A question for physics people:

[bemildred]

Air conditioners emit cool dry air. Evaporative coolers require dry air to work properly. So if you put an evaporative cooler on output end of a compressor driven air conditioner, would you not get a more efficient cooling system, and a system that works better particularly when the air conditioner is having trouble keeping up with the heat?

TIA.

[NYC_SKP]

1. Typically, the air blowing out through an AC condenser is hotter than ambient...

...so it would have to drop significantly further in temperature through the pads of the evaporative cooler to deliver cool air than the ambient air on a hot summer day.

Still, I like the way you think.

However, if one could take the drier air coming from the AC condenser and allow it to cool before getting to the swamp cooler, it probably would work pretty well, especially where it's really humid.

[bemildred]

3. I was thinking of the other end of the AC system, the cool dry air going into your building.

A bit more explanation below.

[NNadir]

2. Many large air conditioners on large buildings do, in fact, have cooling towers.

Often you can see these on roofs, although often architects conceal them.

A variation on this theme is to pump heat to or from ground water. Often these types of systems are called "geothermal energy" although in reality they are no such thing.

An evaporative cooler works anywhere that the humidity is less than 100%, although obviously not as well at high humidity.

On very large power plants, cooling towers are almost always required.

Here for instance is a photograph at one of the largest coal plants in Europe, the Drax coal plant in Great Britain:



http://en.wikipedia.org/wiki/Drax_Power_Station

This plant, by the way is considered the most efficient coal plant in Europe.

It releases over 20 million tons of dangerous fossil fuel waste each year, generally in the form of carbon oxides and nitrogen oxides.

The plant produces about 3,500 MW of electricity, but to satisfy the scientifically illiterate public's fantasies, Drax plans to build a biomass fueled plant 1/10th the size on the same site.

Carnot efficiency is a function of temperature differences between heat reservoirs and the heat of the environmental reservoirs. Thus if one has lower temperatures, owing to (possibly) a higher heat capacity fluid (usually but not always water), one can raise efficiency.

[bemildred]

4. I don't think Carnot efficiency applies to evaporative cooling. It does to compressor AC of course.

As I understand it, evaporative cooling is based on the heat water takes up to become vapor. So an AC system is actually losing some of it's cooling capacity by getting heat from the water that it condenses out of the air as it cools it, condensation heating you might call it.

I used to live in the desert, and was impressed with how cheap, easy to work on, and effective evap coolers are in hot dry weather.

And I've been working on my AC system, which was dribbling water into my house because of a disconnected pipe. So for some reason I put those two ideas together and got to wondering.

I knew that if I ran the cool dry AC air through an evap cooler it would cool it further, and humidify it, by evaporation, but I was not sure if it would be worth the extra cost of the evap cooler. The question would be how much cooler would it make the air going into my house, and would that save or cost money/energy overall. You seem to have examples where it pays, even in a damp place like the UK.

Evap coolers as I remember them tend to shove lots of air through, great big blowers, more than my house AC system I think. So I figured you want an AC system that will handle high volumne and do a good job of drying the air, then you can pump that through an evap cooler and really freeze your house. You might even go in the direction of something that drys your air well, as opposed to getting it really cold, so you can feed it to an evap cooler, if you could figure out how to do that without a compressor and carnot cycle, or more efficiently with, that could be really cheap.

[Aragorn]

6. btw

based on your possible intentions you should consider changing to retro window units. I have and my bills dropped like crazy. I conclude they are more cost-effective, and after being in the attic where the central unit and ducts are, I can see why. In my older neighborhood I am now in the majority, and btw this is humid rural east texas.

[bemildred]

8. Yeah, I agree. I have central air, and it's a mess up there.

I'm 64 and crawling around in the attic makes me grumpy for days.

If I do it over, I'm likely to do window units in the rooms that matter and screw the central air in the attic. I'm tempted to get a window unit and an evap cooler and jury rig some way for them to fornicate to see how it works out.

[madokie]

17. Once again

What a load of hooey you've typed here. Most times when one doesn't know anything about something it is better to keep ones mouth shut and listen, or at the very least ask questions instead, rather than show your ignorance. A lesson BTW you would be well served to learn.

As time goes on it's getting more and more difficult to not put you permanently on ignore. The reason I haven't yet is because I keep hoping I will somewhere, somehow gain a smidgen of knowledge out of all that intelligence you claim to have. Which most times, BTW, comes through loud and clear as stoopid bullshit :hi:

[wtmusic]

5. You would - but the air coming out would be more humid

so from a comfort standpoint it would provide less relief.

[bemildred]

7. I think that would be a concern in a humid climate.

In a dry climate it could be a good thing to add some humidity. When I lived in the desert, we had 5% humidity, and the damp air coming from the evap cooler was nice. It comes back to how much the AC system drys out the air before it goes into the evap cooler, which also affects how well the whole thing works or doesn't work.

In a dry climate I think this hybrid would work very well, but in a dry climate you could just have an evap cooler, you don't really need AC; but people might still like the result of the hybrid, you would get really cold air with elevated humidity coming out the business end.

The thing that got me interested was that it could allow use of evap coolers in climates where one would not normally find them effective, because the AC would dry the air out enough to make the evap cooler work well, and the overall result would be to make cooling cheaper and more efficient. Cooling with conventional AC can be very expensive and uses lots of electricity. I live in LA, and I would like to be able to use an evap cooler here, they were still common when I was young, but it is often too humid for them to work well. On the back end of an AC system, that could change, and it could save money and energy too.

[wtmusic]

9. AC doesn't typically arify output air to desert humidity

You really need extremely low humidity and high temperatures for evaporative cooling to be effective.

Your skin is cooled by two different mechanisms: conduction and evaporation of perspiration. Although air from an evaporative cooler is lower in temperature than ambient air, when you add moisture to it less evaporation from your skin takes place - so that part of the equation is basically a wash.

Bottom line: the juice ain't worth the squeeze. Not to mention it requires water, which is in short supply nearly everywhere this scheme might actually work.

[bemildred]

10. Well, you put your finger on the areas that I'm concerned about.

And I agree it's not going to change the world unless you have a good cheap way to arify the air, without heating it, which conventional AC is not, that is not even the intent with conventional AC, it's an "unfortunate" side effect.

With evap coolers, I found that "wind chill" had an effect too, I don't know how that would balance out with the higher humidity causing less evaporation.

[Ready4Change]

15. Yah, depends on your outside climate.

In an extremely arid area (Arizona, maybe?), adding humidity to the already cooled air would be a plus.

But in an extremely humid area (think Georgia) an AC system is likely to be used as much as a dehumidifier as an air cooler. Adding moisture would defeat much of the purpose.

Personally, I can stand hot-ish temps, so long as it isn't humid. But if humidity is above 65% or so I'm generally uncomfortable, no matter WHAT the temp is.

[DCKit]

11. When water is sprayed on the hot coils outside with a fan evaporating the water...

they will chill the coils to the dewpoint of the outside air. It's why swamp (evaporative) coolers work in dry climates (without the compressor).

It's relative to humidity - temp + humidity = dewpoint

Someone is making evaporative condensers with a water jacket and claim a 60% savings over conventional air cooled units.

[bemildred]

12. Yeah, I do that myself to make my AC system work better on hot days.

My system is old and I'm going to have to do something about it. I'll see if I can look that system up.

[madokie]

13. Isn't it the evaporation that gives the cooling in a evaporative cooler?

So you would need dry hot air for one to work very good and not dry cool air. Not much evaporation going on when the temp is low no matter the moisture content of that air. As a kid we cooled our house with an evaporative cooler and it worked the best when it was setting out in the hot sun so as to increase the evaporation of the water from the water saturated filter. The hotter as well as dryer the air is the more evaporation there is and with each bit of water turned to vapor goes with it heat being remove from the air making it cooler. The amount of cooling is directly proportional to the amount of evaporation so increase that and you have more cool air so I don't think your idea will help much but will only increase the complexity and cost of cooling. Use an air conditioner in humid places and a evaporative cooler in less humid places but for the most part not the two together except where noted.

From wiki: http://en.wikipedia.org/wiki/Main_Page

An evaporative cooler (also swamp cooler, desert cooler, and wet air cooler) is a device that cools air through the simple evaporation of water. Evaporative cooling differs from air conditioning by refrigeration and absorptive refrigeration, which use vapor-compression or absorption refrigeration cycles. In the United States, the use of the term swamp cooler may be due to the odor of algae produced by early units.<1> Air washers and wet cooling towers use the same principles as evaporative coolers, but are optimized for purposes other than air cooling.

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. For example, in the United States, the western/mountain states are good locations, with swamp coolers very prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso, Tucson, and Fresno where sufficient water is available. Evaporative air conditioning is also popular and well suited to the southern (temperate) part of Australia. In dry, arid climates, the installation and operating cost of an evaporative cooler can be much lower than refrigerative air conditioning, often by 80% or so. However, evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal cooling results. Some evaporative coolers may also serve as humidifiers in the heating season.

In moderate humidity locations there are many cost-effective uses for evaporative cooling, in addition to their widespread use in dry climates. For example, industrial plants, commercial kitchens, laundries, dry cleaners, greenhouses, spot cooling (loading docks, warehouses, factories, construction sites, athletic events, workshops, garages, and kennels) and confinement farming (poultry ranches, hog, and dairy) all often employ evaporative cooling. In highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides.

The bolding is mine and I'm not sure where they are going with that. Maybe it's what you're talking about but I suspect it is used to remove some of the heat from the hot side of an air conditioner, in other words the same as locating the airconditioner in a cooler location where more heat can be liberated from the hot outside coils which would increase the amount of cooling on the inside coils.

From this I will say that the two will work together for a common cause if the evaporative cooler is located on the condenser (hot) end of the air conditioner and not on the evaporater {cool} end.

[bemildred]

14. Agreed, that is what I'm talking about.

To use an evap cooler as a humidifier, the air from it would have to wind up inside the house. This would be different from simply irrigating the outside coils or putting some irrigated excelsion on them to make them more efficient by increasing the temperature differential.

As I remember water takes up and loses large quantities of heat in changing from liquid to vapor and back, several times what it takes to get from 32 degrees F to 212 degreees F, so as long as evaporation is occurring, there is some effect. This is why you sweat, and why sweating doesn't work so good in humid climates.

So I think you could do it either way, and the result will depend somewhat on climate. Also, cooling the outside coils looks simpler to do, but putting evaporative cooling before the inside coils might offer advantages in some circumstances. I am tending to conclude that putting evaporative cooling after the insides coils would not be the way to do it.

I'm going to see if I can look up some commercial systems and see how they do it.

Thanks for your comments,

[madokie]

16. Thats the way I see it too

use an evaporative cooling system to lower the temperature the hot outside condenser coils see as the ambient temperature and you will have a cost savings.

This summer we replaced our one large through the wall air conditioner with three smaller window units located in different sections of our house and the end result is its cheaper to operate, due to better EER and I suspect the fact the cooled air doesn't have to travel as far to get to the areas where we spend our time. The units we have now have a 10.8 EER where the one we replaced was something like 8 point something, been too long since we bought it to remember. These new units are draining off hardly any water that is being condensed like the old one did and I suspect that is because in these new units they are using the evaporation of that condensed collected water as a bonus which is in effect making these units more efficient, ie a higher EER rating. When I look at the units I can see the piping that goes to the outside coil from the compressor is routed along the bottom where the water collects in a multiple S type configuration allowing part of the heat in that piping to raise that collected waters temperature to rise allowing more evaporation which does what I'm saying when I suggest to use the evaporative cooler on the outside hot coils to increase the efficiency of the unit.

We originally thought we'd put in a central unit but in my research on that we decided to go with the separate units located in the sections where we spend our time and we see the results in a somewhat, looks like a lower electric bill, but mostly in the comfort level we gained. I say 'what looks like a lower electric bill' as its hard to compare one month to the next, years apart, due to the weather is always different from year to year or month to month and day to day.

[DCKit]

18. Just a bit of trivia about the "power of water":

One pound of ice will cool one pound of boiling water to 32F.