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Jack, it is unusual to find anyone who has lived aboard with a water cooled Danfoss BD condensing unit that likes it. You are right there are no simple black and white answers about selection ice box conversion refrigeration units.
The days of selling products on the merits of their actual performance is over the real pioneers in boat refrigeration manufacturing and service are gone and it is now a catalog sales business. I guess I see the darkest side, those boaters that are dissatisfied with the unit or the people they bought it from. I have tried to cover in detail the small 12 volt refrigeration story in my 12/24 Volt Refrigeration Manual, the book also evaluates the manufactures. On the subject of selecting equipment the slide show on my web site is as good of an explanation as you will find on the subject and its free.
European manufacturers of these small 12 volt systems concentrate on the volume sales markets, small boats in non tropical waters. If a box to be converted to refrigeration is over four cubic feet and used in tropical waters Isotherm and WAECO do not have a suitable unit for a true refrigerator conversion. Seawater temperature plays an important role in selecting the type of refrigeration because it effects the boats interior temperature and the refrigerators performance. For more on this see FAQ 33 on my web site.
When a small 12 volt refrigeration unit is selected for a specific boat an application engineer or in most cases the boat owner should review all the available unit specifications. The important items effecting the equipment selection are:
Box Size and Shape
Even with today’s high output alternators and smart voltage regulators there is a limit to how large a box that can be refrigerated with 12 volt power. When the daily total boat 12 volt amp hours exceed 100 its time to consider some type of alternative energy source or another type of refrigeration unless the engine alternator is to be run several hours a day.
The box’s size, amount of insulation and the owners intended use will provide enough information to determine daily required capacity. Manufacturers are not always helpful in providing system capacity information here is what one sales brochure says. ( BD50 compressor will handle a 8.1 cu. ft. box consuming One amp per hour average. In small print; With ambient water temperature of 72 degree F.) Bad info.
There are those that try to convince you that it is possible to use figures from a mechanical engineering book to determine the Btu heat load of a pleasure boat ice box, bad advise these boxes are not walk in coolers.
The shape of a box and lid opening in most cases will determine the type and size of the evaporator. The evaporator should be mounted high in the box. A freezer evaporator should surround the food product as much as possible. A small box or one with a tapered side is difficult to convert to a true refrigerator which means it also has a freezing section.
On the small boxes where the lid is large enough a chamber evaporator (bin) can convert the box into a true refrigerator with a freezing section.
Condensing Unit Location
The air cooled condensing unit must be located in a spot where the air that passes through it is not allowed to pass through it again.
Condensing units that are water cooled do not have a fan to assist in compressor cooling. This means that the only source of compressor cooling is the returning refrigerant that may be insufficient if the compressor is in a confined space.
Required Compressor Capacity
Select a compressor and its most efficient speed that will provide less than a 50% duty cycle.
Required Evaporator Capacity
It there are times when a compressor is to be operated at max capacity the evaporator or holding plate must be sized to except that capacity. Example, a BD50 compressor at 3500 rpm needs a evaporator/holding plate with 500 square inches of surface skin area to be efficient.
A large holding plate may be selected as a means of storing addition energy from alternative power sources.
Gas to Liquid Condenser Capacity
It is the job of the condenser to see that liquid is delivered to the expansion device near the evaporator in the correct pressure range. If the air or water used as a cooling medium across the condenser coil is too cold or too warm, system efficiency is lost. A properly installed Danfoss compressor air cooled system can be left unattended for months this is not possible with a water cooled system.
There is a lot of boat refrigeration information on my web site including a refrieration forum at http://www.kollmann-marine.com