Boat­works

The time has come when the prospect of cold drinks and long-term food stor­age has you think­ing about up­grad­ing your ice­box to DC-pow­ered re­frig­er­a­tion. Dun­can Kent has been there and done that, and has some ad­vice

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Ev­ery­thing you need to know be­fore up­grad­ing or in­stalling a fridge on­board

Fresh food must be kept at a re­frig­er­ated tem­per­a­ture of 40 de­grees F to stay edi­ble for a week, whereas a freezer needs to be able to re­tain a con­stant 0 de­grees F. The suc­cess­ful op­er­a­tion of any re­frig­er­a­tor is al­most en­tirely de­pen­dent on the ef­fi­ciency of the in­su­la­tion sur­round­ing the ice­box. In­su­la­tion that is too thin or of the wrong ma­te­rial can dou­ble its op­er­at­ing time and hence its power con­sump­tion.

If you are plan­ning a new in­stal­la­tion and cal­cu­lat­ing the phys­i­cal di­men­sions of the box, you must al­low for the thick­ness of in­su­la­tion re­quired to con­struct a ther­mally ef­fi­cient unit. If you al­ready have an un­re­frig­er­ated ice­box that you plan to con­vert, check the thick­ness and type of in­su­la­tion first, as it will al­most al­ways re­quire up­grad­ing. The min­i­mum in­su­la­tion thick­nesses rec­om­mended for the North­ern Hemi­sphere are: ( fridge/ freezer) 2in/ 3in ( sides), 3in/ 4in (bot­tom) and 1in/2in (lid). For trop­i­cal cli-

mates, add an­other 1-2in all round.

Pro­pri­etary in­su­lated boxes, as stocked by nu­mer­ous spe­cial­ist agents and chan­dlers, will of­ten fit your avail­able space with a few mi­nor mod­i­fi­ca­tions. How­ever, many do not meet the rec­om­mended in­su­la­tion stan­dards, as most are de­signed for use in trucks where power con­sump­tion is not an is­sue, so you’ll of­ten need to fix ad­di­tional in­su­la­tion around the out­side of the box.

You can also make your own liner by con­struct­ing a mold, lam­i­nat­ing it in fiber­glass and then coat­ing the in­side with gel­coat or paint. It’s not a dif­fi­cult project and can be a lot of fun. The WEST Sys­tem web­site is a great re­source if you want to try this.

An es­pe­cially easy route may be to use one of the re­frig­er­a­tor boxes made by com­pa­nies like Isotherm, where the fridge box and com­pres­sor are com­bined in a sin­gle unit. They are avail­able in a num­ber of shapes and sizes, and are in­creas­ingly used by pro­duc­tion boat­builders.

COOL­ING UNITS

Your choice of cooler will make the dif­fer­ence be­tween a “vaguely cool” and “prop­erly cold” fridge. The two main meth­ods of cool­ing are thermo-elec­tric (aka: Peltier-ef­fect, as found in many bud­get por­ta­ble cool­ers) and the more ef­fec­tive com­pres­sor-driven units.

Peltier-ef­fect fridges work by fan­ning air over a solid-state thermo-cou­ple. They are com­pact and in­ex­pen­sive, but won’t cool food suf­fi­ciently in hot cli­mates and can only be used with ice­boxes up to 1.25ft³ ca­pac­ity. They are fine for short pe­ri­ods if you load them with prechilled food and/or throw in a block of ice. How­ever, they will con­sume a con­sid­er­able amount of power if the am­bi­ent tem­per­a­ture is high and food at room-tem­per­a­ture is loaded into them, mak­ing them less than per­fect in many sit­u­a­tions.

A proper marine fridge re­quires a com­pres­sor-driven cool­ing unit, as with a do­mes­tic fridge, where an in­ert gas is cy­cled through an evap­o­ra­tor plate in­side the fridge, col­lect­ing heat that is then re­leased via a con­denser out­side the fridge. (Once the heat is dis­persed, the coolant is pumped back to the evap­o­ra­tor, af­ter which the process is re­peated.) Con­densers are com­monly air-cooled, so in a closed en­vi­ron­ment such as a boat it is vi­tal to en­sure it is adequately ven­ti­lated or it can lose up to 50 per­cent of its ef­fi­ciency.

Boats in south­ern climes or trav­el­ling to the trop­ics may also want to con­sider fit­ting a wa­ter-cooled fridge. Th­ese most com­monly uti­lize a hull-mounted, sin­tered-bronze con­dens­ing heat ex­changer (aka: keel plate), which can be at least 30 per­cent more ef­fi­cient than air-cool­ing. Al­ter­na­tively, the Isotherm SP sys­tem comes with a spe­cial gal­ley sink drain skin fit­ting with an in­te­gral cool­ing ma­trix for the fridge.

Th­ese days the com­po­nents of a com­pres­sor-driven cool­ing sys­tem can be bought in kit form. They come with pre-gassed pipes sport­ing sealed end-valves, al­low­ing them to be used as soon as the pipes are con­nected to the var­i­ous other bits.

When you’re plan­ning your sys­tem it is also im­por­tant to know that the greater the sur­face area of the evap­o­ra­tor plate, the more heat it can ab­sorb and the quicker it will cool the box down, so chose the largest size that will fit the box. The plate can also be bent dur­ing man­u­fac­ture, which might al­low you to cover more than one side.

SAV­ING EN­ERGY

Fridge com­pres­sors driven from a 12-volv DC sup­ply usu­ally draw

be­tween 3.5 and 5.0 amps while op­er­at­ing (usu­ally around 20 min­utes ev­ery hour in mod­er­ate cli­mates), so a 48W/12V fridge can con­sume around 32 amp hours (Ah) of bat­tery ca­pac­ity (or 40Ah in­clud­ing bat­tery in­ef­fi­cien­cies) over a 24-hour pe­riod. Most new mod­els in­cor­po­rate a low-bat­tery cutout switch, which will dis­con­nect the fridge should the bat­tery volt­age drop be­low 11.5 volts. Many can also switch au­ti­mat­i­cally be­tween 110V AC and 12/24V DC to help pre­serve your bat­ter­ies when you’re hooked up to shore power.

Though th­ese are not so com­mon on sail­ing boats any­more, you can also in­stall an en­gine-driven com­pres­sor, pow­ered by a belt from the crank­shaft, to bring the fridge tem­per­a­ture down rapidly. Al­though th­ese ide­ally re­quire the en­gine to be run for one or two hours a day, the ad­di­tion of a hold­ing (eu­tec­tic) plate (such as the CoolBlue sys­tem by Tech­nau­tics), or a thermo-elec­tric cooler will al­low it to re­tain the cold for the long pe­ri­ods in be­tween. This can be es­pe­cially use­ful at night, when a noisy com­pres­sor would oth­er­wise keep you awake.

Hold­ing plates work by us­ing a fluid that freezes at a tem­per­a­ture above that of wa­ter. The coolant runs through a ma­trix within the plate, freez­ing the fluid and turn­ing it into a “cold reser­voir,” much like a reg­u­lar freezer block.

The down­side with en­gine-driven com­pres­sors is that they’re not as con­trol­lable as a purely elec­tric fridge and the tem­per­a­ture in the box tends to vary from very cold to cool be­tween en­gine boosts. How­ever, most new elec­tric fridge com­pres­sors, such as the Isotherm ASU and SX cool­ers, also in­cor­po­rate a “Smart En­ergy” volt­age sen­sor that au­to­mat­i­cally switches the com­pres­sor pump to full speed whenecer it de­tects the en­gine run­ning or the al­ter­na­tor charg­ing. When the en­gine is switched off it then drops back down to half speed, which in turns halves the power con­sump­tion. This sys­tem works ex­cep­tion­ally well when com­bined with a hold­ing plate.

Of course, cold air sinks, so top-open­ing, rather than front-open­ing fridges don’t lose nearly as much cold air when opened—the ex­cep­tion be­ing the very lat­est “drawer style” front-open­ing fridges (Dometic and Vitrifrigo for in­stance), where the food is stored in a sealed drawer, as in a do­mes­tic freezer, to stop the cold air spilling out ev­ery time the door is opened.

A fridge also needs a drain, but a sim­ple open hole at the bot­tom will al­low the cold air to quickly seep out. To pre­vent this, drains should have a pump, tap, plug or “U” bend filled with wa­ter.

AL­TER­NA­TIVES

Like many “marine” items, DC fridges are ex­pen­sive when com­pared to their do­mes­tic AC coun­ter­parts. In fact, some opt to in­stall the lat­ter on board us­ing a DC-AC in­verter, de­spite the in­her­ent in­ef­fi­cien­cies of such a method. Do­mes­tic fridges, how­ever, are not de­signed to op­er­ate when heeled over and can run in­ef­fi­ciently or even fail com­pletely if pow­ered up when sail­ing. If you only day­sail and switch your fridge on once you’re set­tled at an­chor or in a ma­rina, you might just get away with it, but you’d have to re­strict the num­ber of times you open it dur­ing the day—par­tic­u­larly if it is front-open­ing with no draw­ers.

Mo­torhome and RV fridges are cheaper and can of­ten run on AC, DC or some­times even propane. But once again, th­ese don’t like work­ing on an in­cline and us­ing them on propane would re­quire an ex­ter­nal flue. Also, their in­su­la­tion is usu­ally in­ad­e­quate.

Por­ta­ble ice­boxes, ei­ther thermo-elec­tric or com­pres­sor­driven, are pop­u­lar too and usu­ally come with both 110V AC and 12V DC leads. Ideal for week­ends or short trips, they can be filled with cold food from home, kept cool on the jour­ney via your car’s 12V aux­il­iary power socket and then plugged into the boat’s AC/DC sys­tem when aboard. They do, how­ever, con­sume a lot of power (5 to 8 amp[s) when on DC con­tin­u­ously, es­pe­cially mod­els with­out a ther­mo­stat. s

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