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  1. #27
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    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.
    Lets talk.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  2. #28
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    Feb 2005
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    In a van by the river
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    May I add that it would be impossible to supply 120-140 degree air at a rate of 2700 cfm with a 20 KW heater. You'd be looking at a 24 degree delta roughly so 80 degrees would be more realistic. These units, electric heat, would not reevaporate the water on the coil as I would assume that the heat strips would be located downstream as well. In essence, if electrically the circuit was big enough, you'd be able to run units in Ac mode with heat strips as reheat coils.
    ## + years in the field never made you a know-it-all This industry is far more diverse than you are

  3. #29
    Join Date
    Aug 2011
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    97
    Quote Originally Posted by teddy bear View Post
    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,
    I found through a very unscientific test (Placing my inferred thermometer flush with the floor.) that at the point right when the floor dried the surface temperature was 8-12* cooler than the air, in various locations. But it did not stay cool for long. right after the floor dried it began to warm up. Then bead water. Looked like it was condensing. This is with both units on Elec-heat. As I discussed with my instructors about the situation they suggested that the water was more likely to be permeating the floor. (Soaking through) And the slab would be working like a heat sink from the ground. The inferred thermometer has a 5+/- variable in reading so the floor test was discontinued. Seeing how the problem was satisfied and no more measurements could be taken, we settled on the air condensing the moisture in a (fog). As the units got hotter the ice on the coils melted(these are forced air. so the heat strips are in the return side) and the damp ducts would evaporate (Left that out of the presentation. Just forgot it.) making the humidity rise.

    Maybe with the water evaporating off the floor cooled the floor below the dew point?

  4. #30
    Join Date
    Dec 2003
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    Adam, I was in a gym playing basketball a few weeks ago on a cold rainy night and the floor started sweating. The floor was slick! I turned the gas heaters on and this raised the temperature of the air which lowered the rh of the air. This didnt change the amount of moisture in the air, but since the rh was lower the water in the floor was absorbed into the air and the floor dried out. Warming the air cant make the floor wet.

    You did the right thing running the cooling and the heat at the same time, but I dont think you have a total grasp of why. Dont fuss at medic as he is giving you some good information. Keep studying on this topic.

    And a shout out to Dr Willis Carrier who developed the psych chart. We struggle to understand it. He took a blank sheet of paper and drew it.

  5. #31
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    OK guys, I want to learn something here, and as far as I can tell, some of those here are those who should be able to teach us what happened during each phase of the OP's experience, and why. We just need to break it down succinctly and try to make it easily understood. Remember, if you can't teach something in a way that it is understood by others, you don't understand what you are teaching enough.

    As I see it, we are discussing an area that is being made very humid by still wet concrete. We have two decent sized cooling systems to remove moisture from the air, but in doing so, the air becomes colder and therefore contains less moisture.

    At the same time, as we remove moisture from the air, the moisture laden concrete floor is giving up its moisture to the air as it dries out. A problem is that while drying out the air, the air became too cold as well as the floor became too cold. The colder air contained less moisture to remove and the cold floor beaded up the moisture being brought to the surface without that beaded up water being able to evaporate into the cooled air.

    So, heat is turned on on one system, heating the air enough that it could absorb the excess moisture in the room, becoming heavily laden with moisture. At the same time, the other system was still cooling, so that the now warmer, more moisture laden air moving across the evap coils condenses the moisture content before being moved back into the room as cooler, less moisture laden air.

    With each cycle, the cooling system removes moisture from moisture laden air being heated by the other system. As the moisture in the air is continually being removed, the moisture in the concrete floor is moving to the surface, beading up and then evaporating into the now warmer and dryer air, once again saturating the air to go through the cooling system for more moisture removal.

    Is there anything I am missing for the process that was going on with this specific issue?
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  6. #32
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    Quote Originally Posted by bigtime View Post
    Adam, I was in a gym playing basketball a few weeks ago on a cold rainy night and the floor started sweating. The floor was slick! I turned the gas heaters on and this raised the temperature of the air which lowered the rh of the air. This didnt change the amount of moisture in the air, but since the rh was lower the water in the floor was absorbed into the air and the floor dried out. Warming the air cant make the floor wet.

    You did the right thing running the cooling and the heat at the same time, but I dont think you have a total grasp of why. Dont fuss at medic as he is giving you some good information. Keep studying on this topic.

    .
    When a surface temperature of the floor material is 1^F below the dew point of the air, condensation forms on the surface. By raising the temperature of the material above the dew point of the air, the condensation stops and the moisture evaporates.
    Or if you lower the dew point of the air 1^F below the surface temperature, the moisture evaporates.
    Occupants in a gym humidify the space, raising the air dew point. Raising the air temperature warms the temperature of the floor. Warming the air does not raise the dew point of the air. Temperature of the floor and the dew point of the air determines wet or dry floor.
    Trying to keep it simple.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  7. #33
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    Quote Originally Posted by RoBoTeq View Post
    OK
    As I see it, we are discussing an area that is being made very humid by still wet concrete. We have two decent sized cooling systems to remove moisture from the air, but in doing so, the air becomes colder and therefore contains less moisture.

    At the same time, as we remove moisture from the air, the moisture laden concrete floor is giving up its moisture to the air as it dries out. A problem is that while drying out the air, the air became too cold as well as the floor became too cold. The colder air contained less moisture to remove and the cold floor beaded up the moisture being brought to the surface without that beaded up water being able to evaporate into the cooled air.

    So, heat is turned on on one system, heating the air enough that it could absorb the excess moisture in the room, becoming heavily laden with moisture. At the same time, the other system was still cooling, so that the now warmer, more moisture laden air moving across the evap coils condenses the moisture content before being moved back into the room as cooler, less moisture laden air.

    With each cycle, the cooling system removes moisture from moisture laden air being heated by the other system. As the moisture in the air is continually being removed, the moisture in the concrete floor is moving to the surface, beading up and then evaporating into the now warmer and dryer air, once again saturating the air to go through the cooling system for more moisture removal.

    Is there anything I am missing for the process that was going on with this specific issue?
    As the moisture evaporates from the concrete surface/water cools and the air dew point rises. The cooler the surface temp, the slower the evaporation rate. The higher the dew point of the air, the slower evaporation rate. When the temperature of the water lowers to the dew point of the air, evaporation stops. If the temperature of the water/concrete becomes lower than the dew pont of the air, condensation of moisture in the air will add to the surface water. Concrete will not dry to a wet surface or bead up. Warming the concrete will warm the air and raising the temp/dew point of the air.
    Warming concrete and reducing the dew point of the air are basic. By high velocity air circulation at the surface of the concrete, the evaporation of the moistue and warming of the concrete are increases dramatically. Also consider that wet bulb temperatures are even lower than the air temperature.
    Drying materials is a science. Warming of materials and reduction of the dew of the air are the key. Air velocity at the surface of the material plays critical role in the drying rate.

    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  8. #34
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    Dec 2003
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    I think you described it pretty well robo.

    And if you only had one unit you could have ran the ac and heat strips at the same time. If it was cold outside during this process you prolly wouldnt need to run the ac if you are introducing some oa into the building. Seems like i remember Adam saying in the video the air in the building was 53deg/50%rh which is pretty dry. I wonder if the coil was actually wet as you would have to get 53d/50% air pretty cold to remove any water from it. You could plot the dew point of 53/50% air and if you ac unit is not getting the air colder than this running the ac would be doing more harm than good. On a winter day I would of prolly just cranked up the heat.

    I dont see how you could predict how long the drying process would take if the concrete is giving up moisture. That just depends on how much moisture is in the concrete. All you can do is put warm dry air next to it.

    This is a good discussion topic and thanks for posting your video to get it started! There was a member named Carnak who passed away a few years ago that could really explain this kind of stuff. TB and robo are pretty good at explaining it to.

  9. #35
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    I take a back seat to others on this thread because I simply do not deal with as much diversity with humidity issues that some others here do on a daily basis. It seems that every time I get it figured out for a specific situation, by the time something similar comes up again, I've forgot or get mixed up about what I previously learned. There are quite a few variables when changing substances from liquid to gas and from gas to liquid. Temperature values change simply because of the latent heat required to change the state of a substance. In this case, the floor surface cools due to the evaporation of the moisture on it, which changes the evaporation process at the surface of the floor even though the temperature has not changed. At the same time, the dew point of the air is changing as the floor surface moisture evaporates. So the evaporation process in this situation is constantly changing fairly rapidly. This is what makes plotting the conditions of this area on a psychrometric chart more interesting, and why doing so allows for a better understanding of why what is occurring is occurring.

    As a novice, I would have a much better understanding of how a psychrometric chart is used for a practical application after having listened to the OP's presentation.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #36
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    Aug 2004
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    Just to throw a monkey wrench into the discussion:

    Masonry materials such as concrete, brick, mortar and stucco are massive water storage systems. You can dry the surface and not appreciably affect the total water storage capacity. Moisture will move by vapor diffusion from a high mc to a lower one. When you dry the inside, you attract water stored in the concrete. Therefore, as fast as you're drying the surface of the concrete more and more water is passing by into the room air----until you hit dewpoint. If you have soggy concrete and lay ceramic tile over it, then dry the air above it, you will attract moisture to the surface of the tile and conceivably cause the tile to fail soon after installation. Solutions are first to keep the water out of the building then manage the drying process. Are there rain gutters on the roof? Do they discharge at the foundation or well away from the building? Is the grade away from the building or does it tend to trap water against the foundation? Are there plants, bedding and mulch against the building that store water? how is the surface water run-off? Have the DVW pipes been scoped and a leak detection dye been used?

    Building dehumidification is fine but only when it has a chance to succeed.

    As for the presentation, I thought that as a technical trainer, he did rather well. Work on not fidgeting. Use a laser pointer against a screen rather than your arm. Use a large white board or telestrator to sketch. When not speaking about a slide, use the 'B' button to blank the screen dark or 'w' to make it white and brighten the room. Leaving slides in place tends to draw protracted attention to the slide rather than to what you are saying. Use dramatic pauses strategically to make a point. Don't ask vague questions because nobody wants to look like a fool even when they offer an intelligent answer that happens to differ from what you wanted.

  11. #37
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    Quote Originally Posted by hearthman View Post

    Masonry materials such as concrete, brick, mortar and stucco are massive water storage systems. You can dry the surface and not appreciably affect the total water storage capacity. Moisture will move by vapor diffusion from a high mc to a lower one.
    Several years ago I took a two hour class class given by a cement guru ans was informed that cement took around around 20 years to fully cure. My assumption was that cure and dry were one in the same, this is were i could be wrong Ive honestly not given it much thought over the years. Anyhow this statment leads me to belieave that concrete once hardened does not readily give up or absorb moisture, atleast not on a pratical time scale.

  12. #38
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    Quote Originally Posted by hearthman View Post
    When you dry the inside, you attract water stored in the concrete. Therefore, as fast as you're drying the surface of the concrete more and more water is passing by into the room air----until you hit dewpoint.
    That only applies for recirculating air that is not providing any dehumidification, correct?
    ## + years in the field never made you a know-it-all This industry is far more diverse than you are

  13. #39
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    http://www.cement.org/tech/cct_floors_moisture.asp

    http://www.buildingscience.com/docum...floor-problems

    http://www.buildingscience.com/docum...bout-diffusion

    A lot depends upon how the concrete was laid, with or without vapor barrier, etc. Note Dr. Joe's cure of a wet slab by using Ditra dimpled membrane under the tiles.

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