Air Con Workings

[KannikaP]

I read so many posts about A/Cs and would like to clarify a few things.

I have conventional Mr Slim a/cs in my home, 4 x 9600BTUs. 

According to an expert on here, they SHOULD take about 10kW but go on & off 50% of the time depending on the temperature set on the remote.

As I see it, the unit draws in air at the top, obviously at the room's ambient temperature, runs it past the cooling vanes and out at the bottom. Those vanes are cooled by the refrigerant passing through them supplied by the compressor.

The air coming OUT of my a/c is at 16 degrees C (verified with a thermometer) and no matter what temperature I set my remote, will run until the air being drawn IN at the top, ie the ambient room temperature reaches that.  Is this correct?

I know that Inverter a/cs vary the speed of the compressor to maximise efficiency, but mine are conventional.

 

[Crossy]

Yeah, for a conventional unit that's pretty much it, although your 9,600BTU units will be drawing more like 1kW when the compressor is running.

 

How often it cycles depends upon the set temperature, ambient temperature and the size of your room along with any heat sources in said room, humans, computers and of course windows etc.

 

I usually say a duty-cycle of 30% is about right for a correctly sized unit that's not set for arctic temperatures.

 

[gamb00ler]

I would really appreciate if an A/C expert would explain the difference in the operation of A/C units on the de-humidify (DH) vs the cooling mode.

 

Obviously both modes produce both cooling and de-humidification.

 

I think @KannikaP provided a good explanation of operation in cooling mode.

 

I have read that in DH mode the compressor runs more frequently but at a lower output to keep the cooling unit at just a low enough temperature to constantly cause condensation.  When walking past the compressor unit it seems that on DH mode the exhausted air is not as hot as when the unit operates in cooling mode.

 

 

 

[KannikaP]

6 hours ago, Crossy said:

Yeah, for a conventional unit that's pretty much it, although your 9,600BTU units will be drawing more like 1kW when the compressor is running.

 

How often it cycles depends upon the set temperature, ambient temperature and the size of your room along with any heat sources in said room, humans, computers and of course windows etc.

 

I usually say a duty-cycle of 30% is about right for a correctly sized unit that's not set for arctic temperatures.

 

Sorry Crossy, another typo from me. Yes, 1kW is correct, as it was you who told me.

[bankruatsteve]

3 hours ago, gamb00ler said:

I have read that in DH mode the compressor runs more frequently but at a lower output to keep the cooling unit at just a low enough temperature to constantly cause condensation.  When walking past the compressor unit it seems that on DH mode the exhausted air is not as hot as when the unit operates in cooling mode.

The "DH" or dry setting is usually the lowest setting the AC will operate.  Whether it condenses or not depends on the humidity in the room.  My Daikin inverter on dry will keep the compressor on until the ambient temperature drops below about 22C or so.  In any case, after it has been running normal for a few hours the RH drops into the 40's or even 30's and there is little left to condense.  At that point, it just puffs cool air into the room and maintains about 25C.

[tjo o tjim]

Dehumidification mode also usually slows down the fan on the indoor unit so the air has more contact time with the coil. 
 

Some units do actually vary the coil temperature depending on space temperature and/or the delta between set point and space; dehumidification mode puts it in the lowest coil temperature. 

[Yellowtail]

"As I see it, the unit draws in air at the top, obviously at the room's ambient temperature, runs it past the cooling vanes and out at the bottom. Those vanes are cooled by the refrigerant passing through them supplied by the compressor."

 

Correct

 

"The air coming OUT of my a/c is at 16 degrees C (verified with a thermometer) and no matter what temperature I set my remote,..."

 

Sort of correct. The temperature of the air coming out, is largely dependent on the temperature of the air going in. Once the room cools down, this should be somewhat constant while the compressor is running, but when the compressor cycles-off, the air coming out should start to warm, and continues warming until the compressor cycles back on.

 

"...will run until the air being drawn IN at the top, ie the ambient room temperature reaches that. Is this correct?"  

 

Reaches what? The setting on your remote or the temperature of the air coming out? The compressor should run until the return-air (going into the top) is at the lower-level of the temperature you set with your remote, at which time the compressor should cycle-off, and the air coming out should start warming, and should continue warming until the return-air (going in the top) reaches the upper level of the temperature you have selected with the remote. 

 

"I know that Inverter a/cs vary the speed of the compressor to maximise efficiency, but mine are conventional."

 

An inverter unit will generally vary the speed of the fan as well. 

[digbeth]

If the experts are in here, if you don't mind another question:

does the Thermal mass of the radiator (and refrigerant in the loop?) matter when the compressor cycles on and off? like are there any noticeable difference between light aluminium radiator vs heavy copper ones in one being able to be cycle off longer and the room temp won't fluctuate? 

[KannikaP]

42 minutes ago, Yellowtail said:

The compressor should run until the return-air (going into the top) is at the lower-level of the temperature you set with your remote, at which time the compressor should cycle-off, and the air coming out should start warming, and should continue warming until the return-air (going in the top) reaches the upper level of the temperature you have selected with the remote. 

In my case, I set ONE temperature on the remote.

[Yellowtail]

2 minutes ago, digbeth said:

If the experts are in here, if you don't mind another question:

does the Thermal mass of the radiator (and refrigerant in the loop?) matter when the compressor cycles on and off? like are there any noticeable difference between light aluminium radiator vs heavy copper ones in one being able to be cycle off longer and the room temp won't fluctuate? 

I'm no expert, but I'm going to say yes, that (all things the same) the mass of the "radiator" and refrigerant will affect how often how effective the air is cooled, which will affect when the compressor cycles on an off. 

 

That said, the room temperature is going to fluctuate regardless.

 

And to be clear, aluminum and copper would be an example of all things NOT being the same, as the thermal conductivity of copper is much higher than that of aluminum, so an aluminum coil needs to be much larger (40-50%?) than a copper coil to provide the same heat exchange.

 

There are other benefits of copper as well. For air conditioning, as far as I know,  the only benefit of aluminum coils over copper is cost. Copper coils cost significantly more than aluminum coils. 

 

 

 

[KannikaP]

51 minutes ago, Yellowtail said:

Reaches what? The setting on your remote or the temperature of the air coming out?

As I said in my first post, the air COMING OUT of the A/C is always the same, ie 16C. So until the air going IN at the top is at the temperature you set on the remote, the compressor will run.

If I put it on Economy Cool, the air coming out is at 19C.

However these things work though, the cooler you set on your remote, the size of the A/C unit and room, and the length of time you have it on ALL TAKE MORE ELECTRIC = MORE MONEY.

[Yellowtail]

9 minutes ago, KannikaP said:

In my case, I set ONE temperature on the remote.

Yes, but the unit measuring the return air has an upper and lower level which signal the compressor off and on, otherwise it would cycle off and on continuously.

[Yellowtail]

9 minutes ago, KannikaP said:

As I said in my first post, the air COMING OUT of the A/C is always the same, ie 16C. So until the air going IN at the top is at the temperature you set on the remote, the compressor will run.

If I put it on Economy Cool, the air coming out is at 19C.

However these things work though, the cooler you set on your remote, the size of the A/C unit and room, and the length of time you have it on ALL TAKE MORE ELECTRIC = MORE MONEY.

If you're trying to reduce cost, always run the fan on high. 

[KannikaP]

4 minutes ago, Yellowtail said:

Yes, but the unit measuring the return air has an upper and lower level which signal the compressor off and on, otherwise it would cycle off and on continuously.

Oh, you didn't say that earlier. Yes I now understand. I still set say 28 on my remote, but the A/C will kick in at 30 and off at 28C

[KannikaP]

5 minutes ago, Yellowtail said:

If you're trying to reduce cost, always run the fan on high. 

Really, why please? If the fan is pushing air across the vanes faster, will it not have less time to cool it. Just using logic. 

[Yellowtail]

2 minutes ago, KannikaP said:

Oh, you didn't say that earlier. Yes I now understand. I still set say 28 on my remote, but the A/C will kick in at 30 and off at 28C

Sorry I was not clear.

[Yellowtail]

1 minute ago, KannikaP said:

Really, why please? If the fan is pushing air across the vanes faster, will it not have less time to cool it. Just using logic. 

And using the same logic, if you turned the fan off, the air around the coil would get really cold, yes? The room? Not so much. 

 

The idea is to get the maximum amount heat out of the air in the shorted mount of time. 

[tjo o tjim]

14 hours ago, digbeth said:

does the Thermal mass of the radiator (and refrigerant in the loop?) matter when the compressor cycles on and off?

Not really.  You have an impact on temperature gradient between the coil and air impacting rate of heat transfer, but for the compressor that is a non-issue.  

 

The main factor is the load on the air conditioner.  Here are two meter readings from an air conditioner-- first one is more heavily loaded than the second.  This is an inverter unit; the second one will look more like what you have as it is at the minimum inverter setting.

 

 

[tjo o tjim]

13 hours ago, Yellowtail said:

And using the same logic, if you turned the fan off, the air around the coil would get really cold, yes? The room? Not so much. 

 

The idea is to get the maximum amount heat out of the air in the shorted mount of time. 

I might say it a little differently-- it is more efficient because you are taking less water out of the air with the fan on high.  With an inverter unit there are other options as well, like reducing the refrigerant pressure delta.

[digbeth]

13 hours ago, KannikaP said:

Really, why please? If the fan is pushing air across the vanes faster, will it not have less time to cool it. Just using logic. 

wind chill effect on the skin, making you feel colder for less cost? 

[KannikaP]

15 minutes ago, digbeth said:

wind chill effect on the skin, making you feel colder for less cost? 

So walk around naked for greater effect!   LOL

[KannikaP]

1 hour ago, digbeth said:

wind chill effect on the skin, making you feel colder for less cost? 

And that will only apply if some of your bare skin is in direct line with the fan.