314 CHAPTER XVIII 



tivity at entry. Orrok, referring to surface condensers used with steam 

 turbines, expresses the relation thus : let P, be the partial pressure due to 

 the steam, and P, be the total pressure ; then the coefficient of transmission 



varies as [ — ' ] ; to the exponent n, values varying from 2 to 5 have been 



assigned, Orrok's experiments pointing to the latter value. 



In tubular condensers the length and the diameter of the tube have 

 an influence on the transmission of heat, the generally accepted formula 



being K = — , where c is a constant and d and I are the diameter and length 



respectively. With decreasing diameter it is easy to see that the thickness 

 of the wall of liquid through which heat has to be transmitted decreases ; 

 it is not so easy to realise what influence length will have. In vertical 

 submerged tube evaporators, however, increase in length of tube increases 

 the hydrostatic head or pressure under which the lower layers of liquid boil, 

 and also increases the length of time taken for a drop of water to trickle down 

 from the top to the bottom of the tube. 



The passage of heat to the atmosphere from a steam pipe, a tank full of 

 hot juice, an evaporator, pan, or juice heater, may be considered as a special 

 case of the transfer of heat through a partition. In the case of a bare pipe 

 the coefficients a and b in Peclet's equation may be considered as of the same 

 order as those found in surface condensers. By the substitution of air for 

 water or boiling juice the value of c is many times decreased, and when a 

 non-conducting material is placed round the partition the value of b is also 

 decreased. The question is complicated by the dissipation of heat being also 

 due to radiation and convection, as well as conduction ; in any case, however, 

 the loss of heat cannot be greater than what can pass through the partition. 

 In the case of a hot body separated from air by a partition, heat wiU pass 

 through, and eventually, if there is no loss of heat, both sides of the partition 

 will be at the same temperature and no more heat will pass. As soon as 

 heat is lost by radiation and conduction, the temperature of the external 

 side falls, and heat again begins to pass ; this process will continue until the 

 external side is at such a temperature that the heat which passes under the 

 temperature difference is exactly balanced by that given off by radiation 

 to and conduction by the air. The dominant factor controlling the loss of 

 heat will be the final difference in temperature between the external side 

 of the partition and the surrounding air ; this in turn will be controlled by 

 the conductivity of the partition, the temperature of and circulation in 

 the air, and the nature of the external surface of the partition. The combined 

 effect of all these influences, called the exterior conductibility or surface emis- 

 sivity cannot be combined in one general formula, although a number of 

 empirical formulae have been suggested. For small and nearly related 

 temperature differences the loss is directly proportional to the temperature 

 difference, but the loss for mx degrees is more than m times the loss for x 

 degrees when m is large, the proportionate difference increasing as m 

 increases. 



As the loss in steam pipes is partly controlled by the value of a in Peclet's 

 equation, or conductibility between steam and partition, an explanation 

 is afforded of the less loss found with superheated compared with saturated 

 steam. Although the former is at a higher temperature, its conductibility 



