Refrigeration - Evaporator vs. Holding Plate

[caldawson]

Originally Posted by RobMinton

So I'm looking for the most efficient icebox conversion kit for the most reasonable cost. We'll be leaving after storm season this year for the Caribbean, Panama, and beyond so our primary cruising areas will be tropical/very warm. I've just started this bit of research and open to suggestions on the direction I should take our refrigeration, of which we don't require very much. But our existing icebox is about 9 CuFt. We don't require freezer space but evaporators do allow just enough space to freeze some cubes or whatever. We're a lower energy demand boat and hoping to keep it that way. I think a water cooled system is the way to go in the tropics. The cost of the evaporator systems vs. holding plate systems is very attractive. Can anyone lend advice or input on this? Thanks in advance!

Hi Rob, I have a cooling plate on one side of a double drop icebox, it is very efficient and works quite well.
We have actually taken out the thermostat in hotter climates so we can use one side as a freezer and the other side as a refrigerator, have said this, it does freeze over very quickly and we do have to turn the fridge off once every couple of days, so everything doesn’t freeze, if you use a thermostat this can be avoided and you can just jury rig a sensor, something else we have tried and tested and this works just as well.

[SV THIRD DAY]

Originally Posted by missourisailor

Richard,
Thanks for the kind words.
But my fridge/freezer really pisses me off!!
For the exact reason I highlighted above.

I can see if our sailing/cruising consisted of weeks or months from land based supplies where I would have a good to very good setup. One time loading then frozen until needed.

But as coastal sailors, we tend to load up when ever we come ashore. Which is usually every three to five days.

When we load the "high temp" frozen food into the freezer, the system goes nuts trying to bring the temp down. Usually it will run nonstop for 36 to 48 hours until the stat shuts it down.
Honestly this is really no big deal because our house bank can handle the load. But it drives me nuts when it is not cycling.

IMHO I think the driving cause for this long cool down time is directly related to the surface area of the holdover plate. Small surface area equals less BTU transfer.

Pete's test showed that with an empty box, the holding plate system is more efficient. But if a half full box was brought down to temp, then more items added and the system was allowed to recover. I would bet that the results would swap. At least until it recovered.

Most holding plates just don't have the surface area that the thin plates have. At least not the ones I've seen.

Heat Transfer from the Box to the Plate has more to do with the Volume to Surface Area ratio of the set-up than the plate construction material. The worst thing you can do with a Holding plate or Cyclic Evaporation plate is to let the frost monster build up to more than 1/4".
Three things you can do to increase the "pull down time" with a holding plate system are:
1. Add a second holding plate in series, that's the $$ Solution or course.
2. Be sure to run your compressor on high speed rather than 1/2 or 3/4.
3. Keep the plate frost down to below 1/4. Scrape it with a SS spatula frequently when the show frost is fluffy before it turns to hard ice. In our testing that can slow down the heat transfer rate and lead to warmer box temps and more daily power usage.

Originally Posted by newhaul

the plate on the la ruina is 13x24x2.5 stainless steel so about 5 sq ft of surface area and if memory serves the last evaporation plate unit I installed was about the same but only 3/8 inch thick aluminum.

True....

[SV THIRD DAY]

Originally Posted by RobMinton

I think a water cooled system is the way to go in the tropics.

The Air vs Water Cooling Myth takes a lot of work to debunk, but it's worth it in getting away from the evils of sea water maintenance.

https://www.youtube.com/watch?v=YL_xj_O4h3c

[a64pilot]

Ref how cold you can get one to go.
I have a Cool Blue system that is custom, it has a BD80 compressor and dual cold plates in series. I have a 14 cu ft split over, approx half freezer and half fridge. These numbers are actual box air temp numbers, not cold plate temps. this is with no open holes in the divider. This wasn’t equilibrium, I had added food to the fridge, but not the freezer. Normally I cannot hold this differential the fridge would go below freezing as well. This was an attempt to see just how cold it would go with the thermostat turned down to max and compressor run was continuous. I think this is about as cold as you can get R-134, and likely well past it’s efficiency knee if you will .

[BigBeakie]

Originally Posted by kmacdonald

I have a new engine designed to run on salt water. I'm giving them away to the first 100 takers. Would you like one?

Show us the data and the test methodology whereby you can make such a statement, and then we might listen to you after we have examined your data and experimental method, and come to some conclusions. You know, like Pete did in his experiment.

Of course, then again we could just make a pronouncement like you did, and display our lack of critical thinking skills....

[missourisailor]

Originally Posted by SV THIRD DAY

Heat Transfer from the Box to the Plate has more to do with the Volume to Surface Area ratio of the set-up than the plate construction material. The worst thing you can do with a Holding plate or Cyclic Evaporation plate is to let the frost monster build up to more than 1/4".
Three things you can do to increase the "pull down time" with a holding plate system are:
1. Add a second holding plate in series, that's the $$ Solution or course.
2. Be sure to run your compressor on high speed rather than 1/2 or 3/4.
3. Keep the plate frost down to below 1/4. Scrape it with a SS spatula frequently when the show frost is fluffy before it turns to hard ice. In our testing that can slow down the heat transfer rate and lead to warmer box temps and more daily power usage.

Rich,
Comments in reverse to your suggestions.

3) Frost is always kept at a minimum. Still has frost but no where near 1/4".

2) 1500 Ohm resistor and a current draw of about 4.5~5 A, shows high speed.


1) Left this for last for a reason.
A year or so ago, we had this same discussion, and I agree that a second evap would be the best solution. But at that time I was not convinced that a Holding plate system was indeed the best approach. I thought that a well designed and built thin plate system might be a better option. Like what John builds. So I was considering contacting him about building a couple of his evaps to replace the holding plate that I have.
But it never got much past the "thinking" point.

Now Pete has thrown a monkey wrench into this whole thing, with his test.
As accurate and/or flawed as his test is, I'm convinced that it still shows that holding plates are better than thin plates. To what degree is still debatable.
And your statement about a holding plate still absorbing heat with the compressor off, is an aspect that I had not thought of.

So now I'm back to dual holding plates.

Now a question for you.

Can my small BD35 compressor handle two holding plates?

[letsgetsailing3]

Some great ideas in here. Looking to modernize, so I'm in the process of reading through this thread.

[SV THIRD DAY]

Originally Posted by missourisailor

Can my small BD35 compressor handle two holding plates?

Yes....we have plenty of dual holding plate systems out there. The plates are plumbed in series with the TXV on Plate No 1 and the TXV sensing bulb is strapped to the outlet line of Plate No 2. That way from the compressor point of view (and TXV for that matter) the two plates are acting as one larger plate.

So what's the negative?
Initial warm plate start-up. It will take longer for the two holding plates and the system to reach equilibrium and start it's normal cycling with two plates because obviously you have the additional eutectic solution to initially freeze down from the second plate. Another Negative...Space. Another holding plate takes up box volume. Lastly...cost. a second holding plate and the interconnecting copper lines is $900.

What's the advantage?
Lower box temps. This is because you will be essentially doubling the amount of surface area inside the Box that will be absorbing heat. Another plus, longer compressor OFF times because you have more BTUs of heat removal stored inside your Box.

[missourisailor]

Originally Posted by SV THIRD DAY

Yes....we have plenty of dual holding plate systems out there. The plates are plumbed in series with the TXV on Plate No 1 and the TXV sensing bulb is strapped to the outlet line of Plate No 2. That way from the compressor point of view (and TXV for that matter) the two plates are acting as one larger plate.

So what's the negative?
Initial warm plate start-up. It will take longer for the two holding plates and the system to reach equilibrium and start it's normal cycling with two plates because obviously you have the additional eutectic solution to initially freeze down from the second plate. Another Negative...Space. Another holding plate takes up box volume. Lastly...cost. a second holding plate and the interconnecting copper lines is $900.

What's the advantage?
Lower box temps. This is because you will be essentially doubling the amount of surface area inside the Box that will be absorbing heat. Another plus, longer compressor OFF times because you have more BTUs of heat removal stored inside your Box.

THX for the pro's and con's.
But I would assume that the BD35 compressor has a max evap size. And by stringing two in series, at what point do we need to upsize the compressor?

[kmacdonald]

Originally Posted by BigBeakie

Show us the data and the test methodology whereby you can make such a statement, and then we might listen to you after we have examined your data and experimental method, and come to some conclusions. You know, like Pete did in his experiment.

Of course, then again we could just make a pronouncement like you did, and display our lack of critical thinking skills....

I'm sorry you mistook the infomercial for a scientific "experiment". An excellent display of our lack of critical thinking skills and judgement.

[SV THIRD DAY]

Originally Posted by missourisailor

THX for the pro's and con's.
But I would assume that the BD35 compressor has a max evap size. And by stringing two in series, at what point do we need to upsize the compressor?

The BD35 can handle two of our standard holding plates in series.
Once you go over that you could run into long term compressor issues by having too long a copper line inside the holding plate to keep proper oil flow that continually cycles through the refrigeration cooling loop.

[Richard Kollmann]

Originally Posted by missourisailor

THX for the pro's and con's.
But I would assume that the BD35 compressor has a max evap size. And by stringing two in series, at what point do we need to upsize the compressor?

One of my test boats had two freezer eutectic plates and one refrigerator eutectic plate connected to one BD2.5 compressor using a separate TXV for each plate The three plates contained a total of eight gallons of eutectic solution. This system required separate compressor cooling and small liquid orifices before TXV This system operated 10 years before boat was sold. It was not possible to freeze all plates at the same time but once plates were frozen the BD2.5 maintained eutectic plate desired temperatures.

When I built kits requiring two plates for BD compressor systems I cut 1/4 in OD copper tube 50 foot roll using less tubing in smaller plate when plates were in series.

[hellosailor]

missouri-
If you strike the words "second holding plate" and replace them with "a holding plate that is twice as large", which is essentially the same thing, well then duh? Of course the pros and cons are obvious, with twice as much holding plate to cool down it WILL take longer to get that cooling going. And it will provide more cooling, and cost more money, as more equipment always does. But being a "second" plate or a "larger" plate, there's no real difference except the bit of tubing to plumb it in, is there?
Sizing the compressor to the job, whether that's one big plate, three small plates, a big icebox, a small one...all the same thing in the long run, no?

[Richard Kollmann]

Originally Posted by hellosailor

missouri-
If you strike the words "second holding plate" and replace them with "a holding plate that is twice as large", which is essentially the same thing, well then duh? Of course the pros and cons are obvious, with twice as much holding plate to cool down it WILL take longer to get that cooling going. And it will provide more cooling, and cost more money, as more equipment always does. But being a "second" plate or a "larger" plate, there's no real difference except the bit of tubing to plumb it in, is there?
Sizing the compressor to the job, whether that's one big plate, three small plates, a big icebox, a small one...all the same thing in the long run, no?

My example was three separate boxes on one compressor Refrigerator, freezer and a 4 gallon plate in a 96 quart drink cooler. Was this system energy efferent since compressor ran continuously? No. The point is when there are mutable eutectic plates they perform much better when not in series and each plate has its own TXV refrigerant control.

[hellosailor]

Richard-
"The point is when there are mutable eutectic plates they perform much better when not in series and each plate has its own TXV refrigerant control."
Yes, that too. Each plate will perform better with a separate controller, since each plate is being optimized for a local environment (so to speak) that way. Being kept matched to the particular load it sees, the amount of food, the volume of the box...all the variables that are particular to just that one plate.
Yes, by all means.