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I don't think it's that simple at all. The principal human-appreciated effect of AC is the dehumidifying of the air when it's cooled. The dry interior air all

I don't think it's that simple at all. The principal human-appreciated effect of AC is the dehumidifying of the air when it's cooled. The dry interior air allows human perspiration to regulate temperature effectively. The AC doesn't have to cool all the bodies, it just has to keep the air dry enough.

the8472 7y ago
Don't ACs dehumidify by condensing the water? If so then it means they need to get rid of the heat of vaporization, the same amount of energy the humans shed by evaporative cooling.
- Scoundreller 7y ago
  Right, but it does mean you just need to be in a somewhat sealed room to remove the humidity instead of constantly fighting the ever-warming walls/floors/ceiling. Of course you need to exchange CO2 and O2 to stay alive.
  - wongarsu 7y ago
    But the people in the room will create more humidity that you have to constantly remove again. Humans burning calories to stay alive creates heat, and thermodynamics that you spend energy to remove it or see the temperature rise. Whether that heat is hot air or bound as humidity shouldn't make a large difference.
- newnewpdro 7y ago
  In a closed system sure, but I'd expect a larger building to have significant fresh-air return systems. In a scenario where you've filled the place with people, the interior humidity is going to exceed that of the exterior air and you'd just start dumping the moisture outside exchanging with the fresh, dryer air. It's a comfort regression sure, but not a everyone dies situation. The survival of the people doesn't come down to the AC heat pump keeping up, it comes down to supplying them enough water to drink and disposing of the humidified air. My point is just that it's the relative humidity that matters, and the AC system has some options there. I've lived through a desert summer in a small space without any AC at all. I'd just keep the windows closed as deep into the day as possible riding out the night's cooling until the air gets too humid indoors from sweat and respiration for the fan to have any cooling effect. At that point I'd open the windows for relief. The air coming in was usually much hotter than the interior air, since the space was well insulated and nights get cool, but it didn't matter because it's very dry air and perspiration resumes functioning. The situation is easily survived unless you run out of water for your person. Of course if the outdoor air is something like 100% humidity there's only the heat pump option, but that simply doesn't happen in deserts when it's hot out. It's a desert, not the tropics. In the desert the relative humidity shoots up when it cools down, the humid nights are the worst. But it's because they cooled down. Again, not a survival problem since it's not that hot, just uncomfortably muggy. One saving grace with deserts is they don't generally have any mosquitoes, so these humid nights can easily be spent outdoors without any annoyance.
  - wongarsu 7y ago
    > In a scenario where you've filled the place with people, the interior humidity is going to exceed that of the exterior air and you'd just start dumping the moisture outside exchanging with the fresh, dryer air. It's a comfort regression sure, but not a everyone dies situation. That's equivalent to opening a window without the AC doing anything. If that would work we wouldn't be in a "crowd everyone into a shopping center" type situation, the people would be just fine outside in the shade. > It's a desert, not the tropics. While half of India is in the tropics, this part indeed isn't. But next month the monsoon will put humidity at approximately 100% [1]. 1: https://en.climate-data.org/asia/india/delhi/new-delhi-30/ https://en.climate-data.org/asia/india/delhi/new-delhi-30/
    - newnewpdro 7y ago
      > That's equivalent to opening a window without the AC doing anything. Even when the air temperature at your fresh air intake is cooler than the air at your condensor coil? Wouldn't we assume no system would draw its fresh air from near the condensor coil? You still have a heat pump in the system, and there's a temperature difference between the air inlet and the pump outlet.
- saeranv 7y ago
  Yes, ACs are heat pumps, which means they're just moving heat from one area to another. So yes, the heat shed by human beings via evaporative cooling, is then dumped outside via heat of vaporization. Specifically the ambient indoor air heat is absorbed by refrigerant which is transformed into a superheated vapor saturated with moisture. When this vapor either touches a cooler surface, or has cooler air blown past it, it will dehumidify and exhaust the heat it has absorbed indoors to the outside. The mechanical work done by the AC (fans, pumps) is also being released as heat. So the total energy being dumped outside is therefore the heat that's taken from the inside plus the energy used by the AC released as heat.
saeranv 7y ago
It's more complicated then that. Take a look at this psychrometric chart showing the environmental bounds at which human comfort is guaranteed: http://comfort.cbe.berkeley.edu/ http://comfort.cbe.berkeley.edu/ You can see there that even at extremely dry conditions, if the operative (ambient + radiant) temperature is hot enough, it will cause heat discomfort. Even if the human starts sweating, that'll only reduce the temperature a bit so dehumidification isn't the only role of air conditions, sensible cooling is also a factor.

the8472 7y ago

Don't ACs dehumidify by condensing the water? If so then it means they need to get rid of the heat of vaporization, the same amount of energy the humans shed by evaporative cooling.

Scoundreller 7y ago
Right, but it does mean you just need to be in a somewhat sealed room to remove the humidity instead of constantly fighting the ever-warming walls/floors/ceiling. Of course you need to exchange CO2 and O2 to stay alive.
- wongarsu 7y ago
  But the people in the room will create more humidity that you have to constantly remove again. Humans burning calories to stay alive creates heat, and thermodynamics that you spend energy to remove it or see the temperature rise. Whether that heat is hot air or bound as humidity shouldn't make a large difference.
newnewpdro 7y ago
In a closed system sure, but I'd expect a larger building to have significant fresh-air return systems. In a scenario where you've filled the place with people, the interior humidity is going to exceed that of the exterior air and you'd just start dumping the moisture outside exchanging with the fresh, dryer air. It's a comfort regression sure, but not a everyone dies situation. The survival of the people doesn't come down to the AC heat pump keeping up, it comes down to supplying them enough water to drink and disposing of the humidified air. My point is just that it's the relative humidity that matters, and the AC system has some options there. I've lived through a desert summer in a small space without any AC at all. I'd just keep the windows closed as deep into the day as possible riding out the night's cooling until the air gets too humid indoors from sweat and respiration for the fan to have any cooling effect. At that point I'd open the windows for relief. The air coming in was usually much hotter than the interior air, since the space was well insulated and nights get cool, but it didn't matter because it's very dry air and perspiration resumes functioning. The situation is easily survived unless you run out of water for your person. Of course if the outdoor air is something like 100% humidity there's only the heat pump option, but that simply doesn't happen in deserts when it's hot out. It's a desert, not the tropics. In the desert the relative humidity shoots up when it cools down, the humid nights are the worst. But it's because they cooled down. Again, not a survival problem since it's not that hot, just uncomfortably muggy. One saving grace with deserts is they don't generally have any mosquitoes, so these humid nights can easily be spent outdoors without any annoyance.
- wongarsu 7y ago
  > In a scenario where you've filled the place with people, the interior humidity is going to exceed that of the exterior air and you'd just start dumping the moisture outside exchanging with the fresh, dryer air. It's a comfort regression sure, but not a everyone dies situation. That's equivalent to opening a window without the AC doing anything. If that would work we wouldn't be in a "crowd everyone into a shopping center" type situation, the people would be just fine outside in the shade. > It's a desert, not the tropics. While half of India is in the tropics, this part indeed isn't. But next month the monsoon will put humidity at approximately 100% [1]. 1: https://en.climate-data.org/asia/india/delhi/new-delhi-30/ https://en.climate-data.org/asia/india/delhi/new-delhi-30/
  - newnewpdro 7y ago
    > That's equivalent to opening a window without the AC doing anything. Even when the air temperature at your fresh air intake is cooler than the air at your condensor coil? Wouldn't we assume no system would draw its fresh air from near the condensor coil? You still have a heat pump in the system, and there's a temperature difference between the air inlet and the pump outlet.
saeranv 7y ago
Yes, ACs are heat pumps, which means they're just moving heat from one area to another. So yes, the heat shed by human beings via evaporative cooling, is then dumped outside via heat of vaporization. Specifically the ambient indoor air heat is absorbed by refrigerant which is transformed into a superheated vapor saturated with moisture. When this vapor either touches a cooler surface, or has cooler air blown past it, it will dehumidify and exhaust the heat it has absorbed indoors to the outside. The mechanical work done by the AC (fans, pumps) is also being released as heat. So the total energy being dumped outside is therefore the heat that's taken from the inside plus the energy used by the AC released as heat.

saeranv 7y ago

It's more complicated then that. Take a look at this psychrometric chart showing the environmental bounds at which human comfort is guaranteed: http://comfort.cbe.berkeley.edu/ http://comfort.cbe.berkeley.edu/ You can see there that even at extremely dry conditions, if the operative (ambient + radiant) temperature is hot enough, it will cause heat discomfort. Even if the human starts sweating, that'll only reduce the temperature a bit so dehumidification isn't the only role of air conditions, sensible cooling is also a factor.