Understanding RV Refrigerators
Absorptive & Compressor Fridges

Understanding RV refrigerators is crucial not only when you buy a new one, but also necessary to operate them safely so they can serve you well and last long.

These days it will be difficult to imagine RV journey without an “on-board” refrigerator. 

It is not only for the pleasure of having your afternoon bottle of cold beer or coke but first of all, to prevent your food from being spoiled.

Having say these obvious reasons let’s see what solutions for RV refrigeration are available on the market, what are their pros and cons and how to make the optimal choice that will suit your personal needs.

Gas Absorption Refrigerators

Almost all RV refrigerators are using the same principle of operation: gas absorption.  As a refrigerant they commonly use ammonia along with water and hydrogen.  Absorption refrigerating systems have many advantages – they are small, operate quietly, do not have moving parts and are inexpensive.  On top of that they can run on either electricity or any other source of heat like natural gas, LP gas, kerosene etc…  making such system very versatile. 

Summarizing – being energy efficient, versatile and “theoretically robust” (thanks to simplicity of construction) absorption refrigerators dominate the RV world.

For most of us it may be surprising that the cooling cycle in absorptive refrigerators starts with heating (see basis of operation below). Indeed, the essential element of any absorption refrigerator is a heater.

Depending on the source of the heat there are two different types of absorption refrigerators:

a)    LP-gas-run refrigerators.
In these units the necessary heat is provided by LP burner fed from your camper’s bottle used for kitchen stove/heating etc…

b)    Electricity-run refrigerators.
In these units the heat is provided by an electric heater (normally 110V AC).

Usually RV refrigerators are designed as “2-way” units being able to run either on LP gas (on the road and in rustic camping parks) or on 110V AC (whenever available). You can select the operation mode manually, or it can be done automatically by refrigerator’s controller. They are programed to detect the presence of 110V AC and then automatically switch to electricity as the preferred source of operation. Similarly, when the electricity is disconnected, the controller automatically turns-on the gas burner to keep the continuity of the cooling process.

You will also find so-called “3-way” RV refrigerators. The main difference - they have two electric heaters.  While one will run on 110V AC as in the classic “2-way” fridge, the second one is designed for 12V DC operation directly from your battery.

3-way refrigerator has some merits – you can efficiently keep your fridge running when you are on the road. Your battery is constantly being charged by your car’s engine (alternator) so you do not have to worry that it may get discharged.  And the clear bonus is the security as you can close LP-gas valve in your camper. Obviously the “shaking” environment when on the road (especially in off-road” conditions) is always a bit risky!

Please note that even the LP-gas refrigerator still needs electricity (usually 12V DC from the battery). Electricity is necessary to operate the control system (electronics) and to power the interior light (LED in modern units).

To make it clear - 2-way refrigerator in Auto Mode fed by 110V AC must will need 12VDC to automatically switch to the LP-gas mode. If 12V DC isw missing (connection to the battery) then the refrigerator will stay off with known impact on your food!

How an absorption process works on a propane fridge

Now let's summarize this video:

Simplified Theory of Operation (just to give you some idea):

1.    Heat is applied to Generator (either from the gas burner or the electric heater). The solution of water and ammonia starts boiling releasing ammonia gas.

2.    Ammonia gas flows upward into Condenser where it releases heat and converts to the “pure” liquid ammonia.  Condenser is a sort of radiator located externally at the back of the fridge (similarly like in household fridges);

3.    Liquid ammonia flows down into Evaporator. It mixes there with hydrogen gas and then starts evaporating.  Evaporation is an endothermic process absorbing heat and as the result cooling “surroundings”. In the refrigerator, the Evaporator makes cooling coils inside of the “cold-box”!

4.    Ammonia and hydrogen gases flow to the Absorber containing water, where ammonia mixes with the water creating water solution, while released hydrogen gas flows up to the Evaporator.

5.    Ammonia-water solution flows back to the Generator where it is heated…..

The whole cycle is continuously repeating as long as the external heat is applied to the Generator.  The refrigerant “medium” flow and separation is ensured by convection and gravity. The tubing system containing refrigerant (ammonia, water and hydrogen) is hermetically sealed to prevent any leaks and to make it “maintenance free”.

Graphical presentation of how works an ammonia absorptive refrigerator.

Source: Christopher D. Bryant, Bryant Services, LLC

http://bryantrv.com/reefer.html

On top of simplicity and low cost, the main advantage of absorptive refrigerators is their low power consumption (electricity and gas). And it really counts when you are not connected to the AC power! Given the fact that most RVs have gas system already installed (kitchen stove), it is not surprising that absorption refrigerators are the most popular choice.Most of them in normal weather conditions consume modest 12-20g of LP gas per hour. To keep this number at the low end, try to place your camper in a shade, or at least keep the camper wall on the fridge’s side in the shade.

In theory, due to such simple construction and lack of moving parts absorption refrigerators should be reliable, safe, easy to maintain in operation and long lasting.

Unfortunately it is not the case, first of all ammonia is known for its toxicity.

Although leaks are relatively easy to spot due to the characteristic odor, it does not make everybody feel comfortable. 

Adding “salt to the injury” – hydrogen gas is explosive!  But that is not the end of problems – ammonia based refrigerators may experience several problems like overheating, disruption of the cooling process etc…

Bottom line – suddenly you may find yourself without the working fridge in the middle of the hot summer. Needless to say, that since long we lost our natural ability to “survive” in such situation.

Some manufacturers instead of hydrogen use helium gas (see Atwood on RV Refrigerator Brands). Helium-based absorption refrigerators are not only safer but also more reliable. But nothing comes free. Unfortunately, the cooling range of helium-based absorptive refrigerators is narrower compared to those using hydrogen. As the result, they are not able to bring fridge’s interior temperature down in hot ambient environment as well as hydrogen-based units (not mentioning compressor-driven ones). Frankly, it is not really that bad and in normal circumstances and no direct exposure to sun, you will not see difference. Big bonus is the safety of helium!

The idea of not having moving parts in absorptive RV refrigerators excludes the fans. Unfortunately this does not add much to the overall reliability, but certainly worsens interior temperature distribution.  Under such circumstances the fridge’s interior is cooled by convection instead of by circulation like in household-type fridges. And frankly, especially if the shelves are filled with food, convection does not work well (if at all). To solve this problem on your own, you can add small battery-operated fan inside of the fridge.

Short presentation by Dometic (company that in 1923 patented the absorptive cooling technology)

In order to guarantee proper operation of absorption refrigerators you have to pay attention to few details:

a)    When ran on a gas burner, you have to make sure that the burner is clean to be able to “deliver” required amount of heat. Other obvious conditions are: sufficient gas pressure and access of oxygen (air).

b)    The back of the fridge is cooled by natural air-convection. This cooling is essential for the whole process to work. You have to make sure that the space behind the fridge is clear and vent on the roof not clogged. What may help (especially in hot days) is an extra fan to improve the heat dissipation by the external radiator (Condenser);

c)    At higher altitude (typically above 5500 ft) both – the burner efficiency and the external convection cooling will be lower.  That is why at higher altitudes manufacturers usually suggest using AC or DC power to run 2/3-way refrigerators.

d)    Although absorptive refrigerators are more tolerant to leveling, too much of inclination may disrupt the cooling process.  Usually they can operate quite well within few degrees off-level in any direction (some manufacturers claim that their fridges can work up to 30 degrees off-level). Regardless these claims, try to level your camper “reasonably” well. As a practical rule, you may assume that if the RV level is good enough for you to live comfortably in the camper it will be also good for the RV absorptive fridge.

Summarizing - none of the absorptive refrigerators can compete with household (compressor-type) refrigerators in terms of cooling capabilities and surprisingly with their reliability, despite the fact that the latter are much more complex and have several moving parts (compressor, electric engine, fans…). But there are no miracles – we have to understand that “camper’s environment” (vibrations, shocks, off-level position and bigger exposure to weather will impact the operation and reliability of all RV fridges.

Compressor Refrigerators

Typically, compressor-based refrigeration is used in household refrigerators.  The basic operation can be explained as follows:

a)    Compressor constricts refrigerant gas into high pressure then pushes it up into external radiator at the back of the fridge (meanders of tubes and fins). 

b)    Radiator helps to release the heat from the compressed refrigerant, as the result the refrigerant changes its gaseous form to the liquid one.

c)    Liquefied refrigerant flows along high pressure side to the Expansion Valve next to the Cooling Section.

d)    Expansion of refrigerant’s volume (pressure changes quickly from high to low) as well as subsequent evaporation of the refrigerant are two endothermic processes.  They absorb heat from “surroundings” and as the result lower the temperature. These two processes take place in cooling coils inside of the fridge.

e)    Refrigerant in form of gas goes back to the compressor and the cycle continues.

Basics of the compressor-driven refrigerators

Till recently compressor-based refrigerators used Freon (CFC) as the refrigerant. However since it was discovered that it destroys layers of ozone in the upper atmosphere, Freon is being replaced by other, less harmful refrigerants (In any case, from user's point of view the most important difference compared to absorptive refrigeration is lack of explosive hydrogen gas!). 

Due to their high reliability and great cooling capabilities (much wider temperature range between external and internal temperatures), compressor refrigerators are often considered as the right choice for RVs especially “high-end” units like motor-homes, large fifth-wheels etc…

Their main disadvantage is much higher power consumption compared to absorptive refrigerators (about 10 to 20 times more).   It makes it impractical to power them through Inverter from your 12V DC batteries (you can do it eventually during daytime driving, but once you have to turn on your headlights, your power budget may become negative and you will risk to drain your car’s battery.  But adding extra battery pack periodically re-charged by Generator will do the job keeping your refrigerator running!

Once in the camping park, obviously RV can be connected to the available electrical AC hook-up.  On the road and when camping in rustic areas you may consider using a Diesel (gas) Generator and eventually to be more “eco-friendly” - solar power. Obviously these are realistic solutions for high-end and luxury RVs.

Please note however that original household refrigerators may not be robust enough for RV-ing. Unless designed for RV “environment” they may be last as long as expected due to vibrations, shocks and off-level working conditions!

So let's summarize the refrigeration process:

In absorptive refrigerators the refrigerant pressure is lowered by mixing ammonia with hydrogen. In compressor-driven refrigerators the refrigerant is first compressed so it can be easily decompressed by pushing it through expansion valve. In both cases when pressure is suddenly lowered the liquefied refrigerant evaporates. And because the evaporation process is endothermic (needs heat), the "pipe" where evaporation takes place absorbs the required heat from "surroundings" (coil in the freezer).

Peltier's Coolers

To make this article complete we have to also mention small portable refrigerators. They will run on 12V/24VDC directly from the battery and 110VAC with the help of an AC/DC adapter. Most of them will make use of the so-called “Peltier” cooler (heat absorption semiconductor effect). It’s a very simple and reliable design (no moving parts, no refrigerant of any kind), but it is not very power efficient. That is why Peltier’s devices are used mainly for small, portable coolers.



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