Running freezer as a fridge

[4arch]

My boat has separate fridge and freezer boxes, each with its own Danfoss BD35 compressor. For various reasons, it would be nice to have the option to run the freezer box as a refrigerator at certain times. I’ve been able to do this to some extent by turning the freezer on and off manually, but have ended up freezing items I didn’t want frozen. I’ve found some electronic control modules that provide broad temperature range control but they are expensive and probably add more complexity than I need/want. Any advice on whether this is possible to accomplish easily and without breaking the bank?

[ColdEH]

Stc1000 find them on eBay

I use many of these on my boat , simple cheap and reliable .

Also what kind of system does the freezer have ?

Regards


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[fallingeggs]

Originally Posted by typhoon

Stc1000 find them on eBay

I use many of these on my boat , simple cheap and reliable .

Also what kind of system does the freezer have ?

Regards


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Thanks for that suggestion. I always wondered why digital controllers are $250! Maybe Third Day will chime in and explain.

As for the STC1000, it appears to be AC only, but I did find a 12v version after a quick search. You're completely on your own at figuring out how to make it work as it is directly from China. That said, for $16 it might be worth the hassle.

https://www.amazon.com/All-Purpose-F.../dp/B008KVGWT4

[ColdEH]

No they are actually very simple to use , they come with instructions and there are many youtube videos showing there function as well.

Mine all say 120 vac , but they work just fine on 12 v and even 13.8 volt , for 16 bucks you cant go wrong . You can set the delay , the range , and many other things as well . I have them controlling my furnace, fridge , freezer , even my heater fans , So right now i have 5 of them running and keep 3 for spares but have not had to change any out as yet .

https://www.youtube.com/watch?v=TQjicdtDVrQ

Regards

[a64pilot]

Originally Posted by typhoon

Also what kind of system does the freezer have ?

Regards


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This is a good question, are they holding plates?

[SV THIRD DAY]

DC 12V Digital F Fahrenheit Temperature Controller Greenhouse Thermostat Control | eBay

Bingo....
You just can't beat $11.45 for a digital degs F thermostat/controller.
They come in degs C for the rest of the world that uses degs C.
At this price, you should buy two, one for the Freezer and one for the Refrigerator...heck I added a 3rd one for my boat ambient temp.

Why do some cost $250...ah...well, how do I say this politely...because they are HOSING YOU. Ok ok...maybe I shouldn't say that because the Corell stat/controller for $250 may have some extra features, dual temp probes, extra programing and a fancy company logo on it....gulp. We stopped carrying the high priced Stat/Controllers because I just can't justify the price difference. So now I just send the above Ebay link out to clients who don't like the manual 1-7 old school dial temp setting/control. I recommend they install and keep the old school dial for if/when the digital controller dies, that way they are not SOOB (Sh#! out of Beer). But being able to see your box temps at a glance in a panel mounted LED read out is NICE... But beware...if you are using this to control your evap plate or holding plate you will not be seeing/displaying the Box temp, but rather the evap/holding plate temp, which will be colder than the average box temps.

If you run a holding plate system with the eutectic solution designed for a Freezer as a Refrigerator, then you could be using more amps due to the warmer temps of a refrigerator allowing for the eutectic solution to defrost and then pay the energy "penalty" of the liquid/solid phase change when your compressor turns back on. If your system is an evaporation plate, then you don't have this worry and it's just a simple thermostat setting to go from a Freezer to Refrigerator.

[CGirvan]

Originally Posted by SV THIRD DAY

DC 12V Digital F Fahrenheit Temperature Controller Greenhouse Thermostat Control | eBay

If you run a holding plate system with the eutectic solution designed for a Freezer as a Refrigerator, then you could be using more amps due to the warmer temps of a refrigerator allowing for the eutectic solution to defrost and then pay the energy "penalty" of the liquid/solid phase change when your compressor turns back on. If your system is an evaporation plate, then you don't have this worry and it's just a simple thermostat setting to go from a Freezer to Refrigerator.

Its interesting you write this as I was reading up on Ozefridge's website recently and it seemed like they were saying the exact oposite.

Quoting from there website:

If the eutectic solution doesn’t freeze on each run cycle and then thaw completely during off periods, then the system can not operate correctly. It is as simple as that!
The Ozefridge Sure-Thaw control system allows the user to digitally set the compressor cut-out below the eutectic freeze point and cut-in above the eutectic thaw temperature, with a controller that reads the actual eutectic temperature. This unique control method guarantees that each on / off cycle of the refrigeration system provides proper phase change and benefits of the eutectic principle.

[SV THIRD DAY]

Originally Posted by CGirvan

Its interesting you write this as I was reading up on Ozefridge's website recently and it seemed like they were saying the exact oposite.

Quoting from there website:

If the eutectic solution doesn’t freeze on each run cycle and then thaw completely during off periods, then the system can not operate correctly. It is as simple as that!
The Ozefridge Sure-Thaw control system allows the user to digitally set the compressor cut-out below the eutectic freeze point and cut-in above the eutectic thaw temperature, with a controller that reads the actual eutectic temperature. This unique control method guarantees that each on / off cycle of the refrigeration system provides proper phase change and benefits of the eutectic principle.

Very good catch and to answer your question...They are saying the exact oposite....but it's due to their different system design and operational philosophy. The Ozefridge folks post on this site, so perhaps they will chime in but Ozefridge operates their eutectic holding plate more like the approach used by the old school engine driven systems where you have longer compressor off cycles and are willing to pay the higher energy price of phase changing the eutectic solution from liquid back to a solid. Our approach is different because of the nature of the 12v compressor units we would rather not have to spend the energy on the phase change and work the compressor. It makes sense when you have an overpowering engine driven or AC compressor but our test data shows it's not worth doing with the smaller capacity 12v compressors. There are arguments pro and con to both approaches. The need for a larger compressor, larger temps swings, but there is some merit to both approaches. Coming into play is the nature of a glycol based eutectic solution as well not being a "true eutectic" solution and the reality/real world ways that a eutectic holding plate "thaws or defrosts". It's not a complete uniform event in a holding plate and as "clean and complete" as their quote makes it sound. I'll let them go into the techy details on why they think their operational approach is better and not speak for them. But as I've said before, we have tested the Ozefridge unit side by side to ours and it's a good unit.

[StuM]

Originally Posted by CGirvan

Its interesting you write this as I was reading up on Ozefridge's website recently and it seemed like they were saying the exact oposite.

Quoting from there website:

If the eutectic solution doesn’t freeze on each run cycle and then thaw completely during off periods, then the system can not operate correctly. It is as simple as that!
The Ozefridge Sure-Thaw control system allows the user to digitally set the compressor cut-out below the eutectic freeze point and cut-in above the eutectic thaw temperature, with a controller that reads the actual eutectic temperature. This unique control method guarantees that each on / off cycle of the refrigeration system provides proper phase change and benefits of the eutectic principle.

Not the opposite, just two different ways of looking at the problems of using a eutectic freezer as a fridge. (And SV is a little out since he is implying that there will be a phase change taking place at fridge temperatures.)

If the maintained temperature is well above the PCM state change temperature (which is frequently configured to around -20°C) then you will see NONE of the advantages of using a eutectic media. You will never get the increase latent heat storage/release.

Edit: Rich posted while I was composing. I see he is in effect using the eutectic chamber as a normal heat exchanger and not making use of the prime advantage of a eutectic solution - the high latent heat on phase change around the holding temperature. But since his systems are not focused on the need for infrequent compressor runs and long hold times , in his situation - that doesn't matter.

[SV THIRD DAY]

Originally Posted by StuM

SV is a little out since he is implying that there will be a phase change taking place at fridge temperatures..

Just one correction, there will be a phase change at refrigerator temperatures, that is if we didn't match the eutectic solution and thermostat to keep that from happening.

In a refrigerator plate, the eutectic solution is mixed to freeze around 25-degs F so that a box temp of 35-deg F is targeted.
In a freezer plate, the eutectic solution is targeted to freeze between -10 to 0 degs F.

Different thermostats are used so that the freeze/thaw (ie phase change) temperature point is controlled. That's why if you put a freezer holding plate in a refrigerator no matter how high (or warm) you set the thermostat you will always freeze you produce. It just wont let the plate get warm enough to get into the refrigerator temp ranges because that would allow the eutectic solution to defrost, something we design against.

On Edit: StuM was editing while I was posting and he nailed it when he said:

Originally Posted by StuM

Edit: Rich posted while I was composing. I see he is in effect using the eutectic chamber as a normal heat exchanger and not making use of the prime advantage of a eutectic solution - the high latent heat on phase change around the holding temperature. But since his systems are not focused on the need for infrequent compressor runs and long hold times , in his situation - that doesn't matter.

I guess we are both too fast on the post for our own good...ha ha ha...
Cheers Amigo.

[4arch]

Thanks for the suggestion on the STC1000. I'll try one, if I can figure out how to wire it up in place of the current mechanical thermostat. For the record, this is an evaporator plate system.

[ColdEH]

I assume you are using a danfoss compressor , if you are then just connect it across the C, T, connectors on the control box and put the temp sensor midway in the box .

Regards


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[CGirvan]

advantages of using a eutectic media. You will never get the increase latent heat storage/release.

So can I assume this latent heat storage/release advantage is only gained in the thawing phase change and can be matched by an equal energy disadvantage on the freeze phase change which is what Rich is refering to when he talks about saving energy by not letting the eutectic thaw. If this is the case this makes sense but was not evident on Ozefridges website which seemed to imply that the gain was in both the phase changes,

[SV THIRD DAY]

Originally Posted by CGirvan

advantages of using a eutectic media. You will never get the increase latent heat storage/release.

So can I assume this latent heat storage/release advantage is only gained in the thawing phase change and can be matched by an equal energy disadvantage on the freeze phase change which is what Rich is refering to when he talks about saving energy by not letting the eutectic thaw. If this is the case this makes sense but was not evident on Ozefridges website which seemed to imply that the gain was in both the phase changes,

You pretty much got it, there is no free lunch...in terms of heat removal, it's a pay me now or pay me later. The energy of heat absorption you gain by the phase change from solid to liquid will then need to be paid back when you go back from a liquid to a solid. There is a SMALL amount of energy savings by doing the full phase change, but frankly to get all of it you would have to let your box temperature swing too much. That box temp swing issue is one of the big reasons we take the other approach of not messing with the frost/defrost cycle and just keep the plate frozen (or at a hard slush). In a holding plate there isn't really a clean solid/liquid temperature...it's more of a sliding scale of Rock Hard....Partially Slush/Hard...partially slush/liquid....Liquid. So properly controlling and managing that is more difficult than it sounds without fulling instrumenting the holding plate with 5-10 temp probes. One temp probe no matter if it's inside or "special" can deal with the facts that the plate defrosts from the outer edges to the core and the defrost rate will also be different for different boxes...ugg.

[W3GAC]

The principal behind letting true holding plate solutions totally thaw before the next 'freeze up' cycle starts is because they want/ need the solution to circulate around the very cold 'gas' tubes so that ALL the solution cools reasonably uniformity to the freeze/ state change temp. If some of the solution is still frozen... likely it will be the interior area... which is still surrounding the cooling tubes. As the new freeze cycle begins the 'cold extraction' is less efficient/ not uniform because of the restricted fluid flow, i.e. the very cold cooling tubes don't have direct contact with the thawed outer fluid. Instead, the cold tubes must super-cool the already frozen state fluid and it in turn has to extract the heat from the outer/ non-contact fluid... a process that is not as efficient/ fast. So if it would normally take a 40 minute pull-down and that's what the user is use to before manual or an automatic shutdown, not all the outer fluid would have undergone the state change and the 'life' of the cold temp in the box will have been reduced. Also some holding plate auto monitoring circuits can monitor/ sense the state change happening inside the holding plate (return gas temp, compressor head pressure/ electrical load) which triggers a timer for additional preset addition runtime. If the core is still in the frozen state, the sending circuits can be fooled into thinking it's all in the state change phase and shutdown prematurely. if in the auto mode, a repeating short cycling pattern is set up with the plate never reaching its full 'cold capacity.' This results in more wear and tear on compressor/ motor and of course less holdover cold time.

All of that said, many of us nevertheless adjust/ modify our temp setting to convert the freezer to just frig temp. For weekend cruising where we don't need a months worth of deep frozen meat,... but rather just a frig and what was the 'spillover' refrig space now becomes a cool pantry for storage of fruits, bread, and pre-cooling/ thermo-mass of extra water bottles/ soft drinks, beer, & wine. Never heard of any real damage resulting from this mode other than the theoretical extra start-up wear and tear mentioned before.

Over the years of 'switching' back and forth between the freezer and frig modes I directly attached a direct contact digital thermometer/ controller to the plate itself and with a little trial (& error) we've adjusted it so that it doesn't cause freezing of food, but seems to have some reasonable 'holding' state change performance. This is a moving target by the way, if starting out with a hot box, or a lot of new food that got warned up by the drive to the boat... we can let the plate go colder before switching off. Once everything is near uniform temp, then there is less adjustment for factoring in outside temp/ high use/ opening - closing of the lid. Once at this stable status we can go all night w/o needing to pull down the plates. Morning inside temps about 45 degrees during hot Summer months. By day with use, hotter day temps we pull down after breakfast use, after lunch use, hour or so before turning in... the timing isn't critical and we often skip the lunch pull down. Our compressor is AC and usually pull downs are done when generator is started for other reasons, which happens mostly in morning/ evening for 30 minutes to top off batteries, and bigger creature comforts like AC, water heater, hairdryer, microwave/ toaster, ...


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