Selecting a refrigeration system for your boat involves a number of considerations. The size of the box, its insulation, heat load, cooling and/or freezing requirements, ambient air and water temperature, and how often the box is opened are all factors in determining the British Thermal Units (BTUs) you will need. A northern-based weekender for two may be well suited by a very small refrigeration system, while a liveaboard cruiser with four adults in the tropics may require a powerful refrigerator/freezer combination.
Refrigeration systems include many combinations of compressors, power sources, condensing units, cooling systems, evaporators or holding plates, and refrigerant types. Since CFC 12 refrigerant is no longer used in products manufactured for use in the US, the most common replacement has been HFC 134a, also known as R-134a. Newer refrigerants on the market are R-409a and R-404a, and it is important to use only that refrigerant for which the compressor was designed.
COMPRESSORS
Compressors pump the cooling gas, or refrigerant, through the system and are available in DC, AC, and engine-drive configurations. In addition, electrical compressors are built in modular format or hermetically sealed units. Belt-drive systems are available, but are uncommon aboard sailboats.
DC compressors come in standard voltages of 12, 24 or 32 volts.
- The advantage of a DC system is that alternative battery charging, from wind generator or solar panels, may be used at anchor without having to run the auxiliary engine or generator each day.
- With battery banks adequately sized to meet the refrigerator’s electrical needs, the boat can be left unattended for a period of time.
- Refer to the SailNet Pilot electrical section for tips on sizing the battery bank.
- DC compressors normally run evaporator or holdover plates.
AC compressors are available in 110 and 220 volts.
- Are ideal if you’re dockside most of the time and if you have a generator to run at anchor.
Engine-drive compressors are used primarily for holdover plate systems.
- The compressors are large and the engine needs to be run once or twice a day for an hour or so to freeze the cold plates.
- If running the engine twice a day at anchor is not ideal, dual system holdover plates and a DC compressor might be a consideration.
- Dual systems are manufactured with engine-drive and AC combination, or engine-drive and DC power.
CONDENSING UNITS
The condensing unit removes heat from the refrigerant and is either air-cooled, water-cooled, or keel-cooled.
Air-cooled condensing units require a well-ventilated space for installation and begin to lose efficiency as ambient air temperature rises above 80° F. The advantages include:
- No water lines clogged by debris sucked into the water intake line.
- Easier installation with no additional thru-hull fittings.
- Less power draw without a water pump.
- No salt water corrosion inside the condensing unit.
Water-cooled condensing units can be installed in closed spaces and begin to gain efficiency over air-cooled units in ambient temperatures over 100° F.
Keel-cooled condensing units are essentially heat exchangers using a closed liquid system. Copper tubing is run from the condenser to a hollow plate mounted outside the hull below the waterline, using the ambient temperature of the water to dissipate heat. The advantages include:
- No additional thru-hull fittings
- No clogged water lines
- No corrosion inside the condensing unit
- Efficiency is based on the ambient temperature of the seawater
INSIDE THE BOX
Evaporative systems
- Circulate evaporating refrigerant gas, which absorbs heat directly from the refrigerator/freezer box when the compressor is running.
- Evaporative plates have no holding capacity to maintain box temperature.
- Evaporative plates are constructed in many different configurations including flat, L-shaped, U-shaped, and box-like, the latter being able to serve as a small freezer.
- Mountings can be vertical or horizontal.
- The advantage is a more constant temperature, although in order to maintain this temperature, the compressor will run at frequent intervals.
- The limitations are that smaller amounts of BTUs are removed per hour resulting in higher energy consumption than most holdover systems.
- Evaporative plates are best for smaller refrigerators and freezers.
Holdover plate systems
- Are composed of a metal tank, usually stainless, with tubing inside that refrigerant gas is circulated through to freeze a solution in the tank.
- The solution may be either saline, referred to as eutectic, or an antifreeze/water combination.
- The compressor runs long enough to freeze the solution in the plate. The box temperature is then maintained for an extended period of time.
- Eliminate the on-and-off cycling of the compressor that is found in evaporative systems.
- Holdover plates may have two sets of refrigerant coils inside to support the operation of two separate compressors. Thus, a combination AC/engine-drive or DC/engine-drive holdover system is available, and is ideal for larger refrigerators and refrigerator/freezer combinations.
SIZING A REFRIGERATOR
Most manufacturers provide data to assist in choosing the correct size unit for your boat.
- Know the approximate thickness of insulation and "R" value, if possible.
- The refrigerator and/or freezer size stated in cubic feet
- The desired temperature inside the box.
- Refrigerators usually require 35° to 38° F, while freezers use temperatures below 12°F. The colder the box, the longer fresh foods will last, but the more energy the system will consume.
- The ambient air and water temperature in your sailing area, whether tropical or arctic, has a great effect on system size. The warmer the air and water, the more BTUs will need to be removed from the system to cool the box. This is referred to as heat load, defined earlier.
TIPS
- The more effective the insulation in the box, the less compressor cycling or run time, and energy consumption.
- If replacing or updating an existing system, consider re-insulating the box with new and more efficient products on the market.
- The most effective boxes also have well-insulated door seals and gaskets which prevent moisture from entering and frosting the evaporator.
INSTALLATION
A DC evaporative plate system can be installed by the average mechanically and electrically skilled person with minimal tools, assuming the box is well insulated and no major upgrade is required on the electrical system.
Engine-driven and AC units require more mechanical and electrical aptitude and a complete toolbox.
- Additional engine pulleys or brackets may be required for the compressor, which may need to be manufactured.
- These systems require charging with refrigerant after installation, requiring gauges and possibly a license.
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