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Good Sam Highways Magazine (August-September 2012 Issue) has some information about gas absorption refrigerators on page 28 with Gary Bunzer, the RV Doctor. For those that don’t have access to this publication, I’ve summarized some of the content below and added my own thoughts.
Cooling units consist of the following four major components:
1) Boiler (heated by electric or propane)
2) Condenser (finned tube at highest point in the cooling unit)
3) Evaporators (what cools freezer/refrigerator compartments)
4) Absorber (coils and vessel visible from access panel)
Contained within the tubing system are water, liquid ammonia, ammonia gas, hydrogen and sodium chromate. Sodium chromate is a rust inhibitor which lines the inside of the steel piping to protect it from the corrosive effects of ammonia. It doesn’t play a part in the refrigeration cycle, however, but it can become a problem if it crystalizes.
The refrigeration cycle starts when heat (electric heaters or propane) is applied to the boiler (long vertical tube shielded with insulation). Water and liquid ammonia begin to boil and ammonia vapor rises up into the condenser. Water reaching the condenser condenses and is returned back to the boiler.
Ammonia vapor also condenses and is combined with hydrogen gas in the low temperature evaporator (located in freezer). Heat is absorbed through the evaporator piping during this process which creates the freezing cold (0-5F).
Next, the mixed hydrogen vapor and liquid ammonia pass through the high temperature evaporator (located in refrigerator compartment). More heat is removed at this point but at a higher temperature. This cools but doesn’t freeze. The weakened ammonia liquid flows through the absorber coils where the hydrogen gas rises back to the evaporator sections. Liquid ammonia mixes with water in the absorber vessel where it is held until flowing into the boiler for the next cycle to begin. It’s actually a continuous process driven by the application of heat.
Gravity plays an important part in moving fluids downward from the condenser to the absorber vessel. Off level operation impedes the flow and causes overheating of the boiler section. Continued off level operation in an overheated state crystallizes the sodium chromate which blocks the internal tubing. Blockages result in further heating, damage, piping failure and potential fires. Once sodium chromate crystallizes, the damage to the internal piping is permanent. An exceedingly hot absorber vessel is an early warning sign of excess heat build up. Our Pines RV Refrigeration cooling unit absorber vessel can always be touched without discomfort to the skin.
Out of level operation pertains to stationary vehicles only. Driving creates enough agitation to move fluids through an off level cooling unit. I personally would avoid running your refrigerator while parking in off level spots say during a lunch stop. Be extra safe and turn off your refrigerator during these times. Service stops are another potential problem if your vehicle is jacked up out of level. Be careful about this. The “take away” is to make sure you are perfectly level when operating a gas absorption refrigerator.
Another failure is condensation in the cooling pack that rusts out the evaporator piping. Cooling units see pressures in and around 350 PSI so anything less than perfect piping can develop a leak. An ammonia smell means a leak and should be a call for an immediate shut down of the refrigerator. This failure doesn’t create excessive heating and won’t present the same fire hazard risks as an overheated boiler.
Cooling units can be rebuilt or re-manufactured and depending on who does the work, results are mixed in my opinion. I think it’s best to replace a defective cooling unit with a brand new one. Buy from a company that is in touch with quality, safety and their customer base. Also make sure the company stands behind its products. In short, do your homework before the purchase. A good new aftermarket cooling unit for a full sized refrigerator will cost $1K+.
Inspect your cooling unit often and make sure everything is tight and securely fastened. Norcold committed another boner on our refrigerator by not fastening the condenser assembly properly. One of the screws pulled out leaving the piping to swing back and forth. This was found by accident when I needed to remove the refrigerator roof vent cover to run wires.
It’s my opinion that gas absorption cooling units can be used safely providing the manufacturer puts in the required quality. Norcold dropped the ball on this one which has caused untold amounts of damage, loss and hardship. DON’T BUY JUNK!! Stay in touch with your cooling unit’s operation and install additional safety devices if you feel the need to. And remember that safety is no accident.