How does Marine refrigeration work? The major parts of a DC refrigeration system include the refrigerant, a compressor, and a condenser, a cooling system for condenser, and a plate or plates inside the refrigeration box.
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 through the evaporator plate back to the compressor. The cold 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 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 rises dramatically. This 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.
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.
Evaporator or Holding Plates
Marine refrigeration systems use either an evaporator plate or a holding plate in the boats refrigeration space or freezer space. Each type of plate works differently in drawing heat from the boats refrigerator and ice box space.
Marine Evaporator
Marine evaporator are just like the ones found in household refrigerators. They 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 plate's, vertical plates and rolled plates. Evaporators are constant cycling, or short cycle. Most use a Danfoss marine compressor with H134 refrigerant.
Thermostat controlled evaporator temperature. Turn to lower ice box temperature. Evaporator plates are less expensive, but need Constant power supply.
Marine Holding plates
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.
The main advantage of a holding plate over an evaporator is that they only need to be recharged 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 energy consumption. However 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 of 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 supplies to marine refrigeration systems 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 Power boating 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 110 volt marine refrigeration
These drop in refrigerators are like the one in your home and are commonly seen on larger 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.
DC marine refrigeration with Evaporator plate
One of the cheapest marine refrigeration system and easiest to install is the 12 v or DC system. Air cooling is the simplest. 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.
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 Adler Barbour Cold Machine has been around for around 25 years and provides great refrigeration for small to medium size ice boxes.
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.
Engine powered marine refrigeration
The idea behind an engine driven compressor system is that the engine gets used anyway for at lest an hour or so. If you are Power boating this makes sense, if you are sailing calculate how much time you use the engine.
If this 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.
The compressor is run directly off the engine. Belt driven or direct compressor, There are two plates and you can add more, plus add a separate freezer unit. This creates power and fast cooling of the holding plate. More power than a 110 volt system. Larger systems and multiple plates are possible. Engine driven systems cost more and also involve a labor higher cost
Condenser Cooling
The marine refrigeration systems condenser needs cooling. This is how the refrigerant gets cooled and turned into a liquid. There are 3 ways to cool the condenser;
Air Cooled
Air cooling simplifies installation plus it does not rely on water or adding through 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.
When you cool by air flow you remove heat from the condenser and add it to the ambient temperature. The temperature inside of the cabin only has enough capacity to disperse this heat.
Water cooled Condenser
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 through hull and a pump to get the water to the condenser. The most efficient way to cool the condenser
Keel cooler Condenser
The keel cooler or keel condenser requires no through 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.
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, are a must. 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 comes out easily there is a gap. Get new or better gasket material.
Insulation
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. The Glacier bay Barrier Ultra-R 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.
Water temperature
In the tropics water temperature is a lot warmer than 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.
Plate Thermostat
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.
How big a Refrigeration System?
To calculate how big a refrigeration or marine freezing unit required, 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; 600 BTU per cu. ft.
Freezer daily heat load; 1200 BTU per cu. ft
Lets look at how this works for the 4 cu ft refrigeration system, using the above formula
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, and then calculate amps needed to power the system, and 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 Danfoss DB35.
Amps
To calculate how many amps the system will draw we start by converting BTUs to amps with this formula, using an assumption of 5 BTUs per watt hr of energy used. BTU/5/volts. so say we have 4400 BTUS and 12 volts, 4,400/5/12=74 amp hours/day.
Battery Bank
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.
Conclusion
These days being on the water means keeping food and drinks cold. 12 volt refrigeration units are becoming more popular with technological advances. Greater compressor efficiency and evaporator technology brings 12 volt cooling to the smallest of boats.
