Reefer Madness
Marine reefers cool things by changing refrigerant pressure and keep things chilled in an insulated box. The heat is removed by an evaporator and dissipated by a condenser.
Anyone who's ever been on a charter or an extended passage knows how important that little hole in the galley that keeps the food and beer cold is to happy cruising. So we're going to dive in to see how that mysterious chilly box works and to explain the basics you need to know to choose the right system.
Since this is such an important and somewhat mystical subject, I called on my buddy Roger Hellyar-Brook to shed some light on the dark art of refrigeration. He's an instructor in all aspects of systems and boatbuilding at the Landing School in Maine, and he was Cruising World's systems-specific judge in last year's Boat of the Year program. "Refrigeration is one of my favorite subjects," he says. "It's easy, really. A fridge is like a sponge."
"Ahh, right," I say. "I mean, huh?"
"A refrigeration system," he says, speaking a little slower, "makes things cold by removing heat."
"OK, now we're on to something," I say, furiously scribbling on my pad. That, I'd learn, was only the beginning of the basics.
How It Works
As Roger says, all marine refrigerators remove heat from a box. And all use a gas refrigerant that's condensed (turned into a liquid) and evaporated (turned back into a gas) at different pressures in a closed system. The heat removal-which causes the icebox to get cold-happens when pressurized liquid refrigerant is allowed to quickly decompress in a long, curved tube in the icebox. The pressure drop in the system causes the liquid to evaporate, and as we all learned in high-school science class (OK, maybe some of us), when a liquid evaporates (boils off into a gas), it absorbs a significant amount of heat, at least enough to keep the beer cold.
All refrigeration systems have a high-pressure side and a low-pressure side (see illustration page 88). Heat is removed from the icebox on the low-pressure side, but the pressure must be first increased on the high-pressure side. To accomplish this, all marine refrigeration units are made up of the same four basic components: a compressor, a condenser, an expansion valve, and an evaporator.
The compressor does exactly what you'd think it does-it compresses the gas refrigerant. This pressurized gas then passes through a condenser, a long tube that acts somewhat like a radiator in your car. The condenser allows the temperature of the gas to drop to the point that the gas condenses into liquid. As the gas becomes a liquid, it releases heat.
At this stage, the refrigerant is a cool, pressurized liquid. The cold happens when this cool, pressurized liquid passes through the expansion valve into the evaporator in the icebox. The evaporator allows the pressure of the gas to drop significantly. The expansion valve is essential to control the high-pressure liquid passing into the low-pressure evaporator. The evaporator works on the opposite principle as the condenser: As the pressurized liquid passes into the evaporator, it evaporates-or vaporizes-into a gas. It's like a radiator that absorbs rather than releases heat. As the liquid vaporizes in the evaporator, heat is removed (the temperature drops) from the icebox, and the beer gets cold. The last step in the cycle returns the decompressed refrigerant back to the compressor, and the whole process starts all over again.
Once Roger explained the basics, he moved right on to the choices.
A Constant-Cycle,DC-Powered System
A constant-cycling DC unit acts like the fridge you have at home, except it runs on DC rather than AC power. It's controlled by a thermostat and turns on and off as often as necessary to keep the icebox at a set temperature. The temperature stays consistent throughout the entire box, and in some cases, these units can be installed by an average boat owner. They're often equipped with air-cooled condensers and come sealed and pre-charged with refrigerant. Another benefit that anyone who's fished a soggy, spoiled chicken breast out of the smelly water at the bottom of a simple icebox will appreciate: There's no melting ice and, as a result, no smelly water.
On the other hand, some of the smaller constant-cycle units aren't too powerful, and as you'd expect with something that's constantly turning on and off, their daily power consumption-which can be as high as 20-50 amp-hours-is significant. However, most boats these days are equipped with the robust battery capacity and efficient charging capability to keep up with the increased energy demands. And since these systems draw low power over time instead of requiring higher power for shorter durations, they can be easier on the batteries than a cold-plate system (described next).
These units work great on a boat in a marina or on a weekend cruise within close range of shore power. And they'll be up to the rigors of long-term, offshore cruising provided the boat has ample battery capacity. As with all electric-powered systems, running the engine will be required to keep the batteries charged, but in some cases, wind- and solar-powered chargers can come pretty close to keeping up with the energy demands of these units.
A Cold-Plate-Equipped, DC-Powered System
Instead of keeping the box cool by constantly cycling on and off, these refrigeration systems freeze a small tank-the cold plate-in the fridge itself. The unit runs until the cold plate is frozen or the fridge reaches a certain thermostat-controlled temperature; then it shuts off. The frozen plate then keeps things cold just like a block of ice.
The obvious benefit of this system is that it cycles on only when the cold plate thaws below a certain temperature, and only for as long as it takes to refreeze the plate. As a result, running these systems for 1 to 2 hours per day is often all you need to keep the batteries charged and the fridge cold.
