Why do smaller airconditioners have higher EER?

[zxcvbn]

Why do smaller air conditioners generally have a higher EER and COP?

The question relates, for example, to domestic split system air conditioners.

Take a look at the specs from any manufacturer, and you will see what I mean. The smallest systems of around 2.5kW claim amazingly high EER of up to 4.6, by the time you get to 6kW you won't find better than around 3.4, and go to 8kW and you are lucky to find 3.0.

Of course, the EER numbers I quote use proper units of Watts for power input and cooling capacity. You can go back to the last century and use BTU units if you like, but the conclusion will be the same, the smaller units have the highest EER and COP.

On the face of it, this is the opposite of what I would expect, for generally speaking, larger machines are more efficient. For example, it is a fact that larger electric motors are generally more efficient. All else equal this should mean that the larger units would have a higher EER and COP, and yet exactly the opposite is observed in practice.

This raises 2 questions.

(1) Why do smaller air conditioners generally have a higher EER and COP?

(2) Everyone would prefer a high EER, so why doesn't someone make a larger capacity unit (eg, 6kW domestic split system) with the same high EER of the smaller units? Even if there is some odd reason why smaller systems are inherently more efficient, it is obviously possible to build a larger unit consisting internally of several smaller units, thus giving the same very high EER/COP of the smaller units. I would buy a 6kW split system with an EER=4.6 tomorrow if it was available, even if it cost more. Have the engineers that design these things got rocks in their heads? I doubt it, so what is going on here?

[taz24]

.

I don't know the real answer to your question but EER and COP are calculations that
compare power used in the running of the system to the cooling or heating output.

Could it be as simple as the bigger the system the more power consumed so the ratio
drops a little as the systems get bigger?

I might be simplifying it too much???

All the best

taz

.

[Yuri B.]

A random citation from internet.

Sharp AF-S60MX (6,000 BTU)
This unit is Energy Star compliant with an EER rating of 10.7·
......................................
Kenmore 75121 (12,000 BTU)
This model cools up to 600 square feet. It is Energy Star compliant, has an EER rating of 10.8

[paul_h]

I gather the OP is an end user, not in the trade.
(so free to over simplify as much as you want Taz :) )
Based on your location, this is a good question to pass on to Stef, as I heard pana had some problems with their larger units...


Some engineers and designers do have rocks in their heads for sure, based on some of the units I deal with. But all major manufacturers have problems with this system as well, so I say the rating system is probably not the best.

[RSTC]

Interesting question and a good observation.

A quick glance across the 'net and it seems zxcvbn has a point, although not all manufacturers have the same issue. eg hitachi drop from a 'rated' cop of 3.6 for the 2kw high wall to 2.61 for the 8kw... mitsibuishi seem to remain very much the same for their Mr Slim high wall.

The Mr Slim is inverter driven, so that may have in impact.

I would hazard a guess that manufacturers will strive to have equipment as small as possible and also a standard range of parts eg they may use the same plastic casing for the 2,3 & 4 kw evaporator, so fans will become less efficient or pressure drops higher. I wonder will this be the same for the compressor/capillary tube?

[Yuri B.]

I would hazard a guess that manufacturers will strive to have equipment as small as possible and also a standard range of parts eg they may use the same plastic casing for the 2,3 & 4 kw evaporator, so fans will become less efficient or pressure drops higher. I wonder will this be the same for the compressor/capillary tube?

Yes, for example, the 4 way valve may be the same for all 3 units, but pressure drops will be highest for the 4 kW unit. So, more losses.

[chillerman2006]

Hi zxcvbn

When Oportunity Strikes

I think we should be trading info here, some of our fridgie knowledge for some of your atomic & molecular physics knowledge ;)

for instance I explain pressure testing/leak checking with nitrogen against helium as comparing beach balls to size 5 footballs due to their molecular size

with your far superior understanding of this field could you explain for me/us

why helium will get through smaller gaps than nitrogen

and do both gases have a smaller molecular mass than flouracarbon/hydrocarbon refrigerants
be appreciated :)

R's chillerman

[zxcvbn]

Hi Chillerman,

You are evidently at least as proficient at googling as with refrigeration. ;)

Yes, my area is physics research, but refrigeration is ultimately just physics.

It also turns out that vacuum technology and leak testing are an important part of the work that we do, as much of our equipment operates under high vacuum, and we use helium leak detectors to find the leaks. However, your mechanical vacuum pumps are what we call 'low vacuum'. We use high and ultra-high vacuum pumps and techniques to achieve vacuum in the 10^-6 to 10^-11 Torr range, which is a million times or more lower than you guys need.

I'm not sure exactly what leak checking methods you use, but we use helium leak testers for two reasons. Firstly, you are correct in pointing out that helium leaks easily through small holes, because it is a small molecule. Remember your periodic table, Hydrogen, Helium, Lithium, etc, only hydrogen is smaller and will leak out easier. The other reason our leak testers use helium, is because there is very little helium naturally occurring in the atmosphere. Our leak testers are really a sensitive mass spectrometer tuned to helium, that is, will respond only to helium, which makes it possible to detect minute amounts of helium that leak into the system when a fine jet of helium is passed over a small hole.

I am not familiar with the chemistry of refrigerants, but almost certainly they will be larger molecules than helium, and therefore leak more slowly.

It is lucky that hydrogen is not used as a refrigerant, as hydrogen molecules are so small as to diffuse through most materials, even when there is no hole as such. We have problems with hydrogen that becomes diffused deeply within the metals of our vacuum chambers, which can only be removed by pumping for days or weeks, while heating the entire chamber up to 200 DegC or more, known as a 'bakeout'. You guys have got it real easy.

In respect of my original question, I believe RSTC has pretty much got the answer nailed. I'll talk more about that in another posting, with some evidence to back it up.

Cheers, Colin

[JoeAT50A]

Hi EER and COP required, large capacity with smaller input in power.
Larger capacity required large surface area and respective large air volume.
Smaller input required smaller efficient compressors and motors are required.
Bigger system required not only compressor but also big accumulator and those need an additional crankcase heater or heating element required.
Of course water cooled units have super high COP/EER than air-cooled units.
End of the day, not economically sound to make high COP and EER for bigger units unless maket demand and law could set the bar to something like (MEPS Australia) minimum performance to import. Then you could able to see the units have met your expectation. If not cost of production is rathar too high to compete the compitative market. You could able to see those products in your country soon, your MEPS has been planning for bigger range A/C and HR.