How do I use a holding plate freezer?

[msponer]

We now have an AC holding plate freezer. The temperature gradually rises when the generator is off, then shoots down to 10 or 15 degrees when we run it for about an hour twice a day.

This worked when the freezer was packed completely full-- overnight the temperature would rise about 10 degrees, say from 10 to 20. We had a problem with food near the top third of the freezer that wasn't touching the holding plate not being frozen solid. People have told me one has to rotate food around inside to keep it all frozen, which we will try next time.

But as we've eaten food and the box has gotten more empty -- its now a third full -- the temperature gain between when we run the generator is in the 20 degree range. We often wake up with the freezer approaching 32, which seems like we may be getting unsafe with the meat.

I think I am using this thing wrong. What should we do? Pack it full of water bottles and run the genset for several hours until they are all frozen?

[a64pilot]

I'd look into an inverter and a appropriately sized battery bank so I could run the thing without running the generator, at least get it through the night

There will hopefully be an expert that will chime in, but a holding plate is not more efficient a system, but it does have mass that will act as a sort of flywheel.
You have hit on one possible way in that by increasing mass, it takes longer to freeze and of course can absorb more heat.
But I think you will be better served by finding a way to hold the temp more stable, and that means allowing it to run more frequently.
Of course if you can appreciably increase insulation / sealing, reduce opening times etc., they all help too

[sailormed]

Fill up the empty space with plastic water-bottles, you gain a lot of thermal inertia.

[Dennis.G]

I think you are seeing too much temperature swing and you are allowing your plates to super heat (rise too much above the freeze/melt point). You need to have a temperature sensor as close as possible to the actual plate and make sure you run refrigeration once you have melted the eutectic solution. On our freezer we cycle on at +10 F and off at -9 F plate temperature. These temperatures will vary from box to box depending of sensor location (sensors only give approximate temperature of plate). Freezer box stays at about 15 F mid way down. Put meats and critical foods at bottom of box.

[msponer]

Hi,
Thank you everyone for your suggestions and tips.

What does super heating the holding plate mean or do?

The temperature sensor is on the holding plate, halfway down.

Cycling from -5 to 10 degrees sounds a lot safer for the food. Our thermostat may be set wrong, we never got below 0 degrees when plugged in at the marina.

We have a small Engel that we use for cold drinks. To keep the kids from opening the holding plate fridge or freezer. It readily freezes water. Maybe I'll start freezing big water bottles in it and placing them in the freezer.

We can run the freezer off of the batteries, but it draws 50 amps at 24 volts. I tried it as an experiment. Our genset charges at 50-80 amps (depending on how low the batteries are), so I figured I am not so much saving genset running time as shifting it into later in the future while abusing the batteries.

[stevebein]

The answer was alrady given. Air is the most difficule thing in a freezer. Your freezer holds temps best when it is full. The simple answer, without getting technical, is fill the freezer. When food items are used up, add something else, like bottles of water, which will freeze and make holding temps easier. Simple, effective and easy.

[Scott Berg]

One more thing, check your insulation and for airleaks at the door(s).
Filling empty space with Gin is more efficient than water <grin>

s

[exMaggieDrum]

The elephant in the room, or I should say freezer, is the insulation, or lack thereof. All of the thought above should be checked too but the box should not be getting that warm in the time you say it is IMO. It could be a combination of all of the above too. Just a thought.

[skipgundlach]

Rich Boren could chime in here, but as I understand their system, it has a hysteresis which is measured at the plate; it's kept at the minimum/maximum via thermostat. If your plate is always relatively the same temperature, your box will, as well.

In my box' case, it's actually evaporators in SS boxes, looking like holding plates, but not.

My 6 degree hysteresis translates to a box temperature variation of 2 degrees. My run/off time is about 30/20, and based on the amps currently (pardon the expression) used, on low, on my automatic/computerized control.

YMMV, of course; with hope, Rich will chime in. It could be that your AC system could be adapted in ways similar to his setup (which would involve an inverter and adequate battery, of course, but if you have a genset, perhaps the battery's of no particular issue).

[a64pilot]

Originally Posted by msponer

Hi,
We can run the freezer off of the batteries, but it draws 50 amps at 24 volts. I tried it as an experiment. Our genset charges at 50-80 amps (depending on how low the batteries are), so I figured I am not so much saving genset running time as shifting it into later in the future while abusing the batteries.

I find that difficult to believe, I think you may have been running something else too. That is of course 100 amps at 12V or 10 amps at 120V.
That is I believe 1200 Watts, I don't believe even a home side by side freezer uses that much electricity.

But yes if you use an inverter / battery, you are of course just shifting the total usage to be recharged when the generator is on. There is no free lunch

Look at it this way, you need an energy accumulator to get you through the generator off times, you can do this maybe with thermal mass as in jugs of frozen water or by storing electrical power directly. Either way total power consumption is roughly the same, or you can increase efficiency with better insulation etc.

Normal consumption for a boat sized compressor is around 5 amps at 12 VDC. Yours is 20 times that amount? Yes there are conversion losses in converting DC to AC of course, but they are usually around 20%?

Something isn't adding up, I'd suspect you maybe had your water heater or something like that on.

On edit, possible your battery charger was on? I've done that by mistake, silly of course. I've invented the perpetual motion machine

[sailing_gal]

I have a rather large freezer and fridge on my boat with a SeaFrost holding plate system. I made "bricks" by cutting and gluing together chunks of the pink insulating foam stuff you can buy at Home Depot or Lowe's. As I use up food, I insert those bricks to take up space and they provide insulation for the remaining food and the fridge.

[sailcrazy]

We operate a 3.4cf refer and a 1.9cf freezer on board, both with holding plates. We run our genset 2x/day (it's a single AC-motor-driven open wheel compressor system that cools both boxes via solenoid valves), until both box thermostats are satisfied....averages approx 1.4 hrs run time/day, unless we just added unfrozen meat to the freezer and/or non-cool items to the refer. But we use/prefer an electric stove for cooking too, so morning=coffee and evening=supper prep, so running 2x/day works well for us. We've set our refer to demand at 43F plate temp and turn off at 35F; the freezer demands at 3F, and turns off at 16F plate temp. I installed a Cural digital 2-probe thermostat is each box(1 probe on the plate and 1 midway in the box), and it works well for us. We've found the items at the very top of the freezer do not freeze,or don;t freeze hard, but everything else is OK.....we wrapped a 1"t piece of Styrofoam in a garbage bag(to protect it), and put it as the very top layer in the freezer-it seems to help alot.

[Djarraluda]

I will not get too specific on temperatures etc, because these will vary slightly between systems.
Most refrigeration works by creating Phase Change. We use the energy of gas/liquid; and liquid/gas change (Latent Energy).
The phase change occurs in the refrigerant due to pressure and temperature, so a loss of refrigerant will change pressures, just as well as a loss of heat will.
So where I am going is that the Eutectic plate is also a phase change filling inside a metal box. What a good system does is match the refrigeration to the eutectic and get the settings to suit the requirement (i.e. food refrigeration in this case).
Before getting too involved in tweaking things, check the basics. Check and double check the insulation, check the seals in the lid/door and especially check the seals where the pipes and sensor cables enter the freezer. These are the common leak points.

Typically, poor freezer performance can be either a poorly matched system, or a loss of insulation, next it is a loss of seals to the compartment. Finally, the refrigeration system itself, such as leak of refrigerant and dirt on the condenser. Oh and not forgetting, a damaged or displaced sensor.
Roger

[Symphony]

hey SailCrazy - your system sounds smart. can you describe how you attach and where attach the Cural thermostat probes? do they connect to your compressor controller?

warning---thread drift: - what is your experience/ brand/ warnings with the electric stove and oven?

[Richard Kollmann]

A true well designed holding plate system when plates are frozen solid should hold over for 24 hours. Ten degrees of temperature rise in 24 hours is normal. Most holding plates designed for freezers have a 0 to 6 degree F eutectic solution freeze point inside. Freezer plate surface area is very important and performance is best when there is more than one plate in freezer. Length of time frozen food can be stored will depend on temperature, the colder the longer parish able product can be stored. There are US government charts on line that can guide you on each frozen food type. Plus 22 degrees F is listed in US regulations as limited high temperature for transportation of frozen food products.

Holding plates has short holdover cycles:
Cause could be some of plates eutectic solution has leaked out as indicated by exterior frost line less than 90% of plate’s surface leaving almost a straight line around plate at level of solution.
Air leaking in and out of refrigerated box.
Age of Insulation or lack there of.
Refrigeration unit not freezing all of plate’s solution.


Box temperature stratifying in layers allowing items high in box and product further away from plate are not cold enough:
All refrigerator and freezer boxes need air space around product to allow natural air tumbling. Warm air rises and passing over evaporator holding plate that absorbs heat and the cooler air then descends picking up more heat. Modern Home refrigerators that do not have open wire shelving can not rely on natural air circulation use fans and passageways to control desired box temperatures. Filling open air space with anything suggested here will add to a greater temperature in product away from evaporator.

The best solution to insure a normalized your box temperature is to install one inch wood rails down the evaporator plate wall extended across box bottom and up the other opposing wall. The rails will now permit natural air movement. It is also possible to draw cold air from bottom of box stimulating natural air tumbling. The fan recommended would be 10 Cfm and run continuously 24 hours a day when refrigerator is in use.