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GREEN SYSTEMS e Continued from p 71
return lines drain empty of water. The water that drains from the system is stored in a drainback tank that is located in a freeze protected area. If the collector is sufficiently hotter than the bottom of the main storage tank, the solar pump will turn on and lift the water up the supply piping pushing the air out of the collector and return line into the drainback tank. As this happens the piping begins to fill
with water and eventually will operate similar to a closed loop system once all the air is removed from the piping. In order for a drainback system to
function properly as a freeze protection mechanism, it must drain completely. All the piping has to slope back to the drainback tank at a sufficient pitch (¼” per foot) to ensure that fluid does not get “hung up” in a section of pipe exposed to freezing
conditions. This can be difficult on certain jobs such as ground mounted systems where there may not be enough of a height difference between the collector and the drainback tank. One important element to the operation of the system is that air must be able to reenter the system otherwise it will not drainback. If this is not able to take place, a vacuum will hold fluid in the exposed piping and freezing can occur. It should also be noted that due to the design of the internal flow paths, certain types of collectors simply will not drain completely and therefore cannot be used in these systems. A drainback system
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will also require slightly more energy since the pump has to lift the water up and through the collectors. This energy cost can be minimized through certain piping designs so that once the piping is flooded with water, the system operates with similar pumping requirements as a closed system. All types of freeze
protection have important considerations that must be addressed when installing a solar thermal system. Keeping a system from freezing is only one part of the design. Sufficient research of system types should be carried out before a final method is chosen so that the best method for that installation is carried out. Each application is unique and differing methods of freeze protection have been used in many installations, some of which have been in service for over 20 years. Proper design, installation and equipment selection will help ensure that the system runs safely and efficiently. l
FOR COMPLETE DETAILS, EVENT SCHEDULE AND LODGING INFORMATION, VISIT
www.naohsm.org
888.552.0900 e Circle 45 on reader reply card Eric Skiba is a
technical engineer at Apricus Inc.
phc april 2011
www.phcnews.com
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