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The Ammonia Absorption Cycle

The average person usually thinks of chlorofluorocarbons or hydrofluorocarbons when the word 'refrigerant' is used. Names like Freon, Suva, Forane, R-22, R-12, R-134Aordinarily spring to mind. But ordinary water becomes an excellent refrigerant when it is combined with ammonia or a unique salt made of lithium and bromide.

First, Let's take a look at the behavior of gases in a mixture. Dalton's Law of Partial Pressure is the one key to understanding the ammonia adsorption process:

PTotal= PGas 1+ PGas 2 + . . .

Simply stated, Dalton's Law of Partial Pressure says that the total pressure of a gaseous mixture is the sum of each individual gas in the mixture. These individual pressures are referred to as the "Partial Pressure. Formally then, Dalton's Law of Partial Pressure states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture.

On the surface, Dalton's Law doesn't seem earth-shaking.But, intuition would suggest that each gas in a mixture would be at the same pressure as all the other ones. Wouldn't all the gasses in a balloon be pushing on the wall of the balloon equally? Dalton said, "No". Each gas actually contributes an individual pressure to the mixture. Different gasses would push on the wall of the balloon at different pressures.

Most people are familiar with the odor of ammonia and probably think of it as a liquid. But, at room temperature ammonia is a colorless gas about half as heavy as air. Pure ammonia is a very smelly chemical made up of about 82% nitrogen and 18% hydrogen. Ammonia is usually stored in tanks under high pressure to keep it in a liquid state. The second key to to understanding the ammonia the balloon process is that ammonia absolutely LOVES water. It is almostimpossiblee to see pure ammonia in the natural world because pure ammonia combines with water any chance it gets.

The ammonia absorption cycle depends on a source of heat. In the "generator tank", a solution of water and ammonia is heated. Like in a distillation process, ammonia boils out of the water because its boiling point is much lower than that of the water. The ammonia gas then flows to the "separator".

The "separator" is slightly cooler than the "generator" so any water vapor that happens to be present condenses and is returned back to the "generator tank". What is left is nearly pure ammonia vapor. The next stop for the ammonia vapor is the "condenser". In the condenser, the ammonia gas is cooled so that it condenses to a liquid. Next, the liquid ammonia is pumped to the evaporator.

Here is the secret ingredient: The evaporator contains pure hydrogen gas under a high pressure of about 200 PSI. Remember Dalton's Law; In a mixture of gases, the total pressure of all the gasses is equal to the sum of each partial pressure. In the evaporator before the introduction of the liquid ammonia, the total pressure was equal to the partial pressure of the hydrogen because hydrogen was the only gas present!

PTotal= PHydrogen
PTotal= 200 PSI
Now, imagine what happens when the liquid ammonia is introduced into the evaporator. In order for the ammonia to remain in a liquid state, it must generate IT"S OWN vapor pressure that is equal to the total pressure in the evaporator. The ammonia liquid boils and vaporizes into a gas.

PTotal= PHydrogen+ PAmmonia
PTotal= 200 PSI+ PAmmonia

You see, it is a stacked deck. The ammonia can't reach the vapor pressure necessary to prevent its own vaporization. In order to achieve a vapor pressure of about 200 PSI, the temperature of the ammonia would need to be at about 100 degrees. We made sure that was not the case back in the condensing stage when we cooled the ammonia liquid . Note too, that as the ammonia evaporates, the total pressure also increases. So the target pressure increases because of the addition of the ammonia vapor. It is a race ammonia just can't win. It keeps trying but it never achieves the elevated vapor pressure that would stop the evaporation process.

In this desperate attempt to achieve liquid/vapor equilibrium, the ammonia boils. In order to change state from liquid to gas, the ammonia pulls heat energy from its surroundings. The evaporator cools.

And . . . we have refrigeration.

Next, the ammonia/hydrogen gas mixture are piped to the "absorber". In the "absorber", the water is relatively cool. The ammonia vapor wastes no time rejoining with the love of its life and mixes with the water. This action serves to pull the ammonia out of the ammonia/hydrogen gas mixture. The cool water and ammonia solution are sent back to the "generator tank". The hydrogen is returned to the evaporator. And the cycle repeats.

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