Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-04-14
2001-11-27
Walberg, Teresa (Department: 3742)
Refrigeration
Automatic control
Refrigeration producer
C062S243000
Reexamination Certificate
active
06321549
ABSTRACT:
FIELD OF THE INVENTION
The field of the present invention relates to control systems for transport refrigeration systems. More specifically, the present invention is directed towards alternative means for implementing a control system governing the electronic expansion valve (the “EXV”), to precisely maintain the necessary capacity and/or pulldown during a variety of operating and/or ambient conditions.
DESCRIPTION OF THE PRIOR ART
A transport refrigeration system used to control enclosed areas, such as the insulated box used on trucks, trailers, containers, or similar intermodal units, functions by absorbing heat from the enclosed area and releasing heat outside of the box into the environment. A number of transport refrigeration units, including units currently sold by assignee, employ a reciprocating compressor to pressurize refrigerant to enable the removal of heat from the box. The system further includes an evaporator for drawing heat out of the box by drawing or pushing return air across refrigerant containing coils within the evaporator (the refrigerant is typically supplied to the evaporator from a condenser, which in turn receives refrigerant from the compressor). This step vaporizes any remaining liquid refrigerant flowing through the evaporator, which is then drawn through a suction modulation valve (SMV) into the compressor. Thus, in the evaporator, the refrigerant is superheated to a certain extent(superheat being defined as the difference between the temperature of a vapor and the saturation temperature of the same vapor at the same pressure).
Transport refrigeration systems currently employ a variety of controls to manipulate the operating envelope of a reciprocating compressor. As can be shown by U.S. Pat. Nos. 5,626,027 and 5,577,390, both assigned to the assignee of the present invention, compressors can be operated in a multi-stage mode or in single stage modes depending upon operating temperature. Such references further discuss generally the use of suction modulation for capacity control. Also, other current systems meet load requirements of conditioned space by reducing compressor speed, unloading compressor cylinders, and pulsing hot gas into the evaporator. However, currently available commercial designs, including those sold by assignee, do not offer the combination of discharge pressure control, expansion valve control, and superheat level control to maintain a transport refrigeration system within its designed operating envelope.
In addition, a number of problems have arisen within prior commercial transport systems which present needs addressed by the present invention. A serious problem exists in “floodback,” i.e., when the system sends liquid refrigerant to the compressor, thus adversely affecting compressor life. In addition, a need exists to maximize pulldown capacity (i.e., the system mode which reduces the temperature of the conditioned space) which creates problems due to the limitations on system power. Another such problem is the occurrence of “nuisance” shutdowns during high ambient temperature conditions due to unacceptable engine coolant temperature, compressor discharge temperature, and/or compressor discharge pressure. Yet another problem that exists during pulldown mode is the inability of the expansion valve, by itself, to control product humidity. Simply controlling power by controlling superheat with the expansion valve will cause low coil temperatures and therefore may result in unacceptable product dehydration. Another product control problem within transport refrigeration systems is “top freezing,” a phenomenon which occurs when the refrigeration system operates at or near freezing conditions. The air leaving the evaporator coil in such conditions can be well below freezing and can freeze the top portion of a perishable load unless extraordinary (and costly) insulating steps are taken.
The volume of refrigerant flowing into the evaporator (and thus flowing into the compressor) is controlled by an electronic expansion valve (EXV). Thus, the pressure and temperature of the refrigerant can be controlled via the control of the refrigerant flow rate through the EXV. The applicants have found that, in order to optimize maintenance of the reciprocating compressor of a transport refrigeration system within its design operating envelope, it is desirable to use a controller within the transport refrigeration system which actuates the EXV in response to deviations from a preselected level of superheat (as stored within memory or calculated by the controller), as compared to the superheat level calculated from the evaporator pressure transducer and the evaporator coil temperature sensor. This EXV control, in addition to the selective use of the compressor unloaders and/or engine speed control, is believed to solve the problems mentioned above.
SUMMARY OF THE INVENTION
The control process and system of the present invention uses an evaporator coil temperature sensor (EVAP), an evaporator pressure transducer (EPT), a compressor discharge pressure transducer (CDP), and an ambient temperature sensor (AAT). In a further alternative, the present invention could further include additional sensors to such as an engine water temperature sensor (WTS) and /or a compressor discharge temperature sensor (CDT) to control the shutdown of the transport refrigeration unit.
In essence, the transport refrigeration unit microprocessor (MICRO) uses inputs from the sensors referenced above to control the electronic expansion valve (EXV). Specifically the MICRO reads the EVAP and EPT inputs and calculates or approximates the actual evaporator coil superheat level within the system. The MICRO then compares this calculated superheat level and compares it with a desired superheat level which is stored within memory. The MICRO then generates control signals to close or open the EXV based upon differences between the calculated and desired superheat settings. The various desired superheat levels, in turn, will be set or determined by the MICRO depending upon which mode the transport refrigeration unit is in.
In its “base” setting, the controller monitors and calculates the superheat so as to minimize the level of superheating (short of floodback of liquid refrigerant into the compressor) and thus maximizing the capacity of transport refrigeration system. During pulldown (i.e., the mode in which transport refrigeration units are trying to reduce the temperature of the conditioned space), power is limited—this power limit can be approximated roughly as a constant according to the relationship of the ambient temperature and the compressor discharge pressure. Thus, by controlling the discharge pressure through the adjustment of the EXV, the system can limit compressor discharge pressure such that the maximum power limit is not exceeded. In another variant from the base setting, the cylinders may be loaded based upon box temperature, ambient temperature, or some preselected setpoint. Thus, the unloaders can be used to control the maximum superheat level, thus controlling humidity.
In yet another variant from the base setting, when the operation of the system has caused the temperature of the engine coolant to approach its safety limit, the controller will cause an additional superheat offset to be added to the base level. This system adjustment will effectively reduce the engine load and avoid a high coolant temperature shutdown. Likewise, the control of the EXV can be used to limit compressor discharge pressure, thus avoiding nuisance pressure shutdowns. In addition, the controller can avoid nuisance compressor discharge temperature shutdowns and excessive product dehydration by selectively loading or unloading cylinders in the compressor.
Finally, in another variant from the base setting, the control features of the present invention (specifically, the use of superheat offset) can be used to optimize partial load operation. The use of such a superheat offset reduces capacity and, more importantly, reduces unit fuel consumption.
Accordingly, one object of the present inv
Brendel Thomas Edward
Morse Douglas Herbert
Reason John Robert
Rusert Mead Robert
Carrier Corporation
Haller Timothy J.
Niro Scavone Haller & Niro
Robinson Daniel
Walberg Teresa
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