use an external clamp the clamps can be any material which will function and withstand below ground exposure metal such as aluminum fulfills this requirement but plastic clamps molded so as to conform closely to the bag and conduit are preferred again high density polyethylene is particularly suitable in this regard furthermore both plastic and metal clamps can be held in place by conventional adhesives or mechanical means and of course the clamps should be positioned on the bags in such a manner so as not be puncture the bags the heat transfer fluid conduits can be any material useful for this purpose such as copper or aluminum pipe which can transfer heat to and from the ground as above the pipe size can vary based on installation and thermodynamic design considerations the pipe of the figures has a three inch inside diameter and 1 4 inch wall thickness while the heat transfer fluid in the heat transfer fluid conduit can be the same as that in the heat pipe bag it is preferred to vary the two for example it is preferred to use either water or a 1 1 water ethylene glycol by volume mixture as the first heat transfer fluid in the circulation pipe and freon u00ae 11 ccl 3 f du pont de nemours inc as the second heat transfer fluid in the heat pipe bags however other conventional heat transfer media can be used in either or both the conduit or bag such as alcohols ammonia and other freons the heat transfer fluid in the heat pipe bag preferably has a vapor pressure at or near atmospheric 14 7 p s i g between about 30 u00b0 100 u00b0 f and most preferably a boiling point of about 55 u00b0 f at one atmosphere of.