Refrigeration – With indicator or tester – Condition sensing
Reexamination Certificate
2001-06-21
2002-11-05
Esquivel, Denise L. (Department: 3743)
Refrigeration
With indicator or tester
Condition sensing
C062S196100
Reexamination Certificate
active
06474082
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed at a method of sensing flow and providing a switch to indicate the presence of flow of a fluid that is at very low temperature (−40 C to −273 C). More particularly this invention is intended to sense the flow of a refrigerant at these very low temperatures.
BACKGROUND OF THE INVENTION
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial applications. In particular, very low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, physical vapor deposition vacuum coating processes, and semiconductor manufacturing applications.
This invention relates to refrigeration systems which provide refrigeration (removal of heat) at temperatures between −40 C and −273 C. The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this Patent the term “very low” or very low temperature will be used to mean the temperature range of −40 C to −273 C.
As a customer security measure, closed loop, very low temperature refrigeration systems must provide a refrigerant flow indicator in highly sensitive applications. In general, these systems require that all components withstand operating pressures between 1 and 31 atmospheres, and maintain a leak tight integrity.
Traditional means of sensing refrigerant flow requires moving elements, such as turbine meters, or other means of displacement of the sensing element. In refrigeration systems, small amounts of compressor oil may be present in the refrigerant that may congeal at low temperatures and interfere with the moving parts of these sensors. Flow sensors or flow switches that meet the leak integrity and working pressure requirements are usually very costly and are susceptible to contamination from this congealed oil. Other means of sensing flow, measure the pressure drop across an element such as an orifice or venturi section and then measure the pressure difference between the two points across or within such an element. These methods require knowledge of the flow properties of the fluid as a function of temperature, pressure and flow rate which can be difficult for applications in which the range of flow conditions vary substantially. The present invention provides a means to sense the presence of flow reliably and without a detailed knowledge of the fluid being sensed.
Many industrial applications require the ability to transfer heat in an efficient manner at a relatively fast rate. In applications where the presence of flow of a cooling media are considered essential to the process, a means of flow indication is required. Examples of such critical applications are storage of biological tissue, or the processing of a silicon wafer as part of a semiconductor manufacturing process. Providing a means of sensing flow or lack of flow alerts the user to the fact that flow is no longer present. In some control applications, where for example a heater is used to control temperature, such a flow indicating device can be used as an interlock to assure that heat is applied only when flow is occurring.
Conventional flow switching devices with displacing elements cannot detect flow of fluids at very low temperature temperatures (−40° C. to −273 C.) due to the incompatibility of sensing elements. Such very low temperatures require that the sensing element be suited for operation at very low temperatures. Specifically the materials used must not lose a significant portion of their mechanical strength and must not deform or become brittle. For closed loop refrigeration processes leak tight operation is also mandated. This means that the very low temperature flow switch must also be leak tight. In addition to mechanical strength issues, typical flow switches available have elastomeric seals that leak to the atmosphere at very low temperatures. In addition, the materials used must be chemically compatible with the fluid being sensed. Failure to have proper chemical compatibility results in degradation of sealing material, or moving parts and eventually result in component failure. Flow switches designed to meet all of these various requirements are typically very expensive. The present invention describes a means to provide sensing of a very low temperature fluid while satisfying the requirements for mechanical strength, leak tightness and chemical compatibility. This is achieved using components that are much less expensive than typical flow switches designed to meet these requirements.
There are many vacuum processes which have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber's water vapor partial pressure. Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces that are being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some cases the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly than other means, even though the object's final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases it is to be understood that the evaporator surface is where the refrigerant is removing heat from these customer applications when providing cooling at very low temperatures.
Refrigeration systems can also be used to provide a heating function for industrial processes in addition to providing refrigeration. This is needed when the system must provide a defrost or bakeout function. A defrost function delivers warm refrigerant to the object that is typically being cooled. The purpose is to bring the object to a temperature close to room temperature. This is typical of vacuum processes in which the vacuum chamber must be opened to room air. Having the evaporator surface at room temperature eliminates condensation of room air moisture in the vacuum chamber. A bakeout function also delivers warm refrigerant to the evaporator surface that is typically being cooled. Unlike the defrost function the purpose of the bakeout is to warm the evaporator surface above room temperature, typically +60 C to +110 C for an extended period of time, typically 1 to 24 hours. This can be useful when desorbing water vapor from the vacuum chamber after the chamber has been exposed to moist ambient air. During defrost and bakeout processes it is important to verify that refrigerant is flowing. Therefore for the flow switch to be fully effective its design must provide operation at elevated temperatures in addition to very low temperature. The present invention describes a means to satisfy these requirements.
BACKGROUND PATENTS
U.S. Pat. No. 5,730,216, “Air conditioning and refrigeration units utilizing a cryogen,” assigned to Thermo King Corporation (Minneapolis, Minn.), describes an air conditio
Esquivel Denise L.
Norman Marc
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