Drainback Continued from page 48
the piping. The speed of the water is necessary to push steam through the collector, which will occur during start up in the heat of the day. As an added precaution, balancing valves can be installed on the collector return side to keep the fluid velocity into and through each collector array high during the initial fill. Another consideration is mounting
a drainback pump in a vertical posi- tion rather than horizontal. I have seen horizontal pump installations be
the cause of a troublesome and hard- to-pinpoint problem causing pump cavitation. Another thing which will help with fluid pumping dynamics is to seal the drainback loop and pres- surize it to 15 to 25 psi. There would be no airvent installed anywhere in a drainback loop; the piping and thread joints will be airtight. Adding an air charge to a drainback system would verify any leaks that may be present and help the pump do a better job lowering the potential for bother-
some issues of pump cavitation. The last thing I would like to point
out is that in a drainback system, the collectors and the collector piping can be installed dead level. The collectors
The all-important specification to con- sider for drainback is that the collec- tors and piping must slope to drain back to the drainback reservoir.
and piping will drain even if mounted level. As a safeguard, the drainback fluid would include an antifreeze mix. Some state programs require an antifreeze mix in the drainback reser- voir as a precaution for freeze protec- tion anyway. However, there may be codes and rebate programs which would require that the slope drain at ¼ inch per foot for collectors and piping. I will point out that collectors and pip- ing installed dead level for drainback are being successfully implemented in extremely cold climates and for large commercial systems of 100+ collec- tors. The practice of doing away with slope in order to drain collectors and piping may give drainback a whole new appeal to the solar industry, as well as a whole new set of options for how to (or how not to) install for drainback. n
Peter Biondo is the technical sales Circle 30 on Reader Reply Form on page 65 Page 50/Plumbing Engineer
coordinator for Oventrop Corp. He has been involved in hydronic heating and solar thermal technologies for more than 25 years. His primary work is assisting mechanical engineers and contractors with balanced distribu- tion hydronic systems and solar hot water and space heating. His solar thermal and hydronic heating work- shops are featured in trades shows throughout the country.
March 2011
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