Marine Refrigeration comes in two basic types. The first are self-contained marine refrigeration units, which drop into a spot just like the fridge at home. They work better on larger powerboats that have an abundance of AC power and space.
The second type of refrigerators found on boats, are built in modular fridge systems that use available space in or near the galley. These modular systems use the same components as a self-contained unit but can be mounted within 12 feet of the refrigeration box. This article will cover built in units
First decide what you need, is it to replace ice, so you will need a fridge temp between 34 and 39 degrees F, or do you need partial freeze or full time freeze? Freezer temperatures are from 10-20 F.
Marine Refrigeration parts
The major parts of a DC refrigeration system include the;
- Plates inside the refrigeration box.
- Condenser and Cooling system for condenser
Plates in the fridge space have either expansion valves or capillary tube that separate the low and high pressure sides of the refrigeration system.
The evaporator plate takes the refrigerant from the condenser and here it boils rapidly & evaporates back to a gas, at a very low temperature. This change of state absorbs vast amounts of sensible heat from the evaporator which in turn removes heat from the insulated refrigeration box, thereby lowering its temperature. The BTU is the measurement of heat removed. From the evaporator plate the refrigerant is returned to the low side of the compressor, to start the process again.
The compressor is part of a closed loop pumping refrigerant through the system and through the evaporator plate in the ice box. The compressor has two sides the High side or discharge side. The discharge side pumps refrigerant under pressure to the condenser. The suction side or low side and sucks refrigerant after it passes thru the evaporator plate back to the compressor.
The refrigerant in the compressor starts as a gas. The compressor compresses the refrigerant gas, from low pressure to high pressure between 100-150 psi. When the pressure is increased like this its temperature is raised dramatically. Pressure is proportional to temperature.
hot high pressure refrigerant is then fed to a condenser, where it is cooled and turned into a liquid. The condenser is cooled by either air or water. The refrigerant is now a cool high pressure liquid and is fed to an evaporator plate inside the boats refrigerator box.
Refrigerants. R134 is the most common marine refrigerant and will boil at a very low temperature. And in this process absorbs large amounts of heat. R134 will evaporate when exposed to air and so must be in a sealed container. Many systems made for the DIY enthusiast, come pre charged with R134 and have connectors which will easily connect to each part of the system without letting in air. 404 is a commercial refrigerant and is used in SeaFrosts BDxpx system
Terms; Later in the article you will see BTU in defining the refrigeration box needs to keep contents cool. BTU is defined as the amount of heat required to raise or lower 1 pound of water 1 degree F. BTU is a British thermal unit and this is how a refrigeration system defines its power to absorb heat.
Evaporator plates are constant cycling and need constant power.
Flat Plate or bin evaporators are like the fridge in your home. They are also known as constant cycling DC refrigeration. This is due to the compressor cycling on and off several times an hour to maintain the desired temperature in the box. Therefore they need a constant supply of power due to the cycling.
Evaporators can get quite cold (thermostat setting) and many evaporators have the ability to make ice next to the evaporator plate or inside the evaporator box. Evaporators come in several shapes and sizes, they can be horizontal plates vertical plates and rolled plates. Image bin type evaporator from Dometic. More on evaporator plates
- Constant cycling, Short cycle should cycle between 5 and 30 minutes per hour.
- Most use a Danfoss marine compressor with H134 refrigerant
- Works on total skin area.
- Should have small layer of frost covering the evaporator
- Thermostat controls evaporator temperature.
- Turn to lower ice box temperature.
- Less expensive
- Easy to install
- little effect on volume of the box Size
- Bendable Freezes colder
- Need Constant power supply
- Cooler temperatures require more power limits most units are good to 15 cu ft max
Size limits for evaporators restrict output to between 7,500 and 10,000 BTU for a boat refrigerator and 3,500 to 5,000 for a boats freezer. You can add multiple units however the energy drain on your boats battery bank & DC system will be large.
Holding plates or holdover plates only require periodic charging
Marine Refrigerator Holding Plates act like large blocks of ice and the cold temperature of the holding plate sucks heat out of the boats refrigeration box. Holding plates can keep ice boxes cold for long periods. Holding plates can be made for fridge and freezer. This holding plate is from Rparts.
The main advantage of a holding plate over an evaporator is that it only needs to be recharged (temperature drawn down) 1 or 2 times per day. This charging can coincide with attaching to shore power, running the engine and so the refrigerator does not rely on the battery bank. When incorporated into a properly designed system, holdover plates can significantly reduce average energy consumption. DC holding plates are also possible.
The holding plate is filled with a solution that has a freezing point below 23 degrees F. As the compressor runs, the refrigerant passes through the holding Plates coil, freezing the holding plate solution. The compressor turns off, and as the holding plate thaws out, heat is removed from the box.
Holding plates take time and energy to bring down to temperature from ambient temperature. An engine driven compressor will speed this up. Once drawn down to temperature the boats holding plate is energy efficient, and does not depend on the battery bank.
Generally the refrigeration space will be 10-15 degrees F above the holding plate. That’s why the factory sets the holding plate solution at different temperatures for the freezer and refrigerator.
- Thermal batteries (block of ice)
- Twin coil holding plates available
- Fixed operating temperature, set by manufacturer 24 F for refrigerator 0 F for freezer source
- Long duty cycle
- Battery bank not effected
- large holding plate takes up usable refrigerator space
Marine Holding plates more common for 110 and engine power, however Tecnautics, SeaFrost and isotherm make DC holding plate marine refrigeration systems.
Hybrid marine refrigeration uses a combination of say engine and 12 volt require two coils in a holding plate to save needing two holding plates. You can get twin coil holding plates. One 12 volt compressor runs one refrigerant circuit and an engine driven compressor the other
Powering the Compressor
The power supply to the compressor is one of the key elements of the boats refrigeration system. Refrigeration is one of the largest energy consumers onboard, so the power supply is an important element of the system.
Power supply to a marine refrigeration system include, AC, DC, Shore Power, Engine power, and hybrid systems. Hybrids are combinations of say 12 volt and engine drive, or engine and shore power.
The whole point in looking at power supply to your boats refrigerator is to couple it into onboard power requirements for all your boats marine systems. If you run a generator much of the time then adding on an AC refrigeration unit may make sense, but unless you do, you would be better at looking at 12 V, engine or shore power.
Power can be decided on how you use your boat. Are you tied up at a dock for much of the time and take days trips. Or do you cruise and spend large amounts of time at anchor. Finally are you Powerboating or Sailing will also influence power supply. Sailing means no charging of batteries or power from the engine. Here a solar panel or wind or towed generator can help replenish batteries.
If you spend time at the dock, a DC system has plenty of time to recharge on shore power. If you spend time motoring and at the dock and engine drive with shore assist works well.
AC marine refrigeration
These drop in refrigerators are like the one in your home and are commonly seen on larges boats with an abundance of AC power and space. The AC powers the marine compressor, and the condenser is typically air cooled.
A reliable AC supply is needed in the form of a generator.
This unit from Dometic is also available as a DC unit as are many others.
DC marine refrigeration with Evaporator plate
For most boats with a small box, a single 12V compressor, air cooled condenser, with evaporator type plate will be about the cheapest option.
The DC system combined with an evaporator plate that is thermostatically controlled gives flexibility over cooling requirements.
Many 12 volt systems use the Danfoss compressor. With the increased efficiency of the Danfoss compressor, DC refrigeration onboard is getting more efficient, but is still power hungry
The Adler Barbour Cold Machine (left) has been around for around 25 years and provides great refrigeration for small to medium size ice boxes.
Pros Air cooled systems are relatively cheap and easy to install
Cons Power hungry 150 amps/day so needs a large DC battery bank limiting in size, Air cooled works up to 6 cu ft of fridge space.
DC marine refrigeration with Holding plate
Some manufacturers offer holding plates with DC power. These include SeaFrost, Isotherm and Technautics.
Check with the manufacturer how often the holding plate needs to be recharged. If the holding plate can hold temperature for 10-12 hours then you can recharge it at the dock of you have shore power.
The image shows the Isotherm holding plate DC system
Charging your DC refrigeration system
Any DC refrigeration unit will eat up amps from your battery bank, so you will need a way of replacing those amps.
Engine/Alternator battery charging
The DC refrigeration system will drain your boats batteries, so check battery capacity and charging capacity. Many manufacturers recommend 500 amp hrs battery capacity for a refrigeration system in the tropics. Plus you will need charging ability, so make sure you have a large alternator say 100 amp. A large alternator to charge the battery bank in short order if you only use the engine in limited periods.
Solar and wind Battery charging
If you spend large amounts of time sailing, consider solar power, wind generators or water generators to charge your DC system. This will keep your batteries charged while not at the dock and will save you needing to run the engine.
The same applies for at anchor. While at anchor you don’t want to have to run the engine. Some manufacturers like SeaFrost have systems for alternate energy.
Solar Stik has 2 50 watt panels and can put out up to 89 amp hours per day. This will not supply a large DC refrigeration system but will cut down the amps consumed.
Shore power Battery charging
If you live much if the time at a dock and have shore power, then maintaining battery capacity may not be an issue. With shore power your battery bank should be kept at full capacity even with the 12 volt refrigeration on, and you will only use up amps without replacement while away from the dock. The question becomes do you have enough battery capacity to keep the refrigeration unit powered while away form shore power.
Shore powered marine refrigeration
Shore powered systems are made to maintain the boat’s ice box at set temperature when the boat is at the dock. They offer less power than direct from an engine drive but since you will be at the dock for a while that is not an issue.
If you use a holding plate and shore powered system you can keep the plate cool while away from the dock for 12 hours or so.
The idea behind an engine driven compressor system is that the engine gets used anyway for at least one or two tomes per trip. If you are Powerboating this makes sense. If you are sailing, calculate how much time you use the engine, to see if you can draw down the refrigerator temperature in this time.
If this is is the case an engine drive with a holding plate can draw down the ice box in a short period and after that it can be left for 12 plus hours. If you have engine drive with shore assist you can maintain temperatures at the Marina.
The compressor is run directly off the engine. Belt driven or direct compressor. The picture is of SeaFrosts engine driven holding plate refrigeration. The image shows all the parts except the sea water intake for cooling. There are two plates and you can add more, plus ad a separate freezer unit.
- This creates much power and fast cooling of the holding plate.
- More power than a 110 volt system
- Larger system and multiple plates possible
- Engine driven systems cost more and also involve a labor Higher cost
- Engine driven systems are expensive
- If you are at anchor you may not want to run the engine.
Hybrid Marine refrigeration packages
Hybrid systems combine 2 systems together so you can add power options. You can get holding plates with twin coils for hybrid systems.
Common hybrids may combine shore power and engine. This way of your a power boater you can power the refrigeration system at the dock and under power.
Cooling the Condenser. The marine refrigeration systems condenser needs cooling. This is how the refrigerant gets cooled and turned into a liquid.
3 ways to cool the condenser;
- Air Cooled
- Water Cooled
- Keel cooled
1 Air Cooled
Air cooling simplifies installation plus it does not rely on water or adding thru hulls. It is therefore the cheapest installation. For smaller units air cooling is OK, say 4 cu ft or under 6 cu ft you will get adequate performance. The air cooling unit needs a sufficient supply of re-circulated air for it to work. Ducting and space around the unit will help this.
Cons cooling is limited. When you cool by air flow you remove heat from the condenser and ad it to the ambient temperature. The temperature inside of the cabin only has enough capacity to disperse this heat. On the other hand water cooling has a infinite supply of sea water to disperse the heat from the condenser.
Note if your refrigeration system is larger than 6 cu ft capacity, you will get better performance from water cooling.
2 Water cooled compressors
Water cooled compressors will work better in higher ambient temperatures and are more efficient and can be 30% more efficient. Water cooling may be best and is better for larger installs especially if freezer is concerned. Water cooling needs a thru hull and a pump to get the water to the condenser.
Cons The most efficient way to cool the condenser Needs water pump and through hull Water cooling may get restricted due to fouling of the thru hull. Zinc needs maintenance
3 keel cooled Condenser
The keel cooler or keel condenser requires no thru hull fitting and will not be subject to clogging. The keel cooler is a 3″ x 7″ bronze plate that mounts on the outside of the hull and it is the condenser heat exchanger. The bronze plates are connected direct to the compressor which is the only moving part in the system. The Keel Cooler is for a box up to a 15 cu ft refrigerator or 5 cu ft freezer. Since all the heat is passed into the water outside the boat it will does not heat up the interior. Since it works without a water pump there is never a pump or strainer to maintain and best of all it is nearly silent in operation.
Frigoboat and Isotherm make keel coolers.
Pros No thru hull eliminates a possible clogged system No through hull required so saving install time
Cons Bronze plate on the outside of the hull producing excess drag. Zinc needs maintenance Inlet and outlet are next to each other, reducing temperature gradient Operates better when the keel cooler has water moving past the plate.
Refrigeration Freezer Combinations
There are a couple of way to combine freezing and refrigeration needs onboard. One is to have separate boxes, another is to have one box and a a divider to separate the temperatures called spill over and then there is the evaporator plate which will freeze next the plate and have warmer temperatures away from the plate.
Refrigerator – Freezer
If space allows have separate refrigeration and freezer boxes, outfitted with separate compressors, evaporators and holding plates. Possibly a holding plate system for the fridge and an evaporator system for the freezer.
Separate compressors also provide redundancy in the system system, so you have a backup and still have cold food storage.
A box that is divided into two separate sections with a partition diving the box into 1/3rd freezer section and 2/3 refrigerator that is cooled by air spilt over from the freezer section. This concept provides both a refrigerator and freezer from one compressor. Factors for a spill over design;
The partition separating the two compartments must be well insulated. Design the freezer section around 1/3 of the total volume. Air ducts in the separation panel should be clear to circulate when the freezer is full.
This spill over kit with fan and thermostat from Adler Barbour controls air flow when temperature demands.
The 12 volt bin style aluminum evaporator will provide small freezer space inside the bin with a ice cube tray and the rest of the refrigerator box will act as the fridge. This is a very common arrangement and can be a very energy efficient solution.