Abstract:
A pressure independent control valve contains a valve body with an inlet, an outlet and a flow channel coupling the inlet to the outlet. A hollow piston is arranged in a seat in the valve body, such that the hollow piston is configured to move. The hollow piston has an enclosure, such that the pressure independent control valve maintains different fluid pressures in the flow channel and inside the hollow piston. The pressure independent control valve contains a chamber and a biasing member to urge the hollow piston towards the chamber. The chamber is in fluid communication with the inlet and with the inside of the hollow piston, such that the valve applies substantially the same pressure inside the annular channel, at the inlet and inside the hollow piston.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of European application No. EP 14165902.9, filed Apr. 24, 2014; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an improved control valve. The present disclosure focuses on a control valve wherein the flow of a fluid is a function of the position of a throttle. More particularly, the control valve as disclosed herein achieves a flow rate that is substantially independent of the pressure at the outlet of the valve. 
     Flow control valves are commonly employed in HVAC (heating, ventilation, air conditioning) systems of buildings. These systems typically circulate a fluid such as water through a plurality of conduits in order to provide heating or cooling. The purpose of a flow control valve is to achieve a controlled flow of a fluid through the conduits of the system. 
     The amount of water flowing through the valve is essentially governed by the position of the throttle. A separate flow meter measuring the flow of water through the HVAC system may thus be dispensed with. 
     The amount of delivered energy is then calculated as the throughput of the fluid multiplied with the temperature drop in the system. The flow of water is determined from the position of the throttle and the temperature drop is measured separately. In the context of HVAC systems, the amount of energy is frequently measured in kWh. 
     U.S. Pat. No. 7,128,086 B2 was granted in 2006 and discloses a flow control valve. The valve according to U.S. Pat. No. 7,128,086 B2 contains a hollow piston  110  movable along an axis X 1 . A spring  160  exerts a force on the hollow piston  110  in the direction of the same axis X 1 . A rolling diaphragm is arranged on one side of the piston  110 . The rolling diaphragm is connected to the hollow piston  110  and separates an annular channel  109  from the inside of the hollow piston  110 . The annular channel  109  is in fluid communication with the inlet  106  of the valve through a reference passageway  180 . The inside of the hollow piston  110  is in fluid communication with the outlet  108  of the valve through apertures  192  of the hollow piston  110 . The valve also contains a channel that circumferentially surrounds the hollow piston  110  and is in fluid communication with the flow channel  104  of the valve. 
     The pressure in the annular channel  109  of this arrangement is the pressure p 1  at the inlet  106  of the valve. Similarly, the pressure inside the hollow piston  110  equals the pressure p 3  at the outlet  108  of the valve. The pressure p 2  in the chamber surrounding the hollow piston  110  is the same as the pressure inside the flow channel of the valve  104 . 
     The hollow piston  110  may move under the influence of the pressures p 1 , p 2 , p 3  and under the influence of the spring  160 . As soon as the corresponding forces are balanced, the difference between the pressures p 1  at the inlet and p 2  inside the flow channel predominantly determines the flow rate through the valve. The influence of the pressure p 3  at the outlet  108  of the valve is largely eliminated. 
     The arrangement as disclosed by U.S. Pat. No. 7,128,606 B2 requires an element  118  for guidance of the axial movement of the piston  110 . The piston guide needs to be mounted to the valve body and a seal  130  is necessary to separate the annular channel  109  from the inside of the hollow piston  110 . The seal  130  and the rolling diaphragm separate the annular channel  109  with the highest pressure p 1  from the inside of the piston  110  with the lowest pressure p 3 . The stresses on the seal  130  and on the rolling diaphragm are particularly high along its second convolution  138 . The piston  110  is movable against the guide  118 . Due to the stresses on the seal  130  and on the rolling diaphragm, an adequate choice of materials for these highly stressed parts becomes challenging. 
     The gap in between the rim  117  of the guide  118  and the sleeve  114  of the piston  110  needs to be narrow in order to prevent transverse movement of the piston  110 . Yet the fluid from the inside of the hollow piston  110  must reach the space in between the rim  117  and the second convolution  138 . The second convolution  138  will otherwise not be exposed to the pressure drop between the p 1  and p 3 . Extra design measures will be required to overcome the conflicting requirement of precise guidance through the rim  117  and of full pressure drop across the second convolution  138 . 
     The aim of the present disclosure is at least to mitigate the aforementioned difficulties and to provide a flow control valve that meets the aforementioned requirements. 
     SUMMARY OF THE INVENTION 
     The present disclosure is based on the discovery that technical constraints on a seal adjacent to a piston can be relaxed through an adequate pressure concept. The valve disclosed herein is configured such that the pressure inside the piston is the same as the pressure of an annular channel adjacent to the piston. This measure mitigates the difficulties involved in configuring a seal in between the annular channel and the piston. Further, the pressure concept of the present disclosure avoids extra measures to ensure an even distribution of pressure around a guide element. 
     The above problems are resolved by a pressure independent control valve according to the main claim of this disclosure. Preferred embodiments of the present disclosure are covered by the dependent claims. 
     It is a related object of the present disclosure to provide a pressure independent control valve wherein friction between the movable piston and the guide element is minimized. 
     It is another related object of the present disclosure to provide a pressure independent control valve wherein any hysteresis affecting the movement of the piston is negligible. 
     It is yet another related object of the present disclosure to provide a pressure independent control valve wherein a throttle controls the fluid throughput through the valve to the point where an additional flow meter can be dispensed with. 
     It is another object of the present disclosure to provide a pressure independent control valve configured for measuring a temperature drop across the valve. 
     It is yet another object of the present disclosure to provide a heating, ventilation and air-conditioning system with a pressure independent control valve according to this disclosure. 
     It is another object of the present disclosure to provide a building with a heating, ventilation and air-conditioning system comprising a pressure independent control valve. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a pressure independent control valve, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic, sectional view of a pressure independent control valve according to the invention; and 
         FIG. 2  is a graph showing a fluid throughput versus a pressure difference. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawings in detail and first, particularly to  FIG. 1  thereof, there is shown various principal and optional components of a pressure independent control valve as per this disclosure. 
     The pressure control valve contains a valve body  1  with openings forming an inlet  2  and an outlet  3 . The inlet  2  and the outlet  3  allow a flow of a fluid through the valve. In a preferred embodiment, the fluid is a liquid. In a particularly preferred embodiment, the fluid flowing through the valve is water or a mixture containing water. 
     A flow channel  4  is arranged along the fluid path and in between the inlet  2  and the outlet  3 . At the inlet  2  of the valve, the fluid has a pressure of substantially p 1 . The pressure of the fluid at the outlet  3  of the valve is substantially p 3 . The (overall) pressure of the fluid inside the flow channel  4  substantially is p 2 . 
     A throttle  5  is movably mounted inside a seat  27  in between the inlet  2  and the flow channel  4 . The position of the throttle  5  may change by moving a stem  6  back and forth along the direction indicated by arrow  7 . In a particular embodiment, the stem  6  is rotatable around the axis indicated by the arrow  7 . In an alternate embodiment, the stem  6  is not rotatable around the axis indicated by the arrow  7 . 
     The throttle  5  effectively varies and limits the flow of the fluid through the pressure independent control valve. To that end, the body of the throttle  5  is permeable to the fluid. 
     A bearing  8  restricts the movement of the stem  6  against the valve body  1 . Accordingly, the walls of the valve body surrounding the throttle  5  and the bearing  8  act as guide elements for the throttle  5 . 
     The bearing  8  may be of the ball-bearing type and/or of the friction-bearing type. It is envisaged that the bearing  8  also seals the pressure independent control valve, so that no fluid will leak from the valve. 
     A hollow piston  9  is movably mounted inside another seat in the valve body  1 . The hollow piston  9  has a cover  10  that is exposed to the pressure p 2  in the flow channel  4 . It is envisaged that the shape of the cover may be uneven or may be substantially flat. Those parts of the hollow piston  9  that are exposed to the pressure p 2  inside the flow channel  4  are impermeable to fluid. Consequently, no fluid coming from the flow channel  4  will enter the hollow piston  9 . 
     It is envisaged that the cross-section of the hollow piston  9  may be circular, oval, triangular, quadratic, rectangular. The cross-section of the hollow piston may actually have any shape  9  that technically makes sense. 
     Any movement of the hollow piston  9  is restricted by the seat in the valve body. Preferably, the seat for the hollow piston  9  effectively restricts the movement of the piston  9  to directions towards or away from the throttle  5 . The walls of the seat may hold the hollow piston  9  either through a friction-type bearing and/or through a ball-bearing. It is envisaged that the bearing will allow essentially no fluid to flow through the passage in between the hollow piston  9  and the walls of the seat in the valve body  1 . It is also envisaged that the same bearing is optimized for low friction and/or for minimum hysteresis. 
     The pressure independent control valve contains a further guide element  11  for the hollow piston  9 . The guide element  11  is arranged opposite to a cover  10  and penetrates a bore through the hollow piston  9 . The bore through the hollow piston  9  provides a sleeve  12  that is substantially parallel to the wall of the guide element  11 . The sleeve  12  and the guide elements  11  essentially form a bearing. This bearing may be of the ball-bearing or of the friction bearing type. The passage between the guide element  11  and the sleeve  12  needs not be fluid-tight. It is envisaged that the bearing formed by the sleeve  12  and the guide element  11  is optimized for minimum friction and/or for minimum hysteresis. 
     The sleeve  12  and the guide element  11  restrict the movement of the hollow piston  9  in the same manner as the aforementioned seat in the valve body  1 . It follows that technical constraints as the accuracy of guidance either through the sleeve  12  or through the seat in the valve body  1  may be relaxed to some extent. 
     The guide element  11  is surrounded by a biasing member  13 . In a preferred embodiment, the biasing member  13  is a spring. In a yet more preferred embodiment, the biasing member  13  is a helical spring, in particular a helical compression spring. The biasing member  13  is mounted to an end  14  of the guide element  11 . In a preferred embodiment, the guide element  11  provides a head  14  with a substantially flat surface that compresses the biasing member  13 . 
     An annular channel  15 , in general terms a reservoir  15 , is arranged adjacent to the hollow piston  9 . The annular channel  15  is in fluid communication with the inlet  2  of the pressure independent control valve through a passageway  16 . The annular channel  15  is also in fluid communication with the inside of the hollow piston  9 . The inside of the hollow piston  9  and the reservoir  15  in this context form a chamber. The hollow piston  9  is in general terms a displaceable element  9  or part of a displaceable element that separates the chamber from the flow channel  4 . According to a particular embodiment, the displaceable element provides no holes, orifices or apertures that allow the chamber to be in fluid communication with the flow channel  4 . In other words, the displaceable element provides a simply connected surface within the topological meaning of the term simply connected. 
     One or several apertures  17  are located in the wall of the hollow piston  9  that separates the annular channel  15  and the inside of the hollow piston  9 . Since the inlet  2 , the hollow piston  9 , and the annular channel  15  are all in fluid communication, these parts ( 9 ,  15 ,  2 ,  16 ,  17 ) are exposed to substantially the same pressure p 1 . 
     A rolling diaphragm  18  contributes to separating the pressure p 1  inside the annular channel and the pressure p 2  inside the flow channel  4  of the valve. The rolling diaphragm  18  provides a seal in addition to the aforementioned bearing formed by the hollow piston  9  and the seat in the valve body  1 . In a preferred embodiment, the presence of the two seals implies that the technical constraints for each of the two seals may be relaxed to some extent. If the sealing effect of the rolling diaphragm  18  is sufficient, the interface between the piston  9  and the valve body  1  may be permeable to some extent. Consequently, a ball bearing may be arranged in between the hollow piston  9  and the valve body  1 . The arrangement will then experience even less friction and/or less hysteresis as the hollow piston  9  moves. 
     The rolling diaphragm  18  may be made of any suitable flexible material. In particular embodiments, the rolling diaphragm  18  is made of rubber and/or fabric coated rubber and/or biaxially-oriented polyethylene terephthalate (MYLAR®) and/or polyester film and/or metal foil. 
     During operation, the pressure p 1  will exert a force to drive the hollow piston  9  towards the throttle  5 . The biasing member  13  will urge the piston  9  in the opposite direction away from the throttle  5 . A width of a gap between a rim  28  and (the cover  10  of) the piston  9  is thus allowed to vary to some extent. The amplitude of the movement of the hollow piston  9  depends on the pressure difference between the inlet  2  and the flow channel  4 . 
     The position of the throttle  5  relative to its seat  27  and position of the hollow piston  9  relative to the rim  28  determine the throughput of fluid through the valve. These positions are substantially independent of outlet pressure p 3 , so that the valve achieves a flow rate which is essentially independent of outlet pressure p 3 . The same is indicated on  FIG. 2 , where typical fluid throughput (axis  21 ) is plotted versus pressure difference (axis  22 ). The flow of fluid is essentially constant on the right hand side of a pressure difference  23 . 
     Preferably, the piston  9  provides a surface  10  to separate the chamber from the flow channel  4  and the same surface is larger than the corresponding surface provided by the diaphragm  18 . In a yet more preferred embodiment, the area of the separating surface  10  of the piston  9  is at least twice the separating surface of the diaphragm  18 . In a yet more preferred embodiment, the area of the separating surface  10  of the piston  9  is at least five times larger than the area of the separating surface of the diaphragm  18 . 
     In a particular embodiment, the pressure independent control valve also contains an adjusting bolt  19 . The adjusting bolt  19  connects to a head  14  of the guide element  11  via a telescopic stem  20 . By turning the bolt  19  it is possible to adjust the position of the head  14  of the guide element  11 . Since the head  14  also connects to the biasing member  13 , the bolt  19  can be used to adjust the bias applied by the member  13 . 
     The bolt  19  is employed to alter the balance between the pressure inside the piston  9 , the pressure in the flow channel  4  and the force applied by the biasing member  13 . An adjustment of the bias applied by the member  13  has an effect on the maximum throughput of fluid through the valve. The flow of fluid through the valve will depend on the gap between the hollow piston  9  and the rim  28 . By altering the balance of pressures and forces inside the valve, this gap will also change. Consequently, an adjustment of the bias will affect the maximum flow of fluid through the pressure independent control valve. Arrow  24  on  FIG. 2  indicates possible changes in the rate of fluid flow due to an adjustment of bias. 
     Actually, the flow of fluid through the valve is independent of outlet pressure p 3  as soon as the pressure difference between input  2  and output  3  exceeds a threshold. Any difference between p 1  and p 2  is limited to the difference between p 1  and p 3 . The pressure difference p 1 −p 2  between the inlet  2  and the flow channel  4  cannot exceed that value. If the difference between p 1  and p 2  becomes too small, the flow of fluid through the valve will depend on the pressure difference between inlet p 1  and outlet p 3 .  FIG. 2  illustrates this regime as a line  25  with positive slope. 
     As soon as the pressure difference  22  reaches the onset  23  of constant flow, the throughput of fluid through the valve will essentially be independent of outlet pressure p 3 . By changing the position of the adjusting bolt  19 , the pressure difference required to achieve constant flow will also change. 
     An adjustment of the onset  23  of constant flow and of maximum throughput offers distinct benefits where pressure independent control valves need be accurate within certain limits. This is often the case in applications where a control valve renders a separate flow meter obsolete. Pressure independent control valves are then required to produce constant flow over a given range of pressure differences. Constant in this context means that the flow of fluid through the valve is determined by the position of the throttle  5 . 
     In yet another embodiment, a pressure independent control valve provides a plurality of temperature sensors to determine temperature drop. The temperature sensors can, for instance, be arranged at the inlet and/or at the outlet of the valve. This particular embodiment is particularly useful for metering. 
     By changing the position of an adjusting bolt  19 , the onset of constant flow and hence the useful range of pressure differences of a control valves is set. Likewise, the maximum throughput of fluid through a valve will affect accuracy. Also, for a given building the maximum flow of fluid will depend on the characteristics of the HVAC system employed in that building. The adjusting bolt  19  thus allows a pressure independent control valve to be adapted to the particular HVAC system of a building. 
     It should be understood that the foregoing relates only to certain embodiments of the invention and that numerous changes may be made therein without departing from the spirit and the scope of the invention as defined by the following claims. It should also be understood that the invention is not restricted to the illustrated embodiments and that various modifications can be made within the scope of the following claims. 
     The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 
     REFERENCE NUMERALS 
     
         
           1  valve body 
           2  inlet 
           3  outlet 
           4  flow channel 
           5  throttle 
           6  stem 
           7  arrow indicating possible movements of the stem  6   
           8  bearing surrounding the stem  6   
           9  hollow piston 
           10  cover 
           11  guide element 
           12  sleeve 
           13  bias element 
           14  head 
           15  annular channel 
           16  passageway 
           17  aperture 
           18  rolling diaphragm 
           19  adjusting bolt 
           20  telescopic stem 
           21  axis for the flow rate through the valve 
           22  axis for the pressure difference 
           23  onset of constant flow 
           24  variation of maximum flow 
           25  proportional regime of flow rate versus pressure difference 
           26  variation of onset of constant flow 
           27  seat of the throttle  5   
           28  rim