PIPE AND HEAT EXCHANGER Field Of The Invention The present invention relates to a pipe and a heat exchanger. 5 Background With a certain type of conventional heat exchangers using a cylindrical pipe as a heat transfer pipe, the cylindrical pipe is formed in corrugated shape for improving the heat transfer characteristic (heat transfer efficiency) . When such a heat 10 exchanger is used in the food, bio-technological, and electronics industries and the like, it must meet the requirements that the intraductal liquid be able to be thoroughly discharged to the outside, and that the cleanability be improved and the level of sanitation be enhanced. 15 Especially, horizontal heat exchangers using a pipe formed in corrugated shape, however, have a drawback of that liquid accumulation is produced in the root of the corrugation, which makes it difficult to thoroughly discharge the residual liquid to the outside in draining or cleaning. Especially, for the 20 processes for manufacturing pharmaceuticals, those in the electronics industry that use a cleaning liquid or pure water, the variety of processes in the bio-technological and food industries, and the like, the heat exchanger used must produce no residual liquid, thus, a satisfactory measure has been 25 demanded. Summary Of The Invention Then, the purpose of the present invention is to provide a cylindrical pipe which has an excellent heat transfer characteristic and produces no liquid accumulation, as well as 30 a heat exchanger which uses such a cylindrical pipe. To achieve the above purpose, the pipe according to the present invention is a cylindrical pipe, wherein a flat-shaped portion in strip shape is formed along the pipe axis inside the cylindrical pipe, and said cylindrical pipe is formed in 35 spirally corrugated shape over the entire length except for said flat portion, the depth of the corrugation being gradually 1 increased from said flat portion, and being at maximum in the area opposite to said flat portion. Further, such a pipe is used as a heat transfer pipe, and the heat transfer pipe is disposed such that the flat portion thereof provides the pipe 5 bottom. 1/1 If a cylindrical pipe which is formed in corrugated shape is used as a heat transfer pipe for a heat exchanger, the formation of a turbulence is promoted inside it, which results in the heat transfer efficiency being improved. Further, by 5 disposing the flat portion formed along the pipe axis in the corrugated cylindrical pipe such that it provides the pipe bottom, the residual liquid can be thoroughly removed in draining the liquid inside the pipe or in cleaning the pipe inside, the possibility of contamination being eliminated with 10 the level of sanitation being raised, and the efficiency of blowing off the intraductal liquid with the use of compressed gas being improved. Therefore, the heat exchanger according to the present invention is well suited for the pharmaceutical industry, electronics industry, bio-technological industry, 15 food industry, and the like where no contamination of impurity is permitted. Brief Description Of The Drawings Fig. lis a side view, with a portion broken away for clarity, showing an embodiment of the present invention; 20 Fig. 2 is a cross-sectional view thereof; Fig. 3 is a sectional view of a shell-and-tube heat exchanger; Fig. 4 (A) is a left side view thereof, and Fig. 4 (B) is a right side view thereof; 25 Fig. 5 (A) is a side view of a left pipe plate, and Fig. 5 (B) is that of a right pipe plate; Fig. 6 is a sectional view of the right end portion of a shell-and-tube heat exchanger; Fig. 7 is a front view, with critical portions broken away 30 for clarity, of a double-tube heat exchanger; and Fig. 8 is a front view, with critical portions broken away for clarity, of a coil-type heat exchanger. Description Of The Preferred Embodiments Hereinbelow, the present invention will be specifically 35 described with reference to the drawings showing embodiments thereof. 2 Fig. lis a side view, with a portion broken away for clarity, of a cylindrical pipe 1 according to the present invention, and Fig. 2 is a cross-sectional view thereof. This cylindrical pipe 1 is formed in spirally corrugated shape, however, the center 5 of the spiral (the axis) "0" is offset by a dimension of E with respect to the pipe axis Las shown in FIG. 2, the cross sectional view. Thus, inside the cylindrical pipe 1, a flat portion 2 extending linearly along the pipe axis L is formed. Therefore, when the cylindrical pipe 1 is disposed such that the flat 10 portion 2 provides the pipe bottom, the liquid inside the pipe can be thoroughly discharged from the pipe opening 3, 4 to the outside with no liquid accumulation being produced in the inside. Such cylindrical pipe is well suited for use as a heat transfer pipe for a shell-and-tube heat exchanger 5 (as shown in FIG. 15 3). In other words, the inside diameter (root diameter) of the corrugation of the above-mentioned cylindrical pipe 1 is offset by a small dimension of E below the pipe axis L, as shown in FIG. 2, with the pipe being formed such that a flat portion 2 20 in the shape of a linear strip, which is free from corrugation, is provided along the pipe axis L (see FIG. 1) . The degree of corrugation is gradually lowered (the depth of the corrugation is gradually decreased) as the pipe bottom is approached from the pipe top through either pipe side, and in the flat portion 25 2, no corrugation is given (the depth of the corrugation is zero). Fig. 3 is a sectional view of a shell-and-tube heat exchanger 5 of mono-tube type which adopts the cylindrical pipe 1 as shown in FIG. 1 and FIG. 2 as the heat transfer pipe; Fig. 4 (A) is 30 a left side view thereof, and Fig. 4 (B) is a right side view thereof; Fig. 5 (A) is a left side view of one pipe plate 6, and Fig. 5 (B) is a right side view of the other pipe plate 7; Fig. 6 is a sectional view of the right end portion. Here is a description of the main components of the 35 shell-and-tube heat exchanger 5; 10 indicates a jacket pipe; 8, 9 a cover plate; 11 a baffle; 12, 13 a special ring-shaped 3 gasket provided around the heat transfer pipe 1; 14, 15 a ring-shaped gasket for the circumferential portion of the cover plate; 16, 17 a clamp band for jointing one pipe plate 6 with the cover plate 8, and the other pipe plate 7 with the cover 5 plate 9, respectively; and 18, 19 a bolt and nuts for clamping the clamp band 16, 17. The jacket pipe 10 is of cylindrical, horizontal stationary type, being made of a corrosion resistant metallic material, such as stainless steel. Inside thereof, a plurality of heat 10 transfer pipes 1 are disposed in parallel, a process liquid which heat is to be exchanged being let to flow into the heat transfer pipe 1, and at both ends of the jacket pipe 10, an inlet pipe 20 and an outlet pipe 21 for a heating medium (liquid) are mounted, the heating medium being let to flow from one baffle 15 11 to another provided in the jacket pipe 10, and thus it flowing in a turbulent condition assuring efficient contact with the surface of the heat transfer pipe 1. The heat transfer pipes 1 are made of a corrosion resistant metallic material, such as stainless steel and a titanium 20 material, being disposed in parallel and fixed by means of the pipe plate 6, 7 together with the jacket pipe 10, and each heat transfer pipe 1 is connected to a U-turn flow path 22, 23 in the cover plate 8, 9, respectively, forming a forward or backward feeding heat transfer pipe. As described above, the 25 heat transferpipes are formed in corrugated shape forproducing a turbulence in the process liquid flowing in the pipe, and thus increasing the overall heat transfer coefficient, which provides a measure of the performance of a heat exchanger. Between the pipe plate 6, 7, which supports the heat transfer 30 pipes 1, and the cover plate 8, 9 (see FIG. 6), a special ring-shaped gasket 12, 13 and a ring-shaped gasket 14, 15 are installed, and the flange edges 61 and 81, and 71 and 91, which are formed in tapered shape in the circumferential portions of the pipe plate 6, 7, and the cover plate 8, 9, respectively, 35 are fixed by means of the clamp band, 16, 17, which is tightened with the use of the bolt and nuts 18, 19, such that the jointing 4 surfaces are uniformly and tightly contacted with each other. The special ring-shaped gasket 12, 13 is formed in ring shape which is roughly L-shaped in cross section, being provided for sealing the heat transfer pipe 1. Specifically, as shown in 5 FIG. 6, the right end portion of the shell-and-tube heat exchanger is configured such that the convex of the special ring-shaped gasket 13 is tightly fit into the concave formed in the inlet/outlet surface for the U-turn flow path 23 which is provided in the cover plate 9, therefore, the cover plate 10 9 (or 8) can be opened and closed for easily performing inspection/cleaning of each process flow path without the special ring-shaped gasket 13 dropping out. In addition, because the bore of the special ring-shaped gasket 13 is provided with the same diameter as that of the bore 15 of the heat transfer pipe 1, which is inserted into the pipe hole 72 in the pipe plate 7, both bores are jointed with each other with no step being formed in the junction, thus no accumulation of the process liquid is produced at the jointing surfaces of the pipe plate 7 and the cover plate 9, although 20 they are flat. The left end portion of the shell-and-tube heat exchanger is configured in the same manner as the right end portion. The ring-shaped gasket 14, 15 provided between the circumferential portions of the pipe plate 6 and the cover plate 25 8, and between those of the pipe plate 7 and the cover plate 9, respectively, causes the process liquid (if leaked from a particular special ring-shaped gasket 12, 13) to be discharged to the outside only through a drain/leak detection pipe 26 (shown only for the right end portion) provided in the cover 30 plate 8, 9, thus permitting the proper operation control even to be performed during the running of the shell-and-tube heat exchanger. With such a configuration, the process liquid is fed in at the inlet pipe 24 as shown in FIG. 3, and while it is fed forward 35 or backward through the respective heat transfer pipes 1, which are connected to one another by means of the U-turn flow path 5 22, 23, heat exchange is performed at a high heat transfer efficiency between it and the heating medium in the jacket pipe 10 (outside the heat transfer pipe 1), with the formation of a turbulence being promoted; finally the process liquid is 5 discharged through the outlet pipe 25 at the cover plate 9. The flat portion 2, which is linearly formed along the pipe axis L in the heat transfer pipe 1, is disposed such that it provides the pipe bottom, thus, when the liquid inside the pipe is to be drained after the completion of the heat exchange operation, 10 or when the heat exchanger is to be cleaned, the residual liquid inside the pipe can be thoroughly removed by gravitational drain or forced drain with the use of compressed gas, resulting in the possibility of the so-called intraductal contamination being eliminated, and the level of sanitation being raised. 15 Also, the safety in the subsequent process can be improved. Therefore, the heat exchanger according to the present invention is well suited for food processing, bio-technology processes, manufacture of pharmaceuticals, and production processes, such as cleaning of electronic parts, where no 20 contamination due to even a trace of impurity ispermitted, and thus heating, cooling, sterilization, and cleaning are repetitively performed, a high level of sanitation being often required. 25 30 FIG. 7 shows a double-tube heat exchanger 5; 1 indicating a heat transfer pipe 1; 10 a jacket pipe; 20 a process liquid 35 inlet pipe; 21 a process liquid outlet pipe; 24 a heating medium inlet pipe; and 25 a heating medium outlet pipe. With this 6 double-tube heat exchanger 5, the heating medium and the process liquid are let to flow in a countercurrent flow such that they face to each other, resulting in an increased overall heat transfer coefficient, and in many applications, a few (a 5 plurality of) double-tube heat exchangers 5 are connected to one another with a U-shaped pipe (not shown) to form a single process flow path. Both the heat transfer pipe 1 and the jacket pipe 10 in this double-tube heat exchangers 5 are formed to provide a corrugation such that a turbulence is created in both 10 the heating medium and the process liquid, resulting in the heat transfer efficiency being improved. If a few double-tube heat exchangers 5 are combined into a single processing unit, being disposed at different levels with an upper heat exchanger being connected to a lower one by 15 means of a U-shaped pipe to form a step-like process flow path, for example, (which illustration omitted), the process liquid, the cleaning drainage, or the like can be thoroughly discharged to the outside by gravitational drain from the bottom heat exchanger, which makes it possible to effectively prevent 20 contamination due to liquid accumulation from being caused. Therefore, this system can be used with a highly viscous liquid, a slurry liquid, and the like, and further it allows a residual liquid, such as a pig, to be recovered, and the level of sanitation for the entire unit to be raised. 25 Fig. 8 shows a coil-type heat exchanger 5. With it, a spirally formed cylindrical pipe is adopted as a heat transfer coil 31. 32 indicates a vessel; 33 a heating medium inlet pipe; 34 a heating medium outlet pipe; 35 a liquid inlet pipe; 36 a liquid outlet pipe; 37 a stirring vane; and 38 a rotation shaft 30 for the stirring vane 37. With this configuration, the reaction liquid or the process liquid is introduced into the vessel 32 at the liquid inlet pipe 35, and then is stirred by the stirring vane 37. On the other hand, the heating medium is fed into the heat transfer coil 31 at the heating medium inlet pipe 33 at 35 the top, and is discharged at the heating medium outlet pipe 34 at the bottom, meanwhile, heat exchange is performed between 7 the heating medium and the process liquid at the outside surface of the heat transfer coil 31. With such a coil-type heat exchanger 5, it is well known that, if a corrugation is formed on the surface of the heat 5 transfer coil 31, the heat transfer efficiency is improved in heat exchange, however, in the prior art, when draining the heating medium, or when draining the liquid for cleaning the inside of the heat transfer coil 31, such a liquid is sometimes left at the bottom of the heat transfer coil 31 which is formed 10 in corrugated shape, resulting in the heat transfer efficiency being lowered due to the intraductal contamination, and the heat transfer coil 31 being corroded. Then, with the present invention, a flat potion 2, which is free from corrugation, is continuously formed along the (spiral) pipe axis at the bottom 15 of the heat transfer coil 31, which eliminates the occurrence of liquid accumulation, and thus allows the liquid inside the pipe to be thoroughly discharged to the outside by gravitational drain. Thus, the maintenance can be fully performed, which permits'a high heat transfer efficiency to be always obtained, 20 and the durability to be improved. According to the present invention, the cylindrical pipe 1 is formed in corrugated shape, therefore, the formation of a turbulence is promoted inside it, which results in the heat transfer efficiency being improved. In addition, a flat potion 25 2 is provided along the pipe axis L inside the cylindrical pipe 1, thus, if the cylindrical pipe 1 is disposed such that the flat portion 2 provides the pipe bottom, the residual liquid can be thoroughly removed in draining the liquid inside the pipe or in cleaning the pipe inside, the possibility of contamination 30 being eliminated with the level of sanitation being enhanced, and the efficiency of blowing off the intraductal liquid with the use- of compressed gas being bettered. 8