288 Mr. T. E. Stanton. On the Passage of Beat 



in the theory of boilers and surface condensers, but the difficulty of 

 experimentally determining this rate and the constants involved, for 

 cases which occur in ordinary practice, is so great that, as far as the 

 author is aware, only approximate results have been obtained. 



It was pointed out by Peclet that in tbe case of a metal transmit- 

 ting heat to water in contact with it, this heat had to be trans- 

 mitted by conduction through a film of water adhering to the surface, 

 the thickness of the film depending on the state of agitation of the 

 water, i.e., on the convection of the heated particles of water from 

 the surface. The determination of this rate of convection would 

 in most cases be extremely difficult. 



Another difficulty is the varying temperature of the surface of any 

 plate giving up heat to a liquid, for in this case the problem becomes 

 more complicated owing to the conduction of heat from one point of 

 the plate to another. 



For the experimental study of the subject the best conditions of 

 working would, therefore, seem to be : — 



1. Surface transmitting heat to water moving over it at a known 



velocity. 



2. Temperature of the surface constant. 



The first of the conditions is fulfilled by taking the case of water 

 flowing through metal pipes ; and it occurred to the author that the 

 second would be fulfilled by the following method of working : — 



AB is a thin metal pipe surrounded by a 

 second pipe CD, the space between the pipes 

 being used as a water jacket. If now hot 

 water at a temperature T initially flow down 

 the jacket, and cold water at a temperature t 

 flow down the pipe, then heat is transmitted 

 from the hot water to the cold water through 

 the walls of the pipe. Again if the quantities 

 in each case are the same the rise of tempera- 

 ture in the water flowing through the pipe is 

 equal to the fall of temperature of the jacket 

 water, so that at any cross-section the mean 

 temperature of the water is constant. 



Now the temperature of the wall of the pipe 

 will not necessarily be a mean between the 

 values of T and t at that point, but if the 

 total fall of temperature from one end of the 

 pipe to the other is small, say not more than 

 6° C, we may fairly assume that the ratio 

 of the differences of temperature between (jacket-water and wall) 

 and between (wall and water flowing through pipe) is constant for 



BID 



