Abstract: A bent pipe to which external force, such as an internal pressure and a bending moment, is applied singly or in combination, characterized in that the relationship among the outside diameter D.sub.0 of the bent part, the radius of curvature R of the bent part, the outside diameter D.sub.0 of the straight part, the wall thickness T.sub.0 of the straight part, the central cross section angle .phi. of the bent part and the wall thickness t of the pipe, is in the range defined by the following equations (1) and (2): ##EQU1## whereby the bent pipe has a sectional form of high strength.

Abstract: The present invention provides a pulse tube refrigerator, comprising a regenerator having an inlet port and an outlet port, a pulse tube having one end portion connected in series to the outlet port of the regenerator, a gas compressor connected to the inlet port of the regenerator, a first valve disposed between the discharge port of the gas compressor and the inlet port of the regenerator, a second valve disposed between the suction port of the gas compressor and the inlet port of the regenerator, a first valve controller for selectively opening/closing alternately the first and second valves to permit a high pressure coolant gas discharged from the discharge port of the gas compressor to be guided into the pulse tube through the regenerator and, then, to permit said coolant gas to be sucked into the gas compressor through the suction port thereof via the reverse passageway so as to generate coldness, a third valve disposed between the other end portion of the pulse tube and the discharge port of the gas co