Prove me wrong, please?
edit: thanks for all the great comments, this is really helpful. My main take-away is that it does work, but requires dry air. In humid conditions it doesn’t really do anything.
Spouse bought this thing that claims to cool the air by blowing across some moist pads. It’s about as large as a toaster, and it has a small water tank on the side. The water drips onto the bottom of the device, where it is soaked up by a sort of filter. A fan blows air through the filter.
- Spouse insists that the AIR gets cooled by evaporation.
- I say the FILTER gets cooled by evaporation.
- Spouse says the cooled filter then cools the air, so it works.
- I say the evaporation pulls heat (and water) from the filter, so the output is actually air that is both warmer and wetter than the input air. That’s not A/C, that’s a sauna. (Let’s ignore the microscopic amount of heat generated by the cheap Chinese fan.)
By my reckoning, the only way to cool a ROOM is to transport the heat outside. This does not do that.
We can cool OURSELVES by letting a regular fan blow on us = WE are the moist filter, and the evaporation of our sweat cools us. One could argue that the slightly more humid air from this device has a better heat transfer capacity than drier air, but still, it is easier to sweat away heat in dry air than in humid air.
Am I crazy? I welcome your judgment!
Swamp coolers are a thing. I don’t know about the physics, but they can actually cool a space if the ambient humidity is low.
Using ice water in the gadget you have will improve its performance, obviously.
swamp coolers work through evaporation cooling. the water absorbs some energy from the air as it evaporates. (essentially the water gets hotter, the air gets cooler.)
Too add to that, slot of that energy goes into vaporizing the water, so the average temperature is lower.
it’s not technically a net loss- some of that energy is lost as it escapes the system, but conservation of energy generally means that as the air cools down, the water gets warmer. it’s just that the water has extremely excellent thermal mass, meaning that the air appears to cool much more than the water gains heat. This is especially true if the water itself is cold to begin with. (ie, blowing it over ice cubes.)
This is not the dominant factor, and this system will still work even if the water is warmer than room temperature. The primary energy sink is the vaporization of water.
Vaporizing 1g of water takes 2257 Joules. https://en.wikipedia.org/wiki/Enthalpy_of_vaporization
The water does not change temperature during that process.
Heating 1 g of water 1C takes 4.184 J. To heat it from from 0C to 25C (about freezing to room temp) it thus takes104.6 J, much less than the 2257 Joules required to vaporize it. These numbers could be modified to properly account for the variace with temp, but the effect actually gets larger then I believe.
No energy leaves the system; it goes into changing the state of water and is stored in the water.
That’s a swamp cooler and is very commonly used in dry environments. It will help a lot in Arizona (well, maybe not that tiny thing, but a properly sized one) and not at all in Miami, due to the difference in ambient humidity.
It is a swamp cooler. It works, and works better in drier air, but it is not a heat exchanger. Most of the cooling is gonna be from the moving air.
Technology Connections has a video on exactly these devices that dives into how they work and what they can and can’t do. TLDW; you’re not wrong about the physics of cooling a room, though in some cases this little thing might make you feel a bit cooler.
I was about to share this! Always glad to see Alec get recognition!
It’s evaporative cooling. Grew up with them. They’re great. They aren’t A/C. They don’t worm when it too humid.
Great for dry desert geography.
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OP, where do you live? What is the surrent relative humidity of your house? The lower it is the more likely this thing is to work. It is a legitimate technology but they only work well in dryer places. A dry heat is perfect for this.
These are often called swamp coolers.
swampies down under mate
These work so long as the air around them is not humid already
The issue here is that your wife bought this thinking it’s an ac when it’s an evaporative cooler aka swamp cooler. They do work if you have low humidity. If you are in a humid area this definitely won’t work. Since the unit is small it won’t cool the entire room but she should feel nice and cool about 3 to 5 feet in front of it. She will need to make sure the wicking action is working to get the pads nice and wet, otherwise she will have to manually remove them to wet them.
Edit: I wanted to add that I have had a similar small unit before which is why I know that she needs to be 3 to 5 feet in front of it to hit the little target cool zone.
On point 4, the key part that you are missing is that evaporation /takes/ energy. The standard central air works closer to how you are thinking by the evaporator above your furnace taking heat to then be dumped out by the condenser outside. This is necessary because it is a closed system that must continually reuse the refrigerant.
Sweat, and the swamp cooler you have here, are not closed systems and therefore don’t have to “dump” heat. Energy was transferred to the water molecules to cause them to evaporate. As latent heat exists (Google this if you are still confused) the heat energy has been transferred to “evaporation” energy and so the heat can be reduced without breaking any thermal laws.
Basically the water on your skin or in the swamp cooler is like a wall that heat has to break down. The heat can do this, and does get through but has been reduced by the work and is therefore less strong (lower temperature.
There was no subtraction or addition to total energy when you look at the whole process. Heat energy was transferred to kinetic energy to cause the state change of the water.
Central AC has to dump heat to reuse the refrigerant. The swamp cooler doesn’t have to dump heat but needs to be refilled often as the evaporation of water takes matter away from the system.
Why not just try it for a day or two and see if it makes any difference?
Well, what this thing doe is moisten the air and thereby cooling it. So temperature down, humidity up.
If you live in dry areas, this is good, but if you live in more humid areas, this will only worsen the problem.
Don’t forget to air the rooms regularly (at night, if it is too hot during the day) to get the humidity out again - you don’t want to get over 60% relative humidity for a longer period.
Large versions of these are often used in greenhouses for temperature control, so they do in fact work as advertised.
Water is very good at absorbing thermal energy, far better than air. The moist pads are drawing thermal energy out of the air and storing it in the water, reducing the air temperature. Some water eventually evaporates, but in a gaseous state this water still retains the thermal energy it absorbed from the air, causing a noticeable decrease in air temperature, as well as a slight increase in humidity.
Put a very dry cloth in front of the cooler to trap water vapor, every now and again relocate the now damp cloth outside of the room being cooled, and replace it with a fresh dry cloth.
My partner and I started hanging up our clothes to dry on a dirt cheap clothes rack. Think we spent like $10~$15 on the whole setup. Anyway, we have it stood up on the hot side of the apartment and have a fan blowing on the wet clothes toward the coach / desk area in the living room. The thermostat says the room is about 5 degrees cooler, but the room feels more like 8 or 10 degrees cooler. Not sure how the physics works on all of this, but those dumb desk coolers sound like the same principle
The physics work exactly as you think. You’ve effectively turned your laundry into a swamp cooler.
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