Patent Publication Number: US-2002007736-A1

Title: Deaerator

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a deaerator and to a lubrication system including such a deaerator.  
       [0003] 2. Description of Related Art  
       [0004] Oil may be used within a single machine for many purposes. In the context of a constant speed generator within the avionics environment, the oil may be used to lubricate bearings and other rotating parts, to act as a coolant within the generator, and may also act as a control fluid within a speed conversion system used to ensure that a variable input speed is converted to a near constant generator speed. In general, it is desirable that the oil contains little entrained air since air bubbles are compressible where as oil is not. This becomes particularly important when oil is being used as a pressurized control fluid since the presence of air bubbles within an actuator system can seriously degrade the system&#39;s control performance.  
       [0005] Deaerators are known in the prior art. U.S. Pat. No. 5,085,677 discloses a deaeration device in which a single cylindrical chamber is arranged vertically and has a tangential inlet duct in its lower portion for the introduction of pressurized oil and tangential outlet ducts in its upper portion for the removal of deaerated oil. An axially disposed dipper tube is provided in the upper portion of the chamber for the discharge of air from the chamber.  
       SUMMARY OF THE INVENTION  
       [0006] According to a first aspect of the present invention, there is provided a deaerator comprising first and second deaeration chambers, each chamber exhibiting substantially rotational symmetry about a respective axis, the chambers deriving a supply of pressurized fluid for deaeration from a shared inlet, and each being arranged such that a portion of the supply of pressurized fluid is introduced tangentially into the chamber.  
       [0007] It is thus possible to provide a multi-chamber deaeration device. Splitting the deaeration task into a plurality of smaller deaeration chambers enables deaeration performance to be improved compared with the use of a single chamber deaeration device.  
       [0008] Preferably the chambers are substantially cylindrical. This makes for a simple manufacturing process. However, the chambers may depart from a strict cylindrical shape if this is deemed desirable and space permits. Thus, for example, the radius of the chamber may increase in the vicinity of an outlet region in order to slow the oil down prior to discharge. Similarly, the internal profile of the deaerator may be varied around the vicinity of the inlet aperture if it is desired to cause the surface of the oil during deaeration to vary from the parabaloid of revolution which it would assume during deaeration in a cylindrical deaerator.  
       [0009] Preferably the first and second deaeration chambers are formed in a single housing. The chambers may be arranged in a side by side configuration with an inlet duct located in the region of the junction between the chambers. The inlet duct may contain a separation device, such as a knife edge, in order that the fluid flow from the inlet is directed into the first and second chambers. In such an arrangement, the oil in the chambers forms contra-rotating vortices. The contra rotating flows gives the designer freedom of choice to combine the flows such that the oil momentum combines or subtracts, if desired, at the outlet of the deaerator.  
       [0010] Preferably the outlet region of each deaerator chamber has a cutaway portion. Preferably the cutaway portions facing in separate directions.  
       [0011] Further chambers may be added within a single unit. Thus three or four chambers may be formed together in a group and a common fluid feed line may be tapped off to each of the chambers in order to admit pressurized fluid into the chamber in a tangential direction near the wall and the base of each chamber.  
       [0012] According to a second aspect of the present invention, there is provided a deaeration system comprising a plurality of separation chambers arranged in parallel to receive a source of pressurized fluid for deaeration, wherein a portion of the flow from the source is introduced substantially tangentially into each separation chamber and each chamber has a centrally located air removal path.  
       [0013] The applicant has realized that the provision of a plurality of smaller deaeration chambers enables the deaeration system to be distributed within the free space within a machine containing or requiring deaerated oil. The use of a plurality of chambers also means that each chamber may be made smaller since the throughput of oil in each chamber, compared to a larger chamber is reduced. The reduction in radius of a chamber means that the centrifugal force acting on the oil within the chamber is increased. However in order to realize the true advantage of such a system the flow rate through each chamber should be reduced otherwise the increase in centrifugal force which enhances separation could be defeated by the reduced time that the oil would spend in the chamber during the separation process. Analysis of the fluid dynamics within a deaerator suggests that the diameter of the deaeration chamber should be proportional to the square root of the flow rate. Thus, if two deaerators (or more) are used instead of a single one, each is sized in proportion to the volume of flow that it has to handle. Thus, by using a number of smaller deaerators instead of one large one, the total circumferential distance of the plurality of smaller deaerators is larger then the circumference of the single large deaerator. This increased circumference gives rise to enhanced deaeration performance. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The present invention will further be described, by way of example, with reference to the accompanying drawings, in which:  
     [0015]FIG. 1 is a schematic cross-section through a constant speed generator for use in an avionics environment;  
     [0016]FIG. 2 is schematic diagram of the oil system of the generator shown in FIG. 1;  
     [0017]FIG. 3 is a representation of a twin chamber deaerator, constituting an embodiment of the present invention;  
     [0018]FIG. 4 schematically illustrates the position of the deaeration chamber of FIG. 3 within an oil reservoir;  
     [0019]FIG. 5 is a representation of a modified twin chamber deaerator; and  
     [0020]FIG. 6 shows the modified deaerator in position in an oil reservoir.  
    
    
     [0021] The generator shown in FIG. 1 comprises a housing  1  which encloses a continuously variable transmission utilizing a belt drive, generally designated  2 , a low pressure pump  4 , a high pressure pump  6 , a generator, generally designated  8 , and an oil system disposed throughout the housing  1 .  
     [0022] The belt drive  2  enables the variable speed of an input shaft  10  which receives a drive from a spool of a gas turbine engine to be converted to a near constant speed such that the generator  8  can be run at a near constant speed. In order to do this, a first shaft  12  of the belt drive mechanism carries a flange  14  which defines an inclined surface  16  against which a drive belt bears. The shaft  12  also carries a coaxially disposed movable flange  20  drivingly connected to the shaft  12  via a splined portion (not shown). The movable flange  20  defines a further inclined surface  22  facing towards the surface  16 , which surfaces serve to define a V-shaped channel whose width can be varied by changing the axial position of the flange  20  with respect to the fixed flange  14 . The flange  20  has a circularly symmetric wall  24  extending towards and cooperating with a generally&#39; cup shaped element  26  carried on the shaft  12  to define a first hydraulic chamber  28  therebetween which is in fluid flow communication via a control duct (not shown) with an associated control valve. Similarly, a fixed flange  30  and a movable flange  32  are associated with a second shaft  36  and serve to define a second hydraulic control chamber  34 . A steel segmented belt having a cross-section in the form of a trapezium, with the outer most surface being wider than the inner most surface is used to interconnect the first and second variable ratio pulleys formed between the pairs of fixed and movable flanges, respectively, in order to drivingly connect the flanges. The position of each movable flange with respect to the associated fixed flange is controlled by the hydraulic actuators. Since the interconnecting belt is of a fixed width, moving the flanges closer together forces the belt to take a path of increased radial distance. The interconnecting belt has a fixed length, and consequently as one movable flange is moved towards its associated fixed flange, the other movable flange must move away from its associated fixed flange in order to ensure that the path from an arbitrary starting point, around one of the pulleys, to the second pulley, around the second pulley and back to the fixed arbitrary starting point remains a constant distance.  
     [0023] It is important in such a pulley system that the position of the flanges can be well controlled. It is also important that the compressive force exerted upon the belt can be well controlled since belt wear increases rapidly with compressive force but belt slippage is damaging to both the belt and the pulleys. Thus a controller or control system (not shown) is provided which controls both the drive ratio and the compressive load exerted on the belt. It is important that the controller can rapidly change the hydraulic pressures and fluid volumes within the hydraulic chambers  28  and  34 , and this requires that the hydraulic fluid must have very little or no air entrained therein. This is because air bubbles are by their very nature compressible and the air bubbles will compress when an increase in hydraulic pressure is made in preference to movement of the actuating surfaces. The hydraulic system may require hydraulic pressures in the region of 100 bar. This requires the use of a high pressure pump in order to achieve this hydraulic pressure. The action of pumping fluid to this pressure warms the fluid, and as a result it is not possible, within the limited space available within an aircraft for these components, to utilize a dedicated supply of control fluid since only a small volume of fluid could be provided and this would suffer unacceptable heat rejection problems. Therefore, in order that the heating of the control fluid does not become a problem, the lubricating oil within the generator is also used as the control fluid for the continuously variable transmission. This solves the heat rejection problem, but does require the oil to be highly deaerated prior to its use within the belt control system. However, the oil is also sprayed onto bearings and gears in order to lubricate them and is also used within the rapidly rotating generator  8  as a cooling fluid. These uses allow air to be easily entrained within the oil.  
     [0024]FIG. 2 schematically illustrates the oil system within the power generation system. An oil reservoir  100  acts to contain deaerated oil. The reservoir has a first outlet  102  connected to an inlet of the high pressure pump  6  and a second outlet  104  connected to an inlet of the low pressure pump  4 . An outlet  106  of the high pressure pump  6  provides oil which is ducted towards a primary piston  110  formed by movable flange  20  and the cup shaped element  26  (FIG. 1) thereby defining the first hydraulic control chamber  28 , and a secondary piston  112  (similar to the primary piston) which contains the second hydraulic control chamber  34 . As shown in FIG. 2, both the primary piston  110  and the secondary piston  112  can be regarded as being connected between a high pressure supply line  114  and a low pressure return line  116 . The pressure in the high pressure line  114  is measured by a pressure sensor  118  and supplied to a controller (not shown). The controller uses a measurement of oil pressure, aero-engine drive speed and/or generator speed and electrical demand to schedule and/or control the hydraulic pressure acting in the primary and secondary pistons. The secondary piston  112  is connected directly to the high pressure line  114 . However, the pressure within the high pressure line  114  can be controlled by spilling pressurized lubricant from the high pressure line  114  to the low pressure return line  116  via an electrically controlled pressure control valve  120  connected between the high pressure and low pressure lines, respectively. Thus in order to increase the hydraulic pressure within the secondary piston  112 , the pressure control valve  120  is moved to restrict flow therethrough, and in order to release pressure within the secondary piston, the pressure control valve  120  is opened. A normally closed pressure return valve  122  is connected between the fluid port to the secondary piston  112  and the low pressure return line  116 . The valve  122  is normally closed, but is set to open at a predetermined pressure in order to protect the hydraulic system in the event of system over pressure.  
     [0025] The primary piston  110  receives high pressure fluid from the high pressure line  114  via an electrically operated flow control valve  124 . The valve  124  is in series with the pressure control valve  120  between the high pressure line  114  and the low pressure line  116 , and the primary piston  110  is connected to the node between these valves. This configuration of valves means that the pressure control valve  120  can be used to simultaneously increase the pressure in both the primary and secondary pistons in order to prevent belt slippage, whereas the balance of flow rates through the control valve  124  and the pressure control valve  120  sets the relative positions of the primary and secondary pistons. Oil from the low pressure line  116  is returned to the sump  152 .  
     [0026] An outlet  140  of the low pressure pump  4  supplies oil via supply line  142  to oil cooling jets  144  for spraying oil onto the moving parts of the continuously variable transmission, to jets  146  for spraying oil onto the gear train interconnecting the transmission to the generator. to jets  148  for lubricating the windings and bearings within the generator and also along a cooling path  150  for cooling the stator within the generator.  
     [0027] The generator  8  has a gravity drain to a dry sump  152 . Oil collecting in the sump  152  is pumped out of the sump by a single scavenge pump  154 . The output line from the scavenge pump connects with the low pressure return line  136  via an oil strainer  130 , a remotely mounted oil cooler  132  and an oil filter  134 . A pressure fill connector  156  is in fluid flow communication with the low pressure return line  194  in order to allow the oil system to be filled. An oil cooler by-pass valve  158  is connected between the output from the strainer  130  and the line  136  in order to by-pass the oil cooler and oil filter during cold start or in the event of cooler, filter or external line blockage. The oil by-pass valve is normally closed and set to open at a predetermined over pressure.  
     [0028] In order to drain the system, a drain plug  170  is provided in the reservoir, similarly a drain plug  172  is provided for the sump and a pressure operated vent valve  174  is provided in the generator in order to relieve the excess pressure occurring within the generator. A manually operated vent valve  176  is provided to vent pressure from the generator. An automatic air inlet valve  178  is provided to allow air to enter the generator via an injector pump  196  to provide positive internal pressure.  
     [0029] In use, the oil in the return line  136  flows with a velocity of up to 6 ms 1 . This flow is sufficient to enable the oil to be deaerated within a vortex deaerator.  
     [0030] Vortex deaerators work by exploiting the difference in density between the oil and an air bubble entrained therein. The oil, under pressure, is forced through a restricted aperture in order to increase its velocity. The aperture is arranged to be tangential with the cylindrical wall of the deaerator in order that the oil is spun into a helical path within the deaerator. Clearly, in the steady state, the rate of entry of oil into the deaerator has to be matched by the rate of exit of oil from the deaerator. Assume that the oil exits the restricted aperture with a velocity V and that the deaerator has a radius r.  
     [0031] Analysis of the predominantly circular motion of the oil within the deaerator allows us to calculate that the force acting on unit volume of oil having density ρ is F oil =ω 2 rρ oil . Similarly for a bubble of air, the force is F air =ω 2 rp air .  
     [0032] Thus the force acting on a bubble of air to separate from the oil is F sep =ω 2 t(ρ oil −ρ air ).  
     [0033] The angular velocity ω is related to the exit of velocity V by ω=V/r. From this, it follows that the force acting to separate the air from the oil is proportional to the difference in density between the air and the oil and inversely proportional to the radius of the separator. Thus the separation force increases with decreasing radius. However, merely decreasing the radius alone is not enough. The reason for this is the fluid in the deaerator spins in to a parabaloid of revolution. Considering this parabaloid near the top the deaerator, the oil forms an annulus of depth D around the wall of the deaerator. To a rough approximation, this annulus has an cross-sectional area equal to πrD. This oil has a vertical motion superimposed on its rotating motion, the velocity of the vertical motion being that necessary to ensure that the flow rate from the deaerator matches the flow rate into the deaerator. From this it follows that, if the radius of the deaerator is halved, the axial velocity of oil up the wall of the deaerator is increased, approximately by a factor of 2, such that the product of separating force by separation time remains substantially invariant.  
     [0034] The applicant has realized that benefits can be obtained from using smaller diameter deaerators provided that this does not result in a corresponding reduction of the time upon which the deaeration force acts on the oil.  
     [0035] The provision of a plurality of smaller diameter deaerators each handling a proportion of the total deaeration task provides for better deaeration of the oil than in the prior art. The level of deaeration is important within the context of speed control systems using continuously variable transmission having belt drive of the type described herein before.  
     [0036]FIG. 3 schematically illustrates a suitable deaerator which can be placed within the oil reservoir  100 . The deaerator comprises two cylinders  200  and  202  defined by cylindrical walls  204  and  206 , respectively. The walls  204  and  206  abut and merge together in a central region  208 . An inlet duct  210  extends horizontally from the base of the cylinders. The inlet duct is bifurcated by a knife edge such that it divides into two fluid flow paths. Each path tapers from a cylindrical to a rectangular cross-section, with the long axis of the rectangle extending vertically. The rectangular channel shape intersects with the circular cross-section of the inner of the chambers in order to define an injection region which is tangential with the walls of the chambers. Thus fluid introduced at pressure through duct  210  is split equally into fluid flow paths and injected into the chambers  200  and  202  at increased velocity and tangentially in order to form a vortex.  
     [0037] The uppermost portions of each wall  204  and  206  have semicircular portions cut out from the wall to define semicircular lips of reduced height. One of the lips is indicated generally as  212 . As shown in FIG. 3, the lips face away front each other. These lips provide a controlled discharge route for oil at the uppermost region of the deaerators. Each deaerator also has an axially positioned dip tube  214  and  216  which extends downward into the deaerator and finishes at a plane just above the uppermost portion of the tangential channel. The dip tubes  214  and  216  extend above the upper surface of the deaerator, and in use pass through and extend above a baffle plate.  
     [0038] The position of the deaerator within the oil reservoir  100  is shown in. greater detail in FIG. 4. The deaerator  200  is shown sectioned through the line A-A′ of FIG. 3, and consequently the rectangular nature of the nozzle  220  is clearly shown as is the relative position of the dip tube  214 , and the fact that its lower end  218  lies just above the uppermost portion of the channel  220  through which oil is introduced into the chamber. An upper end  222  of the dip tube vents into a plenum chamber  224  defined above a baffle plate  226 . The plenum chamber vents to the generator via a duct  228 . Once oil has escaped over the upper edge of the deaerator, it collects in the reservoir from where it can flow through the outlet  104  to the low pressure pump and via the pipe  102  to the high pressure pump. The outlet  104  is formed in the lower wall of the reservoir  100 , whereas the feed for the high pressure pump is taken from the center of the reservoir. Thus, should the reservoir become inverted during flight, the feed for the high pressure pump remains within the oil, whereas the feed for the low pressure-pump rises above the oil. This is important since oil supply to the control actuators for the continuously variable drive must be maintained at all times. However, oil flow to the generator must be inhibited since otherwise the generator would start to fill with oil and windage losses would increase dramatically, possibly causing catastrophic failure of the generator. The position of the feed  104  in the lower wall of the oil reservoir ensures that oil flow to the generator is automatically cut should negative G or inversion occur.  
     [0039] The reservoir includes a drain  230  and an overflow  232 . The overflow is significant since it enables the oil system to be filled in a single operation.  
     [0040] In order to fill a previously drained or unfilled lubrication system, overflow  232  is opened and then oil is injected via connector  156  of FIG. 2. This causes oil to flow from the injector up one arm of the low pressure return line through the strainer  130 , the oil cooler  132  the filter  134  and into the reservoir via the line  136  and the vortex deaerator.  
     [0041]FIG. 5 is a schematic representation of a modified twin vortex deaerator. In essence, the deaerator indicated generally as  300  is similar to the deaerator described with respect to FIG. 3, except that it is now in an inverted configuration. Thus, the deaerator comprises two cylinders  302  and  304  defined by cylindrical walls  306  and  308 , respectively. The walls  306  and  308  abut and merge together in a central region  310 . An inlet duct  312  extends horizontally into an upper portion of the cylinders. The inlet duct is bifurcated by a knife edge such that it divides into two fluid flow paths. Each path tapers from a cylindrical to a rectangular cross section with the long axis of the rectangle extending vertically. The rectangular channel shape intersects with the circular cross section of the inner of the chambers in order to define an injection region which is tangential with the walls of the chambers. Thus fluid introduced at pressure through the ducts  312  is split substantially equally into two fluid flow paths and injected into the chambers  302  and  304  at increased velocity and tangentially in order to form a vortex.  
     [0042] The upper end of each cylinder is closed by a respective wall  320  and  322  through which an associated dip tube  324  and  326  extends.  
     [0043] The lowermost portion of each wall  306  and  308  have semicircular portions removed therefrom such that, when the deaerator is disposed inside a oil reservoir with end wall  330  and  332  of the cylinders contacting a wall of the oil reservoir the semicircular portions serve to define oil exit paths such that oil can escape from the deaerator.  
     [0044]FIG. 6 schematically shows the modified deaerator in position within an oil reservoir. The oil reservoir is similar to that described with respect to FIG. 4, and like reference numerals are used for like parts. The main significant difference is that the dip tube  324  is comparatively shorter than the corresponding dip tube  214  shown in FIG. 4. The deaerator  306  is, in use, completely submerged in lubricant within the chamber  340 . Lubricant fed in from the scavenge pump is injected via a vertically disposed aperture  342  positioned towards the upper wall  320  of the deaerator. Oil exits the deaerator via an exit aperture  344 .  
     [0045] Oil received from the scavenge pump will have been passed through an oil cooler and therefore it can be expected to be cooler than the oil already present in the oil reservoir (since heat will lead into the oil reservoir from the generator and gear box surrounding it) and consequently the oil will naturally tend to sink thereby displacing all the oil. The operation of the vortex deaerator and the oil reservoir is substantially as already hereinbefore described.  
     [0046] It is thus possible to provide an oil system which allows the oil to be recovered, filtered deaerated and then the deaerated oil to be stored in a reservoir prior for reuse. This allows a supply of highly deaerated oil to be available for use in both hydraulic control and lubrication applications within the constant speed generator. The provision of a multichamber deaerator enables the height of the deaerator to be reduced and deaeration performance to be enhanced.