Basic Refrigeration Information

     Ice boxes are normally provided on boats, and for a day's cruise, serve the purpose quite well, but for a longer cruise, refrigeration provides a more varied menu and plenty of cold drinks. In an ice box, food can be kept no colder than 32 degrees F. and above the ice level the temperature rises. Perishable foods, such as meats and dairy products, will deteriorate rapidly due to the growth of bacteria and other organisms. By keeping these products at a temperature from 30 to 50 degrees F. they will last much longer. Refrigeration will not kill the bacteria, but it does make them dormant and prolongs the life of perishables.

The most important principle to remember is that refrigeration is a process of removing heat from a matter by taking the heat from one place and transferring it to another. By using refrigerants (liquid gas) piped through the various stages of the system, the heat in one area can be absorbed and carried to another area and released. This is accomplished by the compression, condensation, expansion, and evaporation stages of the refrigerant as it is pumped through the system.

A good boat refrigeration unit should keep frozen foods frozen and other products cold but unfrozen for days or weeks from an available energy source. There are normally two sources of energy on a boat and those are the ship's electrical power or power from the engine. You must match your refrigeration needs and desires with the energy available as well as the box's size and shape. In order to select the correct system for your boat and your needs, you must give careful consideration to the following factors.

REFRIGERATION BOX: The size of the box and amount of insulation must be evaluated by determining the following; the type of insulation, the interior size or cubic footage and the size and location of the lid opening. Boxes of less than three cubic feet are usually good candidates for 12-volt refrigeration, but boxes larger than six cubic feet are normally good candidates for engine drive systems .

Before making a decision, check the insulation. A good refrigerator box normally has three or more inches of polyurethane foam insulation and a good freezer starts out with at least four or more inches of polyurethane foam insulation.

The inside shape of the box also determines how well the refrigeration process will function. A tall rectangular box with a top lid opening usually provides the most even layering of cold. If the evaporator or holding plate can be mounted high in the box natural air tumbling will occur. This will provide adequate cooling at all layers, although, there will still be stratified layers of cold. The bottom of the box will be the coldest and the top of the box the warmest.

When dealing with long flat boxes, the cold radiating from the holding plate or the evaporator tends to cover the area on an angle away from the evaporator or holding plate. This means that the temperature will be much colder up close to the refrigeration source than the far end of the box. The temperature at the far end, high in the box, will be warmer. So some means must be adapted in the design to move the cold into that area. I will feature box design at a later date. So stay tuned!

The size of the lid opening is very important in selecting your equipment, since the evaporator or holding plate that you choose must fit through the lid opening and into the desirable position in the box. I always recommend that before purchasing any system that you construct a cardboard facsimile of the holding plate or evaporator and place it in the box to find out if it will fit through the lid opening, or if it obstructs the access to the box and the food product.

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DEGREE of REFRIGERATION

Before putting together a refrigeration design for a particular boat, I like to know what degree of refrigeration we are talking about. Refrigeration by melting ice provides a temperature from 40-50 degrees F. and any material embedded in the ice will beat 32 degrees F.. If a bag of ice is to be placed in a box with refrigeration the ice will not melt if the temperature in the box, where the ice is stored, is maintained at 32 degrees F. or less. If there is a desire to make ice, the temperature in the box where the ice is to be formed must be maintained at less than 32 degrees F. For example: at 25 degrees F. with one tray it would take 12 to 24 hours for ice to form a solid cube. If increased production is required, it is desirable to have the temperature from +6 to +8 degrees F.. This is the temperature that the new home refrigerator with an automatic ice maker is maintained, and will produce six to eight trays a day. The desire to have solid frozen ice cream on long cruises will decide the degree of refrigeration that will be required. Ice cream will stay in the jelled form below 32 degrees F., but it will not become frozen solid until it reaches 10 degrees F.. Usually the difference in maintaining soft ice cream and hard ice cream is to double the daily energy BTU requirement. This is difficult for many people to understand. In technical terms, when the Delta T is increased, which is increasing the number of degrees of temperature from ambient down to the lower box temperature, the energy requirement is drastically increased.

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THE DAY and WEEKEND SAILOR: The day sailor must decide whether to refrigerate or buy ice. If a boat is left anchored at a mooring unattended during the week, refrigeration would be turned off, and when the owner arrived for his sailing outing, regardless of which system he had, it would have to be run for a considerable length of time to bring the box temperature down. Usually the owner would bring ice to super cool the box. For the day and weekend sailor who keeps his boat at a dock where shore power is available, an electrical system would be best as it would be ready to go when he arrived, and products could be left in the box from week to week.

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FOR THE LIVE ABOARD WHO ANCHORS OUT: There are a couple of options open to the live-aboard. If his box is small enough and well insulated, he could install a 12-volt system. If the daily AMP hour requirements are greater than 50 and less than 140, he could consider enhancing the system with a wind generator. If the boxes are large and the refrigeration requirements high, he would probably go with the engine drive system. This would dictate the need for running the engine on a daily basis. Twelve volt units and wind generators can be used as supplements, but it would be determined by the daily BTU requirement.

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FOR THE LIVE ABOARD at the DOCK:For the live-aboard who only goes sailing one day at a time, a 110-volt system connected to holding plates installed in the ice box will provide more than adequate refrigeration. It is usually best to install small ll0-volt systems of a one-third horsepower or less. This avoids the start up noise in the middle of the night as the smaller units are quieter. If the boat is to be used for cruising periodically, then an engine drive system would be required, or if the box is small with good insulation then a 12-volt unit could be run off the batteries while at sea and off the battery charger at the dock. Again this would depend on the daily BTU requirements.

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FOR THE LIVE ABOARD at FOREIGN DOCKS: Because of the problem of obtaining 60 cycles, 115 volts in foreign ports, I strongly recommend that the world cruiser consider 12-volt or engine drive for foreign travel. The difference between 50 and 60 cycles to the battery charger is usually not a problem, although I would suggest that a check with the manufacturer be made first just to make sure.

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REFRIGERATION for SPECIAL NEEDS: Many beat owners want to put in refrigeration, but some have a special refrigeration requirement. There is the fisherman who wants to freeze his catch, or just to pump cold water through the fish box, and there is the cruiser who requires a lot of cold beverages every day. Other than finding a place to store these products that is insulated and can be refrigerated, the daily Btu requirement must be determined. Again as a daily rule of thumb, using the formula 600 Btu for each cubic foot of refrigerator and 1000 Btu for each cubic foot of freezer will give a fairly good "ball park" figure for sizing the system.