Marine Refrigeration - A Common Sense Guide
Much has been written on the topic of marine refrigeration.
Rather than getting unnecessarily technical I will try to lay out a few general concepts and practical suggestions that will help you arrive at a successful system design. I have arrived at these conclusions after nearly 15 years of selling marine refrigeration systems.
Every aspect of a boat design involves trade offs - let's try to cover a few of the common trade offs in selecting a refrigeration system.
Custom built -in box vs. Drop-in cabinet refrigerator.
A manufactured self contained cabinet refrigerator is similar in many ways to the domestic refrigerator you have in your home. Most are air cooled with the compressor mounted on the back of the unit. The main differences on the marine unit are the power supply is 12 or 24VDC, and the marine unit has a locking latch on the door and fiddles on the shelves to keep the contents from spilling out.
To keep the cabinet dimensions reasonable the amount of insulation is usually 2 inches or less. With the compressor mounted on the cabinet they are easy to install but require good ventilation. The heat generated by the compressor can leak back into the cabinet and cause the system to be less efficient.
A built-in box can usually take advantage of the available space and can be better insulated. A remote mounted compressor can be air or water cooled and is less likely to feed heat back into the refrigerated compartment.
A self contained cabinet will generally be simple to install and maintain, be less expensive but usually less than ideal efficiency and will require good ventilation
A built in box can be larger and more efficient because it can be better insulated and the compressor can be remotely mounted.
If maximizing the size of the refrigerator and minimizing the power consumption are priorities then a built-in box with a remote compressor is usually the best option.
How much is enough? Is more always better? The answer to these questions like so many questions is "it depends". Insulation works on a diminishing return the first inch gives the most benefit and each additional inch provides less benefit.
Heat gets into the cabinet not only thru the insulation but also thru opening the door or putting warm things in to the refrigerator, less than perfect seals, the door frame, etc. The shape of the box is a huge factor since it is the surface area not simply the volume that is important.
In general the rule of thumb goes like this:
For a refrigerator box:
4 cu ft or less use a minimum of 2 inches.
6 cu ft will need 3 inches
More than 8 cu ft needs 4 inches
For a freezer:
2 cu ft or less 4 inches minimum
4 cu ft or less 5 inches
larger than 4 cu ft 6 inches.
What kind of insulation? We recommend extruded polystyrene - such as Dow Blueboard or Owens Insulpink - it is conservatively rated at R-5 per inch and will not absorb moisture so it will maintain its insulation value. It is easy to work with, easy to obtain at a local builders supply and not expensive. Other foams may have higher initial insulation values but may quickly degrade in the wet marine environment. Vacuum panels can achieve much higher insulation values but they are very expensive, fragile and difficult to handle.
Keep the box small and an efficient shape (square is the ideal).
A front loading box is not a problem as long as the door seals are tight.
If you have a separate freezer be sure to have it well insulated and no larger than necessary - empty space in a freezer will still use power.
Air cooled or water cooled compressor?
Which is more efficient ? Once again the answer is "it depends". An air cooled system can be very efficient but it must be well ventilated. If the air temperature gets above 95F regularly the efficiency will be noticeably reduced. There is no doubt that water is more effective way to cool but don't discount air cooling. For a small box ( less than 6 cu. ft.) that is reasonably well insulated and where the compressor can be well ventilated air can be just as efficient as a water cooled system.
But if the box is larger than 6 cu ft or has a separate freezer section then air may not be the best choice. If the compressor has to go in an engine room or tight enclosed space than water-cooling is usually a better option.
OK, if you are considering a water-cooled system, what type of water-cooled system, pump or SP ( Self Pumping ) keel cooler? The SP - keel cooler has the condensing heat exchanger built into a thru hull fitting - usually it can replace the galley sink drain fitting. Water moving in and out of the fitting as the boat moves even slightly will cool the condenser and make an efficient heat exchange. 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.
If the SP keel cooler does suit your application then a conventional water-cooled system with a water pump is a good alternative. Many customers choose to re-circulate fresh water out of a tank, thru the system and back into the tank. This greatly reduces the need to service the strainer and pump, is safer and works when the boat is hauled out on the hard too! Either way, fresh water or more conventional raw water, the water cooled compressors have great capacity and will be more efficient when ambient temperatures climb into the 90's.
Evaporator vs Holding Plate
The conventional wisdom associates holding plates with old fashioned big cast iron belt or engine driven compressors. The conventional wisdom also associates DC compressors with aluminum evaporators. This passes over what we think is one of the best options around the DC holding plate - hybrid approach.
The main advantages of holding plates are the ability to store thermal energy and manage the power consumption for best efficiency and the least amount of energy drawn from the batteries.
DC holding plates also have freezing temperatures that are fixed by the freezing point of the solution they contain. These temperatures are selected for optimum operation and provide a compact freezing area to make ice and store frozen food with efficient use of battery power.
Holding plates will cycle a few times in a day while a conventional aluminum evaporator will usually cycle several times an hour.
There was a time when it was safe to assume that refrigeration was the largest consumer of electrical power on board. However with today's assortment of large inverters, microwave ovens, hair dryers, washer and dryers and stereo HD plasma televisions, those days are gone.
But if maximum efficiency of the refrigeration system and minimum use of battery power is important, then a DC holding plate system can really help. Using an energy management control such as the ASU - Automatic Speed UP will rapidly freeze the holding plates when ever surplus electrical power is available. This will conserve battery power and mange the power consumption for minimum draw on the batteries.
To summarize the differences between evaporators and holding plates:
Conventional Aluminum Evaporators are:
Easy to install
Will freeze colder
Short duty (on/off) cycles (sometimes referred to as constant cycling).
Holding plates are:
Thermal batteries - better at managing the power consumption.
Stainless steel - rugged and durable.
Long duty cycles- A frozen plate can maintain the box temperature for up to 12 hours
Fixed operating temperature
Putting it all together
Most systems are designed by estimating the volume of the refrigerator or freezer and then using this figure to used to select the correct components.
There are many variables involved so this usually is a rough approximation, but normally this all that is needed to arrive at a successful system design.
Remember that the actual power consumption will depend on the shape of the box, the amount and effectiveness of the insulation, ambient temperatures, and good air tight seals.
There are many formulas that attempt estimate the power consumption but to simply this process I like to use the real world example if a 6 cu ft refrigerator with 4 inches of insulation. We have collected many hours of both shop testing and field data for this type of box.
Ambient temperature of 85 F
Freezer temperatures between 10F - 20F
Refrigerator temperatures 34F - 39F
4 inches of R-5 insulation
Consumption will be 24 to 28 amp hours per day. This can be expressed as an average power consumption of 1.1 amps at 12VDC.
Using an energy management device to cycle the compressor while surplus electrical power is available will reduce the use of battery power dramatically.
Refrigerator - Freezer - Combination - Spill Over
Refrigerator - Freezer
If space allows, we recommend separate refrigeration and freezer boxes, outfitted with separate compressors, evaporators and holding plates. Ideally we suggest a holding plate system for the fridge and an evaporator system for the freezer.
The separate compressors are a redundant system, so if anything does go wrong with one system, you still would have cold food storage. Also if the freezer is not needed, the compressor could be turned off, saving energy.
A separate refrigerator and freezer has many advantages but a single box with one system is much more common.
A holding plate or a bin style aluminum evaporator will provide a small freezer and larger refrigerator compartment. This is a very common arrangement and can be a very energy efficient solution. Normally the freezer compartment is limited to less than a cu ft
A box that is divided into two separate sections with a partition will provide a relatively larger freezer section and a refrigerator that is cooled by air ducted from the freezer section. This type of design is attractive because it provides both a refrigerator and freezer section from one compressor but a spill over design is a bit more complicated and usually requires some fine-tuning to get good performance.
In general for a spill over design:
Keep the freezer section between 1/4 to 1/3 of the total volume.
Freezer temperature performance will range from 10 to 20 deg. F (not a true deep freezer).
Insulate the partition between the two compartments (1.5" to 2" )
Locate air ducts so that they will not be covered when the freezer is full.