How the Gas Refrigerator Works

by David S. Lawyer mailto:davylawyer1@gmail.com

2000-2017
Explains in detail how the gas refrigerator works. It cools by evaporating liquid ammonia in a hydrogen atmosphere and then separates the ammonia from the hydrogen so that the ammonia may be evaporated again. This separation is a 4 stage process: 1. ammonia gas (in a hydrogen atmosphere) dissolves in flowing water 2. boiling the water solution by a gas flame emits ammonia gas. 3. the hot ammonia gas is cooled and condenses to a liquid. 4. Liquid ammonia evaporates creating cold. All this is done at the same pressure without any pumping. However, the above explanation is overly simplified. While there are no chemical reactions there are changes of state from liquid to gas and conversely. This is all very elementary "physical chemistry". You'll find here a descriptive explanation with no equations to understand..

1. History

2. Introduction

3. The Cycle in Detail

4. Construction Details

5. No Compression Needed

6. Evaporation by Pressure Drop

7. Energy Supply


1. History

The gas refrigerator is powered by propane gas (or natural gas) instead of electricity. Today they are used mainly in recreational vehicles and in locations where there is no electricity. But in the United States in the 1930's and 1940's and beyond, the Servel brand of gas refrigerator was often found in homes which also had electricity. They were claimed to be better than electric refrigeration since they were not only silent but had no moving parts to wear out.

2. Introduction

The propane gas is burned to produce heat which boils a mixture of water and ammonia, releasing ammonia vapor and "pumping" fluids thru the system. To understand how it works, it helps to understand a little about "physical chemistry" such as "latent heat of vaporization" and "partial pressures" in a mixture of gases. But even if you don't know what these mean, you may learn what they mean by reading this article.

Both the gas and electric refrigerator operate on the same principle of creating cold by evaporating a refrigerant, which makes cold by absorbing heat (known as "latent heat of vaporization"). We all know that evaporating water (like from sweat) causes cooling and so does evaporating a liquid refrigerant.

The cycle in an electric refrigerator is much simpler than a gas one: An electric pump compresses the refrigerant gas and liquifies it. Then the refrigerant liquid, under high pressure, flows into a region of low pressure and thereby evaporates (turns into a gas) creating cold (the refrigerant absorbs heat due to its change of state from a liquid to a gas). Then the gas is compressed again to turn it into a liquid and then evaporated again, etc, etc. During compression the refrigerant becomes very hot and the heat is transferred to the room air via cooling fins (like in an automobile radiator).

Both the gas and electric refrigerator cycle a refrigerant, which for a gas fridge is ammonia. But the gas fridge has 3 fluids in the cycle instead of just one refrigerant, although only ammonia makes it thru the complete cycle. The other two fluids found in parts of the cycle are: 1. water (both as a liquid and gas (vapor) 2.hydrogen gas. There are no chemical reactions, just changes of state between a liquid and gas (except that the hydrogen always remains a gas). All the state changes happen inside pipes. While an electric fridge has a big drop in pressure to get the refrigerant to evaporate, the gas fridge has the same high pressure in all parts of the cycle and thus doesn't need much of a pump to circulate the fluids. Note that both liquids and gasses are "fluids".

2.1 Evaporating ammonia

The first question that comes to mind is: How can ammonia liquid evaporate unless there is a decrease in pressure? This is where the hydrogen gas comes into play since the ammonia liquid is evaporated into an atmosphere of hydrogen (unavoidably mixed in with some ammonia gas which has just evaporated). The total pressure, which remains constant, is just the sum of the pressures of the hydrogen gas and the ammonia gas and these pressures are known as "partial pressures". So the partial pressure of the ammonia gas is less than the total pressure of the ammonia liquid and this lower pressure results in the liquid ammonia evaporating.

It's similar to cooling by water evaporation into air. When water is sprayed into a moving stream of air the tiny water droplets evaporate because they are at atmospheric pressure while the partial pressure of the water vapor in the atmosphere surrounding these droplets is lower since the water vapor is mixed with and air and thus has a lower partial pressure. Of course, since air is primarily a mixture of nitrogen and oxygen gases, there are three gasses in this mixture surrounding a droplet (including water vapor).

In the gas refrigerator, the pipe feeding the liquid ammonia to be evaporated is small in diameter compared to the larger pipe feeding the hydrogen gas. This is something like spraying water into an air stream as described above but in the fridge, liquid ammonia is "sprayed" into a flowing stream of hydrogen gas. But since the pressure in the gas fridge is much higher than atmospheric, the pressure difference between the ammonia droplets and ammonia vapor (partial) is much higher, and thus the evaporation of liquid ammonia precedes at a much faster rate than for water evaporating in air. This all happens inside a cold pipe which zig-zags back and forth inside the freezer compartment and eventually enters the main refrigerator compartment where fins attached to the cold pipe help to cool the food stored there.

2.2 Separating ammonia from hydrogen

Now we have a cold mixture of ammonia and hydrogen gases and we need to somehow separate the ammonia from the hydrogen and liquify the ammonia so as to repeat the process. To do this we use water. The ammonia gas dissolves in water leaving the hydrogen since hydrogen doesn't dissolve in water. Then we remove the ammonia from the ammonia-water solution by boiling the water solution using heat from a gas flame. Then the resulting hot ammonia gas is liquified by cooling it back down to somewhat above room temperature. Now the liquid ammonia is again evaporated via spaying it into hydrogen and thus the cycle repeats itself.

3. The Cycle in Detail

3.1 Absorbing (washing out) the ammonia

Now to explain this process in more detail: As we have seen, cooling takes place by evaporating liquid ammonia in an atmosphere of hydrogen gas resulting in a mixture of ammonia gas and hydrogen. To reuse the ammonia and hydrogen we need separate them and eventually liquefy the ammonia. To separate out the hydrogen, an "absorption unit" consisting of a fin-cooled (by room air) pipe is used where liquid water flows down a sloped pipe to absorb the ammonia from the mixture of ammonia-hydrogen gas flowing up the same pipe. It's a bi-directional flow of fluids: a liquid water solution flows downhill and two mixed gases flow uphill (all inside the same pipe). When the gas reaches the top, it's nearly pure hydrogen because the flowing water has absorbed (or "washed out") almost all the ammonia gas. Thus this fridge is sometimes called an "absorption" refrigerator" (the water absorbs the ammonia gas). The resulting recycled hydrogen gas is now ready to again meet more recycled liquid ammonia so as to evaporate it to create cold.

3.2 Getting the ammonia out of the water

The (washed out) ammonia, now dissolved in water, must be recovered and separated from the water. So the ammonia solution is boiled by a gas flame. Mostly ammonia is evaporated leaving water (which is reused as explained above to wash out more ammonia from the ammonia-hydrogen gas). But since some water has inadvertently been boiled off with the ammonia, it's recovered in a "reflux condenser and rectifier" where water droplets condense on the interior surface of a finned pipe (the fins are kept cool by the air in the room) and water from the condensate (in droplets) flows downward while the nearly pure ammonia gas flows up the pipe (again bi-directional flow). Thus the impure ammonia gas is "cleaned" of water-vapor. Then the pure ammonia gas is further cooled (by passing it thru more finned pipes) to a little above room temperature resulting in liquid ammonia. There's a U trap here (like under the kitchen sink) to make sure no gases get into the liquified ammonia.

3.3 Evaporating the ammonia

Now the ammonia liquid (after the trap) flows into a much larger diameter pipe where hydrogen gas is flowing. The vapor pressure of the ammonia liquid (at the liquid-gas surfaces) is high enough so that it evaporates rapidly in the hydrogen gas atmosphere resulting in freezing cold. It evaporates rather quickly since the partial ammonia gas pressure above the surfaces of the ammonia liquid droplets is low due to the presence of the hydrogen. After exiting the cold pipe inside the fridge, water is used to wash out the ammonia from ammonia-hydrogen gas mixture.

3.4 Bubble pump

The actual system is a bit more complicated since for water to flow downhill (to wash out the ammonia) it must first be raised uphill by a "bubble pump" which at the same time moves ammonia vapor (gas). These bubbles (mostly ammonia vapor) are created by the previously described boiling of the ammonia solution. The bubbles flow up a small tube (which has the same diameter of each bubble) with "slug"s of water between each pair of bubbles. The gas in the bubbles gives the slugs buoyancy and the bubbles rise up like bubbles rise up in a mountain stream, lifting the water slugs up with them. Then at the top, the water and ammonia gas are separated with the water flowing downward by gravity to meet the ammonia-hydrogen gas mixture so it can wash out the ammonia.

3.5 Cycle summary

The summary of the cycle shown below can only be fully understood if you understand the above paragraphs. All steps itemized below happen inside pipes. The name of the part of the fridge that does each step is shown in (...).

  1. Liquid ammonia evaporates in a hydrogen atmosphere. Cold! (evaporator)
  2. Ammonia is removed from the ammonia-hydrogen gas mixture by washing it out with water (absorber)
  3. The liquid solution of water-ammonia is boiled by a gas flame giving off impure ammonia gas bubbles (generator)
  4. The bubbles lift water uphill (bubble pump)
  5. The water is separated from the ammonia gas so it can be reused in step 2. (separator)
  6. The ammonia gas is "cleaned" of water vapor by a cooler pipe surface (reflux condenser and rectifier).
  7. The still hot ammonia gas is cooled to liquify it (condenser)
  8. Go to the top of this list to repeat the above cycle

4. Construction Details

Besides exchanging heat with the air and providing cold, there are "internal" heat exchangers to increase efficiency. The whole system is enclosed and at high pressure (everywhere the same pressure). Some "pipes" are often quite long due to many U bends in the pipe which thus doubles back on itself, thereby increasing the pipe length. This is a type of "zig-zag". For a pipe, which has a liquid which must flow downhill for a long distance, the pipe may be sloped with sharp bends (zig-zags) something like a trail going down a mountain with sharp switchbacks. With high pressure everywhere in the system the density of the gases (hydrogen and ammonia) is quite high, resulting in more cooling being done.

5. No Compression Needed

Since ammonia at atmospheric pressure is a vapor, why doesn't it require compression to liquify it? It's because the high pressure in the gas fridge pipes is high enough to always liquify pure ammonia vapor provided it's not significantly hotter than room temperature. In the gas frig, ammonia can only exist as a vapor if it's either mixed with hydrogen (reduces its partial pressure) or if it's hot (after boiling off the ammonia dissolved in water).

6. Evaporation by Pressure Drop

Both a gas and electric refrigerator cool by the same principle of liquid evaporation (latent heat of evaporation). But in the electric frig, the evaporation happens due to a pressure drop created by an electric-motor driven pump. In a gas fridge, this "pressure drop" is created by reducing ammonia vapor partial pressure by injecting liquid ammonia into hydrogen gas.

7. Energy Supply

While the electric pump uses energy, for the electric frig it might seem that the introduction of hydrogen achieves the pressure drop without using any energy since there is no mechanical pump and total pressure remains constant. But actually, energy is required to recycle the hydrogen and ammonia gas which uses water to separate them by washing out the ammonia with water and then recovering the ammonia from the water by boiling it off. Thus the heating of the water solution to boil off the ammonia does require energy provided by a blue gas flame.