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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
larger boxes can use 12-volt systems if the insulation and/or
the DC power system is upgraded. Boxes larger than six cubic
feet are normally good candidates for engine drive systems .
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here for more information on selecting the right refrigeration
system for your needs.
Before making a
decision, check the insulation. A good refrigerator box normally
has three or more inches of foam insulation and a good freezer
starts out with at least four or more inches of foam insulation.
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here for more information on 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. There is a section on
box design in my book "Do-It-Yourself
Boat Refrigeration"
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.
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