Features October 1, 2002 Issue

Refrigeration Survey 2002

PS readers are almost universally pleased with their choices in DC-powered refrigeration. The technology is well-developed and reliable. Insulation, location, and system balance are still critical.

Grunert's Polarmate is typical of a 12-volt
system that can be set up with an air-cooled
or water-cooled condenser. The basic com-
ūponents - evaporator, compressor, condenser
- are common to all systems.

The British, who have a long history of extended sea voyages, drink their beer warm. And while we can't be sure there's a connection there, it's a fact that keeping food and drink cold aboard a small vessel has, until the last couple of decades, not really been worth the hassle. The icebox, a relic of the last century, is still the solution provided by many boatbuilders.

A well-insulated icebox can actually work pretty well, if you're not venturing out for more than a few days. And if you can find an icehouse to sell you a block (or you can afford the freezer space to make blocks of your own). And if you can deal comfortably with dragging the ice on board - a major problem if you're at anchor. And if you don't mind living with the occasional soggy sandwich.

Most sailors who cruise extensively, though, have come to adopt some type of mechanical refrigeration system. Over the years, these systems have become as much an integral part of life on a cruising boat as they are at home, and more and more boats are being fitted with refrigeration as original equipment.

While marine refrigerators function in much the same way as their land-bound counterparts, the conditions under which they operate are quite different. On a boat, particularly a sailboat, space requirements generally demand that the refrigeration equipment be added to available spaces, with the existing icebox "converted" to house the cold part. On land, 110-volt AC operation permits the home refrigerator designer to use motors and wiring that are physically smaller than would be required for low-voltage DC operation, and power is available as long as the refrigerator is plugged in. Marine power supplies are typically 12-volt or 24-volt DC batteries, charged by an alternator on the engine or a genset, and the power available is severely limited unless the engine or genset is operating. This is a less-than-ideal situation on a sailboat where engine operation is infrequent and battery capacity tends to be limited.

The power requirements for refrigeration are considerable, with the lion's share of the load consisting of compressing a gaseous refrigerant.

One way of dealing with this is to drive the compressor directly from the boat's engine, eliminating the need for electric power. This approach, which was the most practical one for quite a while, only cools things while the engine is operating. To take care of the times when it's not, a large, high-capacity refrigeration unit must be used. This excess capacity allows you to freeze a sizable holding plate - effectively a big block of ice - and to use that "block" to keep the refrigerator's contents cold when the engine is off. This system works quite well, but the large refrigeration module is bulky and expensive. Engine-driven compressors are probably best suited to larger boats where they are included in the initial design. They still have plenty of devotees.

Another similar approach uses a large electric-powered refrigeration unit to chill down a holding plate. The larger the plate, and the lower the temperature to which it's cooled, the longer the refrigerator can be shut down. This system works best when the "off" period for maintaining an adequately low temperature roughly coincides with the period between engine uses. The major advantage of this approach over the direct-drive one is that installation presents far fewer problems; the disadvantages of bulk are similar.

The introduction of DC-powered compressors in the early 1980s proved to be a huge step towards making refrigeration feasible for smaller craft. These compressors, almost all made by Danfoss, are small, quiet, and reliable. Together with a growing trend toward greater battery capacities, refrigeration units utilizing these compressors have made it possible to run a marine refrigerator for extended periods of time without the need for running an engine or genset. In effect, energy storage can be accomplished with batteries instead of a large block of ice. Because refrigerator operation can be spread over many hours, the refrigerator itself can be made smaller - and therefore less expensive - than intermittent-operation holding-plate systems. These longer-running units are often called "evaporator systems" - a not-terribly-descriptive term, but one that lets you distinguish them from holding-plate systems.

At about the same time, another technological development was introduced: tubing connectors (or couplings) with built-in shutoff valves. Before this type of connector was introduced, refrigeration systems were shipped with no refrigerant, and it was necessary to have the system charged once installation was complete. Use of these connectors makes it possible to ship the separate components of a refrigeration system already charged with refrigerant, so that buyers don't have to hire a refrigeration technician to install the system (although connections do have to be made adroitly). Together with the smaller DC-powered compressors, these pre-charged components have helped convert the marine refrigerator from a device requiring professional, custom installation to one that can be installed by the boatowner with a minimum of fuss.

How It Works
A refrigerator is a heat pump, based on two physical principles. First, when you compress a gas, its temperature goes up. Anyone who's pumped up a tire has noticed this. Second, when you allow a liquid to evaporate, it absorbs heat from its surroundings, which then get cold. (That's the idea behind an alcohol rubdown.)

A heat pump in the form of a refrigerator uses a fluid - the refrigerant - that evaporates and re-liquefies at optimum temperatures when it's compressed and then cooled.

Refrigeration doesn't work by putting cold into a space - it works by taking heat out. Reach into your icebox and touch the evaporator, and it's very cold. Yet it's because heat inside the space is being absorbed by the evaporator, which feels cold because the liquid refrigerant inside is evaporating, absorbing heat, and then moving on the system.

Any heat pump has a hot side and a cold side. In a household refrigerator (or window air conditioner) both sides are mounted on the same frame; on a marine refrigerator (or a home central air conditioner) they're physically separated, connected by lengths of copper tubing.

After the liquid refrigerant has evaporated and drawn heat from the icebox, it passes, still in its gaseous state, into the compressor, where it is compressed (and thus further heated). From there it goes to the condenser, either fan-cooled or water-cooled. It loses heat to the air or water that's used to cool it, turns back into a liquid, and heads back to the evaporator to begin the cycle anew.

Clearly, the lower the temperature of the cooling element, the more heat will be extracted from the refrigerant. This is why water-cooled refrigerators outperform air-cooled ones. Of course, increased efficiency comes with greater complexity and expense, and more maintenance problems.

The efficiency of the heat pump is one consideration, but an even more important one is how much heat has to be removed from the interior of the icebox. That consideration has more to do with the efficiency of the insulation. The more (and/or better) insulation you put in, the less heat you have to remove. Of course, the more insulation, the less room you have.

Where We Are Now
The results of our last hands-on test of icebox conversion kits appeared in the December, 1996 issue. The basic facts about marine refrigeration haven't changed much since then, but there have been a few new twists worth mentioning. For example, the reliable Danfoss BD compressor, which has been at the heart of refrigeration units for many years, has been upgraded to the new BDF unit, an "intelligent" compressor that adjusts its output according to the demands of the evaporator it's working with, thus reducing overall power consumption. It's also smaller and lighter than its predecessor.

Many manufacturers now use a pump motor in which 12-volt DC is converted to AC to run the pump, which provides higher starting torque and variable speed.

We've also heard of arrangements in which the boat's freshwater supply is plumbed to the refrigeration system to act as a heat exchanger (in place of a raw-water cooled system).

Our Survey
This spring, Practical Sailor ran a website survey for subscribers only, asking for feedback about 12-volt refrigeration systems. We received responses from 143 people. Of those we've compiled good results from 119. Sincere thanks to all those who took the time and trouble to log in with username and password and fill out that form?it's a big help to your fellow sailors.

Excerpted survey results are shown in the charts below. More quotes from owners are also included below.

Sailors who responded to our survey were generally very happy with their refrigeration systems. Employing a scoring system of 1 to 4, where one equated to "poor, 2 to "fair," 3 to "good," and 4 to "excellent," respondents gave average scores of three or better for every category, regardless of which unit was being rated. These included ability to chill contents quickly (average score: 3.0), ability to keep frozen food frozen (3.1), noise levels (3.3) and ability to maintain chill under normal use (3.4). Similar scores were recorded for ease of purchase (3.5), completeness of equipment on arrival (3.6), explanation of system operation and installation instructions (3.2 and 3.3 respectively), ease of installation (3.3), technical support (3.2), warranty support (3.1) and quality of materials/fit and finish (3.4). Domestic parts availability scored a 3.4; availability of parts in foreign ports was worse, but still a creditable 2.6.

The survey respondents operate their refrigerators for prolonged periods: 206 days a year, on the average. Surprisingly, no fewer than 39% told us that they use their refrigerators 365 days per year. Despite this heavy usage, they seem to hold up well - we can't speculate on life expectancy, but the refrigeration systems reported on have been in use for an average of 6-1/2 years, with almost 19% of them having been used for over 12 years (and up to 21).

Both holding plate and evaporator systems were well represented. 44% used holding plates, while 56% were evaporator types. Both types worked well, though the holding plate types were somewhat worse in chilling contents quickly (2.9 vs. 3.2 for evaporator units) and somewhat better in terms of keeping frozen foods frozen (3.2 vs. 3.0).

Installed costs varied considerably - from $400 to $7,000 dollars. Cost data, though, does not include built-in systems where the refrigerator cost is included in the boat's price. It can be misleading in terms of what the cost of a DIY installation might be (almost a third of the systems reported on were owner-installed).

The least expensive units were from Norcold (average price $894); the most expensive from Glacier Bay (average price $5,417). The Glacier Bay units reported on were, along with the Frigoboat units, the best performers by a slight margin in both speed of cooling and ability to keep frozen food frozen.

We could find no meaningful correlation between the different brands and the nature (or frequency or severity) of any problems encountered. It's not that problems didn't occur, but they were infrequent, scattered, and often didn't refer to specific issues. Loss of refrigerant due to leakage was the most common problem reported (12.6% of the units) but this was often encountered after many years of operation. In two of the 15 reported cases of refrigerant loss, the unit was self-installed and the owner either undertightened or overtightened to the point of breakage.

Despite the cheerful picture painted by our respondents, there are a few issues that should be considered before purchasing a system:

If you have a small icebox with poor insulation, don't expect miracles. A poorly insulated box increases the heat load on the refrigerator, which will degrade the performance of the best refrigeration unit. A small box doesn't permit you to add insulation without losing too much storage space. Your best bet is to consider a new icebox. If the insulation is inadequate, and the box is large, you can often increase insulation without sacrificing too much space.

If you use a single-sideband (SSB) radio, be aware that just about any 12-volt refrigerator can cause RF interference when it's running. Consider going with a holding plate model, which will runs for shorter times than an evaporator model.

If you do your cruising in southern waters, you'll appreciate the increased efficiency of a water-cooled unit.

Don't try to mix or jury-rig refrigeration components. The pros take great care to balance their system parts to work efficiently together, and with your icebox, charging system, and available mounting spaces. They know best. Consult them.

From what we've been able to find out from manufacturers about recent (and near-future) developments in the field, we can only say that an already rosy picture is apt to become slightly rosier. Possibly the most important new trend is that boatbuilders are gaining a better understanding of refrigeration needs, and working with refrigeration manufacturers to create better icebox installations.

This survey can't provide detailed information about any particular brand; there are just too many models, years, and sets of conditions involved to make meaningful comparisons possible. The survey does indicate, however, that if you're shopping for a marine refrigerator, you'll have to work hard to find a clunker.

Also With This Article
Click here to view "12-Volt Refrigeration Survey: Selected Results."
Click here to view "Survey Quotes."

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