Ask Lemmy

I think that the evaporation in theory is able to cool the room, the heat energy is transferred into launching a bunch of water molecules airborne so to speak. Hanging some wet towels around would also do that.

However, the performance of such small devices is probably not sufficient to significantly cool a room, and it has a lot of drawbacks (filter gets mouldy easily, ...)

Here's an excellent video about these swamp coolers: https://youtu.be/2horH-IeurA (he has many videos on heat pumps and stuff)

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.

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's not AC, it's called a "Swamp Cooler", and yeah, it will work for a limited time, to a small degree, especially in dry conditions.

It's like setting up a fan to blow across a block of ice, yeah, it works until the ice melts.

https://www.nytimes.com/wirecutter/blog/do-swamp-coolers-work/

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.

Swamp cooler, they do work, but raise the humidity, which might cause increased mold growth

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.

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.

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.

These work so long as the air around them is not humid already

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.

I have one similar and it does work, granted the ambient humidity is less than around 50% I live in the desert in northern nevada and we cool our house with a huge one and I have a small desk size one I use in my room. Most of the time our ambient humidity is in the mid teens here so they work rather well.

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.

Like others said, this is an evaporative cooler aka swamp cooler. It takes energy to convert liquid water to water vapor. https://en.m.wikipedia.org/wiki/Evaporative_cooler

You're wrong, your spouse is right.

Thermal energy is required both to raise the temperature of a mass of (in this case) water, and additional thermal energy is required to change its state from liquid to gas. This additional thermal energy is spent without creating any actual temperature increase, but it had to come from somewhere.

In this case, the thermal energy for the state change came from the surrounding air. The energy didn't come from changing the state of the air, so it must have come from lowering the temperature of the air.

As others have noted, this only works in low-humidity environments. If the air is already saturated with water vapor, no more evaporation will occur. This is why high-humidity environments feel hotter: your sweat isn't evaporating to cool you off.

Why not just try it for a day or two and see if it makes any difference?

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

That is not an air conditioner. An air conditioner needs a compressor, evaporator and condenser, this has none of them. What you have is a desktop evaporative cooler. Your theory of how it works is correct, the energy to evaporate the water is pulled from the air, that cools the air. But yes, the heat and moist air still stays in the room. Note, this only works in places with DRY air. If you are in a tropical location with humidity, this will not work because the air is already close to saturation with water.

There are large rooftop versions of this called swamp coolers, installed in places that have short hot dry summers, they work because the heat is still transported outside the building.

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.

https://en.wikipedia.org/wiki/Evaporative_cooler

You do do the same thing.

However, I have a small evaporative cooler which can evaporate 5 gallons of water a day. You aren't gonna do that yourself, and it'd drench you in sweat.

They're a couple of times more energy-efficient than air conditioners, though more maintenance heavy and require being in a dry climate. They also (normally) require outside air coming in, which is nice in that it keeps carbon dioxide levels down but means pollen or whatever too unless you filter that.

sauna

If you're using an evaporative cooler correctly, you have to keep (dry) outside air coming in so that it doesn't just act like a giant humidifier.

FWIW, you can actually use what's called an "indirect" evaporative cooler. That has outside air come in, go through an evaporative cooler to cool it, then sends that through a heat exchangerthat dumps heat from inside air into the heat exchanger, then sends the moist air outside without increasing inside humidity.

You can even extend that to use the cooled, humid air as the input to the "outside" side of an air conditioner's heat exchanger that dumps best to the outdoors. That is basically an indirect evaporative cooler plus a heat pump, a "hybrid" air conditioner, which will boost the air conditioner's efficiency.

Unfortunately, I don't see much by way of small indirect evaporative coolers or small "hybrid" air conditioners on the market, though it's not technically-complicated to build one. Seems to be done by large commercial installations.

I expect you're either both right or wrong, depending on how you want to look at it.

Forget quibbling over the filter, the air, the fan, whatever. Just consider yourself in a closed system.

Evaporation is an endothermic reaction. Energy is "used" as part of the state change. This energy comes from the surrounding environment, but the temperature of the water does not change during evaporation.

The ambient energy expended reduces the heat in the environment. Less heat in the same materials will result in a lower temperature, which is to say that evaporative cooling is real. So the Mrs. is correct.

Does THIS device provide enough to actually meaningfully cool your space? Tough to tell. You could weigh how much water you're evaporating, look up how much energy that expended

https://www.engineeringtoolbox.com/water-properties-d_1573.html

And then try and rough out how that would translate to a cooler room with specif heat capacity of air....

But honestly I'd probably just try and ignore all the interactions and just use a thermometer at the output of that thing to see if it's at all different from the ambient temperature of the room.

I have a swamp cooler going right now on my roof. It works just as good as AC but at a lower energy/money cost.

It's not an air conditioner. It's a swamp cooler.

You owe your wife an apology.

https://www.youtube.com/watch?v=URxSoHpwIz8

You sound persnickety. You'll probably like this persnickety video about swamp coolers

Some massive computer data centers have a giant version of this as a backup system in case the main cooling system goes down.

The one place that i know can use more power than 6000 homes and all of that power turns into heat. Cooling is critical and they need lots of it. They have a giant water tower that they can dump through the chillers if they need to.