Air Conditioner Question

too expensive to run in that mode.

Dear Chris W:

As "ms" said it is too expensive to run this way. But they do run this way as the temperature difference gets larger, or the refrigerant charge is lost over time.

David A. Smith

You can do that by controlling the throttling of the expansion valve based on temperature feedback, or even on suction pressure. This affects the amount of torque required from the compressor motor, which would draw less power when not cooling as much. But the amount of energy saved from the lower torque would not make up for running the compressor and fan all the time and the system would not be very efficient.

Don Kansas City

Some of the new dual stage systems do exactly that. Two compressors, little one runs most of the time big one kicks in as needed. $$$$$$$ They sure are not your track home units.

From that and other posts, it sounds to me like the real answer to my question is, it is too expensive to make an air conditioner that can run efficiently at a wide range of outputs. An air conditioner that ran continuously would need to efficiently put out a little cold are, a lot of cold air, or anywhere in between. I'm guessing to do that you would need a variable displacement compressor pump, variable speed fan on both the condenser and evaporator. That would certainly raise the cost a lot. If I'm not mistaken, reducing the speed a fan spins, effects it's efficacy too. so you might even need a fan with a variable pitch. Maybe just multiple fans to cover the surface of the condenser and evaporator and turn on as many as needed.

Anyway, I think I would have the same problem on really hot days. I hate having the cold air blow on me. I guess the only solution is to somehow diffuse the air, by having bigger or more vents, so it doesn't move so fast.

Dear Chris W:

Insulate the walls, and run chilled water along the inside of the walls. Then there is no air blowing.

David A. Smith

Probably wouldn't cool very quickly without any forced convection to help. You would have to rely on the natural convection of the falling cold air along the wall to eventually bring the room to equilibrium.

Don Kansas City

I wouldn't really need to be that fancy. You can control the temperature of the evaporator coil by varying the amount of throttling. The compressor still chugs along merrily and the fan still runs at the same speed. There are all sorts of controllable expansion valves available.

Don Kansas City

Bad idea! Water would condense on the walls and then run down on to the floor. You would end up a damp, moldy room.

Sorry, Paul D Oosterhout I work for SAIC

No, there is just condensation all over the walls, inside and out. What follows quickly thereafter is mold inside and outside the walls, follwed by sickness and maybe death

ASHRAE has criterion for moving air when air conditioning - see the ASHRAE manuals.

If you move air too slowly, room air will stratify, and if you move it too rapidly, it will give you windchill :-)

15 FPM is the recommended flow rate for AC, 5 FPM for heating. (You can drop the size of an AC unit significantly by keeping air moving over 15FPM in rooms vs having it below 15FPM. ) Home units have AC on high speed, and heating on low speed fan settings.

Continuous running provides more even air and fewer complaints about comfort.

Running the fans 24/7 is the least costly, both from maintenance and from energy use, even when not even considering the savings from raising or lowering temperature from nominal by running 24/7. (think starting the entire mass of air in a building moving each time you start a fan vs just overcoming friction as it moves continuously - in a house test moving 25,000 cu ft continuous vs on demand, one month one way the next month the other, the difference was $20 per month lower for

24/7. Then think of the wear on a bronze bushing starting and stopping from dead stop dozens of times a day for years vs non-stop running wear. )

----- And as to vents - there are roughly 5-6 types for each size, for different applications and velocities.

If the air is moving too fast from the vents you have, get a vent that is more diffuse or one that can direct air off the workspace. (Normally, vents for AC are directed upward so the cold air it moves into the room drops gently through the room's exisitng warmer air, while heating vents are directed along the walls to warm them and directed down so the warm air rises through the room's exiith sting cooler air. In one of my rooms that is 25 feet long with lots of glass and wall vents, I had to change the vent types spring -cooling type- and fall -heating type- to get the comfort desired )

fwiw

It works well with hor water *heating*, but there is a potentially larger temperature difference to do the driving.

David A. Smith

Good point. Actually the whole room wouldn't be moldy, only the "condensor". The dew point in the room would be close to the water temperature...

David A. Smith

Dear Harry Andreas:

...

Yep.

Nope. See where I said to insulate the walls above?

Consider that the condenser coils in a refrigeration system do exactly this. Yet "sickness and maybe death" is a rare thing. The metals of construction probably serve to "poison" most biogrowth.

David A. Smith

To the Group:

So the consensus is that this is a bad idea.

How about a wall-sized waterfall? Chilled water, turned on when the room is to be cooled. The humidity in the room will be near the set point of the water temp. And the moving water will be a driver for air flow. Not the most efficient system perhaps... also only a single-room solution. But the air is constantly cleaned, kept at more-or-less constant humidity, and no directed cold air stream.

David A. Smith

David, I like this idea! It would be a great way to to cool a greenhouse or an indoor temperate rain forest exhibit! The waterfall would lower the ambient temperature while keeping the humidity high.

Unfortunately, this would be a poor way to cool and office or a house. The main benefit of air conditioning is that it removes humidity from the air. I don't know about you, but I'm more comfortable in warm dry environment than a cool damp environment.

I live in Virginia where hot, humid summers are the norm. Comfort is all about the dew point. ;-)

I wish I lived near Sante Fe, NM.

Paul D Oosterhout I work for SAIC

It would *lower* the humidity. Where do you think the water comes from?

Don Kansas City

It depends entirely on the temperature of the water, and the dew point of the air it interacts with.