Refrigeration - Evaporator vs. Holding Plate

[missourisailor]

Originally Posted by Richard Kollmann

....It should also be understood that eutectic solution freeze points below 32 degrees F have short holdover phase change time, this is why even freezer plate solution is generally set to freeze no lower than zero degrees F.

Richard,
So lets assume that my system has, as Rich suggested, 1:3 glycol/water ratio. If I was to raise my box temp to 5F (0F hold plate temp), will I see any noticeable difference in initial pull down time?

Now after initial loading of frozen goods, it can take 3 to 4 days for the box to stabilize and run time to drop to 50%.

[Richard Kollmann]

Originally Posted by john61ct

The tubes are very securely sealed/glued. Plus suspended from the cooler lid above the kept-dry space, pretty sure if one did leak we'd notice.

So Richard, what solution do you think best, assuming the freezer can be held efficiently at well below zero, goal being to get cooling effect as long as possible before swapping.

Starting point being seawater (brine), and no corrosion issue with PVC tubes.

I purchased Polypropylene Food grade Glycol in 50 gallon drums full strength. When someone purchase it locally it is available in gallons labeled as potable water antifreeze intended to prevent a Boats or RV water pluming from freezing in winter storage.

Eutectic freeze points below zero are very difficult to freeze using a small compressor with 134a refrigerant, because to achieve phase change temperature needs a volume of refrigerant at twenty degrees below the freeze point. With a BD compressor it is possible to get evaporator coil down to minus 16 degrees F but Btu output is too low to be of value freezing eutectic fluids

As far as freezer brine solution Dole plates were zero degrees F.

Seawater has a freeze point from 14 to 26 degrees as I recall. In Calder's book you can find information on how Dole controls brine plate corrosion.

[newhaul]

Originally Posted by Richard Kollmann

Eutectic freeze points below zero are very difficult to freeze using a small compressor with 134a refrigerant, because to achieve phase change temperature needs a volume of refrigerant at twenty degrees below the freeze point. With a BD compressor it is possible to get evaporator coil down to minus 16 degrees F but Btu output is too low to be of value freezing eutectic fluids
.

We do it every day on the la ruina with a small cubigel compressor ( comparable to a bd35).
Please provide verifiable proof of your assertion.

[john61ct]

Originally Posted by Richard Kollmann

Eutectic freeze points below zero are very difficult to freeze using a small compressor with 134a refrigerant, because to achieve phase change temperature needs a volume of refrigerant at twenty degrees below the freeze point. With a BD compressor it is possible to get evaporator coil down to minus 16 degrees F but Btu output is too low to be of value freezing eutectic fluids

To the extent those statements are true I am sure they are generally valuable to increasing our knowledge.

However I was not asking about eutectic freeze points below zero, and as I stated I have no issues wrt corrosion using PVC only.

My direct and simple question is

>> **what solution** do you think best (**for my use case** of polar tubes in a cooler), assuming the freezer (airspace) can be held efficiently at well below zero, goal being to get cooling effect as long as possible before swapping

Obviously I'd welcome input from others as well.

I suppose seawater brine is at the higher temp end, and 1/3 glycol at the other, colder phase change.

Just to add a twist, what about the collection of tubes having a mix of solid-liquid phase temps?

Just as an example, seawater in a third, 15% glycol in a third, and 30% glycol in a third?

If they are all placed in the cooler at the same time, the fact that the high energy absorption melting point will occur at staggered times may well help maximise the longevity of their useful cooling lifespan?

[Richard Kollmann]

Originally Posted by newhaul

We do it every day on the la ruina with a small cubigel compressor ( comparable to a bd35).
Please provide verifiable proof of your assertion.

I do not have specs on your compressor but 134a is not considered a low temperature refrigerant even though 134a can reach very low temperatures at a vacuum of 25 inches. The BD35 at -30C produces only 19.5 watts at 2,000 rpm and at at 3500 rpm 32.2 watts. The problem is all systems at some point will reach an equilibrium where cooling performance almost stops. Another problem with refrigerant flow being cut back so much when operating at a low evaporator temperature or in a deep vacuum is refrigerant flow velocity is too low to return compressor oil. Most compressor manufacturers offer a engineering application specification guides to follow and Yes people take chances by expanding the design envelope. When Danfoss BD specs were followed compressor lasted 25 to 35 years.

[Richard Kollmann]

Originally Posted by john61ct

To the extent those statements are true I am sure they are generally valuable to increasing our knowledge.

However I was not asking about eutectic freeze points below zero, and as I stated I have no issues wrt corrosion using PVC only.

My direct and simple question is

>> **what solution** do you think best (**for my use case** of polar tubes in a cooler), assuming the freezer (airspace) can be held efficiently at well below zero, goal being to get cooling effect as long as possible before swapping

Obviously I'd welcome input from others as well.

I suppose seawater brine is at the higher temp end, and 1/3 glycol at the other, colder phase change.

Just to add a twist, what about the collection of tubes having a mix of solid-liquid phase temps?

Just as an example, seawater in a third, 15% glycol in a third, and 30% glycol in a third?

If they are all placed in the cooler at the same time, the fact that the high energy absorption melting point will occur at staggered times may well help maximise the longevity of their useful cooling lifespan?

If you freeze the plastic tubes with 2 pounds of 50/50 Glycol potable water antifreeze mixture in a standard freezer you will have 288 BTU stored the same as if it were ice from pure water. How long will eutectic solution at zero stay frozen, not very long.

Freezers in pleasure boats are not likely going to freeze even water fast enough, let alone a eutectic solution.

If you are looking for a flywheel effect to reduce compressor running time use plastic water bottles. External holding tubes are only justified when there is surplus energy. Small compressor do not generate surplus energy.

[Pelagic]

Originally Posted by john61ct

My direct and simple question is

>> **what solution** do you think best (**for my use case** of polar tubes in a cooler), assuming the freezer (airspace) can be held efficiently at well below zero, goal being to get cooling effect as long as possible before swapping

Obviously I'd welcome input from others as well.

Hi John, Looking at it another way....
I think the User should first decide what level of frozen, he wishes to maintain by adjusting that Set Point.

THEN adjust the solution to have turned fully solid at that level.

Part time cruisers really don't need to hold food at -22°C or less to keep the quality of the frozen food better.

But liveaboards who are stocking up for months should.

When first commissioning my Oziefridge freezer, I screwed up on the mix as it was a local concentrate and all I got was a slurry at -28°C.
So I diluted the mix until I got a solid state at -22°C.

I think fine tuning the mix to your holding needs and the power rhythm of your boat is worth considering.

[missourisailor]

Originally Posted by Richard Kollmann

....but 134a is not considered a low temperature refrigerant even though 134a can reach very low temperatures at a vacuum of 25 inches. .... The problem is all systems at some point will reach an equilibrium where cooling performance almost stops.

Richard,
You kinda answered my question with out answering it.

So let me pose the question another way.
With a Bd35 compressor running on 134A, what is a realistic low freezer and/or Evap Temp?

[kmacdonald]

Originally Posted by Richard Kollmann

Pete, I am again not trying to analyze or examine your refrigeration system design hardware or its performance. I will point out the many uninformed comments you make like "capillary systems don't have the well known TEV issue of gas leaking from frozen fittings". I believe there are designs where either cap tube or TXV valves are equal in controlling refrigerant super-heat. I stay away from LPR valves for refrigerant flow control. I found that with eutectic plates plate freezing the first day out with the TXV will lowered refrigerator box temperature much quicker.

And as to your comment about my advice to a boater in trouble regarding low refrigerant high pressure on an Isotherm Danfoss compressor unit, I recommended adding refrigerant to reach 120 psi . Cap tube refrigerant flow control on the isotherm system with your advice of 140 psi was and is still wrong for this system.

Any experienced engineer who looks at your comparison chart comparing evaporator plate against eutectic plate, of watts consumed, would surely question the results as I did.
The big difference is probably in a mismatch in evaporator plate Btu capacity to condensing unit capacity. You know the old saying, If, it is way to good to be true it is probably not true… My test comparison done 15 years ago did show a 6% to 8% energy improvement in favor of eutectic plate. Your test of 4577 watts for eutectic plate and 9940 watts consumed for a standard evaporator. Could anyone believe the eutectic plate is 100% more efficient?

Of course you are correct. Just about any test or research can be manipulated or designed to show exactly what the person doing the test or paying for the test wants it to show. IMHO that is exactly what was done. If anyone thinks a properly designed eutectic system is 100% more efficient than a properly designed evaporator system, god help them.

[newhaul]

My whole reason for going holding plate system is this . I have 200 ah of lifepo4 and I have 400 watts solar and 400 watts potential in wind generator. So holding plate makes the most sense.

[kmacdonald]

Originally Posted by newhaul

My whole reason for going holding plate system is this . I have 200 ah of lifepo4 and I have 400 watts solar and 400 watts potential in wind generator. So holding plate makes the most sense.

Yes it does if you have excess energy during the day. Might as well use it to cool plates.

[daletournier]

Originally Posted by kmacdonald

Of course you are correct. Just about any test or research can be manipulated or designed to show exactly what the person doing the test or paying for the test wants it to show. IMHO that is exactly what was done. If anyone thinks a properly designed eutectic system is 100% more efficient than a properly designed evaporator system, god help them.

I've had both. I went to the Ozefridge on my last boat therefore over the years got to compare the two systems. The oze fridge wins hands down regarding using less electricity. This is full time cruising in the tropics.

To clarify... I mean my batteries were less down in the morning using the Ozefridge, it's that simple. And the reason was I used excess power to freeze the plate right up with excess solar in the afternoon or when I was motoring. Excess power is free.

Is it more efficient if not using excess power? I don't know. In the real world my batteries were up over 14v by lunchtime, the fridge got a free ride, the holding plate would carry the fridge through most of the night.

All I care about is, does the fridge do what it's meant to (stay cold)? Where are my batteries in the morning?......
Ozefridge wins!

When my current fridge fails I'll install a Ozefridge.

[sailcrazy]

Quote:
Originally Posted by kmacdonald View Post
Of course you are correct. Just about any test or research can be manipulated or designed to show exactly what the person doing the test or paying for the test wants it to show. IMHO that is exactly what was done. If anyone thinks a properly designed eutectic system is 100% more efficient than a properly designed evaporator system, god help them.

what a great statement.....posted without a grain of proof or data to support it! IF you such proof, lets see it. If not, .....my BS flag just went up.

[newhaul]

Originally Posted by sailcrazy

Quote:
Originally Posted by kmacdonald View Post
Of course you are correct. Just about any test or research can be manipulated or designed to show exactly what the person doing the test or paying for the test wants it to show. IMHO that is exactly what was done. If anyone thinks a properly designed eutectic system is 100% more efficient than a properly designed evaporator system, god help them.

what a great statement.....posted without a grain of proof or data to support it! IF you such proof, lets see it. If not, .....my BS flag just went up.

I have installed both systems with solar and the holding plate system does win hands down.
It uses approximately 30% less energy over a months time on two comparable installs while cruising together 3 years ago in the pnw. In the tropics both systems would use more power to be sure however with the solar that is on both vessels the holding plate system would use even less of the overnight battery juice. Compared to the evaporation plate system.

[john61ct]

Originally Posted by Richard Kollmann

If you freeze the plastic tubes with 2 pounds of 50/50 Glycol potable water antifreeze mixture in a standard freezer you will have 288 BTU stored the same as if it were ice from pure water. How long will eutectic solution at zero stay frozen, not very long.

Are you saying that for my application there's no point using fluid with a lower freezing point? Doesn't the fact it changes phase at a lower temp make its cooling effect last longer?

> External holding tubes are only justified when there is surplus energy. Small compressor do not generate surplus energy.

Compressors don't generate any energy right? But yes I will be generating surplus electric both from solar and dino juice, and timing the tubes swap to capitalize on that.