There are 2 types of Marine refrigeration for your boat. 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.
How Marine Refrigerators work?
The major parts of a DC refrigeration system include the;
- Cooling system for condenser
- Plates inside the refrigeration box.
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
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.
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 thermostaticaly 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.
Air cooled systems are relatively cheap and easy to install
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
How to charge 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;
SeaFrost air cooled marine refrigeration
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.
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.
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
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.
No thru hull eliminates a possible clogged system
No through hull required so saving install time
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.
Sizing a Refrigeration System
How big a Refrigeration Compressor & plate Combination
Before we pick a size of marine refrigerator we need to understand what factors are involved in keeping the refrigeration box cool. They are mainly box size, insulation and cooling water temperature, number of people aboard and the temperature you are setting the plates for.
Refrigerator Box Volume
This is obvious, the larger the box the more heat removal is necessary. A larger box will need more BTUs of heat removal.
After this basic size issue we have things like, Front opening or top opening. Front opening lets cold air out quickly but does allow you to get to the bottom of the box.
Drains, if you had a drain for you ice box plug it. You will not need to drain water out of the ice box and this will only let cold air out and heat in.
Gaskets, these are a must and must be properly sealed. A trick to identify if there are gaps in the gasket is to put a piece of paper in between the lid or door and the cabinet and close it. Pull on the paper and you should feel some drag if the gaskets are sealing properly. It it comes out easily there is a gap. Get new or better gasket material.
Typical insulation to a fridge or freezer is foam insulation like Dow Blue board. The recommendation is for 3-4 inches for refrigeration and 4-6 inches for freezer for medium sized boxes. Foam has an R value of 5 per inch thickness, R being a thermal unit. This means in terms of thermal units 3-4 inches represents 15-20R value for the refrigeration unit, and 20-30 for the freezer.
There are manufacturers of vacuum panel thermal insulation. Both Glacier Bay and technautics make these panels. The Glacier bay Barrier Ultra-Râ„¢ super-insulation at R-50 per inch provides lots of insulation without taking up valuable volume. These panels are vacuum panels and are sealed to work. It is very important that you do not drill through or puncture these panels. These panels are custom made, so you would need to provide the manufacturer, exact sizes with locations for copper plate tubes to enter the box. These are built into the panels.
In the tropics water temperature is a lot warmer then northern climates. For every degree water temperature increases a corresponding 2 % increase in required BTU. If you are in the Atlantic portions of the east coast US, you have some cool sea water temps, but of you then cruise down to the Caribbean you may strain your refrigeration system.
The evaporator plate temperature is set by the thermostat. Dial the box temperature down and the system will have to work harder.
Number of people aboard
More people means the box gets opened more and the heat build up from more people adds to the ambient temperature.
Sizing a Refrigeration System
How big a Refrigeration System is required
To calculate how big a refrigeration or marine freezing unit needed, you will need to start with an estimation of the BTU requirements of the box. A simple rule of thumb for estimating the BTUs is based on the box volume.
These BTU estimations are based on these assumptions
Insulation has an R value of 30 no leaks.
Water temperatures are tropical in the mid 80s F.
2 people aboard, For each extra person add an additional 1,000 daily BTU
Top opening box, for a front opening door add 15 BTU/inch of door
BTU estimate on refrigeration volume;
Refrigerator daily heat load box temperature between 34 - 39 F 600 Btu per cu. ft.
Freezer daily heat load box temperature between 10 - 20F 1200 Btu per cu. ft
Lets look at how this works for the 4 cu ft refrigeration system, using the above
4 cu. ft. times 600 = 2,400 Btu.
Two additional people on board = 2,000 Btu.
Total required per day = 4,400 Btu
Choosing 12 volt unit with Evaporator
Match this number to the compressor capabilities, then calculate amps needed to power the system, then work on the battery bank capability
Start by using the 4,400 BTU form the above example
The Adler Barbour Cold Machine uses the Danfoss BD50 Compressor is rated at 650BTU / hr based on 25F evaporator temp.
This is well above the 4,400/day we need for the 4 cu ft fridge, using only about 1/3 of the power. We could easily go down the the Danfoss DB35.
BTUs can be converted to amps with this formula, Using an assumption of 5 BTUs per watt hr of energy used. Reference Refrigeration for Pleasureboats Calder
so say we have 4400 BTUS and 12 volts
4,400/5/12=74 amp hours/day
The Amps needed to power the compressor should be 1/4 of the capacity of the house bank. So for the above 74 amp hours needed multiply by 4 to get recommended house battery capacity = 296 amp hrs
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.