Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-03-15
2001-11-20
Walberg, Teresa (Department: 3742)
Refrigeration
Automatic control
Refrigeration producer
Reexamination Certificate
active
06318101
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to the field of compressors used in chillers and/or heat pumps, and in particular, to using an electronic expansion valve to control cooler pinch and to protect discharge superheat.
BACKGROUND OF THE INVENTION
Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop to a relatively cooler side of the circulation loop. Compression of the refrigerant occurs at the hotter side of the loop, where a compressor raises the temperature of the refrigerant. Evaporation of the refrigerant occurs at the cooler side of the loop, where the refrigerant is allowed to expand, thus causing a temperature drop because heat is necessary for expansion. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor air, respectively, to make use of the outdoor air as either a thermal energy source or a thermal energy sink.
The process is reversible, so the heat pump can be used for either heating or cooling. Residential heating and cooling units are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
These types of heat pump systems operate only if there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger to maintain a transfer of thermal energy. For heating, the heat pump system is efficient provided the temperature difference between the air and the refrigerant is such that the available thermal energy is greater than the electrical energy needed to operate the compressor and the respective fans. For cooling, the temperature difference between the air and the refrigerant generally is sufficient, even on hot days. Air conditioners, or chillers, work similarly to heat pumps but only transfer heat from indoors to outdoors. In most other aspects, the cycles are the same.
When the refrigerant passes from the condenser to the evaporator, it passes through a valve such as an electronic expansion valve (EXV). The main flow control in the system is this expansion valve which permits the refrigerant to expand from the high pressure of the condenser to the lower pressure in the evaporator. The expansion causes part of the liquid to evaporate, thereby cooling the rest of the liquid to the evaporator temperature. The refrigerant level inside the evaporator is also controlled by the EXV, withe the refrigerant level determined based on sensor inputs. One sensor input is from a heat sensor inside the evaporator while a second input is from the saturated suction temperature. Based on these two inputs, an estimate of the liquid level inside the evaporator is obtained. A lot of problems arise from mounting the heat sensor inside the evaporator and from the lack of precision in using these two inputs to determine the liquid level within the evaporator.
SUMMARY OF THE INVENTION
Briefly stated, an electronic expansion valve (EXV) used in a refrigeration cycle for a heat pump or chiller is controlled to maintain minimum pinch for ensuring proper flooded cooler exchange performance by monitoring the delta temperature between the cooler fluid and the saturated suction temperature. The discharge superheat is monitored to protect the compressor from liquid slugging. If the discharge superheat is lower than the expected value, the EXV opening is adjusted. A controller monitors certain system variables which are used to determine the optimal position of the EXV to optimize the system performance, the proper discharge superheat value, and the appropriate refrigerant charge.
According to an embodiment of the invention, a method for controlling an electronic expansion valve in a refrigerant cycle system includes the steps of ensuring proper flooded cooler exchange performance by monitoring a change in temperature between a fluid in said cooler and a saturated suction temperature of the system, protecting a compressor of the system by monitoring a discharge superheat of the system, and correcting a size of an opening of the electronic expansion valve when the discharge superheat is lower than a predetermined value.
According to an embodiment of the invention, a method for controlling an electronic expansion valve in a refrigerant cycle system includes the steps of determining a saturated suction temperature (SST) for the system; determining a saturated discharge temperature (SDT) for the system; determining a cooler fluid temperature (CFT) for the system; determining a saturated condensing temperature (SCT) for the system; determining pinch as the CFT minus the SST; determining pinch error as the pinch minus a pinch setpoint; determining pinch rate as a current pinch minus a pinch of a specified time ago; determining discharge superheat as the SDT minus the SCT; determining discharge superheat error as the discharge superheat minus a discharge superheat setpoint; determining a discharge superheat rate as a current discharge superheat minus a discharge superheat of a specified time ago; setting a deadband of a specified temperature; and controlling a movement of the electronic expansion valve based on the pinch error, the pinch rate, the discharge superheat error, the discharge superheat rate, and the deadband.
According to an embodiment of the invention, an apparatus for controlling an electronic expansion valve in a refrigerant cycle system includes means for determining a pinch of the system; means for determining a pinch error of the system; means for determining a pinch rate of the system; means for determining a discharge superheat of the system; means for determining a discharge superheat error of the system; means for determining a discharge superheat rate of the system; means for setting a deadband of a specified temperature; and means for controlling a movement of the electronic expansion valve based on the pinch error, the pinch rate, the discharge superheat error, the discharge superheat rate, and the deadband.
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Pascal Cretin
Pham Ba-Tung
Carrier Corporation
Robinson Daniel
Walberg Teresa
Wall Marjama & Bilinski
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