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Propane fridge: flame in its heart

by Ray Ford

The propane fridge is more than a place to stow last weekend’s leftovers until they start growing things. It’s a near-magical synthesis of seemingly contradictory elements, one that uses a flame to keep food cold, and accomplishes the feat without the aid of moving parts. Inside that box lurks an epic tale of passion and betrayal, fire and ice, ammonia and propane. And you thought it was just a way to keep the beer cold.

The fridge does all this thanks to ammonia, which absorbs heat as it evaporates in a process familiar to anyone who has ever spilled a drink or put rubbing alcohol on a scratch. When alcohol evaporates, it changes from a liquid to a gas, using the surrounding heat – including the heat from your own skin – to fuel the transformation. Ammonia does the same thing, but at a much lower temperature.

The secret to propane refrigeration is to keep the ammonia constantly on the move, changing from a warm gas to a cool liquid, and back to a vapour again that sucks heat from the food compartment. The system does this by placing ammonia under pressure with water and hydrogen. Together these elements act out a love triangle – a love ’em and leave ’em tale of three chemicals and a heat source.
The propane burner gets the relationship cooking, heating a mixture of ammonia and water and producing bubbles of ammonia gas. The gas rises to the top of the fridge through the vapour pipe, shedding a weak ammonia/water mixture and turning its back on its former partner. The water condenses back into a liquid and slinks back to the boiler.

Meanwhile, the ammonia cools and changes from a gas to a liquid, releasing heat as it flows through the condenser at the top of the fridge. Suitably chilled, the liquid ammonia wanders into the evaporator, the singles bar of the fridge, and sees hydrogen – tall, dark, and gaseous – over by the pool table. It’s love at first sight. Ammonia gets all weak-kneed, its pressure drops, and it evaporates again, drawing warmth from inside the cabinet and keeping the butter cold. Ammonia and hydrogen tumble through the coils inside the fridge, and just as the happy couple look ready to settle down in the absorber, along comes a new suitor, the weak ammonia/water solution on the rebound from the boiler. The ammonia vapour clings to the newcomer, cuts hydrogen loose, and the lighter hydrogen gas saunters back to the evaporator.

The ammonia is reunited with water and flows on to the boiler, where it gets all hot and bothered and repeats the process. Since the cooling system is a sealed unit, your only job is to ensure the boiler gets the heat it needs to keep the ammonia moving. Too little heat and the fridge won’t be cold enough. Too much heat overworks the boiler, throwing off the balance between ammonia gas and liquid, and cutting short the fridge’s life.

Worse, a malfunctioning burner could trigger a buildup of carbon monoxide, cutting short your life. Paul Petersen, president of Unique Gas Products and a Kawartha Lakes cottager, recommends cleaning and adjusting the burner, orifice, and flue tube, before start-up in the spring, as well as every time the fridge is relocated. (To ensure you don’t void your insurance, the burner should be serviced by a licensed gas technician.) You should also ensure the gas regulator on your propane tank is set to the manufacturer’s specifications to get the proper flame. Because the system relies on gravity to circulate the fluids, the fridge should be level, and there should be enough ventilation to aid the condenser’s cooling efforts.

While many older fridges were made to vent inside the cottage, new fridges in Ontario must be installed with either a CO2 alarm that shuts down the burner or an outside venting system. Either way, proper maintenance ensures a safe flame, and keeps your refrigeration soap opera, As the Beer Cools, playing on hot summer weekends for years.

 

Published in the July/August 2002 issue of Cottage Life magazine.
Copyright © 2002 by Cottage Life. All rights reserved. Reproduction of any article, photograph, or artwork, for other than personal use, in whole or in part, without the written permission of the publisher is strictly forbidden.