FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Cryostat for superconducting electric power devices
APPLICANTS
Name : Crompton Greaves Limited
Address : CG House, Dr Annie Besant Road, Worli, Mumbai
400 030, Maharashtra, India Nationality : an Indian Company
INVENTORS
Pawar Sumedh and Dixit Manglesh, both of Crompton Greaves Limited, CG Global R&D Centre, Kanjurmarg (East), Mumbai 400042, Maharashtra, India, both Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :

FIELD OF THE INVENTION
This invention relates to a cryostat for superconducting electric power devices.
This invention also relates to a method of maintaining liquid nitrogen in a cryostat at the superconducting temperature.
BACKGROUND OF THE INVENTION
In superconductivity applications of electricity, very low temperatures are to be maintained. This is known as cryogenic temperature, which is generally attained with liquid nitrogen as its boiling point is 77.4 kelvin at normal atmospheric pressure. The conducting coils of superconducting electric power devices like superconducting transformers, motors, magnets or fault current limiters are maintained at superconducting temperatures by keeping them immersed in liquid nitrogen at 77.4 k in a cryostat comprising a thermally insulated vessel. This is known as 'bath cooling' or 'batch cooling'. The liquid nitrogen has to be continuously replenished in the cryostat so as to ensure that the superconducting coils are always immersed in liquid nitrogen and its temperature is maintained at 77.4 k. Liquid nitrogen is filled in the cryostat from a cryocan and in order to do so the cryostat comprises an inlet tube connected to the outlet tube of the cryocan. The inlet tube of the cryostat extends upto the bottom thereof. During filling of liquid nitrogen in the cryostat from the cryocan, part of the liquid nitrogen in the outlet tube, gets converted into vapour phase ( gas bubbles) due to the
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differential temperature existing inside and outside the outlet tube. The liquid nitrogen flows into the cryostat alongwith the vapour phase namely gas bubbles. The vapour phase of nitrogen does not possess the same refrigeration capacity like liquid nitrogen. The vapour phase of nitrogen has only half useful refrigeration ( cooling) capacity as compared to liquid nitrogen. Thus, it tends to raise the optimum temperature of 77.4 kelvin in the cryostat which is required for superconductivity applications. The vapours tend to move upwards as gas bubbles and come in contact with the superconducting coils, and as a result the cooling effect of the coils is reduced and non-uniform cooling of the coils takes place. Since the nitrogen gas bubbles are at a higher temperature than 77.4 k, namely the optimum temperature required for the superconducting coils, the presence of gas bubbles, causes non-uniform temperature distribution in the coil and localized heating thereof. This localized heating of the superconducting coils is known as 'hot spots'. This may lead to untimely burning out of the superconducting coils. Further, the presence of gaseous nitrogen will reduce the dielectric strength of the liquid nitrogen and increase the probability of dielectric breakdown thereof.
OBJECTS OF THE INVENTION
An object of the invention is to provide a cryostat for superconducting electric power devices, which cryostat ensures uniform cooling of the superconducting coils of the devices at 77.4 k.
Another object of the invention is to provide a cryostat for superconducting electric power devices, which cryostat prevents non-uniform temperature
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distribution in the superconducting coils of the devices and development of hot spots thereby increasing the life of the coils.
Another object of the invention is to provide a cryostat for superconducting electric power devices, which cryostat maintains dielectric strength of liquid nitrogen and reduces the probability of dielectric breakdown of liquid nitrogen.
Another object of the invention is to provide a method for maintaining liquid nitrogen in a cryostat at the superconducting temperature by reducing gas bubbles in liquid nitrogen flowing into the cryostat from a cryocan so as to ensure uniform cooling of the superconducting coils of the devices at 77.4 K, prevent non-uniform temperature distribution in the superconducting coils of the devices and development of hot spots and increase the life of the coils and maintain the dielectric strength of liquid nitrogen and reduce the probability of dielectric breakdown of liquid nitrogen.
Detailed Description of the invention
According to the invention there is provided a cryostat comprising a thermally insulated vessel having an inlet tube connected to the outlet tube of a cryocan, the inlet tube extending upto the bottom of the vessel and being provided with a plurality of gas bubble vents at its surface area in the vapour region in the vessel above the liquid nitrogen level so as to vent out gas bubbles in the liquid nitrogen flowing into the vessel and maintain the liquid nitrogen in the vessel at the superconducting temperature.
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Preferably, the gas bubble vents in the cryostat are of sizes upto 1 millimetre and the surface area of the inlet tube in the vapour region comprise 30 to 40 percent of gas bubble vents.
According to the invention there is also provided a method of maintaining liquid nitrogen in a cryostat at the superconducting temperature by reducing gas bubbles in the liquid nitrogen flowing into the cryostat from a cryocan through the outlet tube of the cryocan and inlet tube of the cryostat by venting out the gas bubbles in the liquid nitrogen through gas bubble vents provided in the surface area of the inlet tube in the vapour region in the vessel above the liquid nitrogen level.
The inlet tube of the cryostat and outlet tube of the cryocan are preferably made with glass fiber reinforced plastics ( GFRP).
Brief Description of Drawings
Fig 1 is a schematic crosssectional view of a cryostat according to an embodiment of the invention connected to a cryocan; and
Fig 2 is an enlarged view at X in Fig 1.
The cryostat 1 as illustrated in Figs 1 and 2 of the accompanying drawings comprises a thermally insulated vessel 2 provided with an inlet tube 3 extending down upto the bottom of the vessel and connected to the outlet tube 4 of cryocan 5 through a coupler 6. 7 is the superconducting coil
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assembly immersed in liquid nitrogen marked 8 in the vessel. 9 and 10 are a pressure regulating valve and level sensor respectively provided with the vessel. 11 is a flow control valve provided in the outlet tube of the cryocan. The inlet tube is provided with a plurality of gas bubble vents 12 (Fig 2) at its surface area in the vapour region in the vessel above the liquid nitrogen level. Gas bubbles generated in the liquid nitrogen flowing from the outlet tube of the cryocan into the inlet tube of the cryostat are lighter and escape into the vapour region in the vessel through the gas bubble vents in the inlet tube in the vapour region. These gas bubbles marked 13 are released into the atmosphere through the pressure regulator valve. Therefore, the flow of gas bubbles into the liquid nitrogen in the vessel is practically avoided and the liquid nitrogen in the vessel is practically free from gas bubbles. As a result the liquid nitrogen in the cryostat is maintained at 77.4 k so as to ensure uniform cooling of the superconducting coils of the devices at 77.4 k. Non-uniform temperature distribution in the superconducting coils of the devices and development of hot spots in the coils are prevented. The life of the coil is increased. The dielectric strength of the liquid nitrogen is maintained and the probability of dielectric breakdown of liquid nitrogen is reduced.
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We Claim
1. A cryostat comprising a thermally insulated vessel having an inlet tube connected to the outlet tube of a cryocan, the inlet tube extending upto the bottom of the vessel and being provided with a plurality of gas bubble vents at its surface area in the vapour region in the vessel above the liquid nitrogen level so as to vent out gas bubbles in the liquid nitrogen flowing into the vessel and maintain the liquid nitrogen in the vessel at the superconducting temperature.
2. The cryostat as claimed in claim 1, wherein said gas bubble vents are of sizes upto 1 millimetre and the surface area of the inlet tube in the vapour region in the vessel comprises 30 to 40 percent gas bubble vents.
3. A method of maintaining liquid nitrogen in a cryostat at the superconducting temperature by reducing gas bubbles in the liquid nitrogen flowing into the cryostat from a cryocan through the outlet tube of the cryocan and inlet tube of the cryostat by venting out the gas bubbles in the liquid nitrogen through gas bubble vents provided in the surface area of the inlet tube in the vapour region in the vessel above the liquid nitrogen level.
Dated this 22nd day of August 2007

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Abstract
A cryostat (1) comprising a thermally insulated vessel (2) having an inlet tube (3) connected to the outlet tube (4) of a cryocan (5). The inlet tube extends upto the bottom of the vessel and is provided with a plurality of gas bubble vents (12) at its surface area in the vapour region in the vessel above the liquid nitrogen level (8) so as to vent out gas bubbles in the liquid nitrogen flowing into the vessel and maintain the liquid nitrogen in the vessel at the superconducting temperature. Also a method of maintaining liquid nitrogen in a cryostat at the superconducting temperature by reducing gas bubbles in the liquid nitrogen flowing into the cryostat from the cryocan through the outlet tube of the cryocan and inlet tube of the cryostat by venting out the gas bubbles in the liquid nitrogen through the gas bubble vents provided in the surface area of the inlet tube in the vapour region in the vessel above the liquid nitrogen level (Fig 1).