Running freezer as a fridge

[SV THIRD DAY]

Originally Posted by 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.

That's true for the vast majority of holding plates that just have a few loops of 1/2 copper tubing inside them. And that style/approach worked well in the old school days of engine driven and AC driven compressors where you had WAY more BTU capacity than you really needed. But when you start working with smaller BTU capacity compressors that you can't live with the time it would take the compressor to say in an On Cycle to freeze down the plates with just copper tubes inside. It takes way too long for the heat to be pulled through the ice layer that first forms tubes. So the solution is to spend the extra money and fill the interior of the holding plate with a large high surface area fin/tube condenser. That way you maximize surface area to pull out heat from the eutectic solution.
Here's a photo of what's inside our standard holding plates:

Originally Posted by W3GAC

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, ...

He nailed it here...but I think he also illustrated why we warn folks that it's difficult to use a freezer plate and thermostat as a refrigerator...it's not just a "set it and forget it" type of thing. It requires lots of user monitoring and feedback to keep the veggies from freezing and it can cause an increase in power usage. But if you have the discipline, are willing to live with the the possible higher power usage and I would recommend are using a electronic thermostat, then you can pull it off. My problem as a manufacturer of the equipment, when I give advice and it isn't "EASY" I run the risk of being called crazy or making something too difficult...but as he points out...yes it can be done...you just have to play with it a bit.

[skipgundlach]

Originally Posted by SV THIRD DAY

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


clip....

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.

clip...

Hi, Rich!

Insert primal scream here, as my 2-sentences left reply/question disappeared when I touched the screen again after the admiral interrupted me 15 minutes earlier. I'll try again:

I hereby expose my refrigeration lack-of-chops by wondering why it is that plate temperature matters at all, so long as the box stays the temperature you want/food stays frozen to the degree you desire.

In my previous life, I had a Frigoboat system which worked perfectly until it didn't work at all. It had two Carel electronic thermostats, both measuring box temperatures, and had 2° hysteresis, each.

The freezer thermo controlled the compressor, and the reefer thermo controlled the spillover fan. Both worked entirely perfectly, returning the box to the appropriate low temperature the moment the higher level had been exceeded. Indeed, unless the box was warm loaded, once it got to the desired temperature, it stayed in that range entirely reliably. I have no idea, and had no interest in learning, what the evaporator plate temperature was during any of this. And, indeed, why it should be of interest to anyone is beyond me.

Enter, now, the replacement Sea Frost system which has a smart controller known as ET(T - apparently to distinguish itself from the other ET, with the expanding neck, which apparently was also smart). The plate temperature's cut-off point is relatively simple to set, but its actual temperature is buried deep in an arcane bunch of steps, further enhancing the thought that it is of interest only to refrigeration geeks and not to actual users; the box temperature is what I see on the controller. Interestingly, despite the freezer's hysteresis being 6°, the box range - when it's mostly empty - works out to be right about 2°, just what I'd want.

But...

My March Pump, which is located ~ 2' below the water line, fibrillates often, for reasons I'm not at all able to fathom; in a foul harbor, it's usually the intake line becoming packed with gunge, requiring a very convoluted sequence of disconnection and reaming in a contortionist position. Under way, there is no apparent reason, as the line is clear; the last couple of times it has been a matter of bleeding the pump at its output. It's again stuffed/howling (March Pumps are magnetically driven, need to be fully wet, and make nasty pre-failure noises if they aren't full of water), but as the engine room was very hot, aside from gingerly clambering over the engine to reach the switch on the compressor box to turn it off, I've not resolved this particular stoppage/failure/whatever. As a side note, I can't begin to tell you how angry I am that a water system was included, instead of the immensely effective (it's all I had to dump heat in the Frigoboat system) keel cooler. But I digress...

As there have been several instances over the last few days as we cruised from Fort Pierce to Charleston that the pump has been turned off until I can get to it again, I can tell you that the system is able to cycle even though the condenser is in a hot engine room and no water is helping to cool. And, I've seen the cutoff at 7° (box temp probe) a few times, even though it's usually much higher.

Yet...

I started with a 10° cutoff, and moved down to 7° and then 5°, because...

The box is entirely full. The bottom 1/4 or so is water bottles both for space reduction and a thermal flywheel. The rest is, as desired for coldest to not-quite-as-cold, either up against the evaporator plates, in the middle, or up against the spillover divider.

Despite being right against the evaporator plates, hamburgers are firm but not hard frozen. Bread never gets frozen, anywhere. The hamburger buns come out very cold but not the least bit stiff. Having twice lowered the plate shutoff temperature (a 5°F difference) has not seemed to change matters.

So...

Aside from the intense annoyance at having to rely on a pump which fails me as much as it helps, and plate temperatures (assuming that the probe is correct in its reporting to the computer) being in a range which should produce a box temperature well below freezing, of what use is the temperature of the evaporator plate? If the box isn't cold enough, run the compressor longer, until it IS cold enough.

Or at least it seems that way to me. In my Frigoboat system, with its box-only probe, I had it at 8-10° and it worked a treat, despite nobody, whether Carel or Frigoboat or me, having a clue as to what temperature the evaporator was.

Is there a reason I should care???

(My problems with this system are an entirely different matter and no amount of hair pulling by many competent folk, Rich included, have been able to make it behave any differently, nor explained why it shouldn't work perfectly.)

L8R

Skip

[jbinbi]

So is there a difference between the STC on amazon and the other brand on ebay controller, and if so, which would be the better choice. I want to get some of these. Thanks.

Originally Posted by fallingeggs

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

Originally Posted by SV THIRD DAY

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

[SV THIRD DAY]

Originally Posted by jbinbi

So is there a difference between the STC on amazon and the other brand on ebay controller, and if so, which would be the better choice. I want to get some of these. Thanks.

Nope....no difference but the name on the front...same unit with the same features.

[OzePete]

[QUOTE=SV THIRD DAY;2208799]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.

Exactly, we have been getting ripped of on the price of digitals for decades. And the Carel controllers are now made in Chine and in my opinion are no better than the much cheaper Elitech models mentioned here But I would recommend using the more costly Elitech STC600 which is water proof, has twin probes / control circuits and comes with remote relays. We use thousands of controllers and where the non-sealed types are prone to moisture damaging the PCB, we don't have that problem with the sealed STC600. Anyone with the open type might like to spray some WD40 or similar in and over the controller's PCB.
Cheers Pete Ozefridge