Calculations of Heat Capacity for Seed Drying

When a seed dryer is operated at recommended temperatures, it requires about 2000 BTU of heat output to evaporate one pound of water. The required hourly heat capacity of the dryer in BTU per hour can be computed by the following formula:

Heat Capacity = lbs water removed per bushel
X bu to be dried per hour
X 2000
We can now consider an example of how to estimate dryer requirements:

Example: A farmer wishes to dry 1200 bushel of wheat per day from 22 percent to 14 percent.

a) What is the recommended drying depth?
b) What air flow rate should be recommended and what will be the total air flow required?
c) What will be the static pressure?
d) What actual hourly output must the dryer provide?
e) Select the proper crop dryer.

a) The recommended depth for heated air drying of wheat is 20 inches. For the sake of easy calculations let us take it as two feet.

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b) Air flow rates for heated air drying in batch bins or wagons are from 15 to 40 CFM/bu. The higher the air flow rate, the faster and more uniform the drying. For a starter, an air flow rate just above the minimum may be recommended, with the understanding that is highly desirable that the dryer selected be able to provide more air than this. For a starter an air flow rate of about 20 CFM/bu may be recommended.

c) To estimate the static pressure, the static pressure curves will have to be used in manner described earlier and repeated here.

Step I. Air flow in CFM/sq ft = grain height X CFM / bu X 0.8
= 2 X 20 X 0.8 = 32

Step II. Using read horizontally across on the 32 CFM/sq ft line until the wheat cur is contacted. This is done almost exactly at the vertical line which shows that the pressure drop per foot of speed depth is one inch.

Step III. Multiply this figure by grain depth to get static pressure. Since the seed depth is about 2 ft the static pressure will be 2 inches (2 X 1).

d) To estimate the hourly heat capacity, it is necessary to know the actual hourly per bushel and the quantity of water to be removed per bushel. For twenty- four hour operation, the required capacity shall be 75 bushels (average hourly capacity i.e 1200/24 = 50 X 1.5 to get actual hourly capacity) per hour, the quantity of water to be removed per bushel to dry seed from 22 to 14 percent shall be about 6 pounds. The required heat capacity according to formula described earlier will be

Required heat capacity = 6 X 75 X 2000
= 900,000 BTU / hr

e) Select suitable crop dryer to provide a minimum required air flow (bin capacity in bushels X recommended air flow rate) at 2 inch static pressure and 900,000 BTU/hr. in estimating dryer requirements, it is always better to consider the possibility of increased future demands.