My thoughts, for what they may be worth (!), are as follow.
Cold is basically the absence of heat. Cooling and refrigeration are accomplishing by moving heat
from a target location such as the inside of your refrigerator. An absorption refrigerator relies on a process that requires
heating an ammonia solution to faciltate this. So, we are faced with the challenges of a system where you are purposefully
adding heat at one location in the system in order to
remove it from another location, the inside of the refrigerator.
The diagram below illustrates an absorption refrigeration system. The diagram has arrows indicating where heat is put into the system, and where heat is removed from the system.
The only places that heat is supposed to move
into the system are at the
generator (boiler tube) and at the evaporator. Well, the evaporator is the thing inside the refrigerator that is removing the heat from your beer. Yes, your beer is adding heat to the system! The system’s whole job is to get that heat out of the beer, right?!
However, other than the heat exchanger and the refrigerator contents, there is
no other place where you benefit from heat being
added to the system. So, the goal is to
remove the heat from the system in
all other locations.
Let’s first look at where we are deliberately adding heat to the system to drive the process. The ammonia boiler is housed adjacent to the flue pipe within an insulated shield. This assembly is a heat exchanger intended to take heat
from the electric heating coil or LP gas burner and transfer it
to the boiler tube ("generator" in the diagram below).
The spiral baffle inside the flue basically assists the heat exchange process by keeping the heat from just jetting up vertically through the flue without heating the adjacent ammonia solution boiler tube (generator). This type of spiral baffle is also found in gas water heaters where it serves the same purpose of facilitating a better exchange of heat from the flue to the tank.
Once the heat has been exchanged from the flue to the boiler tube (generator), and the ammonia solution has been boiled, the goal is now
to get rid of the heat from all other parts of the system! (Remember, the only other place the system where the heat is added to the system is from the refrigerator’s contents via the evaporator, so
be sure to stock the refrigerator with an adequate supply of beer. This is a
very important step in the process here!)
Heat is removed from the system by an upward ventilation pattern via 1) natural convection caused by warm air rising, and 2) sometimes additionally by one or more fans assisting the upward airflow. Refrigerator installation diagrams indicate low and zero clearances in order to
facilitate airflow across the refrigerators condenser and absorber (hereinafter, the "tubes and fins").
Many refrigerator installations don't adhere to the recommended clearances which direct the airflow to pass through the "tubes and fins". As an example, my Casita’s factory installation's clearance is 3.5 inches at the top; the curvature in the shell lends to this excessive clearance at the top.
Casita does not baffle this space off and force the airflow to pass through the "tubes and fins". So, the ventilation air passes by at a distance. Air "passing by" at a distance does
not effectively remove the heat from the "tubes and fins".
Another factor is how the wall vents on these smaller refrigerators inhibit the airflow. Reduced airflow reduces the amount of heat removed. Subsequently, the hot air is
trapped adjacent to the box that one is trying to cool! It's like putting your icebox in the warmest and sunniest location you can find!
Zach and me and many others have noted that simply removing the vent grill from the top helps keep the refrigerator cooler inside. I think this constricted ventilation which traps heat
at the same level as the top of the refrigerator is a
big factor in why refrigerators with roof vents generally seem to perform better.
Other than that, there are certainly a lot of factors involving proper LP pressure, a clean jet, thermostat performance, etc. However, I think a
key problem experienced with these refrigerators is simply their limited ability to get rid of the waste heat, particularly in side vent installations, for the reasons described.