374 The Philippine Journal of Science lau 



for different boilers, but with waters containing varying amounts of scale- 

 forming ingredients the efficiency of all boilers is seriously interfered 

 with. The question as to whether a given water is satisfactory can be 

 decided only when its chemical composition is known. Although it is 

 difficult to fix an arbitrary standard, the following classification of waters 

 with reference to their suitability for boiler use serves very well for 

 practical work. The figures show the number of parts of scale-forming 

 golids in a million parts of water. 



Quality of ivater for boiler purposes.' 



Less than 90 parts per million Good. 



90 to 200 parts per million Fair. 



200 to 430 parts per million Poor. 



430 to 680 parts per million Bad. 



Over 680 parts per million Very bad. 



"^ Proc. Am. Ry. Eng. & Maintenance of Way Assoc. (1904). 5. 595. 



If the amount of scale-forming ingredients in water for boiler use is 

 high, the interiors of the shell and tubes of the boilers become coated with 

 scale which offers one hundred times the resistance of steel to heat and 

 seriously interferes with the transmission of the latter. Steel is not a 

 remarkably good conductor of heat when clean, and a very thin coating 

 of scale often markedly shows in loss of heat. 



The effect of scale on the transmission of heat through a boiler tube is 

 extremely variable, the mechanical structure of the scale at least as 

 important a factor as the mere thickness. A hard scale 7 or 8 millimeters 

 thick may result in a reduction of from 15 to 20 per cent in the evapora- 

 tion."' Schmidt and Snodgrass "° have investigated the effect of scale on 

 the transmission of heat in locomotive boiler tubes, and feel warranted in 

 summing up the results of their tests in the following conclusions: 



"1. Considering scale of ordinary thickness, say of thickness varying up 

 to one-eighth inch, the loss in heat transmission due to scale may vary in 

 individual cases from insignificant amounts to as much as 10 or 12 per cent. 



2. The loss increases somewhat with the thickness of the scale. 



3. The mechanical structure of the scale is of as much or more im- 

 portance than the thickness in producing this loss. 



4. Chemical composition, except in so far as it affects the structure of 

 the scale, has no direct influence on its heat transmitting qualities." 



A thick scale may cause overheating by preventing radiation, rapid 

 deterioration, and even blistering and cracking of boiler tubes and shells. 



When the chemical analysis of a boiler water is at hand, its scale-forming 

 properties can readily be determined. In this connection the formulae 

 developed by Stabler " are of special value : 



Scale (in pounds per 1,000 gallons of water) =0.00833 suspended matter 

 -I- 0.0083 colloidal matter (= SiO. -\- ALOs -(- Fe^On) -f- O.OlOTFe -1- 

 0.0157A1 -f 0.0138Mg 4- 0.0246Ca. 



=' Palmer, U. S. Geol. Surv., Water-Supply Paper (1909), 233, 186. 



"Bull. Univ. III. (1907), 4, No. 15, 1. 



" U. S. Geol. Siirv., Water-Supply Paper (1911), 274, 176. 



