The mystry of the the pysch chart!
Having been mystified by it for a good part of my life, I now hope to understand most of it.
Lets review the problem at hand. We have two large cooling coils That are able to reduce temperature and %RH. We end up with a dry cold space materials.
When a/c dries the air, the moisture is collected on the cooling coil. The moisture collects and loads the coil. Typical vertical coils collect about 1 lb. of moisture per ton before becoming loaded and draining to the pan and then down the drain. Two 5 ton coils will collect 10 lbs. of moisture before they start dripping down the drain. Horizontal coils will collect 2 lbs. per ton of capacity.
At the end of the cooling cycle, the moisture on the coil will slowly evaporates back in the ducts and into the space. When activating heating coils in the air stream, the moisture will quickly evaporate into the air stream in ten minutes or so. Ten lbs. of moisture evaporating into the space will raise the dew point of the air enough to get condensation on 56^F concrete. The dew point of the air can be calculated by the using the psych chart.
It is all about the dew point of the coil while cooling, the temperature of the concrete, and the dew point of the air.
Of course the final results when operating one coil as a/c and the other as a heater was as stated. The problem was quickly solved. The moisture was removed from the air and discharged down the drain. The shrinking of the air by cooling has a very small effect. Its more about airs ability to hold much more moisture when heated.
There is plenty of info out there on the psych cart. I am not interested in rewriting it. It may be more productive to talk about the misconceptions.
Lets discuss this more. We have all come down the path not understanding what happens, do not feel bad.
The first part is as someone already mentioned, as air cools, the molecular motion of the H2O molecules slow. 68^F, 50%RH has a 49^F dew point. Exposing this air to any surface below 49^F will cause condensation on the cold surface. If we have a 56^F concrete surface, we must reduce the dew point of the air to less 56^F to avoid condenstion.
So its about molecular energy, dew point, and surface temperatures, not the shrinking or increased density of the air.
So by warming the air, warming the materials that have a condensation potiential, or by reducing the dew point of the air that surrounds material, you will be a hero. Being a hero is what is all about. We have been heros and and than again mistified because it did not turn out as predicted.