Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid
Reexamination Certificate
2002-06-04
2003-08-26
Doerrler, William C. (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Liquified gas transferred as liquid
C062S223000, C062S155000
Reexamination Certificate
active
06609382
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to air conditioning and refrigeration systems and more specifically to cryogenic refrigeration systems.
In previous cryogen based refrigeration systems, a controller using a fuzzy logic scheme controlled the system. While the fuzzy logic scheme is well suited to controlling a cryogen system, it takes a substantial amount of time to generate a prediction of the required motor speed. The prediction time that is required by the fuzzy logic system often allows the motor speed to oscillate. On occasion, the motor speed may oscillate between 1400 revolutions-per-minute (“RPM”) and 1600 RPM, thus bringing the temperature of the conditioned space to an undesired range, and consequently damaging the load. The oscillation may sometimes even cause instability of the system. Furthermore, cryogen systems often have several modes of operation such as Cool, Heat, or Defrost, and also user-programmable control is preferred. However, the current fuzzy logic controllers evaluate system sensors to determine which mode to implement with little or no user input. Therefore, a user-programmable control system that regulates the motor speed in a manageable fashion would be welcomed by users of such systems.
SUMMARY OF THE INVENTION
According to the present invention, a method of temperature control in a cryogenic system, wherein the system includes a cryogen tank, and wherein the cryogen tank contains a cryogen, includes providing a motor speed sensor, the motor speed sensor being operatively coupled to a proportional-integral-derivative controller, the motor speed sensor determining a motor speed and sending the motor speed to the proportional-integral-derivative controller, providing a pressure sensor in the cryogen, the pressure sensor being operatively coupled to the proportional-integral-derivative controller, the pressure sensor determining a pressure at an end of an evaporator coil, and sending the pressure to the proportional-integral-derivative controller, providing a temperature sensor in the conditioned space, the temperature sensor being operatively coupled to the proportional-integral-derivative controller, the temperature sensor measuring a temperature within the conditioned space and sending the temperature to the proportional-integral-derivative controller, providing a deprived integral region in a proportional band to the proportional-integral-derivative controller when the motor speed is close to a motor speed set point, and generating an overriding control signal at the proportional-integral-derivative controller when the temperature and the pressure are beyond a temperature set point and a pressure set point.
In another aspect of the invention, a method of controlling a cryogenic temperature system, wherein the cryogenic system uses a proportional-integral-derivative control, controls the temperature within a conditioned space, and includes a cryogenic tank, includes determining a motor speed, determining a pressure of a cryogen, determining a plurality of temperatures inside the conditioned space, determining a plurality of temperatures out side the conditioned space, determining a new motor speed based on the motor speed, the pressure, the temperatures, and a plurality of predetermined temperature and pressure tables, and actuating the motor based on the new motor speed.
In yet another aspect of the present invention, a method of conserving a heat absorbing liquid in a cryogenic temperature control system, wherein the system includes a controller and a motor, and wherein the controller adjusts a motor speed, includes setting a target motor speed, averaging the motor speed over a predetermined amount of time after the system has entered a temperature controlling mode, regulating the heat absorbing liquid after the predetermined amount of time, resetting the target motor speed to a new target motor speed if the average motor speed is less than or equal to a predetermined speed below the target motor speed, the new target motor speed being set below the average motor speed by a predetermined motor speed value, and adjusting the motor speed such that the motor speed approaches the new target motor speed for a second predetermined amount of time.
In still another aspect of the present invention, a cryogenic temperature control system includes a conditioned space containing a gas and a load, the gas having a gas heat and thereby also having a temperature, a heat exchanger in the conditioned space, the heat exchanger having a heat absorbing liquid, and the heat absorbing liquid absorbing the gas heat within the conditioned space thereby lowering the temperature within the conditioned space, a heat source, the heat source releasing heat into the conditioned space thereby increasing the temperature within the conditioned space, a fan adjacent to the heat source and the heat exchanger, the fan circulating the gas in the conditioned space thereby having a fan speed, a temperature sensor determining a temperature within the conditioned space, a pressure sensor determining a cryogenic pressure at an end of an evaporator coil, and a controller operatively coupled to the fan, the temperature sensor, the pressure sensor, the heat exchanger, and the heat source, the controller receiving the temperature from the temperature sensor, receiving the pressure from the pressure sensor, adjusting the fan speed within the proportional band based on the temperature, the pressure, and the fan speed.
The controller of the present invention uses a proportional-integral-derivative (“PID”) approach coupled with a “wrapper” program. The wrapper program evaluates the status of the refrigeration system, the environmental conditions, and user inputs to determine how the system should operate and in which mode the system should operate. This allows the user to quickly and easily override the program if desired. For example, a system installed on a truck may require quiet operation when passing through residential neighborhoods. The vapor motor fan is likely the largest producer of noise. A classical system using fuzzy logic would determine the fan speed based on the needs of the system. In the present invention, the user can set a lower vapor motor speed to provide quiet operation, and the system will compensate by adjusting other parameters such as cryogen flow.
A cryogen system controllable by the present method includes a micro-processor based controller, a cryogen tank for storage of liquid cryogen, a heat exchanger or evaporator, a heat exchanger fan driven by a vapor motor, a second heat exchanger for heating cryogen, and a heat source. In addition, a system uses valves and sensors throughout the system to control the flow of the cryogen and to monitor system parameters such as temperature at various points within the system and the CO
2
pressure. Cryogen refrigeration systems generally use carbon dioxide (“CO
2
”) or nitrogen (“N
2
”) as the cryogenic fluid, however other fluids can be used.
The control method of the present system allows for the use of a cryogen based refrigeration system in one of several modes. These modes include 1) Heat, 2) Defrost, 3) Cool, 4) Null, and 5) Quench. While these active modes are included in the proposed method, the addition of other modes is contemplated by the present invention. Each of these modes except Null mode uses a PID (Proportional, Integral, Derivative) control method to control the fan speed within the system. In addition to these modes, the system includes several protection algorithms. These protection algorithms include 1) two ambient lockouts, 2) Top Freeze Protection, 3) Superheat Protection, and 4) CO
2
Saver. While these algorithms are included in the contemplated system, the system is in no way limited to these alone.
To determine which mode the cryogen refrigeration system should be operating in at any given instant, a wrapper program or State Machine Program (“SMP”) is utilized. The SMP checks the ambient lockout status, and performs special functions for timer and flag initialization. Th
Glentz Joseph Louis
Schwichtenberg Bryan Edward
Seshadri Jayaram
Stuart William Ehrich
Vander Woude David J.
Doerrler William C.
Michael & Best & Friedrich LLP
Thermo King Corporation
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