Refrigeration – Processes – Compressing – condensing and evaporating
Reexamination Certificate
2002-09-23
2003-12-16
Norman, Marc (Department: 3744)
Refrigeration
Processes
Compressing, condensing and evaporating
C062S197000
Reexamination Certificate
active
06662576
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a high efficiency refrigeration system and more specifically, to a refrigeration system utilizing a bypass path to perform refrigerant de-superheating outside the condenser thereby increasing the overall system efficiency.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram of a conventional refrigeration system, generally denoted at
10
. The system includes a compressor
12
, a condenser
14
, an expansion device
16
and an evaporator
18
. These components are connected together via copper tubing such as indicated at
20
to form a closed loop system through which a refrigerant such as R-12, R-22, R-134a, R-
407
c, R-410a, ammonia, carbon dioxide or natural gas is cycled.
The main steps in the refrigeration cycle are compression of the refrigerant by compressor
12
, heat extraction from the refrigerant to the environment by condenser
14
, throttling of the refrigerant in the expansion device
16
, and heat absorption by the refrigerant from the space being cooled in evaporator
18
. This process, sometimes referred to as a vapor-compression refrigeration cycle, is used in air conditioning systems, which cool and dehumidify air in a living space, in a moving vehicle (e.g., automobile, airplane, train, etc.), in refrigerators and in heat pumps.
FIG. 2
shows the temperature-entropy curve for the vapor compression refrigeration cycle illustrated in FIG.
1
. The refrigerant exits evaporator
18
as a saturated vapor (Point 1), and is compressed by compressor
12
to a very high pressure. The temperature of the refrigerant also increases during compression, and it leaves the compressor as superheated vapor (Point 2).
A typical condenser comprises a single conduit formed into a serpentine-like shape with a plurality of rows of conduit lying in a spaced parallel relationship. Metal fins or other structures which provide high heat conductivity are usually attached to the serpentine conduit to maximize the transfer of heat between the refrigerant passing through the condenser and the ambient air. As the superheated refrigerant gives up heat in the upstream portion of the condenser, the superheated vapor becomes a saturated vapor (Point 2a), and after losing further heat as it travels through the remainder of condenser
14
, the refrigerant exits as saturated liquid (Point 3).
As the saturated liquid refrigerant passes through expansion device
16
, its pressure is reduced, and it becomes a liquid-vapor mixture comprised of approximately 20% vapor and 80% liquid. Also, its temperature drops below the temperature of the ambient air (Point 4 in FIG.
2
).
Evaporator
18
physically resembles the serpentine-shaped conduit of the condenser. Air to be cooled is exposed to the surface of the evaporator where heat is transferred to the refrigerant. As the refrigerant absorbs heat in evaporator
18
, it becomes a saturated or slightly superheated vapor at the suction pressure of the compressor and reenters the compressor thereby completing the cycle (Point 1 in FIG.
2
).
FIG. 3
shows the temperature-entropy curve for the vapor compression refrigeration cycle, in which the de-superheating process in the condenser is indicated explicitly. The pressure of the discharge vapor from the compressor has to be raised such that the phase-change temperature (known as the saturation temperature) at the saturation pressure can be large enough to reject heat at the condenser. This requires that the discharge vapor from the compressor is superheated as the entropy increases slightly over the compressor as shown in FIG.
2
. Typically one-third of a condenser is utilized for the de-superheating process in most air-conditioning and refrigeration systems.
This is a source of significant inefficiency in conventional refrigeration systems as the condenser must be larger and more costly than needed for the heat transfer function involving the phase-change of the refrigerant. Conversely, for a condenser of a given size, if the first one-third does not need to be devoted to de-superheating, greater subcooling could be achieved.
An additional benefit which could be achieved by performing the de-superheating step outside the condenser would be an improved energy-efficiency ratio (EER). This is defined as Qv/Wc, where Qv is the heat absorption by the evaporator of the system and Wc is the work done by the compressor. By increasing subcooling for a given size condenser, a greater quantity of liquid in the refrigerant would enter the evaporator. This would increase the cooling capacity Qv, thus the EER would also increase. Furthermore, as the condenser becomes more efficient, the condenser pressure decreases, reducing the required pressure lift across the compressor, thereby reducing the compressor work and accordingly increasing the EER.
FIG. 4
illustrates a modified temperature-entropy curve showing what would happen if the de-superheating step could be performed between the compressor and the condenser. Heat would be removed from the vapor discharged from the compressor, reducing the temperature of the vapor substantially while the saturation pressure is almost unchanged. Consequently, the vapor from the compressor could enter the condenser at or close to its saturation temperature and pressure. This is illustrated in the modified temperature-entropy curve of
FIG. 4
between points
2
c
and
2
a
. Up to now, however, no suitably cost effective technique has been available to eliminate the need for de-superheating in the condenser.
Therefore, a need clearly exists for a cost-effective way to achieve de-superheating at the inlet side of the condenser. The present invention seeks to meet this need.
SUMMARY OF THE INVENTION
According to the present invention, the de-superheating step is performed on the inlet side of the condenser, rather than in the condenser. To achieve this, a portion of liquid refrigerant exiting from the condenser is diverted into a bypass line from which it is re-injected into the primary refrigerant path at a location between the evaporator outlet and compressor inlet. In the bypass line, a secondary expansion valve is used to throttle the diverted liquid refrigerant from the condenser, thus decreasing the temperature substantially below the condenser outlet temperature.
The cooled refrigerant exiting the secondary expansion valve then passes through a heat exchanger which is thermally coupled to the primary refrigerant line between the compressor outlet and the condenser inlet. The heat exchanger removes heat from the refrigerant vapor exiting from the compressor, thus reducing its temperature. As a result, the refrigerant enters the condenser at or near its saturation temperature, and no portion of the condenser needs to be devoted to de-superheating.
Because the refrigerant pressure in the bypass line at the outlet of the heat exchanger is greater than the pressure at the evaporator outlet, a pressure differential compensating device is used to couple the outlet of the bypass line to the primary refrigerant line. The pressure differential compensating device can be either a vacuum generating device or a pressure-reducing device.
According to a first aspect of the invention, there is provided a refrigeration system including refrigerant compressing means, refrigerant condensing means, expansion means and evaporation means connected together to form a closed loop system with a refrigerant circulating therein, and a bypass line connected between the outlet of the condensing means and the inlet of the compressing means, the bypass line including a secondary expansion means, heat exchanging means to remove heat from the discharge vapor from the compressor between the outlet of the compressing means and an inlet of the condensing means, and a pressure differential accommodating means for mixing two vapors at two different pressures connecting the outlets of the evaporation means and the heat exchanging means to an inlet of the compressing means.
According to a second aspect of the invention, there is provided a refrigerat
Norman Marc
Ostrolenk, Faber, Gerb & Soffen, LC
VAI Holdings LLC
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