CANE SUGAR. 



The transmission of heat is governed by the following Laws : 



1. It is directly proportional to the mean difference of temperature. 



2. It varies with the nature of the material through which the heat 

 passes. The hest conductors of heat are metals, silver and copper in particular ; 

 copper or brass are the two materials most used in practice. 



3. The coefficient of transmission decreases with increasing thickness of 

 metal ; for copper, however, within the thicknesses employed in practice the 

 coefficient is sensibly constant, and even with brass 5 per cent, would be an 

 extreme variation. 



4. The coefficient of transmission is affected by the nature of the 

 substances on either side of the partition ; thick viscid liquids as occur in the 

 last body of a multiple effect greatly retard the passage of heat as compared 

 with water. In an extreme case it may be only one-fifth of what it is when 

 dealing with water. 



5. Incrustations, such as occur on the tubes of a multiple effect, have an 

 enormous and undeterminable effect in reducing the passage of heat. 



6. The larger the heating surface, i.e., the partition separating the 

 substances between which a transfer of heat occurs, the less is its efficiency ; 

 that the efficiency is proportional to the square root of the surface has been 

 established for tubes internally steam heated, and the same relation probably 

 holds for externally heated tubes. 



7. The transmission is also affected by the position of the heating 

 surface ; thus with vertical tubes the condensed water which is formed on the 

 surface of the tubes flows down them and prevents intimate contact of steam 

 and heating surface. With horizontal tubes the condensed water drops off the 

 tubes and the latter are in general more efficient. Any form of apparatus 

 which permits easy and rapid removal of condensed water is per se more 

 efficient than one which does not fulfil those conditions. 



8. The heat transmitted increases with the rapidity of circulation of the 

 liquid being heated, and is greatly increased if the liquid should actually boil. 



The proposition enunciated in Law 1 requires some elaboration, and is 

 only approximately true ; the heat transmitted through a partition, one side of 

 which is at 60C. and the other 58C. is twice as great as that transmitted 

 when one side is at 60C. and the other at 59C., but it does not follow that 

 in the latter case the heat transmitted is the same as when one side is at 

 120C. and the other at 119C., although the temperature difference is exactly 

 the same. Actually, Olaassen has shown that at higher temperatures the coeffi- 

 cient of transmission is much greater than at lower ones; hence it follows 

 that a fall in temperature due to high vacuum in the last body of a multiple is 

 not attended with so much benefit as in cases where the temperatures are more 



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