SECTION 1
Introduction 1.1. Difference between Compression and Absorption Refrige- ration Systems (1)»(2)*
Refrigeration might be defined as the art of producing and maintaining, in a given space, a temperature level which is lower than the surrounding temperature level. The reversed Carnot cycle operating as a heat pump removes heat from a low tempera- ture source and rejects it to a higher temperature region; no other cycle is more efficient than this one for given high ten- perature and low temperature sources. Actual refrigeration cycles, because of their inherent irreversibilities, operate at lower efficiencies than the reversed Carnot cycle. In the following discussions, irreversibility for each process in an actual cycle will be considered instead of the reversible pro- cesses which constitute the reversed Carnot cycle.
Generally speaking, refrigeration systems are divided into two classes —— compression refrigeration systems and absorption refrigeration systems. In a compression refrigeration system (see Fig. la), liquid refrigerant passes through a throttle valve, thereby undergoing a throttling process to a lower pres- sure and temperature. After expansion through the valve, the
fluid is evaporated in the coils by means of the absorption of
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heat from the evaporator. The dry saturated vapor from the evaporator is then compressed to a higher pressure by supplying mechanical work to the compressor. During the compression the temperature of the refrigerant increases. The high temperature vapor is then condensed under constant pressure conditions to the entrance of the throttle valve. This completes the cycle.
Both compression refrigeration systems and absorption re- frigeration systems have the same condensing process, throttling process through the expansion valve and evaporating process, The difference between the two systems is the process between the end of the evaporating process and the entrance of the condensing process, As mechanical work is much more expensive than an equivalent amount of heat, it may be desirable to use heat di- rectly as the operating energy instead of using mechanical work. The most successful system operating almost wholly on an input of heat is the absorption system. The basic absorption refrigera- tion system (see Fig. 1b) uses an absorber, generator, solution valve and a liquid pump instead of the compressor in the compres- sion refrigeration system and uses two fluids (such as ammonia and water) instead of one fluid (such as ammonia) in the compres- sion refrigeration cycle. In accordance with the design pressure and temperature at the absorber, the mixed solution quickly reaches the equilibrium condition and finally becomes a strong concentrated solution. (A strong concentrated solution is one that has a relatively large amount of the refrigerant dissolved
in the absorbing fluid.) During this mixing operation, cooling
water circulates continuously to take off this heat. The strong solution 1s then pumped to the generator where almost pure vapor flows to the condenser and the weak solution expands through a solution valve and flows back to the absorber. In the absorber the absorbent and the refrigerant vapor are brought into contact to facilitate the dissolving of vapor continuously. The basic difference described above can be seen by comparing the left hand sides of Fig. la and Fig. lb. By far the most commonly used binary mixtures for absorption refrigeration systems are ammonia- water, lithium chloride-water, lithium bromide-water.
Of these two systems, the compression refrigeration system is much simpler in mechanism than the absorption refrigeration system. For comparisons, however, we have to consider some other practical and local circumstances, such as the investment cost the availability of heating and cooling systems; we can not as- sert which system is preferable in general. Further detail in the comparison of the absorption and compression systems, by a purely thermodynamic viewpoint with the aid of performance Pato
and coefficient of performance, will be shown in Sec. 2.3.
1.2. Principles of the Absorption Refrigeration System and Its Cyclic Analysis Figure 2 shows a more complete diagram of a conventional absorption refrigeration system than does Fig. lb. The high pressure vapor enters the condenser where it becomes a liquid. The liquid then passes through the expansion valve where it is
throttled to a low pressure, low temperature, and low quality
vapor. This low pressure and low temperature refrigerant flows through the evaporator where it absorbs heat as the liquid is vaporized. The low pressure and low temperature vapor coming from the evaporator flows into the absorber where it comes in contact with the cool weak solution. The absorber operates at a pressure slightly lower than the evaporator pressure. The weak solution absorbs the vapor which comes from the evaporator and thereby becomes a strong solution. The maximum concentration of refrigerant vapor that can be absorbed depends upon the tempera- ture and pressure in the absorber. This maximum absorption is an important concept in investigating absorption refrigeration
systems, Fig. 3()
shows, for example, the maximum composition of ammonia by weight in a solution of ammonia and water as a function of pressure and temperature.
During the absorption process, the heat of absorption, which is the equivalent of the heat of condensation of the liquid, must be removed for the purpose of holding a low temperature in the absorber and thereby maintaining a good absorption rate. Usually cooling water is used to absorb this heat.
From the absorber the strong solution” enters the pump at the system which raises its pressure and sends it through a heat exchanger and to the generator. The generator operates at the
condenser pressure and is supplied with steam or other sources of
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of heat through heating coils which drives off some of the re- frigerant from the liquid, decreasing the concentration of the liquid until it becomes a weak liquor, The vapor distilled from the solution in the generator is composed of refrigerant vapor along with small quantities of absorbent vapor, and when this mixture is cooled in the rectifier, the absorbent vapor saturated with refrigerant is condensed and flows back into the generator. By.this means, part of the refrigerant is vaporized from the
_ Strong solution in the generator; the remaining solution is a mixture of liquid absorbent with a relatively small concentration of refrigerant dissolved in it. This remaining solution is call- ed the weak solution. It leaves the generator, flows through the heat exchanger, the solution valve and then into the absorber.
In the heat exchanger the weak solution which comes from the generator heats the strong solution which is sent up from the — absorber through the pump. This reduces the amount of heat re- quired in the generator. Neglecting the pressure head losses due to friction, we can consider that in an absorption refrigerant system there are two distinct pressure levels which exist within the unit; the high pressure which exists in the heat exchanger, the generator, the rectifier and the condenser; the low pressure which exists in the absorber and the evaporator. After the weak solution has been cooled in the heat exchanger it enters the
solution valve, which acts as a throttle valve, where its pres-
- Such as electrical heating; but steam heating is preferable
due to its lower cost.
sure is reduced from the high pressure to the low pressure, and then flows into the absorber.
The refrigerant which was vaporized in the generator con- tains some absorbent vapor. Since this absorbent vapor will con- dense at a much lower temperature than the refrigerant vapor, this mixed solution is passed through a rectifier to condense the absorbent vapor in order to obtain pure refrigerant vapor. In the rectifier the refrigerant vapor is cooled sufficiently to condense the surplus absorbent which is separated and returned to the generator. It is important to keep the absorbent vapor to a minimum to prevent the accumulation of liquid or solid state
absorbent in the condenser or refrigerant coils.
1.3. Cyclic Analysis of an Absorption Refrigeration System in
Terms of the Circulating Fluids 5)
Attention has been previously directed to the principles of the absorption refrigeration systen in terms of the processes that occur in each piece of apparatus. Now, the system will be analyzed from the viewpoint of the four sub-cycles which consti-
tute the whole system.
(1) Refrigerant Vapor Cycle ‘ After the strong solution in the generator is heated to the point of driving off some of the refrigerant vapor, with a@ small amount of absorbent, this vapor passes through the rectifier where the absorbent is condensed and the refrig-
erant cooled. The absorbent is separated and returned to
10
the generator while the refrigerant vapor passes through the condenser where it is condensed. The liquid then flows through the expansion valve to the evaporator where it takes up heat and is revaporized. From the evaporator it passes into the absorber and is absorbed by the cooled weak solu- tion. This weak solution is thus changed to a strong solu- tion which is pumped by the refrigerant pump through the
heat exchanger to the generator. This completes the cycle.
(2) Weak Solution Cycle When the strong solution is subjected to high tempera- ture in the generator, some of the refrigerant in it is vaporized thus producing a weak solution. This weak solu- tion is collected at the bottom of the generator.
Me ___-In order to maintain constant liquid levels in the generator and the absorber, the flow of strong solution to the generator must be of such value that it equals the sun of the flows of weak solution from the generator and the refrigerant to the condenser. The weak solution flows from the generator to the heat exchanger where it is cooled, It then flows to the absorber where it absorbs the cold refrig- erant coming from the evaporator, and is further cooled. The absorption changes the weak solution to a strong
solution.
(3)
(4)
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Strong Solution Cycle
As mentioned in the refrigerant vapor cycle, the strong solution formed in the absorber by the union of refrigerant vapor and weak solution is picked up by the pump, forced through the heat exchanger where it is warmed, and then
passes to the generator to be heated further.
Cooling Water Cycle
When the cooling water is cold, it is common practice to let the water circulate first through the condenser.
Here it liquefies the refrigerant and comes from the con- denser sufficiently cold to be used in the absorber. From the absorber cooling water is circulated through the rectifier.
In warm weather, where a suitable supply of cold water is not available, separate water supplies are maintained for the condenser, the absorber and the rectifier.
Whether separate cooling water supplies are used or not, in either case the water coming from the cooling systems may be passed to the sewer discharge or carried either to cooling towers or spray ponds to be re-used. Whereas some power is used in sending such water through the cooling process of either spray ponds or cooling towers, there are eextarn advantages in reusing this water. The water used and reused does not scale so badly, since it has only so much hardness to deposit, and once this is separated
the water is practically non-scaling. Further, unless very
12
cheap water is available, it is seldom possible to secure it, either by pumping or by purchase, at as low a cost as by
cooling it either by sprays or cooling towers.
The four sub-cycles which have been described above can be
seen from Fig. 4 on page 13.
13
STCn S
sys
14