Patent Application
20070181208
The present invention relates generally to the process control field, and more specifically, but not exclusively, to a system and method for preventing blow-by of liquefied gases.
Liquefied gases, such as liquid nitrogen (LN2), liquid helium or liquid oxygen, are used in a wide range of cryogenic (extremely cold) applications. For example, LN2 is often used as a coolant for sensitive electronic sensors, low noise amplifiers, semiconductor product testing, and other industrial applications. Also, LN2 is used for cryogenics research, preserving biological materials, freezing food products, and heat transfer applications that require the extremely low temperatures involved.
A significant problem that arises with the use of liquefied gases is commonly referred to as “blow-by”. For example, when LN2 is transferred from one containment vessel to another (e.g., storage tank to a Dewar), a vent is provided in the receiving vessel to bleed off excess nitrogen gas and allow additional LN2 to enter the vessel. The excess nitrogen gas is typically vented to the surrounding environment. However, if the Dewar or other receiving vessel is filled to capacity, excess LN2 (rather than nitrogen gas) begins to blow-by or exit through the vent. In less serious cases, the vented LN2 can rain down as super-cooled droplets that “burn” personnel and damage equipment. In the more severe cases of blow-by (e.g., while filling a Dewar), large amounts of LN2 can be released to the surrounding environment in an uncontrolled fashion and produce a large, dangerous cloud of boiling white LN2 “smoke”. Such large amounts of vented LN2 can result in serious bodily injuries such as bums and asphyxiation, severe damage to equipment, and can require a full-blown response by local authorities along with their hazardous materials cleanup teams. Also, liquefied gases such as LN2 are relatively expensive to procure, and the economic cost due to blow-by is a significant expense to have to incur.
Liquefied gases are also used for heat transfer applications. For example, LN2 is used as a super-cooled refrigerant for certain heat exchangers. Typically, the LN2 is piped into a heat exchanger through an inlet tube, changed to nitrogen gas during the heat exchange process, and the excess gaseous nitrogen is output from the heat exchanger through an exhaust tube. The exhaust tube is often constructed to vent the excess gaseous nitrogen to the outside air. However, if the heat exchanger is overwhelmed with the incoming LN2 (e.g., the heat exchange rate is too slow), the excess LN2 (rather than the gaseous nitrogen) begins to blow-by or exit through the exhaust system to the surrounding environment or the outside air. If the exhaust system is not being monitored, the LN2 source can remain on for an extended period and a significant amount of LN2 can be expelled to the outside air. Monitoring the output of the exhaust is problematic, because these vents are typically located outside the buildings involved, and the vented LN2 can thus be difficult to see or hear. Consequently, a significant amount of LN2 can be released and wasted due to blow-by, which also creates a substantial safety risk. Therefore, a pressing need exists for a system and method that can prevent blow-by of liquefied gases. As described in detail below, the present invention provides such a system and method, which resolves the above-described blow-by problems and other similar problems.
The present invention provides a system and method for preventing blow-by of liquefied gases, which senses the occurrence of blow-by and shuts off the source of the liquefied gas. In accordance with a preferred embodiment of the present invention, a system for preventing blow-by of LN2 is provided, which includes a temperature sensor located near an exhaust line of a heat exchanger, a shutoff valve located in an inlet line, and a controller coupled to the temperature sensor and the shutoff valve. The sensor measures the temperature of the exhaust, and provides a signal to the controller that indicates if the temperature of the exhaust approaches the temperature of LN2 . If the exhaust temperature approaches the temperature of LN2 , the controller sends a signal to the shutoff valve, which causes the valve to close and stops the flow of LN2 to the heat exchanger involved. The controller can also send an alarm signal to a display, which indicates to a user that the blow-by is occurring. If the exhaust temperature increases significantly above the temperature of LN2 (e.g., blow-by is no longer occurring), the controller can send a second signal to the shutoff valve, which causes the valve to open and resume the flow of LN2 .
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
With reference now to the figures,
In operation, for this example embodiment, shutoff valve 106 is normally open, and the liquefied gas (LN2 in this case) is being pumped from a storage tank (not shown) into inlet tube 108 and heat exchanger 114. During a normal heat transfer operation, heat exchanger 114 produces gaseous nitrogen having an average temperature of about 20 degrees Celsius in exhaust tube 110. Temperature sensor 102 couples an appropriate signal to controller unit 104 that indicates the temperature of exhaust tube 110 and the gaseous nitrogen flowing within exhaust tube 110. If the input LN2 flow rate surpasses the rate of the heat transfer process in heat exchanger 114, blow-by begins to occur and LN2 starts to flow through exhaust tube 110. As the excess LN2 passes by temperature sensor 102, it senses the dramatic drop in temperature (e.g., approaching 77 Kelvin) and sends a suitable signal indicating that temperature drop to controller unit 104. If controller unit 104 receives a temperature reading from temperature sensor 102, which indicates (e.g., by a dramatic drop in temperature) that LN2 is flowing through exhaust tube 110, controller unit 104 sends a suitable “off” command signal to shutoff valve 106. In response, shutoff valve 106 closes and terminates the flow of LN2 into inlet tube 108 and heat exchanger 114 and thus prevents blow-by. As an option, controller unit 104 can also send a suitable “alarm” signal to alarm/display unit 116 for a user to see and/or hear. Also as an option, if controller unit 104 subsequently receives a temperature reading from temperature sensor 102, which indicates (e.g., by a substantial increase in temperature) that LN2 is no longer flowing through exhaust tube 110, controller unit 104 can send a suitable “on” command signal to shutoff valve 106, which in response, reopens and allows LN2 to flow into inlet tube 108 and heat exchanger 114.
In operation, for this example embodiment, shutoff valve 204 is normally open, and the liquefied gas (LN2 in this case) is being pumped from a storage tank (not shown) into inlet tube 202 and Dewar 206. As long as Dewar 206 is not being overfilled, gaseous nitrogen having an average temperature of about 20 degrees Celsius is vented through vent tube 208. Temperature sensor 210 couples an appropriate signal to controller unit 212 that indicates the temperature of vent tube 208 and the gaseous nitrogen escaping out of vent tube 208. However, if the Dewar 206 is overfilled, blow-by begins to occur and LN2 starts to flow through vent tube 208. As the excess LN2 passes by temperature sensor 210, it senses the dramatic drop in temperature (e.g., approaching 77 Kelvin) and sends a suitable signal indicating that temperature drop to controller unit 212. If controller unit 212 receives a temperature reading from temperature sensor 210, which indicates (e.g., by a dramatic drop in temperature) that LN2 is flowing through vent tube 208, controller unit 212 sends a suitable “off” command signal to shutoff valve 204. In response, shutoff valve 204 closes and terminates the flow of LN2 into inlet tube 202 and Dewar 206 and thus prevents blow-by. As an option, controller unit 212 can also send a suitable “alarm” signal to alarm/display unit 214 for a user to see and/or hear.
It is important to note that while the present invention has been described in the context of a fully functioning system for preventing blow-by of a liquefied gas, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular system for preventing blow-by of a liquefied gas.
The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. These embodiments were chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
