Focus on refrigeration

Being able to have a cold drink when you want one and store perishables are just two of many reasons good refrigeration comes close to top of the list of cruising essentials, writes Sharon Smallwood.

Cruising is not camping. A cliché? Perhaps. But to quote a famous Pardey mantra: “Food sets the mood”. So if you’re sick of soggy fodder floating in melted ice, or fancy a cold cleansing ale instead of a slightly warm one, read on to unravel the mysteries of the marine fridge.

The big chill

Marine refrigeration might seem like a complex conundrum, but the principle behind it is actually quite straightforward – to eliminate heat from an enclosed space. According to refrigeration expert James Lang, “To understand refrigeration it is important to consider a common energy law, that energy cannot be created or destroyed; energy can only be transferred from one form to another.” Heat is a form of energy. Essentially refrigeration is about the transfer of heat; removing warmth causes cold.

In a boat fridge this typically happens through the use of a number of chemicals known as refrigerant. In scientific terms the refrigerant changes state from a vapour to a liquid and back to a vapour at different pressures. Inside your marine fridge, pressurised liquid refrigerant decompresses; the resulting pressure drop allows the liquid to evaporate by collecting heat from the cabinet and hey presto — cold beer!

The basic mechanics required to make this work are a compressor, condenser, metering device and evaporator. These four primary components form the backbone to marine refrigeration.

The role of the compressor is self-explanatory — it compresses (pressurises) the refrigerant, which then passes into the condenser. Here the temperature falls to the stage that the hot pressurised vapour condenses into a cooled pressurised liquid.

Next in the system is the metering device, commonly known as either a capillary tube or a TX valve, which controls the passing of the high-pressure liquid into a low-pressure evaporator, where it evaporates (changes state), back into a vapour.

The beauty of this system rests not just in its inherent simplicity but alsoin the fact that it is cyclic. At the end of the process vapour is returned to the compressor and the sequence begins again.

Power

So now for the million-dollar question: if marine refrigeration is really so simple why does it have a reputation for fallibility?

One of the answers is that compressors need power. This has to come from either an electric motor or the boat’s auxiliary engine. As James Lang explains, “Engine-driven compressors use the diesel engine to spin a car air conditioner compressor via a V-belt.

“The downside of these compressors is they are prone to leak at the shaft seal, particularly if the compressor is not run on a regular basis. The shaft seal will dry out as it not being lubricated by the refrigerant oil. On starting the engine the shaft seal will tear and allow the refrigerant to leak out.

“Engine-driven compressors may be suitable for commercial vessels that are constantly in use. However, the electric compressor systems available today are cost-effective, reliable and outweigh the expense of an engine-driven fridge system.”

The majority of modern electric compressors are dual voltage, ie able to be driven by either 12/24V DC and/or 110/240V AC.

In James’s experience, electric compressors are generally compact and can be mounted in a variety of locations allowing for greater flexibility of installation.

“Most compressors also have a built-in battery protection system, where they stop running if battery voltage falls below a predetermined level, usually 10.4 volts. This design feature helps to protect the house batteries from depletion and ensures the compressor can run efficiently,” James said.

Keeping your cool

The condensers in marine fridge systems are either air, water, or keel-cooled. Water-cooled condensers use seawater to dispel the heat, while air-cooled condensers are equipped with a fan.

The idea behind the fan is a no-brainer, but the disadvantage is the transfer of heat to other areas of the boat, such as the saloon.

As James points out, “The air-cooled system will need to be installed in a suitable place on board where there is plenty of air flow and ideally cool air available, eg the bilge. Some installations will need ducting depending on the placement of the condenser, as restricted airflow will result in increased energy usage and poor refrigerator performance.”

In James’ opinion air-cooled systems are technically less efficient, but also less expensive to manufacture and install. “Additionally they are easier to maintain, making sure you carefully remove any dust that collects on the condenser or the fan with a soft brush,” he said.

Water-cooled condensers involve extra plumbing, namely a pump, heat exchanger, strainer, and intake and discharge pipes, connected to through-hull fittings.

Keel coolers sit on the outside of the hull and so don’t need an independent pump.

Potential seawater-cooling problems are corrosion, fouling, and electrolysis, which can lead to a loss of refrigerant and system failure. Some manufacturers recommend cycling fresh water from the boat’s tanks rather than using saltwater, to avoid this scenario.

According to James, “The water-cooled system is very efficient due to the thermal mass and surface area conduction. To explain this compare your comfort and tolerance level in an office that is 21 celsius to the comfort of being in a bath that is the same temperature. While you can manage eight hours a day in an office you certainly would not manage eight hours in the bath.”

Evaporators

Evaporators come in three main designs: plate, finned and eutectic.

Plates range in size and shape. “The overall surface area of the plate determines the space it will refrigerate and is also relative to the size of the compressor in the system,” James explained.

Fridges with evaporator plates are usually thermostat-controlled, meaning the compressor runs as often as necessary to maintain a set temperature, which gives the advantage of uniformity inside the fridge box. However, as you might expect from something that’s constantly cycling, the overall power draw can be substantial. If the boat’s battery capacity and charging capabilities are not up to the challenge, problems will occur.

This being said, these systems are relatively kind to marine batteries, in that they exert a consistently low-level pull as opposed to short bursts of high power drain.

Plates are typically made of aluminium, which has potential for punctures and corrosion (if/when the anodised coating becomes scratched). A refrigerant leak will render the entire system ineffective.

Finned evaporators rely on a fan, forcing air over a series of aluminum fins. James surmised these evaporators are typically successful in obtaining a consistent temperature inside fridges that are long or deep. “The time to remove the heat from the fridge is accelerated due to the movement of the air across the contact surface of the evaporator,” he said.

Eutectic systems are often confused with engine-driven refrigeration. However, the true term is the description of the evaporator. “This is a cold-storage tank whose contents are frozen to a very low temperature. Water freezes at 0 degrees C so commonly a solution of 33 percent methylated spirits and 66 percent distilled water is used. This brine solution will now freeze at much lower temperature. The tank is filled to 90 percent capacity to allow for expansion,” James said.

The eutectic tank cools the contents of the fridge just as a block of ice would. However, a eutectic tank also shares some of the more unfortunate characteristics of ice — it takes up usable space, thawing can cause water to collect at the bottom of the fridge compartment (as it can with all evaporators), and only the items immediately next to the tank will be exceptionally cold. Proper phase change (change of state from solid to liquid/ freeze and thaw) on each and every cycle is important for optimum performance.

Eutectic systems can be advantageous where power supply is limited or inconsistent. The beauty of a eutectic system power-wise, is that it needs to cycle on only when the cold-plate, fridge cabinet and contents thaw, and then only for the length of time it takes the tank to re-freeze. However, the power drain is significant during this period.

Since a eutectic tank has greater mass than a thin evaporator plate, it has the capacity for more thermal storage. A phenomenon known as “latent heat” (heat that’s added/removed from a substance to change its state (eg solid to liquid/ice to water) increases the hold-over or off time. However, as previously noted, more significant load is placed on the battery bank during the albeit shorter power-on periods.

While the boat is in a marina and connected to shore power this is less of an issue than at anchor or on a mooring, where running the engine becomes necessary to keep up with the electrical demand.

How to choose

Your choice of system is determined by budget, power and space.

James advises asking: What battery capacity do you have? How much generator/solar power do you have available to run your system after you’ve considered lighting, navigation equipment, entertainment etc? What is the objective — is it to fast chill beer or to have food stocks available for longer voyaging? What is your available space and how big does your fridge/freezer need to be?

“Your budget is determined by all of the above. However, a suitably sized unit that has an air-cooled condenser with an evaporator plate is the most cost efficient,” he said.

Fridge manufacturers generally quote figures for a particular system’s current draw, but bear in mind current draw can be affected by ambient temperature.

“Some manufacturers claim low power consumption while the fridge is operating in 18 degrees (ambient) while other manufacturers supply more realistic power consumption based on an outside temperature of 30 celsuis. The cabins of recreational vessels reach high temperatures during the day especially in the warmer regions of our continent and air-cooled fridge systems generate heat as well,” James said.

The DIY fridge

If you want to start from scratch you can custom-build a marine fridge or modify an existing cabinet for the purpose. Multiply (in cm) width by depth by height then divide by 1000 to determine the cabinet’s capacity in litres.

“When you are measuring a fridge that has sloping sides measure at the point that appears to be the average dimension between the smallest and largest angles,” James advised.

In James’ opinion, “It is always best to choose a system that is above the capacity of your existing fridge space. This will allow for error in measurement and other losses such as inadequate seals or poor insulation.”

Individual components such as factory pre-gassed compressors, condensers and evaporator plates are usually sold complete with all the necessary tubing and quick-fit connectors. Although individual parts may be sold pre-gassed, in some cases the complete system has to be evacuated and re-charged with R134A refrigerant.

James warned, “Before attempting to install these units check your state or federal regulations as click-together units (by law) still require a qualified refrigeration mechanic to make the connections. Refrigerant gas can cause severe burns (frost bite) to hands and skin that will take a long time to recover from and gas leakage may not be covered by warranty.”

When installing or replacing individual parts, it’s worth remembering that components need to be evenly matched. Each link in the system relies on the others to function properly. Just one weak link will result in excessive power consumption and a poor end result.

Another option is to buy a pre-assembled unit that’s ready to be “built in”. These typically come in chest, top-loading, front-loading, or even sliding drawer configurations. Sheer size can make installation a two-person job, but otherwise only basic hand tools are required. DIY systems usually come with comprehensive installation and user manuals.

Insulation

A properly insulated fridge compartment is crucial to any system’s success. The better the insulation, the less work the fridge mechanics have to do and the more efficient the system will be. Types of insulation range from rigid panels to expanding foam.

The major issues are insulation quality, quantity and the effectiveness of seals. As James points out, insulation should be water-resistant “because water is an excellent conductor and if the insulation is saturated it will not work”.

Good seals are especially important to ensure isolation of the low-pressure environment inside the fridge from the hot humid conditions outside, which allow ice to form on the evaporator. There should be no voids and a reliable locking lid or door is a must.

Top versus front-loading fridges

After more than five years of full-time liveaboard cruising, I have to say that top-loading fridges are a pain in the proverbial – that’s my opinion and I’m entitled to it! Having said that, I put up with them (to a degree).

Top-loading fridges generally lose less cool air than front-loaders, provided, of course, that the lid is only open for a reasonably short time. Needless to say, if it’s really hard to get to your food and drinks, the lid will be off for longer and some advantage will be lost. Top loaders therefore require organisation on the part of the chief provisioner/stowage person/galley chef.

Top loaders work well when plastic storage containers are used, remembering to drill holes in them for airflow. “Top loaders can be good for longer-term food storage when combined with a eutectic plate,” James said.

James pointed out that another advantage of a front-loading fridge is the ability to mount an evaporator at the top of the unit. “Some evaporators are manufactured to a square box arrangement with a small door allowing the evaporator itself to be used as a freezer for ice cubes or other small items,” he said.

Stand-alone portable systems

Portable fridge/freezers (such as the ones we carry aboard Brilliant II) have the advantage of being transferable between boats, cars and caravans, making them a versatile travelling companion. A stand-alone fridge/freezer can be strapped into any available space and will generally operate well at an angle of up to around 30 degrees.

PUT TO THE TEST

Rob Antia of SV Mora rebuilt his fridge system in Bundaberg before sailing to SE Asia, where the system has been put to the test.

Rob used three isotherm built-in units (two x BI40 and one x BI75), for 115L fridge/40L freezer space. According to Rob the units were “reasonably priced”. The system uses air-cooled hermetic compressors.

“The BD35 compressors are already attached to the units with flex hoses and come pre-charged,” Rob said. “The space underneath the compressors is ventilated by a thermostatically controlled fan.

“I followed (Nigel) Calder’s advice of not putting a drain in (he points out that the drain causes cold air to escape). I added Frigoboat SSC controllers, which work on the principle that it is more efficient to run compressors slower but for a longer time. For some reason these were 1/2 the price in the UK to Oz. One of the BI40s we use as a freezer. Current draw (12V system) is 3-4 amps per running compressor.

“As to daily amps used — I’m guessing 25-30 amps on the 40L fridge, 40-50 for the 75L fridge, 50 for the freezer, mostly a function of drinks/ice cubes. The ventilating fan isn’t on much and only draws about 0.4 amps so it’s not a significant factor. This is all handled by solar.”

The good points:

•  Pre-formed so easy to install

•  Easy to clean s/s liner inside the evaporator plate, so no space wasted inside the fridge

•  No fridge plumbing

•  Very efficient

•  Small individual units cool fast and evenly

• Much cheaper than custom-building

•  Redundancy of having multiple self-contained units.

The bad points:

• With the high humidity in Asia they get condensation on the top of the freezer overnight. “If I were doing it again I would use a piece of the high-tech insulation on the top,” Rob said.

•  The cushioned rubber seal on one of the lids is splitting.

•  The BI75 would be better designed with a sliding top basket like the smaller units.
 
Rob’s final verdict: “So overall the fridges have worked out great. When I was doing them I thought about using a water-cooled system since we’re in the tropics, but in practice the air-cooled system has worked well, even with ambient daytime temps in the 30s. I’m not convinced water-cooled would have been significantly more efficient. I’d use more solid insulation and less spray if I were doing it again — just because the expanding stuff tries to move everything around so much.”