Ventilation of stacks: heat removal and ventilation
Heat removal
Due to the respiration tubers produce heat.
The temperature in a stack of potatoes gradually rises if this heat is not removed by ventilation.
Natural ventilation
The natural ventilation in a stack of potatoes is caused by convective ventilation. Due to the fact that there is always a resistance to air movement in a potato stack such convective ventilation is rather limited. Ventilation can be improved by placement of a slatted duct under the potatoes or the use of a slatted floor in the store.
Improvement ventilation with slatted floor
The highest temperature in the stack can be found near the top of the stack (T top) and the lowest temperature is found at the bottom of the stack (T low) . It has been estimated that differences in temperatures between the bottom and top of the stack is
+ 2 C for each meter piling height (h) or: (Ttop = Tlow+ 2 x h ) in oC.
Forced ventilation
Forced ventilation is efficient for cooling and drying. In a pile of 3 meters high the temperature difference between
T top and T low may be less than 2 C when forced ventilation is used.
This system is used in stores with high storage capacity.
Forced ventilation versus sub-division in small units
To reach a situation where in a stack the temperature differences are not too large two different systems are used:
- sub-divide potato lots in small units (e.g. crates, piled bags)
- use forced ventilation in bulk storage
Forced ventilation v.s. small units----Forced ventilation in bulk----Storage in containers
The storage capacity of a store is higher using bulk storage than when potatoes are stored in sub-divided small units. But if boxes are stacked high, loading capacity per m2 floor surface is high
Loading capacity
The loading density of potatoes is:
- in bulk: 670-680 kg per m2 floor surface
- in bags (dense packing): 550 kg per m2 floor surface
- in bags (rows of bags and space between rows): 420 kg per m2 floor surface
Example refrigerated store
In the example one bin has a floor surface of 20 m2.
With bulk storage and a piling height of 3.5 m the loading capacity of one of such bins is 20 x 3.5 x 670 = 50 tonnes of potatoes. The storage capacity of the total store then is 8 x 50 = 400 tonnes.
In bags with a dense packing and a piling height of 2.2 meter the loading capacity of one bin is 20 x 2.2 x 550 = near 25 tonnes. The storage capacity of the total store then is 8 x 25 = 200 tonnes.
In bags and piled in rows, with space between the rows and a piling height of 2.2 m the loading capacity of one bin is near 18.5 tonnes. The storage capacity of the total store then is 8 x 18.5 = 148 tonnes.