Gas absorber and refrigeration system using same

An absorption refrigeration system with ammonia as refrigerant and a salt solution as absorbent operates with an improved margin of safety to crystallization of the salt solution by incorporating an absorber having a precooler and narrow nozzles. The precooler serves to additionally lower the temperature of the salt solution before it is caused to absorb ammonia exothermically, and the nozzles are for injecting the salt solution into ammonia gas flows, while maintaining the solution on the upstream side at relatively high pressure. The flow characteristics of the gas-liquid mixture through absorber tubes change from churn flow to slug flow and finally to bubble flow. A blower is provided for recycling a portion of the ammonia gas back to the absorber for optimizing absorption efficiency. The precooler and the absorber tubes are cooled evaporatively to reduce operating temperatures and system operating costs.

BACKGROUND OF THE INVENTION 
This invention relates to a gas absorption refrigeration system that can 
operate at temperatures as low as -40.degree. C. and, more particularly, 
to an improved gas absorber for use in such a system for causing a 
refrigerant gas such as ammonia to be absorbed into an absorbent salt 
solution. 
Mechanical compressor systems have long dominated the field of 
refrigeration, but the ozone-depleting characteristics of 
chloro-fluoro-carbons are a serious problem. Moreover, mechanical vapor 
compression systems are found to have little thermodynamic superiority 
over absorption systems as a result of the need to convert thermal energy 
into mechanical energy at a low temperature for the compression system. 
Consequently, there is currently a renewed interest in absorption 
refrigeration systems. 
Various aspects of this technology have been discussed by Carlos Alberto 
Infante Ferreira in his thesis entitled "Vertical Tubular Absorbers for 
Ammonia-Salt Absorption Refrigeration" published by The Technische 
Wetenschappen aan de Technische Hogeschool, Delft, Holland; Mar. 26, 1985. 
In order to improve the performance of an absorption refrigeration system 
for given working conditions, Ferreira considered firstly to identify 
favorable absorbent-refrigerant mixtures from a thermodynamic standpoint, 
secondly to improve the efficiency of system components, and thirdly to 
effect system modifications to the standard refrigeration cycle. Ammonia 
has been considered as one of the best refrigerants, and many salt 
solutions have been investigated. Ferreira considered sodium thiocyanate 
(NaSCN) solution to be the best but saw a definite advantage in the 
NH.sub.3 --NaSCN--NaI system because possible salting out effects are of 
concern only at lower temperatures and concentrations. Although 
significant efforts have been expended to find preferable salt solutions, 
however, the identification of new salt solutions has been only partially 
successful, and it is therefore an object of the present invention to 
provide an improved system for carrying out a refrigeration cycles to 
expand the capabilities of presently known absorption refrigeration 
systems. 
The temperature lift capability of a salt solution as applied to 
refrigeration systems is typically limited by its solubility 
characteristics. As a 50--50 mol % NaI--NaSCN ammonia solution is heated, 
for example, ammonia is driven off and when the salt concentration is 
approximately 75% (at equilibrium temperatures and pressures), the 
solution begins to crystallize due to insufficient ammonia and solubility 
limits. The higher the pressure, the higher the temperature must be to 
drive off ammonia to achieve the 75% level. If the temperature is lower 
than the equilibrium temperature, the vapor pressure will be lower than 
the equilibrium pressure. If the temperature is lowered further, the 
solution will crystallize at atmospheric pressure. In other words, beyond 
the solubility field of a solution, which is dependent on temperature and 
pressure, crystallization occurs, leading to failure of the refrigeration 
system. 
A critical component of all absorption refrigeration systems is an absorber 
for causing the refrigerant to be absorbed by a refrigerant solution. 
Absorption of ammonia is an exothermic reaction, which requires that the 
absorbing medium be cooled effectively because, if the temperature rises, 
the equilibrium condition of the reaction shifts such that the pressure 
inside the vessel may potentially rise. Two types of absorbers which seem 
to be in general service are a vertical water-cooled, shell-and-tube 
absorber-condenser used in conjunction with a cooling tower water 
circulation system and an air-cooled absorber condenser. Water-cooled 
absorbers which derive cooling water from cooling towers operate at 
relatively high temperature differences between the ambient temperature 
and the absorber outlet and require larger amounts of water flow, if the 
temperature gain in the water is to remain small, as is required to 
maintain the lowest possible absorber pressure necessary for low 
temperature operation. The cooling pumps require much power and the piping 
can become very expensive, if the cooling tower cannot be located near the 
absorber. In addition, fans are required to operate the cooling tower. 
Air-cooled absorbers, on the other hand, require very large heat transfer 
surfaces and also consume a great deal of electrical power to drive the 
cooling fans. 
It is therefore a general object of the present invention to provide an 
efficient absorption refrigeration system. 
It is a more specific object of the present invention to provide an 
absorption refrigeration system with a refrigeration cycle capable of 
operating at temperatures as low as about -40.degree. C. with an improved 
margin of safety. 
It is another object of the present invention to provide an efficient, 
cost-saving and relatively compact gas absorber for such an absorption 
refrigeration system so as to increase the margin of safety to 
crystallization along the process path of the refrigeration system. 
SUMMARY OF THE INVENTION 
An absorption refrigeration system embodying the present invention, with 
which the above and other objects can be accomplished, may be constructed 
largely with prior art refrigerator components such as a generator and a 
separator for heating a refrigerant-rich solution to separate a 
refrigerant gas therefrom and to thereby obtain a refrigerant-lean 
solution, a condenser for emitting heat to thereby condense the 
refrigerant gas from the separator and an evaporator for absorbing heat 
from the environment to thereby evaporate the refrigerant gas condensed by 
the condenser. The refrigerant gas from the evaporator is caused to be 
absorbed into the refrigerant-lean solution in a specially designed gas 
absorber according to the present invention to thereby produce a 
refrigerant-rich solution. In order to improve efficiency of the absorber 
and therefore reduce the absorber heat exchange surface requirements, a 
portion of the ammonia gas which passes through the absorber is recycled 
to the absorber. The refrigerant-rich solution from the absorber is 
returned to the generator to complete an operation cycle. 
The gas absorber according to the present invention is characterized in 
that the margin of safety regarding crystallization of the 
refrigerant-absorbing solution is significantly improved for the 
refrigeration system into which it is incorporated. Generally speaking, 
this is accomplished by removing the process cycle curve for the system 
operation farther away from the boundary beyond which crystallization 
occurs. For this purpose, the absorber is provided with an evaporatively 
cooled precooler for reducing the temperature of the refrigerant-lean 
solution such that the exothermic absorption of the refrigerant gas will 
take place at an additionally lowered absorption temperature, and the 
solution is injected into the refrigerant gas flows through narrow nozzles 
such that the pressure of the solution can be maintained relatively high 
on the upstream side. This ensures that the weak solution remains in 
liquid form upstream of the nozzles. 
In order that the refrigerant gas be quickly absorbed, absorber tubes are 
designed such that the flow characteristics of the mixed gas-liquid phase 
therethrough change from churn flow to slug flow and finally to bubble 
flow. Since bubble flow has lower mass and heat transfer rates, means are 
provided for recycling a portion of the refrigerant gas to be recycled 
back into the absorber in order to overcome the disadvantage of bubble 
flow. The condenser and the absorber may be combined into a single unit 
and evaporative cooled technology adopted for reducing the system 
operating temperature, costs and saving space for installation. 
With such a gas absorber incorporated into an absorption refrigeration 
system, the margin of safety to crystallization of the 
refrigerant-absorbent is significantly improved, and operations at 
temperatures as low as -40.degree. C. becomes feasible with a proper 
choice of absorbent solution.

DETAILED DESCRIPTION OF THE INVENTION 
The process diagram in FIG. 1 and the process cycle diagram of FIG. 2 will 
be referenced simultaneously to explain the basic structure and 
characteristics of an absorption refrigeration system 10 according to the 
present invention, as well as features required of its absorber. In what 
follows, the liquid absorbent to be used in the system for absorbing 
ammonia as refrigerant will be simply referred to as a solution without 
specifically mentioning any salt of which it is a solution. The solution 
is called lean or weak when its ammonia-content is relatively low and its 
salt content is high. It is called rich or strong when its ammonia-content 
is relatively high and its salt content is low. In FIG. 2, the boundary, 
beyond which crystallization of the solution would occur, is indicated by 
a shaded plane. In what follows, this plane will be referred to as the 
crystal boundary. 
In FIG. 1, numeral 12 indicates a separator, through which is passed a 
high-pressure strong solution heated by a generator 14 to drive off 
ammonia. The generator 14 may be of a conventional gas-burning type, and 
the separator 12 serves to separate the ammonia gas from the strong 
solution from which it was driven off, thereby producing a weak solution 
with a reduced ammonia content. This process is represented in FIG. 2 by 
an arrow from Point 8 to Point 1, indicating a slight rise in the solution 
temperature and slight drop in pressure. 
The ammonia gas, separated from the solution in the separator 12, is caused 
to pass through a condenser 16, where it is cooled and condenses. The 
condensed ammonia gas is collected in a container, referred to as an 
ammonia receiver 17 in FIG. 1, and is introduced through an expansion 
valve 18 of a known type into an evaporator 19 where heat absorbed from 
the environment causes the liquid ammonia to evaporate. 
The weak solution from the separator 12 with a low ammonia content is 
passed through a heat exchanger 30 and introduced into an absorber 20, of 
which the basic purpose is to cause the ammonia gas from the evaporator 18 
to be absorbed into the weak solution received from the separator 12 to 
thereby produce a strong solution with a high ammonia content. In FIG. 2, 
the effect of the heat exchanger 30 on the solution, as it is transported 
from the separator 12 to the absorber 20, is represented by an arrow from 
Point 1 to Point 2. Since the absorber 20 is one of the critical elements 
of the improved refrigeration system 10 of the present invention and is a 
key to its successful operation, some of the new features of the absorber 
20 embodying the invention, as well as how such new features contribute to 
the successful operation of the system 10, will be explained first with 
reference only to FIGS. 1 and 2. Its structure will be described more in 
detail further below. 
With prior art gas absorbers, such as those tested and described by 
Ferreira, absorption of ammonia gas is effected by causing a weak solution 
to flow through absorber tubes from a large manifold and injecting ammonia 
vapor into these tubes through small vapor inlet nozzles. With an absorber 
thus structured, the pressure within the solution drops immediately and is 
low the absorption begins to take place. In FIG. 2, this process would 
appear as shown by the dotted line from Point 2 through Points 4' and 5 to 
Point 6. In other words, if a prior art absorber as described above were 
used in the system 10, the process cycle curve would come dangerously 
close to the crystal boundary. If a safety margin of 
8.degree..about.9.degree. C. is required to be imposed, such a system 
would be able to achieve operations only down to about -32.degree. C. What 
is required of the absorber 20 according to the present invention, 
therefore, is that the path from Point 2 to Point 6 in the process cycle 
diagram of FIG. 2 be sufficiently removed away from the crystal boundary 
such that the system 10 can be operated down to -40.degree. C. with an 
increased margin of safety. 
For the reason given above, the absorber 20 according to the present 
invention is provided with a precooler 21 for further cooling the solution 
which has already been cooled by passing through the heat exchanger 30. 
The advantage of thus cooling the solution before it is allowed to mix 
with ammonia vapor should be apparent with reference to FIG. 2, wherein 
the effect of this additional cooling by the precooler 21 is represented 
by an arrow from Point 2 to Point 3. In other words, the solution is at a 
lower temperature when it begins to absorb the ammonia gas, and this 
temperature difference removes the cycle path farther away from the 
crystal boundary. 
The part of the absorber 20 where the ammonia-lean solution from the 
separator 12 which has also passed through the heat exchanger 30 is mixed 
with the ammonia gas from the evaporator 18 is identified in FIG. 1 as a 
mixer 24. Details of the mixer 24 are shown in FIG. 4 with injection 
nozzles 27 through which ammonia-lean solution is injected into the 
absorber tubes and mixed with reacting solution and ammonia. The mixture 
of the solution and the ammonia gas which is being absorbed is then caused 
to flow upwards together through absorber tubes 25, and a resulting 
ammonia-rich solution is introduced into a solution receiver 35. Numeral 
23 in FIG. 1 represents a valve for controlling the flow rate and pressure 
through the precooler 22 and ensuring that the ammonia-lean solution is 
maintained at an elevated pressure on the upstream side thereof. 
The mixer 24 according to the present invention is so designed as to be 
able to keep the solution in a relatively high-pressure condition on the 
upstream side thereof and to keep the pressure drop within the absorber 20 
as low as possible while causing the ammonia gas to be absorbed as quickly 
as possible for allowing lowest temperature operation. In FIG. 2, the 
arrow from Point 3 to Point 4 represents the pressure drop through the 
precooler control valve 23, the arrow from Point 4 to Point 5 represents 
the pressure drop due to mixing with absorber resident solution and the 
absorption of the ammonia gas as the ammonia-lean solution is injected 
into the absorber tubes 25 which are evaporatively cooled on the outside, 
and the arrow from Point 5 to Point 6 represents the absorption of the gas 
by the solution within the absorber tubes 25 and as the solution flows 
into and is collected in the solution receiver 35. In a conventional 
cycle, as explained above, the entire pressure drop would take place 
within a manifold and a control valve. As the ammonia-lean solution with a 
high salt concentration is introduced, its initial contact with ammonia 
may cause a rapid temperature rise and localized crystallization, and such 
local heating may result in an increase in evaporator pressure and hence 
in nonoptimum operation. If there is insufficient back pressure, 
crystallization may also occur in the heat exchanger 30. It is to be noted 
that Point 5 in FIG. 2 is substantially removed from the crystal boundary 
compared to Point 4'. 
The ammonia-rich solution collected in the solution receiver 35 is sent to 
the separator 12 through the aforementioned heat exchanger 30 and the 
generator 14 by means of a solution pump 40 which serves to raise the 
pressure of the strong solution as indicated in FIG. 2 by an arrow from 
Point 6 to Point 7. The heat exchanger 30 is for transferring heat from 
the ammonia-lean solution from the separator 12 to the ammonia-rich 
solution from the solution receiver 35. The arrow from Point 7 to Point 8 
in FIG. 2 indicates the temperature increase of the ammonia-rich solution 
as it travels through the heat exchanger 30 
A dryer 36 and a filter 37 may be provided, for example, as shown in FIG. 1 
for accepting a portion of the strong solution from the solution receiver 
35 to dry and filter the accepted solution. This piping configuration may 
be expected to be sufficient for the purpose of maintaining an adequate 
level of dryness and cleanliness of the solution. This configuration is 
advantageous in that the dryer 36 and/or the filter 37 may be serviced 
while the system 10 is in operation because the system 10 can continue to 
operate for a short period of time without the dryer 36 or the filter 37. 
Next, the structure of a portion of the absorber 20, including its mixer 24 
and lower end portions of the absorber tubes 25 will be described more in 
detail. With reference to FIGS. 3 and 4, the incoming weak solution from 
the precooler 22, of which the flow rate is controlled by the precooler 
control valve 23, is introduced into the mixer 24 through a solution inlet 
manifold 26 and injected into the absorber tubes 25 through small 
injection nozzles 27. The ammonia gas from the evaporator 18 is introduced 
into a gas inlet manifold 28 and drawn into the absorber tubes 25 from the 
gas inlet manifold 28 and through the space with an annular 
cross-sectional shape surrounding the injection nozzles 27. Each nozzle 27 
has a predetermined pressure drop during normal operation such that the 
interior of the solution inlet manifold 26 is maintained at a pressure 
higher than where the absorption actually takes place (as shown by Point 4 
in FIG. 2). The weak solution is introduced at a point approximately 5 to 
20 diameters downstream of where the ammonia gas is introduced from the 
gas inlet manifold 28. If this distance were smaller, the solution would 
be entering in a region with poor heat transfer characteristics. If this 
distance were greater, this would adversely affect the cost of the 
absorber 20. The weak solution need not be introduced in each of the 
absorber tubes 25. The specific system design parameters will determine 
the number of injection tubes 25 required. It is to be noted that, 
according to conventional vertical tubular absorber technology, it is 
usually the gas to be absorbed that is injected into the stream of a 
liquid absorbent. The injection method of the present invention is an 
exact opposite. 
Although the absorber tubes 25 are shown vertical, they may be inclined or 
may have serpentine configurations. What is important is that the flow be 
concurrent and upwards to an upper manifold 29 in which the absorber tubes 
25 terminate. The pressure drop through each tube must be similar and the 
absorber 20 must be installed level or the inlets and outlets to all 
absorber tubes 25 level. The gas inlet piping to the mixer 24 must be 
arranged to prevent "back drainage," that is, the gas inlet piping must be 
routed such that its elevation close to the absorber 20 be higher than the 
absorber tube inlets in close proximity to the absorber installation, as 
shown in FIG. 3, and that any liquid that may accumulate in the gas piping 
drain back to the absorber tubes 25. The upper manifold 29 serves to 
contain an overflow. The overflow is located such that all tubes are 
flooding during normal operation. The overflow is sized sufficiently large 
to be filled to half its flow capacity such that the upper half of the 
upper manifold 29 is filled with gas. 
As stated above, the absorption of ammonia must proceed quickly in order to 
efficiently remove Point 5 of FIG. 2 from the crystal boundary. Absorption 
requires, however, that the gas to be absorbed contact the surface of the 
absorbing medium. Higher absorption can be achieved if the liquid-to-gas 
contact surface is large. Thus, the diameter of the absorber tubes 25 is 
determined so as to establish a churn/slug flow therethrough. The design 
criteria and procedure for this purpose are thoroughly discussed in the 
aforementioned thesis by Ferreira. During the absorption process in a 
vertical tubular absorber, the flow characteristics change from churn flow 
to slug flow, and finally to bubble flow as illustrated in FIG. 5 for 
different tube diameters. The churn flow region is merely an entrance 
effect due to high volumetric gas entrance velocities. In this region, 
there is a co-current upward flow of both phases. In the slug flow region, 
the gas phase rises in the form of bullet-shaped bubbles called slugs, 
large relative to the diameter of the tube and separated by liquid. The 
liquid flows downward at high velocity and forms a film around the gas 
bubbles. The bubble flow region is characterized by single isolated 
bubbles rising in co-current flow with a relatively large quantity of 
liquid. 
It has been shown, however, that the bubble flow region has much poorer 
heat transfer and mass transfer characteristics. The relationship between 
the gas phase mass flow rate and the absorber height is typically as shown 
schematically in FIG. 6. In other words, ammonia is most efficiently 
absorbed into the solution in lower parts of the absorber tubes 25. In 
order to make a more effective use of the absorber tubes 25, therefore, 
the solution receiver 35 is designed to be only partially filled with the 
ammonia-rich solution at maximum flow capacity of the absorber 20. A 
blower 45 is provided for controllably recycling the ammonia gas in the 
solution receiver back to the absorber 20 through its mixer 24 for the 
purpose of optimizing the operation of the absorber tubes 25 in the 
slug/churn flow region. 
Since the absorption of ammonia is an exothermic reaction, it is important 
that the absorbing medium be cooled effectively because the equilibrium 
condition of the reaction shifts as the temperature rises and this may 
cause a rise in pressure within the reaction vessel. For this purpose, the 
absorber 20 according to the present invention is adapted to be cooled 
evaporatively. In FIG. 3, numerals 31 indicate spray nozzles, and the 
absorber unit shown in FIG. 3 is incorporated onto a standard commercially 
available fan housing and sump section (only schematically shown at 21 in 
FIG. 1). 
The present invention has been described above with reference to only one 
example, but this example is intended to be merely illustrative, and not 
as limitative. In particular, FIG. 1 is a schematic process diagram and 
should be so interpreted. Many modifications and variations are allowed 
within the scope of this invention. For example, the condenser 16 and the 
absorber 20 may be combined together as a single evaporative 
absorber-condenser unit. Such a combined unit is advantageous in several 
respects such as lower operating temperature capabilities than are 
possible with cooling tower systems, initial cost savings and space 
saving. Fan horsepower is comparable to cooling tower systems and is about 
one-third of an equivalent air-cooled unit. Because of the low pumping 
head and reduced water flow, water pumping horsepower is approximately 25% 
of that required for the normal cooling tower-condenser-absorber 
installation. Because the cooling tower, heat exchange surface, water 
circulating pump and water piping may be combined in one assembled piece 
of equipment, the cost of handling and installing separate components is 
reduced. Such a combined unit may require only about 50% of the plan area 
of a comparably sized air-cooled installation. 
In summary, such modifications and variations that may be apparent to a 
person skilled in the art are intended to be within the scope of this 
invention.