There has long been a debate about air cooling versus water cooling for marine refrigeration systems. Water cooling has traditionally been generally accepted to be between 25% and 35% more efficient in warm/hot ambient conditions, but some of that efficiency gain has to be offset by the energy required to run a water pump.
The Frigoboat Keel Cooled system is water cooled and does a fantastic job without the need for a fan or a pump. That will be mentioned later, but the following compares air cooled systems to traditional water cooled systems using sea water with a pump.
Many boaters these days seem to be opting for air cooled refrigeration systems even though they’re heading south into hot climates, and despite the notion that air cooled is inherently less efficient than water cooled. Why is that?
First let’s compare the basics.
1. Water conducts heat 24 times better than air.
2. Water is 825 times more dense than air.
3. You’d need a room-full of air to do the same job as a couple of pints of water.
In fact, the fan in an air cooled refrigeration unit has to move over 1,000 times more air than the water pump does water in order to do the same job. But even after all that effort, can air cooling really produce the same results?
Let’s just ponder a minute here to consider the objective. We’re trying to decide whether to use either air or water in a heat exchanger called a condenser to cool down hot refrigerant gas and condense it into liquid.
Ideally we want;
(a) for 100% of the gas to be condensed into liquid, and;
(b) for it also to be cooled to the same temperature as the cooling medium.
Keep in mind that the highest rate of heat transfer occurs when the temperature difference between two substances is greatest, and conversely, the closer the temperatures of the two items, the less heat exchange takes place.
When using sea water we can count on typical maximum water temperatures being in the mid to high 80°F’s, maybe with an odd 90°+F here and there if you’re near a nuclear power station cooling outflow. (You’ll know because you’ll be catching fish with two heads.) Condenser location and installation are immaterial, so there will be little to no variation in performance between a water cooled condenser situated in a cool bilge, or one in a hot engine room with the engine running, because the water temperature will be the same.
In contrast, ambient temperatures at an air cooled condenser will vary widely between one installed on a yacht that’s air conditioned 24/7, and one in a stinkin’ hot cockpit locker at well over 100°F with little to no ventilation. Performance and efficiency between these two installations will vary widely, and heaven help an air cooled unit in an engine room with the motor revving its nuts off.
With a water cooled system, it is most likely that the first requirement of our condenser will be met, i.e. that all of the refrigerant gas will be condensed into liquid. It’s also quite possible that the temperature of the liquid refrigerant will be lowered to be close to that of the sea water, or be only slightly warmer.
But in an air cooled system, if the air temperature is high enough that the difference between it and the refrigerant gas is too small to allow adequate heat transfer to take place, two things can result.
First, not all of the gas will be condensed into liquid resulting in pockets of flash gas heading off to the evaporator together with the liquid (and you thought Flash Gas was a college dorm prank ....) This results in a lack of efficiency, higher evaporator temperatures, and longer compressor run times.
Secondly, the mix of liquid refrigerant and flash gas will be at an elevated temperature, and all this sensible heat has to be removed from the refrigerant before the latent heat can get to work in the evaporator, causing even more work for the system to do.
So what can be done to ensure adequate performance of an air cooled condenser in high ambient heat conditions?
The secret is to start with a well designed air condenser such as found on Frigoboat’s Capri and Paris models. These have two rows of a copper tube serpentine with closely spaced unpainted aluminum fins to provide the maximum possible surface area for efficient heat transfer.
The shroud surrounding the condenser helps maintain even air flow over all cooling surfaces, as does the draw-though (sucking) fan mounted away from the condenser that ensures that no one area gets favored and the rest starved. Even in the hottest of conditions, this design will ensure the least possibility of the onset of the dreaded flash gas, and the liquid refrigerant will be delivered to the evaporator at a reasonable temperature.
Having the best possible design of condenser will make a system as efficient as it can be for an air cooled system, but it can in no way be as efficient as a water cooled model operating under the same conditions, even factoring in the power draw for the pump, unless things are taken to extremes. For example, to try to get even close in performance to a water cooled system in really hot ambient conditions, the air cooled condenser would have to be a massive structure, with a huge, noisy, power-sucking fan whose exhaust could possibly be harnessed for jet-propulsion.
I believe one reason that air cooled refrigeration is being seriously considered for boats heading south these days is that the technology of many boaty-bits has advanced so much in the last couple of decades that the efficiency deficit to water cooled models may not be such a big issue any more. These days we have very efficient devices such as variable speed refrigeration compressors; high-power quick-charging batteries; LED lights, wind, solar and hydro chargers, etc., etc, and so maximum efficiency from the refrigeration system may not be as high a priority as it once was.
Back in the dark ages of the 70’s through early 90’s it would be a huge undertaking for the refrigeration systems of the day just to get an insulated box down to beer cooling temperature. That’s why there was so much focus back then on getting as much insulation around the box as possible. Jumping back to the present, today’s uber- efficient refrigeration systems should have no problem getting boxes with standard insulation thickness down to temperature, and adding one or two inches of extra insulation will typically reap savings of just a few amp/hours a day. Unless the box is seriously under-insulated, adding insulation is generally deemed to be not worth the effort and the resultant loss of space.
So it’s a conundrum. Air cooled plus-points include ease of installation, lower pricing, lack of pump draw, plus freedom from clogged strainers and lack of pump cleaning/repair/replacement. Balance that against such nuisance factors as fan noise and additional heat in the boat, together with lower efficiency and higher overall power draw compared to water cooling.
Now, if you choose to go air cooled and are smart enough to install a high-efficiency Frigoboat system, either for a quick and easy install or if you just don’t feel the need for water cooling right now, you always have the option to add a Keel Cooler later on. This can be accomplished with just a few simple tools and with no messing with the refrigerant, and you’ll then have all the benefits of water cooling but without a pump. The fan can be switched off but ready to be re-activated if you get hauled out or end up sitting high and dry on the bricks waiting for the next tide.