Answers to Frequently Asked Questions
Question: What equipment would be best for my needs? I plan to live aboard in the tropics and my box is smaller than 4 cubic ft, with three inches of good insulation. I also plan to live at anchor away from shore power.
ANSWER: In the tropics with two people on board, I would expect your daily requirement to be 2400-3400 Btu. I would recommend a small 1 2-volt DC refrigeration conversion unit with chamber evaporator. As your daily refrigeration load would be 56 AMP hours, your house batteries should be four times your daily load. Be sure to add AMP hours for the operation of your radio and lights. To reduce engine running time, I would recommend a high output alternator and a smart voltage regulator. Also, a good investment for a live aboard boat that will be at anchor most of the time is a wind generator.
QUESTION: Can I run a 110-volt unit off an inverter: I want to use a ll0-volt refrigeration unit instead of a 12-volt unit. I would want to run the 110-volt unit off an inverter when I am away from the dock. Is this feasible?
ANSWER: 110-volt refrigeration units used on boats are an adaptation from the standard home or commercial refrigeration systems. They normally consist of hermetically sealed compressor units identified as low back pressure units so that low freezer temperatures can be maintained. They can either be air or water-cooled. The big problem with 110-volt units is the amperage required to start the unit. For example: The average 1/2 horsepower will take around 1800 watts of power to start it, and it will run at about 900 watts and produce somewhere around 3000 Btu of cooling per hour at 900 watts on a 90 degree day. If you power this unit through an inverter that was 95% efficient you would have a one hour drain on your 12-Volt batteries of 83 AMP hours. At high amperage, the inverter is much less than 95%. Any time the refrigerator is running the alternator must be running. This system is only recommended for emergencies.
ANSWER: There are some important factors that you must know before determining the size refrigeration unit to install in any icebox conversion. The most important factor to take into consideration is the amount of insulation. Three inches of insulation is recommended for a refrigerator and four or more for a freezer. This insulation must be closed-cell foam with an "R" factor of at least 7 per inch.(see insulation) Also, The size of the box, and shape must be considered. Boxes that are long and low require a different type of evaporator/holding plate than a tall or square box. Other factors would be the number of crew and the climate where the boat will be operated. A boat that is operated in the Northwest Pacific or the Great Lakes or East Coast would require a different size system than a boat that would be operated in the tropics. The number of people on board and any special refrigeration requirements must be taken into consideration. The ability to produce ice cubes requires one level of refrigeration, keeping ice cream solid requires another level, and the desire to quick freeze large quantities of fresh fish another level. So a thorough understanding of Your equipment and needs is required to properly design a refrigeration system.
QUESTION: How big a box can I refrigerate?
Answer: What limits the size box that you can convert into a refrigerator is the difference between the amount of energy required versus what is available. For example: If a box was to be refrigerated with a 12-volt unit, the energy available would be the size of the batteries and the running time needed to recharge them. An engine drive system, which produces the maximum BTU per hour would be limited by the number of minutes or hours that you wanted to run the unit per day. Normally a box or combination of boxes less than 20 cubic feet can be easily converted with holding plates connected to an engine drive system.
ANSWER : The maximum number of holding plates is really determined by the total amount of tubing within the holding plates. I saw a boat that had seven holding plates all connected in series to one engine drive compressor. I believe that once you exceed a hundred foot of tubing in the aggregate plates on a single expansion valve you will run into suction pressure problems.
ANSWER : I have three separate box systems connected to one engine drive compressor, and the system works exceptionally quite common to have two Separate boxes refrigerator and freezer on separate sides of the boat connected to the same compressor. In commercial refrigeration systems, its not uncommon to have from five to seven units connected to the same compressor.
QUESTION: When do multiple boxes need dual controls?
ANSWER: When two boxes are operated at different temperatures such as a refrigerator and freezer box would be, it is desirable to have controls on the refrigerator box to shut off the flow of refrigerant when the evaporator temperature of the refrigerator holding plate has been satisfied. Otherwise, items in the refrigerator section would freeze before the freezer box temperature had been satisfied. To save expense many dual boxes have had refrigerator/freezer conversions without dual controls. They either installed a smaller holding plate in the refrigerator or covered a portion of the refrigerator plate reduce rapid heat transfer, or by adjusting the super heat much lower in the refrigerator box to compensate for the temperature difference.
ANSWER: Yes, if the holding plate has dual coils. This keeps the two systems totally independent of each other.
ANSWER: Yes .
ANSWER: A simple rule of thumb is, if a refrigerator has three inches of polyurethane foam insulation it will take approximately a half gallon of solution in the holding plate for each cubic foot of refrigerator. If the freezer has four inches of good foam, the holding plate will require one gallon of solution for each cubic foot of freezer.
QUESTION: I would like to install a small 12-volt unit with a holding plate in my five cubic foot box. It is well insulated and I've heard that holding plates that are connected to 12-volt systems are very efficient?
ANSWER : Most boat refrigeration systems require storage of energy because the engines are not operated 24 hours a day. Energy can be stored in refrigerator holding plates or it can be stored in batteries. There is a 15% energy efficiency improvement by using the holding plate to store the energy with a 12-volt system, but there are some drawbacks. The energy efficiency is achieved by less cycling time on the 12-volt compressor, but the primary disadvantage is that you give up the evaporator chamber in your ice box where you can keep a small quantity of frozen food or make ice cubes. If you have a small box where the entire temperature is to remain at a constant temperature, the holding plate will do this better than the evaporator, but where you would like to have separate temperature gradients for a combination refrigerator/freezer the evaporator chamber is better. For the weekend sailor, where the boat is connected to shore power all week. The energy could be stored in a holding plate for weekend of sailing. With this arrangement, you would require very little 12-volt power.
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