Refrigeration - Evaporator vs. Holding Plate

[missourisailor]

Originally Posted by Richard Kollmann

If the flywheel energy stored in both plates is partly consumed to keep box temperature from rising half as much as one plate. Why would it not take compressor twice as long to replace the melted eutectic ice.

I'm having a hard time wrapping my head around this.

If it does take "twice" as long to recover, then why should I go with two holding plates if one will recover in half the time?

[missourisailor]

Originally Posted by a64pilot

However if you melt two gallons total between the two plates, or two gallons on one plate, wouldn’t the refreeze time be very close?

Good analogy.

But would you be melting two gallons with two plates or some amount less?

[a64pilot]

Originally Posted by missourisailor

I'm having a hard time wrapping my head around this.

If it does take "twice" as long to recover, then why should I go with two holding plates if one will recover in half the time?

I don’t think it will. I think a bdxx compressor can remove X amount of heat per hour, whether it is in 1 or more plates, heat removed Is the same.
Biggest difference I see is that dual plates give you a bigger energy storage, like a bigger battery bank so that you can draw more energy out of them before the box begins to heat up.
Now as Richard correctly stated the larger surface area will allow the heat to be transferred at a higher rate, but total heat removed is the same, just dual plates will maintain box temp better if you dump in more food as the storage is larger.
For the following think a Lifepo bank, so we can ignore perkeuts effect
Think like a 500 AH battery bank, and a 1000 AH bank. We use 200 AH.
The 1000 AH bank won’t take twice as long to charge, charge time will same for both, cause you only have to replace 200 AH, plus a little more, no perpetual motion machine here either.

However with the 1000 AH bank, you can remove twice the AH before you have to recharge.

[a64pilot]

Originally Posted by missourisailor

Good analogy.

But would you be melting two gallons with two plates or some amount less?

Who knows, depends I guess on which plate was closer to the food, and are both plates the same temp?
On my dual plate system, it seems the first plate freezes first, and the second plate never gets quite as cold as the first, which is what I would expect as the first plate is where the expansion valve is, and right at the expansion valve ought if I understand, be the coldest spot in the system, right where you get the big pressure drop and the liquid boils off to a gas.

See right at the freeze point, you have to remove a LOT of heat to freeze the liquid in the plates, once fully frozen, you can drop the temp of the solid with less energy removal, it’s the phase change that absorbs the huge amount of heat.

Just like a glass of ice water, fully packed with ice just a little bit of water, temp is 32F, then as the ice melts temp remains 32f, in theory it will remain at 32F until all the ice is melted, then temp begins to rise. Cold plates act like that ice, they will keep the temp stable until they are fully melted.
Then of course your compressor has to run for a long time to fully refreeze the plates.

But I contend that if it takes four hours of compressor run time to freeze ten lbs of food, then whether you have a thin evaporator or cold plates, it’s going to take four hours.
A difference is that with a thin evaporator box temp will rise until the food is frozen and then the box can be cooled back down, but the excess thermal mass of a cold plate may hold the box temp more stable, but your still going to have four hours of compressor run time to freeze the plates back down again.

[missourisailor]

Originally Posted by a64pilot

I don’t think it will. I think a bdxx compressor can remove X amount of heat per hour, whether it is in 1 or more plates, heat removed Is the same.

Yes. Yes. Yes...

This is the question that I was trying to ask.

If my BD35 is operating at or near it's capacity, as far as heat removal, is installing a second holdover plate really going to shorting recovery time?

I realize that adding a second plate will increase the thermal mass. And this added mass will lessen the impact of the added food.

But the added food will introduce XX BTU's. Of which all, will have to be removed. And if my compressor is already operating at max capacity, then it will still take about the time for the recovery cycle.

I really think that in my case, upgrading the compressor is the only way that I will see measurable decreased recovery time.
At that time a second plate would make an even greater time decrease.

If I'm wrong, please, educate me.

[missourisailor]

Originally Posted by a64pilot

But I contend that if it takes four hours of compressor run time to freeze ten lbs of food, then whether you have a thin evaporator or cold plates, it’s going to take four hours.
A difference is that with a thin evaporator box temp will rise until the food is frozen and then the box can be cooled back down, but the excess thermal mass of a cold plate may hold the box temp more stable, but your still going to have four hours of compressor run time to freeze the plates back down again.

This too is my train of thought. And why I've been having a hard time understanding why/how, two holdover plates is going to help in my situation.

But when you have two, highly respected people, both of which I take their words as gospel, telling me differently, I have to think that I'm missing something.

[Richard Kollmann]

Originally Posted by missourisailor

Yes. Yes. Yes...

This is the question that I was trying to ask.

If my BD35 is operating at or near it's capacity, as far as heat removal, is installing a second holdover plate really going to shorting recovery time?

I realize that adding a second plate will increase the thermal mass. And this added mass will lessen the impact of the added food.

But the added food will introduce XX BTU's. Of which all, will have to be removed. And if my compressor is already operating at max capacity, then it will still take about the time for the recovery cycle.

I really think that in my case, upgrading the compressor is the only way that I will see measurable decreased recovery time.
At that time a second plate would make an even greater time decrease.

If I'm wrong, please, educate me.

Think of your problem this way you want to keep box temperature from rising when adding warm product to box and you seem to be satisfied with your present system plate and compressor.

You know that the secondary benefit of having energy stored in eutectic plate is that it can be consumed at a far greater rate through plate exterior than the energy from your compressor can produce.

With a second plate frozen in the same box you would have more exterior plate surface area to absorb heat when placing warm product in box.

As solution in plates is melting thermostat may start compressor and try to refreeze plates. But unfortunately if eutectic solution temperature is near zero F the BD35 or even a BD50 cooling capacity is very low.


Assuming adding warm product melted only two pounds of ice solution total from one or both plates it will take a efficient BD35 condensing unit two hours at this eutectic temperature to refreeze two pounds of zero degree eutectic solution.

If compressor is upgraded to a BD50 there may or may not be a measurable difference in the above example. The BD50 would be a major improvement when staring compressors with warm plates. Instead of 250 Btu at zero eutectic solution BD 50 compressor with a warm plate may average 500 Btu output.

[SV THIRD DAY]

Originally Posted by missourisailor

I'm having a hard time wrapping my head around this.

If it does take "twice" as long to recover, then why should I go with two holding plates if one will recover in half the time?

You are confusing "recovery" with "compressor" run time. Two completely different things. It is NOT different with a Cycling Evap unit like ColdEH...but it IS with a Holding Plate that removes heat when the compressor is off because you have stored up the BTU energy in the plate.

Recovery is how fast the temperature will "recover" to your baseline equilibrium. That has to do with plate surface area and stored BTU removal capacity inside the hold over plate.

IMPORTANT:
Your daily power usage will stay the SAME.
Your compressor ON Cycles will be LONGER because you have more Eutectic to refreeze BUT your compressor OFF Cycles will also be LONGER because you have more Eutectic to remove heat before the thermostat kicks the compressor back on.


SO What the hell advantage do you get out of Two plates over one?
1. Same Power Daily Power Usage
2. Lower Box Temps with the increased surface area for heat uptake
3. Faster Box temp Recovery due to the increased surface area for heat update and additional BTU heat removal stored in the Holding Plate.
4. You DO NOT get a difference in your Total Compressor Run TIMES PER DAY. NO. Your Off times and your ON times will both be longer.

How do I know this....we tested the 1" thick Plate, 2.5" thick Plate, 4" thick plate along with 1 and 2 plates in a box years ago....the laws of physics don't change with the fads of the day.

[SV THIRD DAY]

Originally Posted by missourisailor

Yes. Yes. Yes...

This is the question that I was trying to ask.

If my BD35 is operating at or near it's capacity, as far as heat removal, is installing a second holdover plate really going to shorting recovery time?

I realize that adding a second plate will increase the thermal mass. And this added mass will lessen the impact of the added food.

But the added food will introduce XX BTU's. Of which all, will have to be removed. And if my compressor is already operating at max capacity, then it will still take about the time for the recovery cycle.

I really think that in my case, upgrading the compressor is the only way that I will see measurable decreased recovery time.
At that time a second plate would make an even greater time decrease.

If I'm wrong, please, educate me.

Wrong.

First see my post above.

Adding the Second plate will have nothing...repeat NOTHING of a change on your total Power usage....therefore it will have NOTHING to do with reaching some Max BTU removal rate for your Box.

It the current compressor handles your Box now...it WILL handle the box with Two, Three or 5 Plates (exaggeration for affect to make the point). Because it's a TOTAL Heat in and HEAT out question for your Compressor and NOT the number of plates.

[Richard Kollmann]

Originally Posted by SV THIRD DAY

Wrong.

First see my post above.

Adding the Second plate will have nothing...repeat NOTHING of a change on your total Power usage....therefore it will have NOTHING to do with reaching some Max BTU removal rate for your Box.

It the current compressor handles your Box now...it WILL handle the box with Two, Three or 5 Plates (exaggeration for affect to make the point). Because it's a TOTAL Heat in and HEAT out question for your Compressor and NOT the number of plates.

Rich your right in both posting. he does not need the second plate if he is satisfied with one plate. As for as increased capacity of a BD50 it might improve overall energy consumption running a lower Rpm COP, is cost of compressor really worth it?
You are correct the heat absorbed by frozen solution trying to still maintain box temperature is unchanged as long as there is a sizable amount of frozen solution left. The end result is the compressor still needs to run long enough to refreeze melted solution and overcome the normal box heat gain.

[missourisailor]

Originally Posted by SV THIRD DAY

You are confusing "recovery" with "compressor" run time. Two completely different things. It is NOT different with a Cycling Evap unit like ColdEH...but it IS with a Holding Plate that removes heat when the compressor is off because you have stored up the BTU energy in the plate.

Recovery is how fast the temperature will "recover" to your baseline equilibrium. That has to do with plate surface area and stored BTU removal capacity inside the hold over plate.

Yes, when I say "Recover" I mean the time it takes for the box to get back to desired temp. set point.

Currently, after reloading the freezer, the unit (compressor) will run nonstop for 24 to 36 hours. Then once the set point is reached, the unit goes back to the usual 50% duty cycle.

It is the 24 to 36 hours of nonstop running, that I want to reduce.

Originally Posted by SV THIRD DAY

....3. Faster Box temp Recovery due to the increased surface area for heat update and additional BTU heat removal stored in the Holding Plate..

If I'm reading this correctly, then the addition of a second hold plate will help in reducing the 24 to 36 hours of recovery time that I now have.

I'll be back to the boat over Memorial Day week. At that time I will take measurement to determine the available space for a second plate.

Thanks to everyone for the patience and education.

[wsmurdoch]

I can follow the argument that a freezing solution filled tank may be a better alternative to storing "cold" energy than a battery based on cost, weight, size, and usable life. But I can see a couple of detractors as well.

The tank is usually in the cold box using up room that could be used to store food (beer) or alternatively requiring a larger cold box with a greater outside surface area and thus greater "cold" loss. Is this additional "cold" loss included in the battery vs. freezing solution filled tank comparison?

I'll assume that a freezer is to be kept at 20F. I'll also assume that the economic temperature difference between the refrigerant in a flat plate evaporator and the air in the freezer is 15F. The required refrigerant temperature is thus 5F. Next, I'll assume that the economic temperature difference between the refrigerant and the freezing solution in a freezing solution filled tank is also 15F and that the temperature difference between the tank solution and the air in the freezer is also 15F. In this case the required refrigerant temperature is -10F. The capacity and the efficiency (COP) of the refrigeration system would be lower for an evaporator operating at -10F than an evaporator operating at 5F. Would this not favor using a battery instead of a freezing solution filled tank?

The accepted solution for a cold plate seems to be a propylene glycol - water mix richer in water than the eutectic (60v% PG). In that mixture starting at a temperature below the freezing point of water the first bit of the water freezes. The remaining liquid mixture then being richer in propylene glycol, the next bit of water then freezes at a little colder temperature. This continues until either the desired low temperature is reached or until the eutectic freezing point is reached. If the eutectic point (-60C, -76F) is not reached, only part of the water is frozen storing "cold" in its latent heat. The remainder of the "cold" is stored in the much smaller sensible heat of the remaining unfrozen propylene glycol and water. If the tank were filled with a true eutectic solution, it would all freeze at one temperature and store its "cold" in a smaller volume. Glacier Bay (https://www.boatdesign.net/attachmen...ucts-pdf.4216/) used to claim its TSS-8 and TSS+26 solutions were true eutectics. Does anyone know their composition?

Bill

[a64pilot]

Originally Posted by missourisailor

Yes, when I say "Recover" I mean the time it takes for the box to get back to desired temp. set point.

Currently, after reloading the freezer, the unit (compressor) will run nonstop for 24 to 36 hours. Then once the set point is reached, the unit goes back to the usual 50% duty cycle.

It is the 24 to 36 hours of nonstop running, that I want to reduce.



If I'm reading this correctly, then the addition of a second hold plate will help in reducing the 24 to 36 hours of recovery time that I now have.

I'll be back to the boat over Memorial Day week. At that time I will take measurement to determine the available space for a second plate.

Thanks to everyone for the patience and education.

Sorry, just back from inlaws House.
I see though that your questions were answered. The second plate will assist in your box temps being more stable, your electrical consumption will not vary appreciably.
However if I were in your shoes, I’d look at a bigger compressor, cause the second plate takes up a LOT of room in the box. I suspicion with only a 35 compressor that your box isn’t very big to start with, or it’s extremely well insulated for you to get by with a 50% duty cycle.
A bigger compressor of course takes no additional room in the box.
Your energy consumption will not change much with a bigger compressor either, increase some, but not a lot.

If you can freeze the food ahead of time, that helps tremendously. Most meat departments will freeze it for you to pick up next day if you ask.

Continuing to brain storm, if you now have a spill over, ideal is separating the box and having a separate fridge and freezer, second plate, second compressor, plus you have some redundancy in case of one breaking down. I like redundancy, an Engel is my backup.
I likely should have gone that route, but the $$$ scared me.

[Richard Kollmann]

Originally Posted by missourisailor

Yes, when I say "Recover" I mean the time it takes for the box to get back to desired temp. set point.

Currently, after reloading the freezer, the unit (compressor) will run nonstop for 24 to 36 hours. Then once the set point is reached, the unit goes back to the usual 50% duty cycle.

It is the 24 to 36 hours of nonstop running, that I want to reduce.



If I'm reading this correctly, then the addition of a second hold plate will help in reducing the 24 to 36 hours of recovery time that I now have.

I'll be back to the boat over Memorial Day week. At that time I will take measurement to determine the available space for a second plate.

Thanks to everyone for the patience and education.

It appears I must have missed something several days ago of how long it took compressor to catch up 24 hours after adding warm product was put in freezer. Rich came up with the answer Equilibrium when low temperature of refrigerant mass was no longer able to drop temperature at a reasonable rate. An efficient zero degree evaporator and 134a refrigerant with little or no resistance in moving heat from box through plate to refrigerant coil can do so at temperatures below -16 degrees. There at two good reasons why eutectic plates are less efficient at very low temperatures are loss of heat conduction inside some plate designs plate and diminishing Btu from compressor at very low temperatures.

You can not freeze water at 32 degree F in a short time this is also true with zero degree F eutectic solutions. If you want to reduce time to freeze a eutectic plate temperature must be 10 to 20 degrees below eutectic phase change to ice and refrigerant mass flow must be adequate.

[OzePete]

A couple of things:
This previously quoted comment is nonsense "There at two good reasons why eutectic plates are less efficient at very low temperatures are loss of heat conduction inside some plate designs plate and diminishing Btu from compressor at very low temperatures. " The lower the evaporator temperature is on ANY SYSTEM the lower the COP factor is. This is a common factor will ALL refrigeration systems, besides in reality the eutectic system's suction temperature / pressure is always higher than a cyclic system maintaining the same cabinet temperature, therefore a more efficient COP.
Also suggesting simply upgrading by changing to a BD50 without consideration for other factors is also wrong advice.
Follow what Rich from Cold Blue is saying regards adding a eutectic plate and you won't go wrong. He actually does know what he is talking about!

Now back to the thread origins.. May I suggest checking the eutectic / cyclic system comparison that I reported on at http://www.cruisersforum.com/forums/...se-200041.html and you will see that there are TWO key factors that allow a Eutectic system to provide refrigeration at a far lower power consumption rate than a thin aluminium cyclic plate. As stated the difference will vary with other situations but always the eutectic wins the efficiency race hands down.

The key issues found as a result of that 60 day trial were:
Firstly the eutectic system operates at a far more efficient COP.
The second issue was the energy wasted because of the cyclic systems excessive number of start ups per day. (Approx 30 TIMES more often than the eutectic system in the test.)

Now consider how little battery power (if any) is needed if the eutectic system automatically kicks in to do its daily refreeze when power is abundant! Like once the solar / wind has topped up the battery bank and is then otherwise wasted.

That is how the Ozefridge ECO2 eutectic system works and NO other system can claim to provide refrigeration without ANY battery drain... Zero, ziltch, nothing, from the battery bank.. how good is that? And that can be *achieved using a correctly sized plate or plates and a compressor system with enough grunt to refreeze the eutectic mass quickly. (*In average conditions with a properly insulated fridge).

Cheers OzePete Ozefridge | 12 Volt Refrigeration Systems