4.2.2.5.2

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

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

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

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