14 June 2002
effectiveness of 0.5. Raising the required solar loop exit temperature to 125 degrees F
(52 degrees C) decreases the effectiveness to about 0.4. The cost difference at these
levels of effectiveness is not significant for either plate or shell-and-tube heat
exchangers. As the heat exchanger effectiveness is further increased (or as the
required solar loop exit temperature is decreased), heat exchanger costs are affected
more. The designer should use judgment to determine if the cost of increasing
effectiveness is justified. For plate-and-frame heat exchangers, gains in effectiveness
can often be achieved with low additional cost.
Specification. The heat exchanger area should be available from the
manufacturer, along with the pressure drop across each side at various flow rates. A
single-isolation heat exchanger can be used, since non-toxic USP propylene glycol is
required as the heat transfer fluid. All materials used in the heat exchanger must be
compatible with the fluids used. The plate or plate-and-frame types of heat exchangers
are becoming increasingly popular, due to their compact size and excellent
performance, availability in a wide range of materials, and ease of cleaning and
servicing. If a shell-and-tube heat exchanger is used, it should be installed such that
the shell side is exposed to the heat transfer fluid, with the tube side containing potable
water. This design is required because potable water tends to foul the tube bundle, so
it must be possible to remove and clean the bundle. Further discussion of heat
exchangers can be found in APPENDIX F.
Sizing. The collector loop piping to the manifold should be sized small
enough to reduce material costs but large enough to reduce excess pressure drop (and
associated pump and energy costs) and to maintain the fluid velocity below 5 ft/s (1.5
m/s). The upper limit is the size of the array supply and return manifold, while the lower
side is that defined by the 5 ft/s (1.5 m/s) velocity limit. Although an optimization
procedure could be performed to determine the pipe size providing the lowest life-cycle
cost (LCC), experience shows that the supply piping can be sized at least one size
smaller than the supply manifold as long as the fluid velocity restriction is not exceeded.
The pipe size on the storage side of the heat exchanger can also be calculated based
on the storage loop flow rate, pump costs, and the 5 ft/s (1.5 m/s) fluid velocity limit.
Materials. Piping materials are limited to copper. To ensure materials
compatibility, only tin-antimony (Sn-Sb) solders are allowed (Sb5, Sn94, Sn95, and
Insulation. Insulation should withstand temperatures up to 400 degrees F
(204 degrees C) within 1.5 ft (457 mm) of the collector absorber surface, and 250
degrees F (121 degrees C) at all other locations. Insulation exposed to the outside
environment should be weatherproof and protected against ultraviolet degradation.
Pre-formed, closed-cell polyisocyanurate insulation has an excellent history of
withstanding the temperatures and environmental conditions required, and its use is
recommended when possible. The amount of insulation to be used is dependent on
the operating temperature of the pipe; however, a minimum of R-4 should be specified