Package-type screw compressor

Abstract: Compressor bodies (1, 2), an accelerator (3) and a main motor (4) are disposed on a base (21), while an intercooler (5), an aftercooler (6), an oil cooler (7) and a coolant cooler (8) are disposed perpendicular to the axial direction of the motor (4) so that the directions, in which the tube nests of the gas coolers (5, 6) are drawn out, are made to be the same. A control panel (11) having a maintenance display is mounted on a front panel (52) composed of panel portions (52a, 52b) which are mounted pivotally around respective remote or opposite side ends. Portions to be inspected daily are disposed near the front panel (52) and one side panel (54) adjacent thereto.

Description: The invention relates to a screw rotor for a dry screw compressor which screw rotor has spiral and projecting teeth and a corrosion resistant coating film, to a dry screw compressor comprising such a male and female screw rotor, as well as to a method for producing such screw rotors. Usually a dry screw compressor is so arranged that a male rotor and a female rotor are, in a non-contact manner, engaged with each other while maintaining a small gap by a synchronizing gear to compress air in a compression chamber in a casing thereof. Thus, the dry screw compressor has the small gap between the two rotors and between the rotor and the casing and there is no oil in its comression chamber. Therefore, if during dwell periods of the compressor rust is generated on the surface of the rotors due to dew condensation of water in air, the rust serves to secure the surface of the teeth of the male and female rotors to each other or the rotors and the wall of the compression chamber to each other. As a result, there arises a problem in that a rotor-locking phenomenon occurs in which the rotor cannot be rotated. Hitherto, a coating film has been, as a method of improving the corrosion resistance of the rotor, formed by fixedly applying, to the surface of the rotor, a solid lubricant, such as molybdenum disulfide hereinafter called MoS 2 or equally-granulated powder of tetrafluoroethylene hereinafter called PTFE. This prior art is discussed referring to the drawings in which Fig.1 illustrates a process flow for performing conventional MoS 2 coating; Fig.2 illustrates a process of formation of a conventional MoS 2 coating film; Fig.3 illustrates a process of formation of the PFA coating film; As shown in Fig.1 and Fig.2, the shape of the screw rotor was machined to have predetermined demensions followed by subjecting the surface of the teeth of the rotor to cleaning and degreasing 118. After the degreasing 118 and the shot blasting 119 are performed, manganese phosphate treatment 120, water- washing 121 and drying 122 are performed so that a manganese phosphate film 140 is formed on the rotor base 104. Then, brushing 123 is performed, and then MoS 2 coating 124 and drying 125 are performed so that withered-leaf-like MoS 2 particles 141 are stacked. Then, preliminary baking 126 and baking 127 are performed, and the MoS 2 particles 141 are bonded together by a binder. The MoS 2 film has a structure that MoS 2 particles 141 are stacked and the space among them are filled with the binder. If the binder and the manganese phosphate film 140 is subjected to a hot temerature higher than 200°C, their corrosion resistance deteriorates. Therefore, pin holes are easily formed between the MoS 2 particles 141. Since the temperature of air in the compression chamber of the dry screw compressor is sometimes raised to a level higher than 200°C due to adiabatic compression, the MoS 2 coating cannot provide for a satisfactory corrosion resistance effect. If necessary, the steps 124 to 128 shown in Fig.1 are repeated to have a required film thickness to improve the corrosion resistance of the rotor. As a result, locking of the rotor taken place due to the corrosion is prevented. However, the fact that the heat resistance of a binder for use in the MoS 2 coating material and that of a manganese phosphate film are about 200°C raises a problem in that a long-time operation of the compressor under a temperature of air discharged from the compressor exceeding 200°C, deteriorates the corrosion resistance and, therefore, rust can be generated in the rotor due to water in air causing the rotor to be locked. Fig. 3 shows conventional PTFE coating as described in "Latest Fluoropolymer Coating Technology", published by Epote Co, Ltd., p.p.104. The PTFE particles 136 are applied to the surface of the rotor base 104. They are not melted but stacked while maintaining their particle shape to form a PTFE film 135. Reference numeral 137 represents a PTFE primer coating solution, 138 a PTFE primer coating layer which has been preliminarily or temporarily baked, and 139 a main PTFE coating solution. Although the corrosion resistance of the PTFE particles 136 is good, the fact that the PTFE coating film is formed by a simple aggregation of the PTFE particles 136 allows pin holes to be easily formed along regions at which the particles 136 are joined together. Therefore, the thickness of the PTFE coating film 135 must be high in order to have satisfactory corrosion resistance. A rotor having a coating layer of MoS 2 particles bound together by the binder of epoxy resin is proposed in JP-A-2-201072. JP-A-2-75789 and 1-302977 disclose a rotor base which is made of synthetic resin and is provided with a surface layer of ethylene-ethylene tetrafluoride copolymer directly or indirectly reinforced by carbon fiber. JP-A-2-301694 discloses a rotor having a base, a corrosion-resisting coating layer thereon and a solid lubricant surface layer. JP-A-3-290086 shows a rotor having a base of spheroidal graphit cast iron, an electroless nickel plating layer, polyphenylene sulfide resin layer and a surface layer of organic resin such a epoxy resin. Further, a rotor having a base of aluminum or magnesium alloy or the like and a thermo-setting resin layer on the base was proposed by JP-A-3-271586. JP-A-61-190184, for example, teaches the thickness of the coating layer to be formed on the base of the screw rotor. It is the object of the invention to provide a screw rotor of the generic kind the surface of which is corrosion-resistant as well as a method for coating such a screw rotor such that pin holes cannot easily form. This object is achieved with the screw rotor of the generic kind in that said coating film comprises of a continuous and uniformly integrated layer of melted and solidified particles of tetrafluoroethylene-perfluoroalkylvinylether copolymer, in the following discription referred to as PFA. The screw rotor of the invention is provided so that primer coating is applied to lobes of screw rotors each having been machined to have a predetermined teeth profile, followed by performing drying and baking. Then, a coating material comprising tetrafluoroethylene-perfluoroalkylvinylether copolymer, i.e. PFA, is uniformly deposited or applied, followed by performing drying and baking to form a PFA coating film or layer on the surface of the rotor lobes so that the rust generation on the rotor is prevented. It is preferable that the PFA coating film is formed by steps of applying a primer coating solution, in which PFA particles, a binder, pigment and water are mixed and dispersed, to the surface of the rotor base, followed by performing drying and baking; and applying a PFA coating solution made of PFA particles, binder and pigment dispersed in water, followed by performing drying and baking up to a temperature where the PFA particles on the rotor base are melted to form a continuous coating film. If the PFA coating is applied, the surface of the rotor base is covered with a uniform PFA film in which the melted PFA particles form a continuous layer. Pin holes cannot easily be formed. Therefore, a significant rust prevention effect can be obtained even by a thin thickness which is 50 µm or thinner of the PFA film. Since the PFA particles are melted to form a uniformly integrated layer after solidification, there is no deterioration of the coating film due to a drop of the granular powder. Since the PFA has a non-adhesive character, foreign matter, such as dust in air, having invaded into the small gap between the male rotor and the female rotor or between the rotor and the casing can easily be removed. Therefore, there is no problem of locking of the rotors due to accumulation of the foreign matter. As described above, the PFA coating applied to the rotor of the dry screw compressor enables to minimize generation of rust on the rotor. Therefore, locking due to rust generated in the small gap between the rotors or between a rotor and the casing can be avoided. Even if rust is generated in the casing, the non-adhesive character of the fluoro resin suppresses fixation of the rotor with the rust of the casing. As a result, locking of the rotors in the gas compression chamber due to rust can be prevented. The invention will be described in detail with reference to the accompanying drawings, in which Fig.4 is a perspective view which illustrates mating engagement between a male rotor and a female rotor of a screw compressor; Fig.5 is an enlarged view which illustrates coating layers according to an embodiment of the invention, wherein an essential portion of the relationship between the base of the screw rotor and the coating layers is illustrated in an enlarged and cross sectional manner; Fig.6 illustrates a procress flow for performing PFA coating according to an embodiment of the invention; Fig.7 illustrates a process flow for performing MoS 2 coating and Fig.8 is a graph which illustrates an effect of preventing locking of the rotor due to rust obtained in a screw compressor having screw rotor lobes applied with PFA coating in comparison with the effect obtained in the screw compressor having screw rotor lobes applied with MoS 2 coating, where the locking was reproduced in a test for evaluating the locking. As shown in Fig.4, a male rotor 101 having spiral and projecting teeth 101a and a female rotor 102 having spiral and concave teeth 102a are supported by bearings 152, (bearings at the other sides not shown) disposed in a casing 151 that the rotors 101 and 102 are rotated in a non-contact manner in the casing 151 but the teeth 101a and 102a of the rotors 101 and 102 are substantially matingly engaged with each other. The two rotors 101 and 102 are connected to each other by means of synchronizing gears 153 and 154. A lobe 103 of the male rotor 101 and that of the female rotor 102 shown in Fig.4 are heated to the same temperature at which a PFA coating material is baked to degrease the lobes 103. Then, the surface of a base 104 of the rotor made of material such as carbon steel is roughened as designated by the reference symbol R in Fig.5 which shows a part of the base portion of the rotor. The roughed surface 104a is applied with a primer coating 105. Then it is dried. Then, a PFA coating material 106 is applied to the surface of the dried primer coating 105. Then the whole body is subjected to baking. As disclosed in "Latest Fluoropolymer Coating Technology", published by Epote Co., Ltd. p.p 8 and p.p. 304, the PFA has the following structure: where x and n are positive integers. The process for the PFA coating is shown in detail in Fig.6. First, the base 104 ( Fig.5 ) of each lobe 103 of the screw rotors is machined to have a predetermined shape. Then, the base 104 of the lobe 103 ( Fig.5 ) is heated at the temperature, which is the same as that at which the PFA coating is baked, to degrease 107. Then, the base 104 is cooled to the temperature of the treatment room. Thereafter the coating surface of the base 104 of the lobe 103 is roughened by performing alumina shot blasting 108. Then, the roughened surface 104a is cleaned by brushing 109, followed by performing a coating 110 of a primer coating composition composed of materials such as PFA particles, pigment and binder. Then, the primer coating film is dried in step 111. This film is preliminarily baked in step 112 at 300°C to 350°C. Then it is cooled. Thereafter a coating 113 of the PFA film is performed. The PFA film has been dried in step 114, and baked in step 115 at 390°C to 420°C, and cooled in step 116. The thickness of the PFA film 106 is measured. If a predetermined thickness is not achieved, the steps 113 to 116 shown in Fig. 6 are repeated. Fig. 7 shows the process of forming the PFA coating film. In this process, a primer coating solution 132, in which the PFA particles 132, the binder, the pigment and water are mixed and dispersed, is applied onto the roughened surface 104a of the rotor base 104 in the foregoing primer coating step 110. After drying 111 and the preliminary baking 112 are performed, a solution 134 of the PFA particles 131 is applied to the preliminary baked primer film 105a in step 113 (Fig.6) and dried. In the next step 115, the overall body is baked so that the PFA particles 131 on the rotor base 104 are melted to form a continuous coating film 105, 106. PFA coating causes the surface of the rotor base 104 to be covered with a melted and solidified continiuous and uniform layer of PFA particles 131. Therefore, pin holes cannot easily be formed so that a satisfactory corrosion resistance can be obtained even if the thickness of the PFA coating film 106 is very thin,i.e. 50 µm or thinner. Since the PFA particles 131 are melted and integrated in the PFA film 106, there is no fear of deterioration of the coating film 106 due to drop of the granular powder. In other words, the PFA film 106 has satisfactory corrosion resistance even if its thickness is relatively thin. Since the PFA has a non-adhesive character, foreign matter, such as dust in air, invaded into the small gap between the rotors 101, 102 or among the rotors 101,102 and the casing 151 can easily be discharged. Therefore, no problem of locking of the rotors 101 and 102 occurs. By applying the PFA coating to the rotors 101 and 102 of a dry screw compressor rust generation in the rotors 101 and 102 can be prevented and locking due to the rust in the small gap between rotors 101 and 102, or among the rotors 101 and 102 and the casing 151 can be avoided. Therefore, the locking of the rotors 101 and 102 due to rust can be prevented. Fig. 8 is a graph which illustrates the results of measurements of rotor-fixing or locking forces of the rotors of a dry screw compressor comprising lobes applied with the MoS 2 coating according to the prior art and lobes applied with the PFA coating. The measurements are evaluated by the rotational torque required to rotate the rotors after salt water was periodically sprayed followed by leaving to force generation of locking of the rotors due to rust. In comparison with the rotors with the conventional MoS 2 coating, it can be understood from Fig. 8 that the rotors with the PFA coating better prevent rust. Therefore, there is less risk of locking of the rotors due to rust.

Claims: A screw rotor for a dry screw compressor which screw rotor has spiral and projecting teeth and a corrosion resistant coating film, characterized in that said coating film comprises a continuous and uniformly integrated layer (106) of melted and solidified particles (131) of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA). A screw rotor according to claim 1 wherein the thickness of the uniformly integrated layer of melted and solidified particles (131) is 50 µm or less A dry screw compressor comprising a male (101) and a female (102) rotor according to claim 1 or 2 which rotors (101, 102) are supported by bearings (152) disposed in a casing (151), which rotors (101, 102) are connected to each other through synchronizing gears (153, 154), and which rotors (101, 102) are rotated in a non-contact manner while their teeth (101a, 102a) are substantially engaged with each other. Method for coating a screw rotor according to claim 1 or 2 wherein a primer coating is applied followed by drying and baking as well as uniformly depositing or applying thereafter a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) coating solution which is followed by a further drying and baking. Method according to claim 4, characterized in that for the primer coating PFA-particles, a binder, pigment and water are mixed and dispersed, while for baking the PFA coating solution PFA particles, a binder and a pigment are dispersed in water. Method according to claim 4 or 5 wherein for baking the PFA coating solution a temperature is used where the PFA particles are melted to form the continuous coating film. Method according to one of the claims 4 to 6, wherein the primer coating is baked at 300°C to 350°C and cooled thereafter while the PFA coating solution is baked up to 390°C to 420°C where the PFA particles on the rotor are melted to form the continuous coating film after solidification by cooling. Method according to one of the claims 4 to 7, wherein the screw rotor is degreased and roughened before the primer coating is applied. Method according to claim 8, wherein degreasing is carried out by heating at the temperature of baking the PFA coating. Method according to one of the claims 4 to 9, wherein further PFA coating solution films are applied, dried, baked and cooled until the required thickness of the coating film is achieved.

Inventors: HIROSE SHINICHI, KANEKI TADASHI, KIKUCHI KATSUAKI, OKITA JUNJI, TSURU SEIJI, HIROSE, SHINICHI, KANEKI, TADASHI, KIKUCHI, KATSUAKI, OKITA, JUNJI, TSURU, SEIJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: F04C23
IPC codes: F04C18, F04C23