162 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1939 



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FiGUiiE 2. — UseJiui heat output of tbin flat-tank absorber on clear days, September 



23-27, 1935. 



rectly by convection of hot air, or by conduction through a cement bed, 

 or by extended fin surface, the ideal absorber has a continuous black 

 surface covering a thin sheet of water. This type is seen in the center 

 of plate 1. The inherent disadvantage of the flat tank is its inability 

 to withstand even low water pressures without an expensive construc- 

 tion using heavy plate and many staybolts. Its high efficiency, due 

 to minimum heat losses, is usually not economical in comparison with 

 a larger, less efficient pipe absorber. 



Heat output of thin f.at-tanh absorber. — Figure 2 indicates the 

 useful heat obtainable on clear days near the end of the usual solar- 

 heater season. Midsummer values would be much higher. The data 

 for this curve were obtained with water constantly flowing through 

 a flat, thin tank 20.9 square feet in area with a single-glass cover, as 

 shown in the center of plate 1. The rate of flow was maintained 

 constant at about 1 quart per minute by gravity from the float chamber 

 seen in plate 1, above and to the left of G. The discharge was into an 

 open funnel, visible at the upper left-hand corner of the flat absorber. 

 A noon temperature rise of about 40° F. above the inlet temperature 

 of about 80° F. occurred, and the average absorber temperature was 



