VEGETABLE AND FRUIT DEHYDRATION 103 



MECHANICAL MOVEMENT OF AIR IN DEHYDRATORS 



Fans generally used for the mass movement of air in dehydrators 

 can be divided into two classes: (1) Propeller, axial-flow, or disk 

 fans, and (2) centrifugal or rotary impeller fans. Disk fans are 

 generally limited to delivery of air against relatively low static pres- 

 sures, although some special designs are capable of developing a pres- 

 sure of 10 inches of water or higher. Centrifugal fans, on the other 

 hand, can be used against either high or low static pressure. Instal- 

 lations in which low-static-pressure disk fans are suitable include 

 cabinet \ driers, low-performance tunnel driers, and belt-cojnveyor 

 driers. Centrifugal fans can be employed in addition, on high- 

 performance tunnel driers, bin driers, and in general on installations 

 where the static pressures are substantially higher. A centrifugal 

 fan consists essentially of an impeller or fan wheel and a casing or 

 housing. The design of these parts can be varied to suit peculiarities 

 of the load. The casing has either of two functions or both; one 

 is to transform velocity pressure into static pressure and the other 

 is to collect and conduct the air to the point of discharge, or fan 

 outlet. 



Centrifugal or Rotary Impeller Fans 



Centrifugal impellers can be roughly divided into three classes: 

 (1) Those with forward-curved blades, (2) those with straight radial 

 blades, and (3) those with backward-curved blades, sloping away 

 from the direction of rotation. Impellers thus formed are termed 

 slow-speed, moderate-speed, and high-speed types, respectively. 

 Their ranges may be overlapping; therefore, each of these types may 

 be used for the same work. Fan characteristics in each group may, 

 however, be varied by changing the depths of the blades. Thus, fans 

 with deep blades will develop relatively high static pressures, whereas 

 fans having short or shallow blades will handle large volumes at 

 low pressures. This latter type is sometimes designated as a 

 volume fan. 



Although the forward-curved-blade centrifugal fan is widely used 

 on tunnels originally designed as fruit dehydrators, they are not 

 best suited for that purpose. The probable reason for their original 

 adoption lies in the fact that the relatively low shaft speed required 

 for a given discharge was applicable to use with steam or gasoline 

 engines. 



The forward-curved fan has many characteristics that are un- 

 suited to dehydrator service. First, the fan power increases when 

 the static pressure of the fan is reduced and therefore the fan absorbs 

 maximum power at free discharge. However, at the normal operat- 

 ing static pressure, which is usually a condition near the point of 

 maximum mechanical efficiency, the fan power may be less than one- 

 half of free discharge power. Second, the fan has an unstable capacity 

 or volume characteristic when operating at static pressures approach- 

 ing the shut-off condition. Third, the slow speed of rotation necessi- 

 tates an appreciable speed reduction if a standard electric motor is 

 used to drive the fan. 



Systems having variable static pressure loading as found in tunnel 

 dehydrators and bin driers are best served with fans having back- 

 ward-curved blades. This type of fan, which is sometimes known 

 as a limit-load or nonoverloading fan, has operational characteristics 



