THE EVAPORATION OF THE JUICE TO SYRUP. 



Latent Heat. When water or any liquid passes into the form of 

 vapour, a certain quantity of heat is required to perform this change ; this 

 quantity is called the latent heat of evaporation. The heat required to raise 

 the temperature of I Ib. of water one degree Fahrenheit is called the 

 British Thermal Unit, or B.T.V., and to convert lib. of water at 212 F. 

 into steam at 212 F. requires 966 B.T.U. This quantity is the latent heat of 

 evaporation of water. A quantity often used in calculation is the total heat of 

 ateam ; this means the quantity of heat required to convert 1 Ib. of water 

 from the freezing point (32 F. or C.) to steam at the given temperature. 

 Thus to raise the temperature of lib. of water from 32 F. to 212F. 

 requires 180 B.T.TJ. ; to convert it into steam requires 966 B.T.U., so that 

 the total heat of steam at 212 F. is 1146 B.T.U. The metrical unit of heat 

 is the quantity of heat required to raise one kilo, of water through 1 C. ; it is 

 called a Calorie, and is 3-967 times as large as the B.T.TJ. 



Conversely to what has been written above, when steam is condensed to 

 water it gives out the heat which was required to convert the water to steam. 

 If then steam at 212 F. be condensed to water at 180 F. it gives up a quan- 

 tity of heat which is the difference between the total heat of steam at 212F. 

 and of water at 180 F. This heat can be used to raise the temperature of or 

 to evaporate water which is under such a pressure that its boiling point is 

 lower than 180 F. ; the vapour or steam given off by the water thus caused 

 to boil may be used to boil more water under a still lower pressure, and so on, 

 and this process repeated to infinity. It is on these lines that the construction 

 of multiple effect apparatuses is based. 



Standards of Pressure and Vacuum. The normal pressure 

 of the atmosphere is 14-707 Ibs. per square inch and it will support a column 

 of mercury 29 '92 inches high ; in the metric system the corresponding measures 

 are 1-034 kilos per square cm., and a barometric height of '760 metre. Pres- 

 sures are also expressed in atmospheres and we thus have 1 Ib. per square 

 inch = 2-035 inches of mercury = -068 atmosphere. The vacuum in an 

 effect is nearly always given in inches of mercury ; thus, a vacuum of 25 inches 

 means that the excess of atmospheric pressure over the pressure in the effect 

 would support a column of mercury 25 inches high; hence expressed as a 

 pressure a vacuum of 25 inches means a pressure of 29-92 25 inches = 

 4-92 inches = 2-42 Ibs. per square inch. 



Transmission of Heat. The coefficient of transmission of heat 

 is the quantity of heat which will pass in one minute from a warmer to a 

 colder surface of one square foot area under a difference in temperature of 

 1F. The metrical units of reference are the square metre, degree Centigrade, 

 and the calorie. The metric coefficient is then 4 -88 times as great as the 

 British coefficient. 



283 



