Refrigeration – Intermediate fluid container transferring heat to heat... – Flow line connected transfer fluid supply and heat exchanger
Reexamination Certificate
1998-09-21
2001-04-24
McDermott, Corrine (Department: 3744)
Refrigeration
Intermediate fluid container transferring heat to heat...
Flow line connected transfer fluid supply and heat exchanger
Reexamination Certificate
active
06220048
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to freeze drying, and more particularly, to a method and apparatus for improving the precision and efficiency of freeze drying using a reduced amount of cryogen consumption.
BACKGROUND OF THE INVENTION
Cryogenic heat exchanger are attractive design alternatives from the standpoint that they do not use environmentally damaging refrigerants, but instead use a cryogenic heat transfer fluid such as a liquefied atmospheric gas.
Previous work in this area does not address the issue of making efficient use of cryogens. In many cases, the temperature and energy requirements of the cryogen and/or other coolant fluids, heat exchanging apparatuses and heat storage apparatuses do not match, thus causing inefficiencies in the freeze drying method and apparatus.
There has been an attempt to ensure the equal heat distribution in the water-ice condenser which leads to the freeze drying chamber. In U.S. Pat. No. 5,456,084 to Ron Lee, an attempt is provided for a cryogenic heat exchange system in which water-ice build-up on a condenser heat exchanger surface employed in the cryogenic heat exchanger system is more uniform as compared to that of the then prior art heat exchangers which utilize a cryogenic heat exchange fluid. In that sense, attempts were made to provide better control over the temperature in which the heat transfer using the cryogenic heat exchanger system takes place.
In U.S. application Ser. No. 08/709,027 filed Sep. 6, 1996 entitled “Method and Apparatus for Controlling Freeze Drying Process”, which is incorporated herein by reference, there is provided a method and process which utilizes a single heat exchanger, cooled by a cryogenic refrigerant, to deliver cold heat transfer fluid directly to a condenser and, independently, to a freeze dryer or other refrigeration system, either directly or through a heater circuit, for cooling or heating the freeze dryer.
Notwithstanding the above, there is a need in the art for a method and apparatus to refrigerate the chamber shelves and water condenser of a freeze drying chamber utilizing a dispensable cryogen (primarily liquid nitrogen) and to allow the exhaust/waste gas from the cryogen supply to exit from the system at the warmest temperature possible, while at the same time, accomplishing with minimal pumping energy thereby for completing each freeze drying cycle with minimal refrigeration cost.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide a method for improving the matching of the condenser cooling demands with the low demands of the cryogenically cooled heat transfer fluid in the art.
Another object of this invention is to provide a method and apparatus to store excess refrigeration with the heat transfer fluid.
Yet another object of this invention is to provide a method and apparatus for supplying cryogen directly to vacuum condensers to achieve lower temperatures.
Another object of this invention is to provide a method and apparatus for recycling cold gas from the condensers for increased operating efficiency.
Another object of this invention is to provide a method and apparatus for condensing a refrigerant that does not require the mechanical compression and expansion.
SUMMARY OF THE INVENTION
As will be discussed hereinafter, the present invention provides a method and apparatus for improving the match of the condenser cooling demands with the varying demands of the cryogenically cooled heat transfer fluid to that which have been found in the art. This matching of cooling demands during a programmed freeze dry recipe provides a more efficient utilization of the cryogen. The freeze dry cycle process typically includes 1) temperature ramp-down; 2) temperature soak; 3) vacuum induction; and 4) temperature ramp-up. This process will contain heat loads that vary by factors of at least 2:1, and can most economically be handled by choosing the pump and heat exchanger combination that will best fit the heat load. The freeze chamber and shelves must operate at a warmer temperature than the condenser. Therefore, a heater is usually used even during the cool down cycle to form a second heat transfer fluid recirculating loop. Such a process produces a high energy waste. This invention avoids the use of a heater during the cool down cycle, thus improving the efficiency. This selection method prevents the physically larger equipment from operating when not needed, thereby preventing large static and dynamic heat leaks, and allowing the smaller pumps/heat exchangers to handle the smaller heat loads more precisely and efficiently.
This invention is directed to a method for controlling the temperature of freeze drying chamber shelves and chamber in a refrigeration system having a condenser operatively associated therewith. This is done by circulating a cryogen through the condenser and circulating a cryogenically cooled heat transfer fluid through the chamber shelves for controlling the temperature therein. The temperature of the cryogenically cooled heat transfer fluid is regulated by an exchange of heat with the cryogen. The temperature of the cryogenically cooled heat transfer fluid is regulated by the exchange of heat with the cryogen through a plurality of heat exchangers, and further by a heating unit. Circulation of the cryogenically cooled heat transfer fluid is accomplished by using a plurality of pumps and valves. At the beginning of a temperature ramp down cycle, the temperature of the heat transfer fluid is first regulated by passing the heat transfer fluid through a precooling medium. At the middle of the ramp down cycle, the temperature is then regulated by passing the cooled heat transfer fluid through a second heat exchanger cooled with a cryogen. A refrigeration recovery unit may be used to maintain the temperature and to recycle the cryogenically cooled heat transfer fluid. A liquid refrigerant may also pass through the condenser.
This invention is also directed to a method for freeze drying by providing a freeze drying chamber having a condenser operatively associated therewith, circulating a cryogen through the condenser, and circulating a cryogenically cooled heat transfer fluid through the chamber shelves for controlling the temperature therein. The temperature of the cryogenically cooled heat transfer fluid is regulated by an exchange of heat with the cryogen.
This invention is also directed to a freeze drying apparatus comprising a freeze drying chamber for subjecting substances to a freeze drying process in which moisture or solvent contained within the substances is frozen and sublimed into a vapor, a series of shelves within the chamber, a condenser operatively associated with the freezing chamber for freezing the vapor and for accumulating the vapor in solid form. The condenser has at least one pass for receiving a cryogen for freezing the vapor. A plurality of heat exchangers is used to exchange heat between the cryogen and a cryogenically cooled heat transfer fluid. A cryogenically cooled heat transfer fluid circuit in which the temperature of the cryogenically cooled heat transfer fluid is regulated by the plurality of heat exchangers, and in which the cryogenically cooled heat transfer fluid passes through the freeze drying chamber to freeze a substance by separating at least a portion of liquid therefrom. The cryogen circuit in which the cold of the cryogen is transferred to the cryogenically cooled heat transfer fluid through the heat exchangers and the cryogen is passed through the condenser. A plurality of valve means regulates the flow of the cryogen, and at least one circulation means for circulating the cryogenically cooled heat transfer fluid through the cryogen circuit. During the initial part of the temperature ramp down cycle, the temperature of the heat transfer fluid is regulated by transferring cold to the heat transfer fluid by a precooling medium. During the temperature ramp up cycle, the temperature of the heat transfer fluid is regulated by passing the heat transfer fluid through a heating unit. A waste refrigeratio
Cheng Alan Tat Yan
Finan, Sr. Donald Stuart
Drake Malik N.
Lau Bernard
McDermott Corrine
Praxair Technology Inc.
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