3io CHAPTER XVIII 



A vapour, like any other body, may be heated, and a vapour heated 

 above the temperature corresponding to its condensation point at that 

 pressure is said to be superheated, or to have so many degrees of superheat. 

 The condensation point of the vapour and boiling point of the liquid are, 

 of course, the same. 



Heat. Heat is a definite measurable form of energy. The unit used in 

 British and British-derived engineering practice is the British Thermal Unit 

 (B.T.U.) : this is the quantity of heat required to raise i Ib. of water through 

 i F. at a temperature of 62 R* The metric unit is the calorie, based on 

 the kilogram and degree Centigrade ; it is hence 3 967 times as great as the 

 B.T.U. Under this definition, to raise the temperature of water from 32 F. 

 to 212 F. will require 180 B.T.U. ; under atmospheric pressure water at 

 this temperature will boil. To convert all the water to steam will require 

 969-7 B.T.U., and this quantity is said to be the latent heat of steam at 212 F. 

 The sum of the latent heat of steam, and the quantity required to raise the 

 temperature from 32 F. to 212 F. is called the total heat of steam. The 

 latent heat of steam is not constant, but decreases with rise in tem- 

 perature; the total heat of steam, however, shows an increase with 

 temperature. 



The quantity of heat in the same weight of different bodies at the same 

 temperature is not the same. The ratio of the quantity of heat required 

 to raise the temperature of a body i F., to the quantity of heat required to 

 raise the temperature of the same weight of water i F., is called the specific 

 heat, the value assigned to water being unity. The specific heat of a mixture 

 is as computed arithmetically ; thus the specific heat of a 10 per cent, 

 solution of cane sugar is 0-9 X i -f o-i X 0-301 = 0-9301. 



The Transference of Heat. In a certain sense a sugar factory may be 

 considered as a system for the transfer of heat, not only in evaporation, but 

 also in the following departments : Generation of steam in the boilers, 

 heating and evaporation of juices and syrups, cooling of juices on settling, 

 cooling of injection water, cooling of massecuites, drying of sugars. 

 The subject of heat transference will therefore be discussed in some 

 detail. 



Heat may be transferred from a hot body to a colder body by conduction, 

 radiation, or convection. By the last term is meant the currents set up in 

 a fluid, when one portion changes in density owing to change in temperature ; 

 an intimate mixture follows, so that transference by convection is merely 

 a special case of conduction. These means may act independently or in 

 conjunction, some specialized examples being given below : i. The hot 

 body is separated from the cold body by a partition (boilers, juice heaters, 

 and evaporators generally). 2. The hot body is in direct contact with the 

 cold body (injection water in condenser, cooling tower). 3. The transfer 

 takes place solely by conduction (surface condensers, water-jacketed crystal- 

 lizers). 4. Transfer takes place by combined conduction and radiation 

 (steam boiler, loss of heat in steam pipes). 5. The hot body is a gas (steam 

 boiler), a liquid (massecuite), a condensing vapour (steam in evaporator) ; 

 and conversely the cold body is a gas (air and steam pipes), or a liquid 

 (juice in evaporator). 6. The hot and cold bodies may mutually change 



* The variation referred to any other temperature is very small, and for engineering practice may be neglected. 



