Can air conditioners lower relative humidity? If so, how?
True or False:
1) AC’s can lower indoor air RH. T/F
2) Air that has just passed over the cooling coil will often have a very high RH (much higher than room RH), even when condensation is occurring on the coil and moisture is being removed from the air stream. T/F
3) Two air masses that thoroughly mix will form a new air mass that has a dry bulb temperature between the dry bulb temperature of the two initial air masses, and an RH that is between the RH of the two initial air masses, assuming no external energy exchange. Between doesn’t mean halfway between. T/F
4) An AC only lowers RH because the new air mass gains sensible heat from the external environment (i.e. the heat gain of the structure), and therefore the relative humidity can be lower than either starting condition. This takes place over time and effectively means that the final dry bulb temp of the two mixed air masses is higher than if no external heat was gained. The higher dry bulb temp with the given absolute humidity lowers RH. T/F
Are all those true, or are some false?
The last one needs some work, but is my attempt to explain how RH can be lowered, assuming all previous statements are true (which they may not be).
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The air coming out of the air conditioner will be at 100% RH. If you ran the air conditioner continuously and -- and this is a big if -- if you weren't adding heat somehow to the room, eventually all the air in the room will have gone through the air conditioner, and will be at the exit temp of the AC and 100% RH. You will have increased the RH of the room even though you removed moisture.
However, think of an alternative scenario: the AC is running continuously, but the room is being heated an amount equivalent to the sensible heat removal of the AC, so the temperature of the room doesn't change. The AC is removing moisture, the temperature of the room doesn't change, the RH is dropping. Eventually the room will reach the dew point of the air leaving the AC, and no more dehumidification takes place.
"The AC is removing moisture, the temperature of the room doesn't change, the RH is dropping."
I thought of this after posting the question, and it certainly seems like a simpler way to frame it. More obvious.
It skips over needing to ask the third question above, which may be good, but the third question is really what drove this question, hence the round about conceptualization I came up with.
But as far as answering the question in the title, what you say is the better way.
What is your goal with this question?
Just a conceptual question. Practical implications could be discussed but I'm not making any point in that regard.
It's really the third question about the RH of the two mixed air masses that drove this. I believe it is true, but don't know how to prove it. Mostly wanting confirmation on that.
Assuming it is true, it got me thinking about how an AC, which puts out high RH air, could lower room RH. Thus the conceptualization here presented as #4. It's a bit clunky. Maybe others have a more streamlined offering that still retains the concept in #3. Or perhaps the idea in #3 is just an unnecessary complication.
I think my take away (which is obvious in hindsight) is that without the heat load on the structure, cooling the air with an AC (even when reducing absolute humidity via coil condensation) will raise RH.
Perhaps one semi-practical takeaway is that one can overcool (remove more moisture) and then introduce 'reheat' to increase dehumidification capacity. This is also what a standalone dehumidifier essentially does, the reheat being the rejection coil.
A good ol' thought experiment. Got it!
If you need precise control over temp and humidity and the cost of operation is irrelevant. Then one installs a reheat system that reheats the air after the moisture has been lowered. Generally only museum requires and can afford such systems.
Maybe Bill will come along and tell us how they handle this problem in data centers.
Walta
This is what a Daikin Quaternity does. It has two coils in the head, one for heating and one for cooling.
Data centers have so much heat constantly being injected into them that they are basically a constant source of reheat.
My two data center/AC related anecdotes:
- When I used to use public transportation to commute to an office building that also had a data center testing lab, on the hottest summer mornings, I would make a stop in the lab and stand over a perforated floor tile for a quick cool down before going to my desk.
- Years ago for a very critical project, I ended up flying last minute to a major data center and worked in a cage non-stop the entire day. I ended up perhaps as dehydrated as I'd ever been, and I chalk this up to the dry air sucking every bit of moisture out of me.
Contractors who need to dry out a damp home as quickly as possible (for example, after flooding from a plumbing leak) sometimes turn on the air conditioner and furnace simultaneously, at the highest possible settings, and let the two systems fight it out. The result is dry air.
Tyler --
I think this will help you. I've added another sheet to my Dew Point Calculator spreadsheet at
https://docs.google.com/spreadsheets/d/1LONsi_1Fb6u4PsDQjzrbHn7ToX7yQHHEvr5OxExVwQc/edit?usp=sharing
The sheet is called "Sheet4." I model mixtures of air at different temperatures and relative humidities. For each mixture I have two parts, Part A and Part B, and then the resulting mixture for portions ranging from 0% to 100% part A.
I did four scenarios, you're welcome to edit those and put in your own scenario. I could not come up with a scenario where the mixture had a lower RH than both of the inputs. I could come up with a scenario where the RH was higher than both of the inputs. For example, if you mix air at 35F, 50%RH with air at 70F, 50% RH in 50-50 proportions, the resulting mixture is 53.5F and 60%RH.
This is great, thank you.
Looking at the psychrometric chart and seeing the upward bending shape of the RH curve, it makes sense that RH could become higher, but never lower.
What I discovered from your chart that I find really interesting is that if you draw a line from one starting condition point to the other, and then use the combined mix's new temperature (so for a 50-50 mix, half way) the point where that dry bulb temp line crosses the connecting line you've drawn is the new psychrometric point for the mixture. It also works when not a 50-50 mix, so long as you use the correct temperature.
So for the visually inclined, it is easy to see that if you draw a straight line from, say, 70F 90% RH to 50F 90% RH, you'll get a combined RH that is higher than 90% (around 95%), as the RH line drops down away from the straight connecting line.