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Old 05-19-2003
Tom Wood Tom Wood is offline
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Refrigeration—Part II Selecting a System

Upgrading a refrigeration box and estimating each boat's individual Btu usage was examined in Part I of this series, "Calculating Refrigeration Needs."


The business end of marine refrigeration: this evaporator plate gets things cold, and also forms a small freezer box.  A frozen Kit-Kat was found in the bottom of this unit aboard a Sabre 38.
After making an accurate guesstimate of your Btu usage, the time has arrived when a large number of refrigeration system choices must be made. There are four major components of refrigeration systems for boats, each having multiple options with key advantages and disadvantages. Makes and models with nearly any grouping of these components are available, meaning that there are more than 20 possible combinations. Let's take a look at the four major components and at putting it all together:

1. The Cooling Plate    This is the part that goes into the refrigerated space and it can be either an evaporator plate or a holding plate.

Evaporator plates are thin with ridges. They are available in a number of sizes as flat plates, formed into L or U shapes, or bent to form small freezer boxes. These latter come in horizontal boxes with doors or as vertical units where the contents are inserted from the top—the vertical is usually best for sailboats. Because of the variety of shapes and sizes, it is usually easy to find an evaporator plate that will suit the box. The weak spot with evaporator plates is that because they are so thin, they hold very little cold, and the compressor must cycle on and off, or run frequently, to keep the box chilled. It is not unusual for an evaporator plate system to run 10 minutes, shut off for 20, then run for 10 minutes again—the temperature in the box rising and falling slightly with the cycles.

Holding plates are much thicker. They are effectively a small tank, usually stainless steel, filled with a eutectic solution. These solutions freeze at much colder temperatures than water and most retain their frozen state longer than water. You must choose between a freezer plate and a refrigerator plate, since the solutions are different. One advantage of holding plates is that two plates can be run off one compressor, thus providing independent freezer and refrigerator spaces run from one compressor. Another is that holding plates can have two sets of internal cooling coils, allowing two different compressors to drive the same plate. The main advantage is that holding plates retain their frozen state for long periods and can often be run only once every 24 hours, holding the temperature in the box reasonably steady for extended periods.

2. The Cooling System    All refrigeration gasses need a condenser that must be cooled either by air or by water.

Air-cooled systems are simple, inexpensive, reliable, and draw very little power. They work well unless they usually run in ambient air temperatures above 90 degrees, just when the refrigerator has to run the hardest to cool the box. If an air-cooled system is mounted in a small space, forced air will need to be ducted to it so that it will cool properly. They should never be mounted in a hot area such as an engine room. But for the average refrigeration installation, air-cooled systems often make a good choice.


The entire system, minus the ice box: compressor, thermostat, and holding plate. The holding plate retains a frozen state for extended periods and is more robust than evaporator plates.
Water-cooled systems use a small pump to circulate salt water through a double coil with the refrigeration gasses. Where the ambient air temperature is normally very warm, when the compressor must be mounted in an engine room or small, airless locker, or for very large systems, water-cooling is the only option. Obviously, water-cooled installations are more involved due to a water intake seacock, strainer, pump, plumbing, and overboard discharge, and they also use more power. Loss of water flow from an obstruction in the intake can make the system inoperable (we once sucked in a jellyfish and had an option between iced drinks or stings—horrible choice).

A third variant is the closed water-cooled system, often in the form of a keel cooler. These are quite popular outside of the US. They operate like the radiator in your car if they are fan-cooled, or like the heat exchanger on a boat if they are submersed in seawater. Some keel-cooler types require drilling several large holes in the boat's hull. They are a nice balance when water cooling is required but the owner doesn't want the complication of open salt water running inside the boat.

3. Compressor Types    Compressors are the pumps that pressurize the refrigeration gasses and can be either hermetically sealed or modular.

Sealed compressors are what you have in your refrigerator at home. The motor and compressor are in one piece, usually with a unique-looking domed top. They are inexpensive because of their large sales volume and are very reliable, often lasting over 20 years. But they are unserviceable—when they go bad, the only option is to replace the whole unit.

Modular compressors are used in many applications such as automobile air conditioning. They are available in a large range of Btu outputs and are capable of driving very large systems. Since the filters, driers, expansion valves, and power source are all separate pieces, these systems are fully repairable. Many will require periodic service, just as the A/C system on your car does.

4. Power Sources    Power for the system can come from 12-volt, 120-volt, belt drive from an engine, from an open flame, or from a combination of sources. The open flame LPG or kerosene types have limited use, are not very popular, and will not be considered here.

Modern, 12-volt DC compressors are fairly energy efficient, but refrigerators, and more especially freezers, still use lots of power. The nice thing about 12-volt energy is that it is easy to replace with solar or wind-generated power. Twelve-volt refrigeration systems can be left for long periods if renewable energy sources are replenishing the power used. Dockside, power can be replaced by a battery charger.

"The nice thing about 12-volt energy is that it is easy to replace with solar or wind-generated power. 12-volt refrigeration systems can be left for long periods if renewable energy sources are replenishing the power used."
120-volt AC systems are usually adaptations of the sealed compressor under your refrigerator at home, although they can be built with a 120-volt motor belt-driving a modular compressor. They can be run from shorepower, a generator, or an inverter if there is a large enough 12-volt system. They can be powerful and are reliable.

Engine-driven compressors are, of course, the modular type. Their real strength is that they are available in sizes that can produce up to 4,000 Btus per hour for use in large systems. They are more expensive, require more maintenance, and the owner must be on board the boat once every day to run the engine.

Combinations of power systems abound. Norcold makes an AC/DC system that switches between voltages automatically. Of course, any 12-volt system can be run off a battery charger and any 120-volt system can be run from an inverter—these units have automatic combination abilities if the electrical systems are correctly sized to allow for such operation. But other combinations are not unusual, especially in larger, modular or partly modular systems. An engine-driven modular compressor in combination with 120-volt sealed unit makes sense for big systems on long distance cruisers.


Refigeration allows you to reach in, grab a cold one, and transcend the world of melting ice water. This unit, mounted aboard a Wauquiez Pretorian, has been running for 15 years.
5. Putting it All Together Certain combinations of all these components make good sense. Let's take a look at some common scenarios:

Expected usage up to 1,500 Btus per day. This is where the standard 12-volt, hermetically sealed system really shines. They are inexpensive, reliable, and easy to install. Choose air cooled if you can mount it in an area of unlimited airflow outside of the engine room, or water cooled if it has to live next to the iron genny or you live aboard in the tropics. The holding plate models and flat-plate evaporators both make good refrigerators—if you want enough freezer space to make a tray of ice and keep two hamburgers frozen, buy a model with an evaporator plate formed into a box. Most of these systems produce 150 to 200 Btus per hour at an average five-amp/hour draw, so they will cycle up to 10 hours per day and use 50 amps of 12-volt power.

Expected usage from 1,500 to 2,500 Btus per day. Still in the range of a 12-volt system, but you'll want to find a larger system capable of removing 250 to 300 Btus per hour and water cool it for best efficiency. At the top end of the Btu range, one of these units would cycle over eight hours per day at an average seven amp/hour draw, using up to 70 amps per day in an efficient box.

Expected usage from 2,500 to 4,000 Btus per day. We're now beyond the abilities of the small, sealed, 12-volt units. A sealed 120-volt system, modular 12-volt, modular engine driven system, or some combination is called for, and water cooling and holding plates are mandatory. A one-half horsepower 12-volt DC motor draws 40 amps when running and can be arranged to belt drive a modular compressor capable of producing 2,500 Btus—this would result in a one-and-one-half hour run time daily at a 60 amp total draw. Most of these systems should be professionally installed.


Once away from the electrical umbilical cord of the dock, it's important to size marine refrigeration and its electrical demands to the boat's typical energy-producing capabilities.
Expected usage over 4,000 Btus per day. Large 120-volt or engine-driven modular compressors, water-cooled condensers, and multiple holding plates will be required. Unless you're willing to spend a few months in refrigeration school, leave the installation and maintenance of these systems to the pros.

A side note to the above: if a large Btu usage stems from a refrigerator/freezer combination, each can be built as a separate box with its own system. In other words, if the refrigerator and freezer each use 2,000 Btus, they could be made as independent units, each with its own compressor and plate. Two identical large, water-cooled, 12-volt systems could then be used with a total amperage draw of between 90 and 100 amps per day if the boxes were well crafted. This system could be run from shorepower with a good-sized battery charger, from an hour or two of engine run time with a high-output alternator, or a large solar array or wind generator.