A model grain dryer is being used to investigate the drying process. It is discovered that the load cell which supports the dryer indicates that the mass of the dryer and its contentsinitially increases during the heat-up. This phenomenon is to be investigated, and it is proposed that it may be caused by either of the factors:
a. Moisture in the hot drying air condensing on the cooler grain near the plenum of the bin (Hint: The vapor in the ambient will condense out if the temperature of a surface is lower than the incoming air dew-point temperature).
b. There is a false reading in the load cell (because there is no initial condensation taking place in this particular case) Could either (a) or (b) be true? The operating parameters of the bin are:
Air mass flow rate (dry) = 0.15 kg/sec, Ambient dry bulb temperature = 23 ºC, Initial grain temperature = 20 ºC &
Plenum temperature = 40 ºC.
d.A computer simulation of the above dryer shows that towards the end of the drying time, air exits the drier at a relative humidity of 90%. What is the theoretical rate of moisture removal from the grain at this point, in kg/hr (Hint: It can be assumed that there is no heat loss through the walls, so the enthalpy of the plenum air will be the same as the enthalpy of the discharge air)