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Old 10-04-2015, 08:18   #31
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Re: A/C refrigeration

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Originally Posted by accomplice View Post
. . . Holding plate systems take a lot of energy to get them to switch phase in the first place. A 12V system may take a *very* long time freeze the eutectic solution. For example, if fridge and freezer have 10kBTU worth of holding plates, ignoring losses due to insulation, etc., you'll have to put about 3kW*hr of energy in to make that happen. If you're getting 50-100W out of a BD50, that'll be *days* to reach temperature (assuming you ever reach it due to losses).

. . .
Huh! Some seriously wrong with that assertion unless I seriously misunderstand what you are saying.

I run two independent 12VDC systems with holding plates - one at freezer maintaining temperatures and the other at refrigerator maintaining temperatures in my divided 8cu. ft. self built box.

Both cold plates freeze solid in less than an hour and stay there 24/7 unless I turn the system off (like for defrosting). Total amphours used as measured by my "e-meters" (amp hour meters) is about 100 to 150 amp hours per day out of a battery bank of 940 amp hours. The differences having usually to due with the outside ambient temperature where we are cruising - south eastern Caribbean = hot; or northern Bahamas/Florida = temperate/cool.

The used amp hours are easily replaced by the engine alternator or wind generators (if we are in the Trade Wind belt) or shore power (if in a marina).

Maybe your "calculations" are based on theoretical values taken from the spec's of the compressors and other sources - ???? Whatever, as an engineer myself, sometimes "real life" does not always follow our "engineer" calculations which is why "test pilots" make a living.
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Old 10-04-2015, 09:19   #32
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Re: A/C refrigeration

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Originally Posted by accomplice View Post
Mine isn't usually the most popular advice on refrig., but I'll offer it anyways.

Once at temperature, there's going to be some regular heat loss due to finite insulation and occasional door openings. 12V systems will work just to keep up with that.

Holding plate systems take a lot of energy to get them to switch phase in the first place. A 12V system may take a *very* long time freeze the eutectic solution. For example, if fridge and freezer have 10kBTU worth of holding plates, ignoring losses due to insulation, etc., you'll have to put about 3kW*hr of energy in to make that happen. If you're getting 50-100W out of a BD50, that'll be *days* to reach temperature (assuming you ever reach it due to losses).

I find that when cruising we are running the genset or the motor anyways -- whether for propulsion, to make water, or to charge the batteries. Further charging the holding plates (removing heat) is easy, but requires a big enough compressor to do the work in 30 minutes, instead of taking hours.

We find that our 240W solar just barely keeps up with the refrigeration (not freezer) when left unattended for days -- and that is with a moderately powerful (600W?) 24V compressor.
This may sound rude ( not meant to ) if this is what you are experiencing you really need to reevaluate your fridge system something is not working right. Our system goes from ambient to fully frozen and holding in about three hours at 70f ambient temp. It maintained the temps I have posted with about an hours run twice a day it is a 12 volt technautics system.
It has the cubigel multi voltage compressor.
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Old 10-04-2015, 09:23   #33
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Re: A/C refrigeration

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Huh! Some seriously wrong with that assertion unless I seriously misunderstand what you are saying.

I run two independent 12VDC systems with holding plates - one at freezer maintaining temperatures and the other at refrigerator maintaining temperatures in my divided 8cu. ft. self built box.

Both cold plates freeze solid in less than an hour and stay there 24/7 unless I turn the system off (like for defrosting). Total amphours used as measured by my "e-meters" (amp hour meters) is about 100 to 150 amp hours per day out of a battery bank of 940 amp hours. The differences having usually to due with the outside ambient temperature where we are cruising - south eastern Caribbean = hot; or northern Bahamas/Florida = temperate/cool.

The used amp hours are easily replaced by the engine alternator or wind generators (if we are in the Trade Wind belt) or shore power (if in a marina).

Maybe your "calculations" are based on theoretical values taken from the spec's of the compressors and other sources - ???? Whatever, as an engineer myself, sometimes "real life" does not always follow our "engineer" calculations which is why "test pilots" make a living.
My measurements are taken both from real-life observations and datasheets. In my experience, the two correlate fairly well for a system that is running well. BTW, if you're running 2 x12V systems then you're doubling the capacity over a single, obviously.

Your measurement of 100 to 150 amp-hours a day (assuming 12V), represents around 1500W ~= 5000 BTU/day heat loss. This is consistent with my empirical usage as well -- a little more than my freezer alone, but less than my freezer plus fridge. Assuming 2:1 (freezer:fridge) losses, that would represent 1000W*hr on the freezer and 500W*hr on the fridge.

How big (energy storage capacity) are your holding plates?

Many 12V systems are more capable than a BD50, but a BD50(R134a)@-23.3C (my freezer holding plates' eutectic temperature), full speed has only about 60W capacity. That's about 200BTU/hr. My freezer's plates alone are around 5kBTU. No matter how you add it up, it's gonna take a while with a 60W capaicty system to freeze them. If your plates are freezing solid in less than an hour, I would there have to be one or more of the following factors at work:

a. energy storage of your holding plates is much, much less than 5kBTU in the freezer. i think the idea of the holding plate is that you want to size its energy storage to be on the order of the daily heat loss of the box.

b. the compressor on your freezer is not a BD50

c. running a warmer eutectic temp.

d. not using R134a (other refrigerants don't fall-off as steeply in efficiency as the temperature drops)

Or is there some other difference?

My data is also consistent with your points about energy usage varying with ambient temperature. Since we are using a water cooled condenser, I have found that the energy usage varies even greater with water temperature.
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Old 10-04-2015, 09:31   #34
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Re: A/C refrigeration

Take a look at a boat with multiple solar cells, adjustment mechanism to aim them toward the sun, a wind generator, a large arch to support it all, multiple wires running down and thru the boat, controllers etc. Now think of that boat in 40 knots of wind.
Then look at one with just a hi output alternator and engine driven refrigerator. None of that stuff hanging all over the boat that makes it looks like a geo satellite.


Just sayin'.... I can see the logic for some people....it is a viable option. Many marine diesels die an early death from lack of use.....
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Old 10-04-2015, 09:36   #35
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Re: A/C refrigeration

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This may sound rude ( not meant to ) if this is what you are experiencing you really need to reevaluate your fridge system something is not working right. Our system goes from ambient to fully frozen and holding in about three hours at 70f ambient temp. It maintained the temps I have posted with about an hours run twice a day it is a 12 volt technautics system.
It has the cubigel multi voltage compressor.
I was speaking to the use of a BD50 with a holding plate system. I don't use a BD50 now, but rather a (much larger) Masterflux Sierra running R404a that has about a 6kBTU/hr rate.

When installing the current system I looked at using a Danfoss (I preferred to), but based on discussing with many, many people and researching the capacities, I decided the Danfoss was not best for my situation. There is a much bigger BD-350, but I had problems with availability. With the higher capacity I now have, I find I am able to go from ambient to frozen for the plates in an couple of hours.

As for running twice a day, I find that once or twice a day for a combined 45-60 minutes will do it. Since it is a DC system it can run off the batteries, but the solar will barely keep up. When sunny I usually see a max charge of ~8A @ 24V. Assuming 6hrs of sunlight at this peak capacity (probably is fewer hours, at peak, but more hours total), that's only 48A-h @ 24V or 96A-h @12V. This ~100A-h/day is comparable to the requirements of the refrigeration, which is why I say it barely keeps up. Is your refrigeration that much more efficient than 100A-hr@12V/day? If it is cloudy for a few days, each day will see a 100A-h (@12V) drop in the battery level. Our house bank is 800A-h@12V, so it is not difficult for a week with a lot of rain to run down the batteries.

What about this do you think is not working right?
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Old 10-04-2015, 10:05   #36
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Re: A/C refrigeration

Quote:
Originally Posted by accomplice View Post
Mine isn't usually the most popular advice on refrig., but I'll offer it anyways.

Once at temperature, there's going to be some regular heat loss due to finite insulation and occasional door openings. 12V systems will work just to keep up with that.

Holding plate systems take a lot of energy to get them to switch phase in the first place. A 12V system may take a *very* long time freeze the eutectic solution. For example, if fridge and freezer have 10kBTU worth of holding plates, ignoring losses due to insulation, etc., you'll have to put about 3kW*hr of energy in to make that happen. If you're getting 50-100W out of a BD50, that'll be *days* to reach temperature (assuming you ever reach it due to losses).

I find that when cruising we are running the genset or the motor anyways -- whether for propulsion, to make water, or to charge the batteries. Further charging the holding plates (removing heat) is easy, but requires a big enough compressor to do the work in 30 minutes, instead of taking hours.

We find that our 240W solar just barely keeps up with the refrigeration (not freezer) when left unattended for days -- and that is with a moderately powerful (600W?) 24V compressor.
OK what are the sizes of your fridge and your freezer. Also your insulation for the fridge? 10 k of plates is huge just in the space it would take up. (About 1.3 cf including mounting system) 12x24x2 inch plate is what we have in our system and aaccording to the book that's 2500 btu and compressor does 58 kcal and 67 w cooling at 3/4 speed and 4 ah power usage. Also just to be clear on my end we have 200 watts solar on the hard dodger.
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Old 10-04-2015, 10:41   #37
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Re: A/C refrigeration

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OK what are the sizes of your fridge and your freezer. Also your insulation for the fridge? 10 k of plates is huge just in the space it would take up. (About 1.3 cf including mounting system) 12x24x2 inch plate is what we have in our system and aaccording to the book that's 2500 btu and compressor does 58 kcal and 67 w cooling at 3/4 speed and 4 ah power usage. Also just to be clear on my end we have 200 watts solar on the hard dodger.
Is that plate in the fridge or the freezer? That's about the size of the plate in my fridge. In my freezer I have two, slightly larger plates. In total, the plates *easily* take up 1.3cf. I forget the exact measurements on the boxes (they're nowhere near square!), but the total is around 10cf, the freezer slightly larger than the fridge.

2500 btu is 733 Watts. At 67w cooling, that would take about 11 hours to freeze. Full power would be less, but if it takes only 3 hours to freeze that's still off by a factor of 3-4 from what the numbers would seem to indicate. How would you explain the difference?

Was the plate fully frozen? Have you measured, such as with an IR thermometer, the top and bottom of the plate and seen very little gradient?

Also, do you mean to say that your refrigeration compressor only consumes 4amp-hours/day?
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Old 10-04-2015, 11:42   #38
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Re: A/C refrigeration

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Originally Posted by accomplice View Post
Is that plate in the fridge or the freezer? That's about the size of the plate in my fridge. In my freezer I have two, slightly larger plates. In total, the plates *easily* take up 1.3cf. I forget the exact measurements on the boxes (they're nowhere near square!), but the total is around 10cf, the freezer slightly larger than the fridge.

2500 btu is 733 Watts. At 67w cooling, that would take about 11 hours to freeze. Full power would be less, but if it takes only 3 hours to freeze that's still off by a factor of 3-4 from what the numbers would seem to indicate. How would you explain the difference?

Was the plate fully frozen? Have you measured, such as with an IR thermometer, the top and bottom of the plate and seen very little gradient?

Also, do you mean to say that your refrigeration compressor only consumes 4amp-hours/day?
First thing I have a front load fridge with freezer on to just like a house unit the plate is mounted horizontally in the top of the freezer compartment with spill down to the fridge section. The next part is explainable wih as the heat is removed there is less to remove so the curve of heat removal goes up as the temperature goes down. Also no I have not verified with an ir thermometer I'm assuming due to the unit turning off when it reaches the setpoint of the stat. The 4 ah usage is for each hour of operation i m not going to do the math. I just know what the unit is doing and how well it maintained the temperatures I have set it for .
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Old 10-04-2015, 12:29   #39
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Re: A/C refrigeration

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First thing I have a front load fridge with freezer on to just like a house unit the plate is mounted horizontally in the top of the freezer compartment with spill down to the fridge section. The next part is explainable wih as the heat is removed there is less to remove so the curve of heat removal goes up as the temperature goes down. Also no I have not verified with an ir thermometer I'm assuming due to the unit turning off when it reaches the setpoint of the stat. The 4 ah usage is for each hour of operation i m not going to do the math. I just know what the unit is doing and how well it maintained the temperatures I have set it for .
My plates are mounted at a near vertical. When I first turn on the system, as the plates cool, I measure very large temperature differentials from the top to the bottom. With ambient 80 degrees F, on the top I might measure 65degrees, and on the bottom -5 degrees. Condensation on the plates turns to frost at some position, starting near the bottom, and gradually working its way to the top as the solution gradually changes phase. If I only looked at the bottom of the plate, I might think that it had changed phase because of all the frost on it. Are you sure the whole solution has frozen?

When your thermostat stops calling for cooling, it is probably based on some thermocouple (or thermistor) in the box. Is yours monitoring the box temperature, or the plate temperature? I find that parts of the box often gets to temperature long before the plate changes phase.
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Old 10-04-2015, 13:06   #40
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Re: A/C refrigeration

A thermocouple measuring solution temperature not box temp a box temp thermostat reacts to often to ambient box temperature when you open and close the door causing rapid cycling of the compressor perhaps with a horizontal mounted plate you get a more even and consistent cooling of the plate and thereby the solution ( that's an interesting idea perhaps rich would like to run some tests on that theory) I seem to see better results and lower power usage than most ppl using similar systems in more conventional mounting of the plates
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Old 10-04-2015, 13:25   #41
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Re: A/C refrigeration

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I seem to see better results and lower power usage than most ppl using similar systems in more conventional mounting of the plates
Yes. If you're able to keep your box cool wtih 4A for 2 hours a day (= 8 Ahr/day), that is remarkably good performance -- better than most by a factor of 10.
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Old 10-04-2015, 22:44   #42
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Re: A/C refrigeration

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Yes. If you're able to keep your box cool wtih 4A for 2 hours a day (= 8 Ahr/day), that is remarkably good performance -- better than most by a factor of 10.
I actually timed it today and it ran 2.5 hours at 4.2 amps twice a day so 10.5 ah twice a day or 21 ah per 24 hour day and the batteries were full charged by ten thirty. Today I did time the run from 1500 PDT to 1730 PDT and at 1900 or dark the battery was still reading 12.9 after an hour resting disconnected from panels. One consideration that may show for efficiency the water is 57 deg and ambient air was 63 today ( compressor and condenser are mounted in the engine room so its nice and cool there)
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Old 11-04-2015, 09:42   #43
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Re: A/C refrigeration

21Ah, while more than 8Ah, is still excellent performance. My best day is still 2.5x that in energy consumption. You must have a very well insulated box.
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Old 11-04-2015, 09:48   #44
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Re: A/C refrigeration

Boy, not sure how some of you guys are doing it. My best usage for 24 hours with a small box and aerocell insulation was always greater than 70AH's. ... Often 100 -120AH for the whole boat sitting at anchor with other insulated boxes. None very big.
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Old 11-04-2015, 09:56   #45
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Re: A/C refrigeration

I suppose the differences could be the type of fridge unit you are using and the efficiency of the compressor it uses. One thing to mention is the boat is a full time liveaboard
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