EVAPORATION 313 



value of k is - - = 0-0250. If the partition be neglected 



o 067 i 0-042 



i 



altogether, the value of k is ^ = 0-0251, so that the effect of 



o 067 o 042 



the brass tube is barely appreciable. The relative conductivity of copper 

 to brass is about 3 : i, so that substituting copper for brass would only cause 

 the value of k to rise to o 0256. The conductivity of copper is not the reason 

 for its use in evaporators and heat transference apparatus generally ; the 

 real reason lies in its resistance to corrosion, and possibly to some extent 

 in conservatism. As regards the manufacture of white sugar there are in ad- 

 dition other grounds. Brass, which is frequently substituted for copper, 

 has a conductivity substantially the same as that of iron or steel. 



It is easy to see from the above equation that the transference of heat 

 is governed by the low conductivity of any one element, and not by the high 

 conductivities of the others ; hence, if an evaporator is not kept clean, 

 application of principles of heat transference and the skill of the designer 

 are rendered null and void. A badly designed clean evaporator will operate 

 more efficiently than a well-designed machine the tubes of which are allowed 

 to become coated with a deposit of scale. 



As regards velocity of steam flow, most engineers now seem inclined to 

 revert to Osborne Reynolds' hypothesis, 7 namely, that the transmission co- 

 efficient is directly proportional to the product of density and velocity of 

 the fluid, or to the weight passing per unit time and per unit area ; experi- 

 ments made by Jordan 8 with hot air and water confirm this hypothesis, and 

 possibly the variations found between other experiments may be due to 

 neglect of the precautions necessary to keep other controlling conditions 

 constant. 



Jordan states with regard to the passage of heat from air to water : 



(1) For a constant mass flow ( f ' = constant ), the trans- 



\area of passage / 



mission coefficient is directly proportional to temperature difference. 



(2) For a constant temperature difference, the transmission coefficient 

 increases with the velocity under a lineal law. 



(3) Other conditions being equal, the transmission coefficient increases 

 with rise in the absolute scale of temperature. 



(4) The transmission coefficient depends on the area of the passage 

 and increases as the ratio, " area /circumference," decreases. 



The effect of variation in the velocity of the liquid has been studied by 

 a number of investigators who find the relation K = C V n where K is the 

 transmission coefficient, V is the velocity of flow. 



C is a constant, and n varies from zero to unity ; when n is zero, velocity 

 has no influence, and this condition might occur in the case of the presence 

 of some other dominant factor, but generally n is given by different ex- 

 perimenters as one-half or one-third. 



The latest and very detailed experiments of Orrok 9 give K = 308 V*, 

 with V in foot-second units ; this expression relates to design conditions 

 for surface condensers referred to a 27-inch vacuum. 



The presence of air in the steam decreases the value of a or the conduc- 



