Abstract:
A liquid management system for supplying or receiving liquid at a controlled pressure, comprising: a closed reservoir having an inlet for receiving liquid from a first remote location and an outlet for supplying liquid to a second remote location; and a pumped outlet disposed in the reservoir and arranged to remove liquid and gas contained within the reservoir, the pumped outlet being disposed such that the level of liquid in the reservoir can be maintained at a constant height.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 14/408,734, filed on Dec. 17, 2014 which is a national phase application of International Application No.: PCT/GB2013/051717, filed on Jun. 28, 2013, which claims priority to GB 1211573.9, filed Jun. 29, 2012, all of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a liquid management system for supplying or receiving liquid at a controlled pressure, typically for use in devices such as a drop-on-demand printer or a spray head for use in aerosol generation, coating or the like. 
     BACKGROUND 
     In particular, the present invention relates to a liquid management system that enables the pressure of the supplied liquid to be controlled in order to, for example, prime a liquid delivery device, and/or in which the supply of liquid can be provided at a controlled pressure to a liquid ejection location. The liquid may have solid particles suspended or dispersed within it or have other additives added to it, but in all cases the end result is a fluid that behaves substantially like a liquid. Further, the present invention relates to a liquid management system for supplying or receiving liquid comprising heavily sedimenting particles, such as glass frit and/or ink pigment or other solids not well dispersed in the liquid. 
     In various liquid delivery devices such as inkjet printers or spray heads, it is necessary to achieve a consistent ejection of liquid from the liquid delivery device and/or in order to do so, precise control of the static pressure of liquid is required at the ejection location. Precise control of the liquid flow may also be required. Printheads of the type requiring the properties above are described in EP 1224079 and EP 1366901, for example. Other devices which have similar requirements are disclosed in, for example, EP1071559. EP 2076395 teaches that the pressures at the printhead described in EP 1224079 and EP 1366901 need to be corrected to about + or −20 Pa and those periodic variations must be below about + or −2 Pa to eliminate visible variations in print quality. Likewise, printheads of other designs will be able to tolerate pressure fluctuations and fluid flow rates) dependant upon their design. 
     A simple method of controlling the pressure of the liquid supply to a liquid delivery device, such as a printhead, is to use gravity. A liquid reservoir, whereby the surface of the liquid is open to atmospheric pressure, is mounted either above or below the level of the printhead in order to generate a positive or negative liquid pressure, as required by the printhead. The required inlet pressure in the ejection location can be set by mechanically adjusting the relative height of the liquid reservoir with respect to the printhead. The reservoir may also be supplied with liquid by a pump. 
     Some liquid delivery devices require ink to flow continuously through the device and this requires the device to have both an inlet and outlet to allow ink to flow in and out of the device. In these devices, the pressure of the ink at this outlet can also be controlled by gravity by allowing ink to flow to atmospheric pressure from the outlet tube to a defined level below the printhead. This level can also be mechanically adjusted to achieve the correct operating conditions (such as ink pressure and flow rate) at the ejection location. 
     As described in EP 2076395, known disadvantages of a gravity-fed ink supply system (which can be generalised to a liquid supply system) are:
         Changing the pressures requires physical movement of the reservoirs.   The location of the reservoirs is determined by the required pressures.   A large volume of space may be required to accommodate the total adjustable range of the reservoirs.   Priming printheads with ink can be assisted by supplying ink at pressures that are very different from the pressures required during printing.   With a gravity fed system a large amount of space and typically a significant amount time is required to move the reservoirs to achieve these pressures.   The surface of the ink must be open to the atmosphere, increasing the risk of dust or other contaminates polluting the ink.       

     WO 97/441914 and EP 1092548 each describe ink supply systems in which the ink surface is maintained at a constant level or height in the reservoir by use of a weir. Such a system is also described in WO 2006/030235. Such systems can either use gravity to set the pressure of the ejection location or, in the case of WO 2006/030235, the pressure of the ink at the inlet and outlet of a nozzle containing fluid supply apparatus is controlled by controlling the pressure of air above or with air at the inlet and the outlet from the nozzle containing fluid supply apparatus. In order to maintain the functioning weir it is necessary to remove the ink that has flowed over the weir from the reservoir. 
     EP 2076395 describes a further system in which the ink is maintained at a constant height in the reservoir by use of a weir. In this system, ink is pumped continuously from a remote ink tank to two reservoirs, one placed just before the printhead in the fluidic circuit and one just after. The pressure of the fluid in the reservoirs is controlled such that the ink flows through the printhead at a controllable pressure and flow rate. 
     In EP 2076395, it is claimed that it is convenient to measure the pressure in the local reservoir by using a gas pressure sensor mounted above the ink level in the reservoir. Therefore, to control the pressure of the ink in the reservoir based on this pressure management, it is important that the depth of the ink in the reservoirs is kept constant. 
     EP 2076395 uses a weir over which the excess ink pumped into the reservoir flows in order to keep the fluid at a constant height on at the upstream side of the weir. The fluidic path to and from the printhead comes from this ink stored at the upstream side of the weir. The ink that flows over the weir is pumped back to the remote ink tank via a return pump. This return pump is over driven, such that it sucks some air in addition to ink out of the reservoir, thus creating a slight vacuum in the reservoir. The gas pressure sensor and proportional valve are operated in a feedback loop in order to let air leak into the reservoir (usually at atmospheric pressure from outside the reservoir, or alternatively from a positive or negative pressure reservoir) at a rate sufficient to enable the fluidic pressure in the reservoir to stabilise at a user controllable set pressure. 
     However, when using liquids which comprise heavily sedimenting or poorly dispersed particulates, such a system described in EP 2076395 has a number of drawbacks. These include:
         the presence of a weir (especially in the configuration shown in EP 2076395) creates a flow pattern that leads to areas where the flow of liquid is sufficiently low to allow the particles in the liquid to start falling out of suspension. This changes the composition of the liquid such that the liquid delivered to the head is different from that desired. The sediment may also start to fill the reservoir, disrupting or blocking the fluid flow.   A heavily sedimenting liquid typically requires a higher flow rate than non-sedimenting liquid through the printhead and local reservoirs so as to prevent sedimentation when liquid is supplied to a system such as that in EP 2076395, turbulence is created in the reservoir such that the height of the liquid surface above the bottom of the reservoir becomes unstable. Turbulence itself also causes unpredictable variations in fluidic pressure. This in turn causes the pressure of the liquid supplied to head to fluctuate and become difficult to control, even with the feedback system described above.       

     The present invention addresses one or more of the problems identified above. 
     SUMMARY 
     According to the present invention, there is provided, a liquid management system for supplying or receiving liquid at a controlled pressure, comprising:
         a closed reservoir having an inlet for receiving liquid from a first remote location and an outlet for supplying liquid to a second remote location; and   a pumped outlet disposed in the reservoir and arranged to remove liquid and gas contained within the reservoir, the pumped outlet being disposed such that the level of liquid in the reservoir can be maintained at a constant height.       

     The pumped outlet may include a tube which extends into the reservoir. The tube may be substantially horizontal or substantially vertical within the reservoir. The tube may have a tapered opening within the reservoir. The pumped outlet may be an opening in a side wall of the reservoir. The inlet may be located above or below the pumped outlet depending upon the requirements of the system. 
     Means may be provided for controlling a pump attached to the pumped outlet such that the pressure within the reservoir is controlled. The system may further comprise an additional pump arranged, in use, to pump gas into or out of the reservoir. Means for controlling the additional pump may be provided such that the pressure within the reservoir is controlled. 
     An orifice may connect the reservoir to a gas at above, below, or at atmospheric pressure configured to bleed gas, in use, into or out of the reservoir. Means may be provided for controlling the orifice such that the pressure within the reservoir is controlled. 
     The height of the pumped outlet may be fixed or may be variable. 
     A liquid delivery system may include a liquid management system as described above and may further include a liquid delivery device supplied with liquid from the liquid management system. 
     The liquid delivery device may be a printhead or a sprayhead or another liquid delivery device. 
     The liquid delivery device may be the first or the second remote location. 
     A liquid delivery system may include two liquid management systems as described above, wherein one system supplies liquid to a liquid delivery device and the other system receives liquid from the liquid delivery device, thereby controlling the pressure of the liquid supplied to the liquid delivery device and the pressure of the liquid removed from the liquid delivery device, such that the liquid flows through the liquid delivery device at a controlled rate and at a controlled pressure. 
     In a further aspect, the present invention provides a liquid management system for supplying or receiving liquid at a controlled pressure comprising:
         a reservoir having an inlet for receiving liquid from a first remote location and an outlet for removing liquid from the reservoir,   the reservoir having a sloped bottom surface defining, at the lower end thereof, an apex, the inlet or the outlet being located adjacent the apex.       

     The inlet may be located adjacent the apex. The inlet may be pumped to supply liquid into the reservoir. The reservoir may be an open or a closed reservoir. 
     The outlet may be a pumped outlet to a second remote location. The pumped outlet may be at a fixed height within the reservoir. The outlet may be located adjacent the apex. 
     The system may further comprise a re-circulating fluid system connected to the outlet for re-circulating fluid back into the reservoir. 
     A liquid extraction outlet may be provided for supplying liquid to a second remote location. 
     The reservoir may include at least two chambers. One or more of the chambers may have a sloped bottom surface. 
     In another aspect, the present invention provides a liquid management system for supplying or receiving liquid at the controlled pressure comprising:
         a reservoir having an inlet for receiving liquid from a first remote location and an outlet for supplying liquid to a second remote location; and   at least one baffle located between the inlet and the outlet in the reservoir for breaking up the inlet flow.       

     The baffle may comprise one or more sloped surfaces. A plurality of baffles may be provided. The plurality of baffles may be in a substantially planar array or may be in a staggered arrangement. The plurality of baffles may be provided in two or more rows. 
     A pumped outlet may be provided from the reservoir, the pumped outlet being disposed such that the level of the liquid in the reservoir can be maintained at a constant height. 
     The uppermost part of one or more of the baffles may be located below the pumped outlet. 
     The skilled person would readily appreciate that features of the different aspects of the invention could be combined, even if not explicitly recited. For example, the sloped bottom to the reservoir could be incorporated in a system with baffles or in a system with a fixed height pumped outlet, or indeed the baffles could be incorporated into the fixed height pumped outlet arrangement as well. As such, unless otherwise explicitly excluded, any of the preferred features of any aspect of the invention disclosed herein can be incorporated into any of the separate aspects of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various examples will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic cross-section of one example of a system; 
         FIG. 2  shows a cross-section view of a further example of a system; 
         FIG. 3  shows a variation on the arrangement shown in  FIG. 1 ; 
         FIG. 4  shows a further example of a system having a sloped bottom to the reservoir; 
         FIG. 4 a    shows another example of a system having a sloped bottom surface; 
         FIG. 5  shows a variation on the arrangement of  FIG. 4 ; 
         FIG. 6  shows an example of a system utilising both a weir and sloped reservoir bottom; and 
         FIG. 7  shows a further example of a system using a baffle. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a liquid reservoir  10  which is supplied with liquid  1  from a remote location (not shown) through an inlet pipe  11 . Liquid exits the reservoir via an outlet pipe  12  to a liquid delivery device (not shown). In this example, the liquid delivery device could be a printhead, in which case the liquid is typically an ink, a sprayhead, in which case the liquid could be any suitable sprayable liquid, or any other aerosolising liquid delivery device. The liquid is typically a suspension in which sedimenting or poorly dispersed particles are included, although this is not a requirement. 
     The reservoir is provided with a further outlet  13 . The outlet  13  is a pumped outlet which is disposed at a fixed height within the reservoir. The outlet  13  is connected to a pump (not shown) such that, when the pump is operational, excess liquid and/or air from the reservoir  10  is drawn through the outlet  13  and removed from the reservoir  10 . In this way, the outlet  13  ensures that the height of the liquid  1  in the reservoir  10  remains constant, as the height can never be above the outlet  13 . Whilst in the preferred example the outlet  13  is at a fixed height, it is conceivable that the height of this outlet could be variable, such that the user can define the height of fluid within the reservoir  10 . Such a variation would typically be only carried out prior to use, so as to set the parameters of the system. 
     The air pressure in the reservoir  10  above the surface of the liquid is also controlled and can be measured by a pressure sensor  14 . Alternatively and/or additionally, a liquid pressure sensor could be employed. Air can either be bleed into or out of the reservoir  10  through bleed valve  15 , which can be supplied with air at any given pressure or it can be pumped in or out of the reservoir by a pump (not shown). The air pressure above the surface of the liquid can be controlled and set at a desired set point by controlled electronics (not shown) or programmed via a computer (not shown). Although air is described in this example, any other suitable gas could be used. 
     The reservoir can be configured such that the air pump (not shown) is not required to control the air pressure above the surface of the liquid. In this example, the rate of pumping from the outlet  13  is greater than the rate at which liquid is supplied into the reservoir  10  and therefore both liquid and air will always be pumped out of the reservoir  10 . This will reduce the pressure of air in the reservoir  10  and this can then be controlled by bleeding air through the air bleed valve  15  into the reservoir  10  in order to maintain the pressure at the desired set point. The pump connected to the pumped outlet  13  returns the excess liquid back to a main liquid reservoir (not shown) which can then be used to supply liquid to inlet  11 . 
     An alternative example is shown in  FIG. 2  in which the vertically aligned outlet  13  from  FIG. 1  is replaced by a fixed height outlet located in a side wall of the reservoir  10 . The outlet, which may take the form of a tube, may extend into the first reservoir  10  or may simply be an opening in the side wall. In this arrangement, the height of the outlet is fixed and cannot be varied. A further example, of the system of  FIG. 1  is shown in  FIG. 3 , which is identical save for the lower end of the outlet  13 . In this example, the lower end of the outlet  13  has been cut away on a diagonal  16 , thereby creating a tapered opening. Such a tapered opening reduces pressure fluctuations caused by fluid pinning to the tube opening. In this arrangement, the height of the fluid is defined by the highest portion of the cut away at the end of the outlet tube. 
     In all three systems shown in  FIGS. 1 to 3 , the inlet  11  is shown below the surface of the liquid  1 . This can be advantageous if it is desired to prevent turbulence that causes pressure fluctuations and bubble formation in a fluid. Alternatively, given the particular use of the invention, with liquids that have heavily sedimenting or poorly dispersed particles therein, it may be advantageous for the inlet to be located above the height of the liquid, such that the flow of liquid into the reservoir promotes mixing of the liquid that keeps the particles suspended. The optimum location for the inlet will be dependent upon the flow rate and subsequent level of turbulence and surface disruption and therefore the amount of pressure control that is required by the system. 
     A further example is shown in  FIG. 4  having a reservoir  30 , an inlet  31  and a pumped outlet  32 . An outlet to the remote location, such as a printhead or sprayhead, is not shown, but is contemplated. Further inlets may also be provided. The pumped outlet  32  is shown in a similar manner to that of  FIG. 1 , but it can, alternatively, take the configuration shown in either  FIG. 2  or  FIG. 3 . Again, an air pressure sensor  33  and proportional bleed valve  34  are provided for the same purposes as described in relation to  FIG. 1 . 
     In this example, the lower surface  36  of the reservoir is sloped to define an apex  35  at which the inlet  31  is located. The sloped surface of the reservoir may take the form of a cone or pyramid, but may also take the form shown in  FIG. 4 a    in which the lower surface  36  is either a simple slope, i.e. planar surface which is angled relative to the horizontal, or alternatively a v-section channel which directs any sedimenting or poorly dispersed particles to an apex. The reservoir is typically circular or square in cross section, although other cross sections are possible. 
     By virtue of the arrangement shown any sediment that does fall out of suspension drops towards the inlet  31  under gravity, at which point the sediment can be captured and re-suspended by the inlet flow, ensuring that the liquid composition remains constant in the bulk of the reservoir. 
     An alternative configuration is shown in  FIG. 5  in which the reservoir  40  is open and, whilst a sloped bottom  36  is provided in accordance with any of the variations discussed above, the apex  35  is provided with an inlet  41  which connects to a pump  42  and a re-circulating fluid system  43 . A liquid supply line  44  is provided to supply liquid into the reservoir from a remote location. This may be above the level of the liquid as shown, or may be below as in other examples disclosed herein. The reservoir is provided with a liquid outlet  45  through which liquid is supplied to a remote location. 
     The arrangement shown in  FIG. 5  helps to keep the particles in suspension by capturing and re-circulating any particles that sediment at the bottom and by creating further agitation in the main tank at the point of return of the flow into the reservoir. Also, the liquid supplied through supply line  44  causes the bulk fluid in the reservoir to become agitated. 
     The provision of one or more sloped bottoms to a reservoir can be applied, as shown in  FIG. 6 , to an arrangement similar to that disclosed in EP 2076395. In this case, one or each chamber of the reservoir  60 , separated by weir  63  can be provided with a sloped bottom  67  having any of the forms described above. The system has a first chamber  61  and a second chamber  62  and the first chamber  61  is provided with an inlet  64  located at the apex of the bottom of the first chamber and an outlet  65 . The second chamber  62  is provided with a pumped outlet  66  at the apex of the sloped bottom of the second chamber  62 . Bleed valve  68  and pressure sensor  69  are provided as in the other examples 
     A further example is shown in  FIG. 7  which is, for the purposes of the description, the same system as that shown in  FIG. 1 . The only difference is the provision of a baffle  70  in the reservoir  10 . However, the use of one or more baffles could be employed in any of the configurations described above. One or more baffles may be provided and they may be provided in any suitable configuration. The purpose of the baffles is to prevent any liquid that may form splashes from impacting on the pressure sensors and proportional valves placed in the reservoir as part of the pressure control apparatus, to break up the flow and to divert it such that any turbulence has minimal effect on the surface of the liquid and therefore the depth of the liquid from the reservoir, and also to smoothly separate the relatively high velocity liquid emerging from the liquid supply to the reservoir. One or more of these advantages may be achieved depending upon the particular configuration of the system and the location or locations of the or each baffle. 
     As can be seen in  FIG. 7 , the baffle is provided with sloped surfaces  71  which assist disrupting the flow through inlet  72  such that turbulence is created. Further, it discourages any sediment from accumulating on the top surfaces. The location of the or each baffle is important so as to ensure that static regions of the flow are not created, for example, eddys or other regions of low flow, which might mean that heavier particles could start to fall out of suspension, thereby affecting the composition of the liquid supplied from the reservoir. 
     Further features may be applicable to any or all of the examples described. These include:
         A filter placed in line with the re-circulating fluid of either the main reservoir in the supply or return line so as to continuously remove any unwanted particles from the liquid.   The pump selection is very important. The pumped overflow design only works with pumps that can pump gas and liquid simultaneously, such as positive displacement pumps, but many of these are very pulsatile, such as diaphragm pumps or peristaltic pumps. Many pumps exhibit relatively low pulsatility cannot handle sedimenting fluids very easily, such as gear pumps. Therefore, in order to pump sedimenting fluids, it may be necessary to select a pulsatile pump and create fluidic damping in a system to aid the active feedback pressure compensation that is present. This can include the use of dampers designed specifically for the pump by the manufacturer, or other well known passive dampening techniques such as increasing the volume of air above the fluid in the reservoirs.   It is typically advantageous to use a pinch valve with sedimenting liquids, as this minimises the chance of the particulates interfering with or damaging the operation of a valve.   Reversing the pump supplying fluid into the main reservoir may allow the system to be drain efficiently allowing the majority of liquid to be recovered to the main reservoir.