Anhydrous Ammonia (Nh3) has been used for many years to perform all types of refrigeration and air conditioning chores. Anhydrous Ammonia means ammonia without water, Aqua Ammonia is a solution of ammonia and water which is used in refrigeration and air conditioners that are not a vapor compression type. Vapor compression systems use compressors to compress the ammonia vapor. Absorption type systems do not use a compressor but instead use a generator. These systems shall be the main topic of the following chapters. It is assumed by the Author that you have some knowledge of refrigeration and are aware of the cycles and pressures of refrigeration in general. Please refer to our available Browser Books on the subject if you are not. Ammonia Refrigeration Basics is a good source of basic information should you require a better understanding on the subject.


Example:# 101A



The Intermittent absorption system uses a generator charged with water and ammonia. A heat source, usually a kerosene flame, heats this solution in the generator. The ammonia is vaporized and driven off.

A condenser, at the top of the system, condenses the ammonia vapor into a liquid. The liquid flows by gravity into the liquid receiver shown above and then into the evaporator. During the generating cycle, little or no refrigerating effect is taking place. As the system cools, the pressure drops, causing the liquid ammonia in the evaporator to boil (flash off) and absorb heat. The cycle is completed when vaporized ammonia is re-absorbed in the Aqua ammonia solution in the generator.

Example: 101A illustrates the generating cycle, in operation, the kerosene burner tank is filled with just enough kerosene for one cycle. This cycle is usually once a day. The burner is filled and lighted. It heats the water and ammonia mixture (Aqua Ammonia) (Brown Mixture) in the generator. The ammonia vapor (Tan Color) is driven off through the tube, (A) up to the air cooled condenser. There the ammonia gas is cooled and condensed to liquid ammonia (Red Color). The liquid flows into the receiver. Note the one way check valve between the condenser and the liquid receiver, it serves to restrict any back flow of liquid into the condenser. Also notice the small restrictor port which will be covered below during the refrigeration cycle of the system.

When the kerosene has all been burned (usually from 20 to 40 minutes), the generating cycle ends.





The pressure in the system drops as the water cools and absorbs ammonia vapor. Liquid ammonia (Light Blue) flows into the evaporator, begins to evaporate, and cools it. Evaporated ammonia (Aqua Blue) flows back through the tube (B). It is again absorbed by the water in the generator. Refrigeration continues, usually until the next firing of the kerosene burner. The small restrictor perform two functions, it allows the liquid to enter the evaporator at a controlled level. This protects the evaporator from getting flooded. The restrictor also acts as a expansion valve which greats a pressure differential. A pressure differential is essential to the refrigeration cycle as it causes the liquid ammonia to change it's boiling point from a high pressure flash of temperature to a lower pressure flash off temperature. Without this change in pressures the refrigeration would not be performed and the system would simply act as an ammonia container.

This type of refrigerating system is quite simple. The piping is welded steel because the pressures on the generating cycle are quite high. The refrigerating ability is quite good. Kerosene flame heated absorption refrigerators are popular in areas where electric power is not available.

Other means of firing the burner can be propane or natural gas. Many RV unit refrigerators use a combination of propane fired burners and add a small 12 volt fan assembly to improve cooling within the evaporator section of the refrigerator (Ice Box).. Many types of absorption systems are on the market these days and are not limited to small appliances. The theory can and has been applied to very large commercial systems and perform in much the same manner.

Most small RV type refrigerators are not field repairable due to the critical charge ( The right amount of Aqua Ammonia ) and the nature of the system construction. If a small unit develops a leak, the best thing to do is replace it with a new one. Most manufacturers design the units to last for a very long time, but a damaged unit should be replaced (Leaking). Maintenance can and should be performed on the burner assembly and the piping that feed the propane or kerosene. Most Aqua Ammonia leaks are easy to find as are Anhydrous Ammonia leaks the smell is very strong and aids in detecting even the smallest of leaks.




Example:# 101C

There are other types of absorption systems available, the continuous-cycle absorption cooling unit is operated by the application of a limited amount of heat. This heat is furnished by gas, electricity, or kerosene. No moving parts are employed. The operation of the refrigerating mechanism is based on Dalton's Law.



Dalton's Law of partial pressures is the foundation of the principle of operation of one of the absorption type refrigerating systems. The Law States:

The total pressure of a confined mixture of gases is the sum of the pressures of each of the gases in the mixture.
The total pressure of the air in a compressed air cylinder is the sum of the oxygen, nitrogen, and the carbon dioxide gases, and the water vapor pressure.

The law further explains that each gas behaves as if it occupies the space alone. To illustrate, the absorption refrigerator uses two gases, ammonia and hydrogen. The ammonia, at room temperature, is absorbed by the water in the closed sustem.
Heating this solution drives out the ammonia. (The hydrogen is not absorbed by the water and remains as a gas.) Due to the pressure it is under, the ammonia condenses into a liquid in the condenser. The pressure is uniform throughout the system. Total pressure in the system is the sum of the vapor pressure of the ammonia plus the hydrogen pressure. When the pressure of the ammonia vapor is below the pressure corresponding to the vapor pressure for ammonia alone, the ammonia continues to evaporate. It tries to reach a vapor pressure corresponding to the temperature in the absorber.


This refrigeration device is widely used in domestic refrigerators, and recreational vehicles. It is also used in year-around air conditioning of both homes and larger buildings. The unit consists of four main parts the boiler, condenser, evaporator and the absorber. When the unit operates on kerosene or gas, the heat is supplied by a burner. This element is fitted underneath the central tube (A). When operating on electricity, the heat is supplied by an element inserted in the pocket (B).

The unit charge consists of a quantity of ammonia, water, and hydrogen. These are at a sufficient pressure to condense ammonia at room temperature. When heat is supplied to the boiler system, bubbles of ammonia gas are produced. They rise and carry with them quantities of weak ammonia solution through the siphon pump (C). This weak solution passes into tube (D), while the ammonia vapor passes into the vapor pipe (E) and on to the water separator. Here any water vapor is condensed and runs back into the boiler system, leaving the dry ammonia vapor to pass to the condenser. Air circulating over the fins of the condenser removes heat from the ammonia vapor. It condenses into liquid ammonia and then flows into the evaporator.

The evaporator is supplied with hydrogen. The hydrogen passes across the surface of the ammonia. It lowers the ammonia vapor pressure enough to allow the liquid ammonia to evaporate. The evaporation of the ammonia extracts heat from the evaporator. This, in turn, extracts heat from the food storage space, lowering the temperature inside the refrigerator.

The mixture of ammonia and hydrogen vapor passes from the evaporator to the absorber. A continuous trickle of weak ammonia solution enters the upper portion of the absorber. It is fed by gravity from the tube (D). This weak solution flows down through the absorber. It comes into contact with the mixed ammonia and hydrogen gases. This readily absorbs the ammonia from the mixture. The hydrogen is free to rise through the absorber coil and to return to the evaporator. The hydrogen circulates continuously between the absorber and the evaporator.

The strong ammonia solution produced in the absorber flows down to the absorber vessel. It passes on to the boiler system, thus completing the full cycle of operation.
This cycle operates continuously as long as the boiler is heated. A thermostat which controls the heat source regulates the temperature of the refrigerated space.
Since the refrigerant is ammonia, it can produce quite low temperatures. Most systems require electrical devices, so both gas and electricity must be supplied. Except for the thermostatic controls and (in some cases) fans, there are no moving parts.
Service is usually quite simple. The burner and stack must be kept clean. The refrigerator should be carefully leveled before being placed in operation.

The pressure differential in the type of system is created by the hydrogen gas, it causes the ammonia to change pressure and allows it to boil off in the evaporator (flash off).