A BALANCE VALVE OF ASYMMETRIC STRUCTURE BACKGROUND OF INVENTION 1. Field of the Invention The present invention relates generally to a balance valve, and more particularly to an innovative one which is designed with an asymmetric structure to maintain cold water supply function. 2. Description of Related Art Given the fact that conventional dual-temperature ceramic valves may lead to transient overburning and scalding due to manual error in turning the faucet and the difficulty in regulating water temperature due to unstable water pressure, an improved structure of dual-temperature ceramic valve combined with cold-hot water balance valve (hereinafter referred to as a balance valve) has thus been developed in this industry. In spite of currently available ceramic products and relevant patents based on balance valve, some shortcomings and disadvantages still exist. A conventional balance valve generally comprises a fixed valve pipe and moveable spool, of which a through-hole is set separately at both ends of the fixed valve pipe, and an opening arranged separately at its both sides so as to guide cold and hot water; an inner groove is formed separately at both ends of the moveable spool, and separated by a partition; moreover, these two inner grooves are provided with an opening connected with that of the fixed valve pipe; with this structure, when cold, hot water separately pass through two inner grooves of the moveable spool via the openings at both sides of the fixed valve pipe, the moveable spool will yield reciprocating transverse movement with the variation of cold/hot water streams' flow velocity, based on the principle of higher velocity versus smaller pressure and smaller velocity versus bigger pressure as well as the property of water flow; this will change the sectional connection area between the moveable spool and the openings at both sides of the fixed valve pipe, serving the purpose of auto balancing of cold/hot water temperature. The following shortcomings of typical balance valve are found during actual applications: If conventional dual-temperature ceramic valve is combined with a balance valve, the functions of full cold/hot water supply modes are not affected even if the users manually turn the faucet handle to full cold water mode or full hot water mode. If the dual-temperature ceramic valve is switched to cold/hot water supply modes and the hot-water line is blocked due to unpredictable elements (e.g.: cracking/blocking of line, and closing of control valve), the moveable spool of the balance valve is abutted onto the hot water side of the fixed valve pipe, such that the hot water flow channel is blocked and closed; in such case, the moveable spool and the opening at cold water side of the fixed spool are fully staggered to block off the flow channel, that's to say, when the hot water line of typical balance valve is blocked, the cold water supply functions will be lost, exerting great impact on the functionality and convenience of faucet (note: since cold water mode is a most commonly used mode). Thus, to overcome the aforementioned problems of the prior art, it would be an advancement if the art to provide an improved structure that can significantly improve the efficacy. Therefore, the inventor has provided the present invention of practicability after deliberate design and evaluation based on years of experience in the production, development and design of related products. SUMMARY OF THE INVENTION 1. Based on the unique design of the present invention that the "balance valve" is provided with an asymmetric structure, when the dual-temperature ceramic valve is switched to cold/water supply modes, and hot water line is blocked to make the moveable spool abutted towards the first lateral end wall of the holding space, a cold water channel can thus be formed between the cold water opening of the fixed valve pipe and the second end of the moveable spool with the setting of said asymmetric structure, hence cold water supply could be maintained; namely, when the hot water line is blocked, the cold water supply function of the balance valve could still be maintained for improved convenience and applicability. 2. Based on the technical characteristics that inner grooves are set on inner wall of the fixed valve pipe so as to reduce the contact friction area between the moveable spool and the inner wall of the fixed valve pipe, and improve the operational sensitivity of the moveable spool; besides, scale on the moveable spool could be removed automatically through the vertical corner formed by the inner groove and inner wall of the fixed valve pipe, so as to maintain the operational smoothness of the moveable spool and extend the service life; furthermore, a capillary rough edge formed internally on the opening of the fixed valve pipe will be accommodated into said inner groove, such that the operation of the moveable spool is immune to the influence of the rough edge. Although the invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1: a sectional view of the asymmetric balance valve assembly of the present invention. FIG. 2: an exploded perspective view of the asymmetric balance valve assembly of the present invention. FIG. 3: a sectional view of the asymmetric structure of the present invention. FIG. 4: a contrast diagram showing the section& operating status of the first preferred embodiment of the balance valve's asymmetric structure of the present invention. FIG. 5: a contrast diagram showing the section& operating status of the second preferred embodiment of the balance valve's asymmetric structure of the present invention.
FIG. 6: a contrast diagram showing the section& operating status of the third preferred embodiment of the balance valve's asymmetric structure of the present invention. FIG. 7: a contrast diagram showing the section& operating status of the fourth preferred embodiment of the balance valve's asymmetric structure of the present invention. FIG. 8: a sectional view of the preferred embodiment of the present invention that the fixed valve pipe is provided with inner grooves. FIG. 9: an interior sectional view of the fifth preferred embodiment of the balance valve's asymmetric structure of the present invention. FIG. 10: a status view of the present invention that the rough edge of the fixed valve pipe can be accommodated into the inner groove. FIG. 11: an operating status view of the present invention that scale could be removed by the corner set on the fixed valve pipe. DETAILED DESCRIPTION OF THE INVENTION FIGS. 1~3 depict preferred embodiments of an asymmetric balance valve of the present invention, which, however, are provided for only explanatory objective for patent claims; said balance valve 11 is accommodated in a holding space 101 preset into the dual-temperature ceramic valve 10; the holding space 101 (only marked in FIG. 2) comprises of a cold water inlet 102 and a hot water inlet 103 arranged at interval on the bottom, a cold water outlet 104 and a hot water outlet 105 arranged at interval on the top, as well as a first lateral end wall 12 and a second lateral end wall 13 located at both ends; the balance valve 11 comprising: a fixed valve pipe 20 (in collaboration with FIGS. 1, 5), a transverse hollow pipe, comprising of a first end 21 correspondingly to the first lateral end wall 12, a second end 22 correspondingly to the second lateral end wall 13, a cold water opening 23 connected with the cold water inlet 102, a hot water opening 24 connected with the hot water inlet 103; moreover, the first end 21 is connected with the hot water outlet 105, and the second end 22 connected with the cold water outlet 104; a moveable spool 30, transversely assembled into the fixed valve pipe 20 in a transverse movement state; the moveable spool 30 is provided with a first end 31 and a second end 32, of which the first end 31 is recessed to form a hot water slot 33, and the second end 32 is recessed to form a cold water slot 34; the hot water slot 33 and cold water slot 34 are separated by a partition 35; besides, the hot water slot 33 is provided with a hot water punch hole 331 connected with the hot water opening 24 of the fixed valve pipe 20, and the cold water slot 34 is provided with a cold water punch hole 341 connected with the cold water opening 23 of the fixed valve pipe 20; an asymmetric structure 40 (referring to FIG. 3), which is extended asymmetrically by the first ends 21, 31 and second ends 22, 32 of the fixed valve pipe 20 or the moveable spool 30; or expanded asymmetrically by the hot water opening 24 and cold water opening 23 of the fixed valve pipe 20, or expanded asymmetrically by the hot water punch hole 331 and cold water punch hole 341 of the moveable spool 30. Referring to FIG. 2, the bottom of the dual-temperature ceramic valve 10 comprises a long-legged pedestal 50, at bottom 51 of which an inlet pipe groove 52 is set for assembly and positioning of an inlet pipe assembly 53; the inlet pipe assembly 53 comprises of a cold water pipe 531 and a hot water pipe 532; two protruding footstands 521 are set laterally at the bottom 51 of the long-legged pedestal 50; an embedded column 522 is set at bottom of two protruding footstands 521; the cold/hot water pipes 531, 532 consist of lower nozzle 54, upper nozzle 55 mated closely with the inlet pipe groove 52 and open abutting side 56, of which the abutting sides 56 of the cold/hot water pipes 531, 532 are assembled to form a holding space 101 for the balance valve 11. Of which, the inlet pipe groove 52 is shaped by a convex ringed frame 523, and an embedded groove 524 is partially set on the convex ringed frame 523, such that an embedded flange 57 is set laterally on upper nozzle 55 of the cold/ hot water pipes 531, 532 of the inlet pipe assembly 53. Based on above-specified structural design of said asymmetric structure 40, if the dual-temperature ceramic valve10 is switched to cold/hot water supply modes, the hot-water line is blocked, making the moveable spool 30 abutted onto the first lateral end wall 12 of the holding space 101; with the setting of said asymmetric structure 40, a cold water channel X (marked in FIG. 4) is formed between the cold water opening 23 of the fixed valve pipe 20 and the second end 32 of the moveable spool 30 to maintain cold water supply function. Next, the preferred embodiments of the asymmetric structure of the present invention are described below: Referring to FIG. 4, when cold/hot water lines L1, L2 are used for water supply simultaneously, the moveable spool 30 of the balance valve 11 is under a balanced state in the fixed valve pipe 20. The operating status of the first preferred embodiment of the asymmetric structure 40 of the balance valve 11 is illustrated in FIG. 4 (in collaboration with FIG. 2). When the hot water line Li is blocked, the cold water line L2 continuously pushes the partition 35 of the moveable spool 30, so that the moveable spool 30 is abutted towards the first lateral end wall 12 of the holding space 101; in such case, the second end 32 of the moveable spool 30 is extended beyond the second end 22 of the fixed valve pipe 20 to form a sectional difference; and, the second end 22 of the fixed valve pipe 20 is relatively shorter to keep spacing with the cold water inlet 102, such that a cold water channel X is formed within the holding space 101 between the moveable spool 30 and fixed valve pipe 20, thus maintaining continuous cold water supply state. The second preferred embodiment of the asymmetric structure 40 of the balance valve 11 is illustrated in FIG. 5(in collaboration with FIG. 2), wherein the cold water opening 23 of the fixed valve pipe 20 is wider than the hot water opening 24 to form an asymmetric pattern; with this design, when hot water line Li is blocked, and the moveable spool 30 is abutted towards the first lateral end wall 12 of the holding space 101, said cold water channel X is formed to maintain water supply state of cold water line L2 since the cold water opening 23 of the fixed valve pipe 20 is wider than the second end 32 of the moveable spool 30. The third preferred embodiment of the asymmetric structure 40 of the balance valve 11 is illustrated in FIG. 6(in collaboration with FIG. 2). When hot water line LI is blocked, the moveable spool 30 is abutted towards the first lateral end wall 12 of the holding space 101; in such case, said cold water channel X is formed to maintain water supply state of cold water line L2 since the shorter second end 22 of the moveable spool 30 cannot block off the cold water opening 23 of the fixed valve pipe 20. The fourth preferred embodiment of the asymmetric structure 40 of the balance valve 11 is illustrated in FIG. 7(in collaboration with FIG. 2), wherein the cold water punch hole 341 of the moveable spool 30 is wider than the hot water punch hole 331 to form an asymmetric pattern. When hot water line Li is blocked, the moveable spool 30 is abutted towards the first lateral end wall 12 of the holding space 101, said cold water channel X is formed to maintain water supply state of cold water line L2 due to sectional difference between the cold water punch hole 341 of the moveable spool 30 and the cold water opening 23 of the fixed valve pipe 20. The fifth preferred embodiment of the asymmetric structure 40 of the balance valve 11 is illustrated in FIGS. 8, 9, wherein an inner groove 60 is recessed onto inner wall of the fixed valve pipe 20 and set correspondingly to the cold/hot water openings 23, 24 of the fixed valve pipe 20; of which the setting width of the inner groove 60 is equal to or bigger than the width of the cold/hot water openings 23, 24. Referring to FIG. 10, in the event of reciprocating transverse movement of the moveable spool 30 in the inner wall of the fixed valve pipe 20, the contact friction area between the moveable spool 30 and the inner wall of the fixed valve pipe 20 could be reduced via the inner groove 60 recessed on inner wall of the fixed valve pipe 20, thus improving the operational flexibility of the moveable spool 30. Referring also to FIG. 10, a capillary rough edge A formed internally on the cold/hot water openings 23, 34 of the fixed valve pipe 20 will be accommodated into said inner groove 60, such that the operation of the moveable spool 30 is immune to the influence of the rough edge A. Or referring to FIG. 11, in the event of reciprocating transverse movement of the moveable spool 30 in the inner wall of the fixed valve pipe 20, scale C on the moveable spool 30 could be removed automatically through the vertical corner B formed by the inner groove 60 and the inner wall of the fixed valve pipe 20, thereby maintaining the smooth operation of the moveable spool 30.