Patent Publication Number: US-2009232688-A1

Title: Compressor Installation With a Water-Injected Compressor Element

Description:
This invention relates to a compressor installation with a water-injected compressor element. 
     More specifically, the invention relates to a compressor installation with a water-injected compressor element to which an inlet line and an outlet line are connected, an air inlet filter to which said inlet line is connected, a water separator into which said outlet line feeds and a return line between this water separator and the compressor element. 
     In water-injected compressor elements, the rotating parts, and in particular the screws in screw compressor elements, are lubricated with water instead of with oil. This water also serves as a sealant between these rotating parts and ensures cooling, making virtually isothermic compression possible. 
     The humidity of a gas, and of air in particular, is a function of its temperature and pressure. Within the compressor element, the gas is mixed with water and the gas at the outlet from the compressor element is thus always 100% saturated. 
     Depending on the absolute humidity of the intake gas and the pressure at the outlet from the compressor element, a water-injected compressor element will thus either consume or produce water. The higher the absolute humidity of the intake gas and the higher the outlet pressure, the smaller the amount of water that will be consumed. On the other hand, the compressor element will consume more water if the absolute humidity at the inlet falls and/or the pressure in the outlet is lower. 
     This is why known compressor installations with a water-injected compressor element also have a water supply device to add water if necessary. 
     It is customary to inject the water that needs to be added via the return line, directly between the rotating elements or at the compressor element&#39;s inlet. 
     However, this water must meet strict requirements. 
     The water that is consumed, is chemically pure water, so that water consumption will cause the concentration of salts and other such substances in the remaining water to rise if ordinary water is added. This can cause damage to the compressor element, and in particular the seals and bearings. 
     The addition of extra water or the consumption of pure water could also change the pH value of the water in the circuit, which can again in turn contribute to corrosion. 
     For these reasons, it is known to add the water via reverse osmosis, which makes the water supply device relatively expensive. 
     According to WO-A-96/21109, the water that has been separated out in the water separator after the compressor element, is returned to this compressor element through an ioniser in which natural minerals in solution in the water, such as silicon, calcium, magnesium and iron, are converted into hydroxide form, after which they acquire an electrical charge. 
     Because the hydroxide particles have the same polarity, they are prevented from clumping together and hence from precipitating. However, as these particles only retain their charge temporarily, the ionisation has to be continually repeated, and the recycled water has to be pumped through the ioniser several times a minute. Moreover, precipitation is not completely excluded, and this injected water also has to be filtered to prevent other impurities. 
     The aim of the present invention is to offer an answer to one or more of said and certain other disadvantages. 
     To this end, the present invention relates to a compressor installation with a water-injected compressor element to which an inlet line and an outlet line are connected, an air inlet filter to which said inlet line is connected, a water separator into which said outlet line feeds and a return line between this water separator and the compressor element, with said air inlet filter taking the form of a wet filter to which a water supply is connected. 
     Through the injection of water into the air inlet filter, this air is simultaneously humidified and purified, as impurities are captured by the injected water droplets and conducted away. A portion of the injected water will evaporate and be conveyed in the air flow to the compressor element. 
     Because a portion of this water which has been injected into the air inlet filter enters the compressor element along with the intake gas in the form of water vapour, this water is chemically pure, and neither the level of minerals and salts nor the pH of the water for water injection is affected. 
     A compressor installation of this type has the advantage that the compressor element&#39;s inlet air can be humidified to such a degree that during periods when the environmental air is dry, no extra supply of demineralised water or water that has been purified by means of a reverse osmosis unit is needed any longer. 
     Another advantage of such a compressor installation according to the invention is that it can be produced relatively cheaply, and that it displays a considerably higher filter efficiency than traditional air filters without water injection. 
     A further advantage is that the air inlet filter of such a compressor installation according to the invention causes a smaller fall in pressure than traditional dry air filters, thus increasing the compressor efficiency. 
     Yet a further advantage of such a compressor installation is that it enables water to be added relatively simply and inexpensively without reducing the quality of the water present in the circuit and without any risk of damaging the compressor element by increasing the deposition of minerals and similar substances. 
     An additional advantage is that in speed-regulated compressors, no inlet valve or so-called “unloader” is needed any longer between the air inlet filter and the inlet to the compressor element to absorb any water hammer which occurs when the compressor is stopped, as the water hammer can escape via the inlet line, which is introduced between the air inlet filter and the compressor element, where applicable to a water drainage line from this wet air inlet filter. 
     Preferably, said air inlet filter will take the form of a housing in which a substrate is introduced through which the intake air is passed; this substrate should preferably take the form of a material with an open cellular structure, such as polyurethane foam, polyethylene or a similar material. 
     As a result of the air inlet filter having this structure, the contact surface between the water and the intake air is increased, which means that the air is optimally humidified and purified. 
     Another preferred characteristic of a compressor installation according to the invention is that said air inlet filter is provided with an element for stopping water drops in the gas flow, in order to prevent water drops being brought in with the intake air. 
     In the preferred embodiment of a compressor installation according to the present invention, said water supply is constituted by a line connecting to said water separator. 
     This has the advantage that the water derived from this water separator can be used by being injected into this air inlet filter, thereby enabling the supply of extra water to be restricted. 
     A further advantage is that the air inlet filter in such a compressor installation operates on a self-cleaning basis due to being periodically sprayed with pure water derived from the water separator. 
     In a further preferred characteristic of the invention, said water supply to the air inlet filter takes the form of a supply device for extra water. 
     This embodiment has the same advantages as an embodiment in which the added water is derived from the water separator, and also makes it possible to add extra water to the compressor installation. 
    
    
     
       With a view to demonstrating the invention&#39;s characteristics more clearly, in what follows, by way of example and without any limitative intention, a preferred embodiment of a compressor installation according to the invention with a water-injected compressor element is described, with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic representation of a compressor installation according to the invention; 
         FIGS. 2 and 3  illustrate the working of a compressor installation according to  FIG. 1 ; 
         FIGS. 4 and 5  show regulating curves which are followed during the working of a compressor installation according to  FIG. 1 , the first in a normal situation with sufficient water vapour in the intake air and the second for a situation with very little water vapour in the intake air, for example during extreme winter weather. 
     
    
    
       FIG. 1  shows a compressor installation  1  according to the invention which is provided with a compressor element  2 , for example in the form of a screw compressor element, which is driven by a motor  3  and to which an inlet line  4  is connected. To the outlet from the compressor element  2  is connected an outlet line  5  which feeds into a water separator  6 . 
     A return line  7  connects the underside of the water separator  6  with the compressor element  2 . In this return line  7 , a cooling device  8  is set up which in this instance, though not necessarily, is cooled by means of a fan  9  which is driven by a motor  10 . 
     However, said cooling device  8  can also take many other forms, such as that of a liquid-liquid heat exchanger. 
     On top of the water separator  6  is fitted a minimum pressure valve  11  of the known type for compressed air; this opens at a pre-set opening pressure in the water separator and a compressed air line  12  is connected to it for the supply of compressed gas to a mains network. 
     Into said inlet line  4  an air inlet filter  13  is introduced which in this case consists of a housing  14  with an air inlet  15  and an air outlet  16  to which said inlet line  4  is connected. In this housing  14 , a substrate  17  is introduced through which the intake air is passed. 
     Said air inlet filter  13  should preferably also be provided with an element  18  or “demister” to prevent water drops in the gas flow, and a water drainage line  19  for contaminated water. 
     Said substrate  17  and the element  18  should preferably both take the form of a material with an open cellular structure, such as polyurethane foam, polyethylene or a similar material. Obviously, the invention is not restricted to the use of any one of these materials for the production of the substrate  17  and the element  18 , and numerous other materials may also be used. 
     In a preferred embodiment of a compressor installation  1  according to the invention, the substrate  17  and/or the element  18  consists of a material that is doped with one or more bacteriostatic agents, such as nano-silver. It is clear, however, that this is not a requirement according to the invention. 
     In the present instance, said substrate  17  is located at the bottom of the housing  14 , while the element  18  is introduced at the top of this housing  14 . Said air inlet  15  is provided at the bottom of the housing  14 , opposite the substrate  17 , while the air outlet  16  is fitted at the top of the housing  14  of the air inlet filter  13 , opposite the element  18 . 
     Said air inlet filter  13  takes the form according to the invention of a wet filter to which a water supply is connected which, in this instance, takes the form of a line  20 , one end of which is connected to said water separator  6  and the other end of which is connected to said air inlet filter  13  and connected to a sprinkler  21  which issues into this air inlet filter  13 , and more particularly between the substrate  17  and the element  18 . 
     In said line  20  between the water separator  6  and the air inlet filter  13 , a controlled valve  22  is introduced. 
     The compressor installation  1  is also provided with a supply device  23  for extra water which in this instance also forms part of said water supply to the air inlet filter  13  and which, in this instance, is connected via a sprinkler  24  to said air inlet filter  13 . 
     Said supply device  23  consists principally of a supply line  25 , for instance for tap water, into which a controlled valve  26  and a water filter  27  are introduced. 
     Said water filter  27  preferably consists of an absolute water filter with a filter rating of 5 micrometres, and whose so-called β value is 1000, in other words whose efficiency is 99.9%. However, if desired, use can also be made according to the invention of a nominal water filter, although the latter&#39;s efficiency is usually lower than that of an absolute water filter. 
     The term β value refers here, as known, to the ratio between the number of particles of a given size which are present in the water flow before filtration to the number of particles of the same size which are present in the water flow after filtration. 
     In this instance, though not necessarily, said supply device  23  for extra water is additionally provided with a water conditioner which ensures that no calcification occurs in the sprinkler  24 . This water conditioner consists in this instance of two powerful permanent magnets  29  and  30  which are set up at an angle of 90° with their south poles pointing towards the spray nozzle of the sprinkler  24 . 
     At least said sprinkler  24  is preferably made of non-ferrous metal (such as brass or CuNi), so that the water in the nozzle is always in the magnetic field and calcification can never occur in this sprinkler  24 . Preferably, one or both sprinklers  21  and/or  24  will take the form of a so-called “flat jet sprinkler” which has a slot-shaped opening rather than a hole in order to prevent any deposits. 
     Preferably, said sprinklers  21  and  24  both feed into the housing  14  of the air inlet filter  13 , and more specifically between the substrate  17  and the element  18 , and are both made of stainless non-ferrous metal. It is also preferable for both of these sprinklers  21  and  24  to have an internal filter to prevent the very fine nozzle from becoming blocked. 
     In a preferred embodiment said sprinklers  21  and  24  are of the type that discharges 0.9 litres of water per hour at a pressure of 7 bar (7000 hPa). Obviously, however, the sprinklers  21  and  24  are not restricted in this respect, and numerous other types can be used. 
     The compressor installation  1  according to the invention is also fitted in this instance with a level regulator which is not shown in the figure; this is connected firstly with a minimum and maximum level sensor, respectively at levels A and B, in the water separator  6 , and secondly with said controlled valves  22  and  26 . 
     The working of a compressor installation according to the invention is very simple and is shown in  FIGS. 2 and 3 . 
     When the compressor element  2  is driven by the motor  3 , air is drawn in through the air inlet filter  13  and via the inlet line  4 ; it is then compressed by the compressor element  2  and conveyed via the outlet line  5  to the water separator  6 ; it is then sent via the minimum pressure valve  11  into the compressed air line  12 . 
     The water that is separated out from the compressed air by means of said water separator  6 , is sent via the return line  7  through the cooling device  8  and is then injected into the compressor element  2  to lubricate and cool the rotating parts of the compressor element  2 . 
     In the situation shown in  FIG. 2 , the water level in the water separator  6  is at a given maximum level A. This is detected by said maximum level sensor and signalled to the level regulator, which reacts by opening the valve  22 , with the result that, under the influence of the pressure build-up in the water separator  6 , water is forced via the line  20  to the sprinkler  21  and injected into the air inlet filter  13 . 
     Water drops which splash downwards humidify the cellular substrate  17  and fall through this substrate  17  to the water drainage line  19 , as a result of which the substrate  17  becomes completely wet. Water drops which splash upwards, are stopped by the element  18  which also has a cellular structure, with the result that only the underside of this element  18  is humidified and water drops are prevented from finding their way into the inlet line  4 . 
     The intake air which enters the air inlet filter  13  via the air inlet  15 , counterflows against the falling drops in the substrate  17 , as a result of which the air is purified and humidified by the water drops. 
     The vast majority of these drops will, together with dust particles which have been collected in the substrate  17 , be conveyed away via the water drainage line  19 , while a portion of these drops evaporates and enters the air flow. 
     The filtered and humidified air flows through the element  18  to the air outlet  16  of the air inlet filter  13  and is then drawn along the inlet line  4  by the compressor element  2 , where, together with the injected water, it is compressed, and the mix of compressed air and water is conveyed via the outlet from the compressor element  2  to the water separator  6 . 
     Via said minimum pressure valve  11  on the water separator  6 , the compressed gas is finally conveyed to the compressed air line  12 -, possibly via a freeze-dryer or cyclone water separator not shown in the figures to which the compressed air outlet on the water separator  6  is connected. 
     If use is made of such a freeze-dryer or cyclone water separator, a return line may be fitted if required, for example to the inlet on the compressor element  2 . 
     If more water vapour is drained away with the compressed air via the compressed air line  12  than enters with the intake air via the inlet line  4 , which thus means that water is being consumed, the water level in the water separator  6  will fall. 
     For example, this is the case in winter, when the absolute humidity of the environmental air is less than or equal to 2 g/kg. 
     In this instance, as shown in  FIG. 3 , as soon as a given minimum water level B is reached in the water separator  6 , the supply device  23  for extra water is activated by the level regulator opening the valve  26 , while the valve  22  in the line  20  between the water separator  6  and the air inlet filter  13  is closed. 
     The extra water that is brought in via the supply line  25  is first of all passed through the water filter  27 , as a result of which any contamination which may be present in this extra water can be filtered out. 
     However, there may also be minerals contained in the supplied water, such as calcium ions. Such ions can flow through the water filter  27  and become deposited in the sprinkler  24 , causing it to become blocked up. 
     To provide a solution to this, use can be made for example of a water conditioner, which ensures that no calcification occurs, and which is preferably of the type described above, which generates a magnetic field which affects the calcium ions in such a way that they are unable to adhere for a certain length of time, with the result that the sprinkler  24  is protected against calcium deposits. 
     The extra water is then injected through the sprinkler  24  into the air inlet filter  13 , where the substrate  17  is humidified in a manner analogous to that described earlier and the air is purified and humidified by the water drops. 
     Any contamination and minerals still present in the injected water cannot evaporate but adhere to the substrate  17  or to the underside of the element  18 . A portion of the contamination particles is dissolved by the water drops and conveyed away to the water drainage line  19 . 
     The element  18  stops the drops in a manner analogous to that described earlier, so that the outgoing air in the inlet line  4  is free of water droplets and only consists of air and water vapour. 
     Through the supply of extra water via the valve  26 , as shown in  FIG. 4 , the water level L in the water separator  6  will rise until the maximum water level A is reached, whereupon the valve  26  in the supply device  23  is closed again and the valve  22  is opened again. 
     By controlling the valves  22  and  26  it is mostly the case that, when there is more water vapour present in the intake air than in the delivered compressed air, the rise in the water level L in the water separator  6  will occur faster than its fall, as shown diagrammatically by the top curve in  FIG. 4 . 
     While extra water is being supplied via the supply device  23  via supply line  25 , there is no continuous loss of water from the water separator  6 . As a result, the water level L in the water separator  6  will rise. 
     As a result of the extra humidification of the intake inlet air by the air inlet filter  13 , the maximum water level A will quickly be reached. As the time during which extra water is conveyed into the air inlet filter  13  is limited, the consumption of water, for example from the mains network, is also limited. 
     Once the maximum water level A in the water separator  6  has been reached, the valve  26  on the supply device  23  will be closed and the valve  22  on the line  20  will be opened. 
     The period during which the valve  22  is open, will in this instance be considerably longer than the period during which it is closed, as is shown in the centre-most curve D in  FIG. 4 , and conversely, as shown in the bottom curve E in  FIG. 4 , the open position of the valve  26  will be maintained for a shorter period than its closed position. The open and closed positions of the valves  22  and  26  are denoted by O (open), and C (closed), respectively. 
     By contrast, if the absolute humidity of the inlet air is lower than the absolute humidity of the delivered compressed air, as is usually the case in extreme winter conditions, there will be more consumption of supplied water via the supply device  23 . 
     As a result, as shown in the centre-most curve F in  FIG. 5 , the period during which the valve  22  is open will be shorter because, in addition to the continuous leakage of water from the water separator  6  to the sprinkler  21 , the continuous water consumption due to the low absolute humidity of the inlet air will also ensure that the minimum water level B in the water separator  6  is reached more quickly, as is illustrated by means of the top curve in  FIG. 5 . 
     By contrast, the period during which the valve  26  is open will, as shown in the bottom curve G in  FIG. 5 , be longer because the continuous consumption of water from the water separator  6  due to the extremely low absolute humidity of the inlet air has to be compensated for. During these periods, the extra water consumption will be higher than during non-winter periods. 
     This will mainly occur in countries which experience periods of freezing weather in the winter. The closer to the equator, the less frequent this situation will occur, since the absolute humidity of the air increases as one approaches the equator. 
     A compressor installation  1  according to the present invention is not restricted to an embodiment as shown in the figures in which said water supply to the air inlet filter  13  is provided by both the line  20  and the supply device  23 : according to the invention it is also possible for the water supply to this air inlet filter  13  to consist of just one of these mechanisms. 
     According to the invention, the possibility is also not excluded of an additional level sensor being fitted in the water separator  6 , at a level below the minimum water level B and connected with a control mechanism which controls said motor  3 . In this way, where there is extremely high water consumption and the water supply is insufficient to compensate for the consumption or in the event of a fault in the minimum level sensor, the compressor element  2  can be switched off. 
     In all the embodiments, problems of deposition of minerals and salts in the screw compressor element  1  are avoided in a simple and inexpensive way and better volumetric efficiency is obtained. The water quality in the circuit remains constant and is thus not altered by the extra supply of water when water is being consumed. 
     The quality of the extra supplied water is of no importance, as only water vapour and hence chemically pure water ends up in the screw compressor element  1 . This can be very important when the compressor installation is located in places where no pure water is available. 
     By a special characteristic of the invention, the compressor installation is not provided with an inlet valve or so-called “unloader” to absorb any water hammer which occurs when the compressor is stopped. 
     The invention is by no means limited to the embodiments described and shown in the figures in the foregoing: such a compressor installation with a water-injected compressor element may be produced in a number of different variants without going beyond the scope of the invention.