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Old 26-06-2019, 14:01   #1
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Cubigel/Frigoboat fridge power consumption

I have a Cubigel compressor for my fridge, it uses R134a and is similar to the Danfox BD35.

The firmware is set to "smart" mode which reduces the RPMs and extends the run duration to minimize energy consumption. I've written some software which connects to the firmware control unit and have been measuring the run information - it runs for about 20-25 minutes, shuts down for 3 minutes, and repeats that cycle. The RPMs are at a low 1600RPM which, according to the Cubigel documentation, should consume 2.3A. Unfortunately the fridge is consuming just under 4A.

I cannot reach the return line to see if it is frosted over, it is twisted with the feed line so when I measure the temperature back at the compressor they are almost identical. The evaporator is frosted throughout its length.

Could the system be overfilled? Or is there some other likely cause for the problem?
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Old 26-06-2019, 16:58   #2
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Re: Cubigel/Frigoboat fridge power consumption

If the wire size for the run length isn't right, it will draw more amps. Are you getting correct voltage when it's running? Lower the volts - more amps.


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Old 26-06-2019, 17:40   #3
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Re: Cubigel/Frigoboat fridge power consumption

The voltage is correct, there is no voltage drop between the bank and the fridge.
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Old 26-06-2019, 18:42   #4
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Re: Cubigel/Frigoboat fridge power consumption

Do you have any fans that aren't included in the documentation, like inside the box or pushing air across the condenser? Does it have a digital thermostat? I don't have one but I presume it would also use some power. Do we trust the documentation?
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Old 27-06-2019, 04:50   #5
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Re: Cubigel/Frigoboat fridge power consumption

I added a second fan to the compressor a while back, it turns on when the main compressor fan turns on. When running at less than 3000RPM and with an evaporator temperature of under 0c the fan on the compressor never turns on (I've checked). So the current draw for the fans can be ruled out.
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Old 27-06-2019, 06:00   #6
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Re: Cubigel/Frigoboat fridge power consumption

I've programmed my Arduino a bit more to look into the Cubigel's output to see if perhaps the 3 minute "off" period was caused by an internal error. It is not an error, the settings have a minimum 3 minute "off" period if the thermostat shuts off.

This means that the fridge is indeed keeping a really good optimized RPM speed to keep the temperature constant.

Now I just need to figure out why it is using almost double as many amperes as expected to do this...
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Old 27-06-2019, 06:50   #7
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Re: Cubigel/Frigoboat fridge power consumption

The three minute "off" period, once temperature setpoint is reached, is to reduce "short cycling", and I would guess could be adjusted via software. This feature is to reduce starts of the compressor, increasing the life span.


My concern would be the lengthy run time of 20-25 minutes per cycle. This is a long period in my mind, and would first send me back to the basics - unit capacity, insulation, air leaks, installation, etc. If all else fails, run a load calculation to determine actual cooling load. Can you ascertain the actual compressor rpm? Perhaps the 4 amps is correct at current run speed.


What is the current temp setpoint, size of refrigerator box, insulation, gaskets, ambient temperature, etc.? And what does "similar to Danfoss BD35" mean?


P.S. The refrigerant lines are "twisted" together? The liquid line is pretty small on these units and easy to crimp. Does your unit also have a filter/dryer? I'm wondering if there might be an obstruction to flow.
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Old 27-06-2019, 07:53   #8
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Re: Cubigel/Frigoboat fridge power consumption

Quote:
Originally Posted by Zanshin View Post
I have a Cubigel compressor for my fridge, it uses R134a and is similar to the Danfox BD35.

The firmware is set to "smart" mode which reduces the RPMs and extends the run duration to minimize energy consumption. I've written some software which connects to the firmware control unit and have been measuring the run information - it runs for about 20-25 minutes, shuts down for 3 minutes, and repeats that cycle. The RPMs are at a low 1600RPM which, according to the Cubigel documentation, should consume 2.3A. Unfortunately the fridge is consuming just under 4A.

I cannot reach the return line to see if it is frosted over, it is twisted with the feed line so when I measure the temperature back at the compressor they are almost identical. The evaporator is frosted throughout its length.

Could the system be overfilled? Or is there some other likely cause for the problem?
Finding the every day low energy Sweet Spot can not be done by altering the newer variable speed compressors electronic control moduleís software. Software changes in a home refrigeratorís environment maybe, but not with all the variable daily conditions involved in mobile pleasure boat refrigeration without cabin air temperature control.
The problem with fixed soft ware that does not alter Systems Coefficient Of Performance (SCOP) speed day and night, day to day, month to month is like a mechanical clock where the hands never move providing correct time only twice a day. Todayís Tec knowledge of variable speed compressors are designed to permit compressor COP output to be improved by lowering speed if desired cooling is still adequate inside box.

There are many sales marketing gadgets with false claims about improving daily small 12 volt refrigeration energy consumption but they fail to quantify when they are not effective. The only truly automatic small 12 volt compressor correct speed controller module optimizing speed Up and Down each time thermostat cycles compressor are the Danfoss AEO electronic control modules. Sorry the AEO is not designed for off brand compressors.

Refrigerant charge, boxes heat load, ambient temperature changes greatly affects performance of this small system. Thermostat cycling three times per hour and off cycle of 3 minutes is not good performance. Fifteen grams too much or too little of refrigerant can reduce performance.

I would increase compressor speed and adjust refrigerant charge if necessary by adjusting amperage.
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Old 27-06-2019, 08:53   #9
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Re: Cubigel/Frigoboat fridge power consumption

Hello Richard,

I understand that there are a lot of variables involved. The "smart" mode of this fridge seems to doing pretty good with a very long runtime at a low RPM, the fridge is keeping a constant 5C at the moment with relatively cool air (I'm not in the tropics at the moment but in Newport, RI).

But somehow I cannot reconcile using double the expected power for this fridge. The freezer unit, which is also a Cubigel/Vitrifrigo compressor but is keel-cooled, uses much less energy when set to keep the box at 5C (I know, that isn't recommended due to potential condensation issues).

Quote:
I would increase compressor speed and adjust refrigerant charge if necessary by adjusting amperage.
I'm afraid I don't quite understand that, I can set the compressor to run faster but that leaves 2 variables open and I'm still uncertain about which relationship they have.

I can attach a digital thermometer to both the evaporator capillary part and to the point closest to the output side and measure temperatures. If I understand it correctly, the return line temperature should be as close to 0C without going below freezing where the return line exits the evaporator plate.

Looking at the performance data curves it seems that I want the evaporation temperature to be as warm as possible while allowing the fridge to cool to the desired temperature. The chart shows 281kCal/Hr cooling capacity @2500RPM with an evaporation temp of 10C, but at -15C that drops down to 82kCal/HR (COP of 2.98 vs. 1.52).
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Old 27-06-2019, 09:02   #10
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Cubigel/Frigoboat fridge power consumption

I would leave it alone and not do anything, until the weather or water temp gets hot enough that it wonít hold set point, then I would turn up the RPM.
My little basis knowledge is that the lower the RPM, the higher the COP, turn up the RPM and you get more cooling, but based on the belief of there ainít no such thing as a free lunch, that increased cooling comes with lower efficiency.
Even running continuously these little compressors lives seem to be measured in decades, or so it seems.

Now I donít have near the experience Richard does, but I have many times in the past tried tweaking and improving things until I break them, and then realize I should have left a sleeping dog lie.

I believe you have a very firm grasp of the operation principles involved. However the compressor manufacturer cannot know or control how itís installed and installation can add a lot of variation, simply put some installations are more efficient than others.
In other word the installation may change the power requirements
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Old 27-06-2019, 09:08   #11
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Re: Cubigel/Frigoboat fridge power consumption

In fact since everyone pretty much uses the same off the shelf pieces parts, what sets them apart is the installation, or design of the system if you will, some are just more efficient than others, even with identical compressors.
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Old 27-06-2019, 11:11   #12
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Re: Cubigel/Frigoboat fridge power consumption

Quote:
Originally Posted by Zanshin View Post
Hello Richard,

I understand that there are a lot of variables involved. The "smart" mode of this fridge seems to doing pretty good with a very long runtime at a low RPM, the fridge is keeping a constant 5C at the moment with relatively cool air (I'm not in the tropics at the moment but in Newport, RI).

But somehow I cannot reconcile using double the expected power for this fridge. The freezer unit, which is also a Cubigel/Vitrifrigo compressor but is keel-cooled, uses much less energy when set to keep the box at 5C (I know, that isn't recommended due to potential condensation issues).



I'm afraid I don't quite understand that, I can set the compressor to run faster but that leaves 2 variables open and I'm still uncertain about which relationship they have.

I can attach a digital thermometer to both the evaporator capillary part and to the point closest to the output side and measure temperatures. If I understand it correctly, the return line temperature should be as close to 0C without going below freezing where the return line exits the evaporator plate.

Looking at the performance data curves it seems that I want the evaporation temperature to be as warm as possible while allowing the fridge to cool to the desired temperature. The chart shows 281kCal/Hr cooling capacity @2500RPM with an evaporation temp of 10C, but at -15C that drops down to 82kCal/HR (COP of 2.98 vs. 1.52).
The old story that one configuration of a unit fits all evaporator sizes and box heat loads is no longer realistic. Ignoring the fact that your refrigeration has demonstrated poor performance at that Latitude you will surly be disappointing with performance in warmer seawater climates. Running evaporator temperature at 5C is producing not even a good cooler box temperature. A refrigerator evaporator needs to operate at -11 C to perform as a non freezing temperature refrigerator.
If you are concerned about frost build up on evaporator try to stop what must be air infiltration in box.

I assume your objective in altering your systemís design to achieve good performance at the lowest daily energy conserved. You need to understand how the variables can affect your present test performance. Determine present ambient air and water temperatures along with actual daily amp-hrs consumed. When ambient condition change the variables increase energy demand by 4% increase per degree of temperature rise so you can expect an energy increase of 100% in a temperature rise of 25 degrees F.
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Old 27-06-2019, 12:43   #13
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Re: Cubigel/Frigoboat fridge power consumption

Since the return line is bound up with the feed line in the Vitrifrigo evaporator plates, I've attached 2 sensitive thermometers (DS18B20 hooked up to an Arduino) to the following locations:



Will this read the evaporation temperature at the blue circle and the return temperature at the red?

I just measured the freezer and have a -13.44c at the blue but -12.87c at the red just before the thermostat turned off the compressor. The spread between the two is never more than 2 degrees.
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Old 27-06-2019, 17:44   #14
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Cubigel/Frigoboat fridge power consumption

Being as they are both in an aluminum sheet, Iíd be surprised if there is ever much difference.
I donít think it exhibits poor performance, or at least without even knowing box size and insulation, how can we come to that conclusion?

Remember itís operating at the lowest RPM.

I believe Richard evaporator temp was at 5c, not box temp, easy to misunderstand
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Old 27-06-2019, 17:46   #15
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Re: Cubigel/Frigoboat fridge power consumption

a64pilot - the data sheet is explicit in that driving the compressor at around 1500 RPM should use 2.3Amps rather than the 4A it is actually using. The size of box or its insulation doesn't affect that relationship, it just means that if the box is bigger or not well-insulated those 1500RPM wouldn't be enough to cool the box sufficiently.

But I do agree about the aluminum transporting the heat so that the 2 readings are going to be close. I will see if I can move the exit thermometer as far away from the capillary as possible.
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