Does fan speed affect the efficiency of my air conditioner?

Discussion in 'Highly Technical' started by NeoPTLD, Aug 9, 2004.

  1. NeoPTLD

    NeoPTLD Platinum Member

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    I've got a window air conditioner that has low, medium and high fan speed. The fan motor sits in the center of the unit and shaft comes out at both ends. It drives both the condenser and evaporator motor.

    Which fan speed will give the best efficinecy if it makes a difference and why?
     
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  3. imported_elwood

    imported_elwood Senior member

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    Well, i'm not a professional, but my dad is an AC and appliance repairman. From what i have learned, i would say to run your fan at its highest speed. Running it at the highest speed should suck/blow more air throughout the coils, cooling your freon to a lower tempurature and reducing the pressure. With your freon running cooler the compressor doesnt have so much of a load, thus making it work less and burning less electricity. I'm sure, since the freon is cooler, it will be able to absorb more hot air from the inside of your house and dump it outside (throught he cooling of the freon in the outside coils).

    Always keep the coils clean. Once a month, rinse the coils with a garden hose GENTLY! You can easily bend the VERY THIN aluminum fins! It is also recommended to have the system professionally cleaned/serviced once a year. It usually runs around $30-$50 for window units.

    A really great tip for window units is to buy a small roll of window screen and a roll of 1/2 inch magnet. Cut the screen to fit the intakes on the case of the unit (there is usually 2 or 3 diff spots). Cut your magnet to fit the screen and staple it together. You could also tape it but that leaves a residue if you ever want to change it.

    If you end up using screens, make sure to clean them ever 1-2 months since they clog up very fast. I know down here in the "mosquitoe capital of the world" they get chocked solid with mosquitoes and dust within a month.

    I know this isnt the most technical explination, but like i said, i'm not the professional here! :D


    I hope this helps :)
     
  4. 1EZduzit

    1EZduzit Lifer

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    Good question. Of course I'm just guewssing but the more air thru the condenser would be more efficient, but it would also move the the cool air farther away form the air conditioner and cool a larger area. If the room was closed off so it only had a specific area to cool then I imagine it would be a little more efficeint, but it would also be LOUDER. I keep mine on low.
     
  5. Calin

    Calin Diamond Member

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    The efficiency of the "thermic engine" that is an air conditioner varies with the temperatures of the hot source and cold source. An air conditioner is just a kind of thermic engine in reverse.
    A thermic engine transfer heat from a hot source to a cold source. A part of the heat is "lost" in the process and transformed in work at the output shaft of the motor (or engine). The best engine works on the Carnot cycle, and its efficiency is 1 - Tcold/Twarm, where T is the temprerature in Kelvin degrees (absolute temperature). So, having a hot warm source and a cold cold source improves the efficiency.
    Now, a A/C unit absorbs work (power * time) and moves caloric energy ("heat") from a hot source to a cold source. If your system is reversible, then you have also a heat pump, that would take "heat" from outside and move in inside.
    The efficiency of the A/C unit is also dependent on the difference of temperature between inside and outside. It works just like an engine, but with the "warm source" colder than the "cold source". The only way to make such an engine work is to inject power from exterior.

    As a result, having either your inside or your outside unit clogged will decrease the efficiency of the A/C unit. You could put the inside unit in a place where to have great air flow, but the noise considerations limit the fan speed for it. So, only the exterior unit can have lots of forced airflow.
    Also, the heat transfer rate between the outside air and the fins of the outside part of the unit is dependent on their temperature difference, and while the A/C unit is on "slow" you gain almost nothing with the outside on high speed

    Calin
     
  6. 1EZduzit

    1EZduzit Lifer

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    Wow, you sure give a good explanation. But all of the window units I have seen are dirven with one motor that has a shoft on each side of it. When the unit is on low, it slows down both the outside and the inside fans. I think it also runs the compressor and would low it down also.

    So wouldn't it being on low make it use less electricity and cool a smaller space, but be more ineffecient?

    Conversley, high speed would be more efficient but it would also be cooling a larger area?

    I just had to replace my thru the wall unit and purchased a Whirlpool 24,000 BTU unit, but I wish I would have bought the Fridgidare unit instead. I didn't notice it when I bought it, but the Whirlpool puts out more CFM of air on low (545 CFM) then the Fridgidare does on high (505 CFM). It is ungodly loud and I wrote to Whirlpool asking if I could put a rheostat in it to slow it up. All they said was
    Not a lot of help. Since reducing the power to the motor will also reduce the power to the compressor I'm not sure if it would be a good idea. I suppose I will have to sell it and get a quiter one or cut another hole in the house and mount it there, which is an option I don't care for since my wife has sever allergies/skin problems and needs the air on even when it is 75 F if the humidity is high. The unit is in the living room and is so loud that the TV needs to be on almost full volume to hear it. You can't even hear the doorbell when watching TV.

    Don't get a whirlpool window air conditioner if you need one 18000 BTU's and noise is a factor. I have since looked at the spec's and they are are all louder on low the the corresponding Fridgidare unit is on high.
     
  7. Tiamat

    Tiamat Lifer

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    This sounds like an exam question I had in my Chem Eng Thermodynamics 2 Course. Smith and VanNess could probably help out here :D

    I assume the efficiency that you are looking for is the cost efficiency of cooling your room.

    Assumptions:
    T inside (final) = 284K (52F)
    T ouside = 311K (100F)

    If your air conditioner worked similar to a simple carnot system, your Efficiency would be 0.09 roughly.
    This is probably the best case scenario. Inefficiencies due to more parts (motor, etc) lower this value ever more. You are probably looking at a max real efficiency of .05 (non-technical guestimation)

    At this low efficiency, it probably doesnt matter what fan speed you use - you will probably maintain extremely similar efficiency. So, the only benefit to high speed on fan is that you cool your room faster at the cost of more noise. Using Low speed on fan will cool your room slowly at a lower noise - both ways costing the same electricity in the end.

    I apologize if ive made a fool of myself and did this totally wrong.

    Hope it helps.
     
  8. sao123

    sao123 Lifer

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    The fan and compressor units are usually driven by separate independent motors.
    most air conditioners have 2 separate controls, fan speed and temperature. The temperature control controls the speed of the compressor, because the faster it turns, the more compression/decompression takes place, and the more heat can be pumped out.

    The fan speed controls how much air moves past the heat exchange area. Obviously the more air moving through the exchanger, the more heat that will be pumped out of the house.

    now since the compressor uses 75% of the electricity coming into the a/c, and changing the fan speed, affects the compressor not. So, without changing the temperature, fan on the highest settings is most energy efficient. Now if you turn down the temperature then it may be more efficient to lower the fan speed. u would have to graph out the volume of air vs the heat transfer cabaility of the pump on all speeds, it is best when they are very close.
     
  9. 1EZduzit

    1EZduzit Lifer

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    I'm not as concerned about the efficiency (unless it is a BIG difference) as I am the noise. My wife has a severe skin condition and is basically allergic to her own sweat so she has the air on whenever it gets warm or humid. Since it is on so much and is so loud (even on low) it requires that the TV be at or close to full volume in order to hear it. I either need to quiet it down or sell it and get something else.

    Yes, central air would be the best but our house has hot water heat, so putting in the air ducts would raise the cost quite a bit. The old unit went out late summer last year and it was very hot at the time so I just bought the first one I could find and put it in for her. Then I found out how loud it was. :(

    If I can quiet this one down enough to live with it this summer, I will look into getting one of those new central air conditioners that have the small insulated vent pipes that can be installed in the attic and put the vents on the ceilings of the rooms. Then there would be no more noise!!! Butr I need to see what all is involved and save up for it a bit.

    Thanks for the help.
     
  10. DrPizza

    DrPizza Administrator Elite Member Goat Whisperer
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    I'd have to say that there's a happy medium... With almost no airflow, of course efficiency is going to be low. But, if you crank the air up to super super fast (beyond the normal capability of an air conditioner), it should be obvious that you're decreasing efficiency by using more energy for the fan without any gain in cooling ability.

    As Sao said above... it has a lot to do with matching the heat transfer capability to the necessary volume of air.
     
  11. jagec

    jagec Lifer

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    fixed, since this question was posted in the summer I can only assume he's using the AC unit to COOL his room, not heat it.
     
  12. 1EZduzit

    1EZduzit Lifer

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    I have to believe that since a Fridgidare unit of the same size only puts out 400 CFM on low speed as compared to this units 545 CFM on it's low speed that the efficency loss wouldn't be that great. I would definitely lose some efficency, but I think i could live with it.

    I was under the inpression that slowing up the fan would slow up the compressor, but that was incorrect. So no one thinks that besides the decrease in efficency that I would harm the fan motor/air conditioner by slowing it up the fans by 25%?

    I think becasue of the high CFM capacity of the Whirlpool unit it would be great for cooling a large room or installing in a back room that doesn't get used much (to get the noise out of the living area). Cold air is hard to move and the high CFM capacity of the unit should help greatly in that situation. I just bought the wrong unit for my purpose and want to get by with it for at least one more season. If I can the reduce the fan noise enough I would keep it longer, but the way it is is unacceptable.

    Thanks for the help guys. :)

    EZ
     
  13. Calin

    Calin Diamond Member

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    jagec, some A/C units can only cool, while some other units can also cool and heat. The unit will work as a "heat pump" during winter, when the outside part will be colder than the outside air, and the inside part will be warmer than the inside air. It is just like a fridge: when you are inside it, it is cold, and when you are outside it, it is warm (the radiator on the backside).
    About the necessary airflow: considering the efficiency of the unit, at the Kelvin scale of temperatures, then I can assume that a 10K increase in temperature differences (something like a 18F) would lower the efficiency with something at most 25-30% relative to the current efficiency.
    If the increase in costs (electricity) is of no concern, then you could reduce the airflow. Check the temperatures of the cooling fins before and after the CFM reduction, and if the difference is something up to 18F you should not worry.
    Simply, the compressor fan will work harder and the airflow fan will work easier. A 25% for each should not disturb you (but the compressor will work at maybe 5-10% extra)

    Calin