312 MARINE PRODUCTS OF COMMERCE 



center at the top. Some reports indicate that differences as great as 20° F 

 (11.1° C) have been noted between the warmest and the coldest parts of the 

 load. The differences of temperature are greatest, naturally, when the level 

 of the cooling mixture in the tanks is low; therefore, it is very important that 

 the bunkers be kept as full as possible. Since the cooling mixture in the lower 

 parts of the bunkers is relatively ineffective, in some cases the lower halves are 

 shelved off and the upper parts only are iced. This is sometimes referred to as 

 "stage icing" or "half-stage icing." In the United States and Canada more or less 

 efficient icing stations have been set up at selected points so that cars can be reiced 

 about every 24 hours in long runs. 



The Fan Car. The inadequacy of the end-bunker car results from the position 

 of the cooling tanks and the relatively long path taken by the circulating air. 

 Since the air movement is induced by convection, the velocity is low, and conse- 

 quently the rate of heat transfer to the tanks is correspondingly low. The recogni- 

 tion of these factors gave rise to the use of circulating fans, which may be operated 

 by a friction drive off the wheels of the car while it is in motion or by electric 

 motors which may be attached to the side of the car where electrical power is 

 accessible when the car is standing still. 



Fan cars have been described in many publications and their application and 

 performances are now well-known. The reports indicate, as would be expected, 

 that the output of the fans is a function of the speed of the train, and therefore 

 varies considerably. The TM-inch (Preco) fan is reported to deliver from 1,600 

 to 2,000 c.f.m. and to require about 1 h.p. when the car is travelling at 30 m. p. h. 

 When these cars are stationary and the fans are not operating, their performance 

 is similar to that of the end-bunker car without fans; therefore, their over-all 

 efficiency depends upon the ratio of running to standing time. The use of attach- 

 able motors during the standing time, of course, improves performance, but many 

 railway yards in the United States and Canada have no electrical outlets con- 

 veniently situated for this service. It is likely that these will be provided, how- 

 ever, if the fan car becomes generally accepted. 



When fan cars are in motion with the fans operating, the temperatures at- 

 tainable in the car are more uniform and somewhat lower than the temperatures 

 in similar end-bunker cars, and ice meltage is correspondingly greater. The effect 

 of the fans, however, is to make the performance less dependent upon ice level in 

 the bunkers; consequently, the bunker capacity could be increased so that the 

 frequency of icing would not be affected by the increased ice consumption. 



The Roof-Tank Car. The illogical position of the cooling bunkers in the end- 

 bunker cars, which resulted in poor temperature distribution, gave rise to the 

 development of the roof-tank car, first in South Africa and more recently in 

 Canada. 



The overhead or roof-tank car now adopted as standard in Canada is 40 feet 

 long inside and is insulated with approximately 4 inches of "Fiberglas" in the 

 walls and ends and approximately 5 inches on the floor and ceiling. It is equipped 

 with 8 roof tanks, holding slightly over 3 tons of ice in total and accessible 

 through 8 hatches in the roof of the car. These tanks are equipped with brine- 

 retaining traps, as well as drain plugs. The brine may be retained at a pre- 

 determined level within the tanks to flood the bottom completely and thus main- 

 tain a uniformly low temperature there, consistent with the concentration of the 



