The liquid solution that fills a cold plate absorbs energy from the evaporator coils while the compressor is operating, dropping its temperature to, ideally, a clearly defined and calculated value. The value of this solution is that, once frozen, it acts like a block of ice, slowly absorbing heat from the box between the times the compressor is operating. Depending on the climate outside the box, the number of people living aboard the boat and the construction of the box, this may be from one to three times a day, or more if any of the aforementioned factors are extreme (100ï¿½ cabin temp, poor insulation, etc).
Like many things in life, however, there is a choice where this all too critical juice is concerned. A true eutectic solution will freeze and melt at specific temperatures in order for the plate, and the box it is installed in, to maintain a constant temperature. (Water is a true eutectic; it will maintain the temperature of its surroundings until it has melted. Unfortunately, its freezing point of 32ï¿½ F is too high for holding-plate use.) This consistency requirement is particularly important for refrigerators, where the acceptable temperature range is small (refrigerated food becomes freezer burned below 35ï¿½ F, and food spoils above 38ï¿½ to 40ï¿½ F). Thus, a true eutectic solution should freeze at approximately 26ï¿½ F for a refrigerator holding plate and between 0ï¿½ and 5ï¿½ F for a freezer. Keep in mind that the box temperature will be roughly 10ï¿½ to 20ï¿½ higher than the holding plate temperature.
There are two schools of thought concerning eutectic solutions. Some holding plates are filled with a true eutectic solution (calcium chloride, sodium chloride, magnesium chloride or other more exotic mixtures), while others use a glycol mixture, essentially antifreeze. The advantage of using a true eutectic is its ability to freeze, completely and efficiently, and then maintain a designated temperature until itï¿½s nearly fully thawed, much like ice except colder. The result of this attribute is a relatively constant box temperature until the eutectic solution within the holding plate is thawed.
Systems using glycol are prone to greater fluctuation in holding-plate temperatures because this solution tends to stratify, which changes its concentration in different regions of the plate, as the freezing process occurs. The result is the plate temperature continues to rise as the solution thaws, making for less than stable box temperatures and increased compressor operation (the temp within the plate continues to plummet as a noneutectic is cooled, which means the average compressor operating temperature is lower and thus less efficient). Glycol-based solutions do have some advantages. They are easy to obtain and refill, should the system develop a leak or require service, and they are anticorrosive by nature.
The true eutectics mentioned above are all more corrosive than antifreeze (some of the exotics claim low corrosiveness; however, I have no field experience with these mixtures) and thus, a plate filled with one of these solutions must be specially designed and constructed to withstand the effects of this liquid. (Dissimilar metals like copper, aluminum and stainless steel may not be used together. Typically, plates utilizing true eutectics are either all stainless steel or all galvanized steel.) Some manufacturers include anticorrosive additives in their eutectics as well as placing a vacuum on the box before it is sealed in order to inhibit corrosion activity. Plates constructed in this manner must be extremely rugged and are accordingly more expensive.
If you are searching for the ultimate in temperature stability with maximum efficiency (read shortest engine run times for either engine-driven compressors or battery recharging for DC holdover systems), and expense is no object, then plates utilizing true eutectics are a must. If, on the other hand, some temperature fluctuation can be tolerated, and economy takes precedence over performance, then glycol solutions usually provide adequate results.
An excellent heat-load and insulation calculator, as well as other useful information, is available on Glacier Bayï¿½s website in the Library section, http://www.glacierbay.com“>http://www.glacierbay.com.
Steve C. Dï¿½Antonio