Any refrigeration experts out there?

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No, assuming the working fluid when through a phase change, it will melt at a constant temperature. The OP indicated -40 degree refrigeration requirement for his cold plates. These store "cold" in the form of a latent heat phase change of the working fluid internal to the plate. It will only increase in temperature once all the working fluid has melted. And only then as a function of the many materials specific heats, and weights, and heat source being added. ignator

On Sun, 31 Aug 2008 04:20:51 -0700 (PDT), ignator
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And just like ice banking for commercial air conditioning to save money using 'off-peak' power to make ice overnight and the ice melt cools the building during the day, that brine phase change cold plate will work fairly well if you need a continuous low rate of cooling but can only supply refrigeration in high-rate bursts because of a restricted power supply - which you would see on a very small vessel like a sailboat, or a small cabin off the power grid.
You'll get solar cell input during the day (but not much), and wind energy when the wind blows fast enough to turn the turbine - and not so fast it trips off the safety and stops. But you only get real high horsepower refrigeration work done when you start a generator set or a propulsion engine, or stoke a boiler and use steam generation, or...
And because of often restricted fuel supplies or the need to manually monitor the prime mover (stoke the boiler) you can't do that 24/7, so you 'bank the cold'. Or you put in a Servel / Dometic ammonia absorption refrigerator and let a small gas flame supply the slow and steady energy input.
But a larger yacht or commercial vessel doing a 5-day run doesn't have nearly the power restrictions as a 18' sailboat trying to do 30 days between ports. You have to be power efficient, but either the main engines or an auxiliary generator set are running most of the time. In that use, IMNSHO the brine plate is going to be FAR more trouble than it's worth.
When you have to fix it yourself with only the supplies and materials on hand, there is one rule: KISS. Keep It Simple, Stupid!
A properly sized 12/24V to 120VAC inverter system (example go look up the "Heart Inverter" perfected in RV use) and a large battery bank can provide seamless switchover between onboard and shore power sources - and a few hours of overnight silence if needed. The inverter can even auto-start the generator when large loads come on or the batteries discharge, then shut it down again when the load drops and the batteries are full.
And if the "Magical" inverter system craps out or the battery string goes flat or open, you thought ahead and built in a bypass. Just throw that big switch and go straight from the generator to the breakers.
--<< Bruce >>--

On Aug 31, 2:13 pm, Bruce L. Bergman
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My point was that R. J. Kinch said "Brines or other fluid mediums cannot work as well, because their temperatures rise as they absorb heat" And my response was that, the melting point of the brine or glycol will result in a constant temperature until all of it is melted.
I've been on sail boats with cold plates, and running the diesel for 2 hours plus twice a day is a PITA. Not to mention that you end up with a solid block or frozen mass that you have to wait to defrost to gain access too, or too damn warm. My last sail had a 12 volt refrigerator, at 5 amps, using battery most of the time and we only ran the diesel long enough to get the amp meter charge down to full enough. The only problem was the Moorings rental either hot wired across the thermostat, or the contacts were welded closed. We cycled the breaker when ever the frost built up a few millimeters. Evidence that this was a problem was two repairs to the evaporator plate where some previous renters used a knife to "defrost". This was the first time sailing that ice would remain frozen as the previous sails, the cold plate either froze everything or the ice melted away. I agree that electric refrigeration is much better. Also Moorings had air conditioning installed in their latest boats, but only ran on shore power, but when your loading provisions, the crew don't get so damn cranky. ignator

ignator writes:
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Listen: fluid mediums. They're not changing phase.
Ice directly against a cold plate works better than an ice/water bath. The liquid water is merely adding thermal resistance to the heat transfer, not sinking heat itself.

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My understanding of the construction of the cold plate with phase change storage media is refrigerant tube ways, conducting the heat from the to metal (aluminum) with phase change storage media in the same plate, as all the same structure, there is no air gap. This plate is install in an insulated "ice chest" that has the door on top to limit the cold air flowing out like a standard refrigerator. Yup the plate is surrounded by air, and any thing in contact with it, like a bag of ice. The idea is this plate has a long off cycle between recharging (re-freezing the phase change energy storage media) via the mechanical refrigerant system. Like a refrigerator, you don't want some foods to be in direct contact with the evaporator, unless you like frozen salads. You don't want everything in the ice chest to be in good thermal contact with this cold plate evaporator. ignator

On Tue, 26 Aug 2008 05:04:22 -0700 (PDT), ignator
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Not necessarily incorrect, but new technologies available might require a rethink on how you accomplish the task. There's always more than one way to do it, and the alternatives may be better than "that's how we always did it."
"Don't raise the drawbridge, lower the river."
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I've seen these big built-up "one compressor" refrigeration systems at supermarkets, and they certainly can be made to work. But consider what happens when that one compressor blows up, or that one massive refrigerant system springs a leak - now you have no refrigeration for the entire boat, and the Chief starts sweating.
Supermarkets can call the local refrigeration service company and have a tech on site in an hour who knows that system inside and out, and with full access to parts and supplies from the local supply house. If you are out in mid ocean...
And supermarkets usually put all the freezers on one combined system and refrigerators on the other, just to simplify the controls. Otherwise they have to start adding check valves and suction regulation everywhere. And it's best to unload the compressor and let it run rather than constant starts and stops - but then you get into run-time wear, and since you have to make your own the power is a lot more expensive on a boat than a building...
Consider that these big systems always have a problem keeping the refrigeration oil in the compressor - it gets carried out into the big accumulator and all the low spots of the lines, and unless you put oil traps and return lines everywhere it never gets back. Then the compressor dies from oil starvation - see "What happens when the one compressor fails" above.
You will be fighting this forever with oil traps and oil makeup tanks and crankcase controls. And trying to keep the velocities in all the return lines right, and getting the slopes and falls right, placing P-traps everywhere to get the oil moving back - and then the boat goes up on plane and all the working angles change...
You do need at least a small accumulator on the compressor suction side as a trap to protect against slugging - gases compress, liquids don't, and you don't want to test the anti-slugging feature on the compressor head too often or it can break the springs. Then refer to "What happens when the one compressor fails" above.
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This was my thought too - Danfoss (http://compressors.danfoss.com ) makes the BD Series 1/20 HP and 1/12 HP hermetic brushless motor compressors that run on 12V or 24V internally, and they have several control boxes to run them on 12/24V battery systems OR variable voltage solar or wind systems OR auto-changeover between 120VAC/240VAC world voltages and 12/24 VDC. And the control boxes have integrated temperature controls - just add a thermistor in the conditioned space and an adjustment rheostat.
Even one control box specifically to control EMI/RFI radiation that could rip the navigation and radar systems on a ship to heck.
And if there is enough demand, I'll bet they could build them bigger without too much trouble. 1/2 HP or even 1 HP isn't out of the question for 12VDC battery supplied systems.
It will be a HECK of a lot simpler to make separate systems that run continually from a deep-cycle battery bank, and each one optimized to it's task - freezer, refrigerator, ice-maker, live-well chiller, etc.
And a whole lot simpler to circulate low pressure condenser water around the ship to separate small heat exchangers at each refrigeration device, rather than high pressure refrigerant.
And it WINS hands down on KISS simplicity and redundancy - if one refrigerator fails, you just move everything to the three others that still work. If your battery bank goes flat, start the generator set and they all switch to 120V.
For space cooling in the cabins, consider either a "Mini-Split" refrigeration based heat-pump separate from the freezers (if you plan to run the gen-set or inverters 24/7), or ducted fan coil units and circulating chilled/heated water from the engine room. Keep all the refrigeration in one compact spot, and you could use a large ice-bank tank low in the ship (they are heavy!) to shut down the gen-set at night.
Heat is easy, I'll bet you have lots of excess heat when those two big main propulsion diesels are running - and when they aren't use the same tankless hot water heater you have for the showers. Or an RV-style forced air propane furnace. Many ways, all depends.
--<< Bruce >>--