42 CIRCULAR 74 ; U. S. DEPARTMENT OF AGRICULTURE 



The greater the distance of air travel between delivery and return 

 openings the greater must be the velocity. 3 One advantage of higher 

 velocities within a room is that locations that might have been high- 

 temperature pockets are swept by moving air and warm air that tends 

 to accumulate in these pockets is carried away to the cooling coils. 

 These higher velocities also take the field heat out of warm fruit faster 

 than relatively still air. / 



When the distance between delivery and return is great, it is par- 

 ticularly important to leave an unobstructed space over the packages 

 and under the ceiling at all points. Otherwise, the air will tend to 

 move along the aisles or other open spaces instead of over the fruit, 

 and the advantage of high velocities will be lost. As in the case of 

 shorter air travel, it is important to have air flow equalized over the 

 length of the ducts and have it directed for equal distribution through- 

 out the stacks of fruit. 



In the design of an air-distribution system for a storage room it 

 is necessary to keep in mind a few points that, if the air volume is 

 adequate, will determine how well a uniform temperature in all parts 

 of the room can be maintained. The air should be both discharged 

 into the room and taken from it at or near the ceiling. The discharge 

 and return openings should be so located that the air is forced to 

 move past all the stored fruit. Installations in which the air is dis- 

 charged along a center aisle and returned at the floor at one end of 

 the aisle usually do not provide for ample circulation along the sides 

 of the room, and any point in the upper part of the room not directly 

 supplied with cold air from the discharge openings is likely to remain 

 too warm. Complicated duct systems with numerous laterals and 

 small openings are to be avoided. They add to the initial expense, 

 build up high resistance to air flow, and tend to result in local warm 

 spots. 



REVERSING DIRECTIONS OF AIR FLOW 



Since it is impossible to avoid having the temperature of the air 

 rise as it passes through the room, the fruit near the openings of return 

 ducts is warmer than that near those of the delivery ducts. If all 

 duct openings can act alternately as deliveries and returns, the warmest 

 fruit will not be so warm and the coldest not so cold. This can be done 

 by reversing the direction of air circulation every few hours by means 

 of a simple set of dampers and special duct arrangement near the fan. 



3 Air velocity and air volume are sometimes considered different expressions of the 

 same thing. They are not. In an extreme case, for example, a long, narrow room, about 

 10 x 100 feet in plan, consider two arrangements of ducts — in one the ducts are along 

 the side walls, so that the air discharged is returned 10 feet away at the other side ; in 

 the other, the air is delivered at one end and picked up 100 feet away in a duct at the 

 other. Now assume that a given volume of air is to be circulated through this room, 

 which is 10 feet high and has a volume of 10,000 cu. ft. If 1,000 c. f. m. of air is to 

 be circulated through this room and it moves from one side to the other, its average velocity 

 will be 1 foot per minute. If, on the other hand, it moves from one end to the other, the 

 same volume will move through the room at 10 feet per minute. In this example, the 

 velocity in one case is 10 times as great as in the other, the volume of air being the same 

 in both cases. 



This is an extreme example of a situation in a storage room. The volume of air re- 

 quired depends upon the quantity of heat to be removed and the tolerable difference in 

 temperature between delivery and exhaust air. It is uneconomical to deliver a larger 

 volume than these considerations require. The velocity at which the required volume 

 of air moves through the room depends upon the length of the path it must traverse. 

 Increasing the distance between delivery and return openings requires that the velocity 

 for a given volume be correspondingly increased. For this reason, velocity and volume 

 are not necessarily dependent upon each other. Volume can be adjusted to the heat load, 

 and velocity can be controlled by the distance between openings. 



