Fridge cool down expectation

[ayates]

I installed a new DC refrigerator, a NovaKool RFU9000. It is currently running with its OEM thermostat. I equipped it with a temperature logger in the refrigerator compartment over the summer, to gather data on its performance. One point of note so far, is that it takes quite a while to cool down food items after being loaded. The attached graph is an excerpt showing typical performance. From a new batch of food being added, I assume, it took 18 hours to bring the temperature back down to 4C. You can even see each time the door is opened.

Now, I realise the compressor is small, and as is the evaporator, but is this the type of performance to be expected. I do not know how much frost was on the evaporator at any point during the summer as I was not present. I am just trying to deduce and analyse based on the available data. The photo of the fridge is the current state, not how it looked when these temperature readings were taken.

Next season I plan to have the Stainless Lobster controller/fan in operation.


Thanks,

Allan.

P.S. The temperature logger is this unit: RHTemp101A, Humidity and Temperature Data Logger

[Stu Jackson]

Originally Posted by ayates

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I do not know how much frost was on the evaporator at any point during the summer as I was not present. >>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Maybe I'm misunderstanding, but does this mean the fridge ran all the time that you weren't there?

Most of us pre-cool material before we stuff it in the box.

[ayates]

Originally Posted by Stu Jackson

does this mean the fridge ran all the time that you weren't there?

I should have been more clear. The ship, St. Lawrence II - Brigantine Inc., is a youth sail training vessel, and sails the great lakes throughout the summer. So the ship, and the fridge, are in constant use, I am just not present (I do volunteer maintenance work in the winter).


Allan.

[OzePete]

Allan, the key to your problem is indicated by the data graph you supplied. The initial pull down, after warm product was added, is quite reasonable because the unit ran constantly but it went bad after that. The reason is that your thermostat is reading evaporator cold plate temperature and causing the unit to cycle on and off well before the cabinet temperature has been reduced to below +4C. Apart from the obvious door opening induced rises you can see that the chart shows on - off cycling. Suggest that you connect a switch across the thermostat, bridging the two thermostat switch wires. This extra switch would only be turned on when you have loaded warm product so as to cause the compressor to run continuously and not on - off. You would switch off once the product and cabinet are down to temperature.

Cheers OzePete from ozefridge.com.au

[Typhoon]

You can't expect to much performance out of a unit like this. Small compressor, large box, poor insulation (inch and a half by the look of it ) .

The BD35 would just barley keep up with the heat that gets into the enclosure through the insulation , once you add a lot of warm food , there is very little power left to bring down the contents temperature , so it takes a long time .

If the ambient temperature inside the boat rises , I would be surprised if this unit could keep up at all. It could run steady and still not get your beer could , add to that a large front loading door that is constantly opened and you have a recipe for poor refrigeration performance.

Regards John.

[Dave Lochner]

Alan,

Sorry I missed you a couple of weeks ago in Oswego. Wasn't able to get the time free to help sail the St Lawrence back to Kingston, but it was a hot sunny day!

Looking at the temp data, it looks like there is a spike in temperature around meal times. Because it is a front loading refrigerator, every time the door is opened all the cold air spills out and the refrigerator needs to cool all the air again.

If the door seal isn't perfect cold air will leak out. Make sure the seal is tight.

[PuttingDoctor]

Originally Posted by Dave Lochner

Alan,

Sorry I missed you a couple of weeks ago in Oswego. Wasn't able to get the time free to help sail the St Lawrence back to Kingston, but it was a hot sunny day!

Looking at the temp data, it looks like there is a spike in temperature around meal times. Because it is a front loading refrigerator, every time the door is opened all the cold air spills out and the refrigerator needs to cool all the air again.

If the door seal isn't perfect cold air will leak out. Make sure the seal is tight.

Bingo! Likely the worst environment for a front loader.

[a64pilot]

Don't forget what Ozepete said, that Is your not cycling based on box temp, but evaporator temp, therefore the box temp will take longer to stabilize, even if you had a way big compressor, cause it cycles off when the plate hits temp.
However from an efficiency standpoint, a compressor with a high duty cycle, one that runs almost continuously, will use less power per BTU removed.
In my opinion from what I see is that your box is performing normally.
I toss in a case of beer and a few steaks in to freeze and it takes more than a day to recover.
There is a huge amount of heat in a case of beer and a couple of steaks, a huge amount, and of course that has to be removed

[ayates]

Originally Posted by Stu Jackson

Most of us pre-cool material before we stuff it in the box.

That unfortunately is not an option, as the ship sails the great lakes (and up the St. Lawrence and down to NYC this past summer) from port to port. Food is purchased once or twice a week, and the perishables loaded into the fridge.


Allan.

[ayates]

Originally Posted by ColdEh Marine

You can't expect to much performance out of a unit like this. Small compressor, large box, poor insulation (inch and a half by the look of it )

I should have mentioned, I laminated 3 1/2" of Styrofoam SM extruded all around the unit before installation. Due to space constraints I could only fit 1 1/2" under the freezer / over the compressor/condenser. I plan to have a similarly insulated exterior door as well, but ran out of time for this season.


Allan.

[ayates]

Originally Posted by OzePete

Allan, the key to your problem is indicated by the data graph you supplied. The initial pull down, after warm product was added, is quite reasonable because the unit ran constantly but it went bad after that. The reason is that your thermostat is reading evaporator cold plate temperature and causing the unit to cycle on and off well before the cabinet temperature has been reduced to below +4C. Apart from the obvious door opening induced rises you can see that the chart shows on - off cycling.

Brilliant observation! Attached is another excerpt, this time with a corresponding batter current graph. This confirms the unit cycling on/off during his period. I would guess my duty cycle is around 60%; that is a lot of wasted cooling time. The original temperature graph corresponded to the batteries being charged, so there compressor cycling cannot be seen in the current readings for that period.


Thanks,

Allan.

P.S. When I had a custom fridge/freezer planned, your units were in the running, but then a floor plan change resulted in a complete commercial unit being a better fit. We are still a bit short on fridge space, so I may still be constructing an additional separate fridge under a bunk. But that won't be happening this year.

[kenbo]

Originally Posted by a64pilot

Don't forget what Ozepete said, that Is your not cycling based on box temp, but evaporator temp, therefore the box temp will take longer to stabilize, even if you had a way big compressor, cause it cycles off when the plate hits temp.
However from an efficiency standpoint, a compressor with a high duty cycle, one that runs almost continuously, will use less power per BTU removed.
In my opinion from what I see is that your box is performing normally.
I toss in a case of beer and a few steaks in to freeze and it takes more than a day to recover.
There is a huge amount of heat in a case of beer and a couple of steaks, a huge amount, and of course that has to be removed

I agree. In fact I think it's operating very well. That's a 9.1 CF box being cooled by a BD35 compressor with only 1.5" of foam insulation! Man those BD35's are good. Imagine how good that would look if it was top loaded with 30R insulation.

[kenbo]

From a system efficiency perspective I would say a duty cycle of 60% is close to ideal. If you don't like it you can always change the resistance between the thermostat and the electronic control unit to change compressor speeds and BTU capability.

[donradcliffe]

Looking at a different way, the danfoss type units are good for about 250-300 btu/hr when they are running. If you wanted to cool 50 pounds of food by 40 degrees F, that's about 2000 btu you need to extract, or the equivalent of 7-8 hours running at 100% duty cycle. Add in the btu lost by the box (if you run a 50% duty cycle in equilibrium, that's about 125-150 btu/hr) , and the btu lost every time you open the door, and you can see why it should take over 24 hours.

[Zep]

[QUOTE=Stu Jackson;2489228]Maybe I'm misunderstanding, but does this mean the fridge ran all the time that you weren't there?

Most of us pre-cool material before we stuff it in the box.[/QUOT

Pre cooling is the only way to go unless you have an engine driven compressor and a eutectic system.
The little compressors only have a refrigeration capacity of about 50 watts. That's if it's a "big one". An engine driven compressor has a refrigeration capacity of about 3000 watts. A big difference.

You are obviously on shore power which is good so it's not taxing your batteries.
Those little units can kill batteries so it's really important to monitor your battery voltage and charge regularly. Otherwise when the batteries are low the compressor will try and start. As soon as it's tries to start the batterie voltage will drop resulting in the built in "low voltage cut out relay" operating switching off the compressor. As soon as the compressor stops the voltage rises again since there's no load. The "low voltage cut out relay" sees that, the compressor tries to start again. By the time that cycle has repeated a few hundred times the result is dead battery.

I suggest getting your elmectrician to put a low voltage lock out on that circuit so that it's necessary to manually reset it. Remember if it's cycling on the low voltage cut out it certainly can't do any cooling. The only hope is to save your batteries. You can have an alarm on that circuit so if you're on board when it happens all you'll have to do is start the engine.

The good thing is today's solar panels are a great help if there's plenty of capacity there then it's not such a big problem. Except Pre Cool if possible.

Have a cold beer!