Moist Air Properties

Heat and moisture enter the warehouse through the walls and with outside air, some heat and moisture may be exchanged with the seed when it is introduced to the warehouse.   An air cooler (refrigeration unit) is designed to remove heat and moisture to maintain the constant temperature relative humidity (rh) during the hot summer months. 

A seed storage warehouse is designed to hold hybrid seed at a constant cool temperature and low relative humidity.     The cost of  warehouse environmental control is justified because hybrid seed is expensive to produce and expensive to discard.   The energy balance for   a warehouse is shown in figure 1.   

A thermostat and a humidistat are used to control the refrigeration unit.    The refrigeration turns on if temperature is greater than design  or the relative humidity (rh) is greater than the design.  It turns off when temperature and rh reach design values.  A low temperature thermostat adds heat if the ambient moisture /heat load does not match the refrigeration performance.   

The thermal performance of the refrigeration system can be shown on a psychrometric chart.    This chart is a graphical plot of the properties of moist air as shown in figure 2.    The horizontal scale is temperature and the vertical scale is humidity ratio.   Humidity ratio is defined as pounds of water vapor divided by pounds of dry air.     Relative humidity appears on the chart as a curved line.    Relative humidity is a measure of moisture in the air divided by the maximum moisture the air can hold at a temperature.  One hundred percent relative humidity means the air is saturated with moisture vapor at the given temperature.      A nominal performance of a refrigeration unit is shown as a straight line  with directional arrow.   The air- cooled refrigeration unit (cooler) is started at a temperature greater than design.  As it removes moisture and heat from the air that passes through the refrigeration coils,  the air may leave at a relative humidity higher than design.   The cooler continues to run, this problem can occur if the temperature and rh are not satisfied simultaneously.   We solved this by adding a low temperature thermostat with a heater.    

 We looked for an improved control system when we switched to a digital control panel.   We found that moisture vapor pressure represented a unique control point and it can be easily calculated from measured values of temperature and relative humidity.     With temperature and moisture vapor pressure as the design control variables,  the warehouse system always hunted for the design point. 

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