Air Conditioners

[toggleswitch]

In light of the current heat-wave in the west of the country, such as the 117 degree record high in Phoenix AZ, a question comes to mind.

At what temperature does air-condtioning generally stop functioning?

Remember heat always flows from more to less.

The condenser coil must be hotter than the ambient air temperature, being the air that is used to dissipate the heat from the condernser coil (i.e, cool it).

IIRC cold condensate water from the evaporator (indoor) coil is pumped out and dumped into the condener (outdoor) coil, but how much can that help in a very dry climate?

If the A/C eventually stops working...what the heck do you do to stay alive in that heat? Swamp coolers?

http://aolsvc.news.aol.com/news/article.adp?id=20050723015309990002&ncid=NWS00010000000001

 

[petek]

Interesting thought

Thing is too, in a dry climate, like here where the relative humidity is normally below 20%, there is no condensate. Don't they make misters, or make your own with one of those patio mister kits to cover the outdoor evaporator unit. Which leads to another problem in many areas you're restricted to using outdoor watering. Wouldn't work on my well water though, the mister valves would be clogged within hours.
BTW if you were a nice guy you'd rip that thru the wall unit out and mail it to me.

 

[gansky1]

On very hot days - yesterday was 102F here - I will go out and cool down the condenser with a little water from the hose periodically. It cools the condenser down very quickly but even with only the fan blowing air over it, it is still cooler than the outside air as the fins on the coils dissapate heat much more rapidly. During the very hot hours of the day, the a/c doesn't keep up as well and the indoor temp may rise a few degrees but 77F inside is still better than 102F outside! One of the best things the a/c does is get rid of humidity, that alone makes it feel cooler - at least in the midwest.

 

[toggleswitch]

Quote: BTW if you were a nice guy you'd rip that thru the wall unit out and mail it to me.

Yes, but then on whom could I depend to come plug my gaping hole and what would I do to cool off afterwards?

Did I mention that it the A/C 18,000 BTU/h capacity (about 4.5 rooms @ 4,000 BTU/h per room), older than the hills, and 220v?

Now, in heat-wave extremes, I have that living room unit to help supplement. Works great with a houseful of people and/or the stoves fired up!

BTW the central unit is purposely undersized to dehumidfy better. It does that by running longer/constantly. Long island, as its name imples, being surrounded by water is moister than the steam-room on QAF.

I even run the basment dehumidifer from 10pm to 10am (on a timer)in the summer to ensure that the central A/C starts more freqeuntly (ultimately all the watts it consumes are turned into heat).

Evenings here can go down to 60 deg F, but then the outdoor relative humidity climbs like a mother!

 

[toggleswitch]

Speaking of misters- (LOL)

Owners of homes with clay red-tile roofs have been known to hose them down (on both sides of the pond) to cool off the house.

The evaporating water takes with it great quantities of heat (latent heat of vaporization).

This is why ice cubes are effective- changing matter from one phase to another (solid to liquid, or liquid to vapor) requires (absorbs) lots of heat!

and vice-versa-

steam (such as steam home heating systems) gives off huge quantities of heat as the steam turn back into water! (latent heat of condensation).

Also explains why dryers stay cool while the laundry is wet. all that evaporating cools the drum!

 

[DADoES]

IIRC (and I probably don't, or misunderstood), when the refrigerant leaves the evaporator and goes through the compressor on its way to the condensor, ~two additional units of "heat" are added to it by the compression process beyond what it has absorbed by "boiling" in the evaporator. The effective temp of the condensor will always be higher than the ambient to faciliate 'giving up' heat there. Of course, there's a balance point in the over-all system at which it can't remove heat faster than it radiates back into the structure through the walls, roof/ceiling, windows, doors, etc. The interior temperature will begin rising, but it should still be lower than the ambient. Whether it's sufficiently lower for comfort may be another matter.

 

[toggleswitch]

Quote: two additional units of "heat" are added to it by the compression process.

I am wondering if this is only the physical compression itself which raises the temperature, or as stated before all wattage consumed is ultimately turned into heat in general.. could this be the compressor's power draw that ends up also in the refrigerant and being discharged by the condenser?

P.S. who remembers Boyle's law--- is it P=V*T
(pressure equals volume times temperature?)

 

[DADoES]

It kinda ends up the same. Wattage consumed by the compressor converts to the physical force that compresses the refrigerant, although there is some loss due to friction of moving parts.

 

[petek]

I find it all cornfusing

The only experience I had with central a/c was about 11 years ago when mom was having it installed in her house and waited for me to come home to get different estimates. The blind leading the blind really. So I called 3 companies, Sears, a Coleman rep and Carrier rep. BTW she had 2 furnaces doing the house and wanted it all down to one. Priority was that she didn't want the house turned into an igloo (old ladies don't like the cold) and that it would get rid of the humidity sufficiently. The Sears rep was an affable old guy but he basically talked a good story around the kitchen table and without doing any calculations or measurements settled on a 2.5 ton unit. The Coleman rep, basically the same thing. The Carrier guy however did measure the house and windows etc, noted the awnings over the living room and bedroom picture windows facing west, the patio roof on the west shading the other two bedrooms and the awning over the dining room and the outdoor evaporator would be on the northside in shade. He came up with a 2 ton unit but with a larger condensor coil to handle the humidty without freezing the house, or vice versa, I can't remember. It was supposedly that the unit would do a good job on humidity but not have to run as much as if the unit was undersized, so we went with the Carrier and a 10 year extended warranty. Has worked like a charm since, never too cold and cycles normally.

 

[petek]

Back around 1970

When I still lived back east with the folks we were one of the few folks even then that didn't have central ac in the house. One small window unit in the family room that barely did much. Finally in desperation I went to a local heating & cooling store to see if they had any used a/c's I could stick in my bedroom window. I got this nice little 6000 btu machine that was really neat and you dont' see anymore, they quite making them. Overall it was much like a regular window unit except that instead of being one "box" it was more like a mini-split.
The inside part and the outside part were seperate except for the bottom right corner where a metal housing contained the hoses. You could then use the unit with either side sliding windows or regular up and down sash windows. Either window slid right between the two units except for about 1 inch where you filled that gap with a vinyl baffle. It was great, since all that was inside the room was the fan and condensor coils, the noisy compressor was basically outside the closed window. My sister took it after I went west and I think she still has it in her garage, not sure if it would still work.

 

[toggleswitch]

They recommended 3.0 tons for this 1,200 square foot three-level house. (@ 12,000 Btu per ton, that is 36,000 Btu).

I had them put in 2.5 tons (30,000 Btu). As I said I have the living room wall unit to supplement.

The unit has no trouble maintaining 77 degrees F, where I keep the thermostat while at home. The humdity hovers around 44% which is very comfortable.

 

[toggleswitch]

The central air kicks in at 4:30pm (from 85 setting to 77), and runs pretty much till bed-time. It is usually not bad in here in that it is dehumidifying constantly.

On rare occasions I DO fire-up the living room A/C when I don't want to wait to cool down the house.

 

[gregm]

My aunt and uncle live southeast of Tucson, AZ and they have a swamp cooler where it actually adds moisture to the air while it "cools it" .......... they open all the windows just about an inch then turn it on and cold water pours over some filter/mesh thing (sorry for lack of words or correct terminology) and the fan pulls air from outside across this water soaked "thing" and cools the air and moistens it before entering the home. Probably works on a similar principle as a "humidifier" ??

Swamp Coolers

How a swamp cooler works

www.rawtimes.com

 

[toggleswitch]

Funny, but my original AOL link now references another article. not the one I origianlly read!

 

[foraloysius]

Overhere airconditioners are quite rare although you see the portables more and more. They are not as effective as the window airconditioners, mainly because there is a constant airflow outside the room, which means that warm air also enters the room. I added a picture of an Everglades airconditioner, the same one as I have.

 

[drmitch]

Sweltering

It`s very hot and humid here, hovering around 100. No air at work, just shop fans. I have a 3.5ton unit in my 1400sq. ft home. I leave my thermostat on 72 during the day and 68 at night not that I need it that cool but it does remove the humidity better the more it runs. Plus I useually have a fan running in whatever room Im in.

 

[petek]

re the portables

There's two types of portables, some like the one you have Louis only have the one vent pipe to exhaust the hot air which naturally pulls in warm air from other rooms. The others have two vent pipes, one exhausts the hot air while the other sucks in fresh air from the outside so they actually work as well as a window unit.

 

[tolivac]

On most of the AC systems I had dealt with-household and commercial-the usual standards is that the system will tray to maintain a temp 20 degrees below that of the outdoor temp up to 90 degrees -Ie maintain a temp of down to 70 degrees in 90 degree outside temp.If the temp goes above 90 degrees-then it will maintain to 20 degrees below the outdoor temp-say 100 degrees outside-the system will maintain it to 80 degrees.If you want it cooler you would need an oversized unit-not prac tical in many situations.would be too expensive to run for those "heat wave" situations.
I have seen up close-the evaporative cooling for very large HVAC systems.You would see it as mist rising from the condensor cooler unit.The exhaust air with the mist is close to 150 degrees.These coolers were cooling the condensers of the 600 ton and 1200 ton capacity chillers in the Wash DC plant I worked at. Would have to go on the building roof to fix equipment we had up there-It always fails during heat waves or cold snaps.when the weather is most severe.The water systems for the coolers recicculated some of the water-an automatic system "made up" for what was lost and put a treating agent into the water(fungicides-algecides) to prevent the growth of those items in the warmed water in the sump tanks.Figure to prevent"Legionares Disease" that came from a contaminated chiller water system.The system in the building I am in now is running at its "satuarated" capacity as well.Thats where the "Air King" fans come in to play.Both compressers of our Carrier 150 ton chillers are running.Each unit has two compressers. In mild weather-one runs-in very hot weather-both run.Occasionally the high temps may trip a "High press" overload-so one of us will reset it(the one noticing it)

 

[DADoES]

The newer two-speed or dual-stage (with two compressors) home systems paired with a variable- or multi-speed blower are excellent at handling both mild/moderate and heavy ambient loads, while still keeping humidity down.

The two-speed Lennox heat pump I had installed in my previous house fit the bill. The contractor recommended 2.5 tons. The upscale system I wanted came no smaller than ~3.25 tons. Fitted with Lennox's optional humidistat, humidity control was fantastic. Rated for 16.45 SEER! It ran on low speed majority of the time, sometimes for hours, just holding the temp at the setpoint (76°F was my usual choice), but could kick into high when needed to "catch up" on those 100°F+ days. Loved it! The new house unfortunately has more middling one-speed 12 SEER Carrier heat pump. 4 tons, it hasn't had any trouble keeping up thus far.