Nature abhors a temperature differential almost as much as she hates a vacuum. Hot air rises, cold air falls, hot coffee gets cold, and ice cream melts because Nature is hard at work trying to make all things equal in temperature. Keeping a space colder than its surroundings is a never-ending task that consumes large amounts of energy. Think of it as bailing a leaky rowboat, but in this case you're bailing out the heat that keeps leaking into the refrigerated space—instead of bailing water, you're getting rid of unwanted Btus. A Btu (British thermal unit) is the amount of energy required to heat (or cool) one pound of freshwater one degree Fahrenheit. A pound of water is a little more than a pint.
So where does the heat come from and where does the cold go, requiring us to keep bailing? This is part of what makes refrigeration decisions so complex—let's take a look.
Warm things put into the box carry heat in with them. Load a case of soda into the box when it's 75 degrees outside and 35 degrees in the box, and the math looks like this: 24 cans at 12 ounces per can equals 288 ounces, or 18.72 pounds, of liquid with a 40-degree temperature differential—18.72 times 40 degrees equals 749 Btus. The more often you load the box, the more heat you put in.
Compounding the Btu usage equation is the fact that you let heat in when you take things out. With front-opening boxes, the cold air simply falls out every time you open the door, allowing warm air to enter. But even taking the lid off top-opening boxes permits some warm air to enter. If your crew is into the box frequently, expect to increase your need to bail the heat out more often. In some cases, simply adding dividers and organizers to the box so that food and beverage items can be found quickly (especially if the lid doesn't have to be lifted completely) is a quick fix. Crew training is also important—the cook needs to have a mental list of all refrigerated items wanted, open the box once and get the ingredients quickly, opening it again only when all items are ready to be put away.
Box inefficiency is a major source of heat leaks. The best insulation between the ambient air and the refrigerated space is a perfect vacuum since it leaves no molecules to transfer heat energy. This is the principle of a vacuum, or Thermos, bottle. But this isn't practical on a boat's icebox. The next best insulation is trapped air such as the bubbles you find in foam. The higher the quality of this foam, the thicker it is, and the more uniform it is, the better it will be at keeping heat from creeping into the box. Insulation is rated in R-factors—a value of R-20 is OK for refrigerators and a minimum of R-30 for freezers, but special (read expensive) space-age materials with R-factors as high as 75 per inch of thickness are available. Other factors involving box performance include:
* Reaching zero air leaks requires a first-class installation of the foam and tightly fitted lids with good gaskets. Many sailboat iceboxes were built with a water drain in the bottom. Since cold air falls, guess where it all goes—down the drain, carrying ice-cold water that still has a lot of Btus left in it. Plug that darn thing and put in a drip pan to catch the water if you're using ice.
* The bigger the box, the worse the leaks. A big box presents more surface area of foam to leak heat, so choosing the smallest box to suit your needs will result in the least amount of Btu bailing possible. Making a large box smaller by adding more insulation to the inside has two benefits —it makes the box smaller and increases the insulation thickness. Even though it's a nasty job, adding insulation inside the box makes such a difference in energy consumption that it is worth doing.
* Temperature differential between the outside and inside the box is a major consideration. A box used to chill milk in Maine in May will not have much leakage. Freezing ice cream in South America in August is a different matter. The curve of energy required to maintain temperature differentials is not linear—it takes nearly four times more energy to maintain an 80-degree temperature differential than one of 40 degrees, which is why most freezers are small.
Actually, ice makes a good test of the efficiency of a sailboat's box since it melts at a known rate of 144 Btus per pound. If you place a 10-pound block of ice on the rack (hopefully at the top of the box) and come back at the same time the next day to discover four pounds are left, the box absorbed 864 Btus in a 24-hour period (six times 144 = 864). But note this is just for the box, and does not include any additions for loading, opening, or a higher temperature differential.
Here are the hard, cold facts that you can use to begin making decisions on what kind of refrigeration system will suit your boat and style of usage:
Sizing a refrigeration system properly begins with considering the size and insulation of the box, the number of crew on board (along with their eating and drinking habits), and the temperature of the anticipated cruising area. Start by measuring the inside of your icebox and calculating the interior volume in cubic feet as precisely as possible—it's easiest to calculate in cubic inches and divide by 1,728, especially if the box has an irregular shape. You can sometimes, but not always, figure out the insulation thickness by subtracting the inside measurements from the outside dimensions. Use the table above as a beginning, adding more Btus for less insulation, indifferent construction, old or less efficient foam, high ambient temperatures, and a larger crew or one that uses the box often. If you have a freezer compartment, treat it as a separate box, adding the refrigeration component to it later.
As an example, we have a couple planning on cruising from San Francisco to Mexico on their old Neversink 32. It has a 10-cubic-foot icebox with a two-cubic-foot spillover freezer. They have stoppered the ice water drain in the bottom and improved the lid seals, but the insulation is of unknown thickness and is as old as the boat (foam can absorb moisture over the years and often becomes less efficient with age). Our tables above show a 2,600 Btu loss for the refrigerator side and another 2,200 for the freezer, bringing the total Btus to 4,800 for 24 hours.
To this, our couple wisely adds 15 percent for going to the heat of the desert, another 10 percent because both of them enjoy cold sodas and beer which involves loading and opening the box often, and another 15 percent for the old, unknown insulation. By adding these three "fudge factors", their total requirement leaps up to almost 7,000 Btus for a 24-hour day.
They are considering buying a small, air-cooled, 12-volt refrigeration system that has been so reliable on a weekender belonging to a friend of theirs. What they haven't seen yet is that the unit only has a capacity of 200 Btus per hour while consuming five amp/hours of 12-volt power. Running 24 hours per day, this little refrigerator is going to swallow a whopping 120 amps of power a day, requiring them to run the engine for two or three hours. And even at a 100 percent cycle time of constant running, it will only bail 4,800 Btus each day. This will not keep the freezer as cold as they would like, and will wear the mechanical parts out in short order. The poor system will be a general source of frustration to its owners through no fault of its own.
In Part Two of this series we'll look at the types of refrigeration systems on the market and provide some guidelines on choosing the right compressors, condensers, cooling systems, and power sources to suit your needs. Hopefully, you can avoid buying too much, or too little, heat-bailing capacity.
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