Refrigeration – With indicator or tester – Condition sensing
Reexamination Certificate
2000-05-31
2001-09-25
Tanner, Harry B. (Department: 3744)
Refrigeration
With indicator or tester
Condition sensing
C062S126000
Reexamination Certificate
active
06293114
ABSTRACT:
TECHNICAL FIELD
This invention relates to apparatus and a method for monitoring a refrigerant state in a refrigeration system and, more particularly, to apparatus and a method in which a charge sensor is positioned adjacent to the outlet of an evaporator and sends a voltage output signal to a controller in response to an input signal from the controller, and in which the controller compares the output signal to a predetermined set point chosen to correspond to a predetermined refrigerant state.
BACKGROUND INFORMATION
As used herein, the term “refrigeration system” includes mobile air conditioning systems, such as automotive, heavy trucking, agricultural, construction, and mining equipment air conditioning systems; stationary air conditioning systems; stationary refrigeration equipment, such as refrigeration and freezer containers and storage refrigerators and freezers; and building heating ventilation and air conditioning systems. A typical refrigeration system includes an evaporator, a compressor, a condenser, and an expansion device. The system may also include additional devices to enhance the functioning of the system. Commonly found devices in refrigeration systems include thermostats, pressure sensors, and switches to engage and disengage components of the system to enhance system performance and/or prevent damage due to system operation under undesirable conditions.
In a refrigeration system, a refrigerant circulates through the system. The evaporator absorbs heat from the area to be cooled, which causes the refrigerant in the evaporator to boil off into a gaseous state. The refrigerant flows from the evaporator outlet to a compressor, in which the refrigerant is pressurized to a high pressure condition. From the compressor the refrigerant circulates first to a condenser, where the refrigerant is cooled to a liquid state, and then to an expansion device, in which the pressure drops down to a low pressure. From the expansion device, the refrigerant circulates back to the evaporator, and the cycle is repeated. Efficient and safe operation of the system requires that proper refrigerant circulation and an appropriate refrigerant charge level be maintained.
It is well known that operating an air conditioning system at a low refrigerant charge condition can cause serious problems. These problems include damage to the compressor due to reduced lubricant circulation since the circulating refrigerant normally carries the lubricant. The problems also include compressor leaks or damage due to low or negative suction pressures, premature compressor clutch failure due to rapid clutch actuation, reduction in fuel economy, loss of air conditioning cooling performance, and operator annoyance. In addition, where the low charge condition is a result of an air conditioning system leak, the condition presents the problem of undesirable emission of refrigerant gases into the environment.
Historically, there have been many difficulties associated with the reliable detection of refrigerant charge levels in mobile air conditioning systems. Because of the wide range of possible operating conditions, both static and dynamic, a low charge state under a particular set of operating conditions looks identical to a full charge state under a different set of operating conditions. Therefore, even devices that appear to function in most cases will sometimes generate unacceptable false low charge alarms. Most known detection systems use a combination of two or more temperature sensors, pressure switches, or pressure transducers. Those that do not tend to be particularly unreliable.
BRIEF SUMMARY OF THE INVENTION
The present invention uses a combination of a charge sensor and a controller to monitor a refrigerant state in a refrigeration system having an evaporator with an outlet.
According to an aspect of the invention, apparatus for monitoring the refrigerant state includes a charge sensor positioned adjacent to the outlet of the evaporator. A controller is electrically connected to the sensor to provide an input signal to the sensor. The sensor produces a voltage output signal in response to the input signal. The controller receives the output signal, processes it, and compares it to a predetermined set point chosen to correspond to a predetermined refrigerant state.
The preferred form of the charge sensor is a self-heated thermistor. In the currently preferred embodiments, the sensor is a self-heated NTC type thermistor. The positioning of the sensor may be varied. Preferably, it is mounted as close as physically possible to the exit of the evaporator outlet. It may be positioned in the stream of refrigerant flow, adjacent to the flow or set back from the flow. Currently, it is preferred that the thermistor be positioned adjacent to the flow exiting the evaporator through the outlet. The optimal position is currently believed to be one in which the sensor is placed radially around the evaporator outlet at about 90° to vertical.
The controller preferably compares the output signal to the set point at preset intervals and computes an average for a predetermined time duration. This averaging of the signal over a period of time helps to prevent incorrect indications of the refrigerant state due to transitory conditions.
The input signal from the controller to the sensor may take various forms. In a first embodiment, the controller applies an at least substantially constant current to a circuit interconnecting the controller and the charge sensor. In a second embodiment, the input signal is a voltage varied to maintain a constant temperature of the thermistor. In a third embodiment, the controller applies an at least substantially constant voltage to a voltage divider circuit to drive the charge sensor. The choice of the type of input signal is determined at least partially on the basis of the purpose for monitoring the refrigerant state in a particular system.
It is currently anticipated that the method and apparatus of the invention will be used primarily for detecting a reduced refrigerant charge. According to an aspect of the invention, the apparatus includes a charge sensor positioned adjacent to the outlet of the evaporator, and a controller electrically connected to the sensor to provide an input signal thereto. The sensor produces a voltage output signal in response to the input signal. The controller receives the output signal, processes it, and compares it to a predetermined set point chosen to correspond to a predetermined refrigerant state indicative of a reduced refrigerant charge.
The apparatus for detecting a reduced refrigerant charge may include one or more of the preferred or alternative features discussed above. When the purpose is to detect a reduced refrigerant charge, the alternative of a voltage divider circuit to which a substantially constant voltage is applied is the preferred option for the input signal from the controller.
Preferably, the controller compares the output signal to a first predetermined set point to determine whether a low charge condition exists. It also compares the output signal to a second predetermined set point to determine whether a very low charge condition exists. This feature allows different warnings or signals to be produced by the controller in response to conditions that the operator should be aware of but that do not present an immediate danger of damage to the system, and conditions that do present a danger of immediate damage. For example, for the former case, a warning signal may be produced. For the latter case, the controller can produce a signal which causes a component of the refrigeration system to cease operation.
One of the major advantages of the apparatus and method of the invention is that they permit determination of a low charge condition on the basis of output from a single charge sensor. Systems which make the determination on the basis of the single charge sensor described above provide significantly improved performance over known systems and, thus, accomplish a major goal of the invention. However, this improved performance can b
Pauly Joan H.
Red Dot Corporation
Tanner Harry B.
LandOfFree
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