TRANSPORTATION OF FISHERY PRODUCTS 309 



The separation of leakage heat from cargo heat has an important bearing on 

 the control of both temperature and relative humidity. A common air-stream 

 which has to take up heat from both sources will naturally undergo a greater 

 temperature rise than if it had to absorb heat from the cargo only; therefore, 

 there would be a greater temperature variation from side to side of the cargo stack. 

 An air-stream passing through fruit tends to reach equilibrium through a balance 

 between the heat given oflF and the water lost by the fruit. On passing through 

 meat, fish, or other materials which do not generate heat, but which do evaporate 

 water, it tends towards complete saturation. In either case if the air-stream has to 

 deal with leakage heat as well as cargo heat, the relative humidity will be lowered. 

 This may or may not be an advantage, but in any case it is uncontrolled. In cer- 

 tain instances, for example where frozen fish are stored, it is of such importance 

 as to have led to the use of the jacketed system of cooling, designed essentially to 

 overcome the difficulty of excessive desiccation. 



Most of the present cooling systems constitute uncontrolled approximations 

 to the ideal of a separation of the two sources of heat. In the grid system the side 

 grids primarily absorb the heat from the walls and floor, while the ceiling grids 

 deal with the heat from the cargo and that coming through the roof. In the 

 battery system a partial short-circuiting of some air around the cargo stack deals 

 with the leakage heat, while the rest of the air penetrates and removes the heat 

 from the cargo. Since in both cases the separation is partial and uncontrolled, 

 some of the leakage heat may gain access to the cargo before being absorbed 

 by the cooling pipes. 



A complete separation of function requires the provision of an interior lining 

 to the room so that the cargo is surrounded by a "jacket" in which the leakage 

 heat is removed. The cooling of the cargo and the jacket can then become separate 

 problems. In the jacketed space the degree of air circulation can be made a 

 function of the amount of heat generated there and the relative humidity desired. 

 If no heat is released in this space, then usually no circulation is necessary. If 

 the jacket is practically gas tight, then the space will become saturated with 

 water vapor evaporated from the product, and no further desiccation can take 

 place. However, if circulation is required, then, wherever practicable, it should 

 be vertical rather than lateral, since in the latter case the superposition of con- 

 vective flow on the forced movement may result in an uneven temperature dis- 

 tribution. The cooling of the jacket itself is best accomplished by circulating 

 through it air which is cooled in an external cooler. The circulation of the cooling 

 air through the cargo and the jacket may be induced by a single fan system, either 

 in parallel or series flow; but the parallel flow is preferable since in this scheme 

 the common air-stream can be divided in any predetermined ratio, depending 

 upon the type and demands of the cargo, by means of a suitable adjustable or 

 automatic proportioning device. 



The principle of the jacket has general application in cold storage and trans- 

 portation services, particularly in the holds of refrigerated ships where space is at 

 a premium. Through careful design the jacket may be reduced to minimum 

 dimensions for the uptake of the maximum leakage load, and the cargo space 

 can be correspondingly increased. This increase in loadable space may or may 

 not result in higher cost. The cost of the jacket may be partially or entirely offset 

 by the saving in piping due to both higher heat-transfer and the shorter length 



