Method and apparatus for condensing and subcooling refrigerant

This invention provides a refrigeration system which includes in a closed loop connection a compressor for compressing a refrigerant into a condenser for condensing the compressed refrigerant into a liquid refrigerant, a control valve to controlling the discharge of the liquid refrigerant from the condenser into a reservoir, a sensor for measuring the temperature of the liquid refrigerant near the control valve, a fan for circulating air thorough the condenser, a sensor for measuring the ambient temperature of the air flow through the condenser, and an electronic control system to control various functions of the refrigeration system, including the flow of the liquid refrigerant through the condenser as function of the temperature difference between the ambient temperature and the temperature of the liquid refrigerant. During operation, a minimal flooding of the condenser is always maintained; i.e., a certain amount of liquid refrigerant is always trapped to thereby subcool the liquid refrigerant before discharging it into the reservoir at all ambient temperatures. The liquid refrigerant flow is decreased when the temperature difference is greater than a predetermined value and is increased when the temperature difference is less than the predetermined value. Further improvements in efficiencies are obtained by controlling air flow through the condenser and compressing refrigerant vapors from the reservoir into the condenser.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to a refrigeration system and more particularly to 
an apparatus and method for improving refrigeration system efficiencies by 
subcooling the refrigerant in the condenser of the refrigeration system. 
2. Description of Prior Art 
It has been known in the art of refrigeration systems that the net 
refrigerating effect in a refrigeration system can be improved by 
producing subcooling of the liquid refrigerant. Subcooling the refrigerant 
means that further energy is taken out of the liquid refrigerant, and as a 
consequence it does not have to be removed by the expansion process in the 
cooling evaporator, thus improving the overall efficiency of the 
refrigeration system. As an example, it has been known that when a 
refrigerant leaving a condenser of a refrigeration system has been 
accumulated in a reservoir, it can then be circulated in the liquid form 
through another cooling section to produce subcooling of the refrigerant 
at a small additional operating cost. This method, however, requires an 
increased amount of refrigerant, which is undesirable. 
In a refrigerator system, it is typical to equip a condenser with a flood 
control means which elevates the condensing pressure of a refrigeration 
system during low ambient temperatures by reducing the effective condenser 
surface that is available for condensing. This is accomplished by filling 
the condenser with liquid refrigerant when the pressure is not sufficient. 
These systems necessarily require increasing amounts of excess refrigerant 
to accomplish this flooding technique as the ambient temperature drops. 
This results in the use of additional refrigerant, which is undesirable 
because commonly used refrigerants, like Chloro-Fluoro-carbons ("CFCs"), 
are believed to increase the ozone depletion problem in the upper 
atmosphere. The additional refrigerant charge is generally lost when a 
leak occurs, which happens on the average several times over the life of a 
refrigeration system. This extra refrigerant usage may dramatically 
increase the amount of leakage of CFCs from refrigeration systems. 
Refrigeration systems currently available also attempt to maximize the 
subcooling effect during the colder periods of the year, i.e., at lower 
ambient temperatures. These systems require increased amounts of 
refrigerant to flood the condenser surfaces. One such system is described 
in U.S. Pat. No. 4,831,385, which performs subcooling during periods of 
low ambient temperature by utilizing a relatively complicated valve 
arrangement. This system ignores the subcooling at some ambient 
temperatures and instigates complicated measures to take advantage of 
subcooling in cold ambient temperatures. Subcooling of refrigerant to be a 
temperature that is closer to the ambient temperature of the systems will 
produce better efficiencies at all times of the year. Thus, subcooling 
within the condenser itself at all times, i.e., at all ambient 
temperatures, is a very desirable feature to have in a refrigeration 
system, which has gone unrecognized in the art. 
U.S. Pat. No. 4,621,505 also describes an arrangement to improve the 
subcooling effects during low ambient conditions. With respect to 
subcooling at higher ambients, this patent suggested that in summer 
operations when the ambient is above 85 to 90 degrees F., the condensation 
temperature and head pressures will be higher and little or no economic 
benefit can be expected. The need to benefit from subcooling has been 
known for some time in the refrigeration industry; however, to date, no 
method for achieving subcooling in a condenser at all ambients (high or 
low) has succeeded in the market place. 
Another type of a subcooling system is disclosed in U.S. Pat. No. 
4,136,528. It describes a system which provides subcooling to a degree 
that is sufficient to insure that the expansion valves operate properly in 
colder ambient conditions. This system fails to recognize that the 
subcooling in the summer time can provide further energy savings. This is 
another example where the need to subcool has been recognized for colder 
ambients, but the value of subcooling in higher ambients has been ignored. 
The attempts of the past to build in subcooling into a condenser have 
failed to recognize the necessity of holding the refrigerant in the liquid 
state for some time before allowing it to leave the condenser. In order to 
make thermal expansion valves function, hold-back valves have been used in 
the condensate line leaving the condenser to elevate the condensing 
pressure during low ambient conditions. This method produces liquid 
subcooling when the condenser is flooded with liquid. Once the industry 
recognized the benefits of subcooling, various methods have been utilized 
to subcool the refrigerant in colder ambient conditions. Hold-back valves 
used for this purpose have throttling ranges from fully open to fully 
closed of 20 to 60 psi have been used, which means that an additional 
inefficiency of higher condensing pressures during higher ambient and 
higher flow conditions have been introduced. 
The present invention provides a refrigeration system and method for 
increasing the subcooling effect of the condenser while utilizing minimal 
amounts of refrigerant. This system provides subcooling of the condenser 
during all ambient temperatures. 
SUMMARY OF THE INVENTION 
The invention provides for a refrigeration system which has in a closed 
loop a compressor for compressing a refrigerant, a condenser for 
condensing the compressed refrigerant into a liquid refrigerant, a control 
valve for regulating the liquid refrigerant flow through the condenser in 
a manner so as to subcool the liquid refrigerant to a desired level at all 
ambient temperatures during normal operations. An electronic control 
system is provided to control various functions of the refrigeration 
system, including the refrigerant flow through the control valve. 
The control system causes the flow through the control valve to increase 
when the temperature difference between the temperature of liquid 
refrigerant and the ambient is less than a predetermined value and causes 
the flow to decrease when the temperature difference is greater than the 
predetermined value. 
Alternately, the subcooling of the refrigerant may be accomplished by 
flooding the condenser by utilizing a passive device. In each of the 
embodiments, the condenser may be partitioned into sections and airflow 
across each section may be independently controlled. condenser.method for 
Examples of the more important features of the invention thus have been 
summarized rather broadly in order that the detailed description thereof 
that follows may be better understood, and in order that the contribution 
to the art may be better appreciated. There are, of course, additional 
features of the invention that will be described hereinafter and which 
will form the subject of the claims appended thereto.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention provides a refrigeration system and method wherein 
improved efficiency is obtained by compressing a refrigerant to a high 
pressure and temperature, condensing the refrigerant within a condenser, 
regulating the flow of condensate leaving the condenser with a flow 
control valve, measuring the subcooling of the condensate and controlling 
the condensate flow out of the condenser so as to obtain a desired amount 
of subcooling. 
The refrigeration system of the invention includes a compressor, condenser, 
evaporator, and control system which provides subcooling within the 
refrigerant condenser by regulating the flow of the refrigerant through 
the condenser in a manner which always floods the condenser during normal 
operation. This is accomplished by not allowing the condensed liquid to 
leave the condenser unless it has been cooled below the condensate point. 
The condenser surface is allowed to drop to a temperature below the 
condensing temperature at the points where subcooling is to take place. By 
selectively allowing the liquid to remain in contact with the condenser 
surface in a controlled manner, further increase in efficiency of a 
refrigeration system during higher condensing temperatures is achieved. 
Electronic control is accomplished by monitoring the temperature difference 
between the subcooled liquid leaving the condenser and the ambient air 
entering the condenser and reducing the flow of the condensed refrigerant 
out of the condenser by means of a flow control valve, and if this 
temperature difference is greater than a predetermined value and 
increasing the flow, if this temperature difference is less than the 
preselected value. This allows subcooling while minimizing the amount of 
refrigerant necessary for the subcooling. In implementing such a control 
strategy, potential control problems arise with a stalled flow condition 
which can elevate the head pressures to an unsafe level. This problem is 
solved by several methods. 
In one method the control strategies are overridden, and unrestricted flow 
of the liquid leaving the condenser is allowed if such a condition is 
detected or suspected. In another, minimum flow points are established. In 
yet another, a safety valve (not shown) is installed in parallel with the 
flow control valve and allows condensed refrigerant to leave when a high 
pressure at the condenser is detected. Still in another, a safety valve is 
installed in parallel with the flow control valve which allows the 
condensed refrigerant to leave when a high differential pressure across 
the flow control valve exists. 
The functions of controlling the pressure based on the pressure and the 
subcooling based on temperature can be interchanged, i.e., the pressure 
can be controlled by controlling the flow valve and the subcooling is 
controlled by controlling the air flow. 
Passive subcooling control is accomplished in several ways. In one 
embodiment, an inverted trap in the tubing leaving the condenser may be 
used to maintain liquid within the condenser and to cause the liquid to 
subcool before it leaves the condenser. The degree of subcooling is not 
controlled, and consequently this is not as efficient as the previously 
described electronic control method; however, it still offers significant 
improvement over the conventional systems. 
In another embodiment a differential valve is placed in the outlet of the 
condenser, and the amount of liquid which is maintained in the condenser 
is dependent on the pressure differential of this valve. The densities of 
most refrigerants are similar, and they produce approximately 0.5 psi per 
foot of standing liquid. 
The effects of these methods can be enhanced by sloping the condenser 
tubing allowing better drainage and heat transfer. When the last pass of 
the condenser is tilted such that the refrigerant is drained from the 
condenser by gravity, the gradient effect of the subcooling allows less 
condenser surface to be used to achieve a given result. 
The equalization of the pressures in the condenser and between the 
condenser and the receiver with equalization tubes allow gravity to be 
used in a controlled fashion to empty the condenser and minimize the 
amount of condensate required to achieve a given effect. 
FIG. 1 depicts an embodiment of the refrigeration system of the present 
invention. The system includes at least one compressor, at least one 
condenser, at least one evaporator with an expansion device, at least one 
cooling fan, a reservoir for holding liquid refrigerant, a pressure sensor 
at the condenser outlet to measure the pressure of the condensed (liquid) 
refrigerant, a control valve to regulate the flow of the subcooled liquid 
refrigerant from the condenser to the reservoir, and a control circuit 
containing a microprocessor to control various functions of the 
refrigeration system including the control valve, secondary compressor and 
cooling fan. The refrigeration system also may contain a secondary 
compressor to controllably compress refrigerant vapor from the reservoir 
to the condenser inlet. 
Referring to FIG. 1, the refrigeration system depicted therein is a closed 
loop commonly piped multiple-stage refrigeration system. A vapor 
refrigerant at a low pressure is passed into compressors 14 and 18 via a 
refrigerant tube 10. The compressors 14 and 18 compress the refrigerant to 
a high pressure gaseous state and discharge it through refrigerant tubes 
22 and 24 into a condenser 28. The high pressure transducer 26 is 
installed in the refrigerant tube 24, which provides an electrical signal 
that is representative of the pressure of the gases in the refrigerant 
tube 24 to a microcontroller circuit 56. 
The microcontroller circuit 56 contains a microprocessor and other 
circuitry which enables it to acquire information from various sensors 
used in the refrigerator system, to process these signals and to control a 
variety of functions of the refrigeration system. 
Still referring to FIG. 1, the condensed refrigerant leaves the condenser 
28 through pipe 38 as a liquid. A temperature sensor 36 is installed on a 
liquid return line which measures the temperature of the liquid 
refrigerant and provides a corresponding signal to the microcontroller 56. 
A flow control valve 40 is installed in the liquid return line 38, which 
controllably discharges the liquid refrigerant from the liquid line 38 
into a main liquid reservoir 44 through a main tube 58. The operation of 
the control valve 40 is controleld by the microcontroller 56. The sequence 
and the method used to control the operation of the control valve 40 is 
described in more detail later. 
Another temperature sensor 34 is provided near the condenser 28 to monitor 
the temperature of the ambient air entering the condenser 28. Sensor 34, 
like other sensors in the refrigeration system, provides an electrical 
signal to the microcontroller which is representative of the ambient 
temperature. 
The liquid from the reservoir 44 flows through a tube 58 into a liquid 
manifold system 57, where it enters a liquid tube that is connected to 
expansion valves 50 and 52. Each expansion valve 50 and 52 is connected to 
separate evaporators 54 and 55, respectively. These evaporators form a 
single temperature refrigeration system wherein the expansion valves 50 
and 52 meter the liquid refrigerant into a gaseous state within its 
respective evaporator at a low pressure and a low temperature. The vapor 
refrigerant is passed to the compressors 14 and 18 through the suction 
refrigerant tube, which completes a refrigerant cycle that is continuously 
repeated during operation 
The operation of the control valve will now be described in more detail 
while referring to FIGS. 1 and 2. As described earlier, the temperature 
sensor 36 measures the temperature of the liquid refrigerant leaving the 
condenser 28, and the temperature sensor 34 measures the ambient air 
temperature entering the condenser 28. When the refrigerator system is 
operating, the high pressure gaseous refrigerant from the compressors 14 
and 18 is cooled in the condenser to condense it into a liquid state. As 
the refrigerant vapor travels through the condenser 28, it begins to 
condense into droplets on the inner walls of the condenser pipes. The 
control valve 40 present the flow of the entire liquid refrigerant from 
the condenser 28 to the reservoir 44, thereby enabling some of the liquid 
refrigerant to accumulate in the condenser pipe 38. The microcontroller 56 
regulates the liquid refrigerant flow through the control valve 40 as a 
function of the difference between the liquid refrigerant temperature 
(ascertained by the temperature sensor 36) and the ambient temperature 
(ascertained by the temperature sensor 34). The temperature difference 
between the liquid refrigerant temperature and the ambient temperature 
("t") is greater than a predetermined value, the microcontroller 56 
decreases the flow through the control valve 40. On the other hand when 
the temperature difference t is less than the predetermined value, the 
microcontroller increases the flow through the control valve 40. A time 
delay between successive decisions to alter the flow through the control 
valve is programmed into the microcontroller to smooth out the operation 
of the control valve. In practice, the microcontroller is programmed to 
regulate the liquid refrigerant flow through the control valve so as not 
to fill the condenser excessively, because that will increase the liquid 
refrigerant pressure at the sensor 26, which in turn will decrease the 
system efficiency. The above described decision making process is 
illustrated in the flow chart of FIG. 2. The operation or the method 
described above ensures that during operation there is always maintained 
an amount of liquid refrigerant in the condenser which is sufficient to 
provide subcooling of the liquid refrigerant before it is discharged into 
the reservoir 44. The liquid refrigerant flow through the control valve 
may be controlled by either pulse modulating or analog modulating the flow 
control valve 40. It is desirable not to let the flow through the 
condenser stop completely, because that can result in loss of control. 
This can be accomplished, in the case of a pulse modulated valve, by 
providing a minimum pulse width or a minimum duty cycle. In summary, the 
continuous flooding of the condenser by controlling flow through the 
control valve 40 provides subcooling at all ambient temperatures, which 
increases the efficiency of the refrigeration system. 
Determining what the desired condensing pressure is may depend upon factors 
such as whether the system is equipped with hot gas defrost or a reheat 
condenser. 
Further improvement in the overall system efficiency may be obtained by 
regulating the fan speed as a function of the discharge pressure of the 
gaseous refrigerant into the condenser. The microcontroller 56 also 
controls or regulates the fan 32 to optimize the condensation of the 
gaseous refrigerant entering the condenser 28. The flow chart of FIG. 3 
shows the control logic for the fan 32. As shown in FIG. 3, when the 
pressure represented by the pressure transducer 26, i.e., the discharge 
pressure of the gaseous refrigerant entering into the condenser 28 is 
above a predetermined value, the microcontroller will increase the fan 
speed, thereby causing it to increase air flow through the condenser. On 
the other hand, when the discharge pressure is below the predetermined 
value, the microcontroller will decrease the fan speed, thereby decreasing 
the air flow through the condenser. Also, a time delay between successive 
speed controls is provided to avoid changing the fan speed too frequently. 
Referring back to FIG. 1, in certain applications it may be more desirable 
to directly transfer the liquid refrigerant from the condenser 28 to the 
reservoir 44. This, however, would allow the liquid refrigerant 48 to 
absorb energy while in the reservoir, especially when the reservoir is not 
sufficiently insulated. This loss of energy will result in a lower overall 
efficiency of the refrigeration system. 
Still referring back to FIG. 1, the present invention provides further 
improvement in overall system efficiency by providing a secondary 
compressor 100 in an equalization line 46 disposed between the reservoir 
44 and the condenser inlet. The secondary compressor 100, when in 
operation, compresses the refrigerant vapor from the reservoir 44 to the 
condenser inlet, where they are mixed with the high pressure gaseous 
refrigerant from the compressors 14 and 18. This evaporation action 
removes heat from the liquid refrigerant in the reservoir 44, and thus 
reduces the temperature and pressure in the reservoir 44, thereby further 
improving the overall efficiency of the refrigeration system. The 
compressor action is controlled by the microcontroller 56, which receives 
input from a pressure transducer 101 and a liquid level transducer 102, 
both of which are mounted on the reservoir. Transducer 102 senses the 
liquid level in receiver 48 and turns off compressor 100, if the level is 
too high. The reduced pressure in the reservoir also improves the draining 
of the liquid refrigerant from the condenser 28 to the reservoir 44. 
Additionally, this method maintains a more constant pressure on the liquid 
line 60, which further improves operation of the expansion valves 50 and 
52. This also allows for more of the energy to be taken out of the 
refrigerant by the compressor 100, which has a higher coefficient of 
performance than compressors 14 and 18. Since high compression ratios are 
not of benefit, compressor 100 may be of a scroll or centrifugal type. 
In the system of the invention described above, the amount of subcooling is 
controlled by the amount of flooding in the condenser, and the condenser 
pressure is controlled by the air flow through the condenser 28. In FIGS. 
2 and 3, temperature pressure may be interchanged so that the condenser 
pressure is controlled by the amount of flooding in the condenser while 
the amount of subcooling is controlled by the amount of air flow through 
the condenser. 
In some applications, it may be more desirable to partition the condenser 
area into zones with sections and to independently regulate the airflow 
through each zone. One such system is shown in FIG. 4. The refrigerant 
enters at the condenser inlet 29 and leaves through the outlet end at 76. 
Here the condenser is sectioned with partitions such as partitions 73A and 
73B. This is done to more evenly and efficiently control the pressure that 
is maintained within the condenser 28. The ambient air TA at 77A, 77B and 
77C flowing into each section is independently controlled by controlling 
fans 75A, 75B and 75C, respectively. In this manner a higher airflow may 
be maintained in one section than another section. An advantage in 
providing a higher rate of air flow through section 77A and less through 
Section 77B and even less through section 77C is that it will aid in 
subcooling the refrigerant leaving the condenser 28. 
In the condenser system of FIG. 4, the amount of subcooling is controlled 
by the amount of flooding in the condenser. Instead of utilizing an 
electrically controlled control valve as shown in FIG. 1, a passive or 
non-reactive type flooding mechanism may be used. An inverted trap 76 is 
provided at or near the end of the condenser pipe. The inversion in the 
trap, extending no higher than one-half the height of said condenser tubes 
as shown in FIG. 4, determines the amount of flooding that is maintained 
in the condenser and thus the subcooling of the refrigerant in the 
condenser. One or more of the condenser tube sections, such as section 74, 
may be sloped as desired. Typically, refrigeration systems utilize a 
plurality of parallel condenser tubes 72 to condense the high pressure 
refrigerant. In such a configuration, the high pressure vapor refrigerant 
enters into one or more manifolds, such as manifold 29, which distributes 
the refrigerant to the various refrigerant tubes. The condensed 
refrigerant from the condenser tubes is discharged into one or more 
manifolds such as manifold 78 shown in FIG. 4A. An inverted trap 76 for a 
multiple-tube condenser system may be installed at one end of each 
manifold 78. It should be obvious that various configurations of the 
inverted trap 76 may be adapted. 
FIG. 5 shows another embodiment of a condenser system of FIG. 4. Here the 
condenser 28 is partitioned in a manner similar to the condenser system of 
FIG. 4. The condenser may contain one or more condenser tubes. The last 
tube or the tubes 80 may be maintained horizontal. An inverted trap 82 is 
used. In a multiple tube condenser configuration system, a configuration 
similar to one shown in FIG. 4 with the inverted trap 82 may be utilized. 
FIG. 6 shows another passive method of flooding the condenser, i.e., the 
control is not reacting to the actual amount of subcooling which is taking 
place in the condenser. Instead, it is maintaining a liquid presence in 
the condenser by means of trapping the liquid. The embodiment depicted in 
FIG. 6 accomplishes this with differential pressure regulating valve 90 
which responds to the weight of the liquid within the condenser. Liquid 96 
has a weight associated with the height of its accumulation, typically on 
the order of 0.5 psi per foot. As the weight of the liquid divided by the 
cross-sectional area of the valve opening exceeds the pressure 
differential established by the differential pressure valve 90, the valve 
opens allowing liquid to be transmitted. The pressure differential can be 
determined by measuring the pressure 92 pH and 94 Pl and computing the 
difference. This pressure differential can be equated to Lh (the height of 
the liquid) 98 which is maintained in the condenser. A further improvement 
in the construction of condenser 28 can be achieved by sloping the last 
pass of condenser tubing 96. Optional equalization tubes 95A, 95B and 95C 
will improve the flow of condensate through the condenser by allowing the 
liquid which condenses in any tube to drain directly out at that point and 
not have to run through all remaining tubes before leaving the condenser. 
It may be desirable to pipe in parallel with flow control valve 40 of FIG. 
1, a differential pressure valve 90 of FIG. 6. This arrangement will 
provide a fail-safe method of preventing an excess accumulation of the 
liquid refrigerant. 
FIG. 7 shows another embodiment of a condenser system wherein a liquid 
level transducer 97 is used to monitor the liquid level in condenser 28. 
This signal from the transducer 97 is used by micro-controller 56 to 
control the liquid level. Flow control valve 40 is used in parallel with 
the differential valve 90. Differential valve 90 serves to protect against 
a failure of valve 40 to operate properly. Liquid level transducer 97 may 
also be used to determine in conjunction with liquid level monitor 102 the 
amount of liquid residing on the condensing side of the refrigeration 
system. 
The condenser should be designed in such a way that the high pressure vapor 
refrigerant entering the condenser does not heat the subcooled refrigerant 
leaving the condenser. Therefore, it is beneficial to control the air flow 
through the condenser by modulating the speed of all the fan motors which 
are running, so as to have air flow over the entire condenser surface. 
This provides a temperature gradient through the condenser and allows the 
bottom passes to subcool the refrigerant more easily. If one of the fans 
is cycled, it is necessary to stop the back feed of air through the fan 
which is cycled. This is typically achieved with swinging dampers located 
in the discharge air of the fans which are held open by the flow of air. 
In the case of a water cooled condenser the method of water flow through 
the condenser can achieve the same optimization. The air flow may also be 
modulated by motorized dampers on the face of the condenser. 
It should be noted that any of the embodiments of the condenser system 
depicted in FIGS. 4-7 may be used in the refrigeration system shown in 
FIG. 1. The amount of subcooling and the control thereupon will depend 
upon the embodiment chosen. 
While the invention has been described in accordance with air cooled 
condensers, one experienced in the art may easily apply the invention to 
water or fluid cooled condensers of all sorts. It is intended that the 
current patent shall apply to a variety of condensers. These embodiments 
have not been specifically described, because they are considered 
redundant in application of the invention in view of the above 
description.