This article presents a brief explanation of how an absorption-cycle refrigerator functions. An example of this refrigerator is the Electrolux, a kitchen refrigerator which was manufactured by Servel, Inc. of Evansville, Indiana. This refrigerator had no compressor or valves, just a small flame or electric heater, and worked continuously without noise or vibration. In spite of their advantages, absorption refrigerators are not much used at present.
First, we should review the operation of the compression-cycle refrigerator, which is by far the most common type. The refrigerant is compressed, and then gives up heat to the environment, condensing to a liquid. This high-pressure liquid is throttled through a valve to a low-pressure region, where it evaporates, absorbing heat from the volume to be cooled. If Q is the heat absorbed at the low temperature, Q' the heat rejected at the high (environmental) temperature, and W the work done on compression, then conservation of energy gives W = Q' - Q. The coefficient of performance P is P = Q/W = Q/(Q' - Q). If the cycle is reversible, which is an idealization, then Q'/T' is equal to Q/T, where T is the high temperature and T' the low. This gives P = T/(T' - T). Typically, if T' = 290K, T = 273K, then P = 16. This means that the heat absorbed, the cooling effect, is 16 times the heat equivalent of the work input.
The absorption cycle refrigerator uses ammonia refrigerant, water absorber, and hydrogen pressurization. The general arrangement is shown in the figure. The whole working volume is at a single high pressure, perhaps 10 atm. There are, therefore, no valves. Three circuits can be identified: ammonia, water and hydrogen. The gas on what would be the high-pressure side of an ordinary refrigerator is mostly ammonia, so the ammonia is at high pressure. It is also warm, and is cooled to the environment in the usual way, upon which it condenses. The liquid passes to the evaporator, which is also at a high total pressure, but now the pressure is mainly hydrogen. The ammonia pressure is kept low by absorption in water. The liquid ammonia then evaporates, cooling as usual. The ammonia-water solution flows to a generator, where it is heated by a small flame or electric heater. This releases the ammonia, which by a percolator action lifts water to a vapor-liquid separator. This lift provides the pressure difference to keep the water flowing. The ammonia replaces the hydrogen on this side, and is condensed in the condenser as described above.
The water, depleted in ammonia, is now cooled and absorbs more ammonia, keeping its partial pressure low in the evaporator. It then goes to the generator, closing the water cycle. This slow movement of the water continues to keep the partial pressure in the evaporator low, and the partial pressure in the condenser high, maintaining the refrigeration effect. The water cycle is the essential action in the refrigerator. The hydrogen mainly fills the low-pressure side, taking no part in the action aside from keeping the total pressure constant.
In this cycle, heat q is absorbed at the high temperature of the generator, as well as heat Q from the evaporator. Heat Q' is rejected to the environment in the condenser, as well as heat Q" in the absorber. Conservation of energy gives q + Q - Q' - Q" = 0. The coefficient of performance is P = Q/q, considering the heat input at the generator to be the input and Q the output. This is not as easily related to the temperatures, since the cycle is considerably irreversible.
M. W. Zemansky, Heat and Thermodynamics, 4th ed. (New York: McGraw-Hill, 1957). pp. 235-237.
Composed by J. B. Calvert
Created 3 December 2004