PASTEURIZATION I5I 



I. Conductivity of the Heating Wall. — The amount of heat 

 that in a given time will pass through the heating wall of a 

 pasteurizer may be expressed by the following formula: 

 Q_ KXA(ti-t.,) ^^^ 



Q represents quantity of heat in B. T. U. conducted. 



K represents the quantity of heat in B. T. U. conducted 

 through one square inch of a wall one inch thick per second 

 when the difference in temperature on the two sides of the wall 

 is 1° F. 



A represents area, in square inches of heating surface. 



ti represents temperature in Fahrenheit degrees in steam 

 chamber. 



t2 represents temperature in Fahrenheit degrees in heating 

 chamber. 



T represents time in seconds. 



d represents thickness of wall in inches. 



The heating wall of a pasteurizer should be constructed 

 from metal which is of high thermal conductivity. Copper 

 has been found very satisfactory for that purpose, its thermal 

 conductivity being 0.72. Only one metal, silver, is a better 

 conductor than copper, its thermal conductivity being 1.096, 

 but its use is prohibited on account of its cost. Aluminum is 

 next to copper in thermal conductivity, being 0.343, while tin is 

 only 0.152. 



Taking for granted that copper will be used for the construc- 

 tion of the heating wall of the pasteurizer we know that 0.72 

 calorie ' of heat will be conducted per second through one 

 square centimeter of a copper wall one centimeter in thickness 

 when the difference in temperature on the two sides of the wall 

 is one degree Centigrade. 



The British thermal unit is a measure for heat which is mostly 

 used in practical work. As one pound is equal to 453.6 grams 



then one B. T. U. is equal to 453.6 X = 252 calories. 



180 



' One calorie is the amount of heat required to heat one gram of water one degree 

 Centigrade from 4° C. to 5° C. 



