Vapor Pressure and Osmosis

HTMD Engineering was given the opportunity to design and build a cold storage facility for DeKalb Genetics for their hybrid seed corn.  The industry standard design conditions of 50°F and 50% relative humidity were selected, and we chose to insulate the facility with a closed cell spray on insulation which provided both good thermal insulation and a vapor barrier. 

We worked with a supplier to design the refrigeration equipment.  The evaporator coil removes heat and moisture from the air stream.  The minimum moisture content of the conditioned air is fixed by the evaporator coil temperature.  A heater increases the air temperature back to 50F.   The equipment was controlled with a humidistat and a thermostat.   See figure 1.

The system performance is shown on a psychrometric chart (see figure 2).      We reasoned that a heater failure could result in continued operation of the refrigeration system.  Cooling below the design point at constant moisture content increases the relative humidity of the air.   When we changed to a digital control system,  we looked at alternate control strategies.

We selected vapor pressure as a control variable.   Moisture vapor pressure (pw) is uniquely defined by temperature (T) and relative humidity (rh), (pw  = pws(T)  * rh).    Saturated vapor pressure of  water is shown on figure 3.    The refrigeration equipment is controlled by temperature and moisture vapor pressure which do not depend on the operation of the auxiliary heater. 

Later, we developed a model for a stored seed  using vapor pressure as the driving force for moisture transfer between the seed and its environment.   Our model agreed with industry standard practices for stored seeds.    

After our work was challenged, we looked at osmosis. We then wrote the “theory of osmosis” showing that osmotic flow was from a high to a low vapor pressure region.    Osmotic pressure applied to the wine side of the osmosis experiment increased the water vapor pressure in the wine to equal the vapor pressure of the pure water.   

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