Home mobile air conditioning: Is this a contradiction to thermodynamics?

[Edwina Lee]

TL;DR Summary

Why do portable air conditioners work?

I am seeing lots of brands of home portable air conditioners to cool rooms. They are sold through well known nationwide chains such as Currys, Argos in the UK. Customer feedback shows that they really work.

But why would they work??? Surely, these portable aircons need to take away heat from the room, and the heat can only go inside the machine!

Evidence: www.currys.co.uk www.argos.co.uk
search for : portable air conditioners

 

[Frabjous]

They have an exhaust tube that needs to be vented outside.

 

[Edwina Lee]

They have an exhaust tube that needs to be vented outside.

I just chatted to Currys' adviser, and he told me there is no connecting pipe to the outside. So I told him I'll ask a science forum why it works.

 

[Frabjous]

Take a look at the room picture.
https://www.currys.co.uk/products/d...77-eco-portable-air-conditioner-10227452.html

 

[Drakkith]

I just chatted to Currys' adviser, and he told me there is no connecting pipe to the outside. So I told him I'll ask a science forum why it works.

Then he doesn't know his product very well. There must be a way to vent the hot air outside of the room or else you get as much cooling as opening your refrigerator door. None.

 

[Dale]

There are two general types of these portable coolers. One is based on a heat pump and the other is based on evaporation. The heat pumps have a vent to the outside. The evaporative coolers do not.

The evaporative coolers work on the same principle as sweating. The heat is put into water which is converted from liquid to vapor in the process. They only work well in dry air, and when they are working they make the air more humid.

 

[Edwina Lee]

That must be it. They are only evaporation coolers.

 

[Dale]

That must be it. They are only evaporation coolers.

In the future it would help to link directly to a page on a specific example of the type of device you are interested in, rather than generic search instructions that led to multiple devices of different types.

 

[Edwina Lee]

Take a look at the room picture.
https://www.currys.co.uk/products/d...77-eco-portable-air-conditioner-10227452.html

Ah-ha! So there is a thick clunky ventilation tube connecting to the outside.

 

[Edwina Lee]

In the future it would help to link directly to a page on a specific example of the type of device you are interested in, rather than generic search instructions that led to multiple devices of different types.

Point taken, thanks Dale!

 

[russ_watters]

Using the search function on the argos site to look for "portable air conditioner", I see that most products are true air conditioners, but there is one "air cooler", which is an evaporative cooler.

In general evaporative cooling is not helpful for a humid climate like the UK's. But I was thinking it could be good for the rare heat wave such as they had last year when it reached 100F. Such high temperatures usually come with much lower humidity. Plus evaporative coolers are cheaper than air conditioners, so they wouldn't be an expensive item that's rarely used.

 

[sophiecentaur]

A poor arrangement for the exhaust tube to be exiting through a half opened door. The slot width can be estimated by looking at the overlap between the two sliding doors. I wonder if there's a suggestion that a proper vent hole in the wall could be better?

Plus evaporative coolers are cheaper than air conditioners, so they wouldn't be an expensive item that's rarely used.

Worse than useless in the UK. We tried on in a hot, upstairs lab and it just made people cold and sweaty instead of hot and sweaty.

But the AC idea should be the last resort. Shutters and thick curtains and some proper insulation in / on the walls should be the way to go. We have this climate crisis in case people have forgotten. A severe lack of education of the public makes everything worse.

 

[A.T.]

The evaporative coolers work on the same principle as sweating.

Or hanging your laundry to dry indoors.

 

[Mister T]

That must be it. They are only evaporation coolers.

This type is called a swamp cooler. They are not of much use in humid climates, but are effective in an arid climate. If you drive through the older sections of Tuscon, Arizona, for example, you see huge swamp coolers on every roof.

The portable ones you linked to have exhaust ducts and are true air conditioners.

The kind that mount on an interior wall and are connected via coolant lines to an outdoor coil and pump work much better, but are more expensive and require professional installation.

 

[sandy stone]

A poor arrangement for the exhaust tube to be exiting through a half opened door. The slot width can be estimated by looking at the overlap between the two sliding doors. I wonder if there's a suggestion that a proper vent hole in the wall could be better?

Note that whatever volume of exhaust air is vented to the outside, must then be replaced by hot, humid, outside air, no matter how it gets in. That makes this type of A/C only slightly better than useless, particularly in the type of humid environment that I happen to live in.

 

[DaveC426913]

I've had one of these for decades and they work great. I've lived in a virtually windowless loft with only six inches separating me from the blazing sun, and that was only possible because of one of these devices. (The only window is tiny and it's 15 feet from the bed. That's why I went with a portable.)

There's only one connection to outside. They pull air from the room and vent it outside while pouring cold air into the room. The exhaust is very warm (you REALLY want to insulate that vent hose). They are also effective at lowering humidity. They don't have a drain because they use the exhausted heat to evaporate and vent the accumulated water.

 

[sophiecentaur]

Note that whatever volume of exhaust air is vented to the outside, must then be replaced by hot, humid, outside air, no matter how it gets in.

You are saying that a one-pipe system with air coming in from outside is not as good as a system that uses two air cycles (effectively three or four pipes?) because it involves drier air already in the room being re-cycled and further dried? I guess that makes sense.
Thermodynamics can be confusing but is the COP better because the ΔT of the circulating outside air is less for a given heat exchanger temperature (I.e. room cooling power)?
Fridges seem to be a simpler system to appreciate.

 

[sophiecentaur]

I've lived in a virtually windowless loft with only six inches separating me from the blazing sun,

It's quite amazing that you (or a landlord) haven't been totally encouraged to go for serious roof insulation. (The insulation in a domestic freezer does a brilliant job with a few cm of wall thickness.) This is a worldwide problem and 'the system' just don't seem to acknowledge it. I'm not shaming you for your situation because insulation's just not in the culture; you've found a solution and that's good.
Temperature control in homes is a major factor in the energy crisis but the only way that governments seem to want to deal with it is to use heat pumps which are about the most expensive gadgets available. And AC is the archetypal heat pump, of course.

 

[hmmm27]

You are saying that a one-pipe system with air coming in from outside is not as good as a system that uses two air cycles (effectively three or four pipes?) because it involves drier air already in the room being re-cycled and further dried? I guess that makes sense.

"Two pipe"
outside(warm) to outside(hot)
inside(cool) to inside(cold)
(no air mixing)

"One pipe"
inside(cool+warm) to outside(hot)
inside(cool+warm) to inside(cold)
(inside air partially replenished by insiderer air)

You could design a unit that uses water as a hot-transport-medium, that has no air hoses. This will seriously annoy some people.

 

[sandy stone]

I suppose the kind of system that DaveC describes works best in an environment where the outside air being pulled in is comparatively cool and dry (don't you live in Canada?), and the main heat load is radiant from the sun. On the steamy Gulf coast where I live, I don't think they're so great.

The ductless split systems from, for example, Mitsubishi, and commonly retrofitted to older buildings in Europe, use Freon as a hot-transport-medium and have no air hoses. I think they work very well, but obviously they cost more.

 

[DaveC426913]

It's quite amazing that you (or a landlord) haven't been totally encouraged to go for serious roof insulation.

Its 95 years old. We were afraid to touch it.
Also, its a peaked attic, so no room.

 

[sophiecentaur]

Also, its a peaked attic, so no room.

That's awks. I guess silver painted shingles or tiles would not be accepted by the neighbourhood. Our house is early 1900s and has had loads of conversions so we have no possible common solution. We have some cavity walls, some lath and plaster and the top half of the sloping roof now has open cell foam between rafters. We also have two big areas of flat roof of different vintages. I'm very tempted to add an upper layer on top of the flat bits that would be worth a few kWh of insulation in winter but I can't find a solution that's acceptable. They keep worrying about condensation and damp but there has to be a solution.
So far, our problem has been cold in winter, rather than hot in summer but the foam in the loft has helped upstairs in very sunny summer days.

 

[sophiecentaur]

This will seriously annoy some people.

Sorry. I must be dim; why?

 

[DaveC426913]

That's awks. I guess silver painted shingles or tiles would not be accepted by the neighbourhood. Our house is early 1900s and has had loads of conversions so we have no possible common solution. We have some cavity walls, some lath and plaster and the top half of the sloping roof now has open cell foam between rafters. We also have two big areas of flat roof of different vintages. I'm very tempted to add an upper layer on top of the flat bits that would be worth a few kWh of insulation in winter but I can't find a solution that's acceptable. They keep worrying about condensation and damp but there has to be a solution.
So far, our problem has been cold in winter, rather than hot in summer but the foam in the loft has helped upstairs in very sunny summer days.

We lived in that old house for 30 years with only two tiny window air conditioners - one in the loft and one in the dining room. We put on the AC when we needed it.

We just this summer moved into a more modern house with central air for the first time in my life (my parents never had us in central air growing up onacccounta so many of us have allergies.) This is a new experience for me as can be seen here. It's either stuffy or freezing.

 

[hmmm27]

Sorry. I must be dim; why?

You're not the dim one : I didn't specify that I meant an open system : bucket of water into the hopper, hot water out the window (or other) - tiny hose ; exhaust fan/radiator not required. It would be considered a waste of potable water in some regions.

Though, I do wonder why water isn't used in a (closed) system : non-toxic, cheap and - at almost 3x the heat capacity of say R134 - 1/3 the flow rate, at least on the hot side. Run the cold side at partial vacuum.

 

[Mister T]

It's either stuffy or freezing.

Check the indoor relative humidity levels. If it's above 55% most of the time, that could cause this problem. A professional would have to be called in to check the coolant level and possibly propose other solutions. Even a new ac unit can come out of the factory with a low coolant level.

 

[DaveC426913]

Check the indoor relative humidity levels. If it's above 55% most of the time, that could cause this problem. A professional would have to be called in to check the coolant level and possibly propose other solutions. Even a new ac unit can come out of the factory with a low coolant level.

Check the other thread for deets.

 

[sophiecentaur]

Though, I do wonder why water isn't used in a (closed) system

Some while ago I came across Heat Pipes for cooling electronics. They just contained water under reduced pressure and a metal(?) wick, I believe. Safer than fancy CFCs and safe. I guess the only disadvantage would be the need for reduced pressure. I'll have to look up the required pressure to get it to work well.

 

[russ_watters]

Though, I do wonder why water isn't used in a (closed) system : non-toxic, cheap and - at almost 3x the heat capacity of say R134 - 1/3 the flow rate, at least on the hot side. Run the cold side at partial vacuum.

Per unit mass, yes. But checking a steam table, you'd need your steam at about 0.15 psia to get it cold enough to do air conditioning. 2,000 cu ft/lb.

 

[hmmm27]

Per unit mass, yes. But checking a steam table, you'd need your steam at about 0.15 psia to get it cold enough to do air conditioning. 2,000 cu ft/lb.

Ah, so very large coldside radiator required. (For some reason I previously thought it was in the 1/10atm range, not 1/100th : checked)

 

[sophiecentaur]

Ah, so very large coldside radiator required. (For some reason I previously thought it was in the 1/10atm range, not 1/100th : checked)

And this is a snag. Any self contained unit would have to use air because it involves only an air hose (or two), going outside with no heat exchanger 'outside on the wall'. These portable units are just so practical, compared with a whole system and the best system in most cases is surely to use good insulation, so the heat never gets inside in the first place. But you can't drive home with that in the car and begin using it in the minutes.

This issue will affect many people in Europe a lot more than it ever has so a re-think is necessary for house builders and (lol) the government.