kill-a-watt meter, the cat's meow

[JHZR2]

essentially what Im saying from my experience - old fridge with broken fan and no cooling to a new fridge is that when the coils arent cooling, the process runs longer, it doent take substantially more instantaneous power to run it.

Refrigeration is all about the movement of energy. Dirty coils, less movement of energy, the process is dumb so it keeps running to try to get to where it wants to be. The compression/evaporation work is roughly the same, just less energy is being transferred out of the system while the cycle is going on.

I suppose people have sludged up radiators on their cars that still maintain operating temp... maybe you were well within spec of how much heat shedding you needed to have done, given your temperature setpoints, thus the lack of any real change???

JMH

 

[brianl703]

For air conditioners, the hotter the evaporator coil runs the more current the compressor draws.

Refrigerators should be the same.

 

[XS650]

JHZR2 , I believe he was recording total power consumed over a 24 hour periord, so he would have seen an increase in compressor time as additional power consumption.

My speculation, initially because of an article I read few years ago, now reinforced by eljefino's observation is that a reasonable amount of dust on the coils isn't as important as it is usually made out to be. It's still a good idea to clean the coils occasionally and take look at things to see how they are doing.

 

[XS650]

quote:

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Originally posted by brianl703:
For air conditioners, the hotter the evaporator coil runs the more current the compressor draws.

Refrigerators should be the same.

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I believe you meant condensor coil, or were you just testing us?

 

[JHZR2]

quote:

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Originally posted by brianl703:
For air conditioners, the hotter the evaporator coil runs the more current the compressor draws.

Refrigerators should be the same.

---

Interesting... condensor/evaporator... I know what you mean.

But, I have to wonder how much the difference in current really is. I can understand that if a thermodynamic cycle shifts slightly due to operating conditions (like when the fan on my old refrigerator died), that the work required to be performed on the system can change - thus the amperage draw increasing. But I can't tell that it is much.

Since most temperature controllers are stupid devices, or so is my uinderstanding... they turn the compressor and thus the refrigeration cycle 'on' when T != T_setpoint, and leaves it running until T_setpoint is reached. If the system is inefficient in moving the energy (heat) away, the cycle will continue to run (adding a slight amount of heat due to the ineficiency of the process) indefinitely until it ever reaches the setpoint. This is why when the fan on my fridge died, the refrigerator ran non-stop and ate up ~$100 worth of electricity.

But another case in point - when it was 95F outside, my small window AC unit pulled 505W when running. Last night, it was only about 75-80F, but it was muggy, so I ran it, and noted 500W. When the compressor is not engaged, the fans pull ~100W. However, what was notable was that when the temperature inside and outside was lower, it was easier to move the amount of heat required, and so the compressor stayed on a lot less, as one would expect.

Extending this to a refrigerator that we can assume well insulated, so T inside is more or less constant (to within a degree or two, which would cause it to run again), the only varying factors given a cycle are the ambient temperature and the thermal transfer coefficient of the coils + film (dust). Assuming my logic above is true that the compressor is on so long as T != T_setpoint, then the rest should hold true, and since time of compresor engagement is dependent upon how fast the process can move heat, than since the process isnt moving much, itll stay on longer chewing up 500 or 505 or whatever watts for a longer period of time.

The question is, even if compressor draw is higher with dirty coils... if that is the only consideration, then would you rather have a process that pulled 2000W for 15 minutes, or 500W for 4 hours? the 15 minutes vs. 4 hours is a function of the ability to shed heat from the coils, not so much the thermodynamic cycle.

Of course correct me if Im missing something, please!

JMH

 

[brianl703]

quote:

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Originally posted by XS650:
I believe you meant condensor coil, or were you just testing us?

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I meant condensor coil, I wasn't testing you all, honest!

 

[brianl703]

quote:

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Originally posted by JHZR2:
But I can't tell that it is much.

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I've never had the opportunity to measure it, but I hosed down the condensor coils on an outdoor unit equipped with a fan cutoff device for low temperatures (to keep the indoor unit from freezing--this was a household type unit used to cool a datacenter). I was cleaning the pollen off before the unit broke down and caused 50 hard drives to overheat and fail, but I digress...

As I washed the coils with water, the fan shut off. As the coils warmed back up, the compressor made a very noticeable change in sound--it sounded like it was being more heavily loaded as the coils warmed up--until the fan came back on.