Wikipedia:Reference desk/Archives/Science/2019 February 11

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February 11

Dehumidifying ability of these new-fangled inverter air conditioners

I know the regular aircons dehumidify by piping all that condensed water out of the house. But they cannot do it in the winter. Can these new-fangled inverter air conditioners dehumidify in the winter? Anna Frodesiak (talk) 09:19, 11 February 2019 (UTC)

An air conditioner tends to act as a dehumidifier when chilling the air. The difference between normal and inverter air conditioners is discussed here and can be noticed as smooth continuous control of motor speed in inverter models in contrast to abrupt on/off switching in normal models. There is no reason to expect more effective dehumidifying in winter from either type of air conditioner. See Dehumidifier for other types of dehumidifier. DroneB (talk) 10:00, 11 February 2019 (UTC)

• No.

A dehumidifier works by exploiting the concept of relative humidity. The 'carrying capacity' of moist air is related to its temperature: warm air can carry more moisture. So if you pass warm, moist air through a machine that cools it, the reduced carrying capacity may achieve 100% relative humidity, the air is now saturated and the excess moisture condenses out (as frost on the evaporator or 'cold plate').

This becomes difficult when the ambient temperature approaches the temperature of the cold plate: there is little change of temperature to work with, thus little change in saturation humidity. The cold plate may also acquire a coating of ice which is now too thick to melt away completely during the dehumidifier's operating cycle, or a refrigerator needs 'defrosting' as this now-permanent ice layer acts as an insulator.

The inverter system is purely electric. It doesn't change the thermodynamics of the refigerator system at all. For the typical vapor-compression refrigeration system, what it does is to allow the motor's power to be efficiently adjusted to represent the ideal power needed, rather than running at the same power all year round. It can (depending on how much your climate varies) provide a substantial energy saving in Spring and Autumn, especially in South East Asia, when humidity is particularly high but the temperature is not at its Summer peak.

I'm in coastal Europe. Where our Winters are both cold and wet. I have a (large, by local standards, as we don't generally have domestic aircon) set of dehumidifying equipment in my workshop, to control damp. I wish that I could improve its performance in cold weather, approaching freezing. Andy Dingley (talk) 10:35, 11 February 2019 (UTC)

Thank you so much, Andy Dingley. You must find the damp horrible. Is it severe?

So yes, this is what I gathered from the articles, and the rather unclear blogs out there. The people in the appliance shops here are, if you'll pardon my language, full of it.

We also live in a place with a bit of cold weather and terrible damp at that time. January has a daily range of maybe 8-20 C with terrible wet that soaks the floors and rots the walls. Not all of our space matters, but there is about 80 sq m. that would be great to keep dry. So, do these humidifiers work? Many many thanks for any experiences with them you can share. Anna Frodesiak (talk) 12:02, 11 February 2019 (UTC)

• Dehumidifiers can work well, but only if the temperature is at least a few degrees above freezing. Their energy cost is not small either. Mostly it's important to try and dehumidify an enclosed and sealed space, rather than to have more cold damp air coming in from outdoors. If you can seal a space (and for a cheaply constructed workshop, this can be difficult) and keep it warmer, then they can be effective. If you are going to heat that space electrically anyway, and it's going to be heated regularly (i.e. every working day) then the cost of running them isn't too bad, as this energy goes to heat anyway.

My older system, before I insulated the workshop, relied on sealed steel cabinets for tool storage, and just dehumidifying inside those. Andy Dingley (talk) 12:08, 11 February 2019 (UTC)

When I lived in Norway, I just followed what everyone else did. When the sun came up around noon, we'd open all the windows in our apartments and leave them open for about 20 minutes. Then, we'd shut them again. It takes another 20 minutes or so for the heater to catch up. All the humidity we created was sucked out the window and all that dry cold air was sucked in. No problem. I find it odd that the same thing doesn't happen now that I live in the US. I have to run a humidifier all winter because the air is so dry inside that everyone gets sick. 209.149.113.5 (talk) 12:55, 11 February 2019 (UTC)

This depends where in Norway (and in the US) you are, but generally a continental climate will get colder and drier in the winter. A maritime climate will stay moist in the winter. Depending on weather patterns, you might (as we do) get very wet in the winter. Although Norway is largely a continental climate, most Norwegianst live in the coastal strip, where their climate is closer to the maritime. Andy Dingley (talk) 13:06, 11 February 2019 (UTC)

It may also depend on the type of heating system you have. Different heating systems have different effects on indoor humidity. When I grew up, we used a wood-burning stove, which tended to drive all of the humidity up the flue; to compensate most people used to keep large pans of water on top of the stove. Nowadays, I live in a more modern house with a forced-air system, it seems to dry the air less; though I also live in a more generally humid climate, so that may have more to do with it. If you live in a place with steam heating, perhaps that kept the indoors more humid in winter. --Jayron32 13:40, 11 February 2019 (UTC)

Why would steam heating keep things more humid? The outside of the radiator doesn't know there's steam inside? Anna Frodesiak (talk) 00:36, 12 February 2019 (UTC)

Andy, when humidity goes up the flue, doesn't that mean moist air is being sucked into the room to replace that chimneyed air? Anna Frodesiak (talk) 00:36, 12 February 2019 (UTC)

Outside air where it is cold holds less humidity, so as it is sucked into the house, replacing the warmer air sent up the flue/chimney, it will tend to be much drier. The air replacing the warm air is a lot drier. Also, I was speculating based on the idea that heating systems that don't involve a flue/chimney will tend to retain moisture better. --Jayron32 12:15, 12 February 2019 (UTC)

Only where outside air is very cold (continental). Cold outside air (maritime climate) can still be just as wet as inside air. There is no real difference in the carrying capacity of air for moisture until the saturation point is reached. Andy Dingley (talk) 12:35, 12 February 2019 (UTC)

Right, but that carrying capacity is a direct relationship with the temperature. So colder air will tend to be drier than warmer air, with all other things being equal. Colder maritime air is still drier than warmer maritime air, and colder continental air is still drier than warmer continental air, on any given day. No one is picking up their house and moving it thousands of miles to a new climate to make these measurements. No one is time traveling between a humid day and a less humid day to make the comparisons. We're comparing the warmer air inside of a house (with people living and breathing out all of that moist air, water evaporating from toilets and sinks, boiling pots of water for pasta on the stove, etc.) with the air outside the house, on the same day, in the same place. I suppose we could construct a very specific set of conditions where a colder day outside is wetter than the same warm air inside, but that's an unusual set of conditions that doesn't bear mentioning when talking about what is usually going to happen. --Jayron32 13:06, 12 February 2019 (UTC)

• Only the maximum carrying capacity of the air is strongly related to temperature (there's a smaller effect related to the ability of nearby bodies of water to vapourise). So colder air doesn't "tend to be drier", only if it gets cold enough to reach the limit at saturation point. In the maritime climate it's rare to go much below freezing point near the coast, and in that range the absolute humidities don't change by much. It may even feel more moist with falling temperature, as relative humidity increases (for the same absolute). This is a common effect in Wales and in Japan, the coast of Finland anomalously doesn't behave the same (the Baltic is peculiar in several ways), and it's quite different in Chicago because their climate is predominantly continental, even alongside the Great Lakes and so they still see their "cold dry winter" effect. Andy Dingley (talk) 13:19, 12 February 2019 (UTC)

• Yes, but dew point cannot exceed actual air temperature, so even if it only gets down to 5 degrees Celsius, that still means the air has at most the moisture of 5 degree dew point air. It will almost never be moister than it would otherwise be (at the same time and place) of air inside a house at say 25 degrees C. That inside air will always be at least at a 5 degree C dew point, regardless of what the actual temperature is, and for any number of factors, that warmer indoor air will tend to be always at least somewhat higher dew point. 5 degrees C dew point is very dry air at any temperature, in any conditions, so regardless of how close you are to the coast, your air is still drier outside than in, and still dry enough to cause some discomfort to the average person, as noted here and here and here, dew points below about 10 degrees C are starting to become uncomfortably dry. The deal is, the outdoor temperature sets the floor at which the dew point will be, not the ceiling, and since it is warmer inside, the dew point will usually always be higher indoors, leading to in general more comfortable conditions. This will change depending on the HVAC system of the building; depending on how it circulates air, replacing warmer (and usually more moist) air inside with colder (and usually more dry air) from the outside will tend to dry a house faster than a house which circulates the inside air around itself. That's why flue-based circulations (which by their nature, pump inside air to the outside, and as such must replace that air with colder drier outside air) will tend to dry the inside out more than closed-loop heating systems. See this article, which looks at some of the humidity differences between vented and non-vented heating systems. This article has some general recommendations on maintaining home humidity at comfortable levels in both summer and winter. --Jayron32 16:22, 12 February 2019 (UTC)

Thanks folks. I do understand the first part, but I got lost during the second. But don't try to explain it to me further. That would sort of be like trying to explain magnets to a lemur. But I think I get the basic idea now. Many thanks and stay dry. :) Anna Frodesiak (talk) 22:02, 17 February 2019 (UTC)

Aside

Does quora do discussions like this where people like me learn, or only upvote responses to questions? That site is blocked here in lovely but damp Haikou. Anna Frodesiak (talk) 00:36, 12 February 2019 (UTC)

Why do galaxies collide?

I don't quite understand why galaxies collide. If space is expanding, shouldn't galaxies be getting further from each other, not closer? I guess some galaxies are moving faster than others, but what would cause that? A Quest For Knowledge (talk) 18:03, 11 February 2019 (UTC)

You see, Jimmy, when two galaxies love each other very very much, they sometimes...

Seriously, though, the expansion is an average. Individual galaxies may be traveling towards each other. --Guy Macon (talk) 18:45, 11 February 2019 (UTC)

The metric expansion of space is a weak effect, and only affects objects whose gravitational attraction is weak. Within any galaxy cluster, the galaxies are bound by gravity, so by definition are not affected by the metric expansion of space. The four fundamental forces are sufficient to prevent expansion on scales small enough for them to operate, which is why your body is not slowly expanding; the electromagnetic forces of attraction holding your atoms together prevents it. Even the very weak gravitational force is enough, under all but the longest scales, to overcome the expansion. Within, for example, the Local Group, which are the galaxies to which the Milky Way is gravitationally bound, we see that effect is a most pronounced way with our nearest neighbor the Andromeda Galaxy; the two galaxies are racing towards each other at an impressively fast rate, and will collide in roughly 4.5 billion years. We only really see Hubble redshift on objects outside of the Milky Way's local gravitational influence, which probably includes the Local Group, and the rest of the Virgo Supercluster/Laniakea Supercluster, which includes all galaxies which are affected by the Great Attractor gravitational effect. However, though the Laniakea Supercluster contains some 100,000 identified galaxies, there are an estimated 10,000,000 superclusters in the observable universe, and while the Laniakea Supercluster is among the larger ones we observed, you can see how absolutely stupidly huge the observable universe is; while out local group of 100,000 or so galaxies is gravitationally bound, and not necessarily expanding from each other, further galaxies from among the billions and billions of other galaxies are not, and it is in those galaxies that we see the Hubble flow away from us in ways that allow us to observe the expansion of space. I hope that helps! In summation: galaxies will collide because of gravity, which is on a close enough scale, a strong enough force to overcome the expansion of space. --Jayron32 18:54, 11 February 2019 (UTC)

And the last phase of this happens as follows. When galaxies move close to each other the tidal forces will lead to some of the energy from the center of mass motion (kinetic energy of the center of mass plus potential energy) dissipated into kinetic energy of individual stars relative to the center of mass. This can then cause the galaxies to no longer have enough energy to escape from each other and become bound. They'll then enter into an orbit around each other, but they'll keep on losing energy, ending up periodically colliding with each other as is the case with our galaxy and the Andromeda galaxy. Eventually this will result in a complete merger. Count Iblis (talk) 18:58, 11 February 2019 (UTC)

Additionally there have not been that many "collisions". Interacting galaxies are easily found because they visibly "stick out" very much, but in comparison to the total number of galaxies their count seem to small to call "common". 3,000 of 40,000 "interactions" according Galaxy Zoo#Galaxy mergers and interactions and only a fraction of thous actually "collide" or more exactly "merge" if i understood that right. --Kharon (talk) 22:58, 12 February 2019 (UTC)

You've misunderstood quite comprehensively. To take one insignificant example, Galaxy_merger mentions two to four taking place with the Milky Way as we speak. It's a completely normal and expected part of galactic evolution. Henry^Flower 08:52, 14 February 2019 (UTC)