During the past few decades, a technical evolution has made the hardware that powers the refrigeration cycle more efficient and more reliable than ever. Today, almost all new cruisers and racers come with refrigeration, or it is near the top of the options list.
In the May 2007 issue of Practical Sailor, we looked at portable, 12-volt “plug and play” freezer-fridges, with the Norcold MRFT 60 and the Engel MT-35F leading the pack. Here, we review systems from three principal makers, Dometic Corp., Frigoboat, and Sea Frost. These are among the most popular ice-box conversion systems on the North American market today. Each is designed as a do-it-yourself kit that turns the ice box into a full-fledged refrigerator and/or small freezer.
All three manufacturers use Danfoss hermetically sealed compressors, devices that have been engineered to house a miniaturized drive motor and compressor inside a grapefruit-size air-tight tank. The evolution of the units DC controller eliminates the need for a conventional commutator and brush assembly, and two decades worth of improvements have significantly increased the systems efficiency. The result is an easy-to-install fridge for modest-sized sailboats with well-insulated ice boxes, and an easily expandable system (compressors can be piggy-backed) to suit larger boats.
The modern marine refrigeration system can be traced back to ice-cube guru Mike Adler, a cruising sailor who applied his engineering talents to a cause that any modern sailor can appreciate-developing a compact, efficient, and reliable alternative to ice.
Going against the prevailing trend of systems with high-capacity mechanical, engine-driven compressors, Adler staked his reputation on the compact Danfoss brushless 12-volt DC electric compressor. It didn't take long for Adlers system to attract converts. The German-engineered Danfoss compressor was not only reliable, it was truly the wave of the future, as advances in battery capacity and charging systems favored 12-volt systems.
The hermetically sealed electric compressor wasnt a new concept. It was introduced in 1926 by General Electric, and the AC version had dominated household refrigeration for many decades. What differentiates the Danfoss is that it eliminates the inefficient brush-and-commutator design of conventional DC motors. Using an external, solid-state controller and three-phase pulse technology to propel the motor, the Danfoss unit has improved efficiency and reliability.
Hermetic units, like their mechanical drive cousins, have pros and cons. The systems strongest attribute is its reliability, a factor that has no doubt contributed to the sagging demand for “Freon Jockeys,” those mobile fridge-repair artists who once enjoyed superhero status on the cruising circuit.
The history of the system aboard Practical Sailor Technical Editor Ralph Naranjos Ericson 41 illustrates well the robustness of the early Adler Barbours. Naranjo installed an original Adler Barbour unit aboard his sloop, Wind Shadow, in 1982. He replaced the evaporator in 1990 and finally removed the unit in 2009 in order to long-term test some of the newer models. Not alone did the original unit perform exceptionally well; energy for its operation was generated by four ARCO M-55 solar panels that continue today to deliver the same current they delivered in 1982.
What We Tested
The refrigeration systemsPractical Sailor selected for this comparison-the Waeco-Adler Barbour Cold Machine (CU-100) from Dometic Corp., the Frigoboat Capri 35F, and the Sea Frost BD-represent a good cross-section of whats on the conversion-kit market today.
Heir to Mike Adlers brainchild, the Cold Machine is a widely distributed production-boat standard. Its maker, Waeco, was acquired in 2007 by Dometic Corp., a Swedish-based multinational that oversees a huge line of marine products.
The Frigoboat system was developed by Veco SPA, a family-run company based in Italy that has specialized in marine refrigeration (and now air conditioning) for more than 25 years. The Sea Frost BD, the only U.S.-made unit in the review, comes from a relatively small but successful New Hampshire-based company that has been around for more than two decades. The company often gets good reviews fromPractical Sailor readers for its outstanding customer service.
All of these units are compact and easy to install, well suited for a do-it-yourself installation. The units run with a whisper-quiet whir and a faint, reassuring gurgle of refrigerant circulation. All use R-134A refrigerant, which replaced R-12 several years ago. They draw from 3 to 6.5 amps when operating. Their duty cycle, or percentage of time that they remain on, depends upon the climate, size of the box, insulation, and how the box is used.
Most manufacturers recommend picking a system with enough cooling capacity so that the compressor is running 50 percent of the time that the fridge is in use. The importance of efficiency is self-evident once you leave the dock. Even at a modest 2- to 3-amp-hour draw, the current consumption over a 24-hour period can add up to 48 to 72 amp-hours.
Dometic carries on the tradition pioneered by Adler with a line that includes air- and water-cooled 12- and 24-volt DC hermetic units. We tested the CU-100. This system uses the BD50F Danfoss compressor, and testers found it to have approximately 25-percent more cooling capacity than the Danfoss BD35F compressor. The increase in capacity stems from a 25-percent larger displacement, but the BD50F compressor fits into the same shell as the BD35F, and all dimensions remain the same.
The current consumption and capacity vary according to box temp and the motor RPM selected, but along with the 25-percent increase in cooling capacity comes a power demand thats about 40-percent higher than the BD35F. This means that the BD50F tends to have a lower coefficient of performance (COP) than the BD35F, but theres more at stake than the numbers indicate. Efficiency is one factor, but it is equally important that the box and climate constraints are matched with the capacity of the unit. In this respect, choosing a reefer is a bit like selecting a car: The job at hand, rather than fuel efficiency, may define what is best for you. You don't want a Toyota Prius to do the work of a pickup.
On many boats, the box size, insulation efficiency, and the ambient conditions may best suit the smaller, more efficient BD35F system. But sometimes, even a small ice box may need high-capacity cooling due to poor insulation, brutally hot weather, or because hot can drinks are frequently being cycled into the box. These environmental and use factors can challenge a reefer systems capacity. Dometic uses only the BD50F, believing that the 25-percent increase in capacity offsets its decrease in its overall efficiency.
The Danfoss controller offers several new features. Perhaps the most interesting is Automatic Energy Optimizing (AEO)-a smart controller relay that automatically changes the compressors RPM based upon running conditions. The RPM “soft starts” at a slower RPM, then ramps up based upon an algorithm that adds 400 RPM every 12 minutes until the unit either reaches 3,500 RPM, or cuts off after reaching its thermostat setting. The reasoning behind this gradual start is that it makes more efficient use of cooling capacity, but under higher heat loads, the unit automatically ramps up to its maximum RPM.
In our test unit, the larger Danfoss BD50F compressor was teamed with a larger evaporator (VD-152) that provided increased cooling surface area, so this system was able to drop the box temperature a little more quickly than its competitors, but it also demonstrated an increased appetite for amp-hours.
Sizing the evaporator to match the box is important, but it is just as critical that the evaporator matches the output capacity of the compressor. Adler Barbour carefully pairs up its hardware for optimum performance. The larger BD50F compressor is also offered by both Frigoboat and Sea Frost. This compressor is a good fit for boxes over 8 cubic feet or for boats cruising tropical climates, but its greater cooling capacity comes with increased power consumption. The laws of physics allow no free lunches, something worth keeping in mind when comparing these products.
The simplest and most common form of thermal control is the conventional mechanical thermostat, a switching device that uses a probe sensor (gas-filled tube) and a diaphragm-type, mechanical on/off relay. This analog function allows users to select a “general” turn-on and turn-off point to keep the box cycling within a few degrees. We found that these reliable but rust-prone mechanical thermostats could be set to cycle within the 39- to 42-degree range. Over the years, the galvanized mild-steel sheet metal in these thermostats corrode, but they often outlast the evaporator units they regulate.
There were a few construction details that testers didn't like about the new Cold Machine. The first was the one-time-only nature of the self-sealing connectors. Once made fast, the compressor and evaporator components cannot be disconnected without losing the refrigerant. Other systems had more expensive resealable fittings that allowed the components to be disconnected without losing their charge, a feature that adds greater flexibility for the do-it-yourselfer. Also, among the three systems scrutinized, the Cold Machines tube soldering showed the most oxidation, due to what appeared to be a lack of quality control. This may be isolated to our particular unit, but it stood out. Lastly, the connector for the thermostat circuit used the RJ-11-type “telephone” plug to connect the control unit to the compressor. No matter how well you protect this socket, this is a poor connection for the marine environment. Despite these few shortfalls, the system performed flawlessly, offered more capacity than its competitors, and was competitively priced, keeping it in close contention with the others.
Bottom line: The Cold Machine is a good pick for sailors who can live without modest gains in energy efficiency, particularly those with a bigger box, or a less-than-perfect environment for cooling. Its connectors and tubing hold it back in terms of construction quality and user-friendly installation.
Compact, efficient, and extremely well made, the air-cooled Capri 35F squeaked ahead in our tightly competitive group of three. The system is a good example of component miniaturization with a bent toward efficiency.
The Danfoss BD35F compressor, control unit, fan, and condenser fit on a 10-by-8.25-inch stainless-steel tray, and offered the smallest footprint of all the units tested. The rectangular evaporator bin can be tucked neatly into a small 2.5-cubic foot reefer/freezer and is also rated for boxes as big as 10 cubic feet.
One of the features we liked most was the manual mode, an option that lets the user select the most efficient, low-power setting (2,000 RPM), delivering the highest COP. This setting, which runs the compressor at a slower speed but for a longer period, is useful for boats that depend on alternate energy sources such as solar and wind power. The rate at which amp-hours are depleted at this lower setting more closely matches the real-world outputs of onboard renewable energy sources. All the Danfoss units tested have the ability to be run at 2,000 RPM, but Frigoboat makes it easy to accomplish with the manual override Smart Speed Control (SSC), a simple circuit board attachment, or the AEO controller.
The large diameter stainless-steel connectors with O-ring seals proved to be user friendly, and required the lowest torque for tightening. This system, unlike the one used by Dometic, allows you to connect and disconnect couplings without any significant gas loss. The attention to soldering detail and care taken with the overall refrigerant plumbing was exemplary as was the overall design of the system.
The evaporator frosted up in the shortest period of time and even when running on the “high” RPM setting, the return tube did not show signs of frosting, two indicators of a properly charged system. The aluminum evaporator bin is not rugged, but as demonstrated by the 10-year lifespan of similar technology, the efficient cooling surface is appropriate for the job at hand.
Our most significant complaint about the Frigoboat related to the overlap length of the small ?-inch copper tubing that was much longer than the -inch return line. In order to connect it to the compressor, a very careful curve had to be made, avoiding any kink or restriction in the thinner tubing. The purpose of the different lengths of tubing is to allow a more compact cluster of couplings when it comes time to press or pull the tubing through a conduit or tight confine. In this case, a good idea was simply overdone, in our opinion.
During our testing, we used Frigoboats standard mechanical thermostat. This worked quite well, but it was not as efficient as the optional digital model. The latter allows for more precise cut-in and cut-out temperature control. We found the Smart Speed Control to be very useful, allowing us to shape the units current demand based upon cooling needs as well as battery condition. For example, when the charge was diminished (battery voltage under load at around 12.1 volts DC), switching to the 2,000 RPM setting not only gave us the highest COP, but the fridges low 2.9-amp current draw extended the discharge capability of the battery bank.
Bottom line: Test results in the Value Guide on the adjacent page speak for themselves, underscoring that the Capri 35F is both efficient and packs quite a bit of capacity in a small package. It also ranks high with regard to attention to details and construction quality.
The 12-volt DC, air-cooled BD plate system from Sea Frost that we tested uses a Danfoss BD50F compressor the company tucks into a powder-coated alloy box. The compact box protects components and provides efficient ducting for the fan.
Sea Frost founder and owner Cleave Horton prefers to pre-set the BD35F to its highest capacity 3,500 RPM run mode. This is accomplished by adding a 1,500-ohm resistor to the thermostat circuit. Its a Danfoss-approved modification. The upside is more capacity while the downside is a lower COP.
The Sea Frost system uses a half-inch-thick stainless-steel cold plate-style evaporator rather than the more common pressed aluminum evaporator. Both approaches handle the evaporation portion of the cooling cycle, providing surface area where a cold metal can come into contact with warmer air and allow thermal transfer to take place. Another design difference in the Sea Frost system is that the liquid refrigerant squirts through a more efficient expansion valve rather than a simple capillary tube. The drop in pressure at the expansion valve allows the liquid to become a gas, and the cooling process begins. In either type, the more refrigerant you have changing state and the more evaporator surface available, the more effective the system will be.
A look at evaporator efficiency offers a quick lesson in thermodynamics. Testers were interested in how the stainless-steel plate stacked up against the alloy box-type evaporators used by Sea Frosts competitors. A look at the thermal conductivity of metals shows aluminum as more efficient, but Sea Frost contends that because of the slow rate of heat transfer, this is not significant. In addition, the softer aluminum is more prone to damage from blows, corrosion, or an ice pick. So despite its efficiency handicap (as measured in amp-hours), the durable stainless-steel evaporator plate ranks high for user friendliness.
Sea Frost adds value with its close attention to details such as high-quality Aeroquip self-sealing connectors, carefully silver-soldered joints, and the component protection afforded by an enclosed, well-ducted compressor. The stainless-steel evaporator plate comes with spacers for mounting it just off the box wall, leaving the internal space of the ice box well preserved.
The efficient Danfoss BD50F is wired to run full-bore at 3,500 RPM, but the AEO controller protects against overload and under-current operation. Unfortunately, the efficiency of the 2,000-RPM low-current demand cycle is not available with the manual thermostat or with the optional electronic thermostat and thermometer. However, the latter offers precise cut-in and cut-out temperature control that can help reduce unnecessary running time.
A manual throttle up/throttle down effect can be achieved through the use of Sea Frosts BD Speed Control, a three-position switch that changes the resistance in the thermostat circuit. The result is an RPM selection (low, medium, high) that includes the extremes and midpoint of the 2,000- to 3,500-RPM operating range. With zero ohms of resistance, the RPM stabilizes at about 2,000 RPM, and this low speed offers the least cooling capacity, but the highest energy efficiency. A 1,500-ohm resistance placed in the thermostat circuit delivers just the opposite effect. The specified COP identifies the most effective setting for energy efficiency (2,000 RPM), however, it involves longer run times and a lower rate of BTU removal.
Keeping the duty cycle under 50 percent is important, especially with a BD50F compressor at full RPM. Heat, in the ice box as well as in the compressor space, is the systems enemy. And in larger, less efficient ice boxes, theres always the opportunity to step up to the increased capacity of Sea Frosts BDxp unit, with a larger plate, two plates, and/or a water-cooled condensor.
Bottom line: The Sea Frost BD system is a well-made alternative to the alloy evaporator system. The builder has taken extra steps to keep the delicate fins of the condenser well protected and offers an attractive, and ruggedly fabricated stainless-steel evaporator thats proven to outlast alloy bins or plates.
This test targeted three of the most respected manufacturers in marine refrigeration systems, each capitalizing upon advances in the Danfoss compressor. During testing, we quickly saw that each of these three systems were capable of many years of good performance.
To a great extent, individual needs will guide selection. For instance, if the bottom line is energy efficiency and the ability to cool a small, well-insulated box with the least amp-hours possible, Frigoboats Capri 35F gets the nod. If sheer ruggedness is important, Sea Frost takes the lead. Finally, those with a larger ice box planning to cruise in a sweltering climate may be better off with the more powerful BD50F compressor and large evaporator that comes with the Cold Machine.
The Frigoboats Best Choice rating is due in large part to its amp-miser efficiency and compact size. Another big plus was the simple power management system that came with the remote manual/auto control, allowing crew to pick an operating RPM (capacity) so that when the engine was running, the alternators excess power could be used most efficiently. And during the night, when power reduction was a big plus, the 2,000 RPM run speed would maximize the COP.
Testers found Sea Frost and Frigoboat neck-and-neck in quality control and attention to detail. Frigoboat also offers stainless-steel plates, an efficient keel cooler, and several remote digital thermostats/controllers. However, it wasnt these features that clinched it. Instead, it was a combination of efficiency, reliability, and capacity that gave line honors to the Frigoboat.