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Old 19-09-2008, 12:03   #31
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That is correct. All other variables being equal it all boils down to efficiency of the compressor and driving motor which is the definition of COP. The hermetically sealed Danfoss has the higher COP. Hermetically sealed compressors are rated as a single unit. The Blitzer spec sheet does not specify the drive method so that COP of 1 is for the compressor only. Efficiencies lost in the drive method, belt or direct, and the motor must be taken into account.

How much diesel either system requires depends on individual preferences and how you use the engine. It takes a certain amount of fuel just to turn the engine. If it is run just for cooling the refrigerator (either by charging batteries or producing power to freeze a cold plate) diesel consumption will be high per BTU removed. If on the other hand you are also charging the batteries, making water, motor sailing, etc. the amount of diesel attributed to cooling the box will be much lower per BTU.

If you try to freeze the holding plate by battery alone the total efficiency drops even more. That mean old Dr. Peukert keeps reminding us that the faster you draw power from a battery the lower the capacity. That means you have to burn even more diesel to recharge the battery. Assume for a moment that both systems are equally efficient and use 80 Ah a day. The hermetic system draws 4 amps for 20 hours and the 1/2 HP holding plate system draws 35 amps for 2.3 hours. Now say the house bank is a pair of lead acid 8Ds with a 20 hour capacity of 460 Ah. That means it will deliver 23 amps (5% of capacity) for 20 hours. (Peukert's Constant for high quality wet cells averages about 1.2) The hermetic system is drawing at a rate of .89% of capacity while the holding plate system is drawing 7.6%. At a 7% rate that 460 Ah capacity becomes more like 420Ah while at less than 1% it is more than 800 Ah. This is for illustration only as nobody will run a battery stone cold dead and have only the refrigerator drawing power but it does show that capacity goes down as the rate of draw increases which means you have to put more power back into the bank. Here is a spreadsheet so you can play with your own configuration: http://www.smartgauge.co.uk/calcs/peukert.xls

So far we have just talked about the compressor but the evaporator has 3 other advantages over the holding plate. First, with a solar or wind power source it is fairly self maintaining while you are not aboard. Second, the box temperature remains more even throughout the day. Third, the evaporator is slightly more efficient. Ice is a fairly good insulator. As ice builds up around the cold plate refrigerant tubes the compressor has to work harder to freeze the remaining solution. A properly defrosted evaporator does not face that challenge.
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Old 15-03-2009, 08:30   #32
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Quote:
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and the one I'm installing, .
After you finish installing and using it for awhile I would be intrested in knowing what your energy use is. Keep us posted. Thanks
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Old 15-03-2009, 09:25   #33
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I used my 25 year old Danfloss Adler Barbour system for a two year trip through Central America with no problems except that, occasionally, I would freeze everything in the box from its getting too cold when I turned the thermostat up too high (never more than half way). I was told that the system would never work in the tropics and would consume too much power by all the local "experts". Never found that to be true--I used more power in the Tropics, but not a lot. Maybe some of it depends on how well you ventilate the unit. Some folks stick those units in the engine compartment or in a closed space in a locker where they cannot get rid of the hot. My unit is located away from the engine and in the coolest place I could find. I paid some attention to opening the lazarette at night to cool things off. And putting an auxiliary freshwater cooler might be OK, but I wonder if the complexity and extra power to run that would pencil out--seems like it would depend on some variables. Most everyone I met had problems with their complex refrigeration systems. I didn't. Also, I think a lot depends on the insulation of the box, but mine is just the standard foam they put in the old Tartan 37's.

I have not been happy with R Parts. Hard as heck to get anyone to respond to you by email. Finally, told me that controllers for this old Danfloss were not made. Found them easily available elsewhere and at a reasonable price. They used to be the place for parts and very helpful on the phone. Not my experience.
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Old 15-03-2009, 09:44   #34
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If you are interested in finding one of these shaft-driven 12 volt Blitzer compressor combinations try the boating flea markets. The largest used and overstocked boating market in the East is in Dania FL next weekend. There were two complete Blitzer units last year, one that was used only one month. Owner complained of noise and vibration so installer sold him another type unit. The boat refrigeration units that perform well do not show up in flea markets until they reach a very old age.
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Old 17-03-2009, 09:56   #35
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Great Idea

Quote:
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Recirculating your fresh water through the condensor is preferable to sea water (less maintenance), but if you want to eliminate the water pump open up your fresh water tanks and run the liquid line off the compressor through the bottom of the tank. Sort of like an inboard keel cooler.
It would actually be your hot gas discharge line. The liquid line is the one which leaves the condenser and goes to the metering device.

Any idea how much water you need to maintain in order to keep head pressure low and condense refrigerant? How hot does the water get? Sort of like having a constant hot water heater also. Really is a great way of condensing the refrigerant. Does slime or bacteria build up quicker due to increased temperature?
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Old 17-03-2009, 10:06   #36
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It doesn't heat up you fresh water supply? When I was on the hard in trinidad, I tired running my A/c unit using a 5 gal bucket of water to see how long it would run. Overheated in 5 min's. Of course the AC unit is alot bigger than a fridge... but over 24 hours use....?
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Old 17-03-2009, 10:50   #37
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A wood bucket on a paved parking lot would heat up very quickly. A metal pail, standing in a stream, would take much longer to heat up as the heat transferred right through the metal into the larger body of water.

Using your freshwater tank as a cooling tank may or may not work, depending on how much water is in the tank, and how the tank transfers heat out into the cabin or through the hull. Metal tank with good hull contact, good heat sink. Heavy wall small poly tank, not touching the hull, not a good heat sink.
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Old 17-03-2009, 11:11   #38
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What would work is to run the discharge line to the water tank and use it as a desuperheater, then go to a standard air cooled condenser which has a fan. The fan would only be energized upon a high head pressure. This would save on power for the fan, noise, and give you elevated hot water temperatures. There is some loss due to the extra tubing in the water tank but I'd guess not nearly as much as powering the fan.
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Old 17-03-2009, 18:04   #39
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Hi I did the condensor line through the water tank and it's working very well. The tank is only 25 gal. poly, but below the water line and up against the hull, (sea water temp 83) The compressor is a Danfoss 50 which I've sped up from 2000 to 2500 and it is cycling quite cheerfully using a danfoss t/stat c/w adj differential. Water in tank doesn't get hot, but I also have a eng driven compressor sharing the freezer holding plate (seperate evaporator lines) Way more energy efficent than using a pump, seperate consensor, sea water strainer etc. Don't see any reason why it would promote growth in the tank. Cheers George
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Old 17-03-2009, 20:07   #40
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George, in theory critters in the tank will grow faster in a warmer environment. In practice...I think if there's enough critters, and nutrients, for them to grow at all, the water tank needs more maintenance. Or, to be relabelled as the "soup tank".

Soylent green anyone?
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Old 17-03-2009, 20:43   #41
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maybe, I've been cruising since '92 and have used tap/rain and ro water in the tanks. Never had to clean them and have never had the tank water go bad. There's definitely a small amount of sediment at the very bottom, but that doesn't surprise me as I catch rain (and god knows what else frequently) No odd odours and no complaints. Yet. Cheers George
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Old 18-03-2009, 06:40   #42
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I don't know how mainstream this type of install is, but it's one heck of a nice way of cooling your refrigerant. It's perfect for the boat I'm looking at as the water tank (100 gallon) is right next to the refrig. The water tank is in an external locker also so the heat disipating from the water wouldn't heat the cabin - Thanks George.
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Old 18-03-2009, 09:12   #43
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I think its an interesting idea as well...I'll be installing a dedicated fresh water tank for flushing my toilets...seems a good chance to include this.
It won’t be a big deal if something starts to grow ...a little!
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Old 18-03-2009, 09:40   #44
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I guess the flip side of this would be, are the refrigerant lines going to be attacked y whatever you add to the water tank? If the lines are aluminum, they won't like bleach in the tank. If they are copper...they still won't like it much. Long-term, the refrigerant lines in the water tank probably should be stainless steel?
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Old 18-03-2009, 11:14   #45
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I think SS would be OK.

I'm just wondering how far from the compressor would be reasonable to go to the tank and back...I'm looking at 12 feet or so between the two...that 24ft round trip?
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