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Thursday, August 10, 2017

A Different Kind of Cold

While researching the various systems in my RV, I managed to find an installation manual for the refrigerator and it emphasized the differences between a typical household refrigerator and the kind you'll find in an RV. Those of you who have no interest in getting an RV may find some of those differences useful if you're looking for a refrigerator/freezer for a remote cabin or other “off-grid” living quarters.

Most RV refrigerators will run on either 120VAC power or propane (LP), while a few models like mine will add 12VDC as an option. It may sound strange, but they actually cool the inside of the refrigerator by heating up a “boiler” using a propane flame or an electrical heating element. The method of cooling is called “single pressure absorbtion refrigeration” (SPAR) instead of the conventional“vapor compression” method (VC)used in air conditioners and household refrigerators. Here are a few of the differences:

A conventional VC system uses a compressor/condenser unit to turn gaseous Freon (a generic name for a family of fluorocarbons) into a liquid, then pumps the liquid through an expansion valve which causes it to turn back into a vapor. The change from liquid to vapor is known as a “phase change” and requires quite a bit of heat to accomplish. The liquid gets that needed heat by pulling it out of the inside of the refrigerator, cooling the contents. The hot vapor is then routed back to the compressor, which further heats it as it compresses it back into a liquid, and finally to the condenser where the excess heat is exhausted to the outside air.

A SPAR unit uses two gasses (usually hydrogen and ammonia) and a liquid (water) that absorbs one of the gasses, all kept at roughly the same pressure. The ammonia, in liquid form, absorbs heat from the inside of the refrigerator and changes phase to a gas. That gas is absorbed by the water, which is then heated in a small “boiler” to drive it back out of solution. The gaseous ammonia condenses back to liquid form in a condenser and flows back to the heat exchanger inside the refrigerator. The hydrogen serves as a carrier for the ammonia and eliminates the need for a mechanical pump to move the liquid ammonia. SPAR systems have no moving parts, so they will last a long time. There is no compressor, and the condenser is a lot smaller, but in return efficiency takes a dive; SPAR units are only about a third as efficient as VC units, which is why SPAR systems are rare in houses today. 

My grandpa used to buy Servel refrigerators that ran on kerosene, back in the 1930s and 40s. I've heard that they would run on as little as a quart a day. You can still find them for sale on eBay and Craigslist, but they don't meet all of the current safety regulations and the safety nannies are trying to get them off of the market. Servel was the brand name used by Dometic, which is the major maker of SPAR systems in the world, for their household appliances. They still make them, but they are all propane-fired now. A full-sized refrigerator will burn about a pound of propane each day, so a grill-sized 20 pound cylinder would last about two weeks (they never fill cylinders more than 85% full [17 gallons] for safety reasons).

The VC system uses an electrically driven compressor, which requires a big surge of power to start. The SPAR system will have a control board and ignition coil that require electricity, but most of that is 12VDC. RV systems generally have a 120VAC heating coil to replace the propane burner when you are hooked up to the grid or running a generator, mine also has a 12VDC heating coil as an option, but it is not rated for the full cooling capacity and the manual advises using the DC option for no more than 6 hours. Electric heating elements are not efficient, and I avoid them wherever possible.

VC system have a compressor, which is a sealed unit with moving parts inside. There is no maintenance, when it dies it has to be replaced and that usually costs more than a new refrigerator would. Compressors can be noisy when they run, whereas a SPAR system is almost silent (unless you're using a generator to power it).

Cost can be an issue. Since SPAR units are targeted at the RV market, their production is nowhere near that of VC units. Economy of scale knocks the price of VC units down to half or a third of a comparable SPAR unit. SPAR units also tend to be smaller, somewhere between a dorm fridge and a house fridge. The smaller size is a plus for me, it means that the refrigerator will get cleaned out more often and there's less of a chance of leftovers getting lost in the back of a shelf.

If you're on a budget and still trying to put together a bug-out location or a bug-out vehicle with cold food storage, keep an eye out for people scrapping or parting out an older RV. It's hard to kill something with no moving parts, and replacement circuit boards are still being produced for a lot of the older models, so a used RV fridge might fill a niche if you have medication that needs to be kept cold or just want to enjoy a cold beer (or have ice cubes in your lemonade) as you watch the world fall apart around you.

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