Project: Refrigeration

GENERAL BACKGROUND:

It's odd to contemplate the common household refrigerator that runs year in and year out with little or no maintenance and then turn around and witness the myriad approaches taken and problems that cruisers sometimes have with refrigeration.  It's not like we're exploring some unknown technology, I mean this stuff is well understood indeed, but still refrigeration can be one of the more troublesome aspects of operating a cruising sailboat.

Of course, household refrigerators are commodity items, built by the millions on assembly lines to relatively standard designs ... and they're powered by a friendly electrical utility that's happy to provide as much power as you could ever want whenever you want it.  They use hermetically sealed compressors and soldered tubing and have no hoses to let refrigerant seep out or moisture permeate in and no rotating shaft seals where refrigerant can leak out.

Not so amongst cruisers, where every boat has a more-or-less unique installation.  You can see DC powered systems and AC powered systems and engine drive systems and water cooled systems and air cooled systems and a whole alphanumeric soup of refrigerants from which to choose.  Some use evaporators that run on and off all day long while others use holding plates that thaw at colder temperatures and are refrozen once a day.  Some are home-made, some are professionally-built, some are pre-packaged units that're plugged into a semi-insulated box.  And some folks just punt the whole thing and do without and maybe just use ice when they can get it.

All these variations just reinforce the preeminent need to consider the compromises required in designing and operating a cruising yachts' refrigeration system.  For example, if you want to fit a refrigerator and/or freezer into existing cabinet space, the outer dimensions are then a given.  Effective refrigeration relies on having good insulation to minimize the amount of heat that's transferred into the cold space, so the thicker insulation is, the better, everything else being equal.  But the thicker the insulation, the smaller the remaining useable refrigerator or freezer volume.  So whaddyawant, a super-insulated tiny bitty freezer that doesn't require much energy to keep cold or a large freezer with not much insulation that has relatively huge energy demands?  Or maybe you don't want much of a freezer at all but rather want to minimize energy consumption by having a box that'll keep things a bit cool to extend storage of milk, veggies and leftovers.  Consider this: freezer capacity is more important to blue-water cruisers who can spend extended periods away from supplies while, on the other hand, coastal cruisers have frequent access to supplies and might well want to stint on freezer capacity.

Within reasonable limits, the larger the refrigerator/freezer, the better.  Also, at least for freezers, the colder, the better, again within reasonable limits.   The downside to this is that the larger/colder the system is, the more energy is required to keep it operating satisfactorily.  If the system is to be evaporator-based, one that cycles on and off all day that is, it'll require a larger battery capacity and more time spent recharging the batteries (or a bunch of solar panels).  If a holding plate is the order of the day, the faster the pull-down (time to refreeze), the larger the compressor needs to be.  Ya pays your money and ya takes your choice.

DAYDREAMER'S REFRIGERATION:

Well, I'm here to tell you that we did pay some money - around US$ 5500 - and our choices resulted in a superb refrigeration system.  After some initial teething problems, the refrigeration system has been dead reliable, having run for 10 years now without a hitch, and its performance has been exceptional.  On the warmest day, it takes about 1 hour of run time to pull the freezer down to 7°F or so (-13 to -14°C).  It'll freeze two ice cube trays overnight, it'll freeze meat and keep it frozen rock-hard, and it even allowed us to enjoy some ice cream when we crossed the equator.  Now there's a true test of any yachtie's refrigeration system!

The design concept is pretty plain vanilla, based on having a refrigerator compartment separated from the freezer compartment by a barrier with a minimal amount of insulation.  A holding plate in the freezer maintains sub-freezing temperatures there and enough heat transfers through the barrier to keep the refrigerator at a temperature a bit above freezing.  The condenser is water-cooled to maintain performance when in the tropics.

The most notable aspect of Daydreamer's refrigeration is built-in redundancy, on several levels, and a high build quality.  Her refrigeration has two separate, identical refrigeration systems, hence the high cost.  There are two holding plates in the freezer and each holding plate has its own dedicated compressor, condenser, drier etc.  Each holding plate is capable of maintaining an acceptable level of cold by itself but will need to be run twice as often to keep it there.  The cost of her refrigeration was high because we basically installed it twice.

The only piece of equipment shared by the two systems is the condenser pump.  The compressors are identical, 110 volt, hermetically-sealed units so there are no shaft seals to leak.  There are no hoses that contain refrigerant and all tubing connections are silver soldered except for mechanical connections at the expansion valves, driers, and hi/lo pressure switches.  One of the holding plate systems even has an air cooled condenser in series with the water cooled condenser to keep the freezer cold when water is not available, such as when hauled out.

This design approach has a number of benefits beyond system redundancy.  For example, having two holding plates rather than one maximizes heat transfer surface area, which helps insure adequate cooling in the freezer compartment and utilization of all the eutectic material.

It also allows us to use smaller, half-sized compressors, which has additional benefits including the fact that it lets us have power source redundancy.  When away from shore power, our normal operational mode is to run the generator for 1-2 hours a day to charge batteries, pull down the refrigeration, and make some hot water.  The smaller compressors have a lower starting current surge, so starting one and then the other is much easier on the generator than starting a single twice-as-large compressor.  Also, when motoring, the alternator can put out enough power to run one compressor at a time, via the inverter, a sort-of pseudo engine drive.  And unlike true engine driven refrigeration, which requires that the engine be run every day, adding unwanted hours of engine run time, we can plug into shore power and leave the boat for extended periods, knowing that the temperature switches will keep the freezer and reefer cold.  We can even run for a few days from the house batteries via the inverter if we have to.

One other system back-up also comes to mind.  We installed this system during the time that Freon (R-12, R-14 etc.) was being phased out in favor of ozone-friendly refrigerants and decided to stay away from R-12 for both environmental and practical reasons, the later being increasingly scarce and costly R-12 if refills were needed on down the line.  We chose R-404a and carry enough spare refrigerant for multiple system recharges and a refrigeration gauge set to do it with.  (See DESIGN section for more on refrigerant choices). 

ALL THIS, AND AIR CONDITIONING TOO:

The standard process flow for refrigeration is almost identical to air conditioning.  Each system has refrigerant (e.g. Freon, R-134a, R-XXX or whatever) that flows from a compressor through a condenser, then through an expansion valve (or equivalent), through a "chiller", then back to the compressor.  The "chiller" is where heat is transferred from the place you're trying to cool down to the refrigerant.  In a refrigerator, the chiller is the evaporator or holding plate.  In an air conditioner, it's the refrigerant/air exchanger.

So if I wanted to contemplate having air conditioning on board Daydreamer, did I really have to consider having yet another whole set of this stuff on board?  Not really.  Maybe I could use the refrigeration system if I could figure out how to get the cold that happened downstream of the expansion valve out into the interior of the boat instead of the holding plate.  What we did was install a couple of solenoid valves downstream of the condenser that allowed the refrigerant to divert from the expansion valve/holding plate to a different expansion valve and a separate heat exchanger, which is now the "chiller" for the air conditioning system.  It's in the wrong place and too cold for direct exchange with the air, so a glycol/water mixture, which won't freeze, circulates through the other side of this heat exchanger, where it is chilled, then circulates on to a water/air exchanger in the aft stateroom, where it cools air being blown through the exchanger.  This exchanger is like the radiator in a car but instead of heating the air blown through it and cooling the radiator water, it cools the air and warms the glycol/water mixture, which then returns to the chiller.  Voila!  (All this times 2, of course.)  Full Disclosure:  I still haven't completed the hose connection to the to the blower or the condensers, nor wired the blower.  But I'm sure it'll work.

DESIGN:

I really, really want to suggest that anyone wanting a better understanding of boat refrigeration design and operation should get a copy of Nigel Calder's Refrigeration For Pleasure Boats, ISBN 0-87742-286-9.  Although I had years of experience with large industrial process refrigeration systems, I found this book to be invaluable.

When we purchased Daydreamer, she had a stainless-steel-lined compartment divided into refrigerator and freezer sections and a 12 volt Adler-Barbour air-cooled refrigeration system with an evaporator.  It worked pretty well in Alaska (well, big Whoop!) but the liner was off center in the cabinetry and the insulation on the aft side was pretty thin as a result.  Indeed, we could sometimes get condensation on the outside of the aft bulkhead.  It obviously wasn't gonna up to snuff when we got to the tropics.  Something needed to be done, but what?

At a minimum, we needed to pull the liner out and reinsulate with it properly centered.  Re-using the Adler-Barbour system in the tropics looked a bit iffy too, what with the air cooled condenser and all.  Maybe something completely new would be in order?  Time to run some design numbers and see what they said ... Nigel Calder to the rescue. 

Refrigeration for Pleasure Boats presents a set of example design calculations all the way from the size of the box and insulation thickness to condenser, holding plate, and compressor sizing.  Guided by the example in Mr. Calder's  book, used as a "go-by" as it were, it would have been quite possible for me to run through a custom design using just a pencil, paper, and calculator.  But what if I wanted to consider a different refrigerant, or different insulation?  Any change in any parameter would require I repeat the exercise.  This was gonna get real old if I wanted to consider a number of alternatives.

Computers to the rescue.  It turned out to be a fairly straightforward exercise to put the entire design calculation procedure on a spreadsheet.  Should you want to run your own design calculations, I have a copy of the Excell spreadsheet available for download HERE.

I contracted with Sea Freeze (Bellingham, Wa - no relationship here except as a satisfied customer) to put all the bits and pieces together based on these design calculations.  Discussions with the folks there led to the notion of the twin systems, to provide both reduce starting current surge and redundancy, as well as the idea of the air conditioner adaptation.  After we had reached agreement on the scope of work, they proceeded with fabricating the compressor skids, condenser assemblies, holding plates, and pressure switch/wiring assemblies.

I proceeded on with demolition, first removing the counter top and Adler-Barbour refrigeration system, then pulling out the stainless steel liner and all the old insulation.  Feh!

I purchased 4 inch thick polyurethane sheet material and a two-component foam kit from a local vendor.  (Two-part foam is available in several forms, one comes as two separate liquids that are mixed together in some kind of container and immediately poured into place where they expand to fill then often overflow the available space.  The second form, which I used, is two pressurized cylinders, where the two liquids flow through hoses to a nozzle, where they're mixed and sprayed into or onto their destination.)  I also purchased two vacuum panel from R-Parts to cover the forward and aft faces of the freezer section.

Reconstruction started with lining the interior of the cabinetry using a single sheet of heavy plastic to keep any moisture from getting into and condensing in any part of the new insulation.  Two staggered layers of the 4 inch sheet foam in the bottom of the cabinet and one layer each on the inner face and forward and aft faces of the refrigerator section of the cabinet and I was ready to slip the liner back into place, having first attached vacuum panels to the fore and aft face of the freezer section.  Two sets of copper tubing runs for twin-system refrigerant supply and return lines were run from the freezer section of the liner to the space where the compressors and condensers were going to be installed.  Next, using a bit of hose attached to the mixing nozzle, I proceeded to fill the space between the outboard face of the liner and the hull (approx 10 inches wide) as well as the remaining space forward and aft of the freezer, which also foamed into place the vacuum panels.  After trimming off the excess two-part foam, I was able to bring the extra plastic sheeting over the top of the insulation and seal it to the liner.

When the holding plates were finished, they were installed in the freezer section of the liner and the expansion valves and refrigerant supply and return lines connected.  The holding plates were filled with a mixture of water and isopropanol (rubbing alcohol) ... a bit of a discussion on this choice might be in order here.  Typically, holding plates are filled with either a mixture of water and a chloride salt (sodium, calcium or magnesium) as a eutectic solution or with an antifreeze mixture of a glycol and water.  The eutectics do a better job of holding a constant temperature as they thaw and also allow for a smaller holding plate but have the unfortunate side effect of being highly corrosive.  Glycol mixtures over time tend to separate and have areas that wind up high or low in glycol concentration; they can be somewhat toxic, depending on the glycol used.  Alcohols also depress freezing point and act much like the glycols but alcohol and water are both highly mutually solvent and I hoped that property would prevent the gravity segregation seen with the glycols.  It seems to have worked.  Also, the freezing point of the anti-freeze/eutectic mix is a direct function of the concentration of the additives in the water.  I designed the holding plates with fill tubes that extend to the bottom of the plate and a separate top-mounted vent connection to allow the solution to be changed out easily with the holding plate left in place by simply sucking the old mixture out thru the fill line (or blowing it out by pressuring the vent connection),

I replaced the counter top and proceeded to design and assemble the control panel and ran power and control wiring through the boat.  Installation of the compressors, condensers, and pressure switch panels was straightforward.  A bit of a vacuum to get any moisture out, charge the systems with R-404a, and we had refrigeration!

TEETHING PROBLEMS:

All seemed good as we headed down the coast of Washington and Oregon in August, 1995 but we began to notice that one of the plates began to exhibit poor performance.  A check of the sight glass showed that it had lost refrigerant so we had that system topped up in San Francisco and checked all connections for leaks but found nothing.  On to San Diego and the problem started to surface again.  At this point we began to worry a bit, so we arranged another recharge.  The service people there didn't have any R-404a handy and didn't want to buy a full cylinder just to recharge us, so we went ahead and bought a full cylinder.  This time we found a very small leak on a mechanical connection on one of the pressure switches.  A bit a tightening with a wrench took care of that problem; the full cylinder must be working like any good magic amulet as we haven't had any refrigerant leaks since then.

By Spring, 1996, we had been having a lot of problems with our generator and had often been operating the refrigeration using our inverter or from shore power in various Mexican marinas.  Then, one day in May, one of the compressors wouldn't start.  We soldiered on with only one system operating and were patting ourselves on the backs for having redundant systems when, two to three weeks later, the second compressor refused to start.  The failures were so close in time that we were concerned that there was some kind of design flaw, something bad about the operating environment, but we didn't really have a clue.  It might have been high voltage spikes from switching in modified sine wave inverter we'd been using - could that have penetrated the electrical insulation allowing power to leak off to ground?

Fortunately, we were only days away from putting Daydreamer on the hard in San Carlos so we could drive back to Alaska for the summer.  We routed through Bellingham and picked up a couple of replacement compressors.  The folks at Sea Freeze told us then that the compressor manufacturer had recently changed the electrical insulation on their R-404a compressors; there was a suspicion that the original insulation hadn't been totally compatible with the new refrigerant.

That must've been it, the replacement compressors were installed when we returned to the boat that next autumn and they haven't missed a lick since.

OPERATING EXPERIENCE:

In warm locations with temperatures into the 90's F (30's C), such as the Australian summer and winter in the tropics, we will normally run the each holding plate every day for an hour or so.  In the Australian winter, with highs in the 40-50°F range (10°C or so), we can often run 2-3 days between pull-downs.  The control panel allows us the option of automatic or manual operation, so we'll normally just switch to "Auto" and let the thermostatic switches take care of turning off the plates when they're cold enough, then turn the switches to "off" until it's time to pull-down again.  The settings on the thermo switches are slightly offset, so when we're operating in a load management situation, we'll just run the one with the higher shutoff temp first.