Patent Publication Number: US-2021179275-A1

Title: Aircraft environmental control system

Description:
FOREIGN PRIORITY 
     This application claims priority to European Patent Application No. 19215042.3 filed Dec. 10, 2019, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure is concerned with environmental control systems for aircraft which provide pressurised and conditioned air to the aircraft cabin for the health and comfort of passengers and crew. 
     BACKGROUND 
     Environmental control systems (ECS) are provided in aircraft to provide pressurised and conditioned air to the aircraft cabin. Regulations provide for the minimum flow of conditioned air to be fed into the cabin per passenger. The Federal Aviation Authority (FAA) requires that fresh air flow rate to be at least 0.25 kg/min per passenger in order to dilute contaminants generated in the cabin, to provide thermal comfort and oxygen for occupants and to maintain cabin pressure. An ECS must be able to comply with such regulations while maximising efficiency in terms of power consumption but also minimizing overall size and weight of the ECS. 
     Generally, particularly in commercial aircraft, fresh air from the aircraft engine (bleed air) or compressed ambient air is used to provide the ECS air flow. The incoming air is, however, at a relatively high temperature and pressure and needs to be conditioned to the appropriate temperature and pressure before it is fed into the cabin. The way this is usually done is to use ambient air, brought into the system via an air intake device, such as a scoop. This air—so-called RAM air—is used in a system of heat exchangers to cool the bleed air or compressed ambient air. The RAM air is firstly used in a main heat exchanger (MHX) as a heat sink to cool the bleed air or compressed ambient air and then in a primary heat exchanger (PHX). By the time the RAM air has passed through the MHX, its temperature has already increased substantially. The ECS of an aircraft consumes the majority of the non-propulsive power. Much of this energy is consumed in extracting and conditioning the bleed air. 
     There is great pressure on the aircraft industry to improve energy efficiency and to reduce emissions and there is, therefore, a need for a more energy efficient ECS. It would be desirable to reduce the amount of bleed air required by the ECS. 
     SUMMARY 
     According to one aspect, there is provided an aircraft environmental control system, comprising a bleed air input and a RAM air input; heat exchanger means for receiving bleed air from the bleed air input and RAM air from the RAM air input and using the RAM air to cool the bleed air, and means for providing the cooled bleed air to the aircraft, the system further comprising an ejector arranged to receive bleed air from the bleed air input at a nozzle shaped to reduce the pressure of the received bleed air such as to create a low pressure area in the ejector, the ejector having a port arranged such that ambient air is drawn into the ejector due to the low pressure area in the ejector, and wherein the ambient air is mixed with bleed air to provide mixed air that is combined with the cooled bleed air which is recovered to an intermediate pressure and temperature when leaving ejector. This air stream leaving the ejector is conditioned, including compressed and cooled, before combining with another stream of bleed air that is pre-cooled by a primary heat exchanger (PHX). The combined air is then conditioned through the ECS before provided to the aircraft. 
     A corresponding method is also provided. 
     In a preferred embodiment, the mixed air leaving the ejector is compressed by a compressor and then cooled by a heat exchanger before being combined with the bleed air out from the primary heat exchanger, PHX. 
     The mixed air leaving the ejector is preferably compressed wherein the compressor is powered by a turbine extracting power from conditioned air exhausted from the aircraft cabin. 
     Preferably the mixed air is cooled by a cooling fluid in the heat exchanger and the cooling fluid comprises conditioned air exhausted from the aircraft cabin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an environmental control system according to a first embodiment in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows one embodiment of an ECS according to the disclosure. It is envisaged that other configurations could fall within the scope of the invention as described by the claims. 
     In order to reduce the amount of bleed air used by the ECS, the system of the present disclosure uses an ejector to suck ambient air into the system to compensate bleed air flow rate. 
     First, the parts of the ECS also known from conventional ECS systems will be described. 
     The ECS provides conditioned air to the aircraft cabin  20 . The source air is bleed air  0  from the aircraft engine. This is provided to the ECS via a flow control valve  1  which changes position depending on flight conditions of the aircraft. As the bleed air temperature and pressure are too high for the conditioned air, cool RAM air  15  is provided into the ECS. The RAM air is used as the cooling or working fluid in a heat exchange system to which the bleed air is provided as the fluid to be cooled. The heat exchange system can be any available heat exchanger known in the art. In the examples shown, the heat exchange system comprises a main heat exchanger MHX  160  followed by a primary heat exchanger PHX  180 . The RAM air acts as a heat sink, in the heat exchange system, thus cooling the bleed air. 
     A split valve  2  is controlled by a controller  3  to split the bleed air into two streams. One stream goes to the primary heat exchanger  180 , and the other flows to a compressor  230 , then, preferably, through a heat exchanger  220 , and then the streams are mixed at a mixer  225 . The mixed air is then compressed at  235 . 
     After the compression phase, the main stream passes through the main heat exchanger  160  and then passes through the hot side of the reheater, RHX,  250  and condenser, CON,  260  where condensates form. After entering the water separator, WS,  270  the collected condensate is sprayed into the RAM channel to enhance heat transfer effect. The export dry air undergoes temperature and pressure reductions in the turbine, T,  290  after passing through the cold side of RHX. The conditioned bleed air is provided via the cold side of the condenser  260  to the cabin  20  to set the cabin air to the desired temperature and pressure. The cabin air needs to be maintained at the required temperature and pressure, and as the air becomes warm it is fed out of the cabin as exhaust air  13  and is replaced by new conditioned air. As mentioned above, this exhaust air is then usually just emitted to the outside environment as waste. 
     In order to reduce the amount of bleed air used by the ECS, the system of the present disclosure uses an ejector to suck in ambient air as will be described in more detail below, with reference to  FIG. 1 . The system uses an ejector  30  which consists of a converging/diverging nozzle, a mixing chamber and a diffuser, located in the stream of bleed air from the split valve  2  to the compressor  230 . The ejector  30  uses the pressure energy in the bleed air moving through it to convert to velocity energy by means of an adiabatic expansion in the ejector. Due to the pressure drop in the fluid as its pressure energy is converted to velocity energy, a low pressure zone is created before the mixing chamber. This low pressure sucks in ambient air at a port  31  of the ejector  30  which mixes with the bleed air in the mixing chamber. The mixed fluid then enters the diverging portion (diffuser) of the ejector where its velocity energy is converted to pressure energy—i.e. the mixed air is slowed down and the pressure increases to an intermediate pressure. The expanded mixed air also has an intermediate temperature. This air is provided at outlet  32  of the ejector. 
     The air from outlet  32  is then compressed by compressor  230 , after which it is mixed with the first stream of bleed air from the split valve  2  at the mixer  225 . 
     Operation of the rest of the system is the same as for a conventional ECS. Thus, the present system uses the ejector to suck in ambient air to be mixed with bleed air to provide the conditioned air, thus reducing the amount of bleed air needed. 
     An embodiment of the ECS of this disclosure can also provide a further efficiency advantage, making use of exhaust air that would usually be wasted. 
     Usually, when an ECS operates, fresh and conditioned air is introduced into the cabin and an exhaust valve is used to exhaust the correct amount of air from the cabin and maintain the prescribed pressure. During flight, the exhaust air is usually at a higher temperature and pressure than ambient air but has already been cooled by the air conditioning system and is, therefore, cooler than, and at a lower pressure than incoming bleed air or compressed ambient air. The exhaust air is dumped overboard and, although, as described above, much energy has been involved in conditioning that air for use in the ECS, the air exhausted from the ECS is essentially wasted. 
     This exhaust air can be put to use in a system according to this disclosure, as described below. 
     Because the temperature of the air at the output of the compressor  230  may be too high, it can be cooled, before entering the mixer  225 , by a heat exchanger  220 . In a preferred embodiment, and to make efficient use of the exhaust air that is expelled from the cabin  20 , Exhaust air  13  can be used in the ECS to drive a turbine  245  that drives the compressor  230 . Further, the exhaust air  13 , expanded by the turbine  245 , can be used in the heat exchanger as cooling fluid. An exhaust air valve  255  that is usually already present to eject the exhaust air from the cabin  20  may be used to regulate the amount of exhaust air passing through the turbine  245 , thus regulating the power for the compressor  230 . 
     In some cases (such as in ground condition) there is very little difference between cabin pressure and ambient pressure and so the exhausted air could not be expanded enough by the turbine  245 . In that case, the system of this disclosure would close off the stream flow to the ejector  30  and the system would operate as a conventional ECS. 
     As mentioned above, the controller  3  controls the split valve to control the amount of bleed air provided to the ECS via the conventional conditioning arrangement and the bleed air that is directed to the ejector to cause ambient air to be drawn into the system. There may be a default control depending on e.g. flight conditions, aircraft speed or the like. It is also envisaged that the control can be adjusted to obtain optimal efficiency. 
     The system of this disclosure provides several benefits, including a reduction in bleed air extraction due to compensation from ambient air; reduction in fuel burn of the ECS; recovery of exhaust air is possible meaning that no additional energy is required; and the ECS can be operated in a more energy efficient and environmentally friendly way, etc.