the resistance to flow. Of
interest to the solar system designer are the pressure losses across the collector risers,
along a manifold, and along linear uninterrupted pipe. As the ratio of a manifold's
pressure drop to its riser pressure drop becomes smaller, the flow becomes more
uniform. To ensure uniform flow through the collector bank, this ratio should be around
10 percent, and under no circumstances should it exceed 30 percent (for a pressure
drop ratio of 30 percent, the flow in any riser does not deviate from the average riser
flow rate by more than plus or minus 5 percent). It is thus an advantage to choose a
collector with a relatively large pressure drop and to ensure that the pipe diameters
throughout the system are sized correctly to maintain adequate riser to manifold
pressure drop while allowing enough cross-sectional area for the calculated flow rate
and keeping the flow velocity below the 5 ft/s (1.5 m/s) limit for copper pipe.