WO2015024071A1 - Waste heat utilization in gas compressors - Google Patents

Abstract

The specification discloses a gas compressor system (40, 90) having at least one first compressor unit (41) delivering compressed gas as a useful output, the gas compressor system further includes an Organic Rankine Cycle (ORC) arrangement (91, 91a, 91b) including an evaporator (48, 51, 51a, 51b) receiving waste heat to evaporate a working fluid of the Organic Rankine Cycle arrangement, the evaporated working fluid being expanded in at least one expansion device (55), the or at least one expansion device (55) driving a secondary gas compressor (54, 93), said secondary gas compressor (93) being either a refrigerant compressor (93) of a refrigerant circuit (92), or being configured to deliver a component of the compressed gas as said useful output.

Description

WASTE HEAT UTILIZATION IN GAS COMPRESSORS

FIELD OF THE INVENTION

[0001 ] The field of the invention is improvements in gas compressors, particularly to improve their efficiency in use.

BACKGROUND OF THE INVENTION

[0002] Almost all manufacturing factories use compressed air in the operation of their business. The cost of producing compressed air is therefore a significant cost in the conduct of manufacturing businesses and if is desirable to improve the efficiency of compressed air production to reduce costs. It is also known in industry and other commercial applications to compress gases other than air including, but not limited to carbon dioxide, nitrogen and natural gas. The Organic Rankine Cycle is also known for its use of a working fluid being an organic high molecular mass fluid with a liquid-vapour phase change (or boiling point) occurring at a lower temperature than the water steam phase change. The working fluid allows Rankine cycle heat recovery from low temperature sources such as biomass combustion, industrial waste heat, geothermai heat, solar ponds and others.

(00033 When using waste heat, a key parameter is the installed capital cost of the machinery (system). If the heat is low cost or free, the capital cost of the machinery determines the cost of any power produced (or saved). Conventional Organic Rankine Cycle systems cost around $1 ,000 to $3,000 / kw (electricity produced) plus the costs of installation. Until very recently, this has been too high to result in wide spread use. A key issue is the installed cost of the system. Integration of an Organic Rankine Cycle generator into an existing factory installation is complicated by the need to synchronize and control the Organic Rankine Cycle generator current and to connect the Organic Rankine Cycle heat exchangers economically to the waste heat source. On site costs for Installation are significantly more than ex-factory costs.

[0004J A major source of waste heat in typical industrial applications is the heat of compression transferred to the oil / lubricant in flooded rotary air compressors. Other sources include but are not limited to, heat generated by the primary drive engine for the primary compressor unit. Until now it has been uneconomic to utilize this waste heat for any use other than factory heating in cold climates as it is very difficult to synchronize the production and consumption of low temperature waste heat.

[0005] US patent specification nos. 4,342,201 , 8,197,227, 2013/0067951 disclose arrangements aimed at increasing the efficiency of gas compressor plants by utilizing waste heat energ generated in the primary gas compression process. US patent specification nos. 2012/0090349, 6,880,344 and 6,964,168 disclose arrangements for utilizing waste heat for driving an Organic Rankine Cycle arrangement or arrangements which in turn drive a refrigeration compressor of refrigeration system, but do not relate to gas compression systems.

[0006] The invention aims to provide an economic means of converting waste heat generated by a gas compression system to produce a compressed gas, particularly but not exclusively, compressed air. A further aim of the present invention is to provide improved efficiencies in a flooded rotary gas compressor system by utilizing aspects of an Organic Rankine Cycle to use at least some of the waste heat generated while producing the compressed air in the compressor system. A still further preferred objective is to achieve the foregoing while minimizing the capital cost of the system. SUMMARY OF THE INVENTION

[GGG7J According to the present invention, a gas compressor system is provided having a primar gas compressor arrangement including at least one first gas compressor unit delivering, in use, compressed gas as a useful output, said gas compressor system further including an Organic Rankine Cycle arrangement including evaporator means receiving waste heat to evaporate a working fluid of said Organic Rankine Cycle arrangement, said evaporated working fluid being expanded in at least one expansion device, the or at least one said expansion device driving a work device.

[0008] The at least one first compressor unit may be a reciprocating compressor or any suitable rotary compressor including a turbo compressor and flooded compressor devices including screw, vane and scroll compressors.

[0009] The work device may be at least one secondary gas compressor which may either comprise a refrigerant compressor forming part of a refrigeration circuit or may be a gas compressor contributing to the compressed gas as a useful output.

[0010] Preferably the waste heat utilised in the conduct of the present invention is waste heat created by the gas compressor system when producing the compressed gas. Conveniently, the waste heat may be produced, at least in part, by operation of the at feast one first gas compressor unit. The compressed gas directly exiting the first compressor unit is normally heated and needs to be cooled in a cooler unit. The cooler unit can be a suitable source of waste heat. Alternatively, in a flooded rotary gas compressor system, the return liquid lubricant from the separator vessel also needs to be cooled and is also a suitable source of waste heat. The waste heat might also be sourced, at least in part, by heat generated via a drive means or engine driving the first gas compressor unit, the waste heat originating from the drive means itself or from a cooling system therefore. [00113 In one aspect, the present invention provides a gas compressor system having a primary gas compressor arrangement including at least one f irst gas compressor unit delivering, in use, compressed gas as a useful output, said gas compressor system further including an Organic Rankine Cycle arrangement including evaporator means receiving waste heat to evaporate a working fluid of said Organic Rankine Cycle arrangement, said evaporated working fluid being expanded in at least one expansion device, the or at least one said expansion device driving a work device, said work device being a refrigerant compressor in a refrigerant circuit, said refrigerant circuit creating, in use, a cold medium passed in heat exchange relation with a gas inlet flow to said primary gas compressor arrangement in a first heat exchange means to cool gas inlet flow to said primary gas compressor arrangement.

[0012] Conveniently th aforesaid refrigerant circuit creates a cold medium passed in heat exchange relation with a compressed gas discharge flow from said primary gas compressor arrangement in the first heat exchange means to cool the compressed gas discharge flow. The compressor system may further include a moisture separator to receive copied compressed gas flow from said first heat exchanger, said moisture separator being capable of removing condensed moisture from said compressed gas flow. The compressed gas flow downstream of said moisture separator being passed through a second heat exchanger means whereby said compressed gas flow is in heat exchange relationship with gas flow to be compressed entering said primary gas compressor arrangement.

[0013] Conveniently the or at least one said first gas compressor unit is formed by a reciprocating piston gas compressor device. Alternatively, the or at least one said first gas compressor unit might be formed by a rotary gas compressor device including a turbo compressor and flooded rotary compressor devices such as screw, vane, and scroll compressor devices, in the latter ease, liquid Iubricant collected in a separator is returned via a liquid lubricant cooler, the liquid lubricant cooler forming the evaporator means for the Organic Rankine Cycle arrangement.

[001.4] In accordance with another aspect of the present invention, there is provided a gas compressor system having a primary gas compressor

arrangement including at least one first gas compressor unit delivering, in use, compressed gas as a useful output, said gas compresso system further including an Organic Rankine Cycle arrangement including evaporator means receiving waste heat to evaporate a working fluid of said Organic Rankine Cycle

arrangement, said evaporated working fluid being expanded in at least one expansion device, the or at least one said expansion device driving a work device, said work device being arranged to drive a secondary gas compressor arrangement including at least one second gas compressor unit delivering a component of the compressed gas as said useful output from the gas compressor system. The secondary gas compressor unit may be a reciprocating piston compressor unit or it may be a rotar compressor unit, in the latter case, the secondary gas compressor unit may be a flooded rotary compressor unit delivering a mixture of compressed gas and liquid lubricant to the separator. Conveniently, two or more said secondary gas compressor unit, each being a said flooded rotary gas compressor units are provided. Preferably compressed gas discharged from each said secondary gas compressor unit is delivered to a common separation vessel, said common separation vessel also receiving compressed gas from said primary gas compressor arrangement. The compressor system may include multiple said expansion devices driving a said second compressor unit through a coupling device.

[0015] In accordance with a still further aspect of this invention, there is provided a gas compressor system having a primary gas compresso

arrangement including at least one first compressor unit delivering, in use, compressed gas as a useful output, said gas compressor system further including an Organic Rankine Cycle arrangement including evaporator means receiving waste heat to evaporate a working fluid of said Organic Rankine Cycle arrangement, said evaporated working fluid being expanded in at least one expansion device, the or at feast one said expansion device driving a work device, the or at least one said first gas compressor unit is formed by a flooded rotary compressor device delivering a mixture of compressed gas and liquid lubricant to a separator to separate said compressed gas from said liquid lubricant, said liquid lubricant being collected in said separator is returned to at least said flooded rotary gas compressor device via said cooler means, said cooler means forming the evaporator means for said Organic Rankine Cycle arrangement, said work device being arranged to drive a secondary gas compressor arrangement including at least one second gas compressor unit delivering a component of the compressed gas as said useful output from the compressor system, said compressed gas discharged by the o each said second gas compressor unit being delivered to said separator.

[OOT63 Preferably, the aforesaid liquid lubricant cooler may be divided into sections, each said section being connected to a separate expansion device of the Organic Rankine Cycle arrangement, the separate expansion devices being arranged in line to drive a said second gas compressor unit, in a possible alternative arrangement the sections of the liquid lubricant cooler are connected in parallel to a separate expansion device of the Organic Rankine Cycle to drive a said second gas compressor unit via a coupling drive train. Conveniently a circulation pump of the Organic Rankine Cycle arrangement is driven by a liquid turbine located in a liquid lubricant return line from the separator vessel in a rotary flooded gas compressor configuration.

[0017] The apparatus disclosed and claimed herein may be used to compress any gas including air, carbon dioxide, nitrogen and natural gas.

[0018] The invention will be better understood from the following description of preferred embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig 1 is a schematic circuit diagram of a very basic conventional flooded rotary compressor system;

[0020] Fig 2 is a schematic circuit diagram of a basic conventional Organic Rankine Cycle system;

[0021 Fig 3 is a schemati circuit diagram of a compressor system according to a preferred embodiment of the present invention;

[0022] Figs 4a, 4b and 4c show diagramrnaticafiy, possible variations to the circuit diagram of Fig 3;

[0023] Fig 5 is a schematic circuit diagram of a compressor system according to a stiil further preferred embodiment of the present invention;

[0024] Fig 6 is a schematic circuit diagram of a compressor system according to yet another preferred embodiment of the present invention;

[0025] Fig 7 is a further schematic circuit diagram with some parts omitted according to yet another preferred embodiment of the present invention; and

[0026] Fig 8 shows a view similar to Fig 7 including another possible variation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Fig 1 shows a typical prior art conventional flooded rotary gas compressor system, typically a screw compressor system but of course other types of flooded rotary compressors could equally be used in this system. The gas compressor system is typically used to compress air but may be used to compress other gases including but not limited to carbon dioxide, nitrogen and natural gas. The following description refers to air compression but it will be understood that other gases may be substituted for air as the gas to be compressed. Fig 1 shows a rotary compressor unit 10 driven by a motor 12. The motor 12 may be an electric motor or may be other forms of motors including internal combustion engines. Atmospheric air is drawn in at 13 into the rotary compressor unit 10 to be compressed while being mixed with lubricant or oil introduced at 14, The mixture of compressed air and lubricant is discharged into a separator 15 where the lubricant and compressed air undergoes a primary separation with the lubricant falling to be collected in a pool 16 in the base of the separator 15. Pressure of the compressed air in the separator 15 forces lubricant from the pool 16, via a lubricant cooler 17 and possibly an oil filter (not shown), to be reintroduced at 14 into the rotary compressor unit 10, High pressure compressed air exits the separator 15 via a filter 18 and is cooled in an after cooler 19 before being discharged from the system at 20.

[0028] The lubricant cooler 17 exchanges heat to a cold fluid 21 which is heated and discharged at 21. Similarly a cold fluid 22 enters the after cooler heat exchanger 19 to cool the compressed air and be heated by the time it reaches 23 on discharge from the after cooler 19. In conventional rotary screw compressors, this fluid is normally water. If the power supplied to the motor 12 is regarded as 100%, then the equivalent to roughly 75% of the motor power is exchanged to the cold fluid in the lubricant cooler 17 and 20% in the after cooler 19,

[0029] Whilst Fig 1 illustrates the basic features of a conventional flooded rotary compressor system, most such systems will include other modifications to improve performance of the compressor system in some way. For example, it is known in prior art compressor systems to remove moisture from the compressed air discharged from the system. One known method of doing so is by utilizing a separate refrigerated air dryer on the compressed air discharge. This separate refrigerated air dryer may use 2-3% of the power of the compressor system and as a result, efficiency is decreased, although in many applications suc moisture removal is essential.

[0030J Fig 2 illustrates a typical prior art Organic Rankine Cycle system having an evaporator / boiler 24 where heat boils a low boiling point working fluid entering at 25. The tow boiling point working fluid may be HFC's (R134a, R245fa, R1245f), butane, isobutane, pentane, propane or similar.

[0031 ] Vapour leaving the evaporator 24 at 26 is passed to an expander 27 which may be any suitable rotary machine including a turbine, screw, vane, scroll or equivalent. The expander 27 conventionally drives a generator 28, Vapour expanding in the expander 27 generates work, Low pressure vapour leaving the expander 27 is passed to and condensed into a liquid in a condenser 29. This liquid form of the working fluid is recycled to the evaporator (boiler) 24 and is increased in pressure by a pump 30. Heat is supplied to the evaporator 24 by heating fluid supplied at 31. The condenser 29 is cooled by a cold fluid supplied at 32.

[0032] Whilst it is possible to use the compressor heat from the water cooling the oil cooler 17 and the water cooling the after cooler 19 (Fig 1) to heat the evaporator 24 which, until now, has been located remotely, this has not been economic to do. To do so, two separate packages were required, that is, pumps and heat transfer loops and duplication of coolers and evaporators (boilers).

[0033] Fig 3 illustrates schematically a diagrammatic circuit arrangement of a flooded rotary compressor system 40 for a preferred embodiment of the present invention, the arrangement 40, preferabiy being formed as a single package. The arrangement 40 includes a first or main flooded rotary compressor unit 41 receiving atmospheric air at 42 for compression. The rotary compressor unit ma be a screw, vane or scroll type compressor unit or any other form of flooded rotary compressor. The compressor unit 41 is driven by a motor 43 which may be an electric motor or an internal combustion engine. As with the conventional system (Fig 1 ), fine 44 delivers a mixture of liquid lubricant (oil) and compressed air to a separator 45 where a primary separation of the liquid lubricant occurs to form a pool of hot lubricant 46 in the base of the separator 45. Compressed air from the separator 45 Is passed through a filter 47 to have any remaining liquid lubricant removed and then via an after cooler 48 to be discharged from the arrangement 40 at 49. Hot liquid lubricant is returned from the poo! 46 along line

50, through a lubricant cooler 51 , and via Sine 52 to the inlet region of the compressor unit 41. The cooled liquid lubricant in line 52 is also supplied via line 53 to an inlet region of a secondary compressor unit 54 that also receives atmospheric air at 73 to be compressed. The secondary compressor unit 54 may be of the same or similar type to the first or main compressor unit 41 but would normally be of smaller capacity. The secondary compressor unit 54 is preferably driven by a form of Organic Rankine Cycle arrangement 91 forming part of and contained within the package 40. The Rankine Cycle arrangement 91 includes a working fluid which may be any working fluid used in known Organic Rankine Cycle arrangements including R134a, propane, butane, isobutane, pentane R245fa, R1245f or similar. The secondary compressor unit 54 is driven by an expander 55 (turbine, screw, scroll or any other rotary expander machine). The after cooler 48 and the lubricant cooler 51 act as the evaporator (boiler) of the Organic Rankine Cycle arrangement 91 , i.e. 24 in Fig 2. The evaporated fluid is passed via line 62 to the expander 55 that drives the secondary compressor unit 54. The working fluid of the Organic Rankine Cycle arrangement 91 is passed via lines 63 through the condenser 56 and via a pump 58 and line 60 back to the after cooler 48. If desired, fluid of the Organic Rankine Cycle arrangement 91 can foliow line 61 and bypass the after cooler 48 returning directly to the oil cooler

51 . in another possible arrangement line 61 could be omitted and bypass line 64 included whereby only the after cooler 48 acts as the feed heater for the Organic Rankine Cycle arrangement 91. Conveniently, the pump 58 can be driven by a suitable mechanical connection 59 driven by a liquid turbine 57 in the lubricant return line 50. The liquid turbine 57 might be placed in other locations in the lubricant return lines. The condenser 56 may be air cooled, water cooled, or cooled by water in a closed circuit including a cooling tower. [0034] Compressed air and entrained liquid lubricant Is passed from the secondary compressor unit 54, via tines 65 directly back to the separator 45. Conveniently .driving the secondary compressor unit 54 by the expander 55 (of the Organic Rankine Cycle arrangement 91 avoids the need to have any mechanical or electrical connection between the secondary compressor unit 54 and the first or main compressor unit 41. With the arrangement shown in Fig 3, an extra 5 to 10% compressed air flow may be achieved from the integrated package compared to a typical conventional rotary flooded compressor system (Fig 1 ). A significant advantage of the arrangement shown in Fig 3 is that the Organic Rankine Cycle (ORG) arrangement 91 and secondary compressor unit 54 can run at optimum and variable speeds, unconstrained by the need to match ORG speed to the first or main compressor speed (compressor unit 41 ). This is of particular importance at part load where the ratio of waste heat produced varies compared to the amount of air compressed by the first or main compresso unit 41 ,

[0035] Figs 4a, 4b and 4c illustrate possible further alternatives to the arrangements shown in Fig 3. In Fig 4a, the secondary compressor unit 54 is replaced by a second compressor unit 54a and a third compressor unit 54b, each respectively driven by an expander machine 55a or 55b. The compressed air and liquid lubricant exiting along the lines 65a, 65b from the compressor units 54a, 54b is directed into the separator 45. The ORC 91 working fluid flowing in line 62 is split to be shared via lines 62a, 62b. The expanded fluid exits the expanders 55a, 55b, via lines 63a, 63b. Fig 4 shows illustratively, multiple expanders 55a 55b in the ORG section driving a single secondary compressor unit 54 via a drive train 67, the drive train 67 being gears, shafts or belts. Fig 4c shows yet another possible arrangement where the oil cooler 51 is split into two (or more) sections 51 a, 51 b and the working fluid of the ORC91 might be boiled at different and optimum pressures whereby the expander (55a, 55b) pressure ratios recover more energy from the hot lubricant flow 50. [0036] Referring now to Fig 5, yet another preferred embodiment is illustrated schematically. The gas compressor system 90 includes a gas compressor unit 41 receiving air to be compressed at 42. The compressor unit 41 may be driven by a motor 43 and may be any type of flooded rotary type compressor unit including screw, vane, scroll or any other. The compressor unit 41 delivers a mixture of compressed air and liquid lubricant as is known, via line 44 to a separator vessel 45. Liquid lubricant typically settles out into a pool 46 maintained in the lower regions of the separator vessel 45 and is returned via line 50 / 52 via a pressure differentia! to the inlet end of the compressor unit 41. The line 50 / 52 includes a lubricant cooler 51. As with other embodiments this embodiment also utilizes an Organic Rankine Cycle arrangement 91 comprised of a work device 55, a condenser 56, a circulation pump 58 and an evaporator being the liquid lubricant cooler 51. In this manner the working fluid of the Organic Rankine Cycle arrangement is evaporated in the lubricant cooler 51.

[0037] The embodiment of Fig 5 further includes a refrigeration cycle arrangement 92 including a refrigerant compressor 93 driven by the work device 55 (rotary motor) of the Organic Rankine Cycle arrangement 91. The

refrigeration cycle arrangement 92 includes a condenser 94, an expansion valve 95 and an evaporator 96.

[00383 Cold medium produced by the refrigeration cycle arrangement 92 can be put to a number of different individual uses or to multiple such uses in combination. Firstly, it might be used to chill or coo! the compressed gas discharged at 49 from the compressor system 90, Secondly, or in addition, It might be used to chill or cool air entering at 42 to be compressed. In hot climates particularly, inlet chilling of the gas to be compressed increases the density of the inlet gas by up to 15%. As a result 15% more mass of gas is compressed for the same volume but the shaft power of the motor 43 is unchanged. Typically, cooling inlet gas from 35°C 60% RH to 5 'C / 90% RH provides a 14.7% improvement in compressor flow, !n addition, the coo! medium can be utilized for a variety of externa! purposes, including for producing chilled water for a variety of cooling applications in manufacturing processes, for use in air conditioning and similar.

[0039J We would further state that the variety of modifications and variations described in earlier embodiments with reference to Figs 3, 4a, 4b and 4c can also be employed with the embodiment of Fig 5 if desired.

[0040] Fig 6 illustrates another possible preferred embodiment according to the present invention. The compressor 41 may be any type of compressor including piston, screw, scroll, turbo, vane or any variation thereon. In the configuration illustrated the compressor 41 might typically be a reciprocating piston type compressor. The compressor 41 is driven by a motor 43. The compressor 41 receives air at 42 to be compressed and delivers hot compressed air at 44 to a cooler / evaporator 51. The embodiment of Fig 6 further includes an Organic Rankine Cycle (ORG) arrangement 91 of which the cooler / evaporator 51 forms part. The hot compressed air gives up heat to the ORC working fluid in the cooler / evaporator 51. The ORG 91 working fluid is supplied via pump 58 from a common condenser 97. The ORG 91 working fluid boils and vapour is supplied to the ORC expander device 55 which drives a refrigeration compressor 93 of a refrigeration circuit 92. Liquid from the common condenser 97, which is cooled by an air or water stream 98, is supplied to an expansion valve 95, The expanded refrigerant working fluid is then passed from the expansion valve 95 to an evaporator 96 forming part of an inlet air cooler 99 whereby incoming low pressure air at 42 is cooled before entering the compressor 41. At the same time outgoing compressed air via line 103 is also cooled in the inlet cooler 99, The chilled higher density Inlet air passes to the compresso 41. The outgoing compressed air is chilled such that water vapour is condensed and separated from the compressed air stream in a moisture separator 101 with the condensed water being discharged at 102. The dry and chilled compressed gas leaving the moisture separator 101 via line 104 may be passed in counter flow relationship through a further heat exchanger 100. In the heat exchange 100, the incoming fow pressure air is pre-chilled and the outgoing compressed air is warmed before being discharged at 49.

[0041 J As noted above, the compressor 41 could be any type of a flooded rotary type compressor and in which case a liquid lubricant separator vessel 45 would be included with the cooler 51 being in the liquid lubricant return line 50 from the separator vessel 45 rather than in the compressed air outlet line, generally as shown in Fig 5 and other embodiments.

[0042] The ORG arrangement 91 driving the refrigeration compressor 93 does not necessarily use all of the waste heat energy capable of being given up by the hot compressed air discharged through the cooler 51. In consequence, a second stage air compression arrangement can be used as shown in simplified schematic format in Fig 7, parts of the arrangement being omitted for the sake of simplicity. In this arrangement an ORG arrangement 91a drives a refrigerant circuit 92 with a secondary compressor unit 54 being driven by an ORG arrangement 91 . Each of the ORG arrangements 91 a, 91 are driven by waste heat from a compressed gas discharge from the primary compressor 41 , the waste heat being sourced from a coole 51 or cooler sections 51a, 51 b. item 108 represents schematically the heat exchangers 99, 100 (Fig 6) and the moisture separator 101 (Fig 6). Of course, if the compressor 41 is a flooded rotary compressor, the arrangement changes in accordance with previously described embodiments.

[00433 The embodiment shown in Fig 8 is still another possible arrangement, useful when the engine 43 driving the primary compressor unit 41 does itself generate a significant amount of waste heat This may be the case if the engine is an internal combustion engine such as a diesel engine or the like. Such engines may include their own coolant arrangements from which waste heat can be withdrawn and delivered to an ORG arrangement 91 a, 91 b or perhaps both of 91a and 91b. Similarly, waste heat may be extracted from the cooler 51 and delivered to either one or both of the ORG arrangements 91a, 91 . Other variations and modifications described with reference to earlier embodiments may also be utilized in the embodiments described with reference to Figs 6, 7 or 8.

[0044J It will be apparent to those skilled in this art that still further

modifications of the apparatus disclosed above and in the annexed drawings may be made within the scope of the claims annexed hereto. The subject matter of the claims is made part of the disclosure of this specification by this reference thereto.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A gas compressor system having a primary gas compressor arrangement including at least one first compressor unit delivering, in use, compressed gas as a useful output, said gas compressor system further including an Organic Rankine Cycle arrangement including evaporator means receiving waste heat to evaporate a working fluid of said Organic Rankine Cycle arrangement, said evaporated working fluid being expanded in at least one expansion device, the or at least one said expansion device driving a work device.

2. A gas compressor system according to claim 1 wherein the work device is a secondary gas compressor arrangement.

3. A gas compressor system according to claim 2 wherein said waste heat originates, at least in part, from a cooler means operable with said at least one first compressor unit, said cooler means forming said evaporator means of said Organic Rankine Cycle arrangement.

4. A gas compressor system according to claim 2 or claim 3 wherein said waste heat is produced, at least in part, by a drive means driving the at least one first compressor unit,

5. A gas compressor system according to any one of claims 2 to 4 wherein said secondary gas compressor arrangement is a refrigerant compressor of a refrigerant circuit, said refrigerant circuit further including a condenser, an expansion valve and an evaporator.

6. A gas compressor system according to claim 5 wherein said refrigerant circuit creates a cold medium passed in heat exchange relation with a gas inlet flow to said primary gas compressor arrangement in a first heat exchange means to cool the gas inlet flow to said primary gas compressor arrangement.

7. A gas compressor system according to claim 5 wherein said refrigerant circuit creates a cold medium passed in heat exchange relation with a

compressed gas discharge flow from said primary gas compressor arrangement in a first heat exchange means to cool said compressed gas discharge flow.

8. A gas compressor system according to claim 7 further including a moisture separator to receive cooled compressed gas flow from said first heat exchanger, said moisture separator being capable of removing condensed moisture from said compressed gas flow.

9. A gas compressor system according to claim 8 wherein the compressed gas flow downstream of said moisture separator is passed through a second heat exchanger means whereby said compressed gas flow is in heat exchange relationship with gas flow to be compressed entering said primary gas

compressor arrangement.

10. A gas compresso system according to claim 3 or any one of claims 4 to 9 when appended to claim 3 wherein the or at least one said first gas compressor unit is formed by a reciprocating piston gas compressor device.

1 1 . A gas compressor system according to claim 10 wherein the cooler means is located in a compressed gas discharge from the or at least one said first gas compressor unit.

12. A gas compresso system according to claim 3 or any one of claims 4 to 9 when appended to claim 3 wherein the or at least one said first gas compressor unit is formed by a rotary compressor device.

13. A gas compressor system according to claim 12 wherein the rotary gas compressor device is a flooded rotary gas compressor device delivering a mixture of compressed gas and liquid lubricant to a separator to separate said

compressed gas from said liquid lubricant.

14. A gas compressor system according to claim 13 wherein liquid lubricant collected in said separator is returned to at least said flooded rotary gas compressor device via said cooler means, said cooler means forming the evaporator means for said Organic Rankine Cycle arrangement.

15. A gas compressor system according to any one of claims 2 to 14 wherein said secondary gas compressor arrangement includes at least one second gas compressor unit delivering a component of the compressed gas as said useful output from the compressor system. 6. A gas compressor system according to claim 15 wherein the or at least one said second gas compressor unit includes either a reciprocating piston compressor unit, or a rotary gas compressor unit, or both a reciprocating piston compressor unit and a rotary gas compressor unit.

17. A gas compressor system according to claim 16 when appended through one of claims 13 or 14 wherein the or each said second gas compressor unit is a flooded rotary gas compressor unit delivering a mixture of compressed gas and liquid lubricant to said separator.

18. A gas compressor system according to claim 17 wherein two or more said second gas compressor units are provided.

19. A gas compressor system according to claim 13 wherein liquid lubricant collected in said separator is returned to at least said flooded rotary gas compressor device via said cooler means, said cooler means forming the evaporator means for said Organic Rankine Cycle arrangement, said work device being arranged to drive a secondary gas compressor arrangement including at least one second gas compressor unit delivering a component of the compressed gas as said useful output from the compressor system, said compressed gas discharged by the or each said second gas compressor unit being delivered to said separator.

20. A gas compressor system according to an one of claims 15 to 18 further including multiple said expansion devices driving a said second gas compressor unit through a coupling drive train.

21 . A gas compressor system according to claim 4 or any one of claims 15 to 20 when appended through claim 14 wheretn said liqutd lubricant cooler is divided into sections, each said section being connected to a separate expansion device of said Organic Rankine Cycle arrangement, said separate expansion devices being arranged in line to drive a said second gas compressor unit.

22. A gas compressor system according to claim 14 or any one of claims 15 to 20 when appended through claim 14 wherein said liquid lubricant cooler is divided into sections, each said section being connected to a separate expansion device of said Organic Rankine Cycle arrangement in parallel to drive a said second gas compressor unit via a coupling drive train.

23. A gas compresso system according to claim 14 wherein a liquid turbine is located in a liquid lubricant return line from said separator and is driven by liquid flow in said return line, said liquid turbine driving a circuiation pump in the Organic Rankine Cycle arrangement.