Desktop A/C unit. This can't possibly work! Prove me wrong?

PlutoniumAcid@lemmy.world to Ask Lemmy@lemmy.world – 151 points –

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.

  1. Spouse insists that the AIR gets cooled by evaporation.
  2. I say the FILTER gets cooled by evaporation.
  3. Spouse says the cooled filter then cools the air, so it works.
  4. 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!

78

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)

Aw. I was going to post the link to his video, but you beat me to it.

But yeah, Technology Connections makes some excellent and informative videos. To anyone else who sees this: If heat pumps, refrigeration, or climate control technology aren't your cup of tea, he also covers older technology based around electromechanical designs (as in, pre-dating microcontrollers and programmable logic) and analog media recording devices.

I mean even if they aren’t your thing you should check out his videos.

I remember watching a 20 or so min video on an antique toaster and since then I’m also pissed at the inferior toasters of today.

His videos are gold.

Can someone please explain why toasters aren't made like that anymore?? I would happily forgo the led and the obnoxious ding they make to have them make the toast perfectly every time

That may have been my intro video to him. Can't go wrong with any topic, no matter how trivial it may sound. You will come back afterwards saying, wow, never knew that.

, and it has a lot of drawbacks (filter gets mouldy easily, …)

The more immediate drawback is after running it for a little bit, you'll lose the ability to sweat.

Well, you'll still sweat, it just won't evaporate due to high humidity.

It's at best a very short term solution before it starts making it worse

Plus if you have AC then the AC has to dehumidify the air first before it can cool it.

That isn't how AC's work. They work by using the fact that a phase change between a liquid and a gas is endothermic. It turns a refrigerant into a gas and that sucks heat from the air in your house and then pumps that gas outside to cool off with your compressor, moving heat outside. (Someone correct me please if I got details wrong). The act of pulling that heat from the air into the phase change cools down air and water condenses out of the air, dehumidifying it. Fun fact AC's weren't designed for our comfort, some facrory needed less humid air for their product, us lazy workers cooling down a little is a side effect.

Unless you are in a dry climate. Our house is cooled almost entirely off of a swamp cooler (small window unit for the bedroom) and the humidity is never noticeably high.

Gotta live in a desert for that. If not yeah swamp coolers are very limited.

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'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’s not really like that. As long as you have a limitless supply of dry air, and a limitless supply of water, it will work indefinitely. If you seal the room, you will eventually hit 100% humidity. If you run out of water, well that’s the end of that. But if you don’t, then you will get cooling that is the result of water phase changing from a liquid to a gas. With a small unit you won’t cool a room, but it will feel nice blowing on you, and it will be cooler than a fan. How much cooler I can’t say. I wouldn’t personally bother with a small unit like this.

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.

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

It's been crazy hot and humid this past month where I live. This thing would kill me

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.

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.

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?

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

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.

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.

I'm renting a house with a swamp cooler. I'm sure they can be nice, but the effectiveness of mine ranges from "barely" to "not at all."

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.

You owe your wife an apology.

I have very good conversations with my spouse, and this is an ongoing one to figure out what's really at play here. No apologies needed in either direction.

That's good, with a sense of humor like that you need to be very careful.

1 more...

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.

I've had one before. They are def not a good enough replacement for AC when you really need it, but they're not useless, either. They certainly don't work like AC does, you are right. They can make you feel a little cooler if you're sitting in front of it. lol

Pro tip: Dowse the pad insert in water and put it in the freezer. You can also put an ice cube or two in the water reserve. Both of these things maximize the "cooling" power, but again, you can't really expect it to cool a whole room.

Thank you for this. I admit that I smiled at your tip to freeze the soaked filter, because freezing it requires the fridge/freezer combo to do so, and where does that unit vent its heat? Into our living space, of course.

So it's a closed loop with losses, and I found that funny.

What do you think they do in prison when the AC burns out?

Fix the AC?

AC to run a prison can cost millions of dollars and take weeks to get shipped new parts. That's the DOC for you, they are very slow to approve anything.

I think you're exactly right. That's a humidifier.

Unfortunately the spouse is right in this case. Water takes heat/ energy to evaporate. We use the same mechanism to cool ourselves with sweat. The evaporating water pulls heat out of the air and cools it.