# Patent

## Title
Refurbishment process of the pumping unit in a volumetric screw compressor of the ‘oil free’ type

## Abstract

A refurbishment process for a volumetric screw compressor of the ‘oil-free’ type, which comprises a male rotor and a female rotor, is described. The process comprises visually checking the wear condition of the rotors, treating their surface for removing the previous coating, and applying a new coating on the surface. The composition of the coating applied on the surface of the rotors consists of the following materials:













Material
Amount (g)








Polytetrafluoroethylene
750 ÷ 850



(954G 303 C Teflon, DuPont)




Amorphous graphite powder
300 ÷ 400



Thinner for spray cleaning apparatuses
200 ÷ 270



(8595 thinner, DuPont)




Methyl ethyl ketone (MEK)
170 ÷ 220



Cellosolve acetate coating additive
200 ÷ 300



(Syn Fac 800 resin).

## Background
The present invention relates to a refurbishment process of the pumping unit in a volumetric screw compressor of the ‘oil-free’ type, i.e. without pumping unit lubrication oil.
Many activities in the field of pharmaceutical or food production, in precision electronics or in other sensitive applications require the use of compression units which deliver excellent air quality in order to ensure perfect end products and production processes.
Being specifically developed for applications requiring maximum purity levels, ‘oil-free’ compressors compress the air without lubrication oil, and thus prevent the introduction of oil into the compression process, thus eliminating the risk of product contamination and alteration, damage to corporate reputation and delays, which are, in turn, cause of further expenses.
Rotary ‘oil-free’ volumetric compressors are known, in particular of the ZR type made by Atlas Copco, in which the pumping unit comprises a pair of screw-shaped rotors. In such compressors, known as screw compressors, the rotors are externally provided with reversing helical screws and are arranged side by side to mate with each other. By rotating within the cylindrical seats obtained in the pumping unit, the screw rotors create a compartment therebetween and the body in which they are accommodated, which progressively moves from the intake zone to the discharge zone, decreasing the volume and thus compressing the air entrapped between the two rotors and the walls of the compartment. By means of the rotation of the rotors, the volume incorporated between them is reduced, thus increasing the pressure until the air is pushed towards the discharge mouth, and thus ejected.
The absence of the action of a lubricant means that the mechanical parts of the ‘oil-free’ compressor are inevitably subject to wear. Careful maintenance is needed in order to keep up the performance level in particularly demanding industrial processes, like those listed above. When a pumping unit wear occurs, the only possible solution is to replace the concerned components with new genuine components.
Various documents are known, which illustrate maintenance services for pumping units, such as for example US patent application 2003113221(A1), which describes a treatment of the rotor surfaces so as to reduce the clearance between the surfaces, or the website http://www.airhire.co.uk/acatalog/The_Refurbishment_Process.html, which describes a refurbishment process for screw compressors.
In the light of the prior art, it is the object of the present invention to provide a maintenance service for pumping units of screw compressors of the ‘oil-free’ type which ensures performances similar to those which would be obtained with genuine spare parts, but with a considerable saving of costs.
In accordance with the present invention, said object is achieved by means of a process for generating the pumping unit of a screw compressor of the ‘oil-free’ type, as claimed in claim 1.



The features and advantages of the present invention will be apparent from the following detailed description of a practical embodiment thereof, illustrated by way of non-limitative example in the accompanying drawings, in which:
FIG. 1 shows by way of example a perspective overview of a volumetric screw compressor of the ‘oil-free’ type, to which the refurbishment process of the pumping unit according to the present invention may be applied;
FIG. 2 shows the same compressor according to another perspective;
FIGS. 3-19 show a sequence of operating steps of the process according to the invention, when used by way of example for refurbishing the pump unit of the compressor in FIG. 1.



The figures refer to a typical example of an ‘oil-free’ screw volumetric compressor, commercially known as ZR compressor made by Atlas Copco, an overview of which is shown in FIGS. 1 and 2, and which is identified as a whole by reference numeral 1.
The refurbishment process according to the present invention can equally be used for other ‘oil-free’ screw volumetric compressors of the same or other manufacturers.
As shown in FIGS. 1 and 2, compressor 1 comprises a pumping unit 2, a header 8 and a synchronization gear casing 13.
The pumping unit 2 is more clearly shown in FIGS. 16 and 17, where it is shown comprising an outer body 200 and an inner body 100 forming two seats 3a, 3b, which house respective male 4 and female 5 connectors. The inner chamber 100 includes as a whole a first side opening (not shown in the drawings) adapted to aspirate air, and a second side opening 201 adapted to eject the air, said openings allowing chamber 100 to communicate with the exterior.
The rotors 4, 5 include respective shafts 40, 50 in a single body, which are parallel and appropriately spaced apart from each other, and respective external reversing helical screws 80, 81 which mesh each other and form an air pumping and compression compartment with the inner wall of chamber 100, which compartment extends from the inlet opening to the outlet opening of chamber 100. The helical screws of rotors 4, 5 are typically made of carbon steel C45/C50.
Header 8 is fixed by means of a plurality of screws 30 to a first side 6 of the body 200 of the pumping unit 2, commonly known as low-pressure side. A seal 9 (FIG. 15) and centering pins 140 (FIGS. 1-9, 16 and 17) are interposed between header 8 and the side 6 of body 200.
Two sealing assembles 10 and 11 are accommodated in respective seats in header 8 and are surmounted by respective radial bearings 121a and 121b, in which a respective end of the shafts 40, 50 of rotors 4, 5 is inserted (FIGS. 9 and 16).
Similarly, on a second side 7 of the body 200 of the pumping unit 2, commonly known as high-pressure side 7, two sealing assemblies 110 and 111 (FIG. 19), surmounted by respective radial bearings 120a and 120b (FIGS. 14, 15 and 18), receive other ends of the shafts 40, 50 of the rotors 4, 5. The rotation of the rotors 4, 5 is allowed by coupling the respective shafts 40, 50 with each pair of hearings 121a, 120a and 121b, 120b (FIGS. 4 and 5).
A plurality of elements are inserted over bearing 121a through a first end of shaft 40, in particular that coupled to bearing 121a on the low-pressure side 6, respectively: a compression spring 21a, an axial spacer 22a, a resting ring 20, a radial bearing 19a with bearing holder 20a, a synchronization gear 18a and a further spacer 17a. A screw 16a, inserted into the end of shaft 40, is adapted to lock the aforesaid plurality of elements and is surmounted by a compensation assembly formed by a tablet 14 and a spacer 15.
A plurality of elements are inserted over bearing 121b through a first end of shaft 50, in particular that coupled to bearing 121b on the low-pressure side 6, respectively: a compensation spring 21b, an axial spacer 22b, a radial bearing 19b with bearing holder 20b, a synchronization gear 18b and a further spacer 17b. A screw 16b, inserted into the end of shaft 50, is adapted to lock the aforesaid plurality of elements.
An oil injector 33 (FIGS. 5-7 and 9) lubricates the synchronization gears 18a and 18b without concerning the pumping unit 2 by virtue of the presence of the sealing assemblies 10 and 11.
The synchronization gear casing 13 (with seal, not shown in the drawings) is fixed to header 8 by means of a plurality of screws 150 so as to cover all the external components with respect to the low-pressure side 6 of the pumping unit 2.
A plurality of elements are inserted over bearing 120a through a second end of shaft 40, in particular that coupled to bearing 120a on the high-pressure side 7, respectively: a spacer 28a, a calibrated shim 25a, a flexible pin 24a, an angular contact bearing 27a, a control gear 29 and a spacer 23a. A screw 14a, inserted into the end of shaft 40, is adapted to lock the aforesaid plurality of elements.
A plurality of elements are inserted over bearing 120a through a second end of shaft 50, in particular that coupled to bearing 120b on the high-pressure side 7, respectively: a spacer 28b, a calibrated shim 25b, a flexible pin 24b, an angular contact bearing 27b and a spacer 23b. A screw 14a, inserted into the end of shaft 40, is adapted to lock the aforesaid plurality of elements.
An oil injector 26 lubricates gear 29 without concerning the pumping unit 2 by virtue of the presence of the sealing assemblies 110 and 111.
When worn, the pumping unit 2 can be refurbished by using the process according to the present invention.
The process initially requires to visually check the wear of bearings 27a, 27b on the high-pressure side 7. Once compressor 1 has been stably fixed to a work bench, it can start being disassembled by removing the screws 150 and then extracting the casing 13 and the respective seal (FIG. 3).
The compensation assembly, consisting of a tablet 14 and a spacer 15, is then removed (FIG. 4), allowing to loosen the fastening screws 16a, 16b of the synchronization gears of both shafts 40, 50 for removing the spacers 17a, 17b (FIG. 5).
The synchronization gears 18a, 18b (FIG. 6), the resting ring 20 (FIG. 7), the bearings 19a, 19b with respective bearing holder 20a, 20b (FIG. 8) and finally the compensation springs 21a, 21b and the axial extractor.
At this point, the pumping unit 2 with header 8 is rotated to face the high-pressure side 7 upwards (FIG. 10). The fastening screw 14a of gear 29 (FIG. 11) is loosened, and the spacer 23a (FIG. 11) and the gear 29 of shaft 40 (FIG. 12) are removed.
The coaxial rotation of the rotors 4, 5 is verified with a dial gauge, checking clearance and any misalignment of the bearings.
The fastening screw 14b and spacer 23b (FIG. 13) are then removed and the angular contact bearings 27a, 27b (FIG. 14) are removed with an appropriate extractor, followed by the flexible pins 24, the calibrated shims 25a, 25b, the oil injector 26 and the spacers 28b, 28b (FIG. 15).
At this point, the assembly is rotated to arrange the low-pressure side 6 facing upwards again. Once the fastening screws 30 of header 8 have been loosened on the low-pressure side 6 of the pumping unit 2, it is possible to extract header 8 (FIG. 16), including the bearings 121a, 121b. Seal 9 is eliminated and replaced during reassembly. The oil injector 33 is also removed.
The rotors 4, 5 are extracted one at a time with a roto-translating motion (FIG. 17) with great care and being careful to prevent contacts between them and the seats 3a, 3b of chamber 100.
Once the pumping unit 21 has been tipped again, the bearings 120a, 120b (FIG. 18) and the sealing assemblies 110, 111 (FIG. 19) are removed from the high-pressure side 7 with the aid of an appropriate extractor. The same operation is carried out on header 8, by removing the bearings 12a, 12b and the sealing assemblies 10, 11.
The four sealing assemblies 10, 11, 110 and 111 are then disassembled and the state of components is checked.
The wear condition of the profiles of the rotors 4 and 5 is visually checked to evaluate the refurbishment feasibility thereof. The rotors must be handled with care being careful not to cause shocks and/or stress of any type.
If the profiles are worn, the rotors can either be replaced or conservatively overhauled according to the present invention.
Firstly, the inner rings 122a, 122b, 123a, 123b of the bearings 120a, 120b, 121a, 121b are removed (FIG. 17), and then the previous coating is removed from both the helical springs 80, 81 and the shafts 40, 50 of the rotors 4, 5.
A preliminary treatment is carried out before applying the new coating, which consists in sandblasting the rotor surfaces 4, 5 using fine grain corundum in order to increase roughness and promote wettability. After such an operation, the rotors are degreased with a thinner (e.g. acetone) and dried in appropriate ovens at 50°/60° C. so as to completely evaporate the thinner. Checking that the temperature is not higher than 40° C. before application is needed.
At this point, a new coating according to the invention is applied on the surface of the helical screws of the rotors 4, 5.
The composition of the new coating according to the present invention consists of the following materials:













Material
Amount (g)








Polytetrafluoroethylene
750 + 850



(954G 303 C Teflon, DuPont)




Amorphous graphite powder
300 + 400



Thinner for spray cleaning
200 + 270



apparatuses




(8595 thinner, DuPont)




Methyl ethyl ketone (MEK)
170 + 220



Cellosolve acetate coating additive
200 + 300



(Syn Fac 800 resin)









For example, a particular formulation of the new coating may be as follows:













Material
Amount (g)








Polytetrafluoroethylene
800



(954G 303 C Teflon, DuPont)




Amorphous graphite powder
360



Thinner for spray cleaning
240



apparatuses




(8595 thinner, DuPont)




Methyl ethyl ketone (MEK)
195



Cellosolve acetate coating additive
240



(Syn Fac 800 resin









The various materials are mixed for about four hours with a slow gear system, which is capable of eliminating any clots or traces of graphite in suspension and does not create thermal imbalance in the mixture (grinding generates heat, which evaporates the MEK as it is highly volatile).
At this point, the coating is sprayed by means of a dry compressed air gun onto the helical screws 80, 81 of the rotors 4, 5, protecting the coupling surface with the bearings. They are then pre-cured at 60°/70° C. for about 30 minutes, and the quality and thickness of the paint coating is checked by means of an appropriate ultrasound instrument. The typical thickness is from 70 to 100 μm.
The shafts 40, 50 of the two rotors are spray-coated by means of a common PFTE (polytetrafluoroethylene) based coating.
The rotors 4, 5 are then put back into the ovens and cured by means of a temperature ramp up to 230° C. for about 30/45 minutes. Before extracting the rotors 4, 5 from the ovens, it is necessary to wait for the temperature to decrease uniformly for an optimal paint coating quality.
It is then checked that the water passages inside body 200 of the pumping unit 2 are free from build-ups or foreign bodies, and that the lubrication and cooling conduits in the pumping unit and in header 8 are clean. Once this check has been completed, the operations of coating removal, preparation and painting are repeated on the body 200 of the pumping unit 2, and on header 8.
Compressor 1 is now reassembled. The sealing assemblies 10, 11 and 110, 111 are refitted with the assistance of a small press in header 8 and on the high-pressure side 7, respectively, paying attention to the correct positioning of the right and left assemblies, intended to receive the coated shafts 40, 50. The same also occurs for both pairs of bearings 121a, 121b and 120a, 120b. The oil injectors 26, 33 are refitted.
With the opening of the compression chamber 100 facing upwards, the male rotor 4 is inserted into its respective seat 3a, delicately rotated to test the lack of interference and then extracted again. In the case of interference/excessive resistance to rotation, the coating thicknesses are checked again and possibly modified. The same operation is carried out on the female rotor 5 in the respective seat 3b. The two rotors 4, 5 and the respective helical screws 80, 81 are meshed and inserted into seats 3a, 3b, and are delicately rotated to test for lack of interference once again. The shafts 40, 50 under the helical screws 80, 81 engage the sealing assemblies 110, 111.
At this point, the low-pressure header 8 is fitted once a new seal 9 and the respective centering pins 140 have been inserted.
The rotors are manually rotated again to test for lack of interference and then the fastening screws 30 are inserted into header 8. The compensation springs 21a, 21b and the axial spacers 22a, 22b are then inserted.
The assemblies 19a, 19b are inserted with the aid of a small press and the resting ring 20 of the compensation assembly 15 is inserted on the male rotor 4.
The synchronization gears 18a, 18b are inserted after induction heating on the male rotor 4, and a service bushing on the female rotor 5, and then the spacers 17a, 17b are inserted and the screws 16a, 16b are fastened over the respective shafts 40, 50 of the rotors 4, 5.
At this point, the pumping unit 1 is rotated to arrange the high-pressure side 7 facing upwards. The spacers 28a, 28b and the angular contact bearing 27a, 27b are inserted with the aid of a small press.
A service bushing instead of gear 29 is inserted on the male rotor 4, and spacer 23a is then inserted and thus the fastening screw 14a is tightened. Similarly, spacer 23b is inserted on the female rotor 5 and screw 14b is inserted. The flexible pins 24a, 24b are then driven.
Coaxial rotation of the rotors is evaluated with the aid of a dial gauge, thus testing the clearance or misalignments of the radial bearings 12.
At this point, compressor 1 is turned upside down and the service bushing on the female rotor 5 is replaced with the respective synchronization gear 18b by removing and re-inserting spacer 17b and screw 16b. 
The compensation assembly 14, 15 is reassembled on the male rotor 4, lastly followed by the synchronization gear casing 13 which is reassembled on the low-pressure side 6.

## Claims

1. A new coating for a refurbishment process of an oil-free volumetric screw compressor comprising a pumping unit with an outer body and an inner chamber comprising a first and a second seat adapted to accommodate respective male and female rotors provided with respective reversing helical screws meshing each other, said process comprising gradually disassembling the components of the compressor up to extraction of a rotor at a time from the respective seats of the chamber, visually checking the wear condition of the rotors, treating the rotor surface to remove the previous coating, applying said new coating on the surface of the rotors, repeating the operations of treating and applying a coating on the outer body of the pumping unit, inserting and then extracting one rotor at a time into the respective seat and checking for lack of interference, reassembling the pumping unit by meshing and inserting the two rotors inside the seats with further checking for lack of interference, reassembling the remaining components of the compressor,
wherein said new coating consists of a mixture of the following materials:














Material
Amount (g)








Polytetrafluoroethylene
750-850



Amorphous graphite
300-400



powder




Thinner for spray
200-70



cleaning apparatuses




Methyl ethyl ketone
170-220



Cellosolve acetate
200-300



coating additive.

2. The new coating according to claim 1 for a process as defined above, wherein said treatment of the rotor surface comprises a sandblasting process adapted to increase roughness and promote wettability, a degreasing process by means of a thinner, and finally a drying process adapted to evaporate the thinner in appropriate ovens.

3. The new coating according to claim 2 for a process as defined above, wherein fine grain corundum is used in said sandblasting process.

4. The new coating according to claim 1, wherein it is applied by means of a dry compressed air gun adapted to spray the coating on the rotors, the rotors with coating are pre-cured in appropriate ovens, the quality and thickness of the paint coating is checked by means of an appropriate ultrasound instrument, the rotors inside the ovens are cured by means of a temperature ramp up and cooled at uniform temperature for an optimal paint coating quality.

5. The new coating according to claim 1, wherein its formulation is a mixture of the following materials:













Material
Amount (g)








Polytetrafluoroethylene
800



Amorphous graphite
360



powder




Thinner for spray
240



cleaning apparatuses




Methyl ethyl ketone
195



(MEK)




Cellosolve acetate
240



coating additive.

6. The new coating according to claim 1, wherein said materials are mixed for about four hours using a low speed gear system.

7. A coating for refurbishing an oil-free a-volumetric screw compressor, wherein the coating consists of a mixture of the following materials:














Material
Amount (g)









Polytetrafluoroethylene
750-850



Amorphous graphite powder
300-400



Thinner for spray cleaning
200-270



apparatuses




Methyl ethyl ketone
170-220



Cellosolve acetate coating additive
 200-300.

8. The coating according to claim 7, wherein the coating is a mixture of the following materials:














Material
Amount (g)









Polytetrafluoroethylene
800



Amorphous graphite powder
360



Thinner for spray cleaning
240



apparatuses




Methyl ethyl ketone
195



Cellosolve acetate coating additive
 240.

9. A refurbishment process of an oil-free volumetric screw compressor, said compressor comprising a pumping unit with an outer body and an inner chamber, comprising a first and a second seat adapted to accommodate respective male and female rotors provided with respective reversing helical screws meshing each other, said process comprising gradually disassembling the components of the compressor up to extraction of a rotor at a time from the respective seats of the chamber, visually checking the wear condition of the rotors, treating the rotor surface to remove the previous coating, applying a new coating on the surface of the rotors, repeating the operations of treating and applying a coating on the outer body of the pumping unit, inserting and then extracting one rotor at a time into the respective seat and checking for lack of interference, reassembling the pumping unit by meshing and inserting the two rotors inside the seats with further checking for lack of interference, reassembling the remaining components of the compressor,
said process being wherein the composition of the coating applied on the surface of the rotors consists of a mixture of the following materials:















Amount



Material
(g)








Polytetrafluoroethylene
750-850



Amorphous graphite powder
300-400



Thinner for spray cleaning
200-270



apparatuses




Methyl ethyl ketone 
170-220



Cellosolve acetate coating
200-300



additive.

10. The process according to claim 9, wherein said treatment of the surface of the rotors comprises a sandblasting process adapted to increase roughness and promote wettability, a degreasing process by means of a thinner, and finally a drying process adapted to evaporate the thinner in appropriate ovens.

11. The process according to claim 10, wherein fine grain corundum is used in said sandblasting process.

12. The process according to claim 9, wherein applying the coating comprises painting by means of a dry compressed air gun adapted to spray the coating on the rotors, pre-curing the rotors with coating in appropriate ovens, checking the quality and thickness of the paint coating by means of an appropriate ultrasound instrument, curing the rotors inside the ovens by means of a temperature ramp up and cooling the rotors at uniform temperature for an optimal paint coating quality.

13. The process according to claim 9, wherein the formulation of said coating applied on the surface of the helical screws is a mixture of the following materials:













Material
Amount (g)








Polytetrafluoroethylene
800



Amorphous graphite
360



powder




Thinner for spray
240



cleaning apparatuses




Methyl ethyl ketone
195



Cellosolve acetate
240



coating additive.

14. The process according to claim 9, wherein said materials are mixed for about four hours using a low speed gear system.