The Physics of Air Conditioning

[Craig krup]

There's surprisingly little on the web about how air con works and (more importantly) why - given certain things - it has to be a lot cheaper than apparently similar heating.

I was very sceptical that 50 or a 100 baht could get you working air conditioning, so I sweated like a pig for a month in Isaan in July before I gave in and paid the extra. Wow! Full bore effective air con that had to be turned off in 20 minutes. That got me thinking. The web has lots of people in places like Arizona comparing the cost of heating with their summer air con bills. But thinking about the physics has to be a better way of going about it. Air con only has to strip the heat out of the air in a room. You aren't buying gas or coal and breaking the chemical bonds, you are only stripping the heat out and pumping it outside. The room is basically a fridge-freezer, and fridge-freezers keep a 20 degree Celsius difference between themselves and the outside world on 60-100 watts.

So if the room is sealed your only costs are, 1) the cost of the fan inside the air con which draws the air out of the room, and 2) the cost of compressing the Freon (or whatever) inside the unit. Of course this only applies if the room is sealed.

As soon as you start pumping heat out, though, you set up a big temperature gradient between the room and the outside, so you need thick walls as well. Given that apartment blocks typically have thick walls, and far less external wall in relation to their internal volume, you'd think it's going to be an awful lot easier to chill a studio flat than a detached house.

So, basically, air con should be at least as effective as a heat-air pump in terms of bringing about temperature changes. If you have good insulation it should be about 30% or less of the cost of equivalent (i.e. same temperature change) heating.

So

Am I basically right?

Has anyone tried to create a room within a room, given that sleeping is the main thing? A room of glass bricks or double glazing units within a bedroom, with the bed and nothing else inside, would be very cheap to run.

[Naam]

how many Chang?

[tifino]

factor in that the walls, floor, ceiling will begin the exercise at the temperature of the environment outside - you will have a lot of catching up to do.

put a load of ice into a hot esky, and see how much ice is melted off, before the esky is useable, would a comparable small-scale of what you need to find out

[GuestHouse]

Reaches for copy of "Eastop and McConkey" while wondering if its on the banned books list.

[cdnvic]

I'll put this in a more suitable forum. Good luck.

[Eddy B]

The problem is not just the physical room but also that most Thai houses have quite thin walls without insulation or a breathing gap between walls like most western houses. The next problem is the ceiling. A lot of Thai house's have a false ceiling tiles and above them is just the tiled roof. Unlike most western houses these spaces would be insulated against heat/cold but not here. The house I am renting has about 5 inch thick walls, zero roof insulation and if I poke my head up through the false ceiling tiles I can see daylight through gaps where they did not seal the roofing properly. No wonder I have an expensive electrical bill every month.

[NeverSure]

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

[thierryviteau]

And do you know why my aircon leaks ?

[NeverSure]

"The room is basically a fridge-freezer, and fridge-freezers keep a 20 degree Celsius difference between themselves and the outside world on 60-100 watts."

The trade-off is that the unit heats the room. The heat that's removed from inside the unit has to go somewhere.

"For every action there's an opposite and equal reaction." - Some genius.

"There ain't no such thing as a free lunch." -Some genius.

"If you remove heat and/or air from somewhere, it has to go somewhere else." Me

[ExpatOilWorker]

To have a comfortable indoor climate you need to change the air in the room ever 2nd hours. Heat capacity of air is 1,005 J/kgxC. The Ac also need to condense a lot of humidity in the air. The heat of vaporization is 2,260,000 J/kg. You do the math.

Here are two words you have never heard before "olf" and "decipol": http://en.wikipedia.org/wiki/Decipol

[Naam]

To have a comfortable indoor climate you need to change the air in the room ever 2nd hours. Heat capacity of air is 1,005 J/kgxC. The Ac also need to condense a lot of humidity in the air. The heat of vaporization is 2,260,000 J/kg. You do the math.

Here are two words you have never heard before "olf" and "decipol": http://en.wikipedia.org/wiki/Decipol

To have a comfortable indoor climate you need to change the air in the room ever 2nd hours.

this claim, respectively the 10 l/s fresh air demand, is in my [not so] humble opinion pure refined BS, in this specific case one could call it "PPOLFS" (Professor P. Ole Fanger Shit)

[IMHO]

factor in that the walls, floor, ceiling will begin the exercise at the temperature of the environment outside - you will have a lot of catching up to do.

put a load of ice into a hot esky, and see how much ice is melted off, before the esky is useable, would a comparable small-scale of what you need to find out

Actually for a small scale test approximating a Thai house, make that a wooden crate instead of an esky, and make sure you park it in the sun - then see how long the ice lasts

[ExpatOilWorker]

To have a comfortable indoor climate you need to change the air in the room ever 2nd hours. Heat capacity of air is 1,005 J/kgxC. The Ac also need to condense a lot of humidity in the air. The heat of vaporization is 2,260,000 J/kg. You do the math.

Here are two words you have never heard before "olf" and "decipol": http://en.wikipedia.org/wiki/Decipol

To have a comfortable indoor climate you need to change the air in the room ever 2nd hours.

this claim, respectively the 10 l/s fresh air demand, is in my [not so] humble opinion pure refined BS, in this specific case one could call it "PPOLFS" (Professor P. Ole Fanger Shit)

Indore Climate is a science of highest caliper. It originated back in the 70'es when the oil crises forced building designers were forced to isolate and seal the buildings, but architects were still using materials, paint, carpets and furniture that gave off, in some cases toxic, vapors. Hence the sick building syndrome, where office worker went down like flies.

Changing ALL the air every second hour sound like a lot, but actually it is not as there is a lot of dead corners and static air in a room. A more general rule of thumb, if you step into a house from the outside in the morning and CAN'T smell what the host had for dinner last night, your home is well ventilated.

[Naam]

A more general rule of thumb, if you step into a house from the outside in the morning and CAN'T smell what the host had for dinner last night, your home is well ventilated.

not necessarily. when you come to my home and neither smell what we had for dinner last night nor my farts or the farts of my dogs then it could be that i forgot the daily whole-house ventilation but the job is done by ozone generators mounted near the aircons.

note: when you were a guest in my home (2 years ago?) the gadgets were not yet mounted.

[noise]

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

Unless you are using an exhaust fan, I would suggest your room air conditioner is circulating the hot room air over cooler/cold coils which in turn cool that same air and returns it to the room. Think of your car air conditioner; most of them have a recirculate or use outside air switches. Room air conditioners do not have that option. A room air conditioner is like having a fan blowing across a large block of ice (you get colder air) but without the added humidity.

[morrobay]

https://www.physicsforums.com/threads/why-do-gases-cool-on-expansion.470311/

Surprised you cannot find anything on internet searches for this? Same principle as a refrigerator: A compressor compresses a gas which is released and expanded into cooling coils. As the gas expands it absorbs heat energy to overcome molecular attractions. Of course by conservation of energy that heat is released on compression.

Edited May 4, 2015 by morrobay

[ExpatOilWorker]

A more general rule of thumb, if you step into a house from the outside in the morning and CAN'T smell what the host had for dinner last night, your home is well ventilated.

not necessarily. when you come to my home and neither smell what we had for dinner last night nor my farts or the farts of my dogs then it could be that i forgot the daily whole-house ventilation but the job is done by ozone generators mounted near the aircons.

ozone generator.jpg

note: when you were a guest in my home (2 years ago?) the gadgets were not yet mounted.

Are you telling me that my pristine lungs were breathing normal air-conditioned air when I visited you?

It is amazing the hardship I have to endure for a few hours of good company!

2 years, time is passing fast at our age.

[Naam]

Are you telling me that my pristine lungs were breathing normal air-conditioned air when I visited you?

It is amazing the hardship I have to endure for a few hours of good company!

2 years, time is passing fast at our age.

relax. besides a daily one hour very forceful ventilation before sunrise our home is continously ventilated by half a dozen small fans placed strategically. that aficionados of airconditioning have to shell out an additional bundle in a tropical country can't be avoided.

but it doesn't bother me too much as i use secretly the money of my heirs to pay the electricity bill

[Craig krup]

The problem is not just the physical room but also that most Thai houses have quite thin walls without insulation or a breathing gap between walls like most western houses. The next problem is the ceiling. A lot of Thai house's have a false ceiling tiles and above them is just the tiled roof.

Yeah, but the condos are often built like a WW2 German submarine pen. Eight inch thick floors, therefore (except for the top floor) eight inch thick ceilings, breeze block walls....

If you can sort out the windows and the door onto the patio it should be pretty cheap in an apartment.

[Craig krup]

https://www.physicsforums.com/threads/why-do-gases-cool-on-expansion.470311/

Surprised you cannot find anything on internet searches for this? Same principle as a refrigerator: A compressor compresses a gas which is released and expanded into cooling coils. As the gas expands it absorbs heat energy to overcome molecular attractions. Of course by conservation of energy that heat is released on compression.

Yeah, but the relative cost, and what impacts on the cost, isn't very well explored. What you get is a load of anecdote from people who have both a winter heating bill and a summer air con bill. Working through the physics tells you that if you get it broadly right it has to be a lot cheaper to cool than to heat, because - fundamentally - buying enough electricity to strip the heat out of (say) 100 cubic meters of air, and pumping it outside, has to be cheaper than generating heat.

[Craig krup]

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

Unless you are using an exhaust fan, I would suggest your room air conditioner is circulating the hot room air over cooler/cold coils which in turn cool that same air and returns it to the room. Think of your car air conditioner; most of them have a recirculate or use outside air switches. Room air conditioners do not have that option. A room air conditioner is like having a fan blowing across a large block of ice (you get colder air) but without the added humidity.

The only way you get heat out of a room is to put it somewhere. Air con heats the street. In New York it ups the city street temperature by a measurable and significant amount. In Houston the units outside the back of the shopping mall are red hot, as they - effectively - blow the solar gain from the building back out into the environment.

The Mono Place in Udon had a completely sealed room, with the air con blowing the heat into the bathroom. I opened the door to take a leak and I thought war had broken out and I'd just experienced the nuclear flash.

[NeverSure]

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

Unless you are using an exhaust fan, I would suggest your room air conditioner is circulating the hot room air over cooler/cold coils which in turn cool that same air and returns it to the room. Think of your car air conditioner; most of them have a recirculate or use outside air switches. Room air conditioners do not have that option. A room air conditioner is like having a fan blowing across a large block of ice (you get colder air) but without the added humidity.

If your aircon was exhausting the heated air into the room you'd be worse off than with no aircon because friction in the operating unit also creates heat. Aircon is the process of moving heat from one place to another.

Let's see if I can describe this. When a gas (refrigerant) is compressed it heats. That's a scientific principle. That's also how an aircon can became a heater called a heat pump. Just push the cold outside and exhaust the heat inside in reverse of aircon. Turn your wall/window aircon around and it will cool the outside and heat your room. It can actually remove heat from cold air by lowering the temp of the cold air even farther.

When a liquid turns to a vapor it cools. This is shown by sweat evaporating on your body. This is why we like to sit in front of a fan or have a breeze when it's hot. Immediately after the compressor in the aircon compresses the refrigerant turning it into a hot liquid, a fan blows across it to cool it by blowing the heat outside. THEN the refrigerant passes to where it loses pressure and vaporizes and super cools. This resulting cold is pushed into the room by fans.

It's a constant loop of refrigerant compressing and decompressing, thereby heating and cooling and exhausting the heat to another place. The resulting super-cooled refrigerant on the low pressure side is the source of the cooling for the room.

[jerojero]

I

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

Not entirely correct. Window AC units yes, but not split units that have refrigeration coils (evaporator) in the room, and compressor/condensor outside the room or building. Split units are most common in Thailand. The duct lines, tubes, piping between these two components carry away the room heat via the liquid refrigerant, freon, and the condensor fan dispels the heat outside while segregated coil fan inside continuously recirculates room air only. Edited May 6, 2015 by jerojero

[Naam]

I

"If the room is sealed." That's not possible. As mentioned, the room exhausts the heat from the air by exhausting heated air. It must replace that air with new outside air. It's a loop.

Not entirely correct. Window AC units yes, but not split units that have refrigeration coils (evaporator) in the room, and compressor/condensor outside the room or building. Split units are most common in Thailand. The duct lines, tubes, piping between these two components carry away the room heat via the liquid refrigerant, freon, and the condensor fan dispels the heat outside while segregated coil fan inside continuously recirculates room air only.

even window units do not exhaust more than 5% of the room air when switched on ventilate.