Patent Application
20140331857
This invention relates to an on board inert gas generation system for generation of inert gas on board an aircraft to facilitate inerting of the fuel tanks and other areas on board the aircraft.
In this specification the widely accepted terminology is employed with the term ‘inert gas generation’ meaning the generation of an oxygen depleted or ‘nitrogen-enriched atmosphere’ (NEA). In recent years the move towards the use of composites in the construction of aircraft wings has meant that the temperatures within the fuel tanks is greater than that of wings of conventional material due to the lower thermal conduction of the composite. Thus there is an even greater need for effective inerting of the aircraft fuel tanks in composite wings due to the greater temperatures experienced. It is well known to use one or more filters or ‘air separation modules’ (ASMs) which allow separation of a supply of inlet air into a nitrogen-enriched air portion (NEA) and an oxygen-enriched air portion (OEA). In order to run air separation modules efficiently, they need to be supplied with inlet air at a relatively high pressure (typically 40 psig (2.76×105 Pag) or more). It is possible to operate at lower pressures but this would mean that more air separation modules would be required with the consequent increase in weight and complexity, which is undesirable. By way of illustration if the air supplied to an ASM is at 15 psig, then ten ASMs would be required each weighing approximately 27 kg. But if the inlet air is at 56 psig only two ASMs are required to provide the required NEA capacity. In the past, the air separation modules have been supplied with high pressure bleed air from the main aircraft power plant. This has been bled off the compressor, cooled, filtered and then supplied to the ASM or ASMs. This system works well but there is an increasing demand on aircraft manufacturers to reduce the specific fuel consumption (SFC) of the aircraft. It is known that bleeding high pressure air from the compressor has an adverse effect on SFC and so there is now a trend to cease use of high pressure bleed air so that the engine performance can be optimised. This means that an alternative source of fluid for supply to the air separation module needs to be found and at an elevated pressure for the reasons given above.
US2006/0117956 describes an on board inert gas generation system which uses two compressors or stages arranged in series to provide compressed air to the air separation module. In order to provide high pressures to the air separation module, whilst coping with the severe strictures imposed by compressor rotor blade design limitations, US2006/0117956 provides a system in which two centrifugal compressors are run in series. The compressed air from the second stage is passed to an air separation module, but a vent is provided between the second stage compressor and the air separation module to enable the flow from the second compressor to be increased, which results in the second compressor having an increased output pressure whilst using the same compressor rotor blade design. Although this provides the centrifugal compressor with a wider operating range of output flows, it does mean that the operating efficiency is very poor at low flow rates because excess flow is vented through an anti-surge valve to prevent the impeller being damaged. Since the aircraft operates at cruise during the major part of its operation, this means that for the majority of the time the centrifugal compressor arrangement is operating at well below its optimal operating efficiency.
Thus the inherent characteristics of a centrifugal compressor are ill-adapted for the operating regime and variations in the flow rates and pressures required during the cycle of ascent, cruise and descent of an aircraft and have resulted in unnecessarily complex solutions such as those set out above, which only partly tackle the issues. As noted, the ASM operates effectively at pressures above 40 psig (2.76×105 Pag). Lower pressures require a larger ASM or several ASMs (and therefore increase weight) for a given duty, whilst higher pressures may exceed the maximum working pressure of the ASM. The flow requirement for an inerting system varies with flight phase. Descent requires the maximum NEA flow-rate as the inerting system is required to re-pressurise the fuel tanks to equalize the tank and ambient pressures. Cruise requires minimum flow-rate as the NEA flow-rate is only required to make up the increase in ullage volume created by fuel burn. The ratio between maximum descent flow and cruise flow is typically up to 6:1 depending on aircraft type, cruise altitude and descent rate. This does not fit well with typical centrifugal compressor characteristics which have a very narrow flow range bounded by the surge limit and the diffuser ‘choking’ limit. In a centrifugal compressor flow can be increased by increasing speed but the pressure generated increases as the square of the speed, and the power required increases by the cube of the speed. The additional pressure must be regulated to avoid damage to the ASM. This makes it very inefficient over the flow range required by an inerting system.
In an arrangement such as is described in US2006/0117956 a centrifugal compressor is driven at a strictly limited speed range and the compressor is run on maximum load condition. NEA flow to the fuel tanks is controlled by a NEA flow control valve and, at part load, excess flow from the compressor is dumped to atmosphere through a surge control valve. This is an inefficient use of the motor and arises because when using a centrifugal compressor, the motor speed is strictly limited due to the surge constraint.
An aspect of the invention provides an on board inert gas generation system for use in an aircraft including a source of low pressure air, the system comprising: a rotary positive displacement compressor including an inlet configured to receive a portion of the low pressure air, and an outlet; an air separation module (ASM) in flow communication with the ASM, the ASM being configured to, in use, deliver a nitrogen-enriched air (NEA) fraction and an oxygen-enriched air (OEA) fraction, the NEA fraction being supplied to a space to be inerted; a motor configured to drive the rotary positive displacement compressor; a sensor configured to monitor a composition of at least one of the NEA fraction delivered by the ASM, the OEA fraction delivered by the ASM, and a further fraction in the space to be inerted; and a controller responsive to the sensor, the controller being configured to control a motor speed in accordance with the composition monitored.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
We have found that the characteristics of a positive displacement type compressor are very well suited to provide the large variations in flow, because they provide a flow rate generally proportional to speed, at a pressure sufficient to supply the pressure required by the ASM and without the substantial pressure increases at higher flow rates, which can reduce ASM life. Therefore we have designed an on board inert gas generation system which is intended to obviate some of the problems encountered with centrifugal compressor based systems.
Accordingly, we have designed a system where the flow delivered by the compressor can be modulated across a wide range without substantially compromising efficiency of the compressor or the ASM. An important advantage of this system is that it allows the on board inert gas generation system to be controlled by varying the compressor speed in accordance with the purity of the detected atmosphere in the space to be inerted, and/or in accordance with a required operating pressure in the ASM.
Accordingly, in one aspect, this invention provides an on board inert gas generation system for use in an aircraft having a source of low pressure air, said gas generation system including a rotary positive displacement compressor having an inlet for receiving a portion of said low pressure air, an outlet in flow communication with an air separation module which in use delivers a NEA fraction and an OEA fraction, the NEA fraction being supplied to a space to be inerted, a motor for driving said rotary positive displacement compressor, a sensor for monitoring the composition of at least one of the fractions delivered by said ASM and/or the composition in said space to be inerted, and a controller responsive to said sensor for controlling the motor speed in accordance with said monitored composition.
Preferably the positive displacement compressor is a rotary device providing a substantially constant and continuous flow in use.
Preferably said sensor monitors the composition of the NEA. This may be done by monitoring either the oxygen content or the nitrogen content, or both.
Preferably said system includes a flow control valve in the flow path downstream of the ASM, and the controller is operable to control the flow control valve to vary the flow through said passage in accordance with NEA requirements. Thus, where the NEA is supplied to a fuel tank, the desired flow may be determined on the basis of the atmosphere composition in the fuel tank, the ullage volume and the flight condition. The flight condition and Oxygen content in the fuel tank determines the NEA purity and NEA flow required during that phase.
In another aspect this invention provides an on board inert gas generation system for use in an aircraft having an on board source of low pressure air, said gas generation system including a positive displacement compressor having an inlet for receiving a portion of said low pressure air, and an outlet in flow communication with an air separation module which in use delivers an NEA fraction and an OEA fraction, a flow control valve in the flow path downstream of the ASM, a sensor for monitoring at least one of the fractions delivered by said ASM and/or the composition in the space to be inerted, and a controller responsive to said sensor for controlling the flow valve in accordance with the monitored composition.
In another aspect this invention provides an on board inert gas generation system for use in an aircraft having an on board source of low pressure air, said gas generation system including a positive displacement compressor having an inlet for receiving a portion of said low pressure air, and an outlet in flow communication with an air separation module, which in use delivers an NEA fraction and an OEA fraction, with the NEA fraction being supplied to a space to be inerted, and the OEA fraction being stored and/or delivered for use on board the aircraft.
The term ‘low pressure air’ used herein means air which is below the inlet pressure required by the air separation module, is generally at a pressure less than 40 psig and typically in the range of from 20 psig to 30 psig. In one scheme the low pressure air may be low pressure engine bleed air. In another scheme the low pressure air may be ram air.
In one arrangement, in order to provide at least some of the power to drive the compressor, the gas generation system may include a turbine for receiving and expanding a portion of cabin air. The turbine may be drivably connected to said positive displacement compressor to provide direct mechanical drive. Instead, or additionally, the turbine may be drivably connected to an electrical generator.
In a motor-driven configuration, an electric motor may be drivably connected to said positive displacement compressor, which conveniently receives electrical energy from said generator or an energy storage arrangement associated therewith. Furthermore, said electric motor may be connectable to receive electrical energy from an aircraft electrical supply. The motor may provide all the power required, or a portion thereof, with the balance being provided by shaft power, for example from a turbine as above.
A power controller may be conveniently provided for selectively receiving electrical energy from said generator (or an electrical storage arrangement associated therewith), and electrical energy from the aircraft electrical supply, and for controllably supplying electrical energy to said electric motor.
The inert gas generation system may include a heat exchanger in the flow path between said positive displacement compressor and said air separation module, the heat exchanger having heating and cooling passes for fluid, with the air from said positive displacement compressor being passed along said cooling pass thereby to reduce the temperature of air supplied to said air separation module. The heat exchanger may receive relatively cool ram air from a ram air duct. The system may include a duct for supplying cabin air to the heating pass of said heat exchanger and a duct for supplying said heated air from the heating pass of the heat exchanger to the input of said turbine. In this case a valve may be provided for selectively supplying relatively cool ram air or cabin air to said heat exchanger.
In another aspect, this invention provides an on board inert gas generation system for use in an aircraft having a source of low pressure air, said inert gas generation system including a compressor having an inlet for receiving a portion of low pressure air and an outlet in flow communication with an air separation module, and a further portion of low pressure air to a turbine for receiving and for extracting therefrom at least a proportion of the energy required for driving the compressor. The low pressure air may be ram air or low pressure bleed air from the aircraft power plant.
In yet another aspect, this invention provides a method for operating an on board inert gas generation system in an aircraft having a source of low pressure air (e.g. ram air or low pressure engine bleed air), which comprises the steps of:
In yet another aspect, this invention provides a method for operating an on board inert gas generation system in an aircraft having a source of low pressure air (e.g. ram air or low pressure engine bleed air), which comprises the steps of:
In yet another aspect, this invention provides a method for operating an on board inert gas generation system in an aircraft having a source of low pressure air (e.g. ram air or low pressure engine bleed air), which comprises the steps of:
The invention also extends to an aircraft incorporating an on board inert gas generating system as set out above.
Whilst the invention has been described above, it extends to any inventive combination or sub-combination of any of the features disclosed herein alone or jointly with others.
The embodiments described below employ a variable speed mechanically and/or electrically driven positive displacement boost compressor to supply air at suitable pressure and flow to an air separation module to inert the fuel tanks of aircraft. An energy recovery turbine is combined with the compressor to reduce electrical power drain by using cabin air supply for both compressor and turbine.
The embodiments make use of passenger cabin air which is provided by the aircraft Environmental Control System (ECS) which requires power from the propulsion engines and increases engine specific fuel consumption. Having circulated through the cabin the air is then vented to atmosphere through overboard vent valves as a waste product. Using this air for fuel tank inerting purposes incurs no additional increase in Specific Fuel Consumption (SFC) as this has been paid for by the ECS. Cabin pressure is typically 11 or 12 psia at cruise altitude, which is too low for the air separation module (ASM) which separates the air into Nitrogen Enriched Air (NEA) and Oxygen Enriched Air (OEA) and which as noted typically operates at pressures in excess of 40 psig. From the ASM the OEA is vented overboard as a waste product and the NEA is passed to the fuel tanks to provide an inert ullage atmosphere. The embodiments below use a turbine to generate power during the cruise phase by using ‘free’ cabin air to provide power to a variable speed positive displacement compressor.
In the first embodiment, illustrated in
Referring to
Referring now to
An important benefit of the various embodiments described herein is that they reduce SFC at cruise altitude, where aircraft economics are most critical. Descent is a relatively short period where power consumption is less critical and, in any event, sufficient power may be available as large electrical loads (e.g. galley ovens) are not in demand in the descent phase, so the use of electrical power to drive the compressor does not impose constraints on aircraft electrical generator sizing.
Referring now to
In some situations such as where the aircraft is on the ground or low speed flight the ram air pressure may be insufficient to drive flow through the heat exchanger and in such conditions an ejector may be used. Thus a portion of the air from the compressor 40 may be tapped from the path between the supply check valve 60 and the heat exchanger 46. The tapped flow passes to an ejector 76 which operates to draw a cooling stream of ram air through the heat exchanger 46 via a control valve 78 and then exhausts the flow overboard via a ram ejector control valve 80. Alternatively a fan may be provided to draw the ram air through the heat exchanger 46.
Referring now to the embodiment illustrated in
The desired NEA flow depends on the oxygen content in the fuel tank, the ullage volume and the flight condition. During cruise the NEA flow is low as it only has to make up the volume created by fuel burn. In this phase the NEA purity is high in Nitrogen and the Oxygen content in the tank steadily reduces with time to a level much lower than the maximum allowable level. During descent the NEA flow is required to recompress the fuel tank ullage to increasing atmospheric levels to prevent air at 21% Oxygen entering. During this phase the NE purity may be higher than the maximum allowable but it mixes with the low tank levels to provide a net level within allowable limit. Having determined the required NEA flow, a controller controls a flow control valve to adjust the NEA flow rate accordingly. As the NEA flow rate is varied, this will affect the pressure in the NEA flow path from the ASM. This is compensated for by adjusting the speed of the compressor to restore the desired pressure in the flow path. The pressure in the ASM depends on the required mass flow rate and required oxygen concentration and thus the desired pressure in the ASM line will depend on the flight phase. As the NEA flow is increased at the start of descent the ASM inlet pressure will drop due insufficient supply air flow. The controller will sense this pressure drop and signal the compressor motor to increase speed and therefore delivery air flow to return the ASM inlet pressure to the controlled level to match the increased flow demand.
Referring now specifically to
Referring now to
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B, and C” should be interpreted as one or more of a group of elements consisting of A, B, and C, and should not be interpreted as requiring at least one of each of the listed elements A, B, and C, regardless of whether A, B, and C are related as categories or otherwise. Moreover, the recitation of “A, B, and/or C” or “at least one of A, B, or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B, and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 3417/DEL/2011 | Nov 2011 | IN | national |
| 1201893.3 | Feb 2012 | GB | national |
This application is a U.S. National Stage application under 35 U.S.C. §371 of International Application No. PCT/EP2012/073685, filed on Nov. 27, 2012, and claims benefit to Indian Patent Application No. 3417/DEL/2011, filed on Nov. 29, 2011, and British Patent Application No. 1201893.3, filed on Feb. 3, 2012. The International Application was published in English on Jun. 6, 2013, as WO 2013/079466 A2 under PCT Article 21(2).
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/073685 | 11/27/2012 | WO | 00 | 5/27/2014 |
