UFC 3-440-01
14 June 2002
4.2.2.5.2
Pressure Drop Across Banks and Rows. The pressure drop across a
bank of collectors must be determined in order to calculate the pipe sizes necessary to
achieve balanced flow in the array. Once the array layout is determined and assuming
that the pressure drop across each collector unit at the recommended flow rate is
known, the pressure drop associated with each branch extending from a manifold can
be determined. When internal-manifold collectors are banked together in groups of
seven or less, it can be assumed that the pressure drop across the entire bank is equal
to the pressure drop across a single collector. This information will be used in sizing
the pipe, as described below.
4-2.2.6
Pipe Sizing. Sizing of the piping in the solar array is critical to system
performance. Flow throughout the array should be in balance at the proper flow rates,
while maintaining a maximum velocity limit of about 5 ft/s (1.5 m/s). These two criteria
impose constraints on the minimum pipe diameter possible, while material and labor
costs pose a constraint on excessively large piping. Another consideration is pumping
power. Specifying pipe diameters that are larger than the minimum can sometimes
lower the system life-cycle cost. By doing so, pumping power requirements are
reduced and the savings over the system lifetime can exceed the initial material and
labor costs of the larger pipe. This situation however is not important for the sizes and
types of solar systems discussed in this guidance.
4.2.2.6.1
Volumetric Flow Rates. The manufacturer's recommended collector flow
rate, CFR, and the piping schematic should be used to determine the design flow rates
throughout the collector sub-system. The total array flow rate, AFR, is determined by
multiplying the CFR by the actual number of collectors, N. Bank flow rates (BFR) and
row or other branch flow rates are determined by multiplying the CFR by the number of
collectors per bank (n) or per row. These flow rates were previously illustrated in Figure
4-2.
4.2.2.6.2
Pressure Drop Models and the Fluid Velocity Constraints. The fluid
velocities in the various pipe branches should be kept below 5 ft/s (1.5 m/s) to prevent
erosion of the copper piping. Below this value, fluid velocity is of no great concern. The
fluid velocity for a given flow rate is dependent on the fluid properties, internal pipe
diameter, the pipe material, and its internal surface characteristics. Empirical
expressions have been developed to model the flow rate, pressure loss, and velocity
behavior of different liquids flowing through various types of pipe. These expressions
are widely available in graphical form for water (usually at 60 degrees F (15 degrees C)
and for turbulent flow) and standard practice dictates their use. For this reason, they
are not presented in this guidance. Although more precise methods can be considered,
the designer can easily correct the pressure drop for water to account for propylene
glycol solutions by the use of Table 4-1. The pressure drop correction is more
important than the velocity correction since there is an increasing effect on the pressure
drop. Use of the velocity result for water is conservative and as such requires no
correction. This velocity correction calculation assumes similar turbulent flow
characteristics for water and propylene glycol solutions (an incorrect assumption in
many cases). Due to the viscosity differences of water and propylene glycol solutions,
flow of the solution is often laminar. This fact can be neglected and the turbulent water
4-9
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