JPH0566508B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a piston structure of a reciprocating expander used in a cryogenic refrigerator such as a He liquefier using a Claude cycle.

[Prior Art] For example, in a Claude cycle helium liquefaction device, when the flow rate is small and the device is small, a reciprocating type expander is used, which is advantageous in terms of efficiency. In this type of expander, in order to minimize heat intrusion from the outside and increase system efficiency, the cylinder is made of a strong metal material such as stainless steel with as low thermal conductivity as possible. Pistons are often made of polymeric materials such as PTFE (polytetrafluoroethylene), which is lightweight and has even lower thermal conductivity.

However, when the cylinder and piston are made of different materials, not only do they have different thermal conductivities (for example, the thermal conductivity of SUS304 is 9~9).
15w/m℃, while that of PTFE is 0.24w/m
℃), and at the same time, the coefficient of thermal expansion is also significantly different (for example, the linear expansion coefficient of SUS304 is 1.3×10 ^-5 /℃, while that of PTFE is 5 to 10×10 ^-5 /℃), so the amount of thermal expansion and contraction of both is This imbalance causes the inconvenience that the characteristics of the expander change depending on the temperature. Specifically, as this expander cools down, the amount of heat contraction of the piston is relatively larger than that of the cylinder, so the tip of the piston gradually moves away from the bottom of the cylinder, and as a result, the piston top dead center This increases the volume of the expansion chamber at the location, which in turn causes a change in the expansion ratio, making it impossible for the expander to maintain an optimally efficient expansion ratio.

In addition, recently, in order to make the expander more compact as a whole, reciprocating type expanders have been equipped with intake and exhaust valve mechanisms that are configured to be opened and closed directly by the reciprocating movement of the piston at the bottom of the cylinder. It is being done. However, in an expander equipped with such a piston-directly actuated suction and exhaust valve mechanism, a particularly disadvantageous problem arises when there is a large difference in the relative amount of thermal expansion and contraction between the piston and the cylinder. In other words, in this type of intake/exhaust valve mechanism, the tip of the piston approaches and separates from the valve body etc. at predetermined height positions corresponding to the intake/exhaust cycle of the expander to switch between opening and closing the valve. This is because if the relative position of the tip of the piston with respect to the bottom of the cylinder changes for the reasons mentioned above, the timing of opening and closing these intake and exhaust valves will be shifted.

[Problems to be Solved by the Invention] The present invention has been made with attention to these problems, and is effective in preventing heat from entering from the outside into a piston for a reciprocating expander as a whole. The object of the present invention is to provide a structure that can reduce the difference in the amount of thermal expansion and contraction relative to the cylinder due to temperature changes while maintaining the thermal insulation properties.

[Means for solving the problems] In order to achieve such objects, the present invention has the following features:
In a reciprocating expander in which a piston is slidably fitted into a cylinder, the piston is
A piston core body is formed of a material with a piston tip connected to one end and has a coefficient of thermal expansion equal to or similar to that of the cylinder; It is characterized by being constructed as a composite body consisting of a piston molded body that is wrapped in a state in which the core body is not restrained from thermal expansion and contraction.

[Function] If a piston has such a composite structure, the cross-sectional area of the piston core may be small, compared to conventional pistons that are entirely made of a low thermal expansion material such as a polymer material. In addition, the thermal insulation properties are hardly impaired even when the cylinder and piston are heated, and the problem that the amount of thermal expansion and contraction between the cylinder and the piston is greatly different can be effectively solved. That is, since the core of this piston is made of a material having a coefficient of thermal expansion equal to or similar to that of the cylinder, the axial length of the piston core expands and contracts in accordance with the degree of thermal expansion and contraction of the cylinder. A piston tip is attached integrally or separately to this piston core, and the surrounding piston molded bodies with different coefficients of thermal expansion are not restrained and can freely expand and contract. Because of this structure, the relative positions of the piston tip located at the tip and the cylinder bottom are always kept substantially constant regardless of temperature changes.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 and FIG. 2 show the main parts of the reciprocating type expander according to the present invention, and its basic configuration is as usual,
A rod-shaped piston 2 is slidably fitted into a bottomed cylindrical cylinder 1 via a seal 3. The cylinder 1 is made of a material that meets the requirements for strength and heat insulation, such as stainless steel. The cylinder bottom 1a is connected to the cylinder bottom 1a through valves which will be described in detail later.
An intake port 5 and an exhaust port 6 are opened to communicate with an expansion chamber 4 formed between the piston and the piston tip.

On the other hand, the piston 2 is a composite body consisting of a small-diameter solid piston core 2a penetrated through the center and a large-diameter hollow piston molded body 2b fitted around the core 2a. It is made up of the body.
The piston core 2a plays the role of a structural member that supports the force applied to the piston 2, and has a flange-shaped piston base end that connects a reciprocating rod 7 connected to a crankshaft (not shown) at one upper end thereof. A flange-shaped piston tip 2 that faces the cylinder bottom 1a and defines an expansion chamber 4 is provided at one lower end thereof. and,
The piston core 2a is made of a material having a coefficient of thermal expansion equal to or similar to that of the cylinder 1, at least the shaft portion 2 excluding the flanges 2, 2 which are integrally or separately connected to both ends of the piston core 2a. (For example, the cylinder 1 is made of the same or similar stainless steel). In the illustrated example, the entire piston including the reciprocating rod 7 connected above to the piston core 2a is made of the same material. On the other hand, the piston molded body 2 on the outer periphery
b plays the role of maintaining the heat insulation properties of the piston 2 as a whole and giving it a predetermined outer diameter, and is made of a particularly low thermal expansion material, such as the above-mentioned material.
Made of PTFE. The piston molded body 2b is formed to have a shorter dimension than the axial length of the piston 2, that is, the length of the core body shaft portion 2, and is formed around the piston core body 2a to prevent thermal expansion and contraction of the piston core body 2a. Encased without restraint. Specifically, in this case, the piston molded body 2b
is slidably and loosely fitted around the core shaft portion 2 between the tip end portion 2 and the base end portion 2 of the piston core body 2a via a sliding seal 8, and is integrally attached to the upper end surface of the piston core body 2a. A spring 10 (a disc spring in the illustrated example) that biases the piston molded body 2b toward the tip end 2 is inserted into the gap created between the attached spring seat 9 and the base end 2. are doing. Therefore, in the piston 2 made of such a composite body, when the temperature changes due to the operation of the expander,
The core body 2a and the molded body 2b can freely expand and contract without restricting each other due to their respective coefficients of thermal expansion, and the molded body 2b around the core body 2a is
Due to the force of the spring 10, it is always in close contact with the brim-shaped tip 2 and does not vibrate, and moves around the core 2a (shaft 2) integrally with the core 2a.

Next, the structure of the intake valve and exhaust valve provided at the cylinder bottom 1a will be explained. First, regarding the intake valve, a valve chamber 11 that expands in diameter inward of the intake port 5 opened at the center of the cylinder bottom 1a is provided, and a valve seat 12 is formed at the end of the valve chamber 11 on the piston side. A valve body 13 made of a hollow body is inserted into the chamber 11 while being biased toward the valve seat 12 by a first spring 14, and then passed through the opening of the intake port 5 from inside the valve body 13. The piston abutting body 15 protruding into the expansion chamber 4 is biased toward the piston by a second spring 16 (having a more dominant spring force than the first spring 14) interposed within the valve body 13. It is constructed so that it can be retracted and expanded with a predetermined stroke. In such an intake valve, the piston abutting body 15 and the piston 2 come into contact with each other, and the intake valve is automatically opened and closed as follows. That is, when the piston 2 approaches the cylinder bottom 1a side during the exhaust process of the expansion chamber 4 (the state shown in FIG. 1), its tip 2 comes into contact with the piston abutting body 15 and pushes it down.
When the piston 2 reaches the top dead center, the piston contact body 15
One end hits the opposite wall within the valve body 13, and at the same time as the exhaust process is completed, the valve body 13 is pushed down to open the intake port 5 (the state shown in FIG. 2). Next, when the intake process begins, the dominant second spring 16 remains until the piston contact body 15 separates from the piston tip 2.
The valve body 13 is kept in the open state by the action of
It returns to contact and closes the intake port 5.

In addition, regarding the exhaust valve, the exhaust port 6 is opened in an annular manner on the cylinder bottom surface 20 around the intake port 5, and has a certain height on the cylinder inner peripheral surface from the cylinder bottom surface 20 toward the piston 2 side. A circumferential groove 17 is recessed, and this circumferential groove 1
7, a hollow plate-shaped valve body 18 having the cylinder bottom surface 20 as a valve seat is interposed in the exhaust port 6;
It is configured to be loaded so as to be able to reciprocate in a predetermined stroke while being biased toward the piston 2 side by a spring 19. In such an exhaust valve, when the piston 2 descends to the vicinity of its top dead center during the exhaust process of the expansion chamber 4 (the state shown in FIG. 1) in which the piston 2 approaches the cylinder bottom 1a, its tip end 2
contacts the valve body 18 and forcibly pushes it down against the spring force of the third spring 19, causing the valve body 18 to come into close contact with the cylinder bottom surface 20 (valve seat) at the same time as the piston 2 reaches the top dead center. to close the exhaust port 6 (as shown in Figure 2). Next, when the intake expansion process begins, the expansion chamber 4 is still filled with high-pressure working gas even when the piston tip 2 separates from the valve body 18, so the valve body 18 remains in close contact with the valve seat and closes the valve. If the condition is maintained and the spring force of the third spring 19 is adjusted in advance to counteract the internal pressure of the expansion chamber 4 at the end of the expansion process, the valve body 18 will be able to move toward the piston 2 where the force of the third spring 19 is superior. The valve automatically returns to the floating position near the bottom dead center, and the valve is in the open state.

Now, if the piston 2 having the above-mentioned structure is used in the cylinder 1 equipped with the piston-directly actuated suction and exhaust valves, the characteristics of the expander may fluctuate due to temperature changes, and the bottom of the cylinder may 1a
It is possible to reliably solve the conventional problems in which the opening/closing timing of each valve installed in the valve may be shifted or the valve may become inoperable. That is, since the piston 2 has a structure in which the change in its axial length is exclusively controlled by the thermal expansion and contraction of the core body 2a, which has a coefficient of thermal expansion that is the same or similar to that of the cylinder 1, Even if there is a large temperature change, the difference in the relative expansion and contraction amount with respect to the axial length of the cylinder 1 and the piston 2 can be suppressed to a small amount.
In other words, regardless of the temperature change in the expander,
The relative position between the tip 2 of the piston 2 and the cylinder bottom 1a remains almost unchanged. Therefore, as a result of this, firstly, the volume of the expansion chamber 4, especially the volume at the top dead center of the piston, is kept constant as a function and effect generally exhibited by reciprocating type expanders. The advantageous property is that the expansion ratio can be maintained at an efficient set value at all times. Secondly, especially the piston 2 as described above.
The intake and exhaust valves of the mechanism that are directly opened and closed by the cylinder 1
In the case of an expander installed in an expander, the relative position of the piston tip 2 with respect to the piston abutting body 15 or the valve body 18 on the cylinder bottom 1a side can be regarded as unchanged, so that the piston 2 that comes into contact with and separates from them during the suction/discharge cycle can be considered to be unchanged. The phase of can be maintained at the initial setting state. As a result, there is no difference in timing between opening and closing of the intake and exhaust valves, and the piston abutment body 15 and the valve body 18 are not pressed with their required strokes, and the intake and exhaust valves are closed. Naturally, the situation where opening and closing becomes impossible will no longer occur.

In addition, the piston 2 made of the above-mentioned composite body provides such advantages, and the piston core 2
Since it is sufficient that a has a relatively small diameter, the conventional example
Compared to PTFE rod-shaped integral products, its insulation properties hardly deteriorate.

The present invention has the configuration shown in the above embodiment, but as already mentioned, the piston 2 according to the present invention is not necessarily an expander equipped with a piston-directly actuated intake/exhaust valve mechanism. It is of course not intended to be applied only to reciprocating type expanders of this type, but can also be used generally for reciprocating type expanders other than this type, and provides the above-mentioned unique effects. In particular, when a piston-directly operated valve is adopted, its utility value is doubled. It should be noted that the structure of the piston direct actuation type valve shown in the embodiment is merely an example, and it is possible to apply valves having various structures and operating principles.

In addition, the piston core body 2a and the piston molded body 2b
The piston 2 of the present invention is not limited to the structure in which the molded body 2b is fitted around a single core body 2a inserted through the center as in the embodiment, but can also be arranged in a dispersed manner. It may be of a structure in which the piston molded body is wrapped so as to fill the inner and outer spaces of a plurality of core bodies inserted through the core body. The piston molded body 2b that encloses the piston core body 2a may be any type as long as it does not restrict the thermal expansion and contraction of the piston core body 2a, which connects the piston tip 2 and controls the piston axial length. In addition to the structure in which the spring biasing force is used to stably position and hold the structure, for example, the proximal end surface may be left free while the distal end surface may be fixed by adhesive to the piston tip 2, or both may be machined. It is also possible to fix it by a conventional coupling means.

[Effects of the Invention] As described above, by using the piston according to the structure of the present invention, the inconvenience that the expansion ratio and other characteristics of the expander change due to temperature changes can be eliminated.
In particular, when a piston-directly actuated intake and exhaust valve is employed, the possibility of the valve opening/closing timing being off or malfunctioning is reliably eliminated.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the main structure of a reciprocating expander according to an embodiment of the present invention, and FIG. 2 is a partial cross-sectional view showing the piston in a different position from that shown in FIG. 1. 1... Cylinder, 1a... Cylinder bottom, 2...
...Piston, 2a... Piston core, 2b... Piston molded body, 2... Piston base end, 2...
...Piston tip, 2...Core shaft, 4...Expansion chamber, 5...Intake port, 6...Exhaust port, 7
... Reciprocating rod, 9 ... Spring seat, 10 ... Spring, 11 ... Valve chamber, 12 ... Valve seat, 13 ...
Valve body (intake valve), 14...first spring, 15
...Piston contact body, 16...Second spring,
17... Circumferential groove, 18... Valve body (exhaust valve), 19...
...Third spring, 20... Cylinder bottom (valve seat).

Claims (1)

[Claims] 1. In a reciprocating expander in which a piston is slidably fitted into a cylinder, the piston is formed of a material having a piston tip connected to one end and having a coefficient of thermal expansion equal to or similar to that of the cylinder. A composite body consisting of a piston core and a piston molded body made of a low thermal expansion material and wrapped around the piston core in a manner that does not restrict thermal expansion and contraction of the piston core. The piston structure of the expander is a feature.