[0001] The present invention relates to a steam supply system for a steam process, for example a paper drying system.

BACKGROUND

[0002] Many industrial processes use steam, for example to treat products. In paper-making processes steam is used primarily in the paper-drying process. In particular, steam is used to heat cylinders over which the wet paper is conveyed. The heat from the steam evaporates water from the wet paper and thereby dries the paper. Heated air is passed over the paper to convey the evaporated water away from the paper.

[0003] Typically, a thermo-compressor vacuum system is provided to siphon condensate from the cylinders and steam into the cylinders. Such a thermo-compressor vacuum system operates on steam at 10 Bar(g) and 180 degrees Celsius, and therefore consumes a high amount of energy. In addition, condensate from the cylinders is passed into a vacuum condenser and flashed steam is then condensed at low pressure using a cold water loop. This generates a vacuum (e.g., -0.5Bar(g)) that is used for the paper making process, but can account for up to 10% of the steam consumption of the paper-drying process.

[0004] Paper-drying processes generate a humid waste air stream and thermal energy can be recovered from the waste air stream to preheat the air stream passing into the drying process, and/or to heat water, and/or to heat the paper mill building(s). This provides some thermal energy recovery. However, to date little has been done to capture thermal energy of the waste steam output from the drying cylinders. A reason for this is the relatively high starting temperatures of the waste steam, which precludes use of conventional heat pumps.

SUMMARY

[0005] In accordance with a first aspect of the present invention, there is provided a steam supply system for supplying steam to a steam process, for example a paper drying system. The steam process includes a high pressure condensate return and a low pressure condensate return. The steam supply system includes a high pressure flash tank arranged to receive the high pressure condensate return and to generate a flash steam output for the steam process. The steam supply system also comprises a low pressure flash tank arranged to receive the low pressure condensate return and generate a flash steam output. The steam supply system also comprises a compressor arranged to compress the flash steam output of the low pressure flash tank and to provide the compressed flash steam to the high pressure flash tank.

[0006] Accordingly, condensate returned from the steam process can be cascaded and compressed up to a pressure where it can be used in the steam process, making better use of the thermal energy contained in the return condensate. In addition, providing the compressor in the steam system will create suction on the upstream side of the compressor (i.e., in the low pressure flash tank and in the low pressure condensate return), acting to siphon the condensate from the steam process without the need of an additional vacuum system. Such a system requires significantly less energy to provide steam to the steam process when compared to known steam supply systems.

[0007] In examples, the high pressure flash tank may comprise a condensate output arranged to convey condensate from the high pressure flash tank to the low pressure flash tank. As the low pressure flash tank operates at a lower pressure and temperature than the high pressure flash tank, condensate returned from the high pressure flash tank to the low pressure flash tank will generate further flash steam.

[0008] In examples, the compressor may comprise a plurality of sub-compressors. For example, the compressor may comprise one or more mechanical vapour recompression compressors, for example a centrifugal compressor. In examples, the plurality of subcompressors may be arranged in series or in parallel.

[0009] In examples, the compressor may be a first compressor, and the steam supply system may further comprise a second compressor arranged to compress a flash steam output of the high pressure flash tank. Accordingly, flash steam output from the high pressure flash tank can be compressed to a higher pressure to provide a high pressure steam supply to the steam process.

[0010] In examples, the second compressor may comprise a plurality of sub-compressors. For example, the second compressor may comprise one or more mechanical vapour recompression compressors, for example a centrifugal compressor. In examples, the plurality of subcompressors may be arranged in series or in parallel.

[0011] In examples, a first portion of the flash steam output from the high pressure flash tank may be provided to the second compressor and then to the steam process at a first pressure, and a second portion of the flash steam output from the high pressure flash tank may be provided to the steam process at a second pressure lower than the first pressure. In this way, the steam supply system provides steam to the steam process at two different pressures, e.g., a high pressure steam supply and a low pressure steam supply.

[0012] In examples, the steam supply system may further comprise a boiler supply arranged to supply boiler steam. The boiler steam may be combined with the flash steam output from the high pressure flash tank. In examples, the boiler steam may be combined with the flash steam output from the high pressure flash tank at a valve or at a thermo-compressor.

[0013] In examples, the steam supply system may further comprise one or more valves arranged to control a suction generated by the or each compressor acting on the low pressure condensate return and optionally also the high pressure condensate return. That is, a valve may be provided upstream of the first compressor. In addition, in examples that includes a second compressor, a valve may be provided upstream of the second compressor. The valve(s) is/are operable to control the suction generated by the compressor(s) and thereby control a rate of siphoning of the condensate from the steam process.

[0014] In examples, the steam supply system may further comprise a heat pump arranged to recover thermal energy from a waste air stream of the steam process and to raise steam with the recovered thermal energy. The heat pump may receive condensate from the low pressure flash tank, boil the condensate to raise steam, and feed the raised steam into the low pressure flash tank. In examples, the heat pump may comprise a condenser, an expansion valve, an evaporator, and a compressor. The evaporator may be arranged to recover thermal energy from the waste air stream, and the condensate from the low pressure flash tank is circulated through the condenser for evaporation to raise steam. In examples, the heat pump has a refrigerant comprising ammonia. In examples, the condenser comprises a welded plate heat exchanger. In examples, the heat pump may further comprise a water loop comprising the evaporator, a refrigerant circuit comprising the compressor, the condenser, and the expansion valve, and an intermediate heat exchanger through which the water loop passes and the refrigerant circuit pass. The water loop advantageously allows the evaporator to be located away from the condenser and compressor, which may be preferable if the heat pump operates on a refrigerant such as ammonia.

[0015] In examples, the steam supply system may further comprise a sub-cooler arranged in the low pressure condensate return upstream of the low pressure flash tank. The sub-cooler may be adapted to heat an air stream using heat from the low pressure condensate return. In examples, the steam supply system may further comprise a bypass line from the low pressure condensate return upstream of the sub-cooler to the low pressure flash tank such that a portion of the low pressure condensate return can bypass the sub-cooler. A control valve may be arranged to control flow of condensate through the bypass line. The control valve can be operated to control a degree of heating of the air stream by the sub-cooler. Advantageously, the sub-cooler acts to reduce a steam content of the low pressure condensate return and also reduces the temperature of the low pressure condensate return. In some examples, the subcooler may be provided in the high pressure condensate return, or a first sub-cooler can be provided in the low pressure condensate return and a second sub-cooler can be provided in the high pressure condensate return.

[0016] In examples, the steam supply system may further comprise a heat exchanger arranged to receive a flash steam output from the high pressure flash tank and to heat air for the steam process. In some examples, the heat exchanger may be provided in the low pressure condensate return, or a first heat exchanger can be provided in the low pressure condensate return and a second heat exchanger can be provided in the high pressure condensate return.

[0017] In accordance with a second aspect of the present invention, there is also provided a method of supplying steam to a steam process, for example a paper drying system, where the steam process includes a low pressure condensate return and a high pressure condensate return. The method includes flashing the low pressure condensate return at a low pressure flash tank, flashing the high pressure condensate return at a high pressure flash tank, compressing a flash steam output of the low pressure flash tank, and feeding the compressed flash steam to the high pressure flash tank.

[0018] Accordingly, condensate returned from the steam process can be cascaded and compressed up to a pressure where it can be used in the steam process, making better use of the thermal energy contained in the return condensate. In addition, providing the compressor in the steam system will create suction on the upstream side of the compressor (i.e., in the low pressure flash tank and in the low pressure condensate return), acting to siphon the condensate from the steam process without the need of an additional vacuum system.

[0019] In examples, the method may comprise operating the steam supply system of the first aspect of the invention as described above.

[0020] In particular, the method may further comprise compressing a flash steam output of the high pressure flash tank. Accordingly, the method may provide steam to the steam process at a pressure higher than the operating pressure of the high pressure flash tank. In examples, the method may comprise providing a first portion of flash steam output of the high pressure flash tank to the steam process as a low pressure steam supply, compressing a second portion of the flash steam output of the high pressure flash tank, and providing the compressed flash steam output to the steam process as a high pressure steam supply. Accordingly, the method can provide steam to the steam process at two different pressures, e.g., a high pressure steam supply and a low pressure steam supply.

[0021] In examples, the method may further comprise feeding condensate from the high pressure flash tank to the low pressure flash tank.

[0022] In examples, the method may further comprise recovering thermal energy from a waste air stream of the steam process and raising steam with the recovered thermal energy. In examples, the method may further comprise using the recovered thermal energy to boil condensate from the low pressure flash tank, and providing the raised steam to the low pressure flash tank. Accordingly, the waste air stream can be used to raise steam for the steam process.

[0023] In accordance with a third aspect of the present invention, there is also provided a paper drying system comprising at least one drying cylinder and a steam supply system configured to provide steam to the at least one drying cylinder and to receive a condensate return from the at least one drying cylinder. The steam supply system includes a flash tank arranged to receive the condensate return from the at least one drying cylinder, and a compressor arranged to compress flashed steam from the flash tank to provide compressed steam to a steam input of the at least one drying cylinder.

[0024] Accordingly, the paper drying system comprises a compressor in the steam loop, which creates suction on the upstream side of the compressor and acts to siphon condensate from the drying cylinder. This obviates the need for a separate vacuum system to provide siphoning, and additionally reduces the pressure in the flash tank, allowing steam to be raised at a lower temperature. For example, if the compressor provides an operating pressure of the low pressure flash tank at about -40 kPa (gauge) (about -0.4 Bar(g)), the flash tank can raise steam at about 85 degrees Celsius.

[0025] In examples, the operating pressure of the flash tank, as defined by the compressor, is a vacuum pressure (i.e., less than 0 kPa (gauge) (0 Bar(g)). For example, the flash tank may be the low pressure flash tank as described with reference to the first and second aspects of the invention. In other examples, the operating pressure of the flash tank, as defined by the compressor, is less than a supply pressure of the steam being supplied to the drying cylinder. By providing the compressor within a steam loop that passes steam into the paper drying cylinder, receives condensate back from the drying cylinder, flashes the condensate and then compresses the flash steam, it is inherent that the pressure upstream of the compressor will be less than the pressure downstream of the compressor, creating the suction needed to siphon the condensate from the drying cylinder and to convey steam into the drying cylinder. In some examples, the flash tank is the high pressure flash tank described above. In this example, the operating pressure of the flash tank, as defined by the compressor, may be between about 100 kPa (gauge) (about 1 Bar(g)) and about 200 kPa (gauge) (about 2 Bar(g)), for example about 150 kPa (gauge) (about 1.5 Bar(g)). A pressure on a downstream side of the compressor (i.e., the pressure of the steam supplied to the drying cylinder) may be between about 300 kPa (gauge) (about 3 Bar(g)) and about 600 kPa (gauge) (about 6 Bar(g)), for example to about 460 kPa (gauge) (about 4.6 Bar(g)).

[0026] In examples, the third aspect of the invention may be combined with any features of the first and second aspects of the invention. In particular, the steam supply system may comprise a second (e.g., high pressure) flash tank that receives a second (e.g., high pressure) condensate return, and the condensate may be cascaded up to a pressure to provide a steam supply for the paper drying system. The steam supply system may comprise a second compressor upstream of the second flash tank. The or each compressor may comprise a plurality of sub-compressors. In addition, the steam supply system may comprise a heat pump that recovers thermal energy from a waste air stream of the paper drying system, as described above. In addition, the steam supply system may comprise a sub-cooler to heat air from the condensate return, as described above. In addition, the steam supply system may comprise a heat exchanger arranged to receive a flash steam output from the flash tank and to heat air for the steam process.

[0027] In examples the paper drying system may further comprise an expansion valve arranged in the condensate return. In examples, the compressor (or each sub-compressor) may comprise a mechanical vapour recompression compressor.

[0028] In accordance with a fourth aspect of the present invention, there is also provided a method of drying paper. The method includes supplying steam to at least one drying cylinder of a paper drying system, returning condensate from the at least one cylinder to a steam supply system, flashing the returned condensate at a flash tank to generate flashed steam, compressing the flashed steam from the flash tank, and providing the compressed steam to a steam input for the at least one drying cylinder.

[0029] Accordingly, the paper drying system comprises a compressor in the steam loop, which creates suction on the upstream side of the compressor and acts to siphon condensate from the drying cylinder. This obviates the need for a separate vacuum system to provide siphoning, and additionally reduces the pressure in the flash tank, allowing steam to be raised at a lower temperature. For example, if the compressor provides an operating pressure of the low pressure flash tank at about -40 kPa (gauge) (about -0.4 Bar(g)), the flash tank can raise steam at about 85 degrees Celsius.

[0030] In examples, the method may comprise operating the paper drying system of the third aspect of the present invention as described above.

[0031] In accordance with a fifth aspect of the present invention, there is also provided a heat pump for a paper drying system having a steam supply system. The steam supply system includes a flash tank arranged to receive a condensate return of the paper drying system and a compressor arranged to compress flash steam from the flash tank such that the flash tank operates at a vacuum pressure. The heat pump includes a refrigerant circuit having an evaporator arranged to receive thermal energy from a waste air stream of the paper drying system, and a heat exchanger configured to transfer thermal energy from the refrigerant circuit to a condensate from the flash tank to raise steam from the condensate. The steam is provided to the flash tank of the steam supply system.

[0032] Accordingly, the heat pump can use thermal energy recovered from the waste air stream to boil the condensate at a temperature below 100 degrees Celsius, allowing more steam to be raised from the recovered thermal energy. In particular, as the compressor and flash tank arrangement means that the condensate from the flash tank is at a vacuum pressure, less thermal energy is required to boil the condensate. It is therefore particularly advantageous to provide a heat pump to boil condensate from the flash tank. In one example, the operating pressure of the flash tank is about -40 kPa (gauge) (about -0.4 Bar(g)), and the heat pump can raise steam from the condensate at about 85 degrees Celsius. The flash tank may be the low pressure flash tank as described with reference to the first to fourth aspects of the invention.

[0033] In examples, the heat pump may be used in conjunction with any of the first to fourth aspects of the invention as described above. In particular, the heat pump may be arranged to receive condensate from the low pressure flash tank of the steam supply system, and to provide steam back to the low pressure flash tank.

[0034] In examples, the heat exchanger may comprise a welded plate heat exchanger. Advantageously, a welded plate heat exchanger can operate with a large pressure difference between the condensate (which is at vacuum pressure) and the refrigerant of the heat pump (which may be up to about 6000 kPa (gauge) (about 60 Bar(g)).

[0035] In examples, the heat pump may further comprise a water loop. The water loop may comprise a second evaporator arranged to receive thermal energy from the waste air stream, and the evaporator may comprise an intermediate heat exchanger configured to transfer thermal energy from the water loop to the refrigerant circuit. Providing a water loop with the second evaporator allows the second evaporator (where the waste air stream is) to be located separately (quite far from) the rest of the heat pump, eliminating the need to have refrigerant pipes (e.g., ammonia pipes) over long distances.

[0036] In accordance with a sixth aspect of the present invention, there is also provided a method of supplying steam to a paper drying system, the method includes flashing, in a flash tank, a condensate return of the paper drying system to generate flash steam and condensate, compressing the flash steam from the flash tank such that the flash tank operates at a vacuum pressure, recovering thermal energy from a waste air stream of the paper drying system using a heat pump, boiling the condensate from the flash tank using the thermal energy recovered by the heat pump to raise steam, and providing the steam to the flash tank.

[0037] Accordingly, the method uses thermal energy recovered from the waste air stream to boil the condensate at a temperature below 100 degrees Celsius, allowing more steam to be raised from the recovered thermal energy. In particular, as the compressor and flash tank arrangement means that the condensate from the flash tank is at a vacuum pressure, less thermal energy is required to boil the condensate. It is therefore particularly advantageous to use the recovered thermal energy to boil condensate from the flash tank. In one example, the operating pressure of the flash tank is about -40 kPa (gauge) (about -0.4 Bar(g)), and the heat pump can raise steam from the condensate at about 85 degrees Celsius.

[0038] In examples, the method may comprise any steps of operating the heat pump of the fifth aspect of the invention as described above. In addition, the method may comprise supplying steam to a paper drying system, or a method of drying paper, comprising supply steam as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0040] FIG. 1 illustrates a paper drying system and a steam supply system.

[0041] FIG. 2 illustrates an example steam supply system for the paper drying system.

[0042] FIG. 3 illustrates a further example steam supply system for the paper drying system.

[0043] FIG. 4 illustrates a further example steam supply system for the paper drying system.

[0044] FIG. 5A illustrates a first example heat pump of the steam supply system.

[0045] FIG. 5B illustrates a second example heat pump of the steam supply system.

[0046] FIG. 5C illustrates a third example heat pump of the steam supply system.

DETAILED DESCRIPTION

[0047] As used herein, the terms high pressure and low pressure will be understood to be relative. That is, a high pressure is greater than a low pressure. As explained below, the actual pressures, and the differences between high and low pressures, may differ according to different uses of the steam. All example pressures are given as gauge pressures and are relative to ambient atmospheric pressure (Bar(g), kPa (gauge)).

[0048] In the example of FIG. 1 the steam process 126 comprises a paper drying system 112. The paper drying system 112 comprises a grouping of low pressure cylinders 104 and a grouping of high pressure cylinders 108. The grouping of low pressure cylinders 104 comprises first to fourth low pressure cylinders 106a to 106d. The low pressure cylinders 106 each receive steam from the steam supply system 102 via a low pressure steam supply 116. Steam passes through the interiors of the low pressure cylinders 106 and heats the low pressure cylinders 106. Paper is conveyed over the surfaces of the low pressure cylinders 106 for drying.

Condensate from the low pressure cylinders 106 is returned to the steam supply system 102 via a low pressure condensate return 118.

[0049] Similarly, the grouping of high pressure cylinders 108 comprises first to fourth high pressure cylinders 110a to 1 lOd. The high pressure cylinders 110 each receive steam from the steam supply system 102 via a high pressure steam supply 114. Steam passes through the interiors of the high pressure cylinders 110 and heats the high pressure cylinders 110. Paper is conveyed over the exteriors of the high pressure cylinders 110 for drying. Condensate from the high pressure cylinders 110 is returned to the steam supply system 102 via a high pressure condensate return 120.

[0050] In examples, the grouping of low pressure cylinders 104 may comprise any number of low pressure cylinders 106, for example one, two, three, or more than four low pressure cylinders 106. In some examples, the low pressure cylinders 104 may comprise up to 20 low pressure cylinders 106. Similarly, the grouping of high pressure cylinders 108 may comprise any number of high pressure cylinders 110, for example one, two, three, or more than four high pressure cylinders 110. In some examples, the high pressure cylinders 108 may comprise up to 20 high pressure cylinders 110.

[0051] In some examples, the low pressure cylinders 104 may be located upstream of the high pressure cylinders 108 in the direction that the paper is conveyed, in other examples the high pressure cylinders 108 may be located upstream of the low pressure cylinders 104 in the direction that the paper is conveyed. In some examples, the order of the low pressure cylinders 106 and high pressure cylinders 110 in the direction that the paper is conveyed may be mixed. For example, the paper may first pass over one or more low pressure cylinders 106 and then over one or more high pressure cylinders 110, and then again over one or more low pressure cylinders 106. Various combinations are possible depending on the desired drying profile of the paper drying system 112.

[0052] In a typical paper drying system 112 the high pressure steam supply 114 may be between about 200 kPa (gauge) (2 Bar(g)) and about 600 kPa (gauge) (6 Bar(g)), for example approximately 450 kPa (gauge) (4.5 Bar(g)). The low pressure steam supply 116 may be between about 100 kPa (gauge) (1 Bar(g)) and about 200 kPa (gauge) (2 Bar(g)), for example approximately 150 kPa (gauge) (1.5 Bar(g)). However, it will be appreciated that some paper drying systems 112 may operate on different (higher or lower) steam pressures. The temperature of the low pressure steam supply 116 and high pressure steam supply 114 may be between about 100 degrees Celsius and about 300 degrees Celsius, for example between about 160 degrees Celsius and about 240 degrees Celsius, for example about 200 degrees Celsius. In some examples, the paper drying system 112 may operate on only a single steam supply (one pressure), or the steam supply system 102 may provide a single steam supply and an additional system may provide an additional steam supply.

[0053] Condensate returned to the steam supply system 102 via the low pressure condensate return 118 and the high pressure condensate return 120 may include condensed water and some blow-through steam, depending on the efficiency of the low pressure cylinders 106 and high pressure cylinders 110. In this respect, the term condensate means return of condensed steam and steam downstream of the steam process (e.g., paper drying system 112).

[0054] The pressure of the low pressure condensate return 118 and the high pressure condensate return 120 are less than the high pressure steam supply 114 and low pressure steam supply 116, respectively, so that the condensate is siphoned from the paper drying system 112 back into the steam supply system 102. As explained below, compressors are provided to control the pressure in the low pressure condensate return 118 and the high pressure condensate return 120.

[0055] As also shown in FIG. 1, air is passed through the paper drying system 112 from an air input 122 to a waste air stream 124. Air at air input 122 is heated and is passed over the paper within the paper drying system 112 to dry the paper and convey evaporated water from the paper drying system 112. The waste air stream 124 has a relatively high temperature and high humidity. For example, at the air input 122 the air may be between about 65 degrees Celsius and about 100 degrees Celsius, and the temperature of the waste air stream 124 will be similar. At at the air input 122 is preferably as dry as possible, and the moisture of the waste air stream 124 may be between about 50 g/kg dry air and about 250 g/kg dry air.

[0056] Additional condensate may be generated within the paper drying system 112, for example condensation forming on the walls of the paper drying system 112. This condensate is not conveyed with the waste air stream 124. This condensate is typically contaminated and not suitable for use in the steam supply system 102 and can be processed separately from the steam supply system 102 illustrated and described herein.

[0057] It will also be appreciated that the steam supply system 102 described herein may be used for other (non-paper drying) steam processes as described further hereinafter.

[0058] Different steam processes 126 may operate on a different pressure steam supplies, with a different value steam pressure being provided by the steam supply system 102. In some examples, steam processes 126 may operate on a single pressure supply and return condensate at two different pressures. As will be apparent hereinafter, the inventive steam supply system 102 provides efficient use of return condensate at different pressures to generate a steam supply for a steam process 126. In particular, the steam supply system 102 recovers thermal energy from two condensate returns to raise steam at a temperature and pressure to feed the steam process 126. This greatly reduces the amount of ‘fresh’ steam required from a boiler and so reduces the energy consumption of the steam supply system 102.

[0059] FIG. 2 illustrates a steam supply system 102 for the steam process 126 illustrated in FIG. 1, particularly a paper drying system 112. The steam supply system 102 illustrated in FIG. 2 is a schematic representation of the main components of the steam supply system 102. More detailed implementations are illustrated in FIG. 3, FIG. 4 and FIGS. 5A, 5B and 5C.

[0060] As illustrated, the steam supply system 102 provides the high pressure steam supply 114 and the low pressure steam supply 116 to the paper drying system 112. The steam supply system 102 receives the low pressure condensate return 118 and the high pressure condensate return 120 from the paper drying system 112.

[0061] The low pressure condensate return 118 is provided as an input to a low pressure flash tank 202. Within the low pressure flash tank 202 the condensate is flashed (pressure is reduced) to produce flashed steam and condensate. Flashed steam is output at a steam output 214. The steam output 214 provides the flashed steam to a first compressor 206, which compresses the flashed steam and feeds into a high pressure flash tank 204. Condensate is output at a condensate output (not illustrated in FIG. 2) and routed elsewhere.

[0062] The first compressor 206 generates suction on the upstream side, and therefore defines the operating pressure of the low pressure flash tank 202 and the pressure of the low pressure condensate return 118. In order to siphon the low pressure condensate return 118 from the paper drying system 112, the suction pressure generated by the first compressor 206 must be lower than the pressure of the low pressure steam supply 116. In preferred examples, the suction pressure generated by the first compressor 206 is negative (i.e., a vacuum pressure). In examples, the suction pressure generated by the first compressor 206 is between about -10 kPa (gauge) (about -0.1 Bar(g)) and about -50 kPa (gauge) (about -0.5 Bar(g)), for example about - 40 kPa (gauge) (about -0.4 Bar(g)). As will be appreciated, the temperature of the flash steam is increased in the first compressor 206. In examples, the temperature of the flash steam may increase by about 40-60 degrees Celsius, for example from about 80-90 degrees Celsius to about 120-150 degrees Celsius.

[0063] The high pressure flash tank 204 receives the high pressure condensate return 120 and, as described above, the flashed steam from the steam output 214 of the low pressure flash tank 202. Within the high pressure flash tank 204 the high pressure condensate return 120 and steam output 214 of the low pressure flash tank 202 are flashed (pressure is reduced) to produce flashed steam and condensate. A condensate output 216 of the high pressure flash tank 204 is fed back to the low pressure flash tank 202. Flashed steam from the high pressure flash tank 204 is output to a first steam output 218 and a second steam output 220. The first steam output 218 of the high pressure flash tank 204 provides the low pressure steam supply 116 for the paper drying system 112. In the illustrated example the second steam output 220 of the high pressure flash tank 204 passes through a second compressor 208 and provides the high pressure steam supply 114 for the paper drying system 112.

[0064] The second compressor 208 generates suction on the upstream side, and therefore defines the operating pressure of the high pressure flash tank 204 and the pressure of the high pressure condensate return 120. In order to siphon the high pressure condensate return 120 from the paper drying system 112, the suction pressure generated by the second compressor 208 must be lower than the pressure of the high pressure steam supply 114. In examples, the suction pressure generated by the second compressor 208 is between about 100 kPa (gauge) (about 1 Bar(g)) and about 200 kPa (gauge) (about 2 Bar(g)), for example about 150 kPa (gauge) (about 1.5 Bar(g)).

[0065] The low pressure steam supply 116 is at the operating pressure of the high pressure flash tank 204, as described above. The second compressor 208 may increase the pressure of the flashed steam, for example to between about 300 kPa (gauge) (about 3 Bar(g)) and about 600 kPa (gauge) (about 6 Bar(g)), for example to about 460 kPa (gauge) (about 4.6 Bar(g)). The high pressure steam supply 114 is at the pressure provided by the second compressor 208. As will be appreciated, the temperature of the flash steam is increased in the second compressor 208. In examples, the temperature of the flash steam may increase by about 20-30 degrees Celsius, for example from about 120-150 degrees Celsius to about 140-180 degrees Celsius.

[0066] It will be appreciated that in some applications the paper drying system 112 may require only one steam input (one pressure), in which case steam may be provided to the paper drying system 112 either by the first steam output 218, or by the second steam output 220 and second compressor 208, the other being omitted.

[0067] It will be appreciated that in some examples more than two flash tanks may be provided, and a compressor may be provided between each flash tank. For example, providing additional flash tanks may provide a higher pressure steam output and/or handle more condensate returns. [0068] Accordingly, the steam supply system 102 cascades the low pressure condensate return 118 and the high pressure condensate return 120 up to a temperature and pressure desired for the the paper drying system 112, particularly the high pressure steam supply 114 and the low pressure steam supply 116. The cascading is provided by first flashing the condensate returns, and then using a compressor to increase the pressure further. Such cascading provides an efficient method of recovering thermal energy from the low pressure condensate return 118 and the high pressure condensate return 120 because it reduces the work required by the compressors to raise the pressure.

[0069] As described further hereinafter, additional steam may be provided from a boiler to make up the high pressure steam supply 114 (and optionally low pressure steam supply 116) to the desired pressures. However, preferably the steam supply system 102 can provide the low pressure steam supply 116 derived entirely on recovered low pressure condensate return 118 and high pressure condensate return 120. The steam supply system 102 may additionally provide at least a portion of, preferably a majority of, the high pressure steam supply 114.

[0070] A control valve 210a is provided to control flow of the low pressure condensate return 118 into the low pressure flash tank 202. A control valve 210b is provided to control flow of the high pressure condensate return 120 into the high pressure flash tank 204. A control valve 210c is provided to control flow of the condensate output 216 from the high pressure flash tank 204 to the low pressure flash tank 202. The control valves 210 can be controlled to balance pressures within the steam supply system 102 to ensure that the steam output 214 at the first steam output 218 of the high pressure flash tank 204 matches a desired pressure of the low pressure steam supply 116 for the paper drying system 112. In addition, the first compressor 206 should be operated to compress the flash steam of the steam output 214 of the low pressure flash tank 202 to the same pressure as the high pressure flash tank 204. This can ensure a balanced system that efficiently recovers thermal energy from the low pressure condensate return 118 and high pressure condensate return 120.

[0071] The control valves 210 additionally act to reduce the pressure of the condensate in the respective pipe, generating flash steam at a position immediately before it enters the respective flash tank.

[0072] In an alternative example, the high pressure flash tank 204 comprises only a single steam output, for example second steam output 220, and the low pressure steam supply 116 is branched from the second steam output 220 upstream of the first compressor 206. [0073] As also illustrated, a sub-cooler 212 is provided in the low pressure condensate return 118 and is used to heat air that is used for the paper drying system 112 or elsewhere in the installation (e.g., for heating). The sub-cooler 212 advantageously also reduces a temperature and a steam content of the low pressure condensate return 118 (i.e., any blow-through steam). Advantageously, the low pressure flash tank 202 operates at a temperature below that of the low pressure condensate return 118, so recovering thermal energy from the low pressure condensate return 118 allows that thermal energy to be used (i.e., to heat air) without affecting the operation of the low pressure flash tank 202. For example, if the low pressure condensate return 118 is at 130 degrees Celsius and the low pressure flash tank 202 operates at 85 degrees Celsius, then up to 45 degrees Celsius of thermal energy can be recovered without affecting the operation of the low pressure flash tank 202.

[0074] Advantageously, the unique cascading arrangement of the low pressure flash tank 202 and the high pressure flash tank 204 reduces the work required by the first compressor 206 (and also the second compressor 208 if provided) because the pressure increase at each of the first compressor 206 and second compressor 208 is reduced relative to a conventional arrangement that does not include such cascading.

[0075] Advantageously, the first compressor 206 generates suction on the upstream side, which acts to siphon condensate from the paper drying system 112 through the low pressure condensate return 118. Similarly, the second compressor 208 generates suction on the upstream side, which acts to siphon condensate from the paper drying system 112 through the high pressure condensate return 120. Accordingly, no additional vacuum/suction system is needed to convey the condensate out of the paper drying system 112. The first compressor 206 and the second compressor 208 therefore provide three advantages: they increase the pressure of cascaded condensate up to the higher pressures for the high pressure steam supply 114 and the low pressure steam supply 116; they generate suction to siphon the condensate from the paper drying system 112, and they also lower the operating pressures of the flash tanks so that steam can be raised at lower temperatures.

[0076] In some other examples, the paper drying system 112 may comprise only a single condensate return (e.g., the low pressure condensate return 118), and the steam supply system 102 may comprise only a single flash tank (e.g., the low pressure flash tank 202). The low pressure condensate return 118 may be provided to the low pressure flash tank 202 and a compressor (e.g., the first compressor 206) can be arranged upstream of the low pressure flash tank 202 to increase the pressure of the flashed steam to the level needed by the paper drying system 112. In such an example, the paper drying system 112 effectively acts as a condenser and the steam supply system 102 as the remainder of a heat pump. Advantageously, such a system includes the compressor (first compressor 206) in a steam loop of the steam supply system 102, acting to compress vapour, so generates its own suction for siphoning condensate from the paper drying system 112. This has the additional advantage of reducing an operating pressure of the flash tank, so that steam can be raised at a lower temperature. In some examples, for example the low pressure flash tank as described above, the operating pressure of the flash tank is a vacuum pressure.

[0077] FIG. 3 illustrates an example steam supply system 102 for the paper drying system 112. As illustrated, the steam supply system 102 provides a high pressure steam supply 114 and a low pressure steam supply 116 as previously described, and receives a low pressure condensate return 118 and a high pressure condensate return 120 as previously described. For ease of understanding, in FIG. 3 conduits carrying condensate are illustrated in dashed lines, and conduits carrying steam are illustrated in solid lines.

[0078] As described with reference to FIG. 2, the steam supply system 102 comprises a low pressure flash tank 202 that receives the low pressure condensate return 118, and a high pressure flash tank 204 that receives the high pressure condensate return 120.

[0079] A first compressor 206 compresses flashed steam from the steam output 214 of the low pressure flash tank 202 and feeds the compressed steam to the high pressure flash tank 204.

The compressed steam output by the first compressor 206 is at the same pressure as the high pressure flash tank 204. In this example, the first compressor 206 comprises first to fifth subcompressors 302a-302e, but it will be appreciated that the first compressor 206 may comprise one or more sub-compressors 302. The sub-compressors 302 are arranged in series and act to successively increase the pressure of the steam. The or each sub-compressor 302 may comprise a centrifugal compressor. The or each sub-compressor 302 may comprise a mechanical vapor recompression (MVR) compressor, for example an MVR blower or MVR fan. In other examples, the or each sub-compressor 302 may comprise a turbo-compressor (turbo-blower), a screw compressor, a reciprocating compressor, a rotary lobe compressor, or a vane compressor. The condensate output 216 of the high pressure flash tank 204 is returned to the low pressure flash tank 202. [0080] A sub-cooler 212 is provided in the low pressure condensate return 118 as previously described. As also shown, a bypass line 312 is provided upstream of the sub-cooler 212 and connected to the low pressure flash tank 202 (and in this example condensate output 216 of the high pressure flash tank 204) via a three-way control valve 210b. The control valve 210b can be controlled to divert a portion of the low pressure condensate return 118 around the subcooler 212 and directly to the low pressure flash tank 202, thereby controlling the air heating provided by the sub-cooler 212.

[0081] In this example, a first steam output 328a of the high pressure flash tank 204 is split at three-way valve 322. A first portion of the flash steam at the first steam output 328a is passed from the three-way valve 322 through the second compressor 208 to provide the high pressure steam supply 114 to the paper drying system 112. A second portion of the flash steam at the first steam output 328a is passed from the three-way valve 322 directly to the high pressure steam supply 114 via control valve 324. A three-way valve 326 combines steam received directly from the first steam output 328a and the steam from the second compressor 208 and can therefore control the pressure of the high pressure steam supply 114 by balancing flashed steam directly from the high pressure flash tank 204 with compressed steam from the second compressor 208.

[0082] In this example the second compressor 208 comprises first to third sub-compressors 304a-304c, but it will be appreciated that the second compressor 208 may comprise one or more sub-compressors 304. The sub-compressors 304 are arranged in series and act to successively increase the pressure of the steam. The or each sub-compressor 302 may comprise a centrifugal compressor. The or each sub-compressor 302 may comprise a mechanical vapor recompression (MVR) compressor, for example an MVR blower or MVR fan. In other examples, the or each sub-compressor 304 may comprise a turbo-compressor (turbo-blower), a screw compressor, a reciprocating compressor, a rotary lobe compressor, or a vane compressor. [0083] The low pressure steam supply 116 is branched from the first steam output 328a of the high pressure flash tank 204 upstream of the three-way valve 322 (and second compressor 208) and provided directly to the paper drying system 112. That is, the flashed steam from the high pressure flash tank 204 directly provides the low pressure steam supply 116. The high pressure flash tank 204 operates at the (low) pressure for the paper drying system 112. In the example of the paper drying system 112, the low pressure steam supply 116 may be between about 100 kPa (gauge) (about 1 Bar(g)) and about 200 kPa (gauge) (about 2 Bar(g)), for example about 150 kPa (gauge) (about 1.5 Bar(g)). This pressure is defined by the second compressor 208 and would be the operating pressure of the high pressure flash tank 204.

[0084] The high pressure steam supply 114, upstream of the second compressor 208, may be between about 300 kPa (gauge) (about 3 Bar(g)) and about 600 kPa (gauge) (about 6 Bar(g)), for example to about 460 kPa (gauge) (about 4.6 Bar(g)).

[0085] As shown, a boiler supply 306 may provide additional steam to the high pressure steam supply 114 to maintain the steam pressures from the steam supply system 102. The boiler supply 306 may be provided at, for example 20 Bar(g). A control valve 324 may be operated to reduce the pressure of the boiler supply 306 to the desired pressure of the high pressure steam supply 114 and combine it with flashed steam received directly from the high pressure flash tank 204.

[0086] In the illustrated example, a second steam output 328b is provided from the high pressure flash tank 204. Flashed steam from the second steam output 328b passes through a heat exchanger 308 where it acts to heat water or air, and is then returned to the low pressure steam supply 116.

[0087] As illustrated, in this example the steam supply system 102 further comprises a heat pump 310, which is described in more detail with reference to FIGS. 5A, 5B and 5C. The heat pump 310 is used to recover thermal energy from a waste air stream of the paper drying system 112. The recovered thermal energy is used to raise steam that is fed into the low pressure flash tank 202.

[0088] As also shown in FIG. 3, a portion of the condensate output 314 of the low pressure flash tank 202 is pumped by pump 316 into the first compressor 206, in particular each of the sub-compressors 302. The condensate output 314 may be pumped into the steam output 214 upstream of each sub-compressor 302, or it may be pumped directly into each sub-compressor 302. Adding water to the steam output 214 at or near the first compressor 206 can prevent superheating of the steam.

[0089] Similarly, a portion of the condensate output 216 can be pumped via water line 320 and pump 318 into the second compressor 208. In particular, the condensate output 216 can be pumped into the first steam output 328a upstream of each of the sub-compressors 304, or directly into each of the sub-compressors 304. Adding water to the first steam output 328a at or near the second compressor 208 can prevent superheating of the steam. [0090] The example of FIG. 3 provides a steam supply system 102 that efficiently cascades condensate from the low pressure condensate return 118 and the high pressure condensate return 120 to generate steam input(s) for the paper drying system 112. It will be appreciated that additional flash tanks and compressors may be provided in a similar arrangement to provide further cascading, for example to handle more than two condensate returns and/or to raise the steam to a higher pressure.

[0091] Additionally, the heat exchanger 308 and/or sub-cooler 212 provides heating of air for the paper drying system 112. Additionally, the heat pump 310 may recover waste heat from the air stream leaving the paper drying system 112 and uses it to raise steam. Therefore, the steam supply system 102 provides a system for recovering and utilising waste heat streams from the paper drying system 112 and reduces use of ‘fresh’ steam from the boiler supply 306, thereby reducing the energy required to operate the steam supply system 102.

[0092] Similar to as described with reference to FIG. 2, in some examples the paper drying system 112 may comprise only one condensate return (e.g, the low pressure condensate return 118), and in such cases the steam supply system 102 may comprise only one flash tank (e.g., the low pressure flash tank 202) and one compressor (e.g., the first compressor 206). As previously described, such an arrangement advantageously generates its own suction (by the first compressor 206) to siphon condensate from the paper drying system 112 and also lower the operating pressure of the low pressure flash tank 202 allowing steam to be raised at a lower temperature.

[0093] FIG. 4 illustrates a similar example steam supply system 102 for the paper drying system 112 as that illustrated in FIG. 3. For ease of understanding, in FIG. 4 conduits carrying condensate (water possibly with some blow-through steam) are illustrated in dashed lines, and conduits carrying steam are illustrated in solid lines.

[0094] Only the differences between the examples of FIG. 3 and FIG. 4 are described in detail. Specifically, the low pressure flash tank 202, high pressure flash tank 204 and first compressor 206 are as described above with respect to the FIG. 3. In addition, a heat pump 310 is provided as described further with reference to FIGS. 5 A, 5B and 5C.

[0095] In the example of FIG. 4, the boiler supply 306 is combined with the first steam output 328a at a thermo-compressor 402. The thermo-compressor 402 advantageously increases the pressure of the first steam output 328a by mixing with the boiler supply 306 to provide the low pressure steam supply 116 at the desired operating pressure. The thermo-compressor 402 of the example of FIG. 4 replaces the control valve 324 of the example of FIG. 3, and may provide benefits in terms of combining the boiler supply 306 and flashed steam from the high pressure flash tank 204 to provide the high pressure steam supply 114.

[0096] Furthermore, the second compressor 208 of FIG. 4 differs to that of FIG. 3 in that it comprises first and second sub-compressors 404a, 404b arranged in parallel, rather than in series as in the example of FIG. 3. Use of the thermo-compressor 402 may reduce the pressure increase required by the second compressor 208 so less compression may be provided. It will be appreciated that second compressor 208 may comprise any number of sub-compressors 404, particularly one or more sub-compressors 404. In addition, the second compressor 208 of FIG.

3 may be used in the steam supply system 102 of FIG. 4, and vice versa the second compressor 208 of FIG. 4 may be used in the steam supply system 102 of FIG. 3. The series or parallel arrangements of the sub-compressors 302, sub-compressors 304, and sub-compressors 404 depend partly on the type of compressor being used, and particularly their capabilities in terms of volumetric flow and pressure increases.

[0097] Like the steam supply system 102 of FIG. 3, the steam supply system 102 of FIG. 4 provides a steam supply system 102 that efficiently cascades condensate from the low pressure condensate return 118 and the high pressure condensate return 120 to generate steam input(s) for the paper drying system 112. It will be appreciated that additional flash tanks and compressors may be provided in a similar arrangement to provide further cascading, for example to handle more than two condensate returns and/or to raise the steam to a higher pressure.

[0098] Additionally, the heat exchanger 308 and/or sub-cooler 212 provides heating of air for the paper drying system 112. Additionally, the heat pump 310 may recover waste heat from the air stream leaving the paper drying system 112 and uses it to raise steam. Therefore, the steam supply system 102 provides a system for recovering and utilising waste heat streams from the paper drying system 112 and reduces use of ‘fresh’ steam from the boiler supply 306, thereby reducing the energy required to operate the steam supply system 102.

[0099] Similar to as described with reference to FIG. 2, in some examples the paper drying system 112 may comprise only one condensate return (e.g, the low pressure condensate return 118), and in such cases the steam supply system 102 may comprise only one flash tank (e.g., the low pressure flash tank 202) and one compressor (e.g., the first compressor 206). As previously described, such an arrangement advantageously generates its own suction (by the first compressor 206) to siphon condensate from the paper drying system 112 and also lower the operating pressure of the low pressure flash tank 202 allowing steam to be raised at a lower temperature.

[0100] FIG. 5 A, FIG. 5B and FIG. 5C illustrate examples of the heat pump 310 mentioned above. The heat pump 310 recovers thermal energy from the waste air stream 124 of the paper drying system 112 (see FIG. 1). The heat pump 310 receives condensate from the low pressure flash tank 202 and raises steam which is provided back to the low pressure flash tank 202 and therefore contributes to the low pressure steam supply 116 and the high pressure steam supply 114 (see FIG. 2, FIG. 3 and FIG. 4). The heat pump 310 operates on a refrigerant that may be water, glycol, or a different refrigerant, for example ammonia. Preferably the heat pump 310 operates on ammonia. In some examples, as shown in FIGS. 5B and 5C, the heat pump 310 may include a water loop to interface with the waste air stream 124, and may additionally be used to heat water for the paper drying system 112.

[0101] The heat pump 310 raises steam from the condensate from the low pressure flash tank 202, which is operating at a vacuum pressure (i.e., less than 0 kPa (gauge) (0 Bar(g)), for example about -40kPa (gauge) (about -0.4 Bar(g))) Therefore, the heat pump 310 raises steam at the pressure of the low pressure flash tank 202, which would be at less than 100 degrees Celsius, for example between about 80 degrees Celsius and 90 degrees Celsius, for example at about 85 degrees Celsius. The heat pump 310 therefore uses thermal energy recovered from the waste air stream 124 and increases the temperature of the refrigerant to a temperature sufficient to raise steam.

[0102] In the example of FIG. 5 A the heat pump 310 comprises a condenser 502, a compressor 512, an expansion valve 534, and an evaporator 510. The refrigerant circuit is completed by a low pressure conduit 514 connecting the condenser 502 to the evaporator 510, a link conduit 516 connecting the evaporator 510 to the compressor 512, and a high pressure conduit 518 connecting the compressor 512 to the condenser 502.

[0103] Refrigerant, preferably ammonia, is circulated through the heat pump 310. The refrigerant is heated by the waste air stream 124 at the evaporator 510, compressed at compressor 512, and then passed through the condenser 502 and the expansion valve 534.

[0104] Condensate from the low pressure flash tank 202 is pumped by pump 504 into the condenser 502 and the ammonia acts to boil the condensate in the condenser 502. Steam raised in the condenser 502 is fed back into the low pressure flash tank 202 at steam input 508. As mentioned above, the condensate is at a vacuum pressure (e.g., about -40kPa (gauge) (about - 0.4 Bar(g))), so the ammonia needs to achieve a temperature of less than 100 degrees Celsius to boil the condensate (e.g., about 85 degrees Celsius).

[0105] The condenser 502 is preferably a welded plate heat exchanger. A welded plate heat exchanger advantageously works with the large pressure differential between the ammonia (which may be at about 6000 kPa (gauge) (about 60 Bar(g))) and the condensate from the low pressure flash tank 202, which is at a vacuum pressure (e.g., about -40kPa (gauge) (about -0.4 Bar(g))).

[0106] In the example of FIG. 5B the heat pump 310 also includes a water loop 536 that includes the evaporator 510. The water loop 536 includes a pump 522 that circulates water between the evaporator 510 and an intermediate heat exchanger 520. The intermediate heat exchanger 520 is positioned in the refrigerant circuit of the heat pump 310, between the expansion valve 534 and the compressor 512. In this way, water heated by the waste air stream 124 in the water loop 536 acts to heat the refrigerant (e.g., ammonia) in the intermediate heat exchanger 520, which is then compressed at the compressor 512 and passed to the condenser 502 where it boils the condensate from the low pressure flash tank 202. It will be appreciated that in this example the intermediate heat exchanger 520 effectively acts as the evaporator of the refrigerant circuit, and the water loop 536 acts to relay thermal energy from the waste air stream 124 to the intermediate heat exchanger 520.

[0107] Advantageously, providing the water loop 536 means that the evaporator 510 can be positioned away from the condenser 502 without having to expand pipework carrying the refrigerant, for example ammonia. This helps to limit the volume of refrigerant in the circuit and also any safety risks associated with the refrigerant.

[0108] In examples, the intermediate heat exchanger 520 may comprise a welded plate heat exchanger, as described above, or other type of heat exchanger as would be known in the art.

[0109] The example heat pump 310 of FIG. 5C is similar to that of FIG. 5B, but provides for heating water as well as raising steam for the low pressure flash tank 202. In particular, as shown, an additional heat exchanger 524 is provided upstream of the compressor 512 via a link conduit 528, and connected back to the intermediate heat exchanger 520 via a link conduit 526. Some of the compressed refrigerant (e.g., ammonia) output by compressor 512 circulates through the heat exchanger 524 and returns to the intermediate heat exchanger 520, while some circulates through the condenser 502 as previously described. The heat exchanger 524 is used 1 to heat water in a water system 530, 532. The heated water may be used for any purpose within the installation, for example building heating or for a different paper-making process such as pulping.

[0110] Accordingly, in the example of FIG. 5C thermal energy recovered from the waste air stream 124 is used to raise steam for the low pressure flash tank 202 and to heat water in the water system 530, 532 for other processes.

[0111] The steam supply system 102 described above efficiently uses condensate returned at two different pressures and temperatures to raise steam for the paper drying system 112. In particular, the cascading of the condensate through the low pressure flash tank 202 and the high pressure flash tank 204, in combination with the first compressor 206 and optional second compressor 208, recovers a significant amount of energy from the condensate and greatly improves the efficiency of the steam supply system 102 compared to just using boiler steam and even compared to a conventional heat pump arrangement. The cascading reduces the work required at each of the first compressor 206 and the second compressor 208.

[0112] Also, the first compressor 206 (and optional second compressor 208) generate suction within the steam system, removing the need for a separate vacuum system and the energy that would consume. In particular, the inventive arrangement of at least one flash tank and a compressor within a steam loop of the steam supply system 102 advantageously removes the need for a separate suction system to siphon condensate from the paper drying system 112 and also lowers the pressure in the flash tank, allowing steam to be raised at lower temperatures. In this arrangement, essentially, the paper drying system 112 acts as a condenser, and the flash tank and compressor complete a form of heat pump to raise the pressure of the condensate back to the required pressure for the paper drying system 112. In such an example the steam supply system 102 may only comprise a single flash tank and a single compressor (which may have multiple sub-compressors) arranged upstream of the flash tank.

[0113] In addition, the heat pump 310 improves thermal energy recovery by using thermal energy of the waste air stream to raise additional steam at the flash tank (e.g., the low pressure flash tank 202). This further reduces the energy consumption of the steam supply system 102. [0114] Therefore, the invention provides numerous benefits for the steam supply system 102 and the steam process (e.g., the paper drying system).

[0115] It will be appreciated that the steam supply system 102 described above could be used for other industrial processes, not only paper-making (paper drying). The steam supply system 102 is particularly suited for generating steam at relatively low temperatures, for example less than 200 degrees Celsius. For example, the steam supply system 102 may be used for food and beverage manufacturing, or chemical processing.

[0116] It will apparent that a key advantage of reducing the amount of fresh steam required for the steam process (e.g., paper drying system 112) is that less input energy is needed to raise steam. Typically, fresh steam is generated in gas boilers, so the steam supply system 102 reduces the consumption of fossil fuels. In some examples the reduced need for fresh steam will allow replacement of gas boilers with electric boilers, allowing renewable energy to be used for the entire steam supply system 102.