UFC 3-440-01
14 June 2002
separate row and/or bank manifolds. The diameter of the array supply manifold will be
larger than the bank manifold, and the bank manifold diameter will be larger than the
collector riser diameter. This design is required to maintain balanced flow through the
array. The actual pipe sizes and layouts to be used depend on many factors, as will be
discussed in the following sections.
4-2.2.2
Flow Balancing. Flow can be balanced by active flow control or by
"passive" piping strategies. For active flow balancing, automatic or manual valves are
installed on manifolds and risers to regulate the fluid flow. In passive flow balancing,
the array plumbing is designed so that uniform flow will occur as naturally as possible in
the array. The most successful passive flow balancing method requires the designer to
consider the fluid path length and the pressure drop along this path. The solar systems
described rely mainly on the passive flow balancing method discussed below. In
addition, manually calibrated balancing valves are included on the outlet of each bank
to adjust for any flow imbalances after construction. Automatic flow control strategies
have been a cause of system failure and are not recommended.
4-2.2.3
Reverse-Return-Piping Layout - The Diagonal Attachment Rule. The
pipe run configuration is important balancing flow, especially with regard to fluid path
length. The reverse-return piping layout provides almost equal path lengths for any
possible flow path that the fluid may take. This design is in contrast to the "direct-
return" system, which results in non-uniform flow through the collector bank due to
unequal path lengths. These two strategies are illustrated for collector banks in Figure
4-3, with vectors on the collector risers to indicate relative fluid velocities. Note that
even for the reverse-return system, the flow is not shown to be perfectly balanced since
pipe resistance is a function of flow rate. The reverse-return strategy of providing
approximately equal length flow paths can be applied to any bank layout or complete
collector array layout by insuring that the supply and return pipes attach to the array at
any two opposite diagonal corners of the array (See Figure 4-3). Use reverse-return
piping strategies for all new design projects (direct return piping strategies will not be
used).
4.2.2.3.1
Reverse-Return Piping Schematics. Figure 4-4 illustrates the steps in
the development of a reverse-return piping schematic, and Figure 4-5 shows some
examples of proper reverse-return piping schematics. Small circles show the
attachment points on opposite sides of the bank in Figure 4-3 and opposite sides of the
array in Figure 4-4 and Figure 4-5. The corner closest to the pipe roof penetrations will
be used as the return point, since this results in the shortest pipe length for the heated
fluid. A slight variation of the diagonal attachment rule is needed if the pipe roof
penetrations are near the centerline of a multiple row, multiple column array with an
even number of columns. For this case, some pipe length can be saved by feeding the
array on the outside and returning the heated fluid from the center of the array. This
case is shown in Figure 4-5(c).
4-5
Previous Page
