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Old 07-11-2014, 10:29   #1
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Refrigeration upgrade

Our boat, a Bermuda 40, still has the original Grunert refrigeration system installed at original launch in 1973. It has been extremely reliable, surviving one sinking, with the original motor running ten years (to date) after that immersion.
Still it won't last forever. The original Tecumseh compressor no longer has repair parts available. It needs to be recharged with Hot Shot every 6 or 8 years, but my real day-to-day complaint is the electrical consumption of the system. At 21 amps, it is an inefficient dinosaur.
What I
would like to do would be to retain the existing coils and lines, which are cupro-Nickel, show no signs of leaks, and substitute a Danfoss or equivalent compressor, continuing to charge the system with the present-style refrigerant.
Is the operation of a Danfoss (or equal) restricted to the newer refrigerants like R122, or does it even care? Knowing little about the finer points of refrigeration, I am at a loss on this.
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Old 07-11-2014, 11:34   #2
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Re: Refrigeration upgrade

I faced a similar situation with an older Grunert. I ended up designing my own system around a hermetic Tecumseh Sierra 24v running R404a, and am very happy with the results. The big issue with the R123a Danfoss is that it is hard to get sufficient output to make the plates change phase. There are dual compressor larger Danfoss, but the lead times were ridiculous (and I think in one case I had to order a few dozen!). A second issue is that the eutectic point of the freezer plate is probably so low that R123a has to run a near vacuum to reach it. This is not very efficient and can require very large volume of refrigerant which will not pass through small tubing without significant losses.




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Old 07-11-2014, 14:05   #3
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Re: Refrigeration upgrade

Since holding plates rarely go bad, it's very common to reuse your holding plates and simply add in a new 12v compressor/condensing unit. We do several of these types of system reworks a month and I have one on the bench in front of me right now.

Before you may have had one compressor running both plates, but that isn't the way to go with these smaller capacity 12v units. So if your boxes are too larger for the classical spill over Freezer/Refrigerator concept, then you would need a two compressor/condensing units...once for each plate and box.

You may need to swap out the expansion valves and change the eutectic solutions in the plate...but that's easy stuff to either do yourself or send in the plate so someone like us to do for you and send the system back bench testing and ready to plug and play back into your box like it was a new system.
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Old 07-11-2014, 16:34   #4
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Re: Refrigeration upgrade

Many of these plates were plumbed with 3/8" or 1/2" tubing. Won't this be an issue for a Danfoss 12V compressor? Even if one made conversion fittings, wouldn't the refrigerant volume in the 3/8" tubing inside the plates be an issue for a Danfoss?

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Old 07-11-2014, 16:54   #5
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Re: Refrigeration upgrade

Our standard fin/tube holding plates have 1/2 copper tubing internal in the plate, so that's not a problem. It doubtful that the old plates have the internal fin/tube condensing unit inside them, more likly just 12ft of 1/2 copper tube. That takes a little away from the efficiency, but it should still work pretty well. It is pretty common to need to change out the eutectic solution just to be safe. We rehabbed a holding plate a few months ago and it was just filled with water....my hope is to open one up and find some good rum inside since that's a standard chat room "I had a buddy that had a friend who was married to a guy who went to school with a guy that said rum was the best eutectic solution to use"....hahaha
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Old 07-11-2014, 19:22   #6
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Re: Refrigeration upgrade

I would love to more fully descibe my system, but since the evaporation unit is concealed inside a wooden built-in cabinet externally and sheet metal housing internally, I can only guess what's there. It has a central freezer unit between two reefer boxes with three thermostats, one for each compartment. Total capacity is 4.5 cu. ft. I know, I know, I should have dealt with this after the Major Refit, (replaced, rebuilt, re-finished just about everything I could except this) but after 5 years you get impatient. And it DOES still work. But for how long?
So is there some compressor out there (Tecumseh hermetic?) that I can drop in place using existing fittings, coils, lines, etc. and charge with an approriate refrigerant and begin cooling my beer? Or is that just one more fantasy
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Old 08-11-2014, 07:23   #7
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Re: Refrigeration upgrade

In my case, the issues wasn't the diameter of tubing *in* the holding plates, it was the tubing to/from the holding plates. To have replaced it with separate runs for fridge and freezer would've required a lot of carpentry and wasn't ideal.

OP, you CAN run both fridge and freezer off a single compressor -- you're doing it now! One issue is the choice of refrigerant. A second issue is that the holding plate designs work well as holding plates, but not as efficiently as regular condensers. Thus, it takes a lot of cooling capacity to bring them down to temp, and then less to keep them there. The Danfoss compressors are efficient, but low capacity -- and thus would take a long time to get holding plates to change phase.

If you don't want to replace the plates or the tubing to the plates, you have several options (If you are willing to replace them, you have many more options). If you change refrigerant, which is probably going to be necessary, you will need to replace not only your compressor but also your TxV's (thermostatic expansion valves), and probably the control valves at the solenoids. You will need to decide on your refrigerant, and on how you want to power the compressor (DC? AC?).

I ended up going for a Tecumseh Sierra SIERRA00027 aka SIERRA03-0982Z1.

Even with 800+Ah of house battery capacity, I like the energy storage of the holding plates. I prefered the hermetically sealed brushless design of the Tecumseh Sierra over the SeaFrost DC5000. I preferred the R404a over the R123a because I could get the high capacity cooling (less time to phase change) without having to change the lines to/from the reefer. I prefered the Tecumseh Sierra over the Danfoss because the Tecumseh was higher capacity than the available Danfoss. Depending on your criteria, you may make a different choice.

The other issue is installation. If you are not doing it yourself, you will be limited to what your chosen installer feels comfortable with.
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Old 08-11-2014, 08:12   #8
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Re: Refrigeration upgrade

Just to avoid some confusion about how holding plate systems efficiently designed should work, you only need to deal with the liquid to solid phase changeat initial start up with a warm plate. The thermostat then kicks the compressor on at a temperature that never allows the eutectic solution to "melt" and go back from a solid to liquid. Why waste the phase change energy with each cycle? That's why a smaller 12v compressor unit can work even with a big/huge holding plate because once the plate is frozen the compressor is just cycling to pull the BTUs of heat out of the box. Sure it will take several hours (4-10) to initially freeze down a warm plate at start up...but this really only matters and is a concern if your system use isn't to just leave the system on, but rather turn it off and on all the time. For that application evaporation plates with no hold over capacity would be the best choice.

The old school operational procedures for a holding plate system where its manually turned on to match a generator or engine driven compressor is pretty out dated. Let the thermostat take care of the system and keep the eutectic solution frozen for a more efficient appljcation where you don't lose that phase change energy.

The reason you can't run two different temperature holding plates on a single 12v danfoss type compressor doesn't have anything to do with the compressors capacity (OK within reason) but is tied to the question of how will the compressor know when to turn on and off based on the thermostat? The freezer plate and refrigerator plate are set up to freeze at different temperatures to hit the target box temps. So if you run two different temp holding plates they will have different compressor on/off temps. Oh sure you can add in the complexity of automatic valves/solenoids but that's a fiasco waiting to happen and we rip out and replace those type of systems monthly. Simplicity wins on a cruising boat.

We do multiple same temperature holding plates off a single 12v danfoss type compressor, especially for freezers all the time without problems.
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Old 08-11-2014, 09:39   #9
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Re: Refrigeration upgrade

Quote:
Originally Posted by colemj View Post
Many of these plates were plumbed with 3/8" or 1/2" tubing. Won't this be an issue for a Danfoss 12V compressor? Even if one made conversion fittings, wouldn't the refrigerant volume in the 3/8" tubing inside the plates be an issue for a Danfoss?

Mark
I missed an important part of your question Mark, about the refrigerant volume. A typical AB type critical orifice rolled aluminum evaporator system uses from 3-4oz of refrigerant and as their name "critical charge" implies, the amount of refrigerant in the system must be precisely matched for the internal volume and even the operational conditions.

But holding plate systems with a TXV (thermally adjusting expansion valve) have a large dryer /receiver added into the refrigerant loop and serves as a resouviur to store the liquid refrigerant for when the TXV needs it. The full 24oz of refrigerant in one of our systems for example is stored in the dryer/receiver when the unit ships. This "internal volume flexibility" let's call it, let's us use 1 or 2 holding plates in series or a 12ft or 30ft copper tube set without having to worry about adjusting the system charge. This also allows systems with a dryer/receiver to use a much larger condensing radiator to better disapate heat from the system. While critical charge systems start losing condensing unit efficiency at ambient temps above 90-deg a system with a larger condensing unit surface area can go up to 120-degs ambient temp without the same loss in efficiency. The units we sell to the US military for Mobil food transport and storage function up to 135-degs ambient temps. A 6ft x 6ft x 6-ft walk in refrigerator box running off two little 12v danfoss air cooled units and a 5ftx 3ftx 3inch thick holding plate. It proves the case of just how important insulation is because this boz is made out of 8" of polyisochanurate foam encased in a fiberglass shell.
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Old 08-11-2014, 10:40   #10
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Re: Refrigeration upgrade

A Danfoss BD-80F w/ R-134a, cooling a -23.3C freezer holding plate, has a rated capacity of about 100W at maximum speed -- or 300BTU/hr.

We don't know the size of the OP's holding plates, nor how well insulated they are. Perhaps they are like mine. I have a ~2500BTU holding plate (-5C)in the fridge and about ~4500BTU holding plate (-23C)in the freezer. The fridge leaks about 2000BTU/day, the freezer about 5000BTU/day.

At this rate, a single BD-80F would run full time at max speed just to keep the holding plates at temperature, and wouldn't have even a hope of being able to get them to change phase.

If the holding plates were smaller, or the refrigerator had more insulation, the situation would be more favorable, but without doing a lot of work a 1973 Bermuda 40 has what it has. And the OP expressed a desire not to make major changes.

Using multiple BD80's would help, but that requires running new lines from the compressor(s) to the refrigerator, something that the OP said he didn't want to do. I believe the OP already has 3 solenoid/valves to run off a single compressor -- it doesn't add complexity to use them in parallel (although the valves would probably have to be changed for a new refrigerant). If anything, it would add work to run new lines from the reefer to the compressor to use multiple compressors. That approach would have the advantage, though, in the event that one of them died -- you would still have refrigeration.

A dual BD-350 has about 3x the capacity of a BD-80, but I question their availability -- I couldn't get one when I tried.

It may be useful to know how the OP intends to use the system. For example, does he leave the reefer off, go down to the boat on a Friday night and turn it on, and expect a cool refrigerator within hours in order to cruise for a long weekend? Or is the reefer always on as part of a live-aboard or long term cruising? Is the reefer powered by solar tricking in? Or is a genset run for an hour a day to charge the batteries?

It isn't "wasting" phase change energy with each cycle to allow the plates to start to melt -- this is what they are designed to do -- this is how they store/release energy. Except for losses in the compressor, etc., any energy put into the system is used to cool the fridge/freezer. The advantage of holding plate systems is that you can store more of the energy in the holding plates, effectively making your storage capacity (battery + plates) larger. The primary disadvantage is that when going from warm to frozen, it takes more energy and time to get there (although you get that energy back as the plates melt). There are disadvantages too -- in terms of greater temperature gradient across the box, for example.

As an aside, although the old-school way to run a holding plate system is to turn it on when one runs the auxiliary or generator, I find that the newer DC compressors are efficient enough to leave it on all the time, thermostatically controlled, and use the battery capacity as a buffer. Still, I have in mind to build a controller that turns on the compressor when it observes high battery voltage -- e.g. charging -- to make even more efficient use of the holding plate's energy storage capacity.
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Old 08-11-2014, 11:14   #11
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Re: Refrigeration upgrade

With all due respect, you are doing a disservice to those with 12v holding plate systems by confusing the different operational properties of having a smaller 12v compressor compared to a big engine driven or generator driven unit. All manufactures of 12v holding plate systems all set their systems to never allow the solid/Liquid phase change to happen. To do so would be to waste the phase change energy and not take advantage of the eutectic solutions ability to change temperature and absorb heat while remaining a solid.

Sure you don't care as much about overall operational efficiencies when the compressor and ability to power it is so overkill, but it matters big time to the systems with smaller 12v compressors . That's the technical point that needs to be flushed out or I know how these things in chat rooms work. The rumor will go out to ignore the tech manuals for Seafrost, Technautics, Isotherm, and the other 12v holding plate systems because they are bozos. Go ahead and set the system up to let the holding plate melt/phase change. 1/4 of my service call time is spent deprogramming myths and half truths people hear online and from their dock expert neighbor, so I'm sensitive to this issue when I see things online that will easily confuse people. Really...I don't try to be an ass, hey I would rather be sanding and varnishing my teak today, but some times I can come off as an ass by not letting something I see as misinformation set unanswered. After 46yrs of Technautics building holding plate systems I may be a bozo, heck even an ass, but there's a little testing and data gathering behind how best to operate a holding plate.
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Old 08-11-2014, 12:05   #12
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Re: Refrigeration upgrade

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... All manufactures of 12v holding plate systems all set their systems to never allow the solid/Liquid phase change to happen. To do so would be to waste the phase change energy and not take advantage of the eutectic solutions ability to change temperature and absorb heat while remaining a solid.
If the eutectic plates never changed phase, then why use eutectic plates at all instead of simply a large thermal mass?

If (partially) changing phase wastes energy, where does that energy go?
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Old 08-11-2014, 14:58   #13
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Re: Refrigeration upgrade

The properties of a eutectic solution allow it to absorb heat without changing temperature - up until its phase change (solid to liquid in this usage). So once it is frozen to a certain temperature, it stays at that temperature until it has absorbed enough heat to change back to a liquid (its eutectic point).

Thermal masses work more like you expect - you cool them to a certain temperature and they continually warm up. These don't work for reefers because the box temperature would go all over the place unless the thermal mass was huge indeed.

Just like a frozen eutectic solution absorbs a lot of heat energy before it changes to a liquid, the liquid solution needs to have a lot of heat energy removed from it to get it to a frozen state. This is where the "waste of energy" comes from (it isn't really a thermodynamic waste, but it is not energy used to cool food so it is a practical waste). Once frozen, keeping it that way only requires removing enough heat to keep it frozen - which is mostly a linear process like an evaporator plate.

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Old 08-11-2014, 17:28   #14
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Re: Refrigeration upgrade

The classic eutectic solution is water -- imagine a glass of ice water, always at 32 degrees. Take out a little more heat, and you increase the proportion of ice to water - still at 32 degrees; put in more heat and more heat and the proportion shifts to more water than ice -- still at 32 degrees. Only when you get all water or all ice can you move above or below 32 degrees.

Ideally, a eutectic plate is brought to its phase change temperature and a majority of it freezes. As it warms, more of it thaws until most of it is liquid again. Then, the compressor is turned on and the heat is removed and it turns more solid. In normal operation it is never 100% solid nor 100% liquid. It spends its time bouncing between those two extremes maintaining a consistent temperature.

Any energy used to move from a slightly frozen to a more frozen state is heat removed from the solution. That energy is "given back" as heat is moved from the food (or contents of the refrigeration) to the solution, producing a slightly less frozen state.

The reason why eutectic plates are preferable to a simple thermal mass is that the amount of heat required to raise the thermal mass by one degree is much, much less than that required to change the phase of the eutectic
solution.
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