1. Technical Field
The present invention relates generally to air conditioning systems and more particularly to an air conditioning system and a method which detects a low-charge state.
2. Discussion
Modern air conditioning systems typically include a compressor, a condenser, a throttling device and an evaporator. Operation of the compressor adds heat to a gaseous refrigerant as well as increases its pressure. High-temperature, high-pressure gaseous refrigerant exiting the compressor is delivered to the condenser where excess heat is removed, causing the refrigerant to condense to a relatively low-temperature, high-pressure liquid refrigerant. The liquid refrigerant is then discharged to the expansion valve.
The expansion valve meters the amount of refrigerant that is discharged to the evaporator, causing the low-temperature, high-pressure liquid refrigerant to change to a lower-temperature, low-pressure gaseous state. A blower forces air over a heat exchanger surface on the evaporator causing the gaseous refrigerant to absorb heat, cooling the air. Gaseous refrigerant is then returned to the compressor.
To maintain the performance of the air conditioning system, it is necessary that the system be properly charged (i.e., the system must have a quantity of refrigerant that exceeds a predetermined minimum amount). If the air conditioning system looses a sufficient amount of refrigerant, the air conditioning system will not cool the air to the maximum extent possible. Furthermore, operation of the air conditioning system in a low-charge state may damage the compressor, which is typically the most expensive component of the air conditioning system.
Conventional air conditioning systems do not include a means for detecting a low-charge state. Consequently, it is necessary to rely on the perception and judgment of the users of these systems to detect symptoms that are characteristic of a low-charge state. The symptom most readily detected with such systems is an output temperature of air exiting the evaporator that is xe2x80x9cwarmer than normalxe2x80x9d. Unfortunately, as the loss of refrigerant from an air conditioning system is usually gradual, the user is not likely to notice the change in the output temperature until a substantial amount of refrigerant has been lost from the system.
Complicating matters is that technicians responsible for trouble-shooting and maintaining these air conditioning systems have no direct means for detecting a low-charge state. As such, the technician is typically forced to employ a decision-making process having several steps of relatively low reliability to develop a plan for dealing with the observations of the air conditioning system user. The process usually includes the verification that the output temperature is relatively high and the re-charging the air conditioning system. Recharging the air conditioning system is a time consuming process, requiring that the refrigerant in the air conditioning system first be evacuated and then a proper quantity of fresh refrigerant be delivered to the air conditioning system. This process typically requires several hours to complete, tying up not only the technician, but also other resources such as the tooling, equipment and possibly even a service bay.
Considering modern standards of accuracy and repeatability, this trouble-shooting process renders it highly likely that some air conditioning systems are being recharged unnecessary. Furthermore, it is also likely that other air conditioning systems may not be being serviced when necessary. To avoid these situations, some air conditioning systems have proposed the use of a dedicated sensor in an attempt to more reliably detect a low-charge state. One such system relies on a low-pressure switch placed between the compressor and the evaporator. This system is premised on the fact that the liquid refrigerant delivered from the evaporator to the compressor will have a relatively lower pressure if the compressor is operated in a low-charge state. Not only does this approach add a considerable amount of cost to the air conditioning system, this approach requires a substantial reduction in the pressure of the refrigerant delivered to the compressor before a low-charge state is detected. Accordingly, it is possible in a system of this type that the low-charge state will go undetected for a considerable period of time, permitting the compressor to be operated repeatedly and damaged.
A second system relies on a sub-cool temperature sensor placed between the expansion valve and the condenser which monitors the temperature of the gaseous refrigerant delivered to the expansion valve. While this arrangement has been shown to be effective at detecting a low-charge state, it is extremely costly, being approximately three times more expensive than the low-pressure switch discussed above. Accordingly, there remains a need in the art for an air conditioning system which is able to detect a low-charge condition in a reliable manner and at a relatively low cost.
It is one object of the present invention to provide a method for detecting a low-charge state in an air conditioning system which provides early yet reliable results.
It is another object of the present invention to provide a method for detecting a low-charge state in an air conditioning system which may be economically incorporated into an air conditioning system.
It is a further object of the present invention to provide a method for detecting a low-charge state in an air conditioning system which employs the slope of the difference between the ambient temperature and the evaporator temperature to determine the existence of a low-charge state.
It is yet another object of the present invention to provide a method for detecting a low-charge state in an air conditioning system which employs the temperature of the evaporator to determine the existence of a low-charge state.
It is a further object of the present invention to provide an air conditioning system which detects a low-charge state in a reliable yet economical manner.
In one preferred form, the present invention provides a method for detecting a low-charge state in an air conditioning system. The method includes the steps of detecting a temperature of air exiting an evaporator and responsively producing an evaporator temperature signal; detecting an ambient air temperature and responsively producing an ambient air temperature signal; detecting at least one operational characteristic of the air conditioning system and responsively producing an operational signal in response thereto; and receiving the evaporator temperature signal, the ambient air temperature signal and the operational signal and responsively detecting a low-charge condition of the air conditioning system. An air conditioning system having a controller which prevents a compressor from cycling on the detection of a low-charge state is also provided.