180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



tems have, of course, considerable flexibility; the unit sections are 

 easily stretched out or compressed a little, and ordinary inaccuracies 

 in pipe lengths will give no trouble. 



Figure 10 shows the six-parallel-pipe absorbers using the standard 

 branch T connection for the terminals. Complicated systems of 

 separate pipe fittings might be used, but the II/2 inlet and outlet 

 fitting is too large to maintain the close pipe spacing shown in figure 

 9. Branch T's were commonly used for industrial wall pipe radia- 

 tors, but because of the present preference for cast radiator sections 

 these branch T's are not always carried in stock by plumbing-supply 

 houses. They are, however, readily obtainable on order and give a 

 much neater pipe absorber. With the branch T's the two pipes con- 

 necting at the ends must be shorter than all the rest to allow for the 

 unions. 



Methods of obtaining greater heat output from, limited absorber 

 area. — Usually the most economical method of obtaining greater heat 

 output is to increase the absorber area and extend the simple pipe 

 coils. When, however, the available space for a solar absorber is 

 limited, the equivalent of about 20 percent greater absorber area can 

 be obtained on cloudless days by embedding the ordinary pipe coil to 

 half-pipe diameter in cement mortar. This arrangement provides 

 a means of transfer for the heat generated in the space between the 

 pipes to the pipes themselves. A suitable concrete mixture is made 

 of 1 part cement and 4 parts sand. After the cement has dried, the 

 whole surface should be painted black. 



Such a bed of concrete is a disadvantage on partly cloudy days be- 

 cause the additional heat capacity delays the warming up of the 

 absorber when the shading passes. 



Thin flat tanks covering the entire absorber area need have only 

 TO percent of the area specified for ordinary pipe coils. This type of 

 equipment can be made to order and might prove advantageous for 

 nonfreeze-type solar absorbers in which the circulating fluid is not 

 subject to high water pressure. The proper thickness of metal and 

 the number of staybolts required depend upon the size of the absorber 

 units and the height of the overflow pipe. 



INITIAL COST AND CARRYING CHARGES OF SOLAR WATER HEATERS 



The sunshine is free, and every surface exposed to the sun is heated 

 at no cost, but to obtain useful energy for heating water some appara- 

 tus is necessary and this usually requires an expenditure which con- 

 notes an interest charge. Then, too, the apparatus deteriorates be- 

 cause of rusting and exposure, so that some annual depreciation 

 charge also should be considered. There are operation costs, even 

 though very low, because of the care required for cleaning the glass 

 and replacing occasional broken panes. 



