176 BUTTER-MAKING. 



(5) Proper utilization of steam turned into the pasteurizer. 



(1) Heating-surface. — It is a well known fact that some 

 metals will conduct heat better than others. The relative heat 

 conductivity of the two substances used chiefly for pasteurizers 

 viz., copper and tin, is .918 and .145 respectively. This 

 means that copper will conduct heat nearly seven times faster 

 than tin of the same tliickness. 



Fig. 116. — The Reid pasteurizer. 



In connection with this it should be mentioned that stabiUty 

 and durability of the substance must also be taken into con- 

 sideration. A heating-surface made from copper may be nearly 

 seven times as thick as that made fiom tin, and still transmit 

 as much heat as the tin surface. From this it can be seen that 

 a heating wall made from copper can be increased slightly in 

 thickness, and thus aid in stability, without affecting the degree 

 of heat conductivity of the wall very much. The heating- 

 surface must be strong enough to withstand a slight steam pres- 

 sure, otherwise the heating wall is likely to collapse or cave 

 in in case of sUght variation in the steam pressure. It is not 



