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Old 07-23-2009, 04:31 PM   #1
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Southwest US A/C Use

Been full timing about a year on the East coast and have used the A/C from PA to FL and understand the the A/C concept. We have vacationed in the Southwest yearly since 1999, but this will be our first visit 9/09-4/10 out west. Since it is a DRY heat how does the A/C actually work since there is virtually NO moisture to extract. Normally it is 7-1% depending on where you are at. Thanks in advance for your replies. Coach is 2008 Winnebago Voyage with basement air.


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Old 07-23-2009, 05:43 PM   #2
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Here is an explanation for you, sorry for the length.

Air conditioners use chemicals that convert from a gas to a liquid and back again. This chemical is used to transfer heat from the air inside of the motor home to the outside air. The machine has three main parts. They are a compressor, a condenser and an evaporator.

The working fluid arrives at the compressor as a cool, low-pressure gas. The compressor squeezes the fluid. This packs the molecule of the fluid closer together. The closer the molecules are together, the higher its energy and its temperature. The working fluid leaves the compressor as a hot, high pressure gas and flows into the condenser.

When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the evaporator the liquid's pressure drops, when it does it begins to evaporate into a gas.

As the liquid changes to gas and evaporates, it extracts heat from the air around it. The heat in the air is needed to separate the molecules of the fluid from a liquid to a gas. The evaporator also has metal fins to help in exchange the thermal energy with the surrounding air. By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It then returns to the compressor to begin its trip all over again.

Connected to the evaporator is a fan that circulates the air to blow across the evaporator fins. Hot air is lighter than cold air, so the hot air in the room rises to the top of a room. There is a vent there where air is sucked into the air conditioner and goes down ducts. The hot air is used to cool the gas in the evaporator. As the heat is removed from the air, the air is cooled. It is then blown into the motor home through the other ducts.

This continues over and over and over until the motor home reaches the temperature you want it cooled to. The thermostat senses that the temperature has reached the right setting and turns off the air conditioner. As the motor home warms up, the thermostat turns the air conditioner back on until the motor home reaches the temperature.

When you first turn on air conditioning and it is hot and humid, and the windows are open, there is a lot of moisture in the air. When you close the windows and turn on the air conditioning, the air conditioner will use about of its energy initially to remove that moisture from the air while it cools. Because there is more moisture in the air to remove at the start, the air conditioner will take longer to cool the motor home down, with only half of its energy going toward cooling. However, after the air conditioner runs for a few hours, the majority of the moisture will be removed. At this point, the air conditioner can cool much more quickly, as well as run for shorter times to maintain that cool temperature.

When in arid conditions, where there is less moisture, more energy is going toward cooling and that is a good thing, because generally the temperatures are higher. In many arid parts of the USA they actually use something called an Evaporative Cooler that adds moisture to the air. The Evaporative Cooler runs water across grids while a large fan blows across the grids. This addition of cool water blowing moisture into the air reduces the temperature.

There you go!


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Old 07-23-2009, 06:20 PM   #3
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The a/c works just fine in a "dry heat". The humidity - or lack thereof - has nothing to do with the efficiency of the a/c cycle that Jim described so thoroughly.

It does, however, affect your comfort by improving the efficiency of your body's cooling system, i.e. perspiration. Sweat evaporates quickly off your skin, helping you cool. You just have to keep drinking liquids to replace the water loss. That's why you feel more comfortable in a "dry heat" than a high humidity environment.
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Old 07-23-2009, 06:51 PM   #4
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Thanks both for your replies. Jim, although my degree is in Economics, I understood every word you said as I did very well in Chemistry in school. I just couldn't get the concept of AC working well in a low humidity scenario, but I can now see how it works. Gary, I became familiar with the "Swamp Cooler" concept from a DESERTUSA site I go to for desert info

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Old 07-23-2009, 07:10 PM   #5
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In the air conditioning process ther is two parts of the cooling...sensible, which is the cooling of the dry bulb temperature and latent cooling which is the lowering of the wet bulb temperature. In low humidity, the wet bulb (or the amount of moisture in the air) is much lower and you will very little if any condensation on the evaporator coil. You can actually have more problems with the evaporator icing in low humidity since the load ins much lower and the compressor will rebalance at a lower suction pressure and temperature which can be below freezing. So run the fan on high if you start to get icing problems.

It is actually more comfortable in a dry climate to use an evaporative cooler (swamp cooler) which will put some moisture back into the air and still drop the temperature way down, depending on just how low the wet bulb is to staty with.

An evaporative cooler is much less expensive to run than refrigerated air.

Been in the refrigeration and A/C business on large industrial systems since 1970.

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Old 07-24-2009, 02:51 AM   #6
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When we stayed in Apache Junction a year, I changed one habit that we do back east here. That is I did not open and run the ceiling fans to vent the moisture out. I kept them closed and used that moisture to bump up the humidity level and stayed quite comfortable as the air conditioner would pull it back out and would run all day long when it got to 110 or more.
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Old 07-24-2009, 06:50 AM   #7
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Jim: OUTSTANDING tutorial on our A/C operation. Thanks. Steve
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Old 07-24-2009, 08:19 AM   #8
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I will second that. an Air Conditioner, well technically an Air Conditioner is anything that changes, hopefully for the better, any condition of the air, (I recall advertisments for a dollar forty nine air conditioner, it was a plug in air freshner)

However in common use we mean air coolers.

They are heat pumps, pumping heat from INSIDE to OUTSIDE,, Just like a slightly different (Reversable) version is called a heat pump. and they work in dry climes as well as wet..

The removal of humidity (Moisture) is a side effect in fact.

NOW... If, like me, when in the Arizona desert you are running on battery power a kilowatt worth of heat pump is not going to be something you want running all night. (Quite hours, no genny)

And. again if you are like me, that dry air does a number on my nasal passages... I need a bit more moisture in the air.


SWAMP coolers,,,, Mine is a "Gator" brand, purchased from E-bay, This unit can put gallons of water into the air in my bedroom during the course of a day, as the water evaporates it cools the air giving me as much as 10 degrees or more of lower temp. plus the moisture is good for me.. Oh, power draw on cool: 56 Watts per the Kill-a-watt

The model I have is also a heater.. Only plug in 2,000 watt heater (They are not kidding, it really draws 2,000, well, 1999.5, watts, I measured it, give me another fraction of a volt and it would have been 2,000) I have ever seen. Takes a 20 amp line (Though it has a 15 amp plug)

Swamp coolers come in 2 flavors, this one is a portable so it humidifies the room (Good for me) the other kind exhausts moist air outside the "house" and uses a heat exchanger to cool the inside. both run on water and very little electricity.

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