Title of Invention

STIRLING COOLER

Abstract

A Stirling cooler (1) consisting of a compressor (2) comprising a compressor piston (3) reciprocally disposed in a compressor cylinder (4). An expander (5) comprises an expander piston (6) rotatably and reciprocally disposed in an expander cylinder (7). A regenerator (8) is located within a cavity (9) formed in an extension (10) at the top of the' expander piston. A circumferential groove (12) is formed on the extension adjoining the bottom wall (9a) of the cavity and the top end of the expander piston. Gas passages (11a, 13) are formed in the top wall (11) and bottom wall (9a) of the cavity. A pair of helical grooves (14, 15) are provided along the length of the expander piston with their ends meeting each other. The pitch of the helix formed by the helical grooves is twice the expander piston stroke. The compressor cylinder is connected to the expander cylinder through a connecting tube (17) and the helical grooves provided along the length of the expander piston and the circumferential groove. At least one seal (16) is provided between the sidewall of the cavity and expander cylinder to prevent gas leakage therethrough. A conductor material cap (18) is provided over the top of the expander cylinder corresponding to the expansion space formed within the expander cylinder between the top end of the extension and the closed end of the cap (Fig 1).

Full Text

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, 2005 COMPLETE SPECIFICATION (See section 10 and rule 13) TITLE OF THE INVENTION Stirling cooler APPLICANTS Name Nationality Address INVENTOR Name Nationality Address Indian Institute of Technology, Bombay an autonomous research and educational institution established in India by a special Act of the Parliament of the Republic of India under the Institutes of Technology Act 1961 Powai, Mumbai 400076, Maharashtrajndia Bapat Shridhar Laxman Indian National Indian Institute of Technology, Bombay Department of Mechanical Engineering, Powai, Mumbai 400076, Maharashtra, India 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 INVENTION This invention relates to a Stirling cooler. Coolers employing the Stirling cycle of operation are known as Stirling coolers. In a Stirling cycle, a working fluid such as helium or hydrogen is first compressed. The compressed hot gas is allowed to cool down to the desired temperature while passing through a regenerator (porous matrix) and then allowed to expand to cool down further to still lower temperature and the cooling or refrigeration effect is extracted and used for cooling applications. During return of the cold gas for the next cycle of compression, it is allowed to pick up heat from the regenerator. Stirling coolers used for cryogenic applications are also known as Stirling cryocoolers. PRIOR ART A single cylinder or in line type Stirling cooler of Alpha configuration comprises a compressor piston and an expander piston (displacer) reciprocally disposed in an open ended common single cylinder from opposite ends thereof. The pistons are separated by a regenerator located within the cylinder. In a cycle of operation of the cooler, compressed gas in the compression space between the compressor piston and regenerator on one side of the cylinder enters the expansion space between the expander piston and the regenerator on the other side of the cylinder through the regenerator, which acts as a thermal sponge, and picks up the heat from the hot gas. The hot gas gets cooled and the cooled gas is allowed to expand and further cool down in the expansion space thereby making the cooling or refrigeration effect available over the cold area spread over the expander piston side of the cylinder starting from the expander piston side end of the regenerator. The cooling effect is extracted and used in the desired manner. While returning to the compression space through the regenerator, the cold gas picks up the heat from the regenerator and gets heated up and undergoes the next cycle of compression. Another type of Stirling cooler of Alpha configuration comprises a compressor piston and an expander piston reciprocally disposed in two separate cylinders. A regenerator is connected to the compressor cylinder on one side and expander cylinder on the other side through connecting tubes. This configuration of the Alpha cooler also works in the same manner as described above. The pistons in both the above configurations are driven by a reciprocating drive arrangement. A common reciprocating drive arrangement can be used in the case of the two cylinders type Alpha cooler but in the case of the single cylinder type Alpha cooler independent reciprocating drive arrangements may be required for the pistons. Alpha coolers have the versatility of being used in low to high temperature cooling applications as the volume of the compressor cylinder can be increased to increase the pressure ratio and cooling capacity of the coolers. The clearance volume of the compressor of the two cylinders type coolers ie the volume from the top dead centre of the compressor cylinder to the regenerator, is fixed. Therefore, variations in the expansion stroke of the expander piston do not affect the clearance volume in the compression space and pressure ratio can be increased by increasing the compressor piston stroke to achieve increased cooling capacity of the coolers. The cold area, where the cooling or refrigeration effect is available, is large in the case of Alpha coolers. This area at cryogenic temperature is required to be insulated with multilayer insulation and maintained under high vacuum in order to prevent leakage of heat from the ambient. Generally an enclosure (bell jar) is provided around the cold area to maintain the vacuum. Provision of multilayer insulation and maintenance of high vacuum over a large area is problematic, especially in the case of miniature Alpha coolers. Therefore, Alpha coolers are not generally preferred for miniature cryogenic applications. A Stirling cooler of Beta configuration comprises a compressor piston reciprocally disposed in a main cylinder whose top end is closed. The compressor piston is provided with a pair of connecting rods. An annular heat exchanger (water cooler) is located within the main cylinder above the compressor piston in spaced apart relationship therewith. An annular regenerator is located within the main cylinder above the water cooler and an annular condenser head is located within the main cylinder above the regenerator. An expander piston is reciprocally disposed within the annular space defined by the water cooler, regenerator and condenser head. This annular space is considered to be the expander cylinder. The expander piston is provided with a piston rod passing through the compressor piston. A mechanical seal is provided in the compressor piston around the piston rod of the expander piston to prevent leakage of gas through the compressor piston. Both the pistons are driven by a reciprocating drive arrangement. The space between the bottom of the expander piston and above the compressor piston forms the compression space. The space between the bottom of the expander piston and the bottom of the water cooler is considered to be the clearance volume, which varies depending upon the position of the expander piston within the annular space. The space between the top of the expander piston and the closed top end of the annular space forms the expansion space. In a cycle of operation of the cooler, the compressed hot gas passes through the heat exchanger and regenerator and gets cooled. The cooled gas enters the expansion space through the gas passages in the condenser head and expands and further cools down thereby making the cooling or refrigeration effect available for extraction at the closed top end of the annular space, which forms the cold area. While returning to the compression space through the gas passages in the condenser head, regenerator and heat exchanger the cold gas picks up heat from the regenerator and heat exchanger and gets heated up. The expansion space is limited to the volume within the annular space between the expander piston and closed end of the annular space. Due to the expander piston being located within the cylinder and expander piston rod passing through the compressor piston, the construction of the cooler is complicated and it is difficult to miniaturise Beta configuration coolers. The volume of the compression space cannot be increased independent of the volume of the annular space ie the expander cylinder as the expander cylinder is located within the main cylinder. Besides, the pressure ratio of the cooler is variable as the clearance volume of the compression space is variable thereby reducing the cooling capacity of the cooler. As the cold area for extraction of the cooling effect is confined to the closed top end of the annular space, extraction of the cooling effect is, however, easy and convenient. The complexity of insulation and vacuum requirements for the cold area is also reduced. Due to the constructional constraints, the Beta Stirling coolers are, however, generally used only for gas liquefaction applications. A Stirling cooler of Gamma configuration comprises a compressor comprising a piston reciprocally disposed in a compressor cylinder. An expander comprising a piston is reciprocally disposed in an expander cylinder. A regenerator is located within a cavity formed in an extension at the top end of the expander piston and a plug is fitted at the open top of the cavity. The expander cylinder is fitted with a cap at the top thereof. The compressor cylinder is connected to the expander cylinder through a connecting tube, which opens into a circumferential groove formed on the extension at the top end of the expander piston adjoining the bottom wall of the socket on the one side and the top end of the expander piston on the other side. Gas passages are provided in the bottom wall of the socket and in the plug. A mechanical seal is provided between the expander cylinder and expander piston towards the top thereof to prevent leakage of gas. Alternatively, clearance seal arrangement is provided between the expander cylinder and expander piston. The space between the top end of the extension and the cap forms the expansion space for the gas. In a cycle of operation of the cooler, the compressed hot gas from the compressor cylinder enters the expander cylinder into the circumferential groove in the extension at the top of the expander cylinder via the connecting tube and travels into the regenerator through the gas passages in the bottom wall of the socket. As the hot gas passes through the regenerator, the regenerator picks up the heat from the gas and the gas gets cooled. The cold gas enters the expansion space through the gas passages in the plug and expands and cools down further thereby making the cooling effect available at the top cap forming the cold area for extraction. When the cold gas returns to the compressor cylinder through the gas passages in the plug, regenerator, gas passages in the bottom wall of the socket, circumferential groove and connecting tube, it picks up heat from the regenerator and gets heated up. The compressor piston and the expander piston are driven by a reciprocating drive arrangement. The cold area for extraction of the cooling effect in a Gamma cooler is limited to the top cap surface. Therefore, extraction of the cooling effect is easy and convenient and insulation and vacuum requirements for the cold area are reduced. The expansion space and the travel of the expander piston are, however, limited by the length of the circumferential groove which maintains the connectivity between the compression space and expansion space. If the expander piston travels beyond a distance greater than the length of the circumferential groove, the connectivity of the compressor to the expander is not maintained and the cooler will not function. The circumferential groove volume acts as an additional component of the clearance volume for the compressor and reduces the pressure ratio. This results into short stroke for the expander piston leading to a limited cooling capacity. Due to the limited cooling capacity, Gamma coolers are generally used as miniature coolers for low capacity cooling applications. OBJECTS OF INVENTION An object of the invention is to provide a Stirling cooler, which is versatile and can have a range of low to high cooling capacities. Another object of the invention is to provide a Stirling cooler, which has reduced cold surface area because of which extraction of cooling effect is easy and convenient and the complexity in insulation and vacuum requirements for the cold area is reduced. DETAILED DESCRIPTION OF INVENTION According to the invention there is provided a Stirling cooler (1) consisting of a compressor (2) comprising a compressor piston (3) reciprocally disposed in a compressor cylinder (4), an expander (5) comprising an expander piston (6) rotatably and reciprocally disposed in an expander cylinder (7), a regenerator (8) located within a cavity (9) formed in an extension (10) at the top of the expander piston, a circumferential groove (12) formed on the extension adjoining the bottom wall (9a) of the cavity and the top end of the expander piston, gas passages (11a, 13) formed in the top wall (11) and bottom wall (9a) of the cavity, a pair of helical grooves (14, 15) provided along the length of the expander piston with their ends meeting each other, the pitch of the helix formed by the helical grooves being twice the expander piston stroke, the compressor cylinder being connected to the expander cylinder through a connecting tube (17) and the helical grooves provided along the length of the expander piston and the circumferential groove, at least one seal (16) provided between the sidewall of the cavity and expander cylinder to prevent gas leakage therethrough, and a conductor material cap (18) provided over the top of the expander cylinder corresponding to the expansion space formed within the expander cylinder between the top end of the extension and the closed end of the cap. The following is a detailed description of the invention with reference to the accompanying drawings, in which: Fig 1 is schematic view of a Stirling cooler according to an embodiment of the invention; Figs 2a, 2b, 2c, 2d and 2e are schematic views of the Stirling cooler of Fig 1 at various positions of the compressor and expander pistons within the respective cylinders and; Fig 3 is isometric view of the expander piston of the Stirling cooler of Fig 1. The Stirling cooler 1 as illustrated in accompanying drawings comprises a compressor 2 comprising a compressor piston 3 reciprocally disposed in a compressor cylinder 4. The piston rod of the piston 3 is marked 3a. 5 is an expander comprising an expander piston 6 rotatably and reciprocally disposed in an expander cylinder 7. The piston rod of the piston 6 is marked 6a. 8 is a regenerator located within a cavity 9 formed in an extension 10 at the top end of the expander piston. Gas passages formed in the top wall 11 of the cavity are marked 11 a. 12 is a circumferential groove formed on the extension adjoining the bottom wall 9a of the cavity and top end of the expander piston. Gas passages formed in the bottom wall 9a of the cavity are marked 13. A pair of helical grooves 14, 15 are provided along the length of the expander piston with their ends meeting each other (Fig 3). The pitch of the helix formed by the helical grooves ie the end to end length of the full helix along the long axis thereof is twice the expander piston stroke. 16 is a mechanical seal provided between the side wall 9b of the cavity and expander cylinder 7 to prevent gas leakage therethrough. Instead of mechanical seal, a clearance seal can be provided between the sidewall of the cavity and expander cylinder. There can be more than one mechanical seal. Such variations are to be construed and understood to be within the scope of the invention. The compressor cylinder is connected to the expander cylinder through a connecting tube 17 and the circumferential groove. A heat conducting material cap 18 is provided over the top of the expander cylinder 7 corresponding to the expansion space 19 formed within the expander cylinder between the top end of the extension and the closed end of the cap. The conductor material for the cap may be any metal with high thermal conductivity such as copper or stainless steel. Drive to the compressor piston is given through a reciprocating drive arrangement (not shown), which may comprise, for example, an electric motor or internal combustion engine associated with a reciprocating drive mechanism. Drive to the expander piston is given through a reciprocating and rotating drive arrangement (not shown) which may be for instance, the drive mechanism described in our patent application No 480/MUM/2000 filed on 25th may 2000 which is incorporated herein by reference in its entirety. The reciprocating and rotating L-shaped member of this application is connected to the expander piston rod. At the beginning of a cycle of operation of the Stirling cooler of the invention, the relative positions of the compressor piston and expander piston and extension thereof with the regenerator are as shown in Fig 2a of the accompanying drawings. The lower end of the helical groove 14 remains connected to the compressor cylinder via the connecting tube 17 and the top end of the helical groove 14 opens into the circumferential groove 12 on the extension at the top end of the expander piston. The compressor piston has described half its downward stroke and the extension at the top end of the expander piston remains adjacent to the cap. During further downward movement of the compressor piston in the compressor cylinder by the remaining half of its downward stroke, the expander piston and extension rotates and moves down in the expander cylinder by half its downward stroke as shown in Fig 2b of the accompanying drawings. While the expander piston is rotating and moving down, the lower half of the helical groove 14 remains continuously connected to the connecting tube and when the expander piston has moved down by half its downward stroke, the centre of the helical groove 14 remains connected to the connecting tube 17. During upward movement of the compressor piston by half its stroke, the expander piston rotates and moves further down describing its remaining half stroke of downward movement as illustrated in Fig 2c of the accompanying drawings. While the expander piston is describing its remaining half stroke in the downward direction, the upper half of the helical groove 15 remains continuously in communication with the connecting tube 17. At the end of the remaining half stroke of the expander piston in the downward direction, the circumferential groove directly faces the open end of the connecting tube. While the compressor piston describes the remaining half of its upward stroke, the gas gets fully compressed and the expander piston rotates and moves up along with the extension and the regenerator thereof by half of its upward stroke as shown in Fig 2d of the accompanying drawings. The upper half of the helical groove 15 remains connected to the connecting tube 17. The compressed hot gas flows into the regenerator through the upper half of the helical groove 15, circumferential groove and the gas passages at the bottom wall of the cavity. When the hot gas passes through the regenerator, the regenerator picks up the heat of the hot gas and the gas gets cooled. The cooled gas emerging from the gas passages 1 la in the top wall of the cavity expands and cools down further in the expansion space. The cooling effect in the expansion space is available over the cap fitted over the upper end of the expander cylinder and is extracted and made use of. When the compressor piston describes its downward half stroke, the expander piston rotates and describes its remaining upward half stroke as shown in Fig 2e of the accompanying drawings. While the expander piston describes its remaining upward half stroke, the lower half of the helical groove 15 remains connected to the connecting tube. The cold gas flows into the regenerator via the gas passages 11a in the top wall of the cavity and exit from the regenerator via the gas passages in the bottom wall of the cavity. While passing from the regenerator, the gas picks up the heat from the regenerator and gets heated up. The hot gas entering the circumferential groove travels back to the compressor cylinder via the helical groove 14 and the connecting tube 17. According to the invention, the connectivity of the connecting tube to the circumferential groove and the regenerator is thus continuously maintained during the entire cycle of operation of the cooler independent of the length of the circumferential groove because of the helical grooves 14 and 15 interconnected at the ends thereof and because of the stroke of the expander piston being half the pitch of the helix formed by the helical grooves. Therefore, the circumferential groove can be of minimal length. As the compressor cylinder is independent of the expander cylinder, the compressor cylinder volume can be independently increased to increase the pressure ratio and cooling capacity of the cooler. The expander piston stroke and expansion space are independent of the length of the circumferential groove and can be considerably increased. Because of all this, the entire gas can be compressed and the cooling capacity of the cooler can be considerably increased. The clearance volume of the compressor ie the volume from the top dead centre in the compressor cylinder to the expander piston side end of the connecting tube remains fixed irrespective of the expander cylinder volume. Due to the clearance volume of the cooler being almost fixed, it is not affected by the variations in the expansion space volume. This increases the pressure ratio and further increases the cooling capacity of the cooler. Therefore, the cooler can be ideally used for low to high temperature cooling applications and is versatile. The cold area for extraction of the cooling effect is limited to the surface area of the cap. Therefore, extraction of the cooling effect is easy and convenient. The complexity of insulation and vacuum requirements for the cold area is reduced. We Claim: 1. A Stirling cooler (1) consisting of a compressor (2) comprising a compressor piston (3) reciprocally disposed in a compressor cylinder (4), an expander (5) comprising an expander piston (6) rotatably and reciprocally disposed in an expander cylinder (7), a regenerator (8) located within a cavity (9) formed in an extension (10) at the top of the expander piston, a circumferential groove (12) formed on the extension adjoining the bottom wall (9a) of the cavity and the top end of the expander piston, gas passages (11a, 13) formed in the top wall (11) and bottom wall (9a) of the cavity, a pair of helical grooves (14, 15) provided along the length of the expander piston with their ends meeting each other, the pitch of the helix formed by the helical grooves being twice the expander piston stroke, the compressor cylinder being connected to the expander cylinder through a connecting tube (17) and the helical grooves provided along the length of the expander piston and the circumferential groove, at least one seal (16) provided between the sidewall of the cavity and expander cylinder to prevent gas leakage therethrough and a conductor material cap (18) provided over the top of the expander cylinder corresponding to the expansion space formed within the expander cylinder between the top end of the extension and the closed end of the cap. 2. A Stirling cooler as claimed in claim 1, wherein the cap is made of copper or stainless steel. 3. A Stirling cooler as claimed in claim 1, wherein the seal is a mechanical seal. Dated this 1st day of March 2006 (Jose M A) of Khaitan & Co Agent for the Applicants

Documents:

299-mum-2006-abstract(6-5-2008).doc

299-mum-2006-abstract(6-5-2008).pdf

299-mum-2006-cancelled pages(6-5-2008).pdf

299-mum-2006-claims(granted)-(6-5-2008).doc

299-mum-2006-claims(granted)-(6-5-2008).pdf

299-mum-2006-claims.doc

299-mum-2006-claims.pdf

299-mum-2006-correspondence(6-5-2008).pdf

299-mum-2006-correspondence(ipo)-(25-2-2008).pdf

299-mum-2006-correspondence-received.pdf

299-mum-2006-description (complete).pdf

299-mum-2006-drawing(6-5-2008).pdf

299-mum-2006-form 1(1-3-2006).pdf

299-mum-2006-form 1(20-3-2006).pdf

299-mum-2006-form 2(granted)-(6-5-2008).doc

299-mum-2006-form 2(granted)-(6-5-2008).pdf

299-mum-2006-form 3(1-3-2006).pdf

299-mum-2006-form 5(21-2-2003).pdf

299-mum-2006-form 8(29-5-2007).pdf

299-mum-2006-form-1.pdf

299-mum-2006-form-2.doc

299-mum-2006-form-2.pdf

299-mum-2006-form-26.pdf

299-mum-2006-form-3.pdf

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