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Air conditioners

  1. Feb 19, 2010 #1
    hi all ,

    i need to find an answer to this question

    you know of course airconditioners draws a high current compared to other devices in apartments , my question is :

    assume you have an air conditioner 1.5 hp and the maximum cooling you can get from it is to make temp at 18 degree .

    is this means that the 1.5 hp will be the full load at 18 to obtain this temperature

    in other words , if i adjust it at 25 degrees , it is going to draw the same current for making temperature 18 degrees ??

    thanks , and i hope every one share his thoughts with me
  2. jcsd
  3. Feb 19, 2010 #2
    In a domestic air conditioning unit there are usually three consumers of power, the compressor (the largest consumer), the condenser fan and the evaporator fan (the one that blows cool air into the room). The control system for these units is fairly basic in that they run until the room thermostat reaches the required temperature and then switch off, (they might leave the evaporator fan running). When the unit is running it will always draw the same current, if you set the desired temperature lower the AC unit will need to run longer to reach the required temperature and consume more power in total. Whatever you do don't do what I found my mother-in-law doing, opening the windows because it was too cold. Note that the air coming out of the AC unit will probably be much colder then the thermostat setting so take care that when you fit a unit that it is not blowing across your favourite chair
  4. Feb 19, 2010 #3
    thanks jobrag for your answer

    so you think airconditioner is always working at full load , but according to setting it will depend how long it will be working for example for 18 degrees it will work continously for 20 minutes , but for 22 degrees it will work continously for 10 minutes [ time i say is just for example of course :D]
  5. Feb 19, 2010 #4


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    Staff: Mentor

    That last part isn't correct. The electrical load on the compressor is dependent on the particulars of the thermodynamics at the time. Since the inside temperature doesn't tend to change much*, the outside temperature is the biggest driver of the compressor wattage. 18 to 22C isn't a very big difference, though...

    *Though if you let your apartment warm up to 30C and it is 30C outside, it will work a whole lot harder than if it is maintaining 20C while it is 30C outside.
  6. Feb 19, 2010 #5
    thank your russ for answering

    and i want to ask you some explanations in order to understand

    let's say the ambient temperature is now 35 and i adjust my airconditioner at 18

    so it will work at full load in order to make the ambient temperature to reach 18 say it works for 1 hour continously

    now case 2 : assume the ambient is 35 , and i adjust the airconditioner to 27

    it will works also at full load but may be for half an hour in this case it draws the full load current

    after that the ambient became 27 , after a while the ambient temperature increases , say became 30 , the AC wants to bring it back to 27 , the important question now is ::::

    is it going to work at full load [ i mean the comperessor] but for a short time

    or ?

    it will work but not at full load [ the comperessor will not draw it's full current ]
  7. Feb 21, 2010 #6
    Consider the the air conditioner as a light bulb with the thermostat as the on off switch.

    A light bulb with a switch for control is either on or off. Your air conditioner is the same. It is either on or off.

    The power consumed by your A/C is resonably constant with in 10 to 15%. The hotter it is outside where the A/C is dumping your room heat, the harder it has to work to get rid of that heat therefore it consumes a bit more power.

    I live in Texas where it can get to 100 deg F frequently in the summer. If I spray a fine mist of water on the condensor coils (the outside coils) by measurement, the power consumed by the compressor drops about 10%. I had been hoping for more.

    Hope this helps with your understanding.
  8. Feb 21, 2010 #7


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    Staff: Mentor

    Sorry, I missed this....
    It will work at less than full load and it will work for a shorter time.
  9. Feb 21, 2010 #8


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    Staff: Mentor

    I'll have to check on that - I'm reasonably sure it is quite a bit more. Some manufacturers publish performance data, but not all do.
  10. Feb 21, 2010 #9
    thank you all for your contribution in this thread , i am very glad

    i agree with you russ because actually i thought same as you

    but i think there is something called evaporator valve or something around this name a friend of mine but in the field of mechanical power talk with me before about something like this

    so if anybody has information about this issue i hope he could share the info with us

    also i will try to bring more information through catalogs

    thank you all :smile:
  11. Feb 21, 2010 #10


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    Staff: Mentor

    Expansion valve. #4 in this link:

    It's a fixed orifice nozzle.

    I'll look for regular air cooled data, but I remember a water source air conditioner that has published wattage data at different temperatures. Here's a link, see page 9 for performance data: http://www.climatemaster.com/downloads/LC505.pdf

    They all are at entering air conditions of 80F (27C) and 51% RH (which is pretty humid at that temperature). Here are wattages at various entering water (or water/glycol mix...) temperatures:

    60F/15.5C: 4.8 kW input, 88 MBH/26kW output: CoP: 5.4
    90F/32C: 6.2 kW input, 81MBH/24kW output: CoP: 3.9
    120F/49C: 8.3 kW input, 67MBH/20kW output: CoP: 2.4

    So input kW goes up quite a bit as the incoming water temp goes up (though you'll see that it hits a low 60F and goes up again). Over the range I showed data for, input kW went up 5% and output went down 0.7% for every 1C temperature rise (I'm sure it isn't linear though).

    Tcarey, I suspect you just didn't have enough water to make a big difference in performance.
  12. Feb 21, 2010 #11


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    Ok, found some air cooled condenser data. Page 30: http://www.docs.hvacpartners.com/idc/groups/public/documents//24acb3-3pd.pdf

    75F/24C - 1.2 kW input, 18.8 MBH/5.5 kW output
    85F/29C 1.4 kW input, 18.0 MBH/5.3 kW output
    95F/35C - 1.5 kW input, 17.1 MBH/5.0 kW output
    105F/41C - 1.7 kW input, 16.1 MBH/4.7 kW output
    115F/46C - 1.9 kW input, 15.1 MBH/4.4 kW output
    125F/52C - 2.1 kW input, 14.0 MBH/4.1 kW output

    For the entire range, that's a 6% rise in input kW and 0.9% drop in capacity per 1C.
  13. Feb 21, 2010 #12


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    I actually need this stuff for a project I'm working on right now, so FYI, I've graphed the data from that first link (in English units). Attached.

    For those who don't know MBH and tons, a ton of refrigeration is 12,000 btu or 12 MBH. kW per ton is a common way to express air conditioner efficiency. It uses the capacity graph to normalize the kW graph. In other words, it gives the input kW assuming a constant output capacity.

    Attached Files:

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