Refrigeration and Planning Guide for Boats, Caravans, Motorhomes and RV's
Refrigeration is generally the single biggest power user and thus getting it right and maximising your fridge’s efficiency will reduce your power consumption.
In this article we will address:
- The different types of refrigeration, DC, AC or engine driven Eutectic and their advantages and disadvantages;
- What to look for in a marine fridge/freezer;
- Planning Guide for refrigeration
- How to build your own refrigerator/freezer box;
- Some tips on how to maximise your refrigerator's efficiency
If you take a walk down most marinas and talk to people about their experiences with marine refrigeration you will have mixed answers. Many people have experiences both good and very often bad. With marine refrigeration there is no simple/cheap way around it, if you want your fridge/freezer to work properly, especially when the ambient air temperature is above 30’C, then a reliable, efficient and properly designed unit needs to be installed.
Insulation is the first key, and the simple rule is the more insulation the more efficiently your fridge/freezer will be. Take this example two eskies of the same volume in an ambient air temp of 25°C. Take a proper icebox such as an Evakool, fill it full of ice now see how long it lasts for quite a few days, now take a cheap esky that you may buy at one of the big discount stores fill it with the same volume of ice and you will probably be lucky if you get one day out of it. Why is this? Well the Evakool has far more insulation, and the insulation itself is a proper closed cell insulation designed with thermal properties while the discount store esky is built to a price and there is far less of a poorer quality insulation.
On boats it is critical that we only use closed cell insulation and the reason for this is that:
- It has far better insulation properties and does not absorb moisture;
- Open cell insulation has less insulating properties and absorbs water which over time causes it to break down and thus the thermal insulating properties are further hampered;
... the Planning Guide below is an excerpt from Isotherm
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 (110L) or less use a minimum of 2 inches.
- 6 cu ft (170L) will need 3 inches
- More than 8 cu ft (225L) needs 4 inches
For a freezer:
- 2 cu ft (56L) or less 4 inches minimum
- 4 cu ft (110L) or less 5 inches
- larger than 4 cu (110L) 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 95°F 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 or 170L) 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 (170L) 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 (F).
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:
- Less expensive
- 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 (170L) refrigerator with 4 inches (100mm) of insulation. We have collected many hours of both shop testing and field data for this type of box.
- Ambient temperature of 85° F (30°C)
- Freezer temperatures between 10°F - 20°F (-12°C to -7°C) NB: not a true deep freezer
- Refrigerator temperatures 34°F - 39°F (1°C to 4°C)
- 4 inches (100mm) 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 (-12°C to -7°C) NB: not a true deep freezer
- Insulate the partition between the two compartments (1.5” to 2” or 38-50mm)
- Locate air ducts so that they will not be covered when the freezer is full.
It is very important that the compressor/condenser unit is well ventilated and that cold air can enter at the bottom, pass behind the fridge and warm air can leave at the top in the area where it is mounted. The natural flow of air from below and upwards behind the fridge can be increased by arranging ventilation openings at the rear. Make sure there is a free area of 15 - 23 sq.in. below and behind the refrigerator to allow ventilation air to pass behind from below (see diagram below).
With boats, there are three common types of power used to run a compressor to cool a fridge freezer box.
12/24 Volt DC Compressors
This is now the most common way of cooling a refrigeration box. Ever since the birth of the humble Danfoss compressor the DC fridge has been the most popular choice in refrigeration. Most marine refrigeration manufacturers use the Danfoss compressor because of their low power consumption, ease of installation as they can can be hooked up to the boat’s DC electrical system, relative small size, reliability and their ability to be easy adapted to various forms of cooling methods such as air cooled or water cooled. The compressors can also be used in front or drawer styled cabinets such as the fantastic units made by Isotherm, Nova Kool and Evakool, or can also be used in Build-In Fridge/Freezer Kits which can be used in converting a built-in ice box to a fridge freezer (please see the section later in this article on how to build your own cold box).
A Danfoss compressor based refrigeration unit will generally consume around 1.5 to 4 amps per hour depending on box size, insulation qualities, ambient air temperature and the amount of food/drink that the unit is loaded with. Companies such as Isotherm have even bettered the power consumption figures down to 0.5 to 1.5 amps per hour with their ingenious ASU and SEC systems.
Isotherm ASU systems are used in installations in existing cool boxes, and in pre-fabricated conventional fridge cabinets, and mainly for use in sailing yachts where battery power is at a premium. The ASU system dramatically reduces power consumption while supplying refrigeration in abundance. It senses when surplus power is available from the engine alternator and speeds up the compressor to rapidly freeze the holding plate. When surplus power is no longer available it reduces its energy-use accordingly. The stainless-steel holding plate can be fitted at any suitable angle high up in the box before being connected to the compressor unit by the three metre long 6mm diameter flexible copper piping fitted with quick-couplings. The compressor unit is small enough to be fitted in stowage within connecting distance from the holding plate. Extremely quiet when running, it can even be fitted under a bunk if needed. After connecting the leads to the battery, the pre-filled system is ready to go. See our article on Isotherm Fridges, Freezers, Kits and Ice Makers.
The only disadvantages that DC refrigeration has is that a single unit can normally only run a fridge up to 350-400 litres in volume and a freezer up to 150 litres in volume and that the DC electrical system has to have adequate battery power to be able to run the refrigeration system. A good quality battery monitor is also an excellent idea in the DC system as this will allow the skipper to know how much battery capacity he has and also let him know exactly how much power the refrigeration system is using. For further information please refer to Bow2Stern #3 article Planning Your Electrical System
This is predominantly achieved by adding an AC/DC module to the DC refrigeration system. These modules run on the highest voltage available (240VAC if available), then automatically switch to 12VDC or 24VDC if 240VAC is no longer available. Should AC be switched back on, the units will switch back to this to save battery consumption.
These can also take the form of a household domestic fridge/freezer or a bar styled fridge, while you get a lot of bang for your buck and large cabinet volumes on these types of refrigeration units they are generally not an ideal option for use on a boat or yacht. Firstly these units are primarily designed for maximum fridge/freezer volume for their size and thus the amount of insulation to fridge volume is compromised and as a result the compressor has more frequent duty cycle. This is normally not an issue when plugged into a constant 240 V power point supply but when this means either an inverter or a generator has to be used more frequently to provide AC power and this means more power is being consumed.
The co-called cost saving in buying an AC refrigeration unit is often offset by upgrading the boat’s AC and DC electrical systems to offset the additional power requirements of the AC refrigeration units, not to mention the additional fuel and maintenance costs because of increased generator run time. Inverters can be used to run the AC refrigeration system but once again careful consideration needs to be given to designing the AC and DC electrical systems, as previously mentioned further information on this subject can be obtained from Bow2Stern #3 Planning Your Electrical System.
Engine Driven Eutectic Systems
These were the in thing a decade or so ago and were most commonly used in retro fitting into built-in ice boxes and converting household AC fridge cabinets. The compressors were mounted to the one of the vessel’s engines and was driven by belts and pulleys from the engine’s flywheel. The compressor then cooled a large eutectic holding plate filled with a special refrigerant glycol which meant the engine only had to be started and run twice time in the day to bring down the holding plate’s temperature. They were, if correctly done, reasonably efficient and could run quite a large sized fridge.However, as time goes on, many people have found the reliability of these types of units often left to be desired, and the maintenance costs of the system scared even the most hardened boaties. These systems are now fitted usually only to large commercial installations where a custom manufacture is required, and regular, professional maintenance can be achieved.
Build an ice box in your boat. It is not difficult at all!
...courtesy of Isotherm
Building or modifying an insulated ice box.
Efficient performance from a refrigeration system depends primarily on a good design for the box with sufficient insulation and an air tight construction.
Many boat builders today are constructing excellent boxes but there are many situations where improvements or modification of the box will dramatically improve the performance. Adding insulation to the bottom or sides of a box will often help improve the insulation value, reduce the volume and improve the shape of an existing box.
Building a new box from scratch is a big project but it can be done with very good results.
Here are a few general suggestions.
Most boats are different but they do tend to have one thing in common and that is that they have very few right angles or regular shaped compartments. There are usually very few boats that can take a ready-made refrigerator or freezer – you just have to make it yourself! These days companies can supply a flat-pack set of ready-cut insulated panels to help you build a customised box. The panels can be covered on both sides with 2.5mm plastic laminate of foodstuff quality and have thick closed cell foam insulation in between. They are easy to cut and glue together using a polyurethane sealant or mould resistant silicone resulting in joints that are leak-proof but not totally inflexible. Some fridge and freezer boxes come pre-fabricated top-loading options.
A top-opening box is usually preferable. The cold air is not leaking out when the box is opened to the same degree as for a front opened fridge. A box can also be designed to fit the shape of the interior and the hull.
Select a suitable place on board where the box can be built. Do not build a bigger box than necessary, it is better economy to have a smaller well filled up than a bigger half full box.
To access the box contents from above should be easy. The lid is normally a storage yard. The lid opening should be not larger than to give access to both hands, when loading or taking out the provisions. It is a great help if the area for the box can be completely made free for moulding and building. Avoid building the box close to a warm engine room or heater compartment. If that cannot be avoided, extra insulation must be added. Also have in mind that the connection pipe to the compressor is 2.5m on evaporators and 3m long on holding plates.
Try to create as many right angles as possible. Make the bottom as big as possible. The bottom shall be made in a size allowing the sides standing on the bottom panel. The bottom must be carefully fastened in the interior; the box filled with provisions must withstand the force during sailing in rough seas. Very often, additional supports and brackets must be manufactured. Normally it is preferable that the top part will lie on top of the sides. The box will be more stable.
The lid is cut out from the top part and can either be hinged or loose. If the lid is made loosely, which is often the preferred method, make it either absolutely square or obvious rectangular to make handling easy. On top of the lid, a plywood or a wooden plate can be mounted to match the interior.
The insulation end of the lid has to be protected with plastic strips, they are 2mm thick, to create a smooth, easy to clean surface.
It is important that the lid is very tight, to prevent warm air entering the box. Mount a rubber sealing strip on the lid.
You can then choose a Build-In fridge or freezer Kit to suit the volume and use of the new compartment.
Some important tips when using your fridge/freezer on board your boat which will reduce your power consumption include;
- If possible minimise the load on the refrigerator when the engine is not running by loading in food stuffs that are already chilled;
- Keep your refrigerator as loaded as possible, it is the vacant air space inside the refrigerator that increases inefficiency. Once again back to the esky principle - load an esky full of ice and close it and see how long the ice lasts for maybe a few days. Now take the same esky and only place a couple of cubes of ice in it. Close it and how long do they last? A couple of hours if we are lucky. This is because solids such as ice/foods and liquids such as water and beer have better thermal retention qualities than air. So fill your refrigerator and presto, you will consume less power than placing only a few items in it;
- Because cooling generates heat, good ventilation of the compressor compartment is essential for air cooled systems. If the temperature in the vicinity of the compressor rises above +40°C, ventilation needs to be improved to allow cool air (preferably from the bilge where the air is coolest) to enter at the bottom and the warm air to leave at the top. A few holes the sizes of a large coin are usually sufficient. All air-cooled Isotherm and Nova Kool units with holding plates are equipped with a fan which speeds up the air flow;
- If you have chosen sea water cooled, such as an Isotherm SP or Magnum models, high temperature inside the boat is immaterial;
- When the refrigerator is not in use, even for a short period, always leave the door or lid slightly open to avoid unpleasant odours;
- The condensation drain in the box should always be closed when the refrigerator is in operation otherwise the heavier cold air will run out at the bottom to be replaced by lighter warm damp air from the boat;
- It is uneconomical to turn off the refrigerator during the night. In addition to an increase in average power consumption when it is started up again, it is also harmful for the food.
So there we have it in a nutshell, how to get the most out of your marine refrigeration system. Marine refrigeration is no differ&am