to the discharge end of the compressor) by a pump, through the valves of which it passes just as it flows through the valves of the compressor piston. In the absorption plant the ammonia (liquor) pump can be made much smaller than the compressor gas pump used in the compression system, because the actual work of compressing the ♦This experiment of Faraday's is described more in detail by Mr. Gardner T. Voorhees in "Ice and Refrigeration," October, 1908. Digitized by V^OOQIC 64 ELEMENTARY MECHANICAL REFRIGERATION ammonia gas to the point at which it can be liquefied by the cooling water in the condenser is performed by the direct heat of steam rather than by the heat generated by the expenditure of energy behind the compressor piston. To facilitate the use of steam for this purpose, the cold ammonia gas returning from the expansion coils is absorbed or dissolved in water in the absorber, and the resulting strong aqua ammonia, known as 'strong liquor,' is pumped into the generator or anmionia boiler, where it is heated by a series of steam coils which drive off the gaseous ammonia at a high pressure, just as water vapor or steam is driven off under high pressure in a steam boiler. Absorption System- A- Discharge ^Tnl 4 End /^ ry Ammonia 3 Compressor Fig. 22. — Elementary Compression and Absorption Machine with Common Condensing and Expansion Units The high-pressure ammonia gas driven off in the generator of an absorption plant, like that discharged from the compressor of a compression plant, is conducted to the condenser, at which point that part of the refrigerating cycle common to both systems begins. The driving off of the ammonia vapor in the generator changes the strong liquor to 'weak liquor,' which, being under a higher pressure than the liquid in the absorber, readily flows back to the absorber. <Callout type="important" title="Important">The weak liquor must be carefully monitored and controlled to ensure proper operation.</Callout> To adapt the elementary absorption machine just described to the requirements of economical commercial apparatus, a number of refinements must be added. Fig. 23 is a diagrammatic representation of a modem absorption machine of well-known make. On account of the similarity of the expansion side of the ab- sorption system to that of the compression system already de- scribed in detail, it will be necessary to trace the working medium through only that part of the cycle between the expansion coils and the condenser. Cooler In the type of machine illustrated in Fig. 23, the expansion side consists of a brine cooler of the vertical cylindrical or shell type, such as is most commonly used in connection with absorption machines, but to some considerable extent with compression machines as well. The brine, usually calcium chloride, is circu- lated through nests of spiral coils within the cylindrical shell of the cooler, and the anhydrous ammonia is expanded into the space between the coils and the shell. The amount of liquid present is readily observed by means of a gauge glass. <Callout type="tip" title="Tip">Regularly check the gauge glass to ensure proper liquid levels in the brine cooler.</Callout> The cold ammonia vapor leaving the top of the cooler passes through the gas-suction line to the absorber, where it is joined by the weak liquor which has just undergone cooling in the 'double-pipe' weak-liquor cooler. Since the absorption of the ammonia gas into the weak liquor takes place with the evolution of a considerable amount of heat, further means for cooling the liquor must also be provided in the absorber. Absorber In some cases the absorber is of the double-pipe type, similar to double-type condensers and brine coolers. The type here illustrated consists of a horizontal cylinder containing coils of pipe through which the cooling water which has previously done duty in the ammonia condenser passes. The cold, weak liquor is ad- mitted at the top of the cylinder, passes down among the cooling coils, and there mingles with and absorbs the cold ammonia gas from the brine cooler. <Callout type="risk" title="Risk">Improperly maintained absorber can lead to inefficiency or failure.</Callout> Since the absorption of the ammonia gas into the weak liquor takes place with the evolution of a considerable amount of heat, further means for cooling the liquor must also be provided in the absorber. Exchanger The exchanger is a horizontal steel shell capable of carrying the full generator pressure through which the comparatively strong liquor is pumped on its way from the bottom of the absorber to the bottom of the generator. This rich liquor is shown entering the shell of the exchanger at the right-hand side, after which it traverses a spiral pipe coil and finally passes out into the top of the analyzer, shown just above the generator. <Callout type="gear" title="Gear">A properly designed exchanger ensures efficient heat transfer.</Callout> The exchanger is provided with nests of pipe coils connected in parallel, through which passes the hot weak aqua ammonia, supplied to the manifold at the top of the exchanger by a pipe running from the bot- tom of the generator. In the countercurrent flow, the hot weak liquor which must eventually be cooled in descending through the pipes gives up a part of its heat to the cooler rich liquor which must eventually be heated when ascending around the coils. By this heat exchange, cooling water is economized in the absorber and steam in the generator. Analyzer Leaving the top of the exchanger shell, the cool rich liquor passes to the top of the analyzer. Here it is allowed to trickle down over a set of metal trays, where, coming in contact with the am- monia vapors rising from the generator, the countercurrent heat- exchanging effect is still further continued. <Callout type="important" title="Important">Regular maintenance of the analyzer ensures optimal performance.</Callout> The hot ammonia vapor, entraining more or less aqueous vapor as it rises from the surface of the liquid in the generator, in passing up through the analyzer on its way to the condenser encounters the descending rain of cool rich liquor on its way to the generator. The former, which must eventually be liquefied in the condenser, is cooled, and the latter, which must eventually be boiled in the generator, is heated. This advantageous heat interchange also has the effect of actually condensing and returning to the generator a large per- centage of the entrained aqueous vapors passing off with the am- monia, and also of liberating some ammonia gas through the heating of the gas-saturated rich liquor. Where ample analyzer capacities are employed, the incoming rich liquor should be within a very few degrees of the evaporating temperature by the time it finally reaches the surface of the boiling liquid. Generator The generator, or 'still,' as it is frequently called, is the am- monia boiler for the evaporation of the weak liquor enriched and changed into strong liquor by the absorption of ammonia gas direct from the expansion coils in the absorber. It consists of a substantial steel shell provided with a heavy cast-iron head through which pass the ends of the steam coils supplying the heat required for driving off the ammonia. <Callout type="warning" title="Warning">Improper operation of the generator can lead to unsafe conditions.</Callout> The strong aqua ammonia enters at the top, where it remains by virtue of its specific gravity being lower than that of the weaker liquor at the bottom of the generator. The water of condensation from the steam used in the generator is usually returned to the boilers. This can be effected either by an automatically controlled pump, or by a suitable high- pressure trap. Rectifier After leaving the analyzer, the hot ammonia gas passes to the rectifier, a water-cooled coil of pipes of sufficient area to insure the condensation of any aqueous vapors that may have escaped con- densation in the analyzer. The liquid condensed in the rectifier is rich, saturated liquor which is returned to the generator by way of the analyzer. Condenser From the rectifier the ammonia gas, which should now be prac- tically free from aqueous vapors, is passed to the condenser. Con- densers for absorption machines, like those for compression ma- chines, may be of either the atmospheric double-pipe or shell type as preferred. The anhydrous ammonia liquefied in the condenser passes to an anhydrous receiver, similar to those used in the com- pression system, from which it is drawn as required for expansion in the brine cooler, its flow through the feed line being regulated by the usual expansion valve. After evaporation in the brine cooler the ammonia gas again passes to the absorber, after which the working cycle is repeated. Cycle Traversed by Ammonia This can be readily traced by following the course of the heavy arrows in Fig. 23. The circuits of both the gaseous and the aqueous components of the aqua-ammonia refrigerant, as well as that of the cooling water, can be more readily followed out by means of the diagram, Fig. 23, in which all mechanical details have been omitted. The several members of the refrigerating system illus- trated in Fig. 23 are represented by shaded areas occupying ap- proximately the same relative positions on the diagram. The path of the ammonia is represented by a heavy solid line; that of the water component of the aqua-ammonia refrigerant by a narrow solid line; and that of the cooling water by a broken line. The direction of travel in each case is indicated by arrows. Weak Liauot CooUag WcUr Fig 24. — Diagrammatic Representation of an Absorption Refrigerating System From this diagram, as well as from Fig. 23, it will be seen that, as 'anhydrous ammonia,' the refrigerant starting from the 'anhydrous receiver' passes to the 'brine cooler,' where in changing to the gaseous state it performs its sole function of ab- sorbing heat from the brine. As saturated low-temperature am- monia vapor, the refrigerant starting from the brine cooler passes to the absorber, where it enters into solution or is absorbed by the weak liquor from the generator, forming strong liquor. As hot strong liquor the refrigerant starting from the absorber passes through the exchanger, where it gives up some of its heat to the weak liquor on its way to the absorber, then on by way of the analyzer into the generator, where the ammonia gas is driven out of the strong liquor solution, under high pressure, by the appli- cation of heat, and passes through the analyzer and rectifier into the condenser, leaving the impoverished aqua ammonia or weak liquor behind in the generator. In the condenser, the heat originally absorbed by the anhydrous ammonia in changing from the liquid to the gaseous state in the brine cooler, as well as that added to increase its temperature and drive it out of solution in the generator, is given up to the cool- ing water, circulated through the condenser, causing the ammonia to retum to the liquid state, after which it flows to the anhydrous receiver, and the cycle is again traversed. The aqueous component of the aqua-ammonia refrigerant, starting from the bottom of the absorber in company with the ammonia in the form of strong liquor, passes through the exchanger and analyzer into the generator. Here it is separated from the greater part of the ammonia and returns through the exchanger and weak-liquor cooler to the absorber. Here it again joins the anhydrous ammonia, forming strong liquor, and retraces the path just described. Path of Cooling Water The cooling water is admitted first to the ammonia condenser, where it performs its most important function of removing heat from and liquefying the anmionia gas. After leaving the anmionia condenser it is still cool enough to be capable of absorbing a con- siderable amount of heat from the strong liquor in the absorber, more from the weak liquor fresh from the generator in the weak-liquor cooler, and still more from the hot ammonia gas fresh from the generator in the rectifier, after which it usually passes to waste. Still another line might have been drawn on the diagram in Fig. 24, indicating the path traversed by the heat from the point of its absorption from the brine in the brine cooler to that of its expul- sion with the cooling water from the condenser. Such a line, how- ever, would coincide with that representing the ammonia from the point where the heat and the vapors of the refrigerant leave the brine cooler, continuing to the condenser, where it would cross over and join that representing the cooling water. It would then follow this line through its circuitous passage to the point where, together with the water, the heat flows away to the sewer. Counter Current Effect It should be noted that throughout the entire system a coun- ter-current effect is carried out between the cooling and the cooled substances. In the absorber the direction of travel of the cooling water is upward through the cooling coils, while that of the cooled strong liquor is downward around the outside of the cooling coils. In the exchanger, the hot weak liquor from the generator passes in one direction through a nest of pipe coils, while the somewhat cooler strong liquor from the absorber flows in the opposite direc- tion around the outside of the cooling coils. In the double-pipe weak-liquor cooler, the cooling water, after passing through the cooling coils of the absorber, passes in one direction through the inner pipes, while the hot weak liquor passes in the opposite direc- tion between the inner and the outer pipes. In the analyzer the hot ammonia gas passes upward' through a rain of cooler strong liquor. Likewise, in the double-pipe condenser a similar counter- current effect is produced. By these counter-current cooling effects, in which the coldest cooled substance gives up its heat to the coldest cooling substance, and the hottest cooled substance to the warmest cooling substance, the outgoing substance is cooled more nearly to the temperature of the incoming cooling substance than would otherwise be possi- ble, thus effecting economy not only in the amount of the cooling substance required but also in the operation of the system through the reduction in the amount of the refrigerating medium required for a given amount of cooling effect.
Key Takeaways
- The absorption system uses steam to compress ammonia, reducing the size and energy requirements of the compressor compared to a compression system.
- Proper maintenance of components like absorbers and exchangers is crucial for efficient operation.
- Counter-current cooling effects enhance the efficiency of the refrigeration cycle.
Practical Tips
- Regularly check gauge glasses in brine coolers to ensure proper liquid levels, which can prevent inefficiency or failure.
- Maintain a consistent temperature range in absorbers and generators for optimal performance and safety.
- Implement counter-current cooling methods to reduce the amount of refrigerant needed and improve overall efficiency.
Warnings & Risks
- Improper operation of the generator can lead to unsafe conditions, including ammonia leaks or fires.
- Failure to maintain proper levels in absorbers can result in inefficient absorption of ammonia gas.
- Inadequate cooling water management can cause system inefficiency and potential damage to components.
Modern Application
While the specific techniques described in this chapter are historical, the principles of counter-current heat exchange and efficient refrigeration cycles remain relevant. Modern survival preparedness often involves understanding basic mechanical systems for emergency situations, such as maintaining food or water supplies during a crisis. The knowledge of how to manage and maintain these systems can be crucial when conventional utilities fail.
Frequently Asked Questions
Q: What is the role of the absorber in an absorption refrigeration system?
The absorber absorbs ammonia gas from strong liquor, forming weak liquor that flows back into the generator. This process is critical for maintaining the cycle and ensuring efficient operation.
Q: How does counter-current cooling improve the efficiency of the refrigeration cycle?
Counter-current cooling allows the hottest substances to give up heat to the coolest ones, reducing the overall temperature difference and thus saving energy. This method enhances the efficiency by minimizing the amount of refrigerant needed.
Q: What are some signs that an absorber might need maintenance or repair?
Signs include inefficient absorption of ammonia gas, increased pressure drops in the system, and abnormal temperatures within the absorber. Regular monitoring can help identify these issues early.