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Old 12-04-2012, 07:40 PM   #21
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Originally Posted by Daddyo View Post
The draw is going to be specific to the fridge, how full it is, how often the door is opened, if there is adequate cooling for the coils, etc. There is no magic in 110V vs 12V. It takes power of either variety to produce cold. 12V in general tends to be slightly more efficient but the conditions I just sighted will trump unit efficiency. Insulate and fill any fridge and the efficiency will go way up
I hate to be a stickler, but I like helping with the right information when I can. Refrigeration systems do not make cold air. The air in the space that a unit is designed to service is drawn into an evaporator, and the heat from that air is given to the refrigerant flowing through that evaporator. That heat causes the refrigerant to flash to a gas, goes to the compressor, then the hot gas is pumped to the condensor where the heat is given off and the cycle begins again. So essentially, we just keep removing heat until the air temperature is where we want it. I have studied Psychrometrics pretty extensively, even in the realm of fluid mechanics, and this is the opinion of all of the professors' and professionals I have encountered. I mean no disrespect, I just like to share the right information. I have had my EPA 608 and 609 refrigeration certification cards for quite a few years and actively pursue refresher training every two years. As for the A/C-D/C issue, like it has been stated, load, insulation, size, frequency of temperature rises, average ambient temperature, even the sun shining on the unit all day can contribute to the amount of amps used per day.
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Old 12-04-2012, 07:56 PM   #22
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Nigel Calder has an excellent book "Refrigeration for Marine Pleasureboats" covering all areas, including the most suitable units in terms of usage, voltage, power supply,etc. Anthonyd`s concise description of how it works is good. So is Calder`s, though eye-glazing and helpful for getting to sleep. I got my copy from Amazon.
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Old 12-04-2012, 08:51 PM   #23
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It is thought provoking that a company, Norcold, that had the bulk of the RV and marine markets did not keep on the cutting edge. The old saying: build a better mouse trap , , , , , ,

If there is one product on a boat that I have heard the most complaints on, it is Norcold. Where was their R & D. It seems that they should have been ahead with patents on most of the new technology. Go figure!
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Old 12-04-2012, 10:13 PM   #24
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My boat has had a propane fridge for 20 years. I'll be replacing it in the spring with a like unit. I get 3-4 weeks on a 20# tank plus no noise.
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Old 12-05-2012, 05:59 AM   #25
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Nigel Calder has an excellent book "Refrigeration for Marine Pleasureboats" covering all areas, including the most suitable units in terms of usage, voltage, power supply,etc. Anthonyd`s concise description of how it works is good. So is Calder`s, though eye-glazing and helpful for getting to sleep. I got my copy from Amazon.
Thanks for the title Bruce. I'm going to check that out.
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Old 12-05-2012, 06:47 AM   #26
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Originally Posted by Anthonyd View Post
I hate to be a stickler, but I like helping with the right information when I can. Refrigeration systems do not make cold air. The air in the space that a unit is designed to service is drawn into an evaporator, and the heat from that air is given to the refrigerant flowing through that evaporator. That heat causes the refrigerant to flash to a gas, goes to the compressor, then the hot gas is pumped to the condensor where the heat is given off and the cycle begins again. So essentially, we just keep removing heat until the air temperature is where we want it. I have studied Psychrometrics pretty extensively, even in the realm of fluid mechanics, and this is the opinion of all of the professors' and professionals I have encountered. I mean no disrespect, I just like to share the right information. I have had my EPA 608 and 609 refrigeration certification cards for quite a few years and actively pursue refresher training every two years. As for the A/C-D/C issue, like it has been stated, load, insulation, size, frequency of temperature rises, average ambient temperature, even the sun shining on the unit all day can contribute to the amount of amps used per day.
Being a "stickler", you should feel right at home with this bunch.

You are correct of course, you cannot make cold, only remove heat. You are also right on the current used per day on a refrigerator. Without an establiched standard on ambient temperature and door openings, it's impossible to compare the efficiency of one unit to another.

Efficiency is most important when running on DC power (usually 12 volts) so the most efficient units are the ones designed for marine use with 12 volt DC compressors and efficient insulation. Inverters are at best, 90% efficient so any unit that depends on an inverter will be less efficient than one with DC powered components.

The most efficient marine refrigerators use shore or alternator power when available to freeze a cold plate and use the cold plate to absorb heat when on battery power.
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Old 12-05-2012, 07:57 AM   #27
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"If you have the space for a regular household fridge that fantastic. Most boats seem to have good house battery banks and inverter setups so it should be no problem."

Most new house fridges will have a tag on them that gives the KW used in an ideal 365 day year.

Most are not accurate as the insulation heating strips are not factored in by the testing regime.

However it IS a start. Divide the annual KW by 365 , then by 10 (for a 12v system) for inverter feed AH per day.

After most do the math it will be about 100AH of 12V DC , in modest sized boxes, per day.

An inverter just adds 3% to 15% amp load to the total required, that's why DC is preferred.

To use 100AH it requires a 200AH battery to not harm the batt for each 24 hours..

For the hook cruiser the alt energy systems are expensive but worth it.

For the Marina > Marina cruisers or 24/7 noisemaker boats,the house stuff is just fine.
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