On Zephyrs, I described the operating principles of The RV ammonia based refrigerator. The key to understanding this is getting a handle on partial pressure in gases. It is this behavior of gases that is being manipulated to provide cooling.
Compressor driven refrigeration uses physical pressure differences with an valve on one side and a compressor on the other to transfer heat. The absorption refrigerator keeps the overall pressure in the loop constant and manipulates concentrations to transfer heat. That requires a careful control of environment by using tubing sizes and shapes that are optimized to the stage of the cycle.
The partial pressure idea means that you have compounds in multiple states at one time. This is just like having humidity in the air above a body of water, for example. When a gas dissolves in water, there is a natural balance between how much is in the water and how much is in the atmosphere above it. The refrigerator case depends upon the fact that ammonia readily dissolves in water but the balance between how much is in the water and how much in the atmosphere above it is highly temperature sensitive. That is utilized to have cooler water in the absorption tank picking up ammonia dropping down out of the evaporator and warmer water in the percolation tube giving up gas going to the condenser.
As for tubing, consider that the condenser needs to cool gas to a liquid at about the same rate as it drips down to the evaporator. That means a big tube coming in to bring the gas from the perc tube and a small tube going to the evaporator to drip a liquid. The evaporator needs to spread that liquid out to help encourage it to evaporate again in the hydrogen atmosphere much like sweat on your skin will evaporate into the air to cool you off on a hot afternoon. In the desert, you get a lot of cooling because the air is dry and your sweat evaporates so fast you don’t notice your sweating. In humid environments it is more difficult to evaporate the sweat so you drip and don’t get as much cooling benefit. The refrigerator uses hydrogen to provide ‘dry’ air for ammonia to evaporate into. The hydrogen is ‘dried’ of ammonia by passing it over water in the absorber where the water has less ammonia in it than needed to balance the amount of ammonia mixed with the hydrogen above it.
Simple concepts carefully integrated together to provide an amazing result that is often take for granted – just how wonderful is that?
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