The invention relates to a method for operating an absorption refrigerating unit including an expeller, condenser, evaporator and absorber with a solvent circulation with a solvent alternating between strong and weak concentrations, a working substance circulation with a condition of the working substance alternating between being dissolved in the solvent, gaseous and liquid, a by-gas circulation with a by-gas alternating between strong and weak concentration of the working substance, wherein the working substance is expelled from the solution rich in working substance with supply of thermal energy and subsequently is liquefied by heat absorption, is evaporated under absorption of heat from a chamber to be cooled and is absorbed from the solution weak in working substance, and wherein the by-gas serves as carrier for the working substance between the evaporation phase and the absorption phase. The invention further relates to an absorption refrigerating unit for carrying out the method as well as a refrigerator equipped with an above-described absorption refrigerating unit.
Absorption refrigerating units in contrast to compression refrigerating units operated with a mechanically operated condenser are operated with thermal energy. The possibility to allow working with a heat generator, like a gas burner, in addition to electrical energy, led to an extended use of absorption refrigerating units of the kind cited in the beginning in small-size refrigerating units for camping vehicles, boats and the like. Furthermore, absorption refrigerating units are used where noise-lessly working refrigerators are required, e.g. so-called minibars for self-service in hotel rooms.
An absorption refrigerating unit comprises the main components schematically indicated in FIG. 2, namely expeller, condenser, evaporator and absorber. Furthermore, it has to be distinguished between three circulations, namely a solvent circulation, a working substance circulation and a by-gas circulation, the solvent in the present field of application being water or an aqueous ammonia solution, respectively, the working substance being ammonia and the by-gas being hydrogen or helium.
The complete lines in FIG. 2 represent the solvent circulation showing to mass circulations of different ammonia concentration, a flow shown in thin line having weak concentration (poor in the working substance ammonia) and a flow shown in thicker line having strong concentration (rich in the working substance ammonia).
The dotted line in contrast thereto shows the working substance circulation. In the expeller the ammonia is separated from the strongly concentrated solution by supply of heat and subsequently is liquefied in the condenser by heat absorption. The evaporation process of the liquid ammonia in the evaporator extracts heat from the cooling chamber on a low temperature level. The gaseous ammonia then flows into the absorber and is absorbed by the solvent weak in ammonia. Thus, the cooling circulation is closed.
The dash-dotted lines in the right-hand portion of FIG. 2 show the by-gas circulation, wherein the by-gas is formed from hydrogen. In the lower portion of the absorber the hydrogen is strongest enriched with ammonia gas, while the ammonia concentration decreases to the upper portion of the absorber, because the ammonia is absorbed from the weak solution flowing in opposite direction. The lower the ammonia concentration the lighter is the by-gas so that it can rise upwardly to the evaporator in the absorber. Almost pure hydrogen is present in the evaporator. In the evaporator the pure by-gas mixes with the evaporated ammonia so that the gas mixture becomes heavy and presses down to the absorber, therein simultaneously pushing the lighter gas mixture from a container in the absorber pipe windings in front of itself. Thus, the by-gas circulation is closed.
As the three circulations are in mutual connection via the pipe system of the absorption refrigerating unit, the change in one circulation causes changes in the other two circulations.
In FIG. 1 further details of the absorption refrigerating unit in accordance with the present invention are shown.
The expeller is built as vapor bubble pump in which a pump is arranged within a cylindrical heater pipe heatable from the outside and in which the rich solution is introduced from bottom side. Upon heating the ammonia evaporates from the water solution and raises upwardly in the pump pipe in form of bubbles. The difference in density between the liquid present in the pump pipe and enriched with vapor bubbles and the rich solvent column, "a" in FIG. 1, causes circulation of the solvent, wherein the solvent reaches the absorber as weak solution via an inlet pipe in counterflow direction to the rich solution.
The rising ammonia vapor contains a small portion of water which is to a great extent expelled in the pipe piece located between the expeller and the condenser. This pipe piece thus serves as water separator.
A line out of two concentrical pipes leads into the input of the expeller. In the inner tube the rich solution is fed to the expeller, while in the outer tube the weakly concentrated solution which was heated on the expeller flows back. The concentrical pipe piece thus serves as liquid counterflow heat exchanger in which the heat from the weak solution is transferred to the rich solution so that the latter reaches the expeller in pre-heated form.
An absorption refrigerating unit of the kind as described above under FIG. 2 is regulated by a thermostat arranged on the evaporator in the cooling chamber. If the temperature in the cooling chamber drops below a given desired value of e.g. 4.degree. C., the heating means of the expeller is switched off so that the temperature in the heater area will drop to such extent that the pure solvent no longer is heated. This means that the solvent circulation is interrupted. As a consequence, no ammonia vapor or liquid ammonia in the condenser, respectively, is made available so that the working substance circulation is interrupted. Thereby, also the gas circulation is interrupted, because no rich, heavy gas mixture is produced, flows downwardly and pushes the weak gas mixture in front of itself to the evaporator any longer. When the operation of the absorption refrigerating unit has stopped, the temperature in the cooling chamber increases. As soon as the temperature reaches a given threshold value, e.g. 8.degree. C., the heating means again is started by the thermostat regulation in order to again increase the meanwhile severely cooled temperature in the area of the heater to the temperature of about 160.degree. to 200.degree.. With maximum power input the heater area now is heated to 160.degree. to 200.degree., still no ammonia being expelled during the heating process. It can be assumed that the operation of the absorption refrigerating unit only is started after 2 to 20 minutes and the above-described circulations have established themselves. Only from this time on cooling power is made available, wherein then cooling is effected down to a lower temperature than the desired temperature of 6.degree. C., before the system is switched off resulting from thermostat control. Thermostat control is effected in the modes:
ON--FULL POWER--OFF.
The advantages of an absorption refrigerating unit of the above-described kind, like simple construction without moving parts and noise-less maintenance-free operation, are opposed by the essential disadvantage of a comparatively high energy demand so that a comparatively low efficiency with respect to cooling power is given. Numerous attempts and proposals to improve efficiency up to now only resulted in success to a very low degree. The reason lies in the complex cooperation of the method and device parameters depending on one another which have to be exactly tuned to match one another. When the refrigerating unit is started, no cooling power will be achieved until, after a longer heating period, ammonia is produced and made available in the evaporator. During this heating period maximal energy has to be supplied. Due to the intermittent regulation of the refrigerating unit it cannot be avoided either that too much liquid ammonia is made available in the evaporator, which will not be evaporated in the evaporator completely but in the gas heat exchanger or in the connecting line between evaporator and absorber. This means that the excess amount of ammonia cannot be utilized for the desired cooling in the cooling chamber but represents a loss.