PDA

View Full Version : "Cool" mode vs. "Dry" mode on my a/c


Thunder
3rd August 2010, 06:55 PM
my a/c has cool mode & dry mode.

i have "dry" mode on now. the air coming our feels cool.

is it just dry air?

how does the compressor make just dry air....or dry and cool air?

please explain.

Roma
3rd August 2010, 07:11 PM
Dry will just dehumidify without cooling even if the air blowing out might feel cooler,
Cool will dehumidify and cool.

Pulvinar
3rd August 2010, 07:16 PM
Most likely your a/c "dry" mode just limits the fan speed to low allowing the air to spend more time going through the evaporator coil and have water condense out of it, as it would on the side of a cold glass of water. You will get exactly the same effect with any other a/c on low speed too, so it's really just a marketing ploy.

The Man
3rd August 2010, 07:38 PM
Well, technically such dehumidifying works by cooling the air, to some degree, so water vapor will condense (dew point (http://en.wikipedia.org/wiki/Dew_point)). It just won’t cool it as much as the cooling mode. A desiccant ( http://en.wikipedia.org/wiki/Desiccant) will also remove moisture from the air by absorption, but not by changing the temperature of the air. Dehumidified air (of the same temperature as the humid air) will also “feel” cool as it can permit more evaporation from the skin, the bodies own cooling system.

The Man
3rd August 2010, 07:45 PM
Most likely your a/c "dry" mode just limits the fan speed to low allowing the air to spend more time going through the evaporator coil and have water condense out of it, as it would on the side of a cold glass of water. You will get exactly the same effect with any other a/c on low speed too, so it's really just a marketing ploy.

Good point Pulvinar, I’m not sure if the compressor actually does less work in such a ‘dry’ mode.

Thunder
3rd August 2010, 07:55 PM
and, somehow the "dry" mode is sensitive to the temperature settings.

which makes no sense..or does it?

tesscaline
3rd August 2010, 07:58 PM
and, somehow the "dry" mode is sensitive to the temperature settings.

which makes no sense..or does it?Does your A/C have a humidifier in it? If it does, it may be that the "dry" setting turns that off, while the "cool" setting leaves it on?

The Man
3rd August 2010, 08:07 PM
Your air conditioner is always "sensitive to the temperature settings", some dehumidifiers ( http://en.wikipedia.org/wiki/Dehumidifier) though do reheat and even elevate the temperature of the air.

Typicallucas
3rd August 2010, 09:37 PM
This:

Well, technically such dehumidifying works by cooling the air, to some degree, so water vapor will condense (dew point (http://en.wikipedia.org/wiki/Dew_point)). It just won’t cool it as much as the cooling mode. A desiccant ( http://en.wikipedia.org/wiki/Desiccant) will also remove moisture from the air by absorption, but not by changing the temperature of the air. Dehumidified air (of the same temperature as the humid air) will also “feel” cool as it can permit more evaporation from the skin, the bodies own cooling system.

The dehumidifying effect your A/C system has on air is the reason why on a hot day you will see water leaking out from under your car when you've just parked it. This is also the reason why A/C units on top of buildings have a drain line, otherwise they would collect a bunch of water and possibly leak into the roof.

The compressed refrigerant gas runs through a heat exchanger (a radiator lookin thingie) near the front of your car and travels through the firewall to a evaporator unit (which looks like another smaller radiator) which allows the compressed gas to expand and chill the evaporator unit. Air from the outside is blown across the vanes of the evaporator unit, cooling the air. Moisture in the air condenses on the evaporator unit, drips down into a tray and through a drain hose and onto the ground. Voila, dry air.

Typicallucas
3rd August 2010, 09:46 PM
Your air conditioner is always "sensitive to the temperature settings", some dehumidifiers ( http://en.wikipedia.org/wiki/Dehumidifier) though do reheat and even elevate the temperature of the air.

Yes. Ambient temperature will make a difference in how effective the heat exchanger is at reducing the temperature of the compressed refrigerant. The cycle is:

Compress refrigerant (refrigerant gets hot!)
Run refrigerant thru heat exchanger (temperature of refrigerant cools to be closer to ambient temperature while remaining compressed)
Run refrigerant thru evaporator (refrigerant is now uncompressed and at a much lower temperature than it started)
Rinse, repeat

The hotter or more humid it is outside the heat exchanger will be less effective at cooling off the compressed refrigerant. This same technology is used in your refrigerator in your kitchen and your air conditioning at home.

I can imagine a situation where I might use the "dry" function. A day that isn't too hot but is god-awful humid. Say a mild, sunny day after a quick rain shower.

Uncayimmy
3rd August 2010, 10:21 PM
I figure this is a good time to mention swamp coolers (http://en.wikipedia.org/wiki/Evaporative_cooler), the ultimate in bush technology. I had never heard fo them until I moved to Phoenix a decade ago. They cool your house by making it more humid.

For those who don't want to follow the link, picture a box on top of your house with sides about three feet tall and wide. The top is sealed. The bottom has a hole leading to your ventilation system. The sides are vented. A fan sucks in air through the sides and pushes it into your house.

The key is that the sides have pads, which are big sponges made out of a straw like material. The bottom of the unit has a tray with water and a feed line with a float like your toilet. A small pump sends water up a pipe that splits off to the four sides so that water falls down through the pads. As air is sucked through the pads, the water inside evaporates. This requires energy, which results in a loss of heat in the air. You end up with cool but moist air coming into your home. Operational costs are minimal since the fan and small pump use very little electricity compared to an A/C.

Of course, they are only good when the air is dry, which is most of the summer here in Phoenix. The weather report always lists the dew point because at about 56F and higher, the coolers don't work all that well. Some people also have A/C, which they switch to when the "monsoon season" arrives in late July or early August. That's when we get a bit of humidity.

The biggest drawback is that the water here has so many minerals in it (calcium, I believe). The pads get crusted and have to be replaced, usually once per year. There's money to be made in an inexpensive solution to pull the calcium out of the water so you don't have to replace the pads - it's not technically difficult or all that expensive, but it gets frigging hot on the roof here in Phoenix.

The Man
3rd August 2010, 10:31 PM
This:



The dehumidifying effect your A/C system has on air is the reason why on a hot day you will see water leaking out from under your car when you've just parked it. This is also the reason why A/C units on top of buildings have a drain line, otherwise they would collect a bunch of water and possibly leak into the roof.

The compressed refrigerant gas runs through a heat exchanger (a radiator lookin thingie) near the front of your car and travels through the firewall to a evaporator unit (which looks like another smaller radiator) which allows the compressed gas to expand and chill the evaporator unit. Air from the outside is blown across the vanes of the evaporator unit, cooling the air. Moisture in the air condenses on the evaporator unit, drips down into a tray and through a drain hose and onto the ground. Voila, dry air.


Exaca-techly, though apparently some modern air conditioners also direct some of the condensed water onto the condenser. The resulting evaporation makes the condenser more efficient at losing heat. A fact I was not aware of until reading it in one of the links I posted, but now I understand why my relatively new AC was basically spraying some of that water onto the condenser.

The Man
3rd August 2010, 10:45 PM
Yes. Ambient temperature will make a difference in how effective the heat exchanger is at reducing the temperature of the compressed refrigerant. The cycle is:

Compress refrigerant (refrigerant gets hot!)
Run refrigerant thru heat exchanger (temperature of refrigerant cools to be closer to ambient temperature while remaining compressed)
Run refrigerant thru evaporator (refrigerant is now uncompressed and at a much lower temperature than it started)
Rinse, repeat

The hotter or more humid it is outside the heat exchanger will be less effective at cooling off the compressed refrigerant. This same technology is used in your refrigerator in your kitchen and your air conditioning at home.

I can imagine a situation where I might use the "dry" function. A day that isn't too hot but is god-awful humid. Say a mild, sunny day after a quick rain shower.

In fact I have been wanting to buy a dehumidifier, for the fall and spring months when the AC is not running. Again based on one of the links I posted, I have a portable AC that should serve that purpose. As the condenser needs to be vented to the outside. Simply do not vent and it is just a dehumidifier (the heat it takes out it puts back in). I now surmise that the original design of the unit was probably as a dehumidified, then modified (by adding the venting capability and removing the larger water storage) to a portable AC. Though I’ll probably buy an intended dehumidifier.

The Man
3rd August 2010, 11:07 PM
I figure this is a good time to mention swamp coolers (http://en.wikipedia.org/wiki/Evaporative_cooler), the ultimate in bush technology. I had never heard fo them until I moved to Phoenix a decade ago. They cool your house by making it more humid.

For those who don't want to follow the link, picture a box on top of your house with sides about three feet tall and wide. The top is sealed. The bottom has a hole leading to your ventilation system. The sides are vented. A fan sucks in air through the sides and pushes it into your house.

The key is that the sides have pads, which are big sponges made out of a straw like material. The bottom of the unit has a tray with water and a feed line with a float like your toilet. A small pump sends water up a pipe that splits off to the four sides so that water falls down through the pads. As air is sucked through the pads, the water inside evaporates. This requires energy, which results in a loss of heat in the air. You end up with cool but moist air coming into your home. Operational costs are minimal since the fan and small pump use very little electricity compared to an A/C.

Of course, they are only good when the air is dry, which is most of the summer here in Phoenix. The weather report always lists the dew point because at about 56F and higher, the coolers don't work all that well. Some people also have A/C, which they switch to when the "monsoon season" arrives in late July or early August. That's when we get a bit of humidity.

The biggest drawback is that the water here has so many minerals in it (calcium, I believe). The pads get crusted and have to be replaced, usually once per year. There's money to be made in an inexpensive solution to pull the calcium out of the water so you don't have to replace the pads - it's not technically difficult or all that expensive, but it gets frigging hot on the roof here in Phoenix.

Again Exaca-techly “only good when the air is dry”. When the air is already saturated it can hold no more water, a drop in temperature can result in a suppersaturation (http://en.wikipedia.org/wiki/Super_saturation) and a precipitation (http://en.wikipedia.org/wiki/Precipitate) of the water as a condensate (http://en.wikipedia.org/wiki/Condensation). When I was in Phoenix some years ago, to visit my brother, was the first time I had encountered the ‘misters’ you have outdoors there.

Uncayimmy
3rd August 2010, 11:20 PM
Again Exaca-techly “only good when the air is dry”. When the air is already saturated it can hold no more water, a drop in temperature can result in a suppersaturation (http://en.wikipedia.org/wiki/Super_saturation) and a precipitation of the water as a condensate. When I was in Phoenix some years ago, to visit my brother, was the first time I had encounter the ‘misters’ you have outdoors there.

Oh, yeh! The misters, for those who may not gather by the name, spray a fine mist into the air. You'll see them about 8 to 10 feet up surrounding something like a patio at a restaurant. The evaporation significantly cools the air.

The el cheapo versions you can get for home don't work well. The mist needs to be very fine. Otherwise you get a bunch of dripping. When the misters are of good quality and the air is dry, no drops of water reach the diners or the ground. Another benefit of the dry air is that the towel you used in the morning after your shower is dry by the time you get home from work.

When I first moved out here I remember riding by a golf course every day in the late evening. When their sprinklers were on, the air was noticeably cooler on the road 30 yards away. By contrast there are some mountain "passes" here in Phoenix where the road was cut through the rock such that on either side of the road was solid rock. Late at night I could feel the heat as I rode my bike through there.

This was all a very drastic change from growing up in the Maryland/DC area. The biggest change of all was being able to see a traffic light a mile ahead. The first time I went back to Maryland I was almost claustrophobic from all the winding rows and tall trees blocking my line of sight.

The Man
3rd August 2010, 11:28 PM
Oh, yeh! The misters, for those who may not gather by the name, spray a fine mist into the air. You'll see them about 8 to 10 feet up surrounding something like a patio at a restaurant. The evaporation significantly cools the air.

The el cheapo versions you can get for home don't work well. The mist needs to be very fine. Otherwise you get a bunch of dripping. When the misters are of good quality and the air is dry, no drops of water reach the diners or the ground. Another benefit of the dry air is that the towel you used in the morning after your shower is dry by the time you get home from work.

When I first moved out here I remember riding by a golf course every day in the late evening. When their sprinklers were on, the air was noticeably cooler on the road 30 yards away. By contrast there are some mountain "passes" here in Phoenix where the road was cut through the rock such that on either side of the road was solid rock. Late at night I could feel the heat as I rode my bike through there.

This was all a very drastic change from growing up in the Maryland/DC area. The biggest change of all was being able to see a traffic light a mile ahead. The first time I went back to Maryland I was almost claustrophobic from all the winding rows and tall trees blocking my line of sight.

The saying there is "Yeah, but it's a dry heat". When my brother was up here visiting one winter he remarked “ Dang it’s cold!!” my reply was “Yeah, but it's a wet cold”.

rjh01
4th August 2010, 12:25 AM
One question that has not been asked. Does the a/c produce water? If it removes water from the air (dehumidify the air) then it would need to get rid of that water. It may drip water outside or something.

gmarshall
4th August 2010, 04:29 AM
If the OP is asking about an automotive AC, "dry mode" could also be mixing the conditioned air with air run through the heater core to bring the temperature back to room temp.

Another question I have about automotive air conditioners is about "Max Cool" or "Max AC". I used to think that button just recirculated the cabin air rather than condition outside air but now I have a vehicle that has both a "Max AC" and a "recirc" button. Is there another control I don't know about?

LarianLeQuella
4th August 2010, 05:30 AM
The saying there is "Yeah, but it's a dry heat". When my brother was up here visiting one winter he remarked “ Dang it’s cold!!” my reply was “Yeah, but it's a wet cold”.


I have used that a few times as well! :D The looks I get are worth it entirely.

rjh01, yes, there is usually condensate that drips out of an A/C system. In some systems, there is even a small pump attached to the system to help direct that water away.

gmarshall, my wife's car has both of those buttons too. Although, as soon as you push the Max AC button, it automatically switches over to recirculate. Not sure if that happens with all vehicles. it may just be a higher fan setting, robbing engine power, on some. Just my anectdotal observation.