THE EVAPORATION OF THE JUICE TO SYRUP. 



When juices are to be heated the temperature of the juice is taken as the 

 mean of its initial and final temperature; when the juices are to be evaporated 

 the temperature of the juice is taken as the mean temperature of the boiling 

 mass. As an example, let there be 30 metric tons of juice to be heated in one 

 hour from 30 C. to 100 C., with steam at 105 C. The average temperature 



of the juice to be heated is 30 -f 10 30 65 C. The difference of 



temperature is 105 65 = 40 C. Taking the specific heat of juice as -9, 

 the heat to be transmitted is 30,000 X "9 x (100 30 J = 1,890,000 calories. 



The coefficient of transmission for diffusion, &c., juices is 5, i.e., per 

 square metre per minute per 1 C. difference of temperature, 5 calories are 

 transmitted. 



Then per hour under the conditions named 1 square metre will transmit 

 5 x 40 X 60 = 12,000 calories, and the heating surface required is 

 1,890,000 -j- 12,000 = 157 square metres, or 1689 square feet. 



Calculations for pans and evaporators may be made on exactly similar 

 lines, using the coefficients of heat transmission given in a preceding section. 



Dimensions of Steam and Vapour Piping. The passage 

 of vapour from the vapour space of one body to the steam drum of the next 

 takes place under a slight difference of pressure ; this pressure amounts to 

 0-3 Ib. per square inch, corresponding to a difference in temperature of about 

 1 F. To calculate from these data the velocity of a gas leaving an orifice 

 requires the use of higher mathematics, which would serve no useful 

 purpose to reproduce here ; under the conditions named, the velocity of a gas 

 is of the order 200 feet per second ; such a velocity is not obtained in 

 evaporators because of the friction of the gas on the walls of the pipes, abrupt 

 changes in direction, especially those due to baffle plates and save-alls, and 

 also to condensation of a part of the vapour. For steam and vapour pipes it i 6 

 is customary to allow a velocity of 100 feet per second, and for the vapour 

 leaving the last body a velocity of 150 feet per second; if velocities higher ^* 

 than these obtain, a very considerable fall in the temperature of the vapour 

 occurs, reducing the efficiency of the apparatus. As an example of this and 

 other calculations, the case of a triple effect with 4000 square feet heating 

 surface, evaporating 6 Ibs. water per square foot per hour, or 2'22lbs. per 

 second per vessel, is taken. 



Steam Pipe. Taking the temperature of the heating steam to be 234 F., 

 1 Ib. will occupy 17-83 cubic feet, and allowing 1 Ib. steam to evaporate 

 2-90 Ibs. water, 2-31 Ibs. (= 41-19 cubic feet) will be required per second. 

 At a velocity of 100 feet per second the area of the pipe must be -4119 square 

 feet, corresponding to a diameter of 8 inches. 



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