Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2003-03-14
2004-09-07
Norman, Marc (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S212000, C062S224000, C062S225000, C062S513000
Reexamination Certificate
active
06786057
ABSTRACT:
The invention relates to an air-conditioning unit, especially for the passenger compartment of a vehicle, and to a method of controlling a refrigerant loop in such a unit, said loop containing a compressor suitable for receiving the refrigerant in the gaseous state and for compressing it, a refrigerant cooler suitable for cooling the refrigerant compressed by the compressor, at approximately constant pressure, by transferring heat to a first medium, an expander suitable for lowering the pressure of the refrigerant leaving the refrigerant cooler by taking it at least partly into the liquid state and an evaporator suitable for making the refrigerant in the liquid state coming from the expander pass into the gaseous state, at approximately constant pressure, by removing heat from a second medium in order to cool the space to be air-conditioned, the refrigerant thus vaporized then being sucked up by the compressor, the loop furthermore containing an internal heat exchanger allowing the refrigerant, flowing in a first path of the internal exchanger, between the refrigerant cooler and the expander, to yield heat to the refrigerant flowing in a second path of the internal exchanger, between the evaporator and the compressor.
To avoid the deleterious effects on the environment of the fluorocompounds conventionally used as refrigerants in the air-conditioning of motor vehicles, the use of carbon dioxide CO
2
is recommended.
This compound has a relatively low critical pressure, which is exceeded during compression of the refrigerant by the compressor so that the refrigerant is then cooled, without any phase change, by the refrigerant cooler which replaces the condenser of the conventional loop. In the absence of a phase change, only the reduction in the temperature of the refrigerant in the cooler allows thermal energy to be dissipated. Since this dissipation generally takes place in a stream of atmospheric air, it is necessary for the temperature of the refrigerant entering the cooler to be substantially greater than the atmospheric temperature. This is why the internal heat exchanger is used, which allows the refrigerant to be warmed up when it flows between the evaporator and the cooler and allows it to be cooled when it flows between the cooler and the expander.
The efficiency &eegr; of the internal heat exchanger, represented by equation [1]:
η
=
T
pi
-
T
eo
T
co
-
T
eo
[
1
]
in which T
pi
, T
eo
and T
co
are the compressor inlet temperature, the evaporator outlet temperature and the cooler outlet temperature respectively, is a decreasing function of the flow rate of refrigerant passing through it, according to equation [2]
&eegr;=
a·Q
b
[2],
a and b being characteristic constants of the internal exchanger.
The foregoing is true only when the internal heat exchanger receives from the evaporator refrigerant entirely in the gaseous state. If, on the contrary, it receives therefrom refrigerant in the liquid state, its efficiency is greatly reduced.
The object of the invention is to optimize the operation of the loop so as to avoid this drawback.
Moreover, in order for the stream of air cooled by the evaporator to be at a uniform temperature, it is necessary for the evaporator not to have an overheating zone, in other words for the refrigerant to vaporize right to the end of its path through the evaporator.
Another object of the invention is to satisfy this condition.
The aim of the invention is especially a method of the kind defined in the introduction and ensures that a first condition likely to reveal the presence of refrigerant in the liquid state in said second path is monitored and that the flow rate of refrigerant in the loop is reduced when said first condition is satisfied.
This mode of regulation, based on a thermodynamic principle, allows the operating conditions of the loop to be rapidly stabilized, without any oscillation. In particular, it avoids the appearance of a cold peak when the vehicle accelerates.
Optional features of the invention, which are complementary or alternative, are given below.
Said first condition consists in that the efficiency &eegr; of the internal heat exchanger, represented by equation
η
=
T
pi
-
T
eo
T
co
-
T
eo
[
1
]
in which T
pi
, T
eo
and T
co
are the compressor inlet temperature, the evaporator outlet temperature and the cooler outlet temperature respectively, is less than a reference value &eegr;
0
.
A second condition likely to reveal the existence of an overheating zone in the evaporator is additionally monitored and the flow rate of the refrigerant in the loop is increased when said second condition is satisfied.
Said second condition consists in that the efficiency &eegr; as defined in claim
2
is greater than or equal to a reference value &eegr;
0
.
The flow rate of the refrigerant is set approximately to the maximum value compatible with an efficiency &eegr; not less than the reference value.
Whatever the value of the flow rate, the value &eegr;
m
, taken by the efficiency &eegr; when the flow rate is a maximum and when said second path does not contain refrigerant in the liquid state, is adopted as reference value.
For a given value Q
p
of the flow rate, the value &eegr;
p
, taken by the efficiency &eegr; when said second path does not contain refrigerant in the liquid state, is adopted as reference value.
The flow rate is set by acting upon the expander.
To evaluate &eegr; on the basis of equation [1], a value measured by means of a sensor in thermal contact with the refrigerant is used for at least one of said temperatures.
To evaluate &eegr; on the basis of equation [1], the temperature of a stream of air having swept the evaporator and constituting said second medium is used to represent T
eo
.
T
pi
is compared with a setpoint value T
pi,set
such that
η
0
=
T
pi
,
set
-
T
eo
T
co
-
T
eo
and &eegr; is considered to be less than and greater than the reference value when T
pi
is less than and greater than said setpoint value, respectively.
The compressor is of the type with a variable swept volume with external control.
The compressor compresses the refrigerant up to a supercritical pressure.
The subject of the invention is also an air-conditioning unit, especially for the passenger compartment of a vehicle, suitable for implementing the method as defined above, comprising a refrigerant loop as defined, monitoring means for monitoring a first condition likely to reveal the presence of refrigerant in the liquid state in said second path and, optionally, a second condition likely to reveal the existence of an overheating zone in the evaporator, and means for controlling the flow rate of the refrigerant in the loop according to the result of this monitoring.
The unit according to the invention may include at least some of the following features:
the monitoring means comprise means for evaluating the temperatures T
pi
, T
eo
and T
co
at the compressor inlet, at the evaporator outlet and at the cooler outlet respectively, means for calculating from the latter the efficiency &eegr; of the internal heat exchanger on the basis of equation [1]:
η
=
T
pi
-
T
eo
T
co
-
T
eo
[
1
]
and means for comparing the efficiency &eegr; with a reference value;
means for determining the flow rate of the refrigerant in the loop and for determining said reference value from the flow rate;
the means for evaluating said temperatures comprise at least one temperature sensor in thermal contact with the refrigerant; and
the means for evaluating the temperature T
eo
comprise a temperature sensor in thermal contact with a stream of air having swept the evaporator.
REFERENCES:
patent: 5685160 (1997-11-01), Abersfelder et al.
patent: 6260367 (2001-07-01), Furuya et al.
patent: 6341496 (2002-01-01), Kettner et al.
patent: 6523360 (2003-02-01), Watanabe et al.
patent: 34 42 169 (1986-05-01), None
patent: 198 29 335 (2000-02-01), None
patent: 1 014 013 (2000-06-01), None
patent: 1 026 459 (2000-08-01), None
Courtney Ronald
Norman Marc
Valeo Climatisation
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